Types of Stainless Steel

the main categories of stainless steels are:

a) austenitic (200 and 300 series) stainless steels

b) ferritic (400 series) stainless steels

c) martensitic (400 and 500 series) stainless steels

d) precipitation-hardening (ph) stainless steels

e) duplex alloys/stainless steel

a) austenitic (200 and 300 series)  stainless steels

austenitic stainless steels are the most common stainless steels containing at least 16% chromium making it suitable for the best corrosion resistance. they contain an austenitic microstructure, which is a face-centered cubic crystal structure. their microstructure is derived from the addition of nickel, manganese and nitrogen.  they cannot be hardened by heat treatment since they possess the same microstructure at all temperatures. moreover, their austenitic microstructure gives them excellent formability and weldability.

austenitic stainless steels can be further subdivided into two sub-groups, 200 series and 300 series:

200 series are chromium-manganese-nickel alloys which maximize the use of manganese and nitrogen to minimize the use of nickel.  200 series posses 50% higher yield strength than 300 series stainless steel due to the addition of nitrogen.

for example: type 201 is hardenable through cold working; type 202 is a general purpose stainless steel.

corrosion resistance will be decreased when nickel content is reduced and manganese content is increased.

300 series are chromium-nickel alloys, and they attain microstructure exclusively by nickel alloying. this series is the most widely used series and is the largest group.

the best known grade is type 304, also known as 18/8 and 18/10 for its composition of 18% chromium and 8%/10% nickel, respectively. the second most common austenitic stainless steel is type 316. the addition of 2% molybdenum provides greater resistance to acids and to localized corrosion caused by chloride ions.

b) ferritic (400 series) stainless steels

ferritic (400 series) stainless steels have 10.5 to 27% chromium and no considerable nickel content, reducing their corrosion resistance. they are considered best for high-temperature instead of high-strength applications.

they are preferred for their resistance to stress corrosion cracking. ferritic stainless steels are magnetic and their grades include 430 and 434.

c) martensitic (400 and 500 series) stainless steels

martensitic (400 and 500 series) stainless steels have the highest hardness. these are magnetic and can be hardened by a combination of cold work and heat treatment.

the martensitic alloys contain 12 to 14% chromium, 0.2 to 1% molybdenum, and no significant amount of nickel. they have lower corrosion resistance than austenitic or ferritic alloys, but are considered hard, strong, slightly brittle, and hardenable by heat treatment.

d) precipitation-hardening (ph) stainless steels

ph stainless steels contain around 17% chromium and 4% nickel. the heat treatment strengthens ph steels to levels higher than martensitic alloys. these steels can develop very high strength by adding elements such as copper, niobium and aluminium to the steel.

e) duplex alloys/stainless steel

duplex alloys have a microstructure which is approximately 50 ferritic and 50% austenitic. they are resistant to stress corrosion cracking.

as their name indicates, duplex stainless steels are a combination of two of the main alloy types. the alloys’ mixture of 19 to 28% chromium, 0 to 5% molybdenum, and 5 to 7% nickel results in a mixed austenitic and ferritic microstructure.