Stainless Steel Types [PDF]

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ES008-01-2001

STAINLESS STEELS TABLE OF CONTENTS

1. Stainless Steels …………………………………………………………………………………………… 2 2. Martensitic Stainless Steels ……………………………………………………………………………… 3 3. Ferritic Stainless Steels …………………………………………………………………………………..

5

4. Austenitic Stainless Steels ……………………………………………………………………………….

6

5. Austenitic-Ferritic Stainless Steels ……………………………………………………………………… 9 6. Other types of stainless steels ……………………………………………………………………..……

9

7. Corrosion …………………………………………………………………………………………………..

10

8. The general corrosion …………………………………………………………………………………….

11

9. The spot-shaped corrosion ………………………………………………………………………………. 11 10. The corrosion through slits ……………………………………………………………………………..… 12 11. The corrosion under stress ……………………………………………………………………………..… 12 12. The inter-granular corrosion …………………………………………………………………………….... 13 13. The corrosion by galvanic pair …………………………………………………………………………… 13 14. The erosion-corrosion …………………………………………………………………………………..… 14 15. The protection against corrosion ………………………………………………………………………... 14 16. Considerations about the choice of stainless steels …………………………………………………... 14 17. Dimensional characteristics of stainless steel plates ………………………………………………….. 15 18. Summary of basic characteristics and care with the stainless steels ……………………………….

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19. Some processing equipment manufactured in stainless steel ………………………………………. 19.1. Processing Tanks …………………………………………………………………………..……… 19.2. Mixers ……………………………………………………………………………………………….. 19.3. Reactors ………………………………………………………………………………………….…. 19.3.1. Heat Exchange ……………………………………………………………………..…...… 19.3.2. Thermal Insulation ………………………………………………………………….…..….

17 17 17 19 19 20

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ES008-01-2001 1. STAINLESS STEELS

Stainless steels consist of a category of steels characterized for having in their chemical composition a high contents of chromium, usually above 12%. As the chromium contents increases in the steel, it turns from a metal of high corrosibility into a practically undestructible metal due to corrosion, so saying that the alloy has become stainless. At high temperatures the same fact may be observed, that is, as the chromium contents increases, the tendency to oxidation decreases. In this case, the most positive chromium effect only develops when its contents is above 20%, so turning from simply stainless steels into heat resistant or refractory steels.* Nickel follows chromium in importance. Its actuation is felt not only in the corrosion resistance improvement but also in the sense of improving its mechanical properties. Its influence is particularly high when its contents is above 6%. Carbon, which is compulsorily present in all sorts of steel, slightly diminishes corrosion on stainless steels, when in diluted state. Molybdenum generally increases the passiveness and resistance to corrosion at high temperatures and pressures. Copper is sometimes added to the stainless steels in order to improve their corrosion resistance to certain agents. Silicium acts in quite the same manner. Its main effect is in the sense to improve oxidation resistance at high temperatures. Manganese only produces outstanding effects when replacing nickel. Tantalum and niobium are sometimes added due to their high avidity for carbon, so avoiding the rising of one of the most harmful kinds of corrosion on stainless steels, the inter-granular corrosion. Nitrogen has been also added to chromium in steels allowing the improvement of their hardness or their soldering capacity and resistance to the inter-crystalline corrosion, depending on the chromium percentage. Stainless steels appeared in the beginning of this century but, only after World War I their production started to grow. As time went by the contents of carbon was being reduced, as the positive influence of chromium, nickel, molybdenum and copper was acknowledged. These steels have had a huge progress and their properties have been improved steadly. Parallelly, stainless steels are always acquiring new applications, in such a way that their consumption has been constantly increased. There are several ways to classify the stainless steels but, according to their metallographic structure, they may be divided into four main groups : !

martensitics

!

ferritics

!

austenitics and

!

austenitic-ferritics

th

* Steels and Cast Irons - ABM - 5 Edition

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ES008-01-2001 2. THE MARTENSITIC STAINLESS STEELS

It is essentially an iron and chromium alloy that comes with a contents varying from 11.5 and 18%. Within this group, three categories may be considered : a) low-carbon martensitic, also called “turbine” type, b) middle-carbon martensitic, called as steels for “cutlery” and, c) high-carbon martensitic, which are the “wearing resistant” ones. In general, their most important characteristics are as follows: !

they may be easily worked, either in hot or cold work, mainly when the carbon contents is low.

!

they are hardenable and the hardness improves their resistance to corrosion and,

!

they are magnetic-iron.

The low-carbon martensenic steels have good mechanical properties and resistance to relatively high corrosion. The “cutlery” types are used where a satisfactory hardness is desired, allied to a reasonable ductility and, the “wearing resistant” ones have a high hardness obtained upon sacrificing the ductility.

Table 1 shows these steels chemical composition and Table 2 summarizes the main applications.

