The mechanical properties of stainless steels are almost always requirements of the product specifications used to purchase the product. For flat rolled products the properties usually specified are tensile strength, yield stress (or proof stress), elongation and Brinell or Rockwell hardness. Much less frequently there are requirements for impact resistance, either Charpy or Izod. Bar, tube, pipe, fittings etc. also usually require at least tensile strength and yield stress. These properties give a guarantee that the material in question has been correctly produced, and are also used by engineers to calculate the working loads or pressures that the product can safely carry in service.
Typical Properties

Typical mechanical properties of annealed materials are as in the graph of Figure 1. Note that the high cold work hardening rate of the austenitic grades in particular results in actual properties of some commercial products being significantly higher than these values. The yield stress (usually measured as the 0.2% proof stress) is particularly increased by even quite minor amounts of cold work. More details of the work hardening of stainless steels are given in the section of this handbook on fabrication.

Figure1. Typical Tensile Properties of Annealed Materials
Yield Strength

An unusual feature of annealed austenitic stainless steels is that the yield strength is a very low proportion of the tensile strength, typically only 40-45%. The comparable figure for a mild steel is about 65-70%. As indicated above a small amount of cold work greatly increases the yield (much more so than the tensile strength), so the yield also increases to a higher proportion of tensile. Only a few % of cold work will increase the yield by 200 or 300MPa, and in severely cold worked material like spring temper wire or strip, the yield is usually about 80-95% of the tensile strength.

As engineering design calculations are frequently made on yield criterion the low yield strength of austenitic stainless steels may well mean that their design load cannot be higher than that of mild steel, despite the tensile strength being substantially higher. Design stresses for various grades and temperatures are given in Australian Standard AS1210 "Unfired Pressure Vessels".
Ductility

The other mechanical property of note is the ductility, usually measured by % elongation during a tensile test. This shows the amount of deformation a piece of metal will withstand before it fractures. Austenitic stainless steels have exceptionally high elongations, usually about 60-70% for annealed products, as shown in figure 2. It is the combination of high work hardening rate and high elongation that permits the severe fabrication operations which are routinely carried out, such as deep drawing of kitchen sinks and laundry troughs.