Post-Weld Heat Treatment (PWHT): Purpose, Temperature, and Standards

Post-weld heat treatment (PWHT) is a controlled thermal cycle applied to a welded assembly after completion of welding. It is one of the most consequential operations in the fabrication of pressure vessels, piping systems, and structural weldments: a correctly executed PWHT reduces residual stress, tempers hard heat-affected zone (HAZ) microstructures, and promotes hydrogen effusion, while a poorly controlled PWHT can induce temper embrittlement, creep damage, or dimensional distortion. This article provides a graduate-engineer-level treatment of PWHT metallurgy, the principal code requirements from ASME BPVC, AWS D1.1, and EN ISO 17663, and the practical parameters governing furnace and local PWHT execution.

What is Post-Weld Heat Treatment?

PWHT is a controlled thermal operation in which a welded joint, or the entire fabricated component, is heated to a prescribed temperature below the lower critical temperature (A₁), held at temperature for a defined period, then cooled at a controlled rate. It is classified as a sub-critical heat treatment: the steel does not undergo austenite transformation, so the hardness and strength are not governed by quench-and-temper mechanisms but by the diffusion-controlled processes of creep, stress relaxation, carbide precipitation, and dislocation annihilation.

The designation “PWHT” encompasses several mechanisms that operate simultaneously during the soak cycle. In engineering codes, the term is used specifically to mean the mandatory heat treatment prescribed for compliance with a fabrication standard — it does not include hydrogen bake-out (post-heat), normalising, or solution annealing, which are distinct operations with different temperature windows and objectives. For a complete treatment of heat treatment terminology, see the MetallurgyZone guide to annealing and normalising in steel.

Metallurgical Purposes of PWHT

1. Residual Stress Reduction

During welding, the weld pool and adjacent HAZ undergo rapid thermal expansion and contraction. The surrounding cooler base metal constrains free thermal movement, generating a complex triaxial residual stress field. In the as-welded condition, tensile residual stresses at the weld surface can approach or equal the material’s yield strength at ambient temperature. For ferritic steels, PWHT reduces these stresses through creep relaxation: at elevated temperature, the yield strength drops significantly, and the constrained metal deforms plastically or by creep until the stress falls to the reduced yield strength. The residual stress level after PWHT is therefore approximately equal to the yield strength of the material at the PWHT soaking temperature.

For a carbon steel with a room-temperature yield strength of 250 MPa, the yield strength at 620°C falls to approximately 30–50 MPa — a reduction of more than 80%. This is why PWHT is so effective: it does not mechanically remove stress but allows the constrained material to yield creep at reduced load capacity until equilibrium is achieved at a far lower stress level.

2. Tempering of HAZ Martensite

In hardenable steels, the thermal cycle of welding can produce martensite in the coarse-grained heat-affected zone (CGHAZ) immediately adjacent to the fusion boundary. This martensitic region is hard, brittle, and susceptible to hydrogen-induced cracking. PWHT acts as a high-temperature tempering cycle: carbon diffuses from the supersaturated martensite lattice to form fine carbide precipitates, dislocations rearrange into lower-energy configurations, and the tetragonality of the martensite crystal structure is relieved. The result is a reduction in hardness and a significant improvement in notch toughness in the CGHAZ region.

For HAZ microstructure in low-alloy steels, the tempering response follows an Arrhenius relationship. The Hollomon-Jaffe tempering parameter (P) is used to equate combinations of time and temperature that produce an equivalent microstructural result:

P = T × (C + log t)

Where:
  T  = absolute temperature, K
  t  = time at temperature, hours
  C  = material constant (typically 14–20 for carbon and low-alloy steels)

Note: P values are equated across different (T, t) combinations.
      Higher P = more complete tempering.

3. Hydrogen Diffusion (Dehydrogenation)

Although the primary mechanism for hydrogen removal is the post-heat (dehydrogenation heat treatment) applied immediately after welding, residual hydrogen that has been trapped in stress-concentration sites can continue to diffuse during PWHT. The higher PWHT temperature accelerates hydrogen diffusivity by orders of magnitude. The diffusivity of hydrogen in α-iron follows the Arrhenius relationship:

At 600°C, D_H is approximately four orders of magnitude greater than at 20°C, meaning trapped hydrogen that would require weeks to effuse at room temperature is removed in hours during PWHT. Nonetheless, PWHT is not a substitute for post-heat; if a component is allowed to cool to below about 100°C before hydrogen can diffuse out, delayed hydrogen cracking may already have initiated.

