Steels_ Metallurgy and Applications, Third Edition

Stainless steels 293

stainless steel has managed to maintain its traditional image as a decorative mate-

rial but is now also regarded as an engineering material for use in applications

where structural integrity is more important that aesthetic appearance. On the

basis of life-cycle costing, stainless steels are also proving to be attractive alter-

natives to mild steel in structures or components that require frequent painting

and maintenance.

Underlying metallurgical principles

As indicated in the Overview, stainless steel grades cover a wide range of

compositions which results in the generation of a variety of microstructures and

mechanical properties. This is clearly illustrated in the following compositions

which represent three of the common grades of stainless steel'

Composition Grade Microstructure

12% Cr 409 martensitic

17% Cr 430 ferritic

18% Cr 9% Ni 304 austenitic

Reference will be made in this chapter to the composition-structure relationships
which show that alloying elements in stainless steels can be divided into two
groups, namely those that promote the formation of an austenitic structure at hot
rolling or solution treatment temperatures and those that promote the formation
of delta ferrite. Chromium is the principal alloying element in stainless steels and
this promotes the formation of delta ferrite at high temperature. However, iron
can accommodate up to about 13% Cr at a temperature of around 1050"C and still
remain completely austenitic at that temperature. On the other hand, the Ms-Me
temperature range of a 12% Cr steel is sufficiently high to allow this material
to transform completely to martensite on cooling to ambient temperature. An
increase in chromium from 12 to 17% brings about a progressive change from
austenite to delta ferfite at high temperature and the ferrite remains unchanged
on cooling to ambient temperature. Nickel is a strong austenite-forming element
and is added to stainless steels in order to preserve an austenitic structure in the
presence of high chromium contents. Thus a steel containing 18% Cr 9% Ni is
completely austenitic at a temperature of 1050"C but the overall alloy content
now depresses the Ms-Mf temperature range to sub-zero temperatures. Therefore
this material retains its austenitic structure on cooling to ambient temperature,
providing a relatively low strength but a high level of formability.
Elements such as silicon, molybdenum and titanium also promote the formation
of delta ferrite at high temperature, whereas carbon, nitrogen, manganese and
copper promote the formation of austenite. Therefore consideration must also
be given to the presence of these elements, in addition to the balance between
chromium and nickel, in determining the structure of stainless steels at elevated
temperatures. However, both austenite- and ferrite-forming elements will depress
the Ms-Mf range and influence the microstructure formed on cooling to ambient
temperature. Therefore the constitution of stainless steels is governed by: