Post Weld Heat Treatment (PWHT)

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Post Weld Heat Treatment (PWHT)


Post weld heat treatment is a post welding heating process used to improve the properties of the weldment. Heat treatment following welding may be carried out for one or more of three fundamental reasons (1):

  • to achieve dimensional stability in order to maintain tolerances during machining operations or during shake-down in service
  • to produce specific metallurgical structures in order to achieve the required mechanical properties
  • to reduce the risk of in-service problems such as stress corrosion or brittle fracture by reducing the residual stress in the welded component 

The need for PWHT is normally driven by code requirements. For Section I these requirements can be found in section PW-39 Requirements for Postweld Heat Treatment. The rules in Section I (2) apply to boiler parts only and not to external piping (see B31.1/31.3).

Most austenitic stainless steel weldments do not require post weld heat treatment (PWHT). However, a heat treatment is sometimes used to improve corrosion resistance or to relieve stresses, or both. Corrosion problems associated with welds in austenitic stainless steels often are localized in the HAZ. If the service environments known to attack sensitized areas containing intergranular carbides or if maximum corrosion resistance is required, a postweld solution heat treatment at the annealing temperature should be used to redissolve the carbides. (3)

ASME Section I - General PWHT rules

PW-39.1 Before applying the detailed requirements and exemptions in these paragraphs, satisfactory weld procedure qualifications of the procedures to be used shall be performed in accordance with all the essential variables of Section IX including conditions of postweld heat treatment or lack of post weld heat treatment and including other restrictions listed below.

PW-39.2 When pressure parts of two different P-Number groups are joined by welding, the postweld heat treatment shall be that specified in Table PW-39 and applicable notes for the material requiring the higher postweld heat treatment temperature, except as noted in PW-39.2.1. When nonpressure parts are welded to pressure parts, the postweld heat treatment temperature of the pressure parts shall control.

PW-39.2.1 Fillet welds, partial penetration welds, and full penetration welds through the tube or pipe thickness, attaching P-No. 5A tubes and pipe to headers of lower P-Number material, may be post weld heat treated at the temperature specified in Table PW-39 for the lower P Number material provided the tubes or pipe comply with all the following conditions:

(a) a maximum specified chromium content of 3.0%

(b) a maximum size of NPS 4 (DN 100)

(c) a maximum thickness of 1?2 in. (13 mm)

(d) a maximum specified carbon content of not more

than 0.15%

PW-39.3 In the procedures that follow, the volume of metal required to be heated, to meet or exceed the minimum post weld heat treatment temperatures listed in Table PW- 39, is defined as the soak band. As a minimum, the soak band shall contain the weld and a portion of the base metal on each side of the weld being heat treated, including the weld heat affected zones. The width of each portion of base metal to be included in the soak band shall be equal to the lesser of the vessel or shell thickness, or 2 in. (50 mm). A greater amount of base material, on either or both sides of the weld, may also be heated to permit temperature gradient control.

The weldment shall be heated slowly to the temperature specified in Table PW-39 and held for the specified time, and shall be allowed to cool slowly in a still atmosphere to a temperature not exceeding 800°F (425°C). Several weldments of varied thickness may be post weld heat treated in the same furnace at the same time. The term nominal thickness in Table PW-39 is the thickness of the weld, pressure retaining material, or the thinner of the sections being joined, whichever is least. For fillet welds, the nominal thickness is the throat thickness, and for partial penetration and material repair welds, the nominal thickness is the depth of the weld groove or preparation. For combination groove and fillet welds, nominal thickness is the total combined thickness of the deposited weld, groove depth plus fillet weld throat. The total depth of a weld repair shall be taken as the sum of the depths for repairs made from both sides of a weld at a given location. The holding time at temperature as specified in Table PW need not be continuous. It may be an accumulation of time of multiple post weld heat treat cycles.

Procedure for determining the need for PWHT

The following information is required to determine the need for PWHT:

  • NPS
  • Schedule
  • Material
  • Weld thickness
  • WPS
  • Material specification
  • Type of weld (load carrying/non-load carrying)

For each of the materials used, go to Table PW-39 in Section I (see Figure 1) and follow the guidelines:

  1. Choose group based on material. See QW-422 in ASME Section IX (4); g. SA-213 T91 can be found on page 80, P-No 5B, Group No. 2. Then proceed to Table PW-39 on page 106 in Section I.
  2. Figure out minimum holding time based on weld thickness.
  3. Step through the notes for exemption rules (no need for PWHT)

Figure 1: Sample Post weld heat treatment rules for P-No 1, Groups 1,2, 3


  1. TWI. Heat treatment of welded joints - Job Knowledge 114. [Online] 05 30, 2016. [Cited: 05 30, 2016.] http://www.twi-global.com/technical-knowledge/job-knowledge/heat-treatment-of-welded-joints-part-1-114/.
  2. ASME. ASME Boiler and Pressure Vessel Code Section I - Rules for Construction of Power Boilers. s.l. : ASME, 2010.
  3. American Welding Society. Welding Handbook - MATERIALS AND APPLICATIONS, PART 1. 2011. Volume 4.
  4. ASME. Section IX - Qualification Standard for Welding and Brazing Procedures, Welders, Brazers, and Weldign and Brazing Operators. 2010.
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