AWS Certified Welding Inspector — FundamentalsWelding MetallurgyMedium
A welding inspector is reviewing a procedure for welding a Cr-Mo steel pipe that will operate at elevated temperatures. The procedure specifies a post-weld heat treatment (PWHT) to be performed after welding. What is the primary metallurgical reason for performing PWHT on Cr-Mo steels?
- ATo refine the grain structure in the heat-affected zone (HAZ).
- BTo dissolve any untempered martensite formed during welding.
- CTo increase the hardness of the weld metal for wear resistance.
- DTo reduce residual stresses and improve ductility.
Show answer & explanationAnswer & explanation
Correct answer: D. To reduce residual stresses and improve ductility.
The primary purpose of PWHT for Cr-Mo steels is to reduce residual stresses induced by welding and to temper any hard, brittle microstructures (like martensite) that may have formed in the HAZ, thereby improving the overall ductility and toughness of the weldment.
Why the other options are wrong
- A. PWHT does not primarily refine grain structure; that's more associated with normalizing or specific thermal cycles during welding.
- B. While PWHT does temper martensite, its primary role is broader: reducing stresses and improving overall ductility/toughness.
- C. PWHT generally reduces hardness by tempering, not increases it, unless specific aging mechanisms are at play in certain alloys, which is not the primary purpose for Cr-Mo steels.
PWHT for Cr-Mo Steels
Post-weld heat treatment (PWHT) applied to Chromium-Molybdenum (Cr-Mo) steels, typically involving heating to a specific temperature and holding, to mitigate adverse metallurgical effects of welding.
- Primarily reduces welding-induced residual stresses.
- Tempering of hard, brittle microstructures (e.g., martensite) in HAZ and weld metal.
- Improves ductility and toughness, reducing risk of brittle fracture.
- Essential for high-temperature service to prevent creep and stress corrosion cracking.
Memory trick: PWHT: Peace, Warmth, Harmony for Welds.