Table 1 - Martensitic steels composition AISI Type

Chemical Composition (%) Cr Ni 11.5 / 13 11.5 / 13.5 11.5 / 13.5 1.3 / 2.5

403 410 414

C 0.15 0.15 0.15

416

0.15

12 / 14

-

420

0.15

12 / 14

-

420 F

0.3 / 0.4

12 / 14

-

431 440 A

0.2 0.6 / 0.7

15 / 17 16 / 18

1.3 / 2.5 -

440 B

0.75 / 0.95

16 / 18

-

440 C

0.95 / 1.20

16 / 18

-

Other elements Si – 0.5 / Mn – 1 Si – 1 / Mn – 1 Si – 1 Mg – 1 Si – 0.5 / Mn – 1 P or S or Se – 0.07 Mo or Zr – 0.6 Si – 1 / Mn – 1 Si – 1 / Zr or Mo – 1 P or S or Se – 0,07 Si – 1 / Mo – 0.75 Si – 1 / Mo – 0.75 Si – 1 / Mn –1 Mo – 0.75 Si – 1 / Mn –1 Mo – 0.75

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ES008-01-2001 Table 2 - Main applications for martensitic steels AISI TYPE Applications turbine type: for turbines wrought blades and compressors and other parts aimed to stand high 403 stresses when in operation. turbine type: of low cost for general applications. Hardened: in pumps framework, axis and 410/414 propellers, screws, nuts, parts for furnaces (up to 750º C), etc. Re-annealed: plates for tanks in the petroleum industry and kitchenware, etc. 416 turbine type: easy for machining, in screws, nuts, valve rods, etc. cutlery type: for surgical and odontological instruments, valve pads, hardened springs, 420 bearings, etc. 420F cutlery type: easy for machining, with the same applications as 420. turbine type: for parts for pumps,conveyors, maritime propeller axis, parts for machinery in 431 food industry, etc. Is the most resistant one to corrosion among the ones in its category. wearing resistant type: for surgical instruments, spheres, bearing tracks and applications 440 A / B / C where high hardness and satisfactory resistance to corrosion are requested.

Table 3 supplies general information about the thermal treatment of martensitic steels and Table 4 shows their average mechanical properties after such treatment.

Table 3 - Characteristics of the thermal treatment of martensitic steels Hardening Tempered Heating Heating time Temperature Bath Type temperature min. °C °C 403/410/416 925 / 1,000 15 / 30 oil 225 / 375 414 975 / 1,050 15 / 30 oil / air 225 / 400 420 975 / 1,050 15 / 30 air / hot oil 150 / 375 431 975 / 1,075 15 / 30 air / hot oil 225 / 400 440 A / B / C 1,000 / 1,075 15 / 30 air / hot oil 100 / 375

Table 4 - Resulting mechanical properties Tensile Hardness Type strength limit 2 kgf/mm HB 403/410/416 360 / 380 130 414 370 / 400 137 420 470 / 530 175 431 370 / 400 137 440 A 500 / 560 189 440 B 520 / 590 196 440 C 540 / 620 200

Flowage limit kgf/mm 98 102 158 102 182 189 193

2

Elongation % 15 15 8 17 5 3 2

Time hours 1/3 1/3 1/2 1/3 1/2

Resistence to shock kgf.m 2.8 / 6.2 4.1 / 8.3 1.1 / 2.1 4.1 / 8.3 0.4 / 0.8 0.3 / 0.7 0.3 / 0.7

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ES008-01-2001 3. FERRITIC STAINLESS STEELS

Ferritic stainless steels have a chromium contents that usually varies from 12 to 18% being able to, however, increase up to 30% in special cases. The presence of carbon seldon exceeds 0.2%. The structure of these steels at room temperature, with any cooling velocity, is always ferritic, therefore, they are not meant to be toughed or hardenable . Ferritic steels are usually cheaper than the austenitic ones and, therefore, are preferable for applications where it is only necessary a moderate level of resistance to corrosion. They have low tenacity or resistance to impact. They are difficult to be soldered and for cold work. Table 5 supplies the chemical composition of the main ferritic steels and Table 6 shows their properties and typical applications.

Table 5 - Chemical composition of ferritic steels Chemical Composition (%) AISI Type C Cr Other elements 405 0.08 11.5 / 14.5 Al – 0.10 to 0.30; Mn – 1 / Si –1 409 0.15 12 /14 Al – 3.5 to 4.5; Mn – 1 / Si – 1 / Ti – 0.75 429 0.12 14 / 16 Mn – 1 / Si – 1 430 0.12 16 / 18 Mn – 1 /Si –1 430 F 0.12 16 / 18 P or S or Se – 0.07; Mo or Zr – 0.06 442 0.20 18 /23 Mn – 1 / Si – 1 443 0.20 18 / 23 Cu – 0.90 to 1.25; Si – 0.75 / Ni – 0.50 446 0.20 23 / 27 N2 – 0.25 / Mn – 1.5 / Si - 1

Table 6 - Main apllications for ferritic steels AISI Type Applications 405 for irradiating pipes, boilers, containers for petroleum industry, etc. 409 for general use, in automobile exausting systems, etc. 429 it has good capacity for soldering, for equipment working with nitric acid. is the most common one, it has easy configuration, for equipment in chemical industry, restaurants, kitchens, outdoor ornaments and decoration, parts for furnaces, etc. Resistant to 430 sea water. 430 F variety of easy machining, for screws, nuts, hardware, etc. it has better resistance to corrosion than the previous ones, for works at high temperature 442 when manufacturing easiness is not requested. 443 high resistance to corrosion, for chemical equipment and apllications at high temperature. shows excellent resistance to corrosion and oxidation up to temperatures of 1150ºC, for parts 446 for furnaces, burners, radiators, recuperators, etc.

The usual thermal treatment for ferritic steels is an annealing for relieving the stresses generated in cold treatment and, for obtaining the maximum ductility. They are usually annealed at 800 / 850º C for one to two hours, following air cooling or even water or oil cooling in order to prevent brittleness that may occur due to the slow cooling.

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ES008-01-2001 Ferritic stainless steels are subject to and acquire brittleness when heated at about 475º C or when slowly cooled throughout this temperature. The phenomenon results in hardness improvement and ductility reduction. Brittleness may be eliminated by re-heating the steel at temperatures over 600º C followed by fast cooling. The main mechanical properties are indicated in Table 7. Regarding this aspect it is necessary to clarify that AISI 430 steel is the unique one in the group which is not entirely ferritic, being feasible to suffer a slight hardening by means of a fast cooling.

