Practical Penetrant Inspection (NDT30P) : World Centre For Materials Joining Technology [PDF]

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WORLD CENTRE FOR MATERIALS JOINING TECHNOLOGY

PRACTICAL PENETRANT INSPECTION (NDT30P)

TWI Ltd, Training and Examination Services

NDT30P PRACTICAL PENETRANT INSPECTION DAY 1 TIME

SUBJECT

9.00 - 9.15

Introduction to the course and administration

9.15 - 10.30

Introduction to NDT

10.30 - 10.45

Break

10.45 - 12.15

The 6 steps of Penetrant Inspection, including practical demonstrations

12.15 - 13.00

Lunch

13.00 – 15.00

First practical exercises of Water washable, Post emulsifiable and Solvent removable techniques

15.00 – 15.15

Break

15.15 – 16.30

The theory of Penetrant inspection Part 1

16.30 – 17.00

Issue of Coursework paper No. 1 Product technology video

DAY 2 TME

SUBJECT

8.45 - 9.00

Review Coursework paper No. 1

9.00 - 10.30

The theory of Penetrant inspection Part 2

10.30 - 10.45

Break

10.45 - 12.15

The theory of Penetrant inspection Part 2 Continued

12.15 - 13.00

Lunch

13.00 – 15.00

Practical exercises

15.00 – 15.15

Break

15.15 – 16.30

Practical exercises

16.30 – 17.00

Issue of Coursework paper No. 2 Product technology video

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NDT30P PRACTICAL PENETRANT INSPECTION DAY 3 TIME

SUBJECT

8.45 - 9.00

Review Coursework paper No. 2

9.00 - 10.30

Product technology theory

10.30 - 10.45

Break

10.45 - 12.15

Instruction and report writing

12.15 - 13.00

Lunch

13.00 – 15.00

Practical exercises including writing an instruction

15.00 – 15.15

Break

15.15 – 16.30

Practical exercises including writing an instruction

16.30 – 17.00

Issue of Coursework paper No. 3 and Product technology paper Product technology video

DAY 4 TIME

SUBJECT

8.45 - 9.00

Review Coursework paper No. 3 and Product technology paper

9.00 - 10.30

Course review

10.30 - 10.45

Break

10.45 - 12.15

Final coursework paper

12.15 - 13.00

Lunch

13.00 – 15.00

Final course practical test

15.00 – 15.15

Break

15.15 – 16.00

Final course practical test continued

16.00 – 16.30

Final coursework review

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PRACTICAL PENETRANT INSPECTION CONTENTS Section

Subject

1.0

PRINCIPLE 1.1 PENETRABILITY

2.0

FLAW ENTRAPMENT EFFICIENCY 2.1 VOLUME 2.2 LENGTH 2.3 CONTAMINANTS 2.4 PENETRANT DYE 2.5 PROCESSING

3.0

PENETRANT PROPERTIES 3.1 WETTING ABILITY 3.2 SPECIFIC GRAVITY 3.3 FLASH POINT 3.4 VOLATILITY 3.5 CHEMICALLY INHERT 3.6 VISCOSITY 3.7 SOLUBILITY 3.8 SOLVENT ABILITY 3.9 TOLERANCE TO CONTAMINANTS 3.10 HEALTH HAZARD 3.11 AVAILABILITY AND COST 3.12 ELECTRICAL CONDUCTIVITY

4.0

FLUORESCENT PENETRANTS AND THE ELECTROMAGNETIC SPECTRUM 4.1 TYPES OF UV-A LAMP 4.2 FLUORESCENT DYE

5.0

DEVELOPMENT 5.1 CAPILLARITY 5.2 LIGHT SCATTERING 5.3 SOLVENT ACTION

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Section

Subject 5.4

6.0

DEVELOPER PROPERTIES

PROCESS SEQUENCE 6.1 PREPARATION AND PRE-CLEANING 6.2 CLEANING METHODS 6.2.1 MECHANICAL METHODS 6.2.1.1 BRUSHING 6.2.1.2 BLASTING 6.2.2 CHEMICAL METHODS 6.2.2.1 HOT SOLVING DE-GREASING 6.2.2.2 VAPOUR DE-GREASING 6.2.2.3 COLD SOLVENT DE-GREASING 6.2.2.4 SOLVENT MATERIALS WITH EMULSIFIERS AND DETERGENTS 6.2.2.5 ALKALINE CLEANING 6.2.2.6 ACID PICKLING 6.2.2.7 STEAM CLEANING 6.2.2.8 PAINT REMOVAL 6.3 APPLICATION OF PENETRANT 6.3.1 SPRAYING 6.3.2 AEROSOL SPRAY 6.3.3 ELECTRO-STATIC SPRAY 6.3.4 DIP AND DRAIN 6.3.5 DIPPING HEATED PARTS 6.3.6 THIXOTROPIC 6.4 REMOVAL OF EXCESS PENETRANT 6.4.1 WATER AS A REMOVER 6.4.2 LIPOPHILIC REMOVER (EMULSIFIER) 6.4.3 SOLVENT REMOVAL 6.4.4 HYDROPHILIC REMOVER (DETERGENT) 6.4.5 SPRAY-SCRUBBER PENETRANT REMOVAL 6.4.6 WATER AND SOLVENT 6.5 DRYING 6.5.1 HOT AIR RE-CIRCULATING OVEN 6.5.2 FORCED WAM AIR 6.5.3 DRY-CLEANED COMPRESSED AIR 6.6 DEVELOPMENT 6.6.1 DRY DEVELOPER

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Section

Subject 6.6.2 6.6.3 6.6.4

6.7

6.8 6.9 6.10 7.0

DRY POWDER DEVELOPER APPLICATION AQUEOUS LIQUID DEVELOPER WATER-SUSPENDED DEVELOPER (WET DEVELOPER) 6.6.5 WATER – SOLUBLE DEVELOPER 6.6.6 SOLVENT BASED DEVELOPER 6.6.7 PLASTIC FILM DEVELOPER INSPECTION 6.7.1 FLOURESCENT INSPECTION 6.7.2 COLOR CONTRAST PENETRANTS 6.7.3 INSPECTION AIDS RECORDING POST CLEANING PROTECTION

SYSTEM CLASSIFICATION 7.1 TYPES OF PENETRANT 7.1.1 COLOUR CONTRAST PENETRANTS 7.1.2 FLOURESCENT PENETRANTS 7.1.3 FLOURESCENT V COLOUR CONTRAST 7.1.4 DUAL PENETRANTS 7.2 EXCESS PENETRANT REMOVAL 7.2.1 WATER WASHABLE 7.2.2 SOLVENT REMOVAL 7.2.3 POST EMULSIFIABLE 7.2.4 HYDROPHILIC REMOVER (WATER-DILUTABLE) 7.2.5 WATER AND SOLVENT 7.3 TYPES OF DEVELOPER 7.3.1 DRY POWDER DEVELOPER 7.3.2 AQUEOUS DEVELOPER 7.3.3 SOLVENT BASED 7.3.4 WATER OR SOLVENT BASED FOR SPECIAL APPLICATION 7.4 SYSTEM CLASSIFICATION 7.4.1 BS EN 571 7.4.2 MIL-L-25135 C 7.4.3 MIL-I-25135 E

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Section

Subject

8.0

CHOICE OF PENETRANT SYSTEM 8.1 SIZE AND TYPE OF DEFECT 8.2 GEOMETRY AND INTRICACY 8.3 SURFACE CONDITION 8.4 OTHER FACTORES TO BE CONSIDERED 8.5 EXAMPLES

9.0

EQUIPMENT CHECKS 9.1 OVERALL SYSTEM PERFORMANCE 9.2 CONTROL CHECKS 9.2.1 WATER WASH TEMPERATURE AND PRESSURE 9.2.2 COLOUR INTENSITY 9.2.3 PENETRANT REMOVER CHECK 9.2.4 DEVELOPER CHECK 9.2.5 UV LAMP OUTPUT CHECK 9.2.6 UV MONITOR CHECK 9.2.7 WATER REMOVABLE PENETRANT, WATER TOLERANCE CHECK 9.3 MAINTENANCE CHECKS 9.3.1 TANK LEVELS 9.3.2 EQUIPMENT CLEANLINESS 9.3.3 AIRLINE CLEANLINESS 9.3.4 PROCESSING UNITS 9.3.5 UV LAMP MAINTENANCE 9.3.6 CLEAN TANKS

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1.0 PRINCIPLE Penetrant testing (PT), alternatively referred to as dye penetrant inspection (DPI), penetrant flaw detection (PFD) and liquid penetrant inspection is often said to function through the phenomenon of capillarity. This is the elevation or depression of the surface of a liquid where it is in contact with a solid, such as the sides of a tube to form a meniscus. It can be most clearly seen when using tubes of a very fine diameter, known as capillary tubes, and is dependant upon the balance between the surface tension of the liquid (cohesive forces) and the wetting of the sides of the tube (adhesive forces). If the adhesive forces exceed the cohesive the surface of the liquid will be concave and the liquid will rise up the tube. In practice the finer the bore of the capillary tube the greater will be the rise seen. If the cohesive forces exceed the adhesive then the surface will be convex and the liquid will fall below level of the surrounding liquid. Penetrant inspection requires the former of these 2 situations.

Capillarity

1.1

PENETRABILITY

Penetrability is however a complicated property and not quite so simple as the analogy with capillary tubes suggests. What is not disputable is that it is influenced by variables such as surface condition and type of the test object, the type of penetrant, the temperature, and the presence or absence of contamination. The formula shown below has been used to link together these factors.

P = P S θ D

= = = =

2S Cos θ D

Capillary pressure Surface tension of the liquid Contact angle Width of the crack

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From the formula it can be seen that high capillary pressure requires high surface tension but it should be stated however that a material with a high surface tension is not necessarily a good penetrant an example being water which has a high surface tension but is a poor penetrant. θ represents the angle formed between the liquid-air interface and the liquidsolid interface. The importance of the ability of a liquid to wet the sides of a tube has been stated previously and the smaller the contact angle is, the higher will be capillary pressure.

