Stainless steels 343conditions in the stressed system, where Fy and Fr' are displaced by an amount
A G~ r'~a', the mechanical energy due to the applied stress. Therefore Ms in the
unstrained system can be moved to a higher temperature, M' s, where the sum of
the mechanical and thermodynamic energies (AG~r-*a' + AFr~a'/M's) is equal
to AF r-*a'/Ms, the critical undercooling energy at Ms. Thus cold working at
temperatures above Ms may result in the formation of strain-induced martensite,
depending upon the chemical composition of the steel and the temperature of
working.
In addition to their influence on the martensite reaction, alloying elements can
also affect the work-hardening characteristics of austenitic stainless steels through
their effect on the stacking fault energy of the system. Stacking faults are planar
imperfections in the normal stacking of the fcc lattice which increase the rate
of work hardening by hindering the movement of dislocations. Elements such as
carbon, manganese and cobalt facilitate the formation of stacking faults whereas
nickel and copper raise the stacking fault energy which inhibits their formation.
The effects of these elements on the true stress-true strain behaviour of 18% Cr
13% Ni-base steels is shown in Figure 4.26. This base composition has suffi-
cient stability to withstand the formation of strain-induced martensite at ambient
temperature and therefore any changes that take place in the work-hardening
behaviour can be attributed to stacking fault effects. Thus cobalt and manganese
raise the rate of work hardening whereas nickel and copper decrease the rate. The
effect of chromium on the stacking fault energy was examined in steels containing
18% Ni and, as illustrated in Figure 4.27, it would appear that chromium reduces
the stacking fault energy and thereby increases the rate of work hardening.Work hardening of commercial grades
The effects of rolling at ambient temperature on the properties of commercial
grades of austenitic stainless steel are shown in Figure 4.28 and 4.29. Type 301100so.j'Base 18/13
10% NI
4% Cu0 0.5 1.0 1.5
Log o -~Figure 4.26 True stress-true strain in 18% Cr 13% Ni steel with alloy additions (After
Llewellyn and Murray 2s )