PWHT Soak Time Calculator — Post Weld Heat Treatment Code Guide
Post weld heat treatment (PWHT) soak time is one of the most consequential numbers in pressure equipment fabrication. Get it wrong — too short or at the wrong temperature — and you risk residual stress-induced cracking, hydrogen damage, or inadequate tempering of hard heat-affected zones. This calculator and guide cover minimum soak time requirements per ASME Section VIII Division 1 (UCS-56), ASME B31.3 Process Piping, and ASME B31.1 Power Piping for the most common P-Number material groups, from plain carbon steel (P-No. 1) through the high-chromium creep-strength enhanced ferritic steels (P-No. 15E, Grade 91). Every calculation includes a full step-by-step formula breakdown so you can verify results against your WPS and PQR.
PWHT Soak Time Calculator
What Is PWHT and Why Is Soak Time Critical?
Post weld heat treatment is a controlled thermal cycle applied to a completed weld assembly. Its primary purpose is to reduce residual welding stresses, temper hard martensitic microstructures in the heat-affected zone (HAZ), and improve resistance to hydrogen-induced cracking (HIC), stress corrosion cracking (SCC), and creep damage in high-temperature service. For pressure-containing equipment in oil and gas, petrochemical, and power generation, PWHT is not optional engineering judgment — it is a mandatory code requirement in many material-thickness combinations.
The soak time (also called holding time or dwell time) is the minimum period for which the entire weld cross-section must remain within the specified temperature band. Too short a soak time leaves the HAZ insufficiently tempered, with hardness peaks that are susceptible to delayed hydrogen cracking and brittle fracture. Too long a soak time or too high a temperature can cause excessive grain growth and reduce the creep strength of advanced alloys such as Grade 91. Both extremes carry real engineering risk; the code tables define the safe envelope.
How Soak Time Is Calculated — The Core Formula
The minimum soak time is a function of the nominal thickness of the weld joint, the material P-Number classification, and the applicable construction code. The governing thickness is generally defined as the thicker of the two base metal sections at the joint, or in the case of fillet welds, the throat thickness. Under ASME Section VIII Div. 1, UW-40(f) defines nominal thickness precisely for different joint configurations.
ASME Section VIII Div. 1 — P-No. 1 Carbon Steel (Table UCS-56-1)
Worked Example — 60 mm P-No. 1 Pressure Vessel Shell
Soak Time Requirements by P-Number and Code
| P-No. | Material Examples | Code | Min. Temp. (°C) | Soak Time Rule | Status |
|---|---|---|---|---|---|
| P-No. 1 | A105, A106 Gr.B, A516 Gr.70 | ASME VIII | 595 | 15 min (≤25 mm); 1 hr (25–50 mm); +15 min/25 mm (>50 mm) | Mandatory >38 mm |
| P-No. 3 | A387 Gr.2, A204 Gr.B | ASME VIII | 595 | 1 hr/25 mm; min 1 hr | Mandatory any thickness |
| P-No. 4 | A335 P11, A182 F11 | ASME VIII | 620 | 1 hr/25 mm; min 1 hr | Mandatory any thickness |
| P-No. 5A | A335 P22, A182 F22 | ASME VIII | 675 | 1 hr/25 mm; min 1 hr; +15 min/25 mm >50 mm | Mandatory any thickness |
| P-No. 5B | A335 P5, A335 P9 | ASME VIII | 675 | 1 hr/25 mm; min 1 hr | Mandatory any thickness |
| P-No. 15E Gr.1 | A335 P91, A335 P92 | ASME VIII | 730 | 1 hr (≤50 mm); +15 min/25 mm >50 mm; min 1 hr | Mandatory any thickness |
| P-No. 1 | A106 Gr.B, A333 Gr.6 | ASME B31.3 | 593 | 1 hr/25 mm; min 15 min | Mandatory >19 mm (unless exempted) |
| P-No. 4 | A335 P11 | ASME B31.3 | 620 | Min 2 hours regardless of thickness | Mandatory any thickness |
| P-No. 5A | A335 P22 | ASME B31.3 | 675 | Min 2 hours regardless of thickness | Mandatory any thickness |
| P-No. 15E Gr.1 | A335 P91 | ASME B31.1 | 730 | 1 hr/25 mm; min 1 hr; min 5 hr >50 mm (B31.1) | Mandatory any thickness |
Heating and Cooling Rate Requirements
Soak time is only part of the thermal cycle specification. Heating rate and cooling rate limits are equally important for preventing thermal shock, distortion, and re-introduction of damaging residual stress. ASME Section VIII Div. 1, UW-40 provides the governing rules.
