Post-Weld Heat Treatment: When and How

## Why Post-Weld Heat Treatment Exists Welding introduces three problems that PWHT addresses: **Residual stresses**: The weld metal contracts as it cools, but is restrained by the surrounding cold base metal. This creates tensile residual stresses in the weld zone that can approach the yield strength of the material. These stresses add to service loads, promote stress corrosion cracking in susceptible environments, and can cause distortion when the part is later machined. **Hard HAZ microstructures**: In hardenable steels (CE > 0.40), the HAZ cools rapidly enough to form martensite or bainite that is harder and more brittle than the base metal. As-welded HAZ hardness can exceed 400 HV in 4140 or 4340 steel, making the joint susceptible to hydrogen cracking and low-toughness fracture. **Hydrogen retention**: Hydrogen dissolved in the weld during welding diffuses slowly at low temperatures. PWHT at moderate temperatures accelerates hydrogen diffusion out of the joint. ## Types of PWHT ### Stress Relief (Most Common) Stress relief heats the entire weldment uniformly to a temperature where the yield strength drops enough for the residual stresses to relax by creep. For carbon and low-alloy steels, the standard range is 580–650 °C (ASME VIII specifies minimums by P-number group). | P-Number | Material Group | Min. PWHT Temp (°C) | Hold Time | |----------|---------------|---------------------|----------| | P-1 | Carbon steel | 593 | 1 hr per 25 mm, 1 hr min | | P-3 | Low-alloy (0.5Cr) | 593 | 1 hr per 25 mm | | P-4 | Cr-Mo (1.25Cr-0.5Mo) | 704 | 1 hr per 25 mm | | P-5A | Cr-Mo (2.25Cr-1Mo) | 704 | 1 hr per 25 mm | Heating and cooling rates must be controlled to prevent thermal shock and new residual stresses. ASME VIII limits heating and cooling rates above 315 °C to 55 °C/hr per 25 mm of thickness (max 220 °C/hr), with a minimum of 55 °C/hr. ### Tempering For quench-and-temper steels, PWHT doubles as a tempering treatment. The temperature must be at or below the original tempering temperature to avoid softening the base metal. For example, AISI 4140 tempered at 540 °C should receive PWHT at no higher than 540 °C. ### Normalizing In rare cases, a full normalizing treatment (heating above Ac3, air cooling) is applied to refine the grain structure of a weldment. This is sometimes specified for heavy carbon steel forgings after repair welding. It resets the entire microstructure but requires re-tempering afterward. ### Hydrogen Bake-Out A lower-temperature hold (200–250 °C for 2–4 hours) immediately after welding allows hydrogen to diffuse out before the joint cools through the cracking-susceptible range. This is not a stress relief but is often specified alongside it for high-strength steels (AISI 4340, HY-80/100). ## When PWHT Is Mandatory Codes mandate PWHT based on material, thickness, and service conditions: - **ASME VIII Div. 1**: Required for P-1 carbon steel above 38 mm thickness, all P-4 and P-5 Cr-Mo steels regardless of thickness, and all vessels in lethal service, caustic service, or hydrogen service. - **AWS D1.1**: Generally does not require PWHT for structural steel but allows the engineer to specify it for fracture-critical applications. - **EN 13445**: PWHT requirements by material group and thickness, generally aligned with ASME but with differences in temperature and hold time. ## Execution Methods **Furnace PWHT**: The entire weldment is placed in a furnace. This provides the most uniform heating and is the preferred method. Temperature uniformity surveys (typically ±15 °C within the soak band) are required. **Local PWHT**: When the component is too large for a furnace, electric resistance heating pads or induction coils are wrapped around the weld zone. The heated band must extend at least 2√(Rt) on each side of the weld (R = radius, t = thickness) to prevent harmful thermal gradients. Insulation must extend further to control cooling rate. **Internal firing**: Large vessels can be PWHT'd by placing gas burners inside. Temperature control is difficult and temperature surveys at multiple points are essential. ## PWHT Risks and Mitigation **Over-tempering**: Holding too long or at too high a temperature softens the base metal below its minimum specified strength. Always verify that the PWHT temperature is at or below the base metal tempering temperature. **Reheat cracking**: Some Cr-Mo-V steels (P-5, 2.25Cr-1Mo-0.25V) are susceptible to intergranular cracking during PWHT at 600–700 °C due to creep-induced grain boundary cavitation. Prevention: control heating rate through the susceptible range, ensure weld toes are ground smooth to reduce stress concentration. **Distortion**: Uneven heating during local PWHT causes bowing and warping. Proper insulation, sufficient heated band width, and symmetric heater placement minimize this. ## Documentation PWHT records include: time-temperature chart from calibrated recorders, thermocouple locations (weld, HAZ, and base metal), heating and cooling rates, soak temperature uniformity, and hold time. These records are part of the permanent fabrication documentation and are reviewed by the Authorized Inspector.