A welding inspector is evaluating a welding procedure for a pressure-retaining component made from 2.25Cr-1Mo low-alloy steel. This material is known for its susceptibility to temper embrittlement when exposed to certain temperature ranges for extended periods. Which of the following elements, if present in higher concentrations, would most significantly increase the susceptibility of this steel to temper embrittlement?
- AMolybdenum (Mo)
- BPhosphorus (P)
- CNickel (Ni)
- DChromium (Cr)
Show answer & explanationAnswer & explanation
Correct answer: B. Phosphorus (P)
Temper embrittlement in low-alloy steels (like 2.25Cr-1Mo) is a form of embrittlement that occurs when the steel is held or slowly cooled through a specific temperature range (typically 750-1100°F or 400-600°C). It is caused by the segregation of certain impurity elements to grain boundaries, with phosphorus (P) being the most potent and common contributor, followed by tin (Sn), antimony (Sb), and arsenic (As).
Why the other options are wrong
- A. Molybdenum is actually added to steels like 2.25Cr-1Mo specifically to *reduce* susceptibility to temper embrittlement by tying up phosphorus and other impurities, not to increase it.
- C. Nickel is an alloying element, but it is not a primary segregating impurity element that causes temper embrittlement; rather, it can sometimes exacerbate the effect of other impurities if present in high amounts.
- D. Chromium is an alloying element that contributes to strength and corrosion resistance but is not a primary impurity element that directly causes temper embrittlement via grain boundary segregation.
Temper Embrittlement
A phenomenon in certain alloy steels where toughness is severely reduced when the material is exposed to elevated temperatures (typically 750-1100°F or 400-600°C) for extended periods. It is caused by the segregation of impurity elements to grain boundaries.
- Occurs in the temper embrittlement range, often during PWHT or service.
- Leads to intergranular fracture and increased ductile-to-brittle transition temperature.
- Caused by impurity elements like P, Sn, Sb, As.
- Molybdenum additions are used to mitigate it.
Memory trick: Temper Embrittlement: Phosphorous is the primary poison at the grain boundary party.