PWHT for Cr-Mo Steels: Post-Weld Heat Treatment of A387 and P91-Class Fabrications

Post-weld heat treatment is part of the weld, not an afterthought, on Cr-Mo pressure equipment. This guide explains what PWHT accomplishes, the holding logic behind the standard ranges, and how grade 91 changed the rules.

Why Cr-Mo welds need it

Welding chromium-molybdenum steels leaves three problems behind: a hard, crack-sensitive heat-affected zone (HAZ), high residual stresses locked in by the weld's thermal cycle, and — for the creep-resistant grades — a microstructure that is not yet the one the design assumes. Post-weld heat treatment (PWHT) addresses all three at once:

It tempers the hard HAZ and weld metal, restoring ductility and toughness.
It relieves residual stress by creep relaxation at temperature, improving dimensional stability and resistance to stress-corrosion and fatigue cracking.
In creep grades it completes the intended tempered-martensite structure.

Construction codes (ASME Section VIII and the piping codes, with material rules in Section II) make PWHT mandatory for most Cr-Mo pressure work above defined thickness thresholds — it is a code requirement, not a shop preference.

Temperature ranges and holding logic

PWHT is a subcritical treatment: the temperature must temper and stress-relieve effectively while staying safely below the lower critical temperature (Ac1), so nothing re-austenitizes. Widely used ranges are approximately:

Grade family	Typical PWHT region (indicative)
1.25Cr-0.5Mo (A387 Gr.11 class)	about 620–680 °C
2.25Cr-1Mo (A387 Gr.22 class)	about 680–730 °C
9Cr-1Mo-V (P91 / A387 Gr.91)	about 730–770 °C, tightly controlled

Holding time scales with thickness — code tables express it as a minimum time per unit thickness, with absolute minima for thin material. The logic is equalization plus relaxation: the thickest section must reach temperature throughout and sit long enough for stresses to creep away. Heating and cooling rates above certain temperatures are also limited by code to control thermal gradients in heavy vessels, and thermocouples on the part — not the furnace dial — are the record that counts.

Grade 91: the strict case

P91's creep strength comes from a carefully tempered martensite with fine precipitates, and its PWHT window is correspondingly narrow. Too low or too short, and the HAZ stays hard and under-tempered; too high, and the treatment over-tempers (or, worst case, locally re-austenitizes, forming fresh untempered martensite on cooling) and creep strength is permanently damaged. For grade 91, temperature measurement accuracy, hold control and PWHT before any hydrotest or service exposure are treated as critical quality steps, and hardness surveys after PWHT are standard verification.

Local PWHT and practical cautions

Large vessels are often locally heat-treated around a weld seam rather than furnace-treated whole. The heated band width, insulation and gradient control then become the quality variables — a poorly controlled local PWHT can bend the shell or leave the stress pattern worse than a competent furnace cycle. Interruption of a cycle mid-hold, and repairs after PWHT (which generally require repeating it), are handled by written procedures for a reason: on Cr-Mo equipment, the heat treatment history is part of the pressure boundary's documented integrity.