Stainless steel is engineering material with good corrosion-resistance, strength and fabrication characteristics. They can readily meet a wide range of design criteria, including load, service life and low maintenance. Selecting the proper stainless steel grade involves weighing four qualities in the following order of importance:
- Corrosion or Heat Resistance, the primary reason for specifying stainless. The specifier needs to know the nature of the environment and the degree of corrosion or heat resistance required.
- Mechanical Properties, particularly strength at room, elevated or low temperature. The combination of corrosion resistance and strength is the basis for selection.
- Fabrication Operations and how the product will be made (e.g., forging, machining, forming, welding, stamping, roll forming, four-slide operations).
- Total Cost, including material and production costs and considering the cumulative savings of a maintenance-free product with longevity.
The corrosion, heat resistance and mechanical properties are all affected by the chemical composition of the stainless steel. As the composition of the steel is varied, so are the properties. The major alloying elements of stainless steel include:
Chromium
- Forms a passive surface film to make stainless steel resistant to corrosion.
- Increases the scaling resistance, tensile strength and wear resistance.
Manganese
- Improves hot-working properties.
- Up to 2% has no effect on strength, ductility and toughness.
- Above 2% increases yield strength and tensile strength (as in the 201 grade).
- Important as a partial replacement of nickel in the 201 grade.
- Stabilizes the austenitic structure.
Molybdenum
- Increases creep resistance; strength at high temperatures; and corrosion resistance, particularly in sulfite, sulfate, acetic acid and acetate solutions and in a salt-water atmosphere.
- Expands range of passivity and counteracts tendency to pit.
Nickel
- Stabilizes the austenitic structure.
- Increases high-temperature strength; ductility, which makes stainless steel easier to form; and corrosion resistance, particularly in industrial and marine atmospheres and the chemical-, food- and textile-processing industries.
Silicon
- Increases scaling resistance by forming a tight initial scale that will withstand cyclic temperature changes; also slightly increases tensile strength and hardness.
- Resists carburizing at high temperatures.
- When 1% or more, improves resistance to strong sulfuric acid but offers little improvement for dilution and is unfavorable in nitric-acid service.
*Note: Ductility decreases as silicon content increases.
Sulfur – Phosphorous – Selenium
- Increases machinability.
- Decreases ductility and transverse tensile strength.
Titanium – Columbium – Tantalum
- Prevents intergranular corrosion by stabilizing the carbon as titanium or columbium carbides.
- Produces finer grain size.
- Reduces stretcher strains from drawing and forming by their addition to type 430.
Stainless Steel Grade Comparison
For a comparison of chemical and mechanical properties of common austenitic and ferritic grades of stainless steel according to ASTM A240, see our website: www.yaang.com
