These are stain less as these have a minimum of 11 to 12% Cr which is having more affinity for oxygen than iron, forms a very thin, continuous protective and stable oxide (Cr2O3) film on the surface. Thus Cr imparts corrosion and oxidation resistance and pleasing appearance. Ni. Mo and Mn enhance strength and improve corrosion resistance. Based on amount and type of alloying elements these steels are classified into 5 types.
Ferritic: (% Cr – 17 × %C) > 12.7 i.e usually 17 to 26 % Cr. These alloys are ferritic in structure up to the melting point. These can’t be heat treated. Cold working is the only strengthening mechanism. These may contain small additions of Mn, Si, Ni, Al, Mo and Ti. Carbon is kept very low to increase toughness to avoid sensitization. Mo is added to improve resistance to pitting corrosion; Nb and Ta are added to stabilize against intergranular corrosion. These are not susceptible to stress corrosion cracking, thus used in chemical plants. These are used as heat-resisting elements in the making of furnace components. Less expensive, good machinability, higher thermal conductivity, low thermal expansion than austenitic. These steels show ductile to brittle transition. Ridging or roping, temper embrittlement, sigma phase and grain growth in HAZ are few problems associated with these steels.
Martensitic: (% Cr – 17 × %C) ≤ 12.7. Heat treatable steels (as these are austenitic at temps 950 - 1000°C). As Cr content is more, air cooling is enough to form martensite which is not brittle due to low carbon content. (1) Low carbon high strength martensitic types are developed to ensure good weldability, formability and impact toughness. Tempering temperature of 440 – 550°C should be avoided (temper embrittlement). These are used in petrochemical, chemical plants, compressors, discs, aircraft structural and engine parts, propeller shafts. (2) High carbon high hardness martensitic types (poor eldability, formability and ductility) are used in cutlery, surgical instruments, springs, high quality ball bearings, razor blades, cool hammers.
Austenitic: (17 – 18% Cr, 8 – 10% Ni, C < 0.03%) these contain sufficient amount of austenitic stabilizers Ni, Mn or N so that steels are austenitic even at RT. To avoid intergranular corrosion and weld decay ‘C’ should be less than 0.03%. Mo is added to improve pitting resistance and sulphuric acid corrosion. Nb and Ti are added to take care of weld decay. Excellent formability, high work hardening (low stacking fault energy), large uniform elongation, non-magnetic, no ductile to brittle transition (tough even at very low temperatures; cryogenic applications). These are not susceptible to temper embrittlement, but shows reduced ductility in temperature range 750 – 950°C due to brittle intermetallic formation (sigma phase). These are susceptible to stress corrosion cracking. These can be strengthened by cold working or solid solution strengthening. These are used in chemical industry, house hold, sanitary, biomedical, architectural, food industries, nuclear and marine applications.
Duplex: these are developed to utilize combination of properties of ferrite and austenite (Toughness, weldability with strength and localized corrosion resistance). These are stronger than austenitic steels. Presence of δ-ferrite cause grain-refinement of austenite hence increases strength. Further refinement is obtained by using controlled rolling at 900 - 950°C which results in very fine dispersion of ferrite and austenite grains. These steels exhibit superplasticity (500% elongation at 950°C). Good corrosion resistance similar to Austenitic type. Not susceptible to stress corrosion cracking and free from intergranular corrosion. These have ductile to brittle transition temperature. These suffer from temper embrittlement and sigma phase embrittlement.
Precipitation hardenable: these offer combination of properties but expensive and difficult to hot-process. These are used particularly for high temperature applications such as power-plant. Strengthening is because of precipitates like Ni3Al, Ni3Ti or Ni3Mo etc. on ageing. These are susceptible to hydrogen embrittlement. The matrix can be (1) martensitic (2) semi-austenitic (3) austenitic.