An alloy steel containing roughly 11% Cr or more, made rust-resistant by a passive surface film. By microstructure it is divided into austenitic, ferritic, martensitic and other grades.
Japanese name: ステンレス鋼
Stainless steel appears in 7 practice questions across 5 topics, including Metals and alloys, Corrosion and surface treatment, Materials processing and joining.
Field: Metals and alloys
Stainless steel resists rusting because a passive film consisting mainly of Cr oxide, a few nanometers thick, forms on the surface and prevents the metal from dissolving. Above roughly 11% Cr the film forms stably, and more Cr gives better corrosion resistance. In environments containing oxygen the film spontaneously re-forms even when scratched, but in chloride-rich environments it is broken at weak points, causing pitting and crevice corrosion. For seawater environments, Mo-bearing grades such as SUS316 (AISI 316) are chosen. Note that stainless steel only resists rust; it does not stay rust-free in every environment.
The representative grade SUS304 (18%Cr-8%Ni, JIS; AISI 304) is austenitic, with an FCC structure even at room temperature because Ni stabilizes austenite; it has good ductility and weldability and is nearly nonmagnetic in the annealed condition. Grades such as SUS430 (AISI 430), with almost no Ni, are BCC ferritic, magnetic and relatively inexpensive. Grades such as SUS410 (AISI 410), with about 13% Cr and higher carbon, are martensitic and harden on quenching, so they are used for cutlery and blades. In addition, duplex grades, roughly half austenite and half ferrite, have excellent strength and resistance to stress corrosion cracking.
When austenitic stainless steel is heated to about 500–800 °C, Cr carbides precipitate at grain boundaries, depleting the surrounding Cr and weakening passivity. This is called sensitization and causes intergranular corrosion in weld heat-affected zones. Countermeasures include low-carbon SUS304L (AISI 304L) and grades in which carbon is tied up as Ti or Nb carbides. Austenitic grades are also susceptible to stress corrosion cracking in hot chloride environments, and countermeasures include removing residual stress or switching to a duplex grade.
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 SUS410 (AISI 410), with about 13% Cr and higher carbon. Ferritic grades such as SUS430 (AISI 430) contain almost no Ni and are BCC and strongly magnetic.
Answer: C. It contains about 11% or more Cr, forming a passive film a few nanometers thick that re-forms by itself
Because stainless steel contains roughly 11% Cr or more (10.5% by the standard definition), a passive film consisting mainly of Cr oxide, only a few nanometers thick, forms on the surface and prevents further dissolution of the metal. What matters is that in environments containing oxygen the film spontaneously re-forms even when scratched. Passivity is therefore readily maintained in oxidizing environments such as nitric acid, but breaks down easily in oxygen-poor environments, chloride-rich environments and reducing acids (such as hydrochloric acid). The potential of the alloy as a whole does not become as noble as gold or platinum; the main role of Ni is to stabilize the austenitic structure. A thick oxide scale several millimeters thick forms by high-temperature oxidation, cracks and spalls easily, and is entirely different from a passive film.
Ferritic and martensitic grades are attracted to magnets. Even nonmagnetic austenitic SUS304 becomes attracted to a magnet when heavily cold worked, because part of it turns into strain-induced martensite.
SUS316 (AISI 316) contains about 2–3% Mo, making it more resistant to pitting and crevice corrosion by chloride ions. In seawater or salty environments, SUS316 and similar grades are chosen.
Martensitic grades harden on quenching, but austenitic and ferritic grades do not transform on cooling and so do not harden by quenching. Austenitic grades are strengthened by cold working.
Because stainless steel contains roughly 11% Cr or more (10.5% by the standard definition), a passive film consisting mainly of Cr oxide, only a few nanometers thick, forms on the surface and prevents further dissolution of the metal.
What is the main reason stainless steel resists rusting in …Austenitic stainless steels do not become martensite on quenching, and Cr evaporation is not the main cause either.
When austenitic stainless steels such as SUS304 are welded,…FCC austenitic stainless steels and aluminum alloys have many slip systems and dislocations remain mobile at low temperatures, so they show no distinct transition and are used for cryogenic applications such as liquefied gas tanks.
Which material shows a distinct ductile-to-brittle transiti…FCC copper and austenitic stainless steels are typical examples with large n.
For annealed copper, the relation between true stress σ and…Classic examples are austenitic stainless steels in hot chloride environments and brass in ammonia-containing environments (season cracking).
Which statement about stress corrosion cracking is correct?Martensite Passivity Heat-affected zone Sensitization Dislocation Stress corrosion cracking Brass Equilibrium oxygen partial pressure Activity Columnar grains