Post Weld Heat Treatment
What is the difference between postheating and PWHT?
Postheating is used immediately after welding to slow cooling and reduce hydrogen-related cracking risk. PWHT is a deliberate heat-treatment cycle performed after welding for stress relief or property modification.
Does PWHT remove all residual stress?
No. PWHT reduces residual stress substantially, but it may not eliminate it completely.
Why is PWHT often done before final machining?
Because the weldment is more dimensionally stable after stress relief, reducing post-machining movement and helping hold tight tolerances on faces, bores, and datums.
Post-weld heat treatment (PWHT) is a controlled thermal cycle applied after welding to reduce residual stress, temper or stabilize the heat-affected zone, and improve dimensional stability and service performance. The weldment is heated below its critical transformation range, held for a specified soak time, then cooled under controlled conditions to prevent distortion or cracking.
On the shop floor, PWHT is typically scheduled after welding and any required pre-PWHT inspection, but before final machining. Heavy weldments and precision assemblies need this sequence because stress relief stabilizes the material. Heating rate, soak temperature, and cooling rate are all controlled to avoid thermal shock and produce uniform stress relief. If a welded fixture or frame goes straight to a CNC mill, locked-in welding stress can release during metal removal, causing the part to walk, shift, or warp between roughing and finishing passes. For millwork-adjacent fabrication such as metal subframes, lift brackets, and structural supports, PWHT prevents post-install movement that shows up as door misalignment, uneven reveals, or binding hardware once load is applied. The practical result is a dimensionally stable weldment that holds tighter tolerances on faces, bores, and datums during final boring, line boring, and clamping.
Machining before stress relief on a precision weldment: welding locks in stress, so the part drifts during PWHT; bores, faces, and holes shift and require rework.
Uneven thermal soak during PWHT: poor furnace loading or thermocouple placement lets one zone lag while another reaches temperature; stress relief becomes inconsistent, producing localized distortion and hardness variation.
Incorrect temperature for alloy or code: too low leaves residual stress intact; too high over-tempers the HAZ, degrades strength, and risks code noncompliance.