How important are heating rate and cooling rate in PWHT?

Critically important — particularly for alloy steels. ASME B31.1 specifies a maximum heating rate of approximately 220°C/hour (400°F/hr) divided by the wall thickness in inches — thicker sections must be heated more slowly. Excessive heating rate creates large thermal gradients and resultant stresses that can crack the component. Rapid cooling after PWHT can re-introduce residual stress or create undesirable microstructures. Thermocouples and a data logger are mandatory to control and document both rates.

What documentation is required to close out the PWHT QA record?

A standard PWHT closure record includes: (1) chart recorder/data logger printout showing temperature versus time with inspector signature, (2) thermocouple quantity and placement per WPS, (3) confirmation that soaking time met the minimum requirement, (4) heating and cooling rates within specified limits, and (5) site photographs tagged with weld number and system reference. Tempilstik® spot-check photographs are a useful supplementary document but do not replace the thermocouple record.

Can Tempilstik® verify PWHT temperatures in the 600°C range?

Tempilstik® is used as an intermediate spot-check tool during PWHT — confirming the component has passed through defined temperature thresholds during the heating phase. For accurate verification of the soaking temperature in the 580–760°C range, thermocouples are mandatory and cannot be replaced. Tempilstik® supplements the thermocouple record; it does not substitute for it.

PWHT — Post Weld Heat Treatment: ASME Requirements & the Role of Tempilstik® in Temperature Verification

Q: Can PWHT be performed in the field without a fixed furnace?

Yes. Field PWHT is commonly performed during plant maintenance and pipeline installation using electric resistance heating (resistance heating wire or ceramic pad heaters wrapped around the weld) or induction heating. Requirements are identical to furnace PWHT: thermocouples, data logging, controlled heating and cooling rates, and adequate soaking time. Tempilstik® is particularly practical for field PWHT because it can be applied to the surface without additional equipment.

Published 27 January 2025 · Fast Group Engineering · 9 min read

Post Weld Heat Treatment (PWHT) is a mandatory heat treatment step under ASME and EN codes for many alloy steels and heavy-section welds. Unlike preheat or interpass temperature control — which are performed during welding — PWHT is an independent thermal cycle applied after the weld is complete: heat the joint to the specified temperature, hold for a defined soak time, then cool at a controlled rate. Thermocouples and a data logger are the primary instruments for PWHT; however, Tempilstik® plays a valuable supplementary role — providing spot-check confirmation that the actual component surface has reached the required temperature, filling in the gaps between fixed thermocouple attachment points.

Tempilstik temperature indicating crayons used for spot-check verification at an industrial plant during post weld heat treatment — During PWHT, Tempilstik® is used for spot-check verification of actual surface temperature at locations not directly monitored by thermocouples — supplementing, not replacing, the thermocouple data record.

What is PWHT — Definition and Metallurgical Purpose

PWHT is a controlled heating process applied to a completed weld after it has cooled to ambient temperature (or to a temperature defined by the applicable code). Its technical purpose is not a single objective — it simultaneously addresses three metallurgical problems introduced by the welding process:

1. Stress Relief

Welding generates extreme thermal gradients over very short time periods. Uneven expansion and contraction introduce residual stress — sometimes approaching the yield strength of the material. PWHT at sufficient temperature softens the crystal lattice, allowing limited plastic deformation to release this stored stress.

2. HAZ Property Improvement

For alloy steels (Cr-Mo, Cr-Mo-V), the welding thermal cycle produces hard, brittle martensite in the HAZ. PWHT tempering transforms this martensite into bainite or sorbite — microstructures with significantly higher impact toughness, suited for high-temperature and high-pressure service.

3. Hydrogen Diffusion

While preheat already assists hydrogen diffusion during welding, PWHT at elevated temperature drives this process more completely — reducing the risk of delayed cold cracking (hydrogen-induced cracking) that can develop hours or days after welding.

Common misconception: PWHT is not "preheat repeated after welding is complete." Preheat, interpass, and PWHT operate at three different points in the welding sequence and address three different problems. Confusing these three concepts is a frequent error in welding QA/QC practice.