Contact Angle A good penetrant must have high wetting ability and hence have a low contact angle typically below 5 degrees. The wetting ability of a penetrant can however vary from one type of surface to another and dwell time should therefore be varied accordingly. Contact Angle

Wetting Ability

Less than 90o

High

90o

Moderate

Greater than 90o

Low

Droplet Shape

Contact Angle, Wetting Ability and Droplet Shape The formula also shows that the width of the discontinuity or opening influences capillary pressure, in fact the narrower the opening, the higher the capillary pressure. Penetrants have been known to enter a space of 0.3 microns width, that is one third of one thousandth of a millimetre. Evidence of this can be seen from capillary rise experiments where liquid will rise higher in narrower tubes although it will take longer for this rise to occur. In terms of a penetrant inspection finer defects will thus require longer dwell times. In the USA a similar formula known as the Static Penetration Parameter (SPP) is utilised. SPP = Practical Penetrant Inspection Rev 0 Dec 2005 Principle Copyright © 2005, TWI Ltd

1.2

SCos θ WORLD CENTRE FOR MATERIALS JOINING TECHNOLOGY

S again represent the surface tension the liquid and Cos θ is the contact angle between the liquid-air interface and the liquid-solid interface. The importance of the ability of a liquid to wet the sides of a tube is again shown and the smaller the contact angle is, the higher will be the SPP value. A fourth factor worth considering is viscosity, which while not significantly affecting a liquid's ability to enter a discontinuity does influence the rate at which it does so. Experience and the formula below for Kinetic Penetration Parameter (KPP) both show that a highly viscous penetrant takes much longer to enter a defect and thus requires a longer dwell time. As viscosity is strongly influenced by temperature, so are penetrant inspections. A viscous penetrant that has been dipped or sprayed will also drain more slowly from a specimen and cause excessive loss of penetrant due to drag out into the wash station.

S Cos θ

η

KPP =

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2.0 FLAW ENTRAPMENT EFFICIENCY A term used to describe the efficiency of a penetrant inspection is "flaw entrapment efficiency" which describes the ability of a penetrant to form an indication large enough to be detected. Some of the factors influencing Flaw Entrapment Efficiency are: •

Volume of a defect

•

Length of defect

•

Contaminants

•

Penetrant Dye

•

Processing

2.1

VOLUME

The size of an indication is based on the volume of the defect it has entered. The larger the discontinuity is in terms of its depth and width, the more penetrant it will hold and the more penetrant there will be present to form the indication in the developer.

2.2

LENGTH

The length of a defect whilst influencing the volume of the penetrant present has a strong influence upon the ability of the human eye to detect the indication. Very fine indications such as those formed by fine fatigue or stress corrosion cracks will have insufficient width to be detected visually and will only be located when they are of sufficient length.

2.3

CONTAMIMANTS

Penetrating fluids will generally enter fine, clean discontinuities more readily than wide and contaminated ones but many in-service inspections will encounter defects contaminated with oil, water and corrosion products. These can both reduce the volume available to the penetrant and in the case of water adversely influence the contact angle of the penetrant. Highly acidic or alkaline contaminants also cause fading of the dye present within the penetrant, likewise heat and prolonged exposure to ultraviolet light can also cause penetrant dyes to lose their brilliance.

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2.4

PENETRANT DYE

The efficiency of a penetrant inspection will be influenced by •

Type of dye

•

Concentration of dye

The most obvious influence of dye type is seen when one changes between colour contrast and fluorescent penetrant dyes, the latter giving much higher sensitivity than the former. Within these 2 categories the brilliance and intensity of the dye colour will strongly influence sensitivity. Penetrant manufacturers will within a classification such as water washable fluorescent penetrants, offer varying sensitivity levels. This can be achieved by altering the dye concentration and in the case of water washable penetrant the level of emulsifier.

2.5

PROCESSING

Dye concentration can be affected by utilising the dip and drain method of processing. This allows more volatile constituents of a penetrant to evaporate during the dwell time and thus increases the concentration of the dye within the remaining penetrant. The degree of penetrant removal must also be considered. Components that are cleaned until there is no background coloration present will offer a high degree of contrast for any penetrant indications. This absence of background coloration is interpreted in some cases as evidence of over-emulsification and over-washing and that penetrant may have been removed from defects. A small degree of background shows that over-washing has not occurred. This should not be excessive however as the brightness of the indication must exceed the background's brightness in order to be detected.

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3.0 PENETRANT PROPERTIES •

Wetting ability

•

Specific gravity

•

Volatility

•

Chemical activity

•

Solubility

•

Solvent ability

•

Tolerance to contaminants

•

Health hazard

•

Flammability

•

Electrical conductivity

•

Availability and cost

3.1

WETTING ABILITY

The wetting ability of penetrants is an important physical property that affects their penetrability and bleed-back characteristics. The contact angle and surface tension of a penetrant control wetting ability.

3.2

SPECIFIC GRAVITY

Specific gravity is a comparison of the density of a penetrant with the density of distilled water at 40C. It is normally not a problem area with oil base penetrants. Penetrants used in a tank system must have a specific gravity less than one in order to ensure that water will not float on top of the penetrant and prevent the penetrant from covering the test object.

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3.3

FLASH POINT

The flash point of a material is the temperature at which enough vapour is given off to form a combustible mixture and a minimum value of around 93°C is typical. Insurance companies and transport regulations are tending to dictate a movement upward in penetrant flash points and this can be a particular problem with solventbased removers and developers that are required to be halogen free. Most manufacturers can, however, supply non-flammable alternatives using chlorinated solvents.

3.4

VOLATILITY

Many materials with good penetrant ability are unfortunately volatile, which means that they would evaporate too quickly to be practical. The penetrant would dry from the inspected surface, leaving it stained, and from any discontinuity, leaving it contaminated with precipitated dye. Volatility is characterised by the vapour pressure or boiling point of a liquid. Low volatility is desirable from a practical standpoint of the evaporation loss of penetrant stored in open tanks.

3.5

CHEMICALLY INERT

Penetrant materials must be as inert and non-corrosive as possible. Maximum sulphur, sodium and halogen levels are often specified by the nuclear and aerospace industries, to avoid the possibility of embrittlement or cracking as failures can occur years after, caused by quite small quantities of contaminant. It is important that the penetrant be chemically compatible with the material being tested and to this end penetrants containing chlorides, chlorine, or sulphur are frequently restricted from use on austenitic steels, titanium, and high nickel steels. When new applications are evaluated, the compatibility of an organic penetrant material or of certain solvents must be considered. The potential reactions must be considered with respect to the operating environment of the product.

3.6

VISCOSITY

Viscosity relates to the thickness or body of a fluid and is a result of molecular or internal friction. Viscosity is an important and easy-to-measure property. Tests to measure viscosity include the Federal Test Method Standard 781 Method 305 and the ASTM Standard D-445 using the Cannon-Fenski viscometer. Practical Penetrant Inspection Rev 0 Dec 2005 Penetrant Properties Copyright © 2005, TWI Ltd

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3.7

SOLUBILITY

All penetrants contain a dye, red or fluorescent, in solution. The penetrant must hold sufficient dye at ambient or high temperature and the dye must not come out of solution if the temperature drops. Red contrast penetrants tend to cause most trouble in this respect.

3.8

SOLVENT ABILITY

Having applied the penetrant, it becomes necessary to remove the surplus from the test specimen to ensure a clean, clear background. Volatile solvents, some flammable, some not, are often used. These must not dissolve the penetrant in defects.

3.9

TOLERANCE TO CONTAMINANTS

Penetrants kept in open tanks will become contaminated after a time, even with the greatest care. Water is the main enemy, especially for water washable penetrants and so checks must be done at intervals to ensure all is well. Oil, grease and solvents as well as many strange objects find their way into tanks. Even though great care in cleaning is taken, contaminants can still remain in a defect. Therefore the penetrant materials must be formulated to minimise such problems. Reduction of fluorescent brilliance by chromate residues on water washable penetrants can be of particular concern, although less so in recent times.

3.10 HEALTH HAZARD Chemists developing new penetrant materials must comply with or exceed the most stringent health and safety requirements. Three of the main problems are of toxicity, odour and skin contact. Again, it is more likely that the solvent-based cleaners, removers and developers will come under closest scrutiny. For instance, halogenated hydrocarbons are extremely dangerous in the presence of heat, so smoking is absolutely forbidden when they are being used. It is very difficult for an operator to remain uncontaminated by penetrants, even when wearing gloves and overalls. Different people are allergic to different materials. Nonetheless, reputable manufacturers will not knowingly use hazardous materials.

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3.11 AVAILABILITY AND COST Materials that are difficult to obtain are unlikely to be used even though they might have big advantages in terms of capillarity. Similarly there is no commercial sense in using components which are very expensive, making the final product uncompetitive in a very open market.

3.12 ELECTRICAL CONDUCTIVITY Electrostatic spraying of penetrant is becoming increasingly popular in large automatic processors, even where electrostatic hand-spray guns are used. The electrostatic spray provides uniform coverage of parts with complicated shapes, reduces over spraying, and requires less penetrant over- all. The basic principle of electrostatic spraying is that the spray gun applies a negative electrical charge to the penetrant as it is sprayed. The test object retains ground potential. The electrostatic attraction between the two opposite polarities causes the penetrant to be strongly attracted to the part. Electrostatic spray systems used manually require penetrants that have high electrical resistance so that there is not a flashback to the operator. The penetrant should have two characteristics to be suitable for electrostatic spraying: 1.

A low viscosity so that the liquid can be readily divided into very small components (i.e., atomised) and be easily attracted to the part

2.

It must readily accept and hold the electrical charge placed on the liquid particles. Most commercially available penetrants have suitable characteristics to be used in electrostatic spray systems.

