348 Steels: Metallurgy and Applicationswork hardening, but the strengthening effect of adding 10% Mn was very small
compared with that produced by a similar level of cobalt.
In the United States, Armco 18-9 LW (18% Cr, 9% Ni, 4% Cu) was devel-
oped for the production of cold-headed fasteners and grades based on 18% Cr,
12% Ni, 2% Cu have been used in the UK. In addition to fastener applications,
materials with a low yield stress and low rates of work hardening are also required
in cold-drawing operations. These characteristics therefore contrast sharply with
the compositions used in stretch forming operations which are designed to work
harden and accommodate the reduction in cross-section via the formation of
controlled amounts of strain-induced martensite. Deep-drawing grades are there-
fore based on compositions with a low level of interstitial dements, sufficient
alloy content to preserve a stable structure and ideally with a high stacking fault
energy. A coarse grain size is also beneficial, consistent with the avoidance of
the orange peel effect. Texture considerations are also important in deep drawing
performance and high r values are associated with strong {111} and/or {110}
planes parallel to the surface of the sheet.
Mechanical properties at elevated and sub-zero temperatures
Although austenitic stainless steels are used primarily because of their high corro-
sion resistance, they also possess extremely good mechanical properties over a
wide temperature range. Unlike ferritic materials, austenitic stainless steels do
not exhibit a ductile-brittle transition and maintain a high level of toughness
at liquid gas temperatures. On the other hand, they also exhibit good creep
rupture strength at temperatures above 600"C, where ferritic and martensitic steels
undergo microstructural degradation. The standard grades of austenitic stainless
steel are therefore used in cryogenic applications, such as liquid gas storage
vessels and missiles, and at elevated temperatures in chemical and power plant
applications. These aspects are discussed on pp. 351-356.
Tensile properties
The tensile properties of the standard grades over the temperature range -200
to 800~ have been examined by Sanderson and Llewellyn 3~ and the data are
summarized in Figure 4.32. This indicates that the tensile strengths of the majority
of the grades fall within a fairly narrow scatter band at temperatures above about
100C. As the temperature falls below this level, there is a marked increase in
tensile strength, the lower alloy steels such as Types 302 and 304 achieving higher
strengths than the highly alloyed grades such as Types 309 and 310. However,
this situation tends to be reversed in the case of the 0.2% proof stress values,
the more highly alloyed grades such as Types 316, 309 and 310 providing higher
strengths than Types 302, 304, 321 and 347. This behaviour is related to the
stability of the austenitic structure in these materials and the relative ease with
which they undergo transformation to strain-induced martensite.
At temperatures above 100C, solid solution strengthening is the main crite-
rion controlling strength and therefore the more highly alloyed steels, such as