Post Weld Heat Treatment (PWHT) Requirement as per ASME Section VIII Division 1

Post weld heat treatment (PWHT) is controlled process in which a material that has been welded is reheated to a temperature below its lower critical transformation temperature, and then is held at that temperature for the specified amount of time. The need of PWHT is mostly due to residual stresses and microstructural changes that occur after welding. During the welding process, a high-temperature gradient is experienced between the weld metal and parent metal. Codes like ASME specify the specific requirements for PWHT of materials. The PWHT requirements discussed in this document are in accordance with ASME Sec. VIII Div. 1.

ASME Sec. VIII Div. 1 specifies the PWHT requirements for materials used in equipment designed as per ASME. Tables UCS-56-1 through UCS-56-11 in ASME Sec. VIII Div.1 provides heating cycle data required for PWHT of materials based on P No. and Gr. No as categorized in ASME Sec. IX. Those tables are followed by notes which provide exemptions. Table UCS-56.1 gives alternative heat treatment requirements for carbon and low alloy steels. Before applying for the detailed requirements and exemptions, satisfactory weld procedure qualifications of the procedures to be used shall be performed in accordance with all the essential variables of ASME Section IX including conditions of post weld heat treatment.

PWHT requirements as per ASME Sec. VIII Div. 1

Except as otherwise specifically provided in notes of tables UCS-56-1 through UCS-56-11 or Table UCS-56.1, all welds in pressure vessels or pressure vessel parts shall be given a post weld heat treatment at a temperature not less than specified in those tables when the nominal thickness including corrosion allowance, exceeds the limits given in those Tables.

When the minimum design metal temperature is colder than -55°F (-48°C), and the coincident ratio defined in Figure UCS-66.1 is 0.35 or greater, post weld heat treatment is required, except that this requirement does not apply to the welded joints, in vessels or vessel parts fabricated of P-No. 1 materials that are impact tested at the MDMT

The exemptions provided in Tables UCS-56-1 through UCS-56-11 or Table UCS-56.1 are not permitted when post weld heat treatment is a service requirement set forth when welding ferritic materials greater than 1/8 in. (3 mm) thick with the electron beam welding process, or when welding P-No. 3, P-No. 4, P-Nos. 5A, 5B, and 5C, P-No. 10, and P-No. 15E materials of any thickness using the inertia and continuous drive friction welding processes.

Electroslag welds in ferritic materials over 11/2 in. (38 mm) thickness at the joint shall be given a grain refining (austenitizing) heat treatment.

Electrogas welds in ferritic materials with any single pass greater than 11/2 in. (38 mm) shall be given a grain refining (austenitizing) heat treatment.

When pressure parts of two different P-Number groups are joined by welding, the post weld heat treatment shall be of the material requiring the higher post weld temperature

When non-pressure parts are welded to pressure parts, the post weld heat treatment temperature of the pressure part shall control

Most commonly used materials for process equipment comes under P.No.-1 & Gr.No.1,2,3,4. PWHT for carbon and low alloy steels- P. No. 1 & Gr.No.1,2,3,4 are given below

PWHT for carbon and low alloy steels- P. No. 1 & Gr.No.1,2,3,4

Postweld heat treatment is mandatory under the following conditions

welded joints over 11/2 in. (38 mm) nominal thickness

For welded joints over 11/4 in. (32 mm) nominal thickness through 11/2 in. (38 mm) nominal thickness unless preheat is applied at a minimum temperature of 200°F (95°C) during welding. This preheat need not be applied to SA 841 Grades A and B, provided that the carbon content and carbon equivalent (CE) for the plate material, by heat analysis, do not exceed 0.14% and 0.40% respectively, where



a. for groove welds not over 1/2 in. (13 mm) size and fillet welds with a throat not over 1/2 in. (13 mm) that attach nozzle connections that have a finished inside diameter not greater than 2 in. (50 mm), provided the connections do not form ligaments that require an increase in the shell or head thickness, and preheat to a minimum temperature of 200°F (95°C) is applied
b. for groove welds not over 1/2 in. (13 mm) in size or fillet welds with a throat thickness of 1/2 in. (13 mm) or less that attach tubes to a tubesheet when the tube diameter does not exceed 2 in. (50 mm). A preheat of 200°F (95°C) minimum must be applied when the carbon content of the tubesheet exceeds 0.22%.
c. for groove welds not over 1/2 in. (13 mm) in size or fillet welds with a throat thickness of 1/2 in. (13 mm) or less used for attaching non-pressure parts to pressure parts provided preheat to a minimum temperature of 200°F (95°C) is applied when the thickness of the pressure part exceeds 11/4 in. (32 mm).
d. for studs welded to pressure parts provided preheat to a minimum temperature of 200°F (95°C) is applied when the thickness of the pressure part exceeds 11/4 in. (32 mm)
e. for corrosion resistant weld metal overlay cladding or for welds attaching corrosion resistant applied lining (see UCL 34) provided preheat to a minimum temperature of 200°F (95°C) is maintained during application of the first layer when the thickness of the pressure part exceeds 11/4 in. (32 mm)