Welder Red Seal · Question
An experienced welder is tasked with welding a thick-section (over 50 mm) 4140 alloy steel pipe. Due to its high hardenability, post-weld heat treatment (PWHT) is specified. What is the primary purpose of this PWHT for 4140 alloy steel?
For alloy steels like 4140, PWHT (typically stress relief or tempering) is performed to reduce high residual stresses developed during welding, temper the hard,
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Question: An experienced welder is tasked with welding a thick-section (over 50 mm) 4140 alloy steel pipe. Due to its high hardenability, post-weld heat treatment (PWHT) is specified. What is the primary purpose of this PWHT for 4140 alloy steel?
Answer options:
- To induce precipitation hardening for increased strength ✅ To relieve residual stresses and improve toughness and ductility
- To achieve a finer grain structure in the weld metal
- To prevent hydrogen embrittlement during welding
Correct answer: To relieve residual stresses and improve toughness and ductility
Explanation: For alloy steels like 4140, PWHT (typically stress relief or tempering) is performed to reduce high residual stresses developed during welding, temper the hard, brittle microstructures formed in the HAZ (like martensite), and improve the overall toughness and ductility of the weld and HAZ. While preheat helps prevent hydrogen embrittlement during welding, PWHT addresses post-weld properties.
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