4. Improvement of Fracture Toughness

The improvement in Charpy impact toughness in the HAZ following PWHT results from the combined effects of martensite tempering, carbide spheroidisation, and partial dislocation recovery. In quenched-and-tempered steels and in normalized fine-grained steels, the toughness of the CGHAZ can improve dramatically after PWHT. The ductile-to-brittle transition temperature (DBTT) is shifted toward lower temperatures.

5. Dimensional Stability

Equipment subject to precision machining after welding (e.g., pressure vessel nozzle flanges, heat exchanger tube sheets) may require PWHT to relieve locked-in stresses before final machining, preventing distortion of critical dimensions in service or during post-weld assembly.

When is PWHT Mandatory?

The requirement for PWHT is determined by the applicable fabrication code, the P-Number of the base material, the nominal weld thickness, the service environment, and any purchaser or engineering specifications that impose additional requirements. The following section summarises the principal code requirements.

ASME BPVC Section VIII Division 1 (UCS-56)

Paragraph UCS-56 governs post-weld heat treatment of pressure vessels fabricated from carbon and low-alloy steels classified under UCS. The key mandatory conditions for P-No. 1 (carbon steel) are:

PWHT Temperatures and Hold Times per ASME VIII UCS-56

ASME B31.3 Process Piping

Table 331.1.1 of ASME B31.3 specifies PWHT requirements for process piping. For P-No. 1 carbon steel, PWHT is mandatory when nominal pipe wall thickness exceeds 19.05 mm (0.75 in). The minimum soak temperature is 593°C (1100°F) with a hold time of 1 h/25 mm (minimum 30 min). The heating and cooling rate requirements reference Table 331.1.3 and are consistent with Section VIII: above 315°C, maximum heating rate is 220°C/h ÷ thickness (mm/25), minimum 55°C/h; maximum cooling rate is 275°C/h ÷ thickness (mm/25), minimum 55°C/h.

NACE MR0175 / ISO 15156 (Sour Service)

For equipment in hydrogen sulphide (H₂S) containing environments, NACE MR0175 / ISO 15156-2 imposes hardness limits on weld metal and HAZ: generally 22 HRC (248 HV₁₀) maximum for carbon and low-alloy steels. HAZ hardness in the as-welded condition frequently exceeds this limit in steels with carbon equivalent (CE) above approximately 0.42 (IIW formula). PWHT is therefore routinely specified for all sour-service carbon steel welds regardless of thickness. Post-PWHT hardness surveys (typically Vickers or Rockwell traverses across the weld cross-section) are required to verify compliance.

Steels with CE_IIW > 0.42 are classified as having poor weldability: preheat and PWHT are essentially mandatory for reliable HAZ toughness and hardness control. For a detailed calculator and discussion, see the MetallurgyZone Carbon Equivalent Calculator.

AWS D1.1 Structural Welding — Steel

PWHT is not routinely mandated in AWS D1.1 for general structural welding; however, the Engineer may specify PWHT for specific applications. Where specified, Clause 5.8 of D1.1 requires a minimum soak temperature of 595°C for A36 and similar mild steels and a hold time of 1 h per 25 mm (minimum 1 h). AWS D10.10/D10.10M provides supplementary guidelines for local PWHT of piping and pressure vessel joints.

Heating and Cooling Rate Requirements

Controlling the heating and cooling rates during PWHT is critical for two reasons: avoiding through-thickness temperature gradients that generate new thermal stresses, and preventing the introduction of microstructural damage (e.g., tempering at too-rapid a rate may leave a stress gradient; cooling too fast through the embrittlement range can cause temper embrittlement in susceptible steels).

ASME VIII UCS-56(c) specifies the following rate limits above 315°C (600°F):

Maximum heating rate = 222°C/h ÷ (nominal thickness in inches)

Minimum heating rate = 56°C/h (100°F/h) in all cases

Example: 50 mm (2 in) wall
  Max rate = 222 / 2 = 111°C/h

Maximum cooling rate = 278°C/h ÷ (nominal thickness in inches)
Minimum cooling rate = 56°C/h

Below 315°C: component may be cooled in still air (no rate limit)

In practice, furnace PWHT controllers are programmed with ramp rates slightly below the code maximum to provide margin for thermocouple lag, furnace inertia, and the temperature gradient from the furnace atmosphere to the component centre. For very thick-walled components (above 100 mm), the actual heating rate achievable in the component interior may be the limiting factor, not the furnace setpoint rate.

