A welding inspector is reviewing a procedure for welding a duplex stainless steel pipe. The procedure specifies careful control of heat input and interpass temperature. Failure to adequately control these parameters can lead to an imbalance in the ferrite-austenite ratio and the formation of undesirable intermetallic phases, significantly impacting the material's ductility and corrosion resistance. Which intermetallic phase is a primary concern for duplex stainless steels when exposed to elevated temperatures, particularly between 1200-1700°F (650-925°C)?
- AMartensite
- BSigma phase
- CGraphite
- DCarbides
Show answer & explanationAnswer & explanation
Correct answer: B. Sigma phase
Duplex stainless steels are designed with a balanced ferrite-austenite microstructure. However, when exposed to elevated temperatures, particularly in the range of 1200-1700°F (650-925°C), they are highly susceptible to the formation of undesirable intermetallic phases, primarily sigma phase. Sigma phase is a brittle, chromium and molybdenum-rich phase that severely reduces ductility, toughness, and corrosion resistance.
Why the other options are wrong
- A. Martensite forms in carbon and low-alloy steels upon rapid cooling, not typically in duplex stainless steels, which are generally non-martensitic.
- C. Graphite formation is a concern in cast irons and some carbon steels, not in duplex stainless steels.
- D. Carbides can form in stainless steels (e.g., sensitization), but sigma phase is a distinct and often more detrimental intermetallic phase in duplex stainless steels at these temperatures.
Sigma Phase Embrittlement
A form of embrittlement occurring in duplex and other stainless steels when exposed to temperatures typically between 1200-1700°F (650-925°C). It involves the formation of a brittle, chromium and molybdenum-rich intermetallic phase (sigma, σ), leading to severe loss of ductility, toughness, and corrosion resistance.
- Forms from ferrite in duplex stainless steels.
- Time-temperature dependent; longer exposure or higher temperatures increase formation.
- Causes a significant increase in hardness and decrease in impact strength.
- Mitigated by controlling heat input, interpass temperature, and rapid cooling after welding.
Memory trick: Duplex steels get Sigma-sad when too hot, losing their balance.