Steels_ Metallurgy and Applications, Third Edition

Stainless steels 347

c

8

"x

0.52 0.51 0.50 ^ .^ ~ Eu

)

o.o ~

002 I I

o s i | ~o 15

I % Martensite at e = 0.2

I.=, ~1

14 -- I ~

I I

15

Figure 4.31 Effect of composition factor and carbon content on the formation of marten-
site at a true strain of 0.2 and on the uniform true strain (~,) (After Gladman et al. 29)

of martensite induced at a true strain of 0.2, the composition being expressed
through the following factor:

CF = 18 (%C) + 13 (%N) + 0.90 (%Ni) + 0.27 (%Cr) + 0.47 (%Mn)

+ 0.53 (%Mo) + 0.97 (%Cu)

By controlling the CF value to within the limits shown, the level of martensite
is controlled to between 3 and 6% at a true strain of 0.2 and, as shown in
the upper diagram, this results in the optimum uniform strain at various carbon
levels. It will also be noted that uniform strain increases with carbon content.
Ordinarily, it would be anticipated that an increase in carbon would reduce the
rate of work hardening by suppressing the formation of martensite. However, for
a given composition factor, and therefore a defined level of austenite stability,
it is proposed that an increase in carbon leads to an increase in work-hardening
rate through its strengthening effect on the strain-induced martensite.
Austenitic stainless steels are also used in fastener applications where they
undergo severe deformation during cold-heading operations. In this context, a
low rate of work hardening is required and the grades involved must have suffi-
cient stability to resist the formation of strain-induced martensite. Beyond this
basic requirement, the performance can be enhanced by the selection of alloying
elements that provide both austenite stability and a high stacking fault energy.
In stable compositions, the rate of work hardening is dictated solely by stacking
fault energy and the effects of alloying elements in an 18% Cr, 13% Ni base were
described earlier (Figure 4.26). Having eliminated stability effects, the addition
of copper has a significantly greater effect than nickel in reducing the rate of