PWHT Methods: Furnace versus Local

Furnace PWHT

Full furnace PWHT provides the most uniform temperature distribution and is the preferred method when the component geometry permits. Batch furnaces (car-bottom or box-type) or continuous roller-hearth furnaces are used depending on component size. Key requirements include uniform furnace atmosphere (typically air, inert gas, or endothermic atmosphere to limit decarburisation), certified calibrated thermocouples (type K or type N per IEC 60584), and continuous data-logger recording at intervals not exceeding 1 minute.

Local PWHT

When a component cannot be placed in a furnace (e.g., field piping repairs, large-diameter vessels, in-situ maintenance), ASME VIII UW-40 and AWS D10.10 permit local PWHT. The heated band must satisfy these minimum dimensional requirements:

• Soak band width: minimum 25 mm (1 in) either side of the weld edge, or equal to the wall thickness, whichever is greater
• Heated band width: must extend sufficiently beyond the soak band to ensure a controlled temperature gradient and prevent high thermal stresses at the edge of the heated zone; typically 75–100 mm beyond the soak band edge
• Insulation: must be applied over the heated and adjacent bands to limit temperature gradients to ≤ 150°C per metre of axial length at the soak temperature

Local PWHT is executed using electric resistance heating blankets (most common), induction heating coils, or gas-fired heating rings. Induction heating offers superior temperature uniformity for pipe PWHT and significantly faster response, but requires specialised equipment and competency. For details on the HAZ thermal cycle and its microstructural consequences, see the MetallurgyZone article on heat-affected zone microstructure.

PWHT of Specific Steel Families

Carbon Steel (P-No. 1): SA-516, SA-106

Carbon steel is the simplest case. The PWHT window of 595–650°C is well below A₁ (~727°C), so no phase transformation occurs. The dominant microstructural change is carbide spheroidisation and partial dislocation recovery. Pearlite lamellae begin to fragment and coarsen. The steel softens slightly (10–20 HBN typical hardness reduction) and the notch toughness of the HAZ improves. For more on pearlite microstructure and carbide behaviour, see the dedicated MetallurgyZone guide.

1.25Cr-0.5Mo and 2.25Cr-1Mo Steel (P-No. 4, 5A): SA-387 Gr.11 / Gr.22

These creep-resistant steels are widely used in refinery and petrochemical hot-service applications. Their HAZ martensite is harder than that of carbon steel due to the higher hardenability from Cr and Mo additions, and it contains a higher density of dislocations and fine M₂₃C₆ and M₂C carbide precipitates. PWHT at 650–760°C dissolves fine M₂C and reprecipitates M₂₃C₆ on prior austenite grain boundaries and on lath boundaries in the tempered martensite. This carbide redistribution is critical for recovery of the creep rupture strength and toughness.

2.25Cr-1Mo (P-No. 5A) is also susceptible to temper embrittlement if PWHT cooling proceeds slowly through 375–560°C or if service temperatures in this range are sustained over many thousands of hours. The Bruscato J factor is used to screen susceptibility:

For a detailed treatment of this topic see the MetallurgyZone article on bainite and tempered martensite microstructure and the Fe-C phase diagram reference.

Grade 91 (P91): 9Cr-1Mo-V-Nb Steel (P-No. 15E)

Grade 91 is an extremely sensitive steel with respect to PWHT. The correct microstructure in service is fully tempered martensite with fine MX (V, Nb carbonitride) and M₂₃C₆ (Cr-rich carbide) precipitates. Any deviation from the prescribed PWHT cycle can leave one or more of the following degraded microstructures:

• Untempered martensite: if the weld is allowed to cool below M_f (~100°C) and PWHT is omitted or insufficiently heated
• Delta-ferrite: if PWHT temperature exceeds 800°C; delta-ferrite has very poor creep ductility and acts as a crack initiation site
• Type IV cracking zone: fine-grained HAZ sub-zone between the CGHAZ and the base metal that undergoes creep cavitation in service due to lack of precipitate strengthening recovery

PWHT for P91 is mandatory regardless of thickness. The specified range is 730–775°C with a minimum of 2 hours (and often 4–8 h for thick sections to ensure uniform tempering). For in-depth guidance see the MetallurgyZone article on creep-resistant steels P91 and P92.