Table 7 - Mechanical properties of ferritic steels Tensile Hardness Type strength limit 2 kgf/mm HB 405 160 / 180 42 430 130 / 165 45.5 430* 255 / 300 105 430 F 150 / 190 49 442 150 / 175 52.5 446 160 / 185 56 * After hardening

Flowage limit kgf/mm 24.5 24.5 77 31.5 31.5 35

2

Elongation % 20 20 / 35 3 15 / 30 30 / 35 25 / 30

Resistence to shock kgf.m 2.8 / 4.8 2.1 / 4.8 2.1 / 4.8 2.1 / 4.8 0.7 / 2.1 0.1 / 0.4

4. AUSTENITIC STAINLESS STEELS

These are the most important ones among the stainless steels. They simultaneously present chromium and nickel, with chromium varying between 15 to 26% and nickel from 6 to 25%. The introduction of nickel considerably increases the resistance to oxidation at high temperature seeing that nickel is more noble than iron and, in addition, it makes an oxide layer that spontaneously protects the steel. It has been already evidenced that the restauration of the protective inert film which has been withdrawn from a Cr-Ni type stainless steel is much faster than the one from an only Cr stainless steel. Their general characteristics are as follows : ! ! ! !

non-magnetic, non-hardenable, they show good conditions for soldering being not necessary pre-heating and, thermal treatment after soldering is rarely required and, their tenacity is superior than the other stainless steels, specially at low temperatures.

These steels show an interesting phenomenon when toughned : the hardness improvement verified is far superior to what would be met, by means of the same deformation, in other steels. The stretching in cold, for instance, of a steel with 18% of chromium and 8% of nickel (o 18-8), may produce a tensile 2 resistance of 250 kgf/mm , with a deformation that, in a common steel, would not generate more than about 2 140 kgf/mm . In most cases, this high hardness is acceptable and, therefore, the austenitic steels are thermically treated before being supplied in order to lead its mechanical resistance to an acceptable level. This treatment, austenitization, consists of heating the temperature, generally between 1,000 and

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ES008-01-2001 1,120º C, followed by cooling, as fast as possible, up to the room temperature. It was observed that as the nickel contents increases, the toughening effect is less emphasized considering the stabilizing action of this element. The most common and known austenitic stainless steel is the o 18-8 whose resistance to corrosion is, in general, far higher than the ferritic stainless steels. If the contents of chromium and nickel is increased and if other alloy elements are added, it may be obtained austenitic stainless steels with a wide and varied range of properties. Titanium and niobium fix the carbon in the form of their respective carbides and, in this way, greatly avoid the chromium carbides precipitation during soldering. When this chromium carbides precipitation occurs, the steel becomes susceptible to the so called inter-granular corrosion. Molybdenum intensively improves the resistance to general corrosion of the austenitic steels in the presence of most acid media. Carbon is an impurifying element and, therefore, is usually kept low in order to prevent the intergranular corrosion. The austenitic stainless steels “L” or “ELC” (extra-low carbon ), which contains a maximum of 0.035% of carbon, are basically immune to inter-granular corrosion. Table 8 shows the composition of austenitic stainless steels and Table 9 presents their general properties and typical applications.

Table 8 - Chemical composition of austenitic steels Chemical Composition (%) AISI Type C Cr Ni 301 0.15 16 / 18 6/8 302 0.15 17 / 19 8 / 10 302 B 0.15 17 / 19 8 / 10 303

0.15

17 / 19

8 / 10

304 304 L 308 309 309 S 310

0.08 < 0.035 0.08 0.20 0.08 0.25

18 / 20 18 / 20 19 / 21 22 / 24 22 / 24 24 / 26

8 / 10.5 8 / 13 10 / 12 12 / 15 12 / 15 19 / 22

316

0.10

16 / 18

10 / 14

316 L

< 0.035

16 / 18

10 / 15

317

0.10

18 / 20

11 / 15

321

0.08

17 / 19

9 / 12

347

0.08

17 / 19

9 / 13

Other elements Mn –2 / Si – 1 Mn –2 / Si – 1 Mn –2 / Si – 2 to 3 Si – 1 / Mn – 2; P or S or Se – 0.07; Mo or Zr – 0.06 Si – 1 / Mn – 2 Mn – 2 / Si – 1 Mn – 2 / Si – 1 Mn – 2 / Si – 1 Mn – 2 / Si – 1 Mn – 2 / Si – 1 Mo – 2 to 3; Mn – 2 / Si – 1 Mo – 3 to 4; Mn – 2 / Si – 1 Mo – 3 to 4; Mn – 2 / Si – 1 Ti – 5 / Mn – 2 / Si – 1 Cb – 10 / Mn – 2 / Si - 1

