Although the corrosion resistance of stainless comes from the presence of Chromium, other elements are added to enhance other properties. These elements alter the microstructure of the steel.
Stainless steels are grouped into families based on their metallurgical microstructure. The microstructure may be composed of the stable phases austenite or ferrite, a “duplex” mix of these two, martensite or a hardened structure containing precipitated micro-constituents.
Austenitic Stainless Steels
Austenitic stainless steels contain a minimum of 16% chromium and 6% nickel. They range from basic grades like 304 through to super austenitics such as 904L and 6% Molybdenum grades.
By adding elements such as Molybdenum, Titanium or Copper, the properties of the steel can be modified. These modifications can make the steel suited to high temperature applications or increase corrosion resistance. Most steels become brittle at low temperatures but the Nickel in austenitic stainless makes it suited to low temperature or cryogenic applications.
Austenitic stainless steels are generally non-magnetic. They are not able to be hardened by heat treatment. Austenitic stainless steels rapidly work-harden with cold working. Although they work harden, they are the most readily formed of the stainless steels.
The principal alloying elements are sometimes reflected in the name of the steel. A common name for 304 stainless steel is 18/8, for 18% chromium and 8% nickel.
Austenitic Stainless Applications
Applications for austenitic stainless steels include:
~ Kitchen sinks
~ Architectural applications such as roofing and cladding
~ Roofing and gutters
~ Doors and Windows
~ Benches and food preparation areas
~ Food processing equipment
~ Heat exchangers
~ Chemical tanks
Ferritic Stainless Steels
Ferritic stainless steels include grades like 430 and contain only chromium as a major alloying element. The quantity of chromium present ranges from 10.5 to 18%.
They are known for their moderate corrosion resistance and poor fabrication properties. Fabrication properties can be improved by alloy modifications and are satisfactory in grades such as 434and 444. Ferritic stainless steels cannot be hardened by heat treatment and are always used in the annealed condition.
Ferritic stainless steels are magnetic. They are also not susceptible to stress corrosion cracking. Weldability is acceptable in thin sections but decreases as section thicknesses increase.
Ferritic Stainless Applications
Ferritic stainless steels are typically used in:
~ Vehicle exhausts
~ Fuel lines
~ Cooking utensils
~ Architectural trim
~ Domestic appliances
Martensitic Stainless Steels
High carbon and lower chromium content are the distinguishing features of martensitic stainless steels when compared with ferritic stainless.
Martensitic stainless steels include 410 and 416. Hardened martensitic steels cannot be successfully cold formed. They are magnetic, have moderate corrosion resistance and poor weldability.
Martensitic Stainless Applications
Martensitic stainless steels are typically used for:
~ Knife blades
~ Surgical instruments
Precipitation Hardening Grades
Precipitation hardening grades contain both Chromium and Nickel. They develop very high tensile strengths with heat treatment. Precipitation hardening grades are usually supplied in a “solution treated” condition that allows the steel to be machined. After machining or forming, the steel can be aged in a low temperature heat treatment process. As the heat treatment is performed at low temperatures, no distortion is induced in the work piece.