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4.0 FLUORESCENT PENETRANTS AND THE ELECTROMAGNETIC SPECTRUM Fluorescent penetrants utilise the ability of certain materials to absorb electromagnetic energy of one wavelength and in response emit light at a different wavelength. Molecules within fluorescent penetrants absorb ultraviolet light, become energetic and then shed their excess energy by emitting yellow green visible light. Indications are viewed under darkened conditions with the operator thus viewing bright indications against a dark background. They are mainly used in factories, on castings, forgings, precision parts, aluminium alloy, stainless steels and so on. Fluorescent penetrants are more sensitive than colour contrast as the indications produced are 10 times more "seeable". Ultraviolet radiation can be divided into three categories based on wavelength, UV-A, UV-B, and UV-C with the shorter wavelengths of ultraviolet radiation being more dangerous to living organisms. UV-A has a wavelength from 400nm Å to about 315nm, UV-B from about 315nm to about 280nm and UV-C from around 280nm to 150nm. Absorbs

10

100

200

300

400

Emits

500

ULTRAVIOLET LIGHT

4.1

600

700

VISIBLE

LIGHT UV-A and Fluorescence

TYPES OF UV-A LAMP

By far the most common type of light source used to inspect components tested with fluorescent penetrant is the mercury vapour arc lamp. In fact the mercury arc lamp is a street or workshop lamp which has a filter over it to reduce the visible light to a minimum but allow the UV-A to be transmitted.

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The filter is called a Woods in UK and a Kopp in the USA. On one type of lamp the filter is integral with the outer envelope but on another, using either a GE or Westinghouse lamp, it is separate. The mercury arc is drawn between electrodes enclosed in a quartz tube. The resistor limits the amount of current in the starting electrode. The quartz tube is mounted and enclosed in the outer glass envelope, which serves to protect it and filter out any possible hazardous radiation. 400 watt mercury vapour arc flood lamps can be used where very large components are tested or to give a background illumination in an inspection area. Background lighting in a darkened area can however, be more economically provided by UV-A strip lights.

4.2

FLUORESCENT DYE

Dye concentration and colour shade influence the overall sensitivity of fluorescent penetrant systems, as with the visible penetrants. There are many other variables that can be controlled, as well as capabilities that can be developed. In general, fluorescent penetrant systems have more potential applications than do the visible dye penetrants. Fluorescent materials absorb energy from light waves in the ultraviolet region of the electromagnetic spectrum. This energy is converted and emits photons of energy at a different light wavelength. Most commonly used in NDT is ultraviolet light (UV) which peaks at 365nm wavelength. This is the UV peak commonly known as black light. Penetrant dyes are selected that absorb energy in the 350 to 400 nm range and emit light in the 475 to 575 nm range. The emitted light is in the visible spectrum in the green to yellow range. The quality of fluorescent dyes is determined by how efficiently they will absorb UV light and convert it into visible light. This is influenced by: •

The intensity of the UV-A light at the surface of the component

•

The ability of the dye to absorb UV-A light

•

The concentration of the dye

•

The ability of the dye to produce visible light

•

Film thickness

Of these factors the inspector has the ability to influence the intensity of the UVA light at the surface, the dye concentration and the film thickness. With respect to the first of these factors, all fluorescent penetrant inspection procedures call for a Practical Penetrant Inspection Rev 0 Dec 2005 Flourescent Penetrants and the Electromagnetic Spectrum Copyright © 2005, TWI Ltd

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minimum level of UV-A measured in μW/cm2. Below the specified level the fluorescence will be insufficient. Dye concentration, as has been mentioned earlier, will increase during the dwell time due to the evaporation of the more volatile penetrant constituents. The final factor, film thickness, is important because fluorescent dyes have a minimum layer thickness below which, they will not fluoresce. Emulsification, washing, and development can influence film thickness

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5.0 DEVELOPMENT As the name implies, images that are invisible are revealed. The function of all developers, regardless of type is to improve the perceptibility of penetrant indications. Developers will achieve this by: •

Drawing out a sufficient amount of penetrant from the discontinuity to form an indication

•

Expanding the width of the indication enough to make it visible,

•

Increasing the brightness of a fluorescent dye above its bulk brightness

•

Increasing the film thickness of the indication to exceed the dye's thin film threshold in order to make it detectable.

Some procedures do not require the use of a developer; however, a high sensitivity penetrant is then used for an inspection where a lower sensitivity material could be used if a developer were applied and requires UV-A levels of 3000 μW/cm2 . Most industrial procedures do however require the use of developers and if correctly applied have been shown to enhance a penetrant image by a factor of up to 600. Thus it is sensible to have knowledge of why and how they work. The basic mechanism of developer action is based on the following: •

Capillarity

•

Light scattering

•

Solvent action

5.1

CAPILLARITY

The most important mechanism for a colour contrast developer is capillarity. The capillary attraction of the developer particles overcomes the opposing attraction of the discontinuity and therefore increases the surface area of the indication. This action spreads the penetrant laterally on the surface, thus widening the indication. Capillary action expands the bulk dye into many thin films around the developer particles to enhance its brightness. Capillary action also works vertically through the developer to expand the thickness of the fluorescent dye. The developer particles must be of a particular size and shape. Too large a size will result in low capillary pressure and too small will cause the particles to block any orifice. Practical Penetrant Inspection Rev 0 Dec 2005 Development Copyright © 2005, TWI Ltd

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5.2

LIGHT SCATTERING

Vitally important when using fluorescent penetrants is the light scattering effect. Through this mechanism the brightness of an indication is significantly amplified per unit area. Each particle of developer provides a bright scattering multiple reflector. The developer particles reflect both the exciting UV-A and fluorescent radiation. This mechanism plus the improved contrast in darkened conditions, gives the marked improvement in sensitivity of fluorescent penetrant systems over colour contrast.

I0 If

If If

If

Light Scattering

5.3

SOLVENT ACTION

This mechanism only applies to non-aqueous (solvent suspendible) developers and dry and water-suspended developers have no capability for drawing the penetrant out of the discontinuity except by capillary action. The developer should be sprayed from a distance such that the developer particles are just damp when they strike the test surface. The remaining solvent on the particle will bridge the gap between the developer particles and the penetrant in the discontinuity. This is especially important with fine tight defects where the developer particles do not necessarily contact the penetrant. The solvent must not however, dilute the penetrant nor significantly reduce the brightness of a fluorescent penetrant. Solvents in the solvent-suspended (non-aqueous wet) developers and in the plastic film developers are also solvents for the entrapped penetrant. The solvent dissolves into the penetrant, lowers its viscosity, and expands its volume. This

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process aids self-development in that the penetrant will flow back to the surface and into the developer to form the indication by capillary action.

5.4

DEVELOPER PROPERTIES

The properties of a good developer used in penetrant testing are numerous. The more important ones are listed below: a)

The material must be absorptive, to perform blotting action.

b)

It must have a fine texture but not be too fine, as this may block imperfections

c)

For colour contrast penetrants it must mask out background contours and colours

d)

It must be easily and evenly applicable

e)

It must form a light and even coat

f)

There must be no fluorescing of the developer when used with fluorescent penetrant

g)

The penetrant bleeding from a discontinuity must easily wet the material

h)

When used with a colour contrast penetrant it must obviously be of a highly contrasting colour. The best colour seems agreed to be white

i)

It must be readily removable after the test is completed

j)

It must be non-toxic and non-irritant

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6.0 PROCESS SEQUENCE •

Preparation and Pre-Cleaning

•

Application of Penetrant

•

Excess Penetrant Removal

•

Application of Developer

•

Inspection

•

Recording

•

Post Cleaning

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Preparation and Pre-Cleaning

Water washable

To assure a valid penetrant inspection it is vital that any discontinuities are free from contamination and open to the surface. The test specimen PostSolvent surface should also be clean. Emulsifiable

Removable

Typical contaminants are: a) b) Water and c) Solvent d) e) f) g) h) i) j) k)

Machine oils Scale Waterand slagWater wash flux spray Welding rinse Corrosion preventatives Paint Oxide films Apply Burnt oil hydrophilic Carbon emulsifier Corrosion products Acid/Alkali Water

Apply lipophilic emulsifier

Apply solvent remover

Water wash

Water wash

The subject is vast and indeed, on some penetrant processes, it takes longer and costs more to prepare a part than to test it. However, whichever cleaning process is used, it must be Penetrant compatible with the material being inspected. Check removal Broadly, cleaning methods can be broken into two, physical and chemical. BS 7773 Cleaning and Preparation of Metal Surfaces should be referred to when in doubt. Dry

Apply dry powder developer

Dry

Apply watersoluble developer

Apply water suspendable developer

Dry

Apply solventbased developer

Inspect

Record

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6.1

PREPARATION AND PRE-CLEANING

Penetrant testing is capable of locating discontinuities open to the surface but will fail to locate these discontinuities if: •

the penetrant is not able to wet the surface of the test object

•

the penetrant is unable to enter a discontinuity due to a blockage

•

the bleed out of the penetrant from a discontinuity is restricted

Shown below are a number of potential causes of such problems: •

Unable to wet the surface of the test object Lubricating oils Water and hydrates left after water evaporation Polishing and buffing lubricants

•

Penetrant is prevented from entering a discontinuity Peening or smearing of the discontinuity Carbon Scale, rust, oxides and other corrosion products Paint and protective coatings Weld metals and flux residues Anodising Penetrant residues

•

Penetrant bleed out is restricted Carbon Varnish Scale, rust and other corrosion products Strong acids and alkalis Anodising

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6.2

CLEANING METHODS

Listed below are some standard cleaning methods. Production rate, volume of parts, and cost will dictate the cleaning method and equipment to be used. Whichever method is used, care should be taken to ensure that it will not react with or attack the base metal. Mechanical Methods Brushing Blasting Chemical Methods Hot Solvent Degreasing Vapour degreasing Cold solvent degreasing Alkaline cleaning Acid pickling Steam cleaning Paint strippers

6.2.1

MECHANICAL METHODS

Physical methods can usually only remove contaminants from the surface and are unable to clean out a flaw. Brushing and blasting are typical methods.

6.2.1.1

BRUSHING

Brushing can be extended to any scrubbing action. Usually a wire brush is used to remove dry scale, flakes of paint, etc. A brush with soft bristles is recommended but often such a brush will not lift stubborn dirt, so a harsh bristle brush ends up being used. Then there is a danger that the scrubbing action will peen the lips of possible defects and so prevent the penetrant entering. If the danger of peening is suspected, a chemical etch process should also be specified to follow the brushing.