Distinguishing Preheat, Interpass, and PWHT

Parameter	When Applied	Type of Control	Purpose
Preheat	Before welding begins	MINIMUM	Slow down HAZ cooling rate; reduce hydrogen-induced cold cracking risk; reduce thermal shock
Interpass	Between each weld pass	MAXIMUM	Prevent overheating — excessive interpass temperature reduces impact toughness and produces adverse microstructural changes
PWHT	After weld is complete and cooled	INDEPENDENT CYCLE	Stress relief, HAZ tempering, hydrogen diffusion — a complete, separate thermal treatment cycle

Preheat and interpass temperature can be verified with Tempilstik® throughout the welding process. PWHT requires a more comprehensive measurement system because the temperatures involved are substantially higher and the soaking time must be fully documented to satisfy third-party inspection requirements.

PWHT Temperatures per ASME — P-Number Classification

ASME classifies weld materials by P-Number (material group) to define PWHT requirements. The following table summarizes stress relief PWHT temperatures for the steel groups most commonly encountered in Vietnam's refinery, petrochemical, and power generation sectors:

P-Number (ASME)	Steel Type	Typical Grade / Spec	PWHT Temperature Range	Min Soak Time	Tempilstik® Spot-Check
P1	Carbon steel	A106 Gr.B, A53, A516	580–620°C (1076–1148°F)	1 hr per 25 mm thickness	#28047 (600°F / 316°C) — verify heating phase
P4	Low Cr-Mo alloy	A335 P11 (1¼Cr-½Mo)	600–650°C (1112–1202°F)	1 hr per 25 mm	#28047, #28057 (1022°F / 550°C)
P5A	Cr-Mo alloy	A335 P22 (2¼Cr-1Mo)	700–760°C (1292–1400°F)	2 hr minimum	#28057 (550°C), #28065 (1450°F / 788°C)
P5B	Cr-Mo-V alloy	A335 P91 (9Cr-1Mo-V)	730–760°C (1350–1400°F)	1 hr minimum (no exceptions)	#28065 (1450°F / 788°C) — mandatory
P8	Austenitic stainless	A312 TP304, TP316	Solution anneal 1040–1120°C	Thickness-dependent	Outside standard Tempilstik® range

⚠️ P91 PWHT is non-negotiable: PWHT at 730–760°C for P91 (9Cr-1Mo-V) is an absolute mandatory requirement under ASME B31.1 and B31.3 — no exceptions by wall thickness, fitting size, or any other parameter. Omitting PWHT for P91 is a direct cause of creep failure after thousands of hours in high-temperature service. See the dedicated article on P91 steel welding requirements for full details.

The Role of Tempilstik® in PWHT

A question frequently raised by QA/QC Engineers: "We already have thermocouples and a data logger — what does Tempilstik® add to a PWHT operation?"

The answer lies in the fundamental difference between the two measurement methods:

Method	Coverage	Output	Primary Use
Thermocouple + data logger	Fixed attachment points — typically on or near the weld surface per WPS	Continuous time-temperature record — mandatory QA documentation	Primary PWHT record — soak time evidence
Tempilstik®	Any accessible surface — applied by inspector at locations thermocouple cannot reach	Go/no-go confirmation that a specific temperature threshold was reached at that location	Supplementary spot-check at complex geometry points

Specific ways Tempilstik® supplements the thermocouple system during PWHT:

Spot-check at thermocouple-free zones: Mark Tempilstik® on flange faces, nozzle base pads, thick fittings, or recessed corners — if the mark melts, that location has reached the required temperature.
Pre-soak confirmation: Before starting the soak time clock, use Tempilstik® to sweep the points farthest from the heat source — confirming that the entire component has reached the target temperature uniformly, not just the thermocouple attachment zones.
Supplementary evidence when thermocouple readings are questioned: If the data logger output is anomalous or a thermocouple becomes loose during heating, the Tempilstik® melt marks on the component surface provide independent physical evidence for the QA record.
Indirect heating rate monitoring: For large components, marking multiple crayons at progressively higher temperatures and recording the order in which they melt gives a practical indication of whether heat is distributing uniformly through the section.