Austenitic Stainless Steels

Austenitic stainless steels (P-No. 8: 304, 316, 321, 347) are not subject to PWHT in the conventional stress-relief sense. Their face-centred cubic (FCC) structure does not undergo martensitic transformation, and conventional stress-relief temperatures would cause sensitisation by precipitation of chromium carbides at grain boundaries, reducing intergranular corrosion resistance. Instead, solution annealing (1040–1120°C, rapid water quench) is used to homogenise composition and redissolve carbides. For weld repair situations where solution annealing is impractical, stabilised grades (321, 347) or low-carbon grades (304L, 316L) are preferred.

For a detailed treatment of delta ferrite content in stainless welds and its effects, see the MetallurgyZone article on delta ferrite in stainless steel welds.

PWHT Procedure and Documentation Requirements

PWHT Procedure (Heat Treatment Procedure Specification)

Most quality codes (ASME VIII, BS PD 5500, EN 13445) require a written Heat Treatment Procedure Specification (HTPS) to be prepared and approved before PWHT is performed. The HTPS must document, at minimum: applicable code, base material P-Numbers and nominal thickness range, minimum and maximum soak temperature, minimum hold time, maximum heating and cooling rates, thermocouple type and calibration interval, furnace or local method, atmosphere control, and acceptance criteria.

Thermocouple Placement and Calibration

Thermocouples must be in intimate thermal contact with the steel, not with the heating element or the furnace atmosphere. Best practice requires direct attachment by percussion welding (stud-welding) of the thermocouple tip to the steel surface within 75 mm of the weld edge. Thermocouples must be calibrated to IEC 60584 tolerances: for Type K at 600°C, the Class 1 tolerance is ±1.5°C. Instruments and recorders must be within their calibration period.

PWHT Records

PWHT records (time-temperature charts or digital data files) are a code-required quality record under ASME VIII and must be retained for the design life of the equipment, typically traceable to individual weld map numbers and the Equipment Datasheet. The record must show: equipment identification, weld map reference, heating ramp, hold period with thermocouple designations, and cooling ramp with times and temperatures for each thermocouple.

Potential PWHT Problems and Their Prevention

Distortion and Warpage

Non-uniform heating or inadequate support of thin-walled components during PWHT can cause permanent distortion. Large flat-plate constructions and skirt-supported vessels are particularly vulnerable. Adequate internal stiffening, uniform heating, and slow heating rates mitigate distortion. Finite-element analysis of thermal gradients is routinely used in critical vessel designs to predict and prevent distortion.

Oxidation and Scale Formation

PWHT in air at 600–760°C forms thin iron oxide scale. For most carbon steel pressure vessels, this is acceptable. For Cr-Mo steels, Cr₂O₃ scale provides some protection. Where bright finish is critical (precision machined surfaces, bore welds), a controlled atmosphere (nitrogen blanket, endothermic gas) is used.

Stress Corrosion Cracking (SCC) Sensitisation in Austenitic Stainless

As noted above, applying conventional ferritic PWHT temperatures to austenitic stainless steel welds will sensitise them by precipitating Cr₂₃C₆ at grain boundaries, creating chromium-depleted zones susceptible to intergranular SCC. This is an error-of-application that must be prevented by clear scope definition in the HTPS.

Temper Embrittlement

For susceptible 2.25Cr-1Mo steels, slow cooling through the 375–560°C embrittlement range during PWHT or exposure in service can shift the DBTT to above the minimum operating temperature, resulting in brittle fracture. Precautions include: procurement of steel meeting the J-factor limit (<100), step-cooling tests per API 934-A, and rapid cooling through the embrittlement range during PWHT where the code permits.

Relationship Between PWHT and Weld Procedure Qualification

Under ASME Section IX, PWHT condition is an essential variable for both WPS and PQR qualification. A WPS that includes PWHT is qualified separately from one without PWHT. The PQR coupon must be subjected to the same PWHT as the production weld. If the production PWHT temperature range or number of PWHT cycles changes, re-qualification may be required. For a full treatment of weld procedure qualification, see the MetallurgyZone preheat temperature calculator and the discussion of hydrogen cracking prevention.