294 Steels: Metallurgy and Applications(i) the balance between austenite- and ferdte-forming elements which controls
the structure at hot rolling and solution treatment temperatures,
(ii) the overall alloy content which controls the Ms-Me temperature range and
therefore the structure and properties at ambient temperature.Austenitic stainless steels can be subjected to severe cold-forming operations,
for example in the cold rolling of hot band to strip gauges and also in the
production of domestic sinks and tableware from annealed strip. This introduces
the topic of strain-induced martensite, whereby a material which is austenitic in
the solution-treated condition can transform partially or completely to martensite
with the application of cold work at ambient temperature. Detailed consideration
is given to this topic later in the chapter, but in essence it relates to the stability
of the austenitic structure, as influenced by the overall alloy content, and the
destabilizing effects due to the magnitude and temperature of cold deformation.
The metallurgy of the 12% Cr martensitic grades is similar to that involved in
the engineering grades, although the presence of such a large amount of chromium
induces a very high degree of hardenability and these steels are capable of devel-
oping a martensitic structure in substantial section sizes, even in the aft-cooled
condition. However, like their low-alloy counterparts, the 12% Cr grades must be
tempered to produce a good combination of strength and ductility/toughness and
both types of steel often incorporate additions of molybdenum and vanadium
in order to improve the tempering resistance through the formation of stable
carbides.
Whereas austenitic stainless steels are used in domestic or architectural applica-
tions, where corrosion resistance and aesthetic appeal are the main requirements,
they are also employed in pressure vessels where both corrosion resistance and
strength are important considerations. As indicated in the Overview, solid solution
strengthening with additions of nitrogen is the main avenue for the production
of higher-strength austenitic stainless steels. Precipitation-strengthening reactions
can also be induced in these grades through the precipitation of carbides and inter-
metallic compounds based on nickel, aluminium and titanium. However, such
materials have found little commercial application, due possibly to weldability
problems and poor corrosion properties.
Stainless steels resist corrosion through the formation of a thin passive film
of Cr203 and, very broadly, the corrosion resistance of these materials increases
with chromium content. However, as illustrated by the previous remarks, marked
variations in microstructure are introduced with the addition of alloying elements
with a marked effect on mechanical properties. Thus the 12% Cr martensitic
grades are capable of developing high levels of strength but with only moderate
resistance to corrosion. In contrast, an austenitic grade, based on 18% Cr 9% Ni,
has a low strength but a significantly higher resistance to corrosion, the latter
being enhanced by the addition of molybdenum. Therefore throughout the broad
range of stainless steels, there will be a compromise between corrosion resistance
and other properties, such as strength, formability and weldability. Additionally,
it is necessary to differentiate between the various types of corrosion in stainless
steels, notably: