Stainless steel

Stainless steel (technically corrosion-resistant steel, in shop talk often just "stainless") is steel with at least 10.5 % chromium and at most 1.2 % carbon, as defined in EN 10088-1. The chromium forms a thin passive layer on the surface that protects the steel against corrosion and re-forms after mechanical damage as long as oxygen is present. Nickel improves toughness and formability, and molybdenum improves resistance to pitting corrosion in chloride environments.

Austenitic grades are the most widespread: 304 (1.4301, X5CrNi18-10) for ordinary environments and 316L (1.4404, X2CrNiMo17-12-2) with molybdenum for chloride environments, food processing and chemical plants. They weld and form well, are non-magnetic in the annealed condition and cannot be hardened by heat treatment. Ferritic grades (for example 430, 1.4016) are cheaper and magnetic but less corrosion resistant. Martensitic grades of the 420 type (1.4021, 1.4034) can be hardened and are used for knives, surgical instruments and shafts. Duplex grades (1.4462, known in the US as 2205) and precipitation-hardening grades (for example 1.4542, known as 17-4PH) combine higher strength with good corrosion resistance.

Austenitic grades have a density of about 7.9 g/cm³, a modulus of elasticity around 200 GPa, a tensile strength typically of 500 to 750 MPa and a minimum yield strength in the annealed condition of 190 to 230 MPa. Duplex grades have roughly twice the yield strength. The thermal conductivity of austenitic grades is only about 15 W/(m·K), around a third of that of carbon steel, and their thermal expansion is roughly a third higher. Both show up in machining and in distortion after welding.

Choose stainless steel where a part is exposed to water, moisture, food, cleaning and disinfecting agents or the outdoors, and a coating on carbon steel would not be enough or would wear off: food processing and pharmaceutical machinery, medical and laboratory equipment, chemical plant components, railings, hardware and fasteners. For indoor and mildly humid environments, 304 is usually sufficient; for salt, chlorides or aggressive sanitation, choose 316L.

Stainless fasteners are marked A2 (the group corresponding to 304) or A4 (the 316 group with molybdenum) together with a property class, for example A2-70 or A4-80 according to ISO 3506. In the US they are more often specified simply as 18-8 (the 304 family) or 316.

Stainless steel only stays stainless under suitable conditions. In chloride environments (sea air, road salt, swimming pools, some cleaning agents), 304 suffers pitting and crevice corrosion. Resistance is also reduced by heat tint left after welding, which has to be removed by pickling and passivation, and by carbon steel particles carried over by tools, grinding wheels or storage, which turn into rust spots. For welded parts, choose low-carbon L grades such as 304L (1.4307) and 316L (1.4404) or titanium-stabilized grades such as 321 (1.4541) and 316Ti (1.4571).

Austenitic grades work-harden during machining and conduct heat poorly, so machining is slower and more expensive than for carbon steel or aluminum. The free-machining 303 (1.4305) contains sulfur and therefore has poorer corrosion resistance and weldability. Stainless bolts in stainless nuts tend to gall; an anti-seize compound helps. A magnet is not a reliable test: austenitic steel becomes partly magnetic after cold working. In your request for quote, state the material number, for sheet also the surface finish (for example 2B or a brushed No. 4 finish), and for critical parts ask for a 3.1 inspection certificate to EN 10204 (the European counterpart of a mill test report).