Maximum Heating Rate (Above 315 deg C)
Maximum Cooling Rate (Down to 315 deg C)
PWHT Requirements for P-No. 15E (Grade 91 / P92) — Special Considerations
Creep strength enhanced ferritic (CSEF) steels such as Grade 91 and Grade 92 require significantly more engineering attention during PWHT than conventional chrome-moly alloys. These materials undergo a martensitic transformation during cooling from welding temperatures, and this fresh (as-welded) martensite must be fully tempered to restore toughness and restore creep strength. Failure to properly PWHT Grade 91 is one of the most common causes of premature failure in high-temperature power piping systems.
The critical additional requirement for P91 and P92, not present for P-No. 4 and P-No. 5 materials, is the mandatory cool-down before PWHT initiation. The martensitic transformation finish temperature (Mf) for Grade 91 is approximately 80°C (175°F). If PWHT begins before the weld has cooled below Mf, retained austenite may not transform to martensite, and subsequent tempering will not develop the correct microstructure. The result is a weld with inadequate creep strength that may pass room-temperature hardness checks but will fail in elevated-temperature service.
- Maintain preheat at 200–300°C throughout welding and until post-weld hydrogen bake-out if specified.
- Allow the completed weld to cool to a maximum of 80°C (not below room temperature) before starting PWHT.
- Heat to 730–775°C at a controlled rate not to exceed 200°C/hr above 300°C.
- Hold at temperature for a minimum of 1 hour per 25 mm, minimum 1 hour, per ASME Section VIII Table UCS-56-4.
- Cool at a controlled rate (max 200°C/hr) to below 300°C, then still-air cool.
- Verify final hardness by Vickers or Brinell test on weld and HAZ — target HV10 ≤250 (HB ≤238).
PWHT Mandatory Thickness Triggers — When Is It Required?
| P-No. | Code | Mandatory PWHT Trigger | Exemption Condition |
|---|---|---|---|
| P-No. 1 | ASME VIII | Nominal thickness > 38 mm (1.5 in) | Preheat to 95°C minimum when t > 25 mm; not applicable for lethal/MDMT service |
| P-No. 1 | ASME B31.3 | Nominal thickness > 19 mm (0.75 in) | Preheat 95°C for t up to 25 mm; higher preheat alternatives per Table 331.1.3 |
| P-No. 3 | ASME VIII / B31.3 | Any thickness | Limited exemptions for thin sections with controlled preheat per code notes |
| P-No. 4 (P11) | ASME VIII / B31.3 | Any thickness | None generally; specific project specifications may permit skip for thin temporary welds |
| P-No. 5A (P22) | ASME VIII / B31.3 | Any thickness | None in standard practice |
| P-No. 15E (P91) | ASME VIII / B31.1 | Any thickness, mandatory | None; PWHT is an absolute requirement for all P91 pressure welds |
| Any P-No. | ASME VIII UW-2(a) | Lethal service, any thickness | No exemption |
| P-No. 1, P-No. 3 | ASME VIII UCS-68(b) | MDMT below −48°C (−55°F) | No exemption |
Heating Methods for PWHT in the Field and Shop
PWHT may be performed by furnace heating (the preferred method for complete assemblies) or by local heating methods for field joints, large vessels, and piping spools. Each method has specific requirements under ASME codes and the welding procedure specification.
Furnace PWHT
Furnace heating is the most uniform method. The component is loaded at a temperature not exceeding 425°C (800°F), the furnace is ramped at the controlled rate, held at the soak temperature, and cooled within the furnace to 315°C before air-cooling. The loading temperature limit prevents thermal shock on components entering a hot furnace. The minimum overlap for two-heat furnace PWHT of a single section is 1.5 m (5 ft), ensuring adequate temperature uniformity at the overlap zone.
Local Resistance and Induction Heating
For field joints in piping systems, resistance heating pads or induction heating coils are wrapped around the weld area. The soak band, heated band, and gradient control band must all be dimensioned correctly. Local heating requires careful thermocouple placement — a minimum of two thermocouples per weld, positioned at the top and bottom of the soak band, to confirm that the entire required volume reaches and maintains the soak temperature. The temperature gradient across the soak band must not exceed 83°C.
Flame Heating
Direct flame heating with gas burners is the least controlled method and is generally limited to preheat and interpass temperature maintenance. It is not recommended for final PWHT of alloy steels because maintaining a uniform temperature band with the required tolerance (±15°C) is impractical with direct flame. Where flame heating must be used for PWHT, shielded burners and continuous temperature monitoring with chart recorders are required.
