Characteristics of Stainless Steels
The characteristics of the broad group of stainless steels can be viewed as compared to the more familiar plain carbon "mild" steels. As a generalisation the stainless steels have:
- Higher work hardening rate
- Higher ductility
- Higher strength and hardness
- Higher hot strength
- Higher corrosion resistance
- Lower magnetic response (austenitic only)
- Must retain corrosion resistant surface in the finished product
These properties apply particularly to the austenitic family and to varying degrees to other grades and families. These properties have implications for the likely fields of application for stainless steels, but also influence the choice of fabrication methods and equipment.
Austenitic Stainless Steels
This group contains at least 16% chromium and 6% nickel (the basic grade 304 is referred to as 18/8) and range through to the high alloy or "super austenitics" such as 904L and 6% molybdenum grades. Additional elements can be added such as molybdenum, titanium or copper, to modify or improve their properties, making them suitable for many critical applications involving high temperature as well as corrosion resistance. This group of steels is also suitable for cryogenic applications because the effect of the nickel content in making the steel austenitic avoids the problems of brittleness at low temperatures, which is a characteristic of other types of steel.
Ferritic Stainless Steels
These are plain chromium (10.5 to 18%) grades such as Grade 430 and 409. Their moderate corrosion resistance and poor fabrication properties are improved in the higher alloyed grades such as Grades 434 and 444 and in the proprietary grade 3CR12.
Martensitic Stainless Steels
Martensitic stainless steels are also based on the addition of chromium as the major alloying element but with a higher carbon and generally lower chromium content (e.g. 12% in Grade 410 and 416) than the ferritic types; Grade 431 has a chromium content of about 16%, but the microstructure is still martensite despite this high chromium level because this grade also contains 2% nickel.
International standards conversion table of DIN, AISI and GOST
|Stainless steel standards||Chemical composition, %|