Post-weld heat treatment (PWHT): when it appears in steel specifications
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Why PWHT is specified, what it can change, which variables must be controlled, and why it must be checked against the material grade, fabrication code and qualified welding procedure.
Post-weld heat treatment (PWHT) is a controlled thermal treatment carried out after welding when the governing code, material specification or project requirement calls for it.
It is often associated with "stress relief", but PWHT can affect more than residual stress. Depending on the material and procedure, it can also temper the weld and heat-affected zone, reduce hardness and change toughness or strength.
The key rule is simple: PWHT is not a generic improvement that should automatically be added to every welded steel component.
Why PWHT may be required
A specification can require PWHT for one or more reasons:
- reduction and redistribution of welding residual stresses;
- tempering of hard weld or HAZ microstructures;
- control of hardness for a particular service environment;
- improvement of dimensional stability;
- compliance with a fabrication or pressure-equipment code;
- material-specific metallurgical requirements;
- project-specific sour-service, fatigue or fracture-control requirements.
The actual requirement normally depends on several variables together, such as material group, thickness, service, welding process and applicable code.
PWHT is different from post-heating
A hydrogen-release or post-heat treatment immediately after welding should not automatically be called PWHT.
A post-heat may be used at a relatively modest temperature to encourage hydrogen diffusion and reduce cold-cracking risk. PWHT normally refers to a controlled thermal cycle with a defined heating rate, soak range, holding time and cooling practice.
The distinction matters because the objectives and qualification requirements are different.
What a PWHT procedure controls
A proper procedure does much more than state one temperature.
Typical controls include:
- target temperature range;
- minimum holding/soak time;
- heating rate;
- cooling rate;
- temperature uniformity;
- heated-band width for local treatment;
- insulation arrangement;
- thermocouple type, number and location;
- calibration of measuring/recording equipment;
- maximum temperature differential;
- number of PWHT cycles;
- recording and acceptance of the time-temperature chart.
ISO 17663 provides quality requirements for heat treatment carried out in connection with welding and forming.
Exact numbers should come from the governing fabrication code, material specification or qualified procedure.
Furnace PWHT vs local PWHT
A complete component can sometimes be treated in a furnace. In other cases only the weld region is heated locally.
Local PWHT can be technically valid, but temperature gradients and restraint become especially important. The heated band and insulated area must be controlled so that the intended weld region receives the required treatment without creating damaging thermal gradients elsewhere.
This is one reason a statement such as "PWHT at 600 °C" is not a complete procedure.
PWHT can change the parent material
The parent steel itself may be sensitive to the thermal cycle.
This is particularly important with steels whose properties were created by a specific delivery route, including:
- quenched and tempered steels;
- TMCP steels;
- precipitation-strengthened alloys;
- some stainless and duplex grades.
An unsuitable PWHT cycle can reduce strength, change toughness or affect corrosion-related properties.
For Q&T material, see Quenched and tempered steel explained.
PWHT and hardness
PWHT is sometimes used to temper hard areas in ferritic steel weldments, but the result must not be assumed.
The effect depends on:
- composition;
- original microstructure;
- weld thermal cycle;
- PWHT temperature;
- holding time;
- cooling;
- location of the hardness measurement.
When a project specifies a hardness limit, the correct approach is to qualify and verify the complete welding/PWHT route rather than assume PWHT will automatically make the weld compliant.
How PWHT connects to the WPS and PQR
If production welds will receive PWHT, the welding procedure qualification normally needs to represent the relevant condition required by the applicable qualification standard or code.
The documentation chain therefore needs to be consistent:
production requirement → WPS → supporting WPQR/PQR → PWHT procedure → production records
A WPS that is qualified only in the as-welded condition should not automatically be assumed valid for a materially different PWHT cycle.
See WPS vs PQR: welding procedure documents explained.
Simulated PWHT on material test specimens
For some projects, the purchaser wants assurance that the base material still meets required properties after the thermal exposure expected during fabrication.
The material order may therefore specify testing after one or more simulated PWHT cycles.
This can matter for:
- pressure-vessel steels;
- Cr-Mo/alloy steels;
- thick plate;
- repeated fabrication heat cycles;
- projects with strict toughness requirements.
The order must define the relevant simulated cycle or refer clearly to the governing specification. It is not enough to request "PWHT tested" without defining the condition.
Procurement questions to resolve before ordering
If PWHT appears in a material or fabrication specification, check:
- Is PWHT mandatory, conditional or only permitted?
- Which code/specification defines the cycle?
- Does the material standard permit the proposed treatment?
- Is testing required after simulated PWHT?
- How many cycles must the material tolerate?
- Does the WPS qualification cover the production condition?
- Are hardness or Charpy tests required after PWHT?
- Is furnace treatment required or is local treatment acceptable?
- What documentation and charts must be submitted?
- Is third-party witnessing required?
These questions should be resolved before buying the steel, not after fabrication starts.
Procurement takeaway
PWHT is part of a qualified fabrication route.
It can reduce residual stress and modify weld/HAZ properties, but it can also change the parent material. The applicable code, grade, WPS qualification and project specification determine whether PWHT is required and how it must be performed.
Never add, omit or change PWHT based only on a general rule about thickness or steel strength.
References and verification sources
Standards referenced
- ISO 17663:2023
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