PWHT and Welding Procedure Qualification Under ASME Section IX
PWHT is a supplementary essential variable under ASME Section IX, QW-407. A change in PWHT condition — from PWHT to no-PWHT, or a change in the temperature or time range — requires requalification of the welding procedure specification (WPS) by a new procedure qualification record (PQR). This is because PWHT fundamentally alters the mechanical properties of the weld, particularly toughness and hardness, both of which are qualified in the PQR test.
Per QW-407.2, the PQR must be subjected to PWHT for at least 80% of the aggregate time at temperature that will be applied in production welding. This means if a vessel will undergo three separate PWHT cycles (initial fabrication, repair, and re-repair), the PQR must have been subjected to the equivalent total time. The 80% rule allows for minor variation but requires careful planning when multiple PWHT cycles are anticipated — a scenario common in heavy fabrication and repair welding under P-Number group qualifications.
Interaction with Impact Testing and Low-Temperature Service
For pressure vessels designed to ASME Section VIII Div. 1 and required to meet Charpy impact requirements per UG-84, the PWHT time documented in the PQR is a qualified variable. The impact test specimens are machined from the PWHT-treated test coupon, so the reported Charpy values only apply when production PWHT matches the qualified conditions. In practice, this means the PQR must document not just the minimum soak time, but also the maximum time (which governs property degradation in over-tempered material), forming the WPS PWHT time range.
Common PWHT Non-Conformities in Fabrication
Site audits and vessel inspection records reveal a consistent set of recurring PWHT deviations. Understanding these failure modes helps quality engineers and welding inspectors establish effective hold points.
- Incorrect governing thickness determination: Using shell thickness instead of attachment thickness when an attachment weld is thicker, leading to insufficient soak time.
- Thermocouple drift or detachment: A thermocouple that lifts off the surface during heating will read air temperature rather than metal temperature, giving a false pass. Verify attachment before and after each cycle.
- Holding at a temperature below the minimum: A reading of 593°C for a P-No. 1 vessel does not satisfy the 595°C minimum. The minimum must be met at every point in the soak band, not just at the control thermocouple.
- Starting PWHT on Grade 91 before Mf is reached: The single most common cause of inadequate tempering in P91 welds. Always verify the weld is at or below 80°C before initiating the PWHT cycle.
- Exceeding the maximum temperature: Overshooting the soak temperature on P91 above 800°C can cause partial re-austenitisation, which destroys the tempered martensite microstructure. If overshoot occurs, the weld must be fully re-normalised and tempered by heat treatment or cut-out and re-welded.
- Chart record gaps or illegibility: PWHT records are quality documents required for the Manufacturer’s Data Report. Gaps in the time-temperature chart record invalidate the cycle. Always use calibrated chart recorders with redundant data logging.
Recommended Books on PWHT and Heat Treatment
AWS Welding Handbook: Welding Science and Technology
Comprehensive reference covering PWHT procedures, heat treatment metallurgy, and thermal cycle effects on weld properties.
View on AmazonASME Boiler and Pressure Vessel Code Section VIII Division 1
The official ASME code governing PWHT requirements for pressure vessels including UCS-56 tables and UW-40 procedures.
View on AmazonHeat Treatment of Welded Joints — WRC Bulletin 452
Welding Research Council bulletin covering local PWHT practice, soak band dimensions, and gradient control for piping welds.
View on AmazonWelding Metallurgy — Sindo Kou
Definitive textbook on the metallurgical principles behind welding, including HAZ microstructure, PWHT effects, and phase transformations in alloy steels.
View on AmazonDisclosure: WeldFabWorld participates in the Amazon Associates programme (StoreID: neha0fe8-21). If you purchase through these links, we may earn a small commission at no extra cost to you. This helps support free technical content on this site.
Frequently Asked Questions
What is the minimum PWHT soak time for P1 carbon steel per ASME Section VIII?
Per ASME Section VIII Div. 1 Table UCS-56-1, P-No. 1 carbon steel requires a minimum hold time that varies by thickness. For sections up to 25 mm (1 inch) the minimum is 15 minutes. Between 25 mm and 50 mm the minimum is 1 hour (60 minutes). Above 50 mm, the minimum is 1 hour plus 15 minutes for each additional 25 mm increment above 25 mm. The minimum holding temperature is 595°C (1100°F) with a maximum of 650°C (1200°F) for most carbon steels. Always verify against the specific material P-Number and the notes accompanying Table UCS-56-1, as exceptions exist for elevated-temperature service and impact-tested material.
What is the PWHT soak time formula per ASME B31.3?
