Steels_ Metallurgy and Applications, Third Edition

Stainlesssteels 317

50-

04 A

,o

v

i=

Ix,

04 20

r

10 ...... I!

750 850 950 1050

Finishing temperature (~

Figure 4.11 Effect of finishing temperature on the properties of austenitic stainless steel
(After McNeely and Llewellyn ?)

offered the prospect of combining the good forming properties of traditional
austenitic stainless steels with the high strength of martensitic grades. The concept
required that the composition of these steels was controlled to within fine limits
such that the Ms-Mf temperature range was just below room temperature.
Following cold-forming operations, the steel could then be transformed to
martensite by refrigeration at a temperature such as -780C (solid CO2). Such
steels became known as controlled transformation stainless steels and their
behaviour can be illustrated by reference to Figure 4.5. This indicates that a
steel containing 0.1% C, 17% Cr and 5.5% Ni is essentially free of delta ferrite
and has an M~ temperature near to ambient. In the solution-treated 1050~ AC
condition, this composition has a hardness of about 220 HV, but on refrigeration
at -78"C, the hardness increases to about 400 HV. This indicates that the steel
has been substantially transformed to martensite, although Figure 4.5 shows that
hardness values approaching 450 HV are obtained on complete transformation
to martensite.
One of the problems encountered in the production of these steels was the very
tight control of composition that was required in order to position the transforma-
tion range just below room temperature. If the alloy content was excessive, the
transformation range was depressed to low temperatures such that transformation
to martensite could not be obtained at -78~ On the other hand, if the alloy
content was too low, the high Ms-Mf temperature range meant that substantial
transformation to martensite occurred on cooling to room temperature. However,
this problem could be eased to some extent by varying the solution treatment
temperature and this facility is illustrated schematically in Figure 4.12(a). In
austenitic steels containing about 0.1% C and 17% Cr, complete solution of
M23C6 carbides can be obtained at a temperature of 1050~ In such a condition,
both chromium and carbon exercise their full potential in depressing the Ms-Mf
range. However, if the solution treatment temperature is reduced, then a propor-
tion of M23C6 carbide is left out of solution and the transformation range is