AWS Certified Welding Inspector — FundamentalsWelding MetallurgyMedium
A metallurgist is designing a new alloy steel for high-temperature service. The alloy needs to maintain strength and creep resistance at elevated temperatures. Which of the following alloying elements is most effective in promoting these properties by forming stable carbides?
- AMolybdenum (Mo)
- BChromium (Cr)
- CManganese (Mn)
- DNickel (Ni)
Show answer & explanationAnswer & explanation
Correct answer: A. Molybdenum (Mo)
Molybdenum is a strong carbide former in alloy steels and is highly effective in improving high-temperature strength and creep resistance. It forms stable carbides that resist coarsening and dissolution at elevated temperatures, which helps to pin grain boundaries and strengthen the matrix, thereby preventing deformation under sustained stress at high temperatures.
Why the other options are wrong
- B. Chromium contributes to hardenability, corrosion resistance, and some high-temperature strength by forming carbides, but molybdenum is generally more effective for creep resistance.
- C. Manganese improves strength and hardenability, but its carbides are less stable at high temperatures compared to molybdenum's.
- D. Nickel primarily improves toughness and corrosion resistance, and is an austenite former, not a strong carbide former for high-temperature strength.
Alloying for Creep Resistance
The addition of specific elements to steel to improve its resistance to permanent deformation (creep) when subjected to prolonged stress at elevated temperatures.
- Involves elements that form stable carbides or intermetallic compounds.
- These precipitates resist coarsening and dissolution at high temperatures.
- Carbides pin grain boundaries, preventing grain boundary sliding.
- Elements like Mo, V, Cr, W are effective for this purpose.
Memory trick: Mo's carbides are the anchors that hold strength at scorching heights.