The document discusses post weld heat treatment (PWHT), which involves reheating welded areas to improve mechanical properties and reduce residual stresses. PWHT is needed to relieve residual stresses from welding, improve toughness and ductility, and fully develop creep strength in thick components. It can be done using fixed furnaces, temporary furnaces built around components, internal firing through openings, or local heating of welds. Proper heating and cooling rates must be followed based on material thickness. Temperature must be carefully monitored and controlled using thermocouples to ensure an even heat profile and avoid thermal shock. PWHT improves weld quality and makes components safer to use in demanding applications like oil and gas.

1. PWHT
BY
T BALAMURUGAN
SENIOR WELDING INSPECTOR

2. 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.

3. PWHT
POST WELD HEAT TREATMENT

4. 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.

5. Applications

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

7. 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.

8. 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.

9. 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.

10. 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.

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

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

13. HOW TO PERFORM POST WELD
HEAT TREATMENT
• FIXED FURNACE
• TEMPORARY FURNACE
• INTERNAL FIRING
• LOCAL HEATING
• PARTIAL HEAT TREATMENT

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

15. FIXED FURNACE

16. FIXED FURNACE

17. FIXED FURNACE

18. 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

19. 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.

20. TEMPORARY FURNACE

21. TEMPORARY FURNACE

22. TEMPORARY FURNACE

23. TEMPORARY FURNACE

24. TEMPORARY FURNACE

25. TEMPORARY FURNACE

26. TEMPORARY FURNACE

27. TEMPORARY FURNACE

28. TEMPORARY FURNACE

29. TEMPORARY FURNACE

30. 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.

31. 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.

32. INTERNAL FIRING

33. INTERNAL FIRING

34. INTERNAL FIRING

35. INTERNAL FIRING

36. 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.

37. LOCAL HEATING

38. LOCAL HEATING

39. LOCAL HEATING

40. 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.

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

42. 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.

43. HEATING RATE

44. SOAKING PERIOD

45. COOLING RATE

46. 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.

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

48. 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.

49. THERMO COUPLES

50. THERMO COUPLES

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