Post Weld Heat Treatment [PDF]

Zitiervorschau

PWHT BY

T BALAMURUGAN SENIOR WELDING INSPECTOR

WHAT IS HEAT TREATMENT? • Heat Treatment is the controlled heating and cooling of metals to alter their physical and mechanical properties, without changing the product shape • PWHT is the one of Heat treatment done after the welding/ machining to improve the mechanical property and material structure. In steel fabrication PWHT is commonly known as stress relieving.

PWHT POST WELD HEAT TREATMENT

WHAT IS PWHT? REFERS TO THE PROCESS OF REHEATING A WELD TO BELOW THE LOWER TRANSFORMATION TEMPERATURE AT A CONTROLLED RATE, HOLDING FOR A SPECIFIC TIME AND COOLING AT A CONTROLLED RATE.

Applications

WHY THE NEED FOR PWHT? • Residual Stress • Effects of Mechanical Properties • Effect on Creep Properties • Other benefits

WHY THE NEED FOR PWHT? • Residual Stress  The development of residual stresses approaching or even exceeding the yield stress is possible when welding thick sections.  Petrochemical, Chemical, Oil and Gas, etc. the existence of residual stress of this magnitude is completely unacceptable  the stress developed due to thermal expansion of pipework can take on a far greater significance than residual stress due to welding.

WHY THE NEED FOR PWHT • Effect on Mechanical Properties As a series of very general statements, the following are the consequences of post weld heat treatment compared with the as-welded condition:  Yield strength is decreased slightly, the effect falling off fairly rapidly with time.  The tensile strength is decreased.  The ductility is increased.  Hardness levels are reduced.  Toughness is slightly reduced at short times but the effect can be significant over longer times.

WHY THE NEED FOR PWHT • Effect on Creep Properties For creep resisting material, post weld heat treatment is required in order to fully develop the creep strength. This is specially true for thicker components such as headers.

WHY THE NEED FOR PWHT • Other benefits Improving the diffusion of hydrogen out of weld metal Softening the heat affected zone and thus improving toughness (although not weld metal toughness) Improving dimensional stability during machining.

WHY THE NEED FOR PWHT • Other benefits…contd Improving ductility. Improving the resistance to stress corrosion cracking. Reducing the effects of cold work.

WHEN TO POST WELD HEAT TREAT • SERVICE CONDITION (Eg. Caustic) • MATERIAL GRADE AND THICKNESS • WHERE HARDNESS LIMITS ARE SPECIFIED.

HOW TO PERFORM POST WELD HEAT TREATMENT

• FIXED FURNACE • TEMPORARY FURNACE • INTERNAL FIRING • LOCAL HEATING • PARTIAL HEAT TREATMENT

FIXED FURNACE • Fixed furnaces tend to be large and expensive to operate. • Gas Fired. • Oil Fired. • Electrical resistance heating elements.

FIXED FURNACE

FIXED FURNACE

FIXED FURNACE

TEMPORARY FURNACE • These are custom-built around a vessel, rather than transport a vessel to a fixed furnace. The idea is to minimise the air space between the vessel and furnace walls, and they allow for faster heating and cooling

TEMPORARY FURNACE ..contd • Heating can be through resistant heating mats placed on a concrete floor or via gas burners placed at each end. • In the case of gas burners, care must be taken to avoid direct flame impingement on the vessel.

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

TEMPORARY FURNACE

INTERNAL FIRING • Vessels of suitable dimensions and arrangement of openings can be post weld heat treated by gas firing through nozzles or manways. • Manways are large enough to accommodate the gas burners, but care needs to be exercised with the diameter and position of nozzles and expert opinion should be sought.

INTERNAL FIRING .. Contd • Care must also be taken to place deflector plates inside the vessel and opposite the burner entry points to avoid direct flame impingement on the shell. • It is not advisable to post weld heat treat vessels that contain internals in this manner. • The outside of the vessel must be completely encased in insulating material, and again, at least a 12 point temperature recorder is advisable.

INTERNAL FIRING

INTERNAL FIRING

INTERNAL FIRING

INTERNAL FIRING

LOCAL HEATING • Circumferential weld seams can be post weld heat treated by heating a band around the weld. • Although not specifically stated, such heating is essentially limited to resistance or induction heating, mainly because of the controls required on heated band width, width of insulation and temperature measurement requirements.

LOCAL HEATING

LOCAL HEATING

LOCAL HEATING

PARTIAL HEAT TREATMENT • There are occasions, for example with very long vessels, when the entire vessel will not fit into a fixed furnace. • This has been catered for in most Standards, and it is permissible to post weld heat treat section of the vessel first, then turn the vessel around and heat treat the remaining section. • As with local heating, there are restrictions in this case as well over the degree of overlap and the longitudinal temperature gradient.

ITEMS TO CONCERN • AREA TO BE HEATED • HEATING AND COOLING RATE • HOLDING TEMPERATURE AND DURATION • TEMPERATURE MEASUREMENT AND DISTRIBUTION • EQUIPMENT CALIBRATION AND CONDITION

HEATING / COOLING RATE

• The PWHT heating and cooling rates above 316°C shall not exceed 222°C/hr divided by the weld thickness in inches, but in no case shall it be more than 222°C/hr. • At least compliant with the necessary code requirements. • Related to component thickness to offer protection against thermally induced stresses (thermal shocks). • With thicker and more complex structures, consider lower rates than required by the code to ensure acceptable temperature profiles and gradients with a view to keeping these thermally induced stresses to an absolute minimum.

HEATING RATE

SOAKING PERIOD

COOLING RATE

THERMO COUPLES • To transmit the surface temperature to the recorder. • If localized PWHT is used, the following minimum number of equally spaced recording thermocouples (T/C) shall be used: a) Pipe diameter of 305 mm or less: 1 T/C. b) Pipe diameter above 305 mm up to and including 610 mm: 2 T/C. c) Pipe diameter above 610 mm: 4 T/C. d) Additional thermocouples are required if multiple heat control zones are used, in which a control zone is not monitored by any of the primary T/Cs.

THERMO COUPLES • Type K (Chromel-Alumel) or Type J (ironConstantan) thermocouples are permitted.

THERMO COUPLES • Thermocouples shall be attached to the component by capacitive discharge welding only. Other methods of attachment are not permitted. • The thermocouples shall not be in direct contact with electrical heating elements or subjected to flame impingement by gas or oil burners.

THERMO COUPLES

THERMO COUPLES

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