Steels_ Metallurgy and Applications, Third Edition

Stainless steels 299

of nickel to a base steel of this composition are illustrated in Figure 4.5. This
shows that the delta ferrite content is steadily reduced and at 1050~ the steels
become fully austenitic with the addition of about 5% nickel. On cooling to
room temperature, the austenite in these low-nickel steels transforms to marten-
site and therefore there is initially a progressive increase in hardness with the
addition of nickel as martensite replaces delta ferrite. However, the addition of
nickel also depresses the Ms-Mf transformation range and at nickel contents
greater than about 4% the M f temperature is depressed below room tempera-
ture. Further additions of nickel therefore lead to a decrease in hardness due to
incomplete transformation to martensite and the formation of retained austenite.
As indicated by the hardness data in Figure 4.5, refrigeration at -78~ causes
the retained austenite to transform to martensite over a limited composition range
until the Ms temperature coincides with the refrigeration temperature. In commer-
cial 18% Cr 9% Ni austenitic stainless steels, the Ms has been depressed to very
low temperatures and little transformation to martensite can be induced, even at
the liquid nitrogen temperature of- 196~

Other alloy additions

Whereas chromium and nickel are the principal alloying elements in stainless
steels, other elements may be added for specific purposes and therefore consid-
eration must be given to the effect of these elements on microstructure. Like
chromium and nickel, these other alloying elements can be classed as ferrite or
austenite formers and their behaviour is illustrated in Figure 4.6 which refers to
a base steel containing 17% Cr and 4% Ni. Thus elements such as aluminium,
vanadium, molybdenum, silicon and tungsten behave like chromium and promote
the formation of delta ferfite. On the other hand, copper, manganese, cobalt,
carbon and nitrogen have a similar effect to nickel and promote the formation of
austenite. A guide to the potency of the various elements in their role as austenite
or ferrite formers I is shown in Table 4.1.

100

A 8O

6o

~ 4o

,Va ~.~ W

Mo

20

Cu and Co

0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8,0

Alloying element (%)

Figure 4.6 Effect of various alloying elements on the structure of 17% Cr 4% Ni alloys
(After Irvine et aL l)