When is PWHT required?

Depends on the code, base metal, thickness, and joint restraint. AWS D1.1 doesn't mandate PWHT for most structural carbon-steel work, but ASME pressure vessel codes require it for many thicknesses and steel grades. AISC and other structural codes may invoke PWHT for thick-plate or restrained applications.

What's the difference between PWHT and stress relief?

PWHT (post-weld heat treatment) is the formal term that covers any heat treatment applied after welding — stress relief, normalizing, tempering, or full annealing. 'Stress relief' is the most common variant: hold at sub-critical temperature, hold for time, slow cool.

Can you skip PWHT if the joint is small?

Sometimes. ASME Section VIII includes exemption tables based on base metal P-number and thickness. AWS D1.1 has thickness-based exemptions for some applications. Read the code carefully — exemptions are specific.

PWHT on a WPS: when post-weld heat treatment is required and how to document it

Post-weld heat treatment (PWHT) relieves residual stress and improves toughness. Here's when it's required, how to specify it on a WPS, and how to verify it in production.

Post-weld heat treatment is the operation that converts a welded joint from "freshly welded with residual stress" to "stress-relieved and stable." For thick-plate, high-restraint, or pressure-vessel applications, PWHT is mandatory by code. The WPS must specify it precisely.

When PWHT applies

PWHT is required by:

ASME Section VIII (pressure vessels) — based on P-number and thickness, tabulated in UCS-56
ASME Section IX (qualification) — when supporting an ASME VIII application
API 1104 (pipelines) — for some applications
AWS D1.1 — for some thick-plate structural work, particularly when contract or AHJ specifies
Owner specifications that mandate PWHT independent of code requirements
AISC certification programs for some bridge work

PWHT is generally NOT required for:

Ordinary AWS D1.1 structural carbon-steel work below ~1.5 in thick
AISC building-construction work in non-seismic regions
Most code-of-record exempted thicknesses (which the code lists explicitly)

What PWHT does

Three effects:

Stress relief. Welding produces residual stresses up to the yield strength of the base metal. Holding at sub-critical temperature for time allows creep-relaxation; stress drops to manageable levels.
Tempering. If the HAZ has hard, brittle martensite, PWHT tempers it — reduces hardness and improves toughness.
Hydrogen diffusion. At elevated temperatures, dissolved hydrogen migrates out of the steel, reducing cold cracking risk.

The temperature, hold time, and cooling rate are all specified by code and project.

How to specify PWHT on a WPS

The PWHT block:

PWHT requirements:
  Required: Yes (per ASME UCS-56 Table for P-No 1 Group 2, 1-1/2 in thickness)
  Temperature: 1,150°F [620°C] ± 25°F
  Soak time: 1 hour per inch of thickness, minimum 1 hour
  Heating rate: 400°F/hr [220°C/hr] maximum above 600°F [315°C]
  Cooling rate: 400°F/hr [220°C/hr] maximum down to 600°F [315°C]; air cool below
  Method: furnace OR local electrical resistance heating with thermocouples
  Documentation: time-temperature chart with thermocouple identification retained as production record

The "method" line matters. Furnace and local methods have different documentation and verification requirements.

Verification in production

PWHT records consist of:

Time-temperature chart — a continuous record from start of ramp-up through end of cool-down
Thermocouple map — locations of TCs on the workpiece (typically multiple TCs to verify uniform heating)
Heat-treatment company's report if done by an outside service
Acceptance by the QC inspector confirming the chart meets WPS requirements

A PWHT chart that shows the part hit the right temperature but didn't hold for the right time is a finding. So is a chart with a heating rate that exceeded the spec.

PWHT impact on the WPS / PQR relationship

PWHT is an essential variable. Adding or removing PWHT from a procedure typically requires requalification:

A PQR welded with PWHT supports WPSs with PWHT
A PQR welded without PWHT does NOT support WPSs with PWHT (and vice versa)
The mechanical-test data on the PQR reflects the post-PWHT condition

Some codes provide exemptions where PQR with PWHT also supports WPSs without PWHT (the assumption being that as-welded properties are at least as good as PWHT properties for most steels). Verify the specific code language.

Common PWHT-related findings

WPS doesn't address PWHT when the code requires it. Direct fail.
WPS specifies PWHT but the supporting PQR was welded without it. Mismatch.
PWHT chart shows temperature ramp too fast. Thermal shock risk; finding.
Local heating used with insufficient TC coverage. Cold spots in the heated zone not detected.
Cool-down recorded but the rate was uncontrolled. Air-cooling fast can re-introduce stress.

When to defer PWHT to a heat-treatment subcontractor

Most shops do small-scale PWHT in-house with electrical resistance heating blankets and a portable thermal controller. Furnace PWHT is typically outsourced to a heat-treat service. Either way, the WPS must specify which method, the documentation requirements, and acceptance criteria. Liability for compliance stays with the fabricator regardless of who runs the heat treat.