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ES008-01-2001 Table 9 - Properties and applications of austenitic steels AISI Type Applications for general application, good workability, ornamentation, home utensils, structural purposes, 301 equipment for chemical, naval, food industry, etc. 302 for decorative applications or of resistance to corrosion as indicated in 301. for parts for furnaces, offers better resistance to the formation of oxide coats at high 302 B temperature due to the presence of silicium 303 type 18-8 of easy machining, for axis, screws, nuts, parts for carburator, valves, etc. 304 type 18-8 for equipment for food processing, cryogenic containers, etc. type 18-8 with lower carbon contents, solderable with less danger of inter-granular corrosion, 304 L same applications as 304. 308 for parts for industrial furnaces, offers higher resistance to corrosion than 18-8. has good mechanical resistance and resistance to oxidation at high temperatures, for 309 equipment for chemical industry, furnaces, kilns, parts for pumps, etc. due to the low percentual of carbon in relation to the 309, has the same applications allowing 309 S soldering with less risk of inter-granular corrosion. resists to oxidation at temperatures around 1050 - 1100º C, for equipment for chemical 310 industry, parts for furnaces, kilns, solder electrodes, etc. offers better resistance to corrosion, for equipment for chemical, paper and cellulose industry, 316 etc. has low contents of carbon, that is why the risk of inter-granular corrosion in the soldering is 316 L lower, has the same applications as 316. 317 better resistance to corrosion than 316 type and, with identical applications. type 18-8, stabilized against inter-granular corrosion at high temperatures, for applications 321 where too much soldering is requested, pressure vessels, expansion joints, etc. 347 type 18-8, stabilized for work at high temperature where soldering is requested, similar to 321.

The main mechanical properties for the austenitic stainless steels after adequately thermically treated, are indicated in Table 10.

Table 10 - Mechanical properties for the austenitic steels Hardness Tensile Flowage limit Type strength limit 2 2 HB kgf/mm kgf/mm 301 155 / 175 70 28 302 140 / 160 59.5 24.5 302 B 150 / 170 66.5 28 303 155 / 175 59.5 24.5 304 140 / 160 59.5 21 308 145 / 165 59.5 24.5 309 165 / 185 63 28 310 165 / 185 63 28 316 140 / 160 56 24.5 317 140 / 160 56 24.5 321 145 / 160 59.5 24.5 347 145 / 160 59.5 24.5

Elongation % 50 / 60 50 / 60 50 / 60 30 / 55 50 / 60 50 / 60 45 / 50 45 / 50 50 / 60 50 / 60 50 / 55 45 / 55

Resistence to shock kgf.m 9.7 / 15.2 9.7 / 15.2 11 / 13.8 9.7 / 15.2 9.7 / 15.2 9.7 / 15.2 6.9 / 13.8 9.7 / 15.2 9.7 / 15.2 9.7 / 15.2 9.7 / 15.2

The most important physical properties of some of these steels are in Table 11 and the admissible stresses transcribed from ASME (Table UHA-23) section VIII, Clause I, are in Table 12.

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ES008-01-2001 Table 11 - Physical properties of austenitic steels Thermal Density AISI Type Conductibility g/cm3 W/m.°C 304 / 304 L 7.9 15 316 / 316 L 8.0 13.5 317 8.0 13.5 321 / 347 7.9 15

Expansion Resistivity at 20° C Coefficient at 20° C -0 10 .Ohm.m -0 -1 10 .°C 18.2 0.7 17.5 0.75 17 0.75 17.5 0.70

Table 12 - Admissible stresses for some austenitic steels in kgf/cm2 - according to ASME Temperature (°C) AISI Type -20 to 40 100 150 200 250 300 304 1,326 1,081 989 918 857 816 304 L 1,102 928 846 775 734 694 316 1,326 1,122 1,030 949 887 836 316 L 1,102 918 836 765 714 673

350 785 663 816 643

5. AUSTENITIC-FERRITIC STAINLESS STEELS These steels have high contents of chromium (up to 26%) a low contents of nickel (between 4% and 5%). They are also known as duplex stainless steels because their structure is made up of a mixture of austenite and ferrite. They are not hardenable and are magnetic, proportionally to the contents of ferrite. They are difficult to work with since their tenacity is relatively low and their flowage limit at 0.2% is high. They present fragility after soldering because the area affected by the heat becomes almost completely ferritic and with coarse granulation, being mandatory the thermal treatment after soldering. The austenitic-ferritic steels are strongly resistant to corrosion under fracturing stress which is, exactly, the conventional austenitic steels weak point. Table 13 brings the approximate chemical composition of two of these steels.

Table 13 - Chemical composition of duplex steels Chemical Composition (%) AISI Type C Cr Ni 327 0.08 26 5 329 0.08 26 5

Mo 1.5

6. OTHER TYPES OF STAINLESS STEELS The event of nickel shortage led to the development of austenitic steels where manganese acts as a partial substitute for nickel. These steels are grouped into the class 200 and their mechanical properties and resistance to corrosion are essentially the same as the ones from serie 300 which they replace.

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ES008-01-2001 There are stainless steels containing from 1.6% to 12.5% of nitrogen, they are called nitronic. They are characterized for having high contents of chromium and manganese besides presenting nickel and eventually, molybdenum, silicium and vanadium. They are austenitic type and have higher mechanical resistance than the type 300 ones and identical resistance to corrosion. The nitronic 50, which is the most alloyed among them, shows better resistance to corrosion. They are indicated for the storage and transportation of liquefied gases and with applications in the chemical industry, in pumps, valves, axis, etc. The hardenable-by-precipitation stainless steels are characterized for simultaneously presenting high resistance to corrosion and high mechanical resistance, consequently being used where both requirements are fundamental, such as in special springs, parts for aviation application, etc. There are special stainless steels for typical applications called of high technology, developed by Sandvik, Uddeholm and Crucible.

7. CORROSION Corrosion is the inverse of the metallurgic process for obtaining a metal from its respective ore.If the iron obtained in this process does not have anti-corrosive protection and gets in contact with the air and moisture, it will return to its original condition, as pointed out in Exhibit 1. It must be emphasized that corrosion is a spontaneous process, in lieu of the metallurgical process that requires the supply of energy for its realization. Corrosion is the deterioration of a material, generally metallic, by means of chemical or electrochemical interation with the medium, allied or not with mechanical efforts. It is a gradual and continuous attack to the metal by part of the neighbouring medium, which may be the contaminated cities atmosphere or a liquid or gaseous chemical medium. The attack velocity and its extension depend not only on the medium nature but also on the type of metal suffering the corrosive action. Corrosion annually costs to the society something evaluated as 3% of the GDP (about US$ 10 billion), not including the costs and the loss of profits caused by temporary maintenance shutdowns. All metallic materials are subject to corrosion if the medium is sufficiently aggressive. Thus, only a study might clarify the corrosive process mechanism, allowing the indication of appropriate protective measures or, the best material for that application.

