Stainless steels 29780.-, 60-"~ 40-20-0 -
12b I
13 14 15 16 17 18
Chromium (%)Figure 4.3 Effect of chromium content on O. 1% carbon steels solution treated at 1050"C
(After Irvine et aL t)high temperature undergoes no phase transformation. Although its structure is
body-centred-cubic, the high-temperature phase is generally called delta ferrite
(8) in order to differentiate it from alpha ferrite (cx), the transformation product
from austenite. As illustrated in Figure 4.3, the delta ferrite content increases
progressively with further additions of chromium and, in the presence of about
0.1% C, the material becomes completely ferritic with the addition of just over
18% Cr. Therefore, between about 13% and 18% Cr, the hardness of these steels
is reduced as the microstructure changes progressively from 100% martensite to
100% delta ferfite. Larger additions of chromium have no further effect on the
microstructure, although such materials become increasingly more susceptible to
the formation of sigma phase.
Although the changes from? ~ F + 8 and Y + 8 ~ 8 occur at chromium
levels of about 13.5% and 18.5% respectively, it must be emphasized that this
refers to alloys containing a maximum of about 0.1% C and at a reference
temperature of around 1050~ Larger amounts of carbon or the addition of
other alloying elements will have a major effect on the microstructure associated
with particular levels of chromium. Additionally, for a given composition, an
increase in the solution treatment temperature above 1050~ will also increase
the amount of delta ferrite at the expense of austenite.Iron-chromium-nickel alloys
Whereas chromium restricts the formation of austenite, nickel has the opposite
effect and, as illustrated in Figure 4.4, the Fe-Ni equilibrium diagram displays
an expanded austenite phase field. In the context of stainless steels, chromium
is therefore termed a ferrite former and nickel an austenite former. Thus having
created a substantially ferritic microstructure with a large addition of chromium,
it is possible to reverse the process and re-establish an austenitic structure by
adding a large amount of nickel to a high-chromium steel.
As indicated in Figure 4.3, a steel containing 17% Cr and 0.1% C will have a
microstructure of about 65% delta ferrite-35% austenite at a solution treatment
temperature of I050~ The various changes that then occur with the addition