A welding procedure for a critical component specifies a post-weld heat treatment (PWHT) at 1150°F (620°C) for 2 hours. The primary metallurgical purpose of this PWHT is to reduce residual stresses and to temper the heat-affected zone (HAZ). Which of the following is an additional, significant benefit of this PWHT, especially regarding potential discontinuities?
- ATo refine the grain structure of the weld metal.
- BTo promote the diffusion of hydrogen out of the weldment.
- CTo increase the hardenability of the HAZ.
- DTo dissolve any untransformed austenite in the weld metal.
Show answer & explanationAnswer & explanation
Correct answer: B. To promote the diffusion of hydrogen out of the weldment.
In addition to stress relief and tempering, a significant benefit of post-weld heat treatment (PWHT) at elevated temperatures is to promote the diffusion of hydrogen out of the weldment. This is particularly important for preventing hydrogen-induced cracking (cold cracking), especially in susceptible steels. The elevated temperature increases hydrogen mobility, allowing it to escape before it can cause cracking.
Why the other options are wrong
- A. PWHT at this temperature and duration does not typically refine the grain structure; grain refinement occurs during phase transformations, usually through specific heating and cooling cycles like normalizing.
- C. PWHT typically reduces hardenability by tempering the microstructure, not increases it. Increased hardenability means a greater tendency to form hard, brittle phases.
- D. PWHT at 1150°F (620°C) is below the austenitizing temperature, so it would not dissolve untransformed austenite. It would primarily affect martensite and bainite.
Post-Weld Heat Treatment (PWHT)
A heat treatment applied to a weldment after welding to improve its properties. Common purposes include stress relief, tempering, and promoting hydrogen diffusion to prevent cracking.
- Reduces residual stresses, minimizing distortion and fatigue failure.
- Tempering effect on HAZ improves toughness and ductility.
- Facilitates hydrogen removal, preventing hydrogen-induced cracking.
- Can also alter microstructure and mechanical properties across the weldment.
Memory trick: PWHT: Relieves stress, tempers, and lets the H-bomb diffuse out.