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6.2.1.2

BLASTING

Surface preparation by blasting is a vast subject in its own right. Grit blasting is the most common method of physically moving debris from a surface. However, exotic solid removers such as walnut shells and plum stones are sometimes used and the remover may also be a liquid. Water under very high pressure is often a most efficient pre-cleaner. As with brushing, the danger of peening the mouth of a suspect defect exists, so chemical etching is often specified as a follow-on procedure.

6.2.2

CHEMICAL METHODS

If possible it is preferable to use one of the chemical processes to remove contaminants. The following are among the most common.

6.2.2.1

HOT SOLVING DE-GREASING

This is probably the most common way to process parts to be batch tested in house. The parts are boiled in a solvent, usually trichloroethane 1.1.1 and the solvent is prevented from evaporating by condensing tubes arranged above the solvent tank. Often an ultrasonic transducer under the tank is fitted to vibrate the parts being cleaned and thus aid the removal of solids and liquids. The liquid becomes dirty and corrosive after a time. Therefore adequate filtration is essential as well as regular checks on water content and acidity.

6.2.2.2

VAPOUR DE-GREASING

The part to be cleaned is suspended above a tank of boiling solvent, usually trichloroethane 1.1.1. The solvent condenses on to the part and removes liquid contaminants. Vapour de-greasers are usually used in conjunction with hot solvent cleaners, as a second stage. The method works well on liquid contaminants but does not dislodge solids. The solvent action continues until the temperature of the part reaches the temperature of the vapour. Since the specimen is hot when removed, drying out water in defects is not a problem.

6.2.2.3

COLD SOLVENT DE-GREASING

This is the most common method of pre-cleaning when a colour contrast system is to be used, especially on site. It is usually sprayed or flushed on to the area to be tested and then swabbed or wiped off. The method is not good at removing Practical Penetrant Inspection Rev 0 Dec 2005 Process Sequence Copyright © 2005, TWI Ltd

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contaminants from potential defects and can only clean surface solids, which are in suspension in grease or oil.

6.2.2.4

SOLVENT MATERIALS WITH EMULSIFIERS AND DETERGENTS

"Gunk" is a trade name, which comes to mind when discussing this type of cleaner. The material is usually a mixture of white spirit or kerosene and emulsifying agents. When brushed or sprayed on to grease and oil-coated parts, the cleaner thins the contaminants. After a period of time the dilute residues can be washed off with water. However, water contamination is a problem therefore drying action must be taken. Detergent cleaners are becoming more common and these are usually diluted with water to be used hot or cold. We will be discussing the difference between emulsifiers and detergents during the penetrant remover section.

6.2.2.5

ALKALINE CLEANING

Alkaline de-greasing may be used hot or cold and has advantages over solvent methods because it will remove soaps and salts. Very thorough washing is necessary after cleaning to remove residues and probably drying after that. Alkaline cleaners are not used on aluminium alloys.

6.2.2.6

ACID PICKLING

Acid pickling or alkali de-rusting solutions may be used to remove rust and scale. After using these chemicals, it is usually necessary to apply a further cleaning method to ensure that discontinuities are also cleaned. Neutralising, washing and drying may then have to be carried out.

6.2.2.7

STEAM CLEANING

This process often works well when large areas have to be cleaned, but as with any process with any process involving water, care must be taken to ensure the part is dry before penetrant application.

6.2.2.8

PAINT REMOVAL

There are a number of proprietary paint removers available, all of which are caustic. Residues will often kill the fluorescent material in a penetrant. Neutralising procedures are necessary after the paint has been removed. Further cleaning must be done to remove paint remover residues from discontinuities. Practical Penetrant Inspection Rev 0 Dec 2005 Process Sequence Copyright © 2005, TWI Ltd

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6.3

APPLICATION OF PENETRANT

Dipping, brushing, pouring, spraying, or essentially any other way to get penetrant onto the surface of a test object is acceptable. The penetrant must be allowed to remain in place, covering any surface discontinuities long enough for the penetrant to enter the discontinuity. The penetrant dwell time can involve a period of submersion and partial draining. This draining period during the penetrant dwell time will enhance the sensitivity of the penetrant due to the evaporation of volatile constituents when exposed to air in a thin film. This evaporation effectively increases the concentration of the penetrant. Two crucial factors to consider when applying penetrant are not how it is done but what is the temperature of the component and how long will it remain on the component. BS EN 571 states that the component temperature should fall between 10 and 50oC. In special cases the temperatures down to 5oC can be used but for temperatures outside this range penetrant product families specially approved for this purpose shall be used. BS EN 571 specifies that penetration time can vary between 5 and 60 minutes. The actual time depends upon the properties of the penetrant, the application temperature, the material being tested and the defects being sought. Under no circumstances shall the penetrant be allowed to dry during the penetration time. Adequate penetration time can, when seeking transgranular and intergranular corrosion cracks, be increased to hours and involve only the most sensitive (Group VI) fluorescent penetrant. Dwelling overnight and processing the next day is a common procedure when such discontinuities are suspected. The method of penetrant application varies a great deal depending on circumstances. The most usual method application for colour contrast penetrants is by spraying from an aerosol. This is not essential and indeed can be quite messy. Providing that contamination by foreign materials can be avoided it matters not how the penetrant is applied when using portable kits on site. Thorough wetting of the surface of interest is the main consideration. For production line work, especially when using fluorescent penetrant materials, application is normally by: • • •

Spraying Dipping and draining Thixotropic

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6.3.1

SPRAYING

Spraying may be more applicable on large items, which are too large to go into a tank, or where an uneconomic amount of penetrant would have to be kept in a large tank. After spraying, drained material is recovered and re-used. Penetrant is usually applied by spraying when spot or local inspections are performed using penetrant kits. Brushing is a good method to apply penetrant to a small local area, especially in hard-to-reach places. A small amount of penetrant can be applied only on the area where it is needed, thus eliminating the need for cleaning overspray. Compressed air spraying using paint spray equipment is still a prominent means used throughout industry for applying penetrant, and particularly when inspecting large parts.

6.3.2

AEROSOL SPRAY

Spraying an object, using material from an aerosol can is expensive, if the price of the container and gas is considered. However, as the aerosol is sealed there is no possibility of contamination of the penetrant while still in the can. It is a total loss system and all excess penetrant is wasted. Aerosol spray is the most common method of application, when local area checks are done with colour contrast penetrant

6.3.3

ELECTRO-STATIC SPRAY

Electrostatic spraying is being more widely used to apply penetrants, and especially in automatic processors. This method of spraying applies very thin coats of penetrant with minimal run-off. The penetrant is expended, but experience has shown that this process is economical. The penetrant is used directly out of the shipping container with no draining losses, contamination, or deterioration from sitting for long periods in a tank. Electro-static spray means that the object to be tested is positively electrically charged with respect to the penetrant coming out of the spray gun. The penetrant is sprayed in a thin even coating upon the object. The method is very good on large, plain, simple shapes. It does not work well on intricate shapes nor does it penetrate holes in such items as castings. Electro-static spraying has a use on stainless turbine blades that have fine cooling holes in them, often bored by electron beam methods. Curiously the penetrant does not readily penetrate the cooling holes, but it does enter adjacent cracks.

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6.3.4

DIP AND DRAIN

The traditional way of applying the penetrant in factory use is to immerse a specimen in a tank of liquid. Once wetting is complete, the item is removed and drained either over the tank or over a separate drain station. In this way the maximum amount of material is recovered and used again. Alternative methods that are useful in reducing mess and waste are thixotropic penetrants and electrostatic sprays.

6.3.5

DIPPING HEATED PARTS

Dipping hot parts directly out of the vapour phase degreaser or those that have been heated in an oven was quite common at one time. This procedure increased the sensitivity by reducing the penetrant viscosity so that it would penetrate faster. The viscosity of commercially available penetrant has been reduced since the late 1960s so that it is now 1/2 to 1/3 of the previous viscosities. Thus, dipping of hot parts provides less of an advantage now that penetrant is readily available with low viscosity.

6.3.6

THIXOTROPIC

Under stable conditions thixotropic liquids increase in viscosity until they gel. When brushed they become mobile (the text book definition being that their viscosity reduces when a shear stress is applied). The usefulness of thixotropic penetrants is that they can be applied by brush overhead, where spraying is difficult and might create a mess.

6.4

REMOVAL OF EXCESS PENETRANT

After the penetrant has been in contact with the test surface for an acceptable period, the excess material must be removed from that surface without affecting the penetrant that has entered a defect. This is a vital stage for the surface must not be cleaned excessively or defects will be leached out. However, without proper removal of excess penetrant a high background will remain, reducing the contrast between potential defects and the test surface. The more common methods of penetrant removal with their BS EN 571 classifications are: Method A

Water

Method B

Lipophilic emulsifier

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Method C

Solvent

Method D

Hydrophilic remover and water

Method E

Water and solvent

6.4.1

WATER AS A REMOVER

A water or air/water spray is used to remove water removable penetrants with fluorescent penetrants being washed under UV-A light (BS EN 571 specifies that such a lamp will have a minimum output of 300μW/cm2). The water should be used as a fairly coarse, high volume, droplet spray with a pressure as low as possible and a temperature below 50°. If air is used it should be maintained at a low pressure and just sufficient to break up the flow of water into droplets. The wash time should be just enough to slightly underwash the part. The penetrant material contains its own emulsifier and is therefore removable by water. There is, however, a big disadvantage. Water will cause the penetrant and emulsifier to be washed out of wide shallow defects. Water washable penetrants are most commonly used on rough parts such as castings or forgings as a rough surface is a series of wide shallow crevices. By lowering the amount of emulsifier in the liquid, finer and tighter defects can be found. This is at a price, for as the sensitivity goes up the washability goes down. So super sensitive water removable penetrants will not wash from anything but smooth surfaces.

