Metals and alloys
Which correctly describes the properties of the 18%Cr-8%Ni austenitic stainless steel SUS304 (JIS; equivalent to AISI 304)?
- AIts high Cr content makes it a martensitic stainless steel, so it hardens greatly on quenching in oil or water
- BDespite its Ni content it is a BCC ferritic steel, strongly attracted to a magnet even in the annealed condition
- CIt is FCC austenite, does not harden on quenching, and is nearly nonmagnetic in the annealed condition
Show answer and explanation
Answer: C. It is FCC austenite, does not harden on quenching, and is nearly nonmagnetic in the annealed condition
SUS304 contains about 8% Ni, which stabilizes austenite, so it keeps an FCC austenitic structure even at room temperature. Because it does not transform on cooling, it does not harden by quenching; it is strengthened by cold working instead. In the annealed condition it is nearly nonmagnetic, but heavy cold working converts part of it to strain-induced martensite, and it becomes attracted to a magnet. Being FCC, it shows no ductile-to-brittle transition at low temperatures and can be used for cryogenic vessels. Steels that harden on quenching are martensitic grades such as SUS420J2 (similar to AISI 420), with about 13% Cr and higher carbon. Ferritic grades such as SUS430 (AISI 430) contain almost no Ni and are BCC and strongly magnetic.