What Is PWHT? Post-Weld Heat Treatment

Post-weld heat treatment (PWHT) is a controlled heating process applied to welded piping, pressure vessels, and equipment after welding is complete. The primary purpose is stress relief—reducing residual stresses locked into the weld zone by the welding thermal cycle. PWHT also tempers hard microstructures in the heat-affected zone (HAZ), reducing hardness to acceptable levels for sour service and improving ductility.

ASME B31.3, ASME Section VIII, and ASME Section I define mandatory PWHT requirements based on material group, thickness, and service conditions.

When PWHT Is Required

Condition	Code Reference	Requirement
Wall thickness exceeds code threshold	ASME B31.3 Table 331.1.1	Mandatory above specified thickness per P-number
Sour service (NACE MR0175)	NACE MR0175/ISO 15156	Mandatory for carbon steel and low-alloy steel welds to meet max hardness (22 HRC)
Low-temperature service	ASME B31.3	May be required to restore toughness after welding
Owner specification	Project-specific	Often mandatory for all welds regardless of thickness
Dissimilar metal welds	ASME B31.3	Based on the material requiring the higher PWHT temperature

PWHT Parameters by Material (ASME B31.3)

P-Number	Material	Min PWHT Temp (deg C)	Hold Time	Thickness Threshold
P-1	Carbon steel (A106, A234 WPB)	593 (1100 deg F)	1 hr/25 mm, 15 min minimum	>19 mm (3/4 in.)
P-3	Low-alloy (A335 P1)	593	1 hr/25 mm	>19 mm
P-4	Cr-Mo (A335 P11, P12)	704 (1300 deg F)	1 hr/25 mm	>13 mm (1/2 in.)
P-5A	Cr-Mo (A335 P22)	704	1 hr/25 mm	>13 mm
P-5B	Cr-Mo (A335 P91)	732 (1350 deg F)	2 hr/25 mm minimum	All thicknesses
P-8	Austenitic SS (304, 316)	Not required	N/A	Exempt (PWHT may sensitize)
P-10H	Duplex SS (2205)	Not required	N/A	Exempt (PWHT degrades properties)

PWHT Process Control

Parameter	Requirement
Heating rate	Max 220 deg C/hr divided by thickness in inches (200 deg C/hr typical for >25 mm)
Soak temperature	Within specified range (+/- 14 deg C of target)
Soak time	Per code table, based on nominal thickness
Cooling rate	Max 280 deg C/hr divided by thickness in inches; furnace cool to 400 deg C, then air cool
Thermocouples	Attached directly to the weld/HAZ; minimum number per code/spec (typically 1 per 3 m of weld)
Recording	Continuous time-temperature chart (digital or paper) for QC records
Temperature uniformity	Max 83 deg C (150 deg F) variation within the soak band

PWHT Methods

Method	Application	Heating Source
Furnace PWHT	Shop fabrication; entire spool piece heated uniformly	Gas or electric furnace
Local PWHT	Field welds; circumferential band of heating elements around weld	Electrical resistance heating pads/ceramic heaters
Induction PWHT	Field or shop; fast heating	Induction coils wrapped around pipe

Local PWHT requires a heated band width of at least 3 times the wall thickness on each side of the weld. Insulation extends beyond the heated band to control the temperature gradient. ASME B31.3 and WRC Bulletin 452 provide guidance on soak band and gradient control band widths.

PWHT Impact on Properties

Property	Effect of PWHT
Residual stress	Reduced by 70-90% (primary benefit)
HAZ hardness	Reduced (critical for NACE MR0175 compliance)
Ductility	Improved (tempered martensite/bainite)
Toughness	Generally improved, but can degrade in some Cr-Mo steels (temper embrittlement)
Creep resistance	Maintained if PWHT temperature is below the lower critical transformation temperature
Corrosion resistance (SS)	Degraded—sensitization in austenitic SS at 500-800 deg C; never PWHT 300-series SS

Common Mistake

Applying PWHT to austenitic stainless steel (304, 316) welds. PWHT at 550-850 deg C causes chromium carbide precipitation (sensitization), depleting chromium at grain boundaries and creating susceptibility to intergranular corrosion. Austenitic stainless steels are exempt from PWHT under ASME B31.3. If stress relief is absolutely required, use solution annealing at >1,040 deg C followed by rapid water quench—a very different process.

PWHT time-temperature records are filed with hydrostatic test reports and mill test certificates in the weld data package, reviewed during pipe inspections.

Read the full guide to non-destructive testing