6.4.2

LIPOPHILIC REMOVER (EMULSIFIER)

A lipophilic material is an emulsifier, which mixes with oil and makes the whole miscible with water. The normal method of application is immersion followed by draining but they can also be applied by dipping, pouring, air spraying and electrostatic spray. Brushing is not recommended since it leaves an uneven application and it mixes the emulsifier, the penetrant, making control over the emulsification time impossible. This method has the advantage of enabling wide shallow discontinuities to be detected because only penetrant oil enters the discontinuity and washing after emulsification should only remove the surplus penetrant from the surface. This method is used on high stress critical parts, one draw back however is that emulsifier contact time is absolutely critical. Most references quote a maximum of three minutes for emulsifier contact time and if exceeded the emulsifier will begin to attack the penetrant in flaws, especially the Practical Penetrant Inspection Rev 0 Dec 2005 Process Sequence Copyright © 2005, TWI Ltd

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wide shallow defects. If a number of specimens are tested in a batch at the same time, then all of them must have exactly the same emulsification and wash time or results will vary. Washing with water takes place after the emulsification phase under the same conditions detailed in paragraph. Lipophilic emulsifiers are used neat and are often viscous. This results in high dragout making it expensive to use. Also when large amounts are being flushed down a drain, pollution control measures may have to be taken. Hydrophilic removers have generally therefore superseded lipophilic emulsifiers.

6.4.3

SOLVENT REMOVAL

The ideal procedure involves using a dry rag to mop up surplus penetrant followed by a lint free rag or soft paper towel moistened with liquid solvent and then wiped over the test area. When removing colour contrast penetrant wiping should cease when the rag is lightly tinged with dye to ensure that the specimen is not over cleaned. When solvent removers are used to clean off excess fluorescent penetrant, removal should again be conducted under ultra violet light (UV-A) to ensure that the background has to be reduced to an acceptable level. Irrespective of the type of penetrant to be removed neat high-pressure solvent must never be sprayed directly at an area of inspection interest as it could enter the surface discontinuity and wash out the entrapped penetrant.

6.4.4

HYDROPHILIC REMOVER (DETERGENT)

A hydrophilic remover is a detergent which, when mixed with water in a tank, has the ability to break down the surface tension of penetrant in contact with a test surface and lift or scrub the penetrant from that surface. Concentrations of emulsifier can range from 5% to 33% in water, the proportion of detergent to water being varied to allow greater control of washing. Experiments have shown that a 5% concentration produces the greatest sensitivity with a continuing decrease in sensitivity up to about 30% concentration. Penetrant tolerance of the emulsifier mix is lower at 5% than at 20% and the tank life is shorter even though the sensitivity is somewhat less. As the remover is diluted, the test parts are given a pre-wash with a spray to remove excess penetrant. This extends the life of the remover and means that there is only a thin film of remover left. In some cases, where an exclusive drain tank is used the pre-wash allows the penetrant to be separated, recovered and perhaps re-used. The hydrophilic remover is usually applied by immersion, although a spray of high dilution remover is sometimes preferred. Remover time is usually a maximum of three minutes, although there is much more flexibility than with a lipophilic emulsifier process. Practical Penetrant Inspection Rev 0 Dec 2005 Process Sequence Copyright © 2005, TWI Ltd

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Washing with water takes place after the remover stage under the same conditions previously. Even though the penetrant used may be called post emulsifiable, hydrophilic removers are by far the most commonly used. It is ultimately the most sensitive system, although it takes longer. It combines flexibility with efficiency, but it is sometimes difficult to remove the penetrant from crevices and the roots of fine threads.

6.4.5

SPRAY-SCRUBBER PENETRANT REMOVAL

Hydrophilic emulsifiers can be used to remove the penetrant by mixing the emulsifier with the rinse water at the spray nozzle. The spray used is a mix of approximately 250 parts of water to 1 part emulsifier. As the emulsifier releases the oil from the surface by detergent action, the water spray scrubs the released oil away. The spray remover serves to scrub layer after layer of excess surface penetrant from the part, continually introducing a fresh detergent/water solution. Spray time depends on the surface roughness, penetrant viscosity, penetrant sensitivity level, complexity and size of the part, and water temperature. The processing procedure is the same as for using the dip tank procedure except that the emulsifier is applied by spraying the emulsifier/water mixture until the surplus penetrant is removed. This is an ideal application on large parts or in an area that does not have large tanks. The pre-rinse, spray emulsification, and final rinse can be accomplished in a single spray booth equipped with a water spray nozzle and an emulsifier-mixing nozzle.

6.4.6

WATER AND SOLVENT

There are some very specialised penetrant applications where a solvent dip is used prior to emulsification. Usually, the solvent dip is used on rough surfaces, such as turbine blades in an as cast condition or with a diffusion coating, or on aluminium parts that have been anodised. A tremendous amount of background coloration occurs in these applications that cannot be removed by emulsification. The procedure normally involves a quick dip in aliphatic kerosene, then a dip into the emulsifier tank. The procedures must be worked out experimentally for each part. In some instances, solvent dip followed by a hydrophilic emulsifier in the spray scrubber mode produce better results than an emulsifier dip procedure.

6.5

DRYING

Drying the test specimen or test surface is an important intermediate stage after penetrant removal. If the solvent removal method is used, the part being inspected will dry quickly as the solvent remover evaporates. Practical Penetrant Inspection Rev 0 Dec 2005 Process Sequence Copyright © 2005, TWI Ltd

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Where water has been used during the removal stage drying can be critical. The essence of drying is that the parts must be dried quickly so that excess penetrant bleed out does not take place. At the same time the part must not get too hot, 50°c is suggested as a maximum temperature, or penetrant will evaporate out of defects. Drying after removal of the penetrant depends on the removal method and developer used. Drying after solvent removal is usually by air drying alone whereas after water removal heat is required to drive off the water. At the same time this will expand the penetrant in any discontinuities and reduce its viscosity to provide better self-development. Heat is also essential when water-suspended developer and watersoluble developers are used since the water must be evaporated from the developer. Self-development from the heat is essential for water-based developers, as they can only provide capillary action. Dryers can be heated by gas, electricity, or steam. It is essential that a fan circulates the air within the dryer or else heat stagnation can build up in the cabinet near the heater. Moving air will break up this stagnation condition and reduce the drying time. Re-circulating the air over the heating system is more economical than continuously heating cold air. Re-circulating hot air dryers are almost always specified.

6.5.1

HOT AIR RE-CURCULATING OVEN

Specimens to be dried are placed in an oven in which the air is moved by a fan. This helps the part to dry evenly. The maximum temperature of the air may be as high as 85°c to promote rapid drying, but as has been said, the specimen must not get that hot. 10 minutes is the maximum drying time. If drying cannot be accomplished in that time an improved oven is recommended.

6.5.2

FORCED WARM AIR

Forced warm air means a large volume of dry heated air enveloping unusually large specimens. For instance large castings which cannot be put into an oven. Hair dryers are seldom recommended as they take too long to dry all but the very smallest specimens.

6.5.3

DRY CLEAN COMPRESSED AIR

Compressed air can often be used to advantage before hot air drying, to blow water droplets from holes and crannies. If a fine nozzle is used, the pressure should be less than 1 bar and the distance should be more than 300mm. If a large volume of compressed air is available, the recommended pressure is not greater than 2 bar. Practical Penetrant Inspection Rev 0 Dec 2005 Process Sequence Copyright © 2005, TWI Ltd

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6.6

DEVELOPMENT

The five classifications of developer listed in BS EN 571 are: •

Form a

Dry

•

Form b

Water soluble

•

Form c

Water suspendable

•

Form d

Solvent based

•

Form e

Peelable

As the name implies, images that are invisible are revealed and the function of all developers, regardless of type, is to improve the perceptibility of penetrant indications. A correctly used developer has been shown to enhance a penetrant image by a factor of up to 600. The application of developer in visible penetrant applications should be in a uniform coat obtained by a series of light passes with the spray with only enough developer as necessary to provide a thin white background being applied. A heavy coat will mask fine indications and some practice is needed before uniform coats can be applied. The can or gun should be held about 12 inches from the surface; the second coat should be applied to the light areas between the passes of the first coat. Sometimes, spraying the second coat across the direction of the first coat is good practice. Developer sprayed over fluorescent penetrant must be carefully done to provide only a very light coat as the surface of the component should not be totally obscured. One coating is usually sufficient, as the developer is not required to provide a contrasting background, as is the case with colour contrast penetrants. If a second coat is applied, only a touch-up of the light spots is needed.

6.6.1

DRY DEVELOPER

Dry developers can be applied by dipping the part in a bin of developer, fogging the part with a gun or in a chamber, or by electrostatic spray. They are only recommended for use with fluorescent penetrants and are best used on parts that have been heated.

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6.6.2

DRY POWDER DEVELOPER APPLICATION

Dry powder developer is applied in a number of ways: a)

Dust storm cabinet.

This is the most common method where the part or parts are sealed in a cabinet, which is probably warmed to keep the powder dry. The powder is agitated and the resulting dust settles on the test areas. Upon removal, the excess powder is either shaken off gently or blown off with a very light air stream. b)

Electrostatic or flock gun spray

Both methods rely on electrically charged particles of powder being attracted to the penetrant exuding from a discontinuity. Spraying is more appropriate to automatic processes and large plane surfaces. Extraction at the developer station is essential. c)

Insufflator.

This method simply relies on puffing the powder on the surface from a rubber or plastic container. It is somewhat wasteful if applied carelessly. Extraction is advised and this method of application is really only suitable for local areas or small parts. d)

Fluidised bed.

Fluidisation is a technique in which a mass of solid particles, i.e. dry powder developer, is brought into a state of suspension by an upward stream of gas blown through it. It is usually only used in large automatic systems, where large numbers of parts are passed through the bed. The station must be sealed from other draughts or the system will fail.

6.6.3

AQUEOUS LIQUID DEVELOPER

Water suspendible and water-soluble developers can be sprayed or flowed on. However, in the vast majority of cases the application is by dip and drain. Dip time is usually no more than 30 seconds. The suspendible type must be thoroughly agitated before the parts are immersed. When aqueous developers are used they are applied before drying and therefore development takes place during the drying phase. Thus there can be a significant saving of processing time.

