An inspector is examining a weld on a quenched and tempered (Q&T) alloy steel component. The welding procedure specifies a maximum interpass temperature of 400°F (204°C). Exceeding this temperature during welding could potentially lead to which of the following metallurgical issues in the heat-affected zone (HAZ)?
- AFormation of excessive amounts of delta ferrite.
- BRe-austenitizing and subsequent embrittlement upon cooling.
- CIncreased susceptibility to cold cracking.
- DOver-tempering, leading to a reduction in strength and hardness.
Show answer & explanationAnswer & explanation
Correct answer: D. Over-tempering, leading to a reduction in strength and hardness.
Quenched and tempered steels derive their strength and toughness from a specific martensitic microstructure that has been tempered to achieve the desired properties. Exceeding the maximum interpass temperature can effectively re-temper the previously heat-treated HAZ, leading to over-tempering. This process reduces the strength and hardness of the HAZ, potentially compromising the component's mechanical properties.
Why the other options are wrong
- A. Delta ferrite formation is typically a concern in stainless steels, particularly duplex stainless steels, and is not a primary concern for Q&T alloy steels in relation to interpass temperature limits.
- B. Re-austenitizing would require heating above the critical temperature, which is generally higher than typical interpass temperatures. While some localized re-austenitizing could occur, the primary concern with exceeding interpass limits in Q&T steels is over-tempering.
- C. Increased susceptibility to cold cracking is more associated with rapid cooling rates and hydrogen, not excessive interpass temperature in Q&T steels.
Over-Tempering
A condition where a quenched and tempered (Q&T) steel is exposed to temperatures higher or for longer durations than intended during a tempering process, or during subsequent thermal cycles like welding. It results in a loss of designed strength and hardness.
- Occurs when tempering temperature is too high or time too long.
- Leads to coarsening of carbides and decomposition of martensite.
- Reduces tensile strength, yield strength, and hardness.
- Can happen in the HAZ of Q&T steels if interpass temperatures are exceeded.
Memory trick: Too much heat on Q&T steel means over-tempering, making it weak.