WLD/Processes/TBT-WLD-012

Preheat and Post-Weld Heat Treatment

Welding & Fabrication › Processes › Preheat and Post-Weld Heat Treatment

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Preheating raises the base metal temperature before welding to reduce the risk of cracking in the weld zone.
Post-weld heat treatment (PWHT) applies controlled heating after welding to relieve residual stresses.
Both processes are specified in welding procedure specifications (WPS) and are mandatory where required.
Preheating is typically done using gas torches, electric resistance heating pads, or induction coils.
PWHT uses electric resistance heaters or furnaces to hold the weld at temperature for a defined period.
Surface temperatures during preheat and PWHT are monitored using thermocouples and temperature-indicating crayons.
Burns from heated metal surfaces, electrical equipment, and radiant heat are the primary injury hazards.
Insulation blankets wrapped around heated components retain heat and can cause burns if touched.
Electrical heating equipment operates at high power — isolation and cable management are critical.
Failure to achieve the correct temperatures invalidates the WPS and compromises the weld integrity.

Weld integrity	Omitting preheat or PWHT where specified causes hydrogen cracking, residual stress, and premature weld failure.
Burn injuries	Metal surfaces at preheat temperatures of 100–300°C and PWHT temperatures up to 750°C cause severe contact burns.
Electrical safety	Heating equipment uses high-current electrical supplies that create shock and fire hazards if poorly managed.

Don't

Follow the WPS requirements for preheat temperature, interpass temperature, and PWHT cycle
Monitor temperatures using calibrated thermocouples attached at the specified locations
Mark heated zones with warning signs and barriers to prevent accidental contact
Wear heat-resistant gloves and avoid touching heated metal, insulation, or heating elements
Inspect electrical heating equipment and cables for damage before each use
Record all temperature data including preheat achieved, soak time, and cooling rates
Ensure gas torches used for preheating are operated under a hot works permit
Allow heated components to cool to a safe temperature before removing insulation
Isolate electrical heating equipment from the supply before adjusting or repositioning
Brief all nearby workers that heating operations are in progress and surfaces are hot

DON'T skip preheat or PWHT where the WPS specifies it — the weld will be defective
DON'T touch heated metal, insulation blankets, or heating cables without protection
DON'T exceed the maximum interpass temperature specified in the WPS
DON'T use damaged heating elements, frayed cables, or faulty temperature controllers
DON'T remove thermocouples during the PWHT soak period — temperature records are mandatory
DON'T cool heated components rapidly by quenching with water unless specified
DON'T leave heating equipment energised and unattended during the treatment cycle
DON'T stack flammable materials near components undergoing preheat or PWHT
DON'T falsify temperature records — non-compliant welds must be reported and addressed
DON'T walk over or under heating cables routed across access areas without protection

See also: Welding Safety Awareness (Comprehensive) | Welding Procedure Specification (WPS) Awareness

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