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6.6.4

WATER-SUSPENDED DEVELOPER (WET DEVELOPER)

The water-suspended developers can be applied by dipping or spraying. References and previous data given provide considerable detail on this method.

6.6.5

WATER – SOLUBLE DEVELOPER

Water-soluble developers become a solution with water. This type of developer can be applied by dipping or spraying. Due to its water base, it is not recommended for use with water-washable penetrants. The appropriate mixtures recommended are available from the suppliers.

6.6.6

SOLVENT BASED DEVELOPER

Solvent suspendible developers are invariably sprayed either from aerosols or paint type sprayers. The developer must be thoroughly agitated, as there is a tendency for the particles to agglomerate. It is wise therefore especially with aerosols, to test the spray quality before applying to the test surface. The developer must be sprayed from a distance exceeding 300mm so that it falls on the area of interest in an almost dry thin even coat. If used with colour contrast penetrant the developer coat should almost obscure the background surface. When applied as part of a fluorescent system, the coating should be only just discernible in daylight conditions.

6.6.7

PLASTIC FILM DEVELOPER

Plastic film developers are supplied in a spray can because of their high volatility. The developers used with visible penetrants are pigmented to provide a white background; those used with fluorescent penetrants are clear. Two spray coats are usually applied for developing. If the developer is to be stripped off for recording, at least three more coats of clear lacquer should be applied over the area to be stripped.

6.7

INSPECTION

Parts should be inspected initially as soon as the developer is applied. This is particularly so when using colour contrast systems and a solvent suspendible developer. The penetrant can sometimes bleed quickly from a crevice or a hole adjacent to a discontinuity and thus mask it. It is difficult to inspect specimens that are placed in a dust storm cabinet very soon after application and impractical on water based developers. As a principle, development time should vary from 0-30 minutes with some authorities suggesting that development times in excess of 10 minutes are excessive. A general Practical Penetrant Inspection Rev 0 Dec 2005 Process Sequence Copyright © 2005, TWI Ltd

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view is that a maximum of 20 to 30 minutes satisfies the requirements and that inspection should be carried out throughout and at the end of the development time. Inspection is a critical part of the penetrant process but no more so than the processing, because improper processing will prevent indications being seen at the proper sensitivity level and thus the inspector cannot detect them. The personnel are the most critical and important elements of the inspection of penetrant indications. Acceptance or rejection of the part is based on the inspector's judgement. The inspector must have very good near vision acuity and be capable of colour discrimination. Eyesight should be checked periodically, and at least once a year. Inspection is monotonous and tiresome; physically, visually, and mentally. A good procedure is for the inspector to alternate between processing and inspection. Inspectors should be properly motivated to realise the importance of their work. Their job should have the same stature, respect, and compensation as other inspection classifications within the same facility.

6.7.1

FLUORESCENT PENETRANT

The room or area where fluorescent penetrant inspection is to take place must be darkened to below 20-lux visible light. Background UV-A strip lights or low output amber lights may be used in order that the inspectors can just see their way round. Actual inspection should take place under UV-A mercury vapour light conditions. For penetrant inspection the minimum level of UV-A light at the test surface must be 1.0mW/cm2 (1000μW/cm2). Before inspecting the person viewing should wait in the viewing area for a minimum of 5 minutes (10 in USA) to allow their eyes to adapt to the low light levels. Photochromatic spectacles must not be worn although the sodium lens type may reduce eyestrain. From cold, mercury vapour UV-A lamps take a minimum of 15 minutes to reach full output intensity. Once switched on they should be left on throughout the working day as constant switching not only wastes time but also reduces lamp life. Inspectors should not view continuously for more than approximately 30 minutes.

6.7.2

COLOUR CONTRAST PENETRANTS

Colour contrast penetrants should be viewed in bright white light conditions. A minimum of 500 lux is recommended in accordance with BS EN 571-1.

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6.7.3

INSPECTION AIDS

Aids to correct inspection should not be spurned but at the same time they should be treated with caution. Hand magnifiers are useful; especially to decide between scratches, scores and cracks but they should not be too powerful, x8 magnification being about the maximum. Illuminated magnifiers, having either miniature UV-A lamps or P bulbs, as appropriate, are also useful. For restricted access, particularly when using penetrants as leak detectors, cold light endoscopes might be considered. These can now be bought with a UV-A light box.

6.8

RECORDING

BS EN 571 lists the following methods for recording indications: •

Written description

•

Sketch

•

Adhesive tape

•

Peelable developer

•

Photography

•

Photocopy

•

Video

6.9

POST CLEANING

It is often unnecessary to clean residues from test material. However, in some cases, such as when a high quality paint surface is to be applied, it is vital to remove penetrant and developer residues. To remove colour contrast residues it is best to first apply a thick wet coating of non-aqueous liquid developer and when it is dry brush the surface clean with a soft bristle brush. Finally the part can be dipped in or sluiced over with a solvent cleaner/remover. If dealing with intricate parts, it is often necessary to scrub them in a warm water detergent mixture, to remove developer residues.

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Specimens tested by fluorescent method using dry powder developer are often just subjected to a dry air blast. This may be followed by cleaning with cold solvent or in a hot liquid/vapour degreaser. It is worth a reminder that solids are difficult to remove with liquid or vapour degreasers. Post-cleaning is usually not necessary if dry developer has been used. Wet developer and solvent- suspended developer need to be cleaned; a water scrub spray will usually suffice. A degreaser or spray solvent can be used in the field. It is desirable that the developer be removed as soon as possible following inspection, as there are some types of developer that become more difficult to remove as time passes. The most difficult developer can usually be scrubbed off with a brush and detergent. Post-cleaning for LOX-use parts requires special procedures, usually specified by the component manufacturer.

6.10 PROTECTION When a penetrant inspection is completed the test surface is invariably vulnerable to outside contamination. Indeed, many test items can be of high value. Protection, even with a light de-watering oil is a wise precaution. The appropriate protective treatment is not within the scope of these notes. Suffice to say it must be compatible with the material, which is applied.

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7.0 SYSTEM CLASSIFICATION Penetrant systems of families are classified according to: •

Type of penetrant

•

Method of penetrant removal

•

Type of developer

7.1

TYPES OF PENETRANT

According to BS EN 571-1 there are three main types of penetrant: Type I

Fluorescent

Type II

Colour contrast

Type III

Dual (Combined colour contrast and fluorescent)

7.1.1

COLOUR CONTRAST PENETRANTS

Colour contrast penetrants are usually dyed red and are mainly used to look for defects when there is adequate day or artificial light or where no power source is available to enable use of an ultra-violet lamp. Colour contrast penetrants are used with spray developers that leave a white background when dry, so the inspector sees a red against white contrast. Colour contrast penetrants are mainly designed for use with either solvent or water removable systems although Mil-L-25135 C does call for post emulsifiable visible penetrant. The solvent removable form is generally used on site or where local areas are to be tested on construction joints and welds. Water washable colour contrast penetrants are mainly used on rough castings although they are used on undressed welds, where a rag moistened with is used to remove excess.

7.1.2

FLUORESCENT PENETRANTS

Fluorescent penetrants utilise the ability of certain materials to absorb electromagnetic energy of one wavelength and in response emit light at a different wavelength. Molecules within fluorescent penetrants absorb ultraviolet light, become energetic and then shed their excess energy by emitting yellow green visible light.

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Indications are viewed under darkened conditions with the operator thus viewing bright indications against a dark background. They are mainly used in factories, on castings, forgings, precision parts, aluminium alloy, stainless steels and so on. Fluorescent penetrants are more sensitive than colour contrast as the indications produced are 10 times more "seeable".

7.1.3 -

FLUORESCENT V COLOUR CONTRAST Fluorescent more sensitive Less operator fatigue with fluorescent More difficulty in monitoring fluorescent penetrant removal

7.1.4

DUAL PENETRANTS

Dual penetrants are combined colour contrast and fluorescent penetrants. These materials are something of a compromise and tend to work best in bright sunlight. They fluoresce at the red/orange wavelengths (650nm) rather than in the green/yellow band. Absorbs

10

100

200

300

400

Dual

500

ULTRAVIOLET

600

700

VISIBLE

LIGHT

LIGHT Dual Penetrant

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7.2

EXCESS PENETRANT REMOVAL

The method of penetrant removal influences the contents of the penetrant itself and although a number of different methods of removal are used, the materials fall into 2 basic types: •

Penetrant chemicals composed of a hydrocarbon and a dye (red or fluorescent) in solution. These can be either post-emulsifiable penetrants or solvent removable.

•

Penetrant chemicals composed of a hydrocarbon, a dye (red or fluorescent) and an emulsifier. These are called water washable or self-emulsifying penetrants.

•

Despite there being only 2 basic types of penetrant five methods of penetrant removal are listed in BS EN 571 part 1, these are: Method A

Water

Method B

Lipophilic emulsifier

Method C

Solvent

Method D

Hydrophilic remover and water

Method E

7.2.1

Water and solvent

WATER WASHABLE

Water washable penetrants contain an emulsifier and are therefore removable by water. There is, however, a big disadvantage as water can cause the penetrant and emulsifier to be washed out of wide shallow defects. Water washable penetrants are most commonly used on rough parts such as castings or forgings as a rough surface is a series of wide shallow crevices. By lowering the amount of emulsifier in the liquid, finer and tighter defects can be found. This is at a price, for as the sensitivity goes up the washability goes down. So super sensitive water removable penetrants will not wash from anything but smooth surfaces. The removal of fluorescent penetrants should be carried out under UV-A light. Advantages

Disadvantages

Useable on rough surfaces Suitable for batch inspection

Susceptible to over washing Least sensitive method

Cheaper than other methods

Requirement for a water source

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7.2.2

SOLVENT REMOVAL

A lint free rag or soft paper towel is moistened with liquid solvent and then wiped over the test area. Before this stage a dry rag can be used to mop up surplus penetrant. If a colour contrast penetrant is being removed then wiping should cease when the rag is lightly tinged with dye. The specimen should not be overcleaned. When solvent removers are used to clean off excess fluorescent penetrant, removal should be conducted under ultra violet light (UV-A) to ensure that the background has to be reduced to an acceptable level. Neat, high-pressure solvent must never be sprayed directly at an area of inspection interest. Advantages

Disadvantages

Portability

Not suited to batch inspections

No water supply required consuming

Requires hand wiping therefore time More expensive than water washable Potentially hazardous chemicals

Solvents used for the purpose of penetrant removal present a number of potential problems. Firstly the compatibility of the remover and the component under test must be considered. If in doubt guidance can be found in BS 7773, the Cleaning and Preparation of Metal Surfaces. Secondly, hydrocarbon solvents and many paint removers are highly flammable and should not be used near open flames. Chlorinated solvents have high flashpoints, but will add fuel to a general fire that exceeds their ignition points. Thirdly, solvents will remove natural oils from the skin. Rubber or plastic gloves should be used when the hands are to be exposed to solvents for any length of time.

