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
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".
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.
Figure 2. Typical elongations of annealed materials
Hardness (measured by Brinell, Rockwell or Vickers machines) is another value for the strength of a material. Hardness is usually defined as resistance to penetration, so these test machines measure the depth to which a very hard indenter is forced into a material under the action of a known force. Each machine has a different shaped indenter and a different force application system, so conversion between hardness scales is not generally very accurate. Although conversion tables have been produced these conversions are only approximate, and should not be used to determine conformance to standards.
It is also sometimes convenient to do a hardness test and then convert the result to tensile strength. Although the conversions for carbon and low alloy steels are fairly reliable, those for stainless steels are much less so.
Mechanical Properties of Wire and Bar
The mechanical properties of the majority of the stainless steel wire and bar products are generally sufficiently described by the tensile strength. These products require mechanical properties which are carefully chosen to enable the product to be fabricated into the finished component and also to withstand the loads which will be applied during service. Spring wire has the highest tensile strength of the wire generally manufactured; it must be suitable for coiling into tension or compression springs without breaking during forming. However, such high tensile strengths would be completely unsuitable for forming or weaving applications because the wire would break on forming. Weaving wires are supplied in a variety of tensile strengths carefully chosen so that the finished woven screen will have adequate strength to withstand the service loads, and yet soft enough to be crimped and to be formed into the screen satisfactorily. Mechanical properties of wire for fasteners are another example where a careful balance in mechanical properties is required. In this type of product the wire must be ductile enough to form a quite complex head but the wire must be hard enough so that the threads will not deform when the screw or bolt is assembled into the component. Good examples are roofing bolts, wood screws and self-tapping screws; to achieve the mechanical properties required for such components requires careful consideration of the composition of the steel so that the work hardening rate will be sufficiently high to form hard threads on thread rolling and yet not so high as to prevent the head from being formed.
For bar products a compromise must also be made; a large proportion of bar will be machined, so it is important that the hardness be not too high, but better load carrying capacity is achieved if the strength is high, and for drawn bar a good bright finish is achieved only by a reduction which significantly increases strength levels.