(^4) Stainless steels
Overview
As chromium is added to steels, the corrosion resistance increases progressively
due to the formation of a thin protective film of Cr203, the so-called passive
layer. With the addition of about 12% Cr, steels have good resistance to atmo-
spheric corrosion and the popular convention is that this is the minimum level
of chromium that must be incorporated in an iron-based material before it can
be designated a stainless steel. However, of all steel types, the stainless grades
are the most diverse and complex in terms of composition, microstructure and
mechanical properties. Given this situation, it is not surprising that stainless steels
have found a very wide range of application, ranging from the chemical, phar-
maceutical and power generation industries on the one hand to less aggressive
situations in architecture, domestic appliances and street furniture on the other.
By the late 1800s, iron-chromium alloys were in use throughout the world but
without the realization of their potential as corrosion-resistant materials. Harry
Brearley, a Sheffield metallurgist, is credited with the discovery of martensitic
stainless steels in 1913 when working on the development of improved rifle barrel
steels. He found that a steel containing about 0.3% C and 13% Cr was difficult
to etch and also remained free from rust in a laboratory environment. Such a
steel formed the basis of the cutlery industry in Sheffield and as Type 420 is still
used for this purpose to the present day.
During the same period, researchers in Germany were responding to pres-
sures for improved steels for the chemical industry. Up until that time, steels
containing high levels of nickel were in use as tarnish-resistant materials but
had inadequate resistance to corrosion. Two Krupp employees, Benno Strauss
and Eduard Maurer, are credited with the discovery of Cr-Ni austenitic stainless
steels and patents on these materials were registered in 1912. However, workers
in France and the United States are also cited as independent discoverers of these
steels.
During the 1920s and 1930s, rapid developments took place which led to the
introduction of most of the popular grades that are still in use today, such as
Type 302 (18% Cr, 8% Ni), Type 316 (18% Cr, 12% Ni, 2.5% Mo), Type 410
(12% Cr) and Type 430 (17% Cr). However, even in the 1950s, stainless steels
were still regarded as semi-precious metals and were priced accordingly. Up
until the 1960s, these steels were still produced in small electric arc furnaces,
sometimes of less than 10 tonnes capacity. The process was carried out in a single
stage, involving the melting of scrap, nickel and ferro-chrome, with production
times in excess of 389 hours. However, substantial gains were achieved with
the installation of larger furnaces with capacities greater than 100 tonnes and
the introduction of oxygen refining techniques also increased productivity very
substantially. Since the early 1970s, the production of stainless steels has been
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