7.2.3

POST EMULSIFIABLE

Penetrants, apart from the self-emulsifying ones used in water washable inspections, are generally oil based and not soluble with water. Since water is the most plentiful and the least expensive type of solvent available the need then is for some chemical that is soluble both in water and in oil that can render them water washable. The chemical utilised is referred to as an emulsifier and can be either oil based or water diluted. The former are referred to as lipophilic and the latter hydrophilic. Both will mix with oil and makes the whole miscible with water and the normal method of application is immersion followed by draining (brushing being Practical Penetrant Inspection Rev 0 Dec 2005 System Classification Copyright © 2005, TWI Ltd

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excluded as it makes it difficult to control the diffusion rate of the emulsifier into the penetrant). This method has the advantage of enabling wide shallow discontinuities to be detected because only penetrant oil enters the discontinuity and washing after emulsifier application should only remove the surplus penetrant from the surface. This method is used on high stress critical parts. However, emulsifier contact time is absolutely critical. Most references quote a maximum of three minutes for emulsifier contact time. If the maximum time is exceeded then the emulsifier will attack the penetrant in flaws, especially the wide shallow defects. If a number of specimens are tested in a batch at the same time, then all of them must have exactly the same emulsification and wash time or results will vary Lipophilic emulsifiers are used neat and are often viscous resulting in high dragout and therefore making it expensive to use. Also when large amounts are being flushed down a drain, pollution control measures may have to be taken. Lipophilic emulsifiers have therefore, generally been superseded by hydrophilic removers. In addition to the emulsifying action other desirable properties are a colour which contrasts with that of the penetrant to enable us to ensure that all the surface of the test item has been covered by the emulsifier. The dye of the emulsifier is also fluorescent so that, when washing under black light, the complete removal of the emulsifier can be verified and water soluble so that the dye will not be left on the surface after the part has been washed.

7.2.4

HYDROPHILIC REMOVER (WATER-DILUTABLE)

Hydrophilic emulsifiers used in penetrant testing are essentially surface-active agents (surfactants) or detergents and the word "hydrophilic" means water-loving or water-soluble. Such emulsifiers are supplied as a concentrate and are mixed with tap water to the desired dilution. Hydrophilic removers have the ability to break down the surface tension of penetrant in contact with a test surface and lift or scrub the penetrant from that surface. In practice the proportion of detergent to water may be varied to allow greater control of washing. The change in concentration varies the activity of the emulsifier and the rate at which it acts. In practice sensitivity tests with specimens of the type to be inspected should be made to determine the optimum level. Low concentration levels near to 5% provide the best sensitivity but can be expected to leave more background than a 20% concentration on rough surfaces. Hydrophilic removers are infinitely water tolerant a fact which enables parts to be given a pre-wash with a spray to remove excess penetrant. This extends the life of the remover and means that there is only a thin film of remover left. In some cases, where an exclusive drain tank is used the pre-wash allows the penetrant to be separated, recovered and perhaps re-used. Practical Penetrant Inspection Rev 0 Dec 2005 System Classification Copyright © 2005, TWI Ltd

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The hydrophilic remover is usually applied by immersion, although a spray of high dilution remover is sometimes preferred. Remover time is usually a maximum of three minutes, although there is much more flexibility than with a lipophilic emulsifier process. Washing with water takes place after the remover stage under the same conditions detailed for water washable systems. Advantages

Disadvantages

Maximum penetrating ability

Not suited to rough surface

Greater control over penetrant removal

More expensive

Ability to locate wide shallow defects

More time consuming

Immersion time not as critical as for lipophilic Even though the penetrant used may be called post emulsifiable, hydrophilic removers are by far the most commonly used. It is ultimately the most sensitive system, although it takes longer. It combines flexibility with efficiency, but it is sometimes difficult to remove the penetrant from crevices and the roots of fine threads. A further advantage in the use of a hydrophilic emulsifier is that the parts may remain in the emulsifier tank from 5 to 20 minutes. This wide range of dwell times provides less dependency on accurate time control. Initial cost of the hydrophilic concentrate is about the same as that of the lipophilic emulsifiers but the high dilution with water provides a substantial cost saving. Experience shows that the tank life of the two types of emulsifier is about the same, depending on the concentration. Less dilution provides somewhat greater tank life for hydrophilic. Operating costs can be lower with hydrophilic emulsifiers because of differences in the processing. The hydrophilic removers are highly water tolerant; this allows for a pre-rinse of water to remove up to 80% of the surface penetrant before emulsification. The pre-rinse thus eliminates much of the penetrant contamination of the emulsifier. The pre-rinse water can be collected and separated to salvage most of the penetrant. The lower viscosity of the hydrophilic emulsifier will drain more rapidly off the parts, resulting in less emulsifier drag-out than for the more viscous lipophilic emulsifiers. Diluted hydrophilic emulsifiers are non-flammable and relatively non-toxic. Lipophilic emulsifiers have a high flash point, but they will add considerably to a general fire if ignited.

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7.2.5

WATER AND SOLVENT

Excess water washable penetrant can be removed by first washing with water and then subsequently by wiping with solvent dampened cloths.

7.3

TYPES OF DEVELOPER

Developers may be divided into five types: Form a

Dry powder

Form b

Water-soluble

Form c

Water suspendable

Form d

Solvent based (non-aqueous wet)

Form e

Water or solvent based for special application

7.3.1

DRY POWDER DEVELOPER

Dry powder developer is almost exclusively used in fluorescent systems in the U.K. It is a light, fluffy, hygroscopic, highly absorbent powder that will cling to dry metallic surfaces. It will cling even better to wet surfaces and therefore will generally stick best to the bleed out from discontinuities. Dry developer is hardly visible in white light so leaves a contrast of bright fluorescence against a dark background in UV-A conditions. By its nature it absorbs moisture so if inhaled or swallowed in significant amounts, will dry out nasal passages and the throat. It is inconvenient rather than harmful but, nonetheless, it is important that care should be exercised in use. Advantages

Disadvantages

Easy to handle

Difficult to see if properly applied

No hazardous vapours

Fine powders can be hazardous

Easy to remove

Do not offer high degree of colour contrast

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7.3.2

AQUEOUS DEVELOPER

Aqueous liquid developers can be sub-divided into suspendable and soluble. The water suspendable type is bought as a white powder and then mixed in the correct proportions. A much more dilute mixture is used when it is to be used in a fluorescent system. It must be constantly agitated to keep it in even suspension. Water-soluble developer is quite common in the USA but has found little favour in Europe. It is sold in white fine granular form and is, in fact, water-soluble salts, which dissolve when mixed with water to make a straw coloured liquid. When the test specimen is wetted with the developer and dried, the surface hardly appears affected in normal light. This type of developer is therefore restricted to fluorescent systems.

7.3.3

Advantages

Disadvantages

No vapours or dust

Difficult to apply evenly

Cheaper than non-aqueous

Require drying after application

SOLVENT BASED

This is often called non-aqueous developer and is a suspension of inert white powders in a volatile solvent. The most common solvent is trichloroethane (1.1.1.), although if sulphur or halogen free materials are specified flammable acetone or naptha based materials are usually used. This is the most common type of developer used with colour contrast penetrant. It leaves a fine white background through which red indications are readily visible. Advantages

Disadvantages

Most sensitive

Hazardous solvents

Useable with fluorescent or colour contrast

Need to be correctly applied Higher cost

7.3.4

WATER OR SOLVENT BASED FOR SPECIAL APPLICATION

When an indication needs to be recorded a peelable developer can be applied which can be carefully removed once development has occurred.

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7.4 SYSTEM CLASSIFICATION Throughout this text the classifications for penetrant chemicals contained in BS EN 571 have been highlighted. These classifications are repeated below along with those outlined in some USA.

7.4.1

BS EN 571 Penetrant Type I Type II Type III

Fluorescent Colour contrast Dual (Combined colour contrast and fluorescent)

Removal Method A Method B Method C Method D Method E

Water Lipophilic emulsifier Solvent Hydrophilic remover and water Water and solvent

Developer Form a Dry powder Form b Water-soluble Form cWater suspendable Form d Solvent based (non-aqueous wet) Form e Water or solvent based for special application

7.4.2

MIL-L-25135 C Group I

Solvent removed, visible, dry, wet or non-aqueous developer

Group II

Post emulsifiable, visible, dry, wet or non-aqueous developer

Group III

Water washable, visible, dry, wet or non-aqueous developer

Group IV

Water washable, fluorescent, dry, wet or non-aqueous developer

Group V

Group VI A

Medium sensitivity, post-emulsifiable fluorescent, wet or nonaqueous developer High sensitivity, post emulsifiable (hydrophilic), fluorescent, wet, dry or non-aqueous developer

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7.4.3

Group VI B

Ultra-high sensitivity, post emulsifiable, fluorescent, wet, dry or non-aqueous developer

Group VII

High sensitivity, solvent removable, fluorescent, non-aqueous developer

MIL-I-25135 E Penetrant Type I Type II Type III

Fluorescent Colour contrast Dual (Combined colour contrast and fluorescent)

Removal Method A Method B Method C Method D

Water washable Post emulsifiable, lipophilic Solvent-removable Post emulsifiable, hydrophilic

Developer Form a Form b Form c Form d Form e

Dry powder Water-soluble Water suspended Non-aqueous Special application

Sensitivity Level 1/2 Level 1 Level 2 Level 3 Level 4

ultra low low normal high ultra-high

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8.0 CHOICE OF PENETRANT SYSTEM The factors influencing the choice of which system to use are: • • •

Size and type of defect Geometry and intricacy Surface condition

8.1

SIZE AND TYPE OF DEFECT

From the previous text it has been mentioned that wide shallow defects (comparing the depth to the width of the opening to the surface) are most likely to be detected by the post-emulsifiable method. Likewise it has also be mentioned that fine defects are best located by fluorescent methods due to the higher sensitivity of the eye to fluorescent rather than colour contrast indications. When seeking very fine wide shallow defects the fluorescent post emulsifiable technique would be the recommended system.