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ES008-01-2001 Iron Ore Energy

Reduction Eneregy

Fusion Energy

Hot or Cold Work

Steel in operation subject to corrosion Energy

Rust Exhibit 1 - The cycle of ore transformation

8. THE GENERAL CORROSION Is the most common type, presenting a uniform attack over all surface of the part in contact with the corrosive medium. Stainless steels do not suffer general corrosion within neutral solutions. This kind of corrosion, in the case of the stainless steels, is associated to the acid media. The most common way to express this corrosion is by the loss of thickness by time unit. In a general way the corrosion tables use the following criterion : ! ! !

corrosion rate lower than 0.1 mm/year, the material is resistant to corrosion. rate between 0.1 and 1 mm/year, the material is not satisfactorily resistant to general corrosion but, may be utilized in certain cases and, rate above 1mm/year, indicating high general corrosion, turning unviable the use of such material for this application.

9. THE SPOT-SHAPED CORROSION The spot-shaped corrosion or by “pit” is an extremely located form of attack which results in holes of small diameter and considerable depth on the metal surface. Usually the “pits” show the botton in angulate

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ES008-01-2001 form and the depth larger than the diameter. This is one of the most dangerous and destructive forms of corrosion, being able to cause unexpected equipment failures by perforation. It is a treacherous corrosion because,in most of the times, it is quite difficult to detect the presence of “pits” since they are hidden by the proper corrosion residues. Stainless steels suffer corrosion by “pits” within neutral or acid solutions containing halogenates, particularly chlorides, bromides and iodides. These ions locally rupture the passive protective film at the points where it is debilitated by the action of dirt, dross or superficial damages caused by machining or utilization. At the points where the passive film is broken , a small anodic area is generated, while the large passive film surrounding the “pit” acts as cathodic area. Molybdenum and chromium have a very positive effect on the stainless steels capacity of resisting to the beginning of such attacks, being that molybdenum has a three-time greater influence than the chromium. Nitrogen or silicium in combination with molybdenum also has a positive effect on austenitic steels, however, sulphur and manganese, in the form of manganese sulphide, have a negative influence. Nickel, copper and silicium slow down the propagation velocity of such attacks.

10. THE CORROSION THROUGH SLITS It is very common and located inside slits and other narrow and small spaces exposed to corrosive media. This kind of attack is associated to small volumes of solution stagnated in screwed joints, in weldings with penetration defect, in flanged joints and in superficial deposit yards such as : sand, mud, the product itself, etc. The contact between the metal and the non-metallic surface as it is in the case of gaskets in flanges that may cause this type of corrosion. This is true since the oxigen exausting conditions existing inside the slit result in the local destruction of the oxide protective film. In principle the corrosion in slits may be avoided or minimized with some cares in the project and in the operation: a) Using solder instead of screws. Top solders with complete penetration are preferred. To remove the superficial oxidation caused by soldering operation. b) Closing up any slit with continuous solder. c) Designing vessels having full drainage, to avoid corner angles and stagnating areas. d) Inspecting the equipment and to clean off eventual deposits. e) The circulation of fluids in pipes must be preferably done at high speed (over 1.5 m/s) so preventing deposits to come out. f) In general, it is not recommended to paint the stainless steels because on occurring the release of parts of this painting, cracks may appear, causing corrosion in slits.

11. CORROSION UNDER STRESS Is defined as the fracture of a material caused by the simultaneous action of tensile stresses and a specific corrosive medium. It is important to observe that the tensile stresses present may be residual, as consequence of a broaching, flexing or soldering operation; or externally applied such as the equipment operational pressure.

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ES008-01-2001 It is a very dangerous type of localized corrosion since the failure, usually catastrophic, happens through the sudden propagation of slits. Whenever possible, regions of stresses concentration must be avoided such as, for instance, notches, or to provide its relief thermal treatment. The most frequently corrosive media associated to this type of corrosion are the solutions containing chlorides (potable water may have them) and strongly alkaline solutions. The austenitic-ferritic steels are far more resistant to corrosion under stress than the austenitic ones and may be considered as a viable alternative to avoid it.

12. THE INTER-GRANULAR CORROSION If a stainless steel is heated between 500 and 850ºC or, if it is allowed its slow cooling within this temperature interval, carbon will combine with chromium creating chromium carbides. In this way the basal metal loses chromium and if its contents stays below 12%, the steel will no longer be stainless; this is the sensitization. When the stainless steel is welded, the critical temperature interval for carbides precipitation will always exist at a distance of few millimiters from the fillet. Therefore, the area susceptible to inter-granular or inter-crystalline corrosion, where there was a chromium exhaustion, is parallel to the solder. There are two ways to prevent this type of corrosion : !

Using a stainless steel containing low carbon contents (L), or

!

Using a stainless steel stabilized with titanium or niobium.

Ferritic steels are more sensible than austenitic ones to inter-granular corrosion and so, they need to have a much lower carbon contents in order to be able to assure an acceptable protection against this type of corrosion.

