As per API RP 571 Section 5.3.2.2 (Sigma Phase Embrittlement):
“Sigma phase embrittlement occurs in austenitic stainless steels (like 300 series) and duplex stainless steels exposed to 1000°F to 1800°F (540°C to 980°C)... This embrittlement results in a significant loss of toughness and ductility.”
Sigma phase is not a concern in Monel (nickel-copper alloy) or 5Cr steels.
400 series are ferritic and not typically affected by sigma phase.
Hence, Option A (300 series stainless steel) is correct.
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