8.2

GEOMETRY AND INTRICACY

Highly intricate components containing a large number of changes in section and those with threaded areas present problems in removing excess penetrant. Post emulsifiable methods whilst more sensitive than water washable would in all likelihood leave behind excessive background coloration on such samples. This would thus reduce the detection of defects.

8.3

SURFACE CONDITION

Likewise rough surface are difficult to fully clean when tested with post emulsifiable methods, and components such as sand castings would be best tested by a water washable method. Fluorescent methods are also less suited to the testing of rough components than visible methods (colour contrast) due to problems in adequately monitoring penetrant removal.

8.4

OTHER FACTORS TO BE CONSIDERED

•

Component material Solvent removable methods may lead to surface damage due to incompatibility between the penetrant and the material under test. The now withdrawn British

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Standard 6443 contained a procedure for determining compatibility involving the partial immersion of components of the same composition for periods of 16 hours in the case of penetrants and developers and 30mins for cleaners and removers.

•

Size and position of the item to be tested On-site welds are unlikely to be tested by fluorescent and water washable methods due to the requirement of darkened conditions in the former and a water source in the latter case.

•

Equipment and expertise available Fluorescent water washable and post emulsifiable test methods generally involve the use of flow lines and are thus more suited to factory use than on site testing.

•

Cost Water washable penetrant methods are obviously much cheaper than solvent removable and post emulsifiable methods due to the availability of the main cleaning fluid.

•

Number of components to be tested Fluorescent methods are recommended for batch inspections due to the higher sensitivity of the eye to fluorescent indications over visible colour contrast ones. This offers greater sensitivity and reduces the fatigue suffered by the operator performing the inspections. Operators are thus able to perform at a higher level for longer periods.

8.5

EXAMPLES

Based upon the criteria listed shown below are a couple of examples showing what test system would be selected and the reasoning behind the selection Example 1: Inspection of a large number of threaded components Batch inspection of components with complex geometry. Method: Fluorescent water washable with dry powder developer • • •

Fluorescent for mass inspections Water washable more suited than solvents to batch inspections Post-emulsifiable difficult to remove from threads

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Example 2: Inspection of turbine blades for fatigue cracks High sensitivity required for a low number of components. Method: Fluorescent post-emulsifiable with non-aqueous developer • • •

Fluorescent more sensitive than colour contrast systems Post emulsifiable more sensitive than water washable Non-aqueous developer most sensitive form of developer

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9.0 EQUIPMENT CHECKS There are few checks on the penetrant materials usually used on site, where the colour contrast solvent remover system is being used, dispensed from aerosols. However, where line processes are in use, controls must be undertaken to ensure that overall system efficiency is maintained. The checks described in these notes are considered to be the minimum needed and are in their simplest form. This is so that they can be easily and economically applied in the average workplace. When material containers are re-filled a sample of each of the materials should be taken and stored away from heat and light in clean glass containers. The containers should be labelled with their identity, batch number and date of filling. These are utilised as controls to check the chemicals in use. These checks can be divided into overall system performance checks which, as the name suggests monitor the entire penetrant system and control checks monitoring individual parts of the process such as UV-A light levels.

9.1

OVERALL SYSTEM PERFORMANCE

This is a check normally done daily or at the start of each shift involving a specially prepared test piece processed through each system and compared with previous checks. Any change in results signifies a change in the chemicals being utilised. These checks typically employ either a quenched aluminium alloy block or chrome plated stainless steel panels. The former of these, while quite useful for comparing the performance of two penetrants, tend to deteriorate quite rapidly after a number of checks. Typically their use will involve processing one half of the block with fresh chemicals stored away from light and extremes of temperature and the other half with penetrant chemicals in daily use. Any difference in the appearance of the two halves signifies a problem with one or all of the chemicals in use. The most common type of test piece is a chrome plated stainless steel panel often referred to as a TAM panel. A number of star shaped artificial cracks of increasing severity are induced by pressing a ball into the obverse side. Different penetrant systems will reveal different numbers of cracks depending upon the sensitivity level. A reduction in the number of cracks revealed will indicate a change in the penetrant chemicals. A comparitor is sometimes used, made as a mirror image to the panel, from a strippable transfer lacquer. One half of the block is often a grit blasted panel used to check washability and remover efficiency. It is essential that test pieces of any kind are thoroughly cleaned and protected after use. Cleaning in a hot liquid and vapour degreaser is usual. Protection is normally effected by immersion in clean solvent before next use the test piece must be thoroughly dried.

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9.2 9.2.1

CONTROL CHECKS WATER WASH TEMPERATURE AND PRESSURE

If water washing is being used, the temperature of the water should be maintained at a constant between 5°c and 50°c. The pressures of the water and air must be as low as possible. These figures conform to BS EN 571 and may vary between specifications. A daily check is recommended.

9.2.2

COLOUR INTENSITY

Every week the colour intensity of the penetrant material should be checked. For colour contrast penetrants a 5% solution of used penetrant mixed with dichloromethane should be compared against a lighted white background with a similar solution of new check material. Any significant difference in colour should be investigated. If the penetrant is fluorescent the brightness of a 55mm x 55mm filter paper soaked in 5% solution of used penetrant and dichloromethane is checked in the UV light monitor. The filter paper is dried and placed on the white light sensor of the monitor. An UV-A lamp, placed at a fixed distance is shone on the filter paper and the white light generated from the paper is noted. A similarly prepared filter paper using new check penetrant is also checked for brightness and the readings from both are compared. Any significant difference in brightness should be investigated. BS6443 annex B.7.1 and B.7.2 give specific details.

9.2.3

PENETRANT REMOVER CHECK

The grit blasted surface adjacent to the chrome-plated strip is sometimes used to check washability and remover efficiency. However, a simple check of hydrophilic remover for contamination by penetrant can be carried out each day before work is started. While the liquid is still, shine the UV lamp on the surface. Any yellow/green fluorescence indicates that the material is saturated and should be discarded BS6443 appendix B.5.3. describes a remover check. If a check of the concentration is called for then a refractometer must be used.

9.2.4

DEVELOPER CHECK

Dry powder developer must remain light, fluffy and dry. It can become contaminated with penetrant carried into the cabinet from test pieces. Therefore a UV light should be shone at the developer weekly and any affected developer must be removed. Practical Penetrant Inspection Rev 0 Dec 2005 Equipment Checks Copyright © 2005, TWI Ltd

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Aqueous suspendible developers must be checked for specific gravity regularly and/or by a settlement check.

9.2.5

UV LAMP OUTPUT CHECK

Mercury vapour lamps deteriorate with age and in proportion to the number of times the arc is struck. Therefore it is best to leave the lamp on all day rather than switch it on and off. Every month the output should be checked using a monitor. The output of the lamp should be recorded at a distance of 400mm, which will reveal deterioration in output (BS4489 paragraph 6.1 refers). The output at the work surface must exceed 0.5mW/cm2 according to BS 4489 and 1mW/cm2 according to BS EN 571.

9.2.6

UV MONITOR CHECK

Every year the monitor, which checks the UV lamps, must be returned to the manufacturer or the National Physical Laboratory for calibration.

9.2.7

WATER REMOVABLE PENETRANT, WATER TOLERANCE CHECK

Contamination of penetrants with water is a problem when water removable penetrant is used. A simple monthly check will prove that the penetrant is fit to use. Ascertain from the manufacturer the water tolerance of the penetrant in use. Take a sample of the used material and add water up to 50% of the tolerance. If the penetrant goes milky and fails to clear it must already have had at least 50% of the allowed water in it. Therefore it would be wise to change the bath.

9.3

MAINTENANCE CHECKS

Included in this section are some common sense checks, which should be applied. The list is not exhaustive; other checks may be necessary in specific systems.

9.3.1

TANK LEVELS

Each day the penetrant tanks must be replenished. Some specifications require that permanent marks be put on the tanks showing maximum and minimum levels.

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9.3.2

EQUIPMENT CLEANLINESS

It is important that the work area is kept clean and tidy for obvious reasons. Also in fluorescent areas the brightness of the area can be significantly affected by spilt penetrant material. This is usually an ongoing job but formally noted daily.

9.3.3

AIRLINE CLEANLINESS

Oil and water traps in factories are frequently overlooked so it is wise to check the cleanliness of the air supply daily. Simply direct the air jet at a strong tissue and check it for moisture.

9.3.4

PROCESSING UNITS

A weekly check on the whole penetrant line or lines is good housekeeping and potential health and safety hazards should be reported. Annoying things like torn or badly hung curtains should be fixed.

9.3.5

UV LAMP MAINTENANCE

Clean each lamp and filter before checking the output value. A significant amount of light can be lost due to dirt. Electrical safety must also be checked.

9.3.6

CLEAN TANKS

It is amazing what can get into penetrant tanks therefore a six monthly clean out is prudent. Although it is rarely necessary to discard the material a drain cock, in the tank side, a few centimetres above the bottom should be fitted and used to drain off the penetrant. A second cock in the bottom of the tank can be used to drain the residue and this small amount is often the only material that needs to be discarded.

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