13. CORROSION BY GALVANIC PAIR It happens when two different metals are placed in contact in a medium creating a galvanic cell, that is, the less noble metal, which means the one having the smaller electro-chemical potential, becomes the anode and the more noble metal, the cathode. It makes the less noble metal to be attacked at a higher corrosion rate than if there were not any contact between the two metals. Stainless stees are very rarely affected by galvanig corrosion due to the fact that they are more noble than most of metallic metals. Graphite gaskets must be prevented from being in contact with stainless steels because it makes a dangerous galvanic pair for stainless steels. The combination of stainless steels with less noble metals, such as zinc, aluminium and carbon-steel will provide a rising in the corrosion rate of these metals. Stainless steels, in certain cases, may be electrochemically protected through the anodic protection method. This method consists of connecting the stainless steel object to the positive pole of an external continuous current source to turn it anodic and, in this way, to make feasible the creation of a passive film of oxides. The anodic protection of stainless steels is used, for instance, in equipment for the sulphuric and phosphoric acids.

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ES008-01-2001 14. THE EROSION-CORROSION It is a form of corrosion that happens when there is a high relative movement between the metal and the corrosive medium. If the solution contains solid particles, the corrosion rate is naturally higher. The applications where the erosion-corrosion comes out are those where draining corrosive media are involved, such as: pipes, pumps, agitators, valves, heat exchangers, etc. Stainless steels have good resistance to erosion-corrosion thanks to the stability of the superficial passive film. However, if the working conditions are severe, that is, if the medium is aggressive and eventually also contains solid particles and, if the draining velocity is high, then certainly even the stainless steels may suffer erosion-corrosion. The stainless steels resistance to erosion-corrosion is generally increased by the same alloy elements that improve the resistance to the general corrosion in the medium in question.

15 - THE PROTECTION AGAINST CORROSION Generally, the protection against corrosion is made by creating over the metal surface a protecting film that will put aside the basal metal from the corrosive medium. This protective film may be artificially or naturally created. Natural protection is provided by the superficial film spontaneous production resulting from the reaction, of certain alloy elements of the metal, with the neighboring medium. The film sticks to the metallic surface through the action of atomic forces. It is called passivity the typical property of certain metals and metallic alloys that make them stay unaltered in the corrosive medium. This inalterability acquired by the material is linked to the formation of an oxide layer or film in the moment that it is exposed to that medium. The film is formed when chromium atoms of the steels superficial layer, which contains this alloy element, absorb oxygen. It is known today that this is the most important alloy element in the resistant-to-corrosion steels. The chromium oxide superficial film, with thickness lower than two centesimals of micron, is indispensable to provide resistance to corrosion. It is also known, as mentioned by Chiaverini, V.* (1982), that either this film thickness or its chromium contents increase as the superficial polishing is improved. Thus, it is concluded, as observed in practice, that the better the superficial treatment, the better will be the resistance to the steels corrosion. The most important metals in the sense of alloying to the iron under economic conditions to provide protective films are, actually, relatively few and do not include, besides chromium, the nickel and, in a lower degree, copper, silicium, molybdenum and aluminium. In order to have this property, chromium shall appear at high contents in the steel, above 10%. None of the alloy elements mentioned, by themselves or combined, would delay corrosion with contents below 1%. Exception is made to copper that already having contents of 0.2% definitely delays the atmospheric corrosion. Anyhow, chromium is the essential element. We may say that the stainless steels science is the chromium science as iron alloy element.

16 - CONSIDERATIONS ABOUT THE CHOICE OF STAINLESS STEELS Choosing the most appropriate steel is a complex task. For the most common corrosive media it may

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ES008-01-2001 be used the previously mentioned tables of properties and applications of the several types of steels , which usually help a lot when selecting the most adequate stainless steel. These tables were elaborated by means of exaustive laboratory tests in which is measured the material loss within several corrosive media. Other forms of corrosion, such as the one under stress and by pit are difficult to be measured and recorded. Only who manufactures and develops products in stainless steel has expertise to choose and capability to advise their clients, but it is essential to present accurate information and, as complete as possible about the working conditions.

17. DIMENSIONAL CHARACTERISTICS OF STAINLESS STEEL PLATES Table 14 shows the dimensional characteristics of stainless steel plates within the most common commercial gauges in the market.

Table 14 - Stainless steel plates - dimensional characteristics Gauge # 3 5 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Inch 1 7/8 3/4 5/8 1/2 7/16 3/8 5/16 1/4 7/32 3/16 11/64 5/32 9/64 1/8 7/64 3/32 5/64 9/128 1/16 9/160 1/20 7/160 3/80 11/320 1/32 9/320 1/40 7/320 3/160 11/640 1/64 9/640 1/80

Gauge in Inch Millimiter Weight Kg/m2 25.40 202.27 22.23 176.99 19.05 151.71 15.88 136.43 12.70 101.13 11.11 88.49 9.53 75.84 7.93 63.20 6.35 53.50 5.56 44.24 4.76 37.92 4.37 35.32 3.97 32.10 3.57 28.90 3.18 25.68 2.78 22.48 2.37 19.27 1.98 15.80 1.79 14.44 1.59 12.34 1.43 11.56 1.27 10.274 1.11 8.99 0.95 7.707 0.87 7.06 0.79 6.42 0.71 5.77 0.64 5.136 0.56 4.49 0.48 3.85 0.44 3.53 0.40 3.20 0.36 2.89 0.32 2.56

Tenth of Inch 1.000 0.875 0.875 0.625 0.500 0.4375 0.375 0.3125 0.250 0.218 0.187 0.171 0.156 0.140 0.125 0.109 0.093 0.078 0.070 0.0625 0.056 0.050 0.043 0.0375 0.034 0.031 0.028 0.025 0.021 0.018 0.017 0.0156 0.014 0.010

