Introduction Additions of chromium (in excess of 12%) to steel compositions, gives rise to a stable surface film of chromium oxide, the stability of the film increasing with increasing chromium content. This oxide film confers corrosion resistance and is the basis on which the stainless steel family is built. Ferritic Stainless Steels Low carbon steels containing 12 to 30% chromium are the ferritic stainless steels (e.g. 430, 409) which are not heat treatable. Increases in mechanical properties can only be achieved by cold working. The corrosion resistance of this group is significantly better than the high carbon high chrome steels.
Martensitic Stainless SteelsHigh carbon high chrome steels are heat treatable as a consequence of the higher carbon content, and are known as martensitic stainless steels (e.g. 410, 416). They do, however, exhibit lower corrosion resistance due to chromium depletion of the oxide film. They exhibit good strength and oxidation resistance up to 750°C, although their creep strength above 600°C is poor.
Austenitic Stainless SteelsAustenitic stainless steels (e.g. 302, 316) result from additions of nickel (usually between 10 to 20%) to low carbon steels containing 18 to 25% chrome. These steels exhibit superior corrosion resistance in a wide range of environments. The properties can only be modified by cold work. They are also significantly more expensive than the straight chromium grades. When mention is made of 'stainless steel', it is generally these non-magnetic steels that are being referred to. While the thermal expansion of these steels is similar to that of copper, their thermal conductivity is less than that of alumina at room temperature.
Precipitation Hardened Stainless SteelsPrecipitation hardened stainless steels (e.g. 17-4 PH, PH 13-8 Mo) are chromium-nickel alloys containing precipitation hardening elements such as copper, aluminium or titanium. The alloys are of two general types; semi austenitic, requiring a dual heat treatment to achieve final strength properties and martensitic, requiring a single heat treatment to achieve final strength properties. The main advantage of these alloys is the low temperature heat treatment required to achieve final strength, which can be as high as 2 GPa, resulting in minimal scaling and distortion, thus enabling parts to be finished machined prior to final heat treatment.
It should be noted that chromium has a tendency to migrate to grain boundaries at elevated temperatures where it forms chromium carbide. This is a serious problem in the heat affected zones of welds. This effect is known as 'weld decay' and causes failure due to corrosion along grain boundaries where there is a depletion of chromium. For welding, a carbon content <0.03% is specified to avoid significant carbide formation. Alternatively, the steel can be 'stabilised' with the addition of titanium or niobium which form carbides in preference to chromium.
Although stainless steels are more corrosion resistant than other steels, they are subject to specific corrosion mechanisms, such as weld decay. Advice must be sought for particular applications.