ABSTRACT
Within a certain range of compositions based on ferritic stainless steels, as
indicated by the diagram in Figure 9.1, martensitic structures can be
developed. The martensitic grades are so named because, when heated
above the critical temperature of 16008F (8708C) and cooled rapidly, a
metallurgical structure known as martensite is obtained. In the hardened
condition, the steel has very high strength and hardness, but to obtain
optimal corrosion resistance, ductility, and impact strength, the steel is given
a stress-relieving or tempering treatment, usually in the range of 300-7008F
(149-3718C). These alloys are hardenable because of the phase transfor-
mation from body-centered cubic to body-centered tetragonal. As with the
low-alloy steels, this transformation is thermally controlled. The Fe-Cr
phase diagram suggests that the maximum chromium content would be
about 12.7%. But the carbon content expands the g region to the extent that
larger chromium contents are possible. Common alloys are 410, containing
12% chromium and low carbon, and alloy 440 of 17% chromium with a high
carbon content. The martensitic stainless steels are the strongest of all
stainless steels, having strength to 275 ksi. However, at such high strength
levels they lack ductility.