Gauge in mm Gauge in mm Weight Kg/m2 50.80 404.540 45.00 358.352 38.10 303.405 32.00 254.828 25.40 202.740 19.00 151.312 16.00 137.461 12.70 101.130 10.00 79.580 8.00 63.758 6.00 50.551 5.00 39.832 4.00 32.343 3.50 28.333 3.00 24.226 2.50 20.327 2.00 15.960 1.50 11.642 1.20 9.708 1.00 8.113 0.80 6.501 0.70 5.689 0.60 4.815 0.50 4.010 0.40 3.200

18 - SUMMARY OF BASIC CHARACTERISTICS AND CARE WITH THE STAINLESS STEELS Stainless steels are, basically, iron-chromium alloys: other metals act as alloy elements but chromium

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ES008-01-2001 is the most important and its presence is indispensable to provide the desired resistance to corrosion. Austenitic stainless steels are characterized for being very ductile and show excellent solderability. They are non-magnetic alloys and cannot be thoughened by thermal treatment. Currently the most popular one is the 304 AISI or ABNT that contains 18% of chromium and 8% of nickel, being that the carbon is limited to 0.08% maximum. They have countless applications in many industrial segments such as chemical and petro-chemical, alcohol, aviation, food, pharmaceutical, medical-odontological equipment, etc. This one and other austenitic steels when heated between 430 and 850ºC are subject to a phenomenon known as sensitization. In these temperatures chromium and carbon are combined to create the chromium carbide which precipitates around the pellet. The consequence is a strong loss of chromium in the areas nearby the pellets surrounds which may end with up to 2% of chromium or less. These areas with loss of chromium, since they have less than 11% of chromium, are no longer stainless and stop resisting to the attack of certain aggressive media. In the area thermically affected by the solder fillet there are layers submitted to critical temperatures that lead to the occurrence of corrosion. The engineering design must be accompanied by a technology involving the solder for this type of steel, working with temperatures above those causing the formation of carbides and, causing cooling in such a way for not allowing enough time for the carbon and the chromium to combine themselves. Titanium and niobium are alloy elements with higher affinity with carbon than with chromium, that is why their inclusion in stainless steels prevents the sensitization. In this way appeared the stabilized steels such as the AISI or ABNT 321 and 347. Another alternative is the utilization of stainless steels with low carbon contents and, in this way appeared the 304L, with a maximum carbon contents of 0.03%. AISI or ABNT 316 is a 304 variation containing a minimum of 2% of molybdenum whose presence allows the formation of a passive layer more resistant to corrosion, mainly in certain media, specially those containing chlorides. The 316 version with extra low carbon contents is the 316 L. Stainless steels meet strict demands regarding cleaning, sterility in service, so preventing contamination by corrosion or bacteriology. Therefore they have lastingness, fulfilling the esthetic aspects with their bright and homogeneous surface. All of it makes the equipment in stainless steel highly competitive causing in most applications the initial investment to be secondary. When exposed to the air, a fine film of chromium oxide is formed on the stainless steel surface, being this film impervious, hard, strong and tough, protecting the basal metal against corrosion. Any damage to the film is not destructive because it quickly self recomposes. However, dirt, dust or grease deposits hindering the contact of the surface with the air for a long period of time may cause localized corrosion. Because of it, a periodic cleaning is fundamental. Use water and detergent and, whenever possible wipe or dry the equipment after washing. When abrasive is used for cleaning, always rub along the polishing lines in order to avoid scratches. Never use common steel wool. Iron particles may adhere to the surface causing corrosion. Use stainless steel wool or a soft brush with vegetal bristles.

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ES008-01-2001 It should not be allowed that saline or acid solutions evaporate or dry off on the stainless steel surface. They may cause corrosion. Traces of these solutions must be eliminated by means of robust washings. The direct and permanent contact with certain materials such as wood or carbon steel must be avoided. In the case of contact the interface between both materials shall be coated for protection against corrosion. The equipment to be acquired must be strong enough to withstand cyclic stresses causing fatigue and/or corrosion by fatigue. Make use of all knowledge and technology acquired by cérebro in the design, manufacturing and treatment of equipment in stainless steel.

19. SOME PROCESSES EQUIPMENT MANUFACTURED IN STAINLESS STEEL The process equipment manufactured in stainless steel designed by Cérebro are always reliable equipment, designed in accordance with international renowned rules. The use of plates is optimized aiming at, within the commercial standards, determining the plates quantity and dimensions that may meet the required capacity. Such procedure eliminates wastes and cuts, meaningfully diminishing the number of soldered joints, bringing advantage to the production cost reduction, eliminating to the utmost the thermically affected areas besides better meeting the sanitary requirements.

19.1. PROCESSES TANKS The relation between the internal diameter and the height of the processes tanks shell is committed with the several inciding costs, with the supporting legs requirements and with the vessel structural stability.This, on the other hand, is committed with the thickness of the plates utilized. The internal and external finishing with the sanitary standard, within the international superficial rugosity rules, is chosen by the customer in funtion of the desired application degree. Although being a standardized line, our engineering is able to meet their customer’s requirements, by means of specific calculations and developments that every process needs.

19.2. AGITATORS Most operations in chemical, pharmaceutical, food and other industries demand product agitation in order to cause mixture or the solids suspension. Agitation may be performed by very quick mixers endowed with fixing clamps that allow fast transfer among several tanks and to lean the rod in any angle in order to obtain the best result in each case. However, such mixers are limited to 2 cv of power. From this range of power the agitators are fixed, vertically assembled in the lateral or still, in some typical applications, placed at the botton of the container.