ASME B31.3 Table 331.1.1 specifies the minimum holding time as 1 hour per 25 mm (1 inch) of nominal weld thickness with a minimum of 15 minutes for carbon steel (P-No. 1). However, for P-No. 4 (1.25Cr-0.5Mo, such as A335 P11) and P-No. 5A (2.25Cr-1Mo, such as A335 P22) materials, B31.3 requires a flat minimum of 2 hours regardless of thickness. B31.3 also permits reduced holding temperatures with correspondingly increased holding times per Table 331.1.3, which is a useful option when the PWHT temperature range conflicts with material restrictions such as dissimilar metal welds.
What is the heating rate limit during PWHT?
Per ASME Section VIII Div. 1 UW-40, the maximum heating rate above 315°C (600°F) is calculated as 205 divided by the thickness in inches (in °C/hr), with a maximum cap of 205°C/hr (400°F/hr) regardless of how thin the section is. Below 315°C, heating rate is not restricted by the code. The cooling rate limit after the soak period is 260°C/hr (500°F/hr) per inch of thickness down to 315°C, after which the component may be cooled in still air. These limits prevent thermal gradients that would re-introduce residual stress and potentially cause distortion or cracking in the cooled structure.
What is the difference between the soak band, heated band, and gradient control band?
The soak band (SB) is the volume of metal — including the weld, HAZ, and adjacent base metal to at least 25 mm on each side — that must be maintained at or above the minimum PWHT temperature throughout the entire soak period. The heated band (HB) is the wider zone surrounding the soak band that is actively heated; it is typically wider than the SB to ensure the edges of the SB do not lose heat by conduction. The gradient control band (GCB), defined clearly in ASME B31P and WRC 452, is the outermost zone where insulation is applied to smooth the temperature gradient and prevent steep thermal stress concentrations. ASME Section VIII specifies SB dimensions; B31P adds formal HB and GCB definitions that B31.3 and B31.1 projects increasingly reference.
Does P91 (P-No. 15E) require a different PWHT procedure from P22?
Yes, significantly. P-No. 15E Group 1 (Grade 91) requires a minimum soak temperature of 730°C per ASME Section VIII Table UCS-56-4, compared to 675°C for P-No. 5A (P22). More critically, P91 requires the weld to cool to below the martensite finish temperature — approximately 80°C — before PWHT is initiated. This step is not required for P22. The purpose is to ensure complete martensitic transformation before tempering begins; initiating PWHT on a partially-austenitic P91 weld will produce an incorrectly tempered microstructure with deficient creep strength. Additionally, for P91 the maximum PWHT temperature is strictly limited to below the lower critical transformation temperature (Ac1), approximately 800–820°C; exceeding this can cause re-austenitisation and catastrophic loss of properties. For more on P91 welding requirements, see our dedicated guide.
When is PWHT mandatory per ASME Section VIII Division 1?
PWHT is mandatory under several conditions in ASME Section VIII Div. 1. For P-No. 1 carbon steel, it is required when the nominal thickness exceeds 38 mm (1.5 in) per UCS-56. For P-No. 3 materials (C-0.5Mo steel), PWHT is mandatory at any thickness. For P-No. 4, P-No. 5, and P-No. 15E (including P91), PWHT is required at all thicknesses with no thickness exemption. Beyond material-thickness triggers, PWHT is also mandatory for lethal service (UW-2), for unfired steam boilers (UW-2(c)), for direct-fired vessels (UW-2(d)), and when the minimum design metal temperature falls below −48°C (−55°F) per UCS-68(b). Learn more about ASME Section VIII requirements through our training module.
What is the governing thickness for PWHT of attachment welds and nozzles?
For weld joints between pressure components of different thicknesses, the nominal thickness for PWHT purposes is generally the greater thickness at the weld joint, per UW-40(f)(3). For nozzle or attachment welds connecting non-pressure parts to pressure parts, the pressure-retaining shell or plate thickness governs the PWHT temperature, but the soak time is governed by the thicker section at the weld — which may be the attachment itself if it is thicker than the shell. ASME interpretations confirm that a trunnion or pad attachment whose wall thickness exceeds that of the shell cannot be ignored when determining minimum soak time. For fillet welds, the controlling thickness is the throat dimension, not the base metal thickness.
Can PWHT be done in multiple heating cycles, and how does this affect WPS qualification?
Yes. ASME Section IX QW-407.2 permits the total aggregate soak time to be applied in multiple heating cycles, provided the PQR was subjected to at least 80% of the cumulative production soak time. This is important for fabrication strategies involving repair welding after initial PWHT. However, multiple PWHT cycles accelerate temper embrittlement and can reduce toughness in alloy steels; the WPS should specify a maximum cumulative PWHT time, not just a minimum. For Grade 91, repeated PWHT cycles are known to coarsen precipitate populations and reduce creep strength; project specifications for power plant components often limit the number of PWHT cycles to two or three for this reason.