For QA documentation: A photograph of a melted Tempilstik® mark on the component surface, annotated with the part number and time of observation, is accepted as supplementary evidence by third-party inspectors (TPI) in addition to the thermocouple record. Some owner specifications (e.g. Shell DEP) explicitly require temperature indicating crayon spot-checks as part of PWHT documentation.

Tempilstik® Part Numbers for PWHT Verification

PWHT temperatures (580–760°C) fall within the Tempilstik® operating range, but selecting the correct part number for a meaningful spot-check requires understanding the verification strategy. The principle: select a part number whose rating is at or below the minimum PWHT soaking temperature — when that crayon melts, you can confirm the surface has passed that threshold.

Part Number	Temperature (°F / °C)	PWHT Application	Applicable Steel Group
#28047	600°F / 316°C	Low-temperature checkpoint during heating phase — verify surface has passed 316°C	P1 (carbon steel)
#28057	1022°F / 550°C	Intermediate checkpoint in P4/P5A heating cycle	P4 (P11), P5A (P22)
#28061	1400°F / 760°C	Spot-check confirming surface has passed 760°C — uniform heating check before soaking	P1 (uniform heat verification)
#28065	1450°F / 788°C	Spot-check in P22 and P91 PWHT — confirm surface ≥ 788°C prior to soaking	P5A (P22), P5B (P91)

Combined use strategy for P91 PWHT (target 730–760°C): Use #28065 (788°C) to confirm all spot-check locations across the component have passed 788°C before starting the soak time clock. When #28065 melts uniformly at all check points, the entire steel mass has exceeded 788°C — the thermocouple system then monitors the soak phase down to the specified 730–760°C range. Note: no standard Tempilstik® is rated in the 730°C range — this is why thermocouples remain the primary instrument and Tempilstik® serves a supplementary role.

→ Full product listing at the Tempilstik® page — filterable by °C or °F. Stock in Ho Chi Minh City and Vung Tau.

Frequently Asked Questions

No. PWHT requirements depend on three factors: material type (P-Number), section thickness, and the applicable code. Thin-section P1 carbon steel typically does not require PWHT under ASME B31.3. P91 steel, however, requires PWHT unconditionally, regardless of wall thickness or fitting type. Always verify against the PWHT requirement table in the applicable code and the approved WPS.

Critically important — particularly for alloy steels. ASME B31.1 specifies a maximum heating rate of approximately 220°C/hour (400°F/hr) divided by the wall thickness in inches — thicker sections must be heated more slowly. Excessive heating rate creates large thermal gradients that can crack the component. Rapid post-PWHT cooling can re-introduce residual stress or produce undesirable microstructures. Thermocouples and a data logger are mandatory for controlling and documenting both rates.

Yes. Field PWHT is routinely performed during plant maintenance shutdowns and pipeline construction using electric resistance heating (ceramic pad heaters or resistance heating wire wrapped around the weld) or induction heating. Requirements are identical to furnace PWHT: thermocouples, data logging, controlled heating and cooling rates, and the required soaking time. Tempilstik® is particularly practical for field PWHT because it can be applied to any accessible surface without additional support equipment.

A standard PWHT QA closure record includes: (1) chart recorder / data logger printout showing temperature versus time, with inspector signature; (2) confirmation of thermocouple quantity and placement per the approved WPS; (3) verification that soak time met the minimum required duration; (4) heating and cooling rates within the specified limits; (5) site photographs tagged with weld number and system reference. Tempilstik® spot-check photographs serve as useful supplementary documentation but do not replace the thermocouple record.

Tempilstik® is used as an intermediate spot-check tool during PWHT — confirming that the component surface has passed through defined temperature thresholds during the heating phase. For accurate soaking temperature verification in the 580–760°C range, thermocouples are the mandatory primary instrument and cannot be replaced by Tempilstik®. The two methods are complementary, not interchangeable.

Need Tempilstik® for PWHT verification or welding temperature control?

Fast Group Engineering is the authorized Tempil® distributor in Vietnam — direct import from the USA, with C/O, C/Q, and VAT invoice. Stock in Ho Chi Minh City and Vung Tau.

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