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ES008-01-2001 For many, agitation is still an empiric matter, however, it may be mathematically represented, what allows us to find the best agitation option for each case, with the least possible power consumption. A certain minimum dynamic response is required in order to satisfactorily solve an agitation issue. This response is represented by the magnitude of the fluid speed to be applied by the mixer. The fluid speed created in a tank by a vertical blade agitator has three components : (a) a RADIAL COMPONENT actuating in the perpendicular direction to the axis-spindle; (b) a LONGITUDINAL COMPONENT parallelly actuating to the axis-spindle and, (c) a ROTATING COMPONENT actuating in the tangencial direction to the axis-spindle rotating circle. Both the radial component and the longitudinal one effectively contribute for the mixture what does not happen with the tangencial one. This tangencial component produces a rotating flow layer around the axis-spindle, in general in laminar drainage and that practically hinders the longitudinal movement. The results is that the tank contents only spins round without carrying out any mixture action. The power absorbed by the liquid is very limited since the relative speed between the blades and the liquid is low. This tangencial component may give place to the formation of a vortex on the liquid surface, which will be deeper as the agitator rotation increases. When the vortex reaches the blade suction area, the power tranferred to the fluid suddenly decreases due to the air dragging into the interior of the product. Another inconvenient associated to the rotating flow is that eventual solid particles may set apart because of the action of centrifugal forces. To avoid the creation of vortexes and other inconvenients from the rotating flow it may be feasible to descentralize the agitator in relation to the tank or, to place diffusers (wave-breakers) made out of soldered vertical blades in the tank shell. Up to 20 years ago the agitation intensity was divided into three categories : low, average and high, or still, minimum, average and violent. These terms are used until now. Currently the fluid velocity in the container is established within the range of 6 to 60 feet/min (1.8 to 18.3 m/min). A scale divided from 1 to 10 was established to cover this range. The agitation level is defined by dividing the fluid velocity by 6, as shown in Table 15.

Table 15 - Agitation Level Low Agitation Fluid Speed Scale (feet/min) 1 6 2

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Average Agitation Fluid Speed Scale (feet/min) 3 18 4 24 5 30 6 36

High Agitation Scale 7 8 9 10

Fluid Speed (feet/min) 42 48 54 60

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ES008-01-2001 Agitation in levels 1 and 2 is characteristic of applications requiring the minimum possible movement to the product. Levels 3 to 6 characterize the great deal of industrial applications. From 7 to 10 are the processes requiring high velocity. Besides establishing the agitation level other factors are important for determining the best system : the specific density, the product viscosity at the operational temperature and the tank own geometry. The relation between the shell height and the diameter may vary from 0.5 to 1.5, being the value 0.87 recommended for most applications, what will also meet the structural stability criteria. For high tanks is is necessary an impeller element for every height diameter of the container. The conventional agitators, with straight blades inclined at 45º, produce a smooth action which is usually desired for most of the substances. They are useful for mixing operations involving miscible liquids or preparation of solid products dissolutions. For very viscous liquids, it is used anchor-type agitators in order to promote a higher heat exchange in heating or cooling operations in reactors and, to minimize the formation of deposits. Naval agitators spin at high speed and produce mainly longitudinal and rotating streams. Despite their small diameter they are quite effective in small tanks. Due to the predominantly longitudinal nature of the flow streams the naval propellers shall be assembled on vertical axis-spindles discentered in relation to the tank diameter and, preferably making a certain angle in relation to the vertical. In large reservoirs they may be laterally assembled but without positioning them in the center. They are more effective in mixing little viscous liquids due to the cutting and shearing action.

19.3. REACTORS

19.3.1. HEAT EXCHANGE

Heat exchange in agitation vessels depends on the agitator type (conventional, anchor or naval) and the system offerred to the heating or cooling fluid (integral jacket, half-stalk coil or internal coil). The agitator type, location and rotation usually depend on the parameters chosen to meet the product agitation needs, however, in many cases these parameters are established to maximize the heat exchange. In the heat flow circuit the agitation is related to the internal film resistance which is one of the resistances making part of the heat exchange global coefficient. On the other side, in the most common situation, there is the saturated steam slowly losing heat. The steam pressure in the coils or in the reactor jacket must be the lowest possible since they meet the product temperature requirements, the counter-pressure in the line and the area needed for the heat exchange. In this condition the steam has a larger quantity of latent heat to yield. With the largest quantity of latent heat coming from low pressure, there is a lower outflow, shorter time of product retention in the reactor and mainly, less fuel consumption for generating this steam. The jacket provides larger surface for heat exchanging, on the other hand, requires the shell with external pressure demanding a thicker wall and reinforcement rings; the half-stalk coil in its turn offers lower convective resistance, allows the use of thinner plates strengthening the sheel.

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ES008-01-2001 19.3.2. THERMAL INSULATION Thermal insulation in reactors aims at two objectives :

1. Energy Savings Nowadays there is a considerable cost involved with the generation of 1 Kg of steam : cost of the fuel utilized and direct and indirect operational costs. So, practically all thermal energy contained in the steam has to be carried to the product with the least possible wastage, assuring a heat exchange with the maximum efficiency.

2. Personal Protection With the thermal insulation it is guaranteed a nearly room temperature on the reactor external surface. These advantages are however counter-balanced by the cost of the thermal insulation used. Therefore, the insulation thickness depends on considerations of economic character. For the range of temperature found in the reactor it is recommended the use of rock-wool or glasswool. The insulation protection is made with stainless steel plate AISI 304, with 1.5 mm of soldered thickness.

© Copyright - 2001 - Engineer Edimilson Souza – All rights reserved.

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