A welder is assigned to weld a critical component made of 4140 alloy steel, which is known for its high hardenability. To prevent cracking, a preheat of 250 °C is applied, and the component is immediately moved to a Post-Weld Heat Treatment (PWHT) furnace at 300 °C after welding. What is the purpose of this immediate transfer and holding at 300 °C before ramping up to the full PWHT temperature?

Question: A welder is assigned to weld a critical component made of 4140 alloy steel, which is known for its high hardenability. To prevent cracking, a preheat of 250 °C is applied, and the component is immediately moved to a Post-Weld Heat Treatment (PWHT) furnace at 300 °C after welding. What is the purpose of this immediate transfer and holding at 300 °C before ramping up to the full PWHT temperature?

Answer options:

• To quickly cool the weld to refine the grain structure. ✅ To prevent hydrogen-induced cracking by allowing hydrogen to diffuse out.
• To increase the hardness of the heat-affected zone rapidly.
• To ensure the weld metal solidifies quickly to minimize distortion.

Correct answer: To prevent hydrogen-induced cracking by allowing hydrogen to diffuse out.

Explanation: After welding high-hardenability steels like 4140, rapid cooling can trap hydrogen in the microstructure, leading to delayed hydrogen-induced cracking. Holding the component at an elevated temperature (often between 250-350 °C, or an intermediate temperature soak) immediately after welding, and before full PWHT, allows hydrogen to diffuse out of the weld and HAZ, significantly reducing the risk of cracking. This is often referred to as a dehydrogenation bake.