Stainless steels 315niobium which prevent the formation of chromium carbide. This gives rise to
grades such as Type 430 Ti (17% Cr-Ti) and Type 430Nb (17% Cr-Nb).
As discussed earlier, 12% Cr steels are normally martensitic and, as such,
tend to have poor forming and welding characteristics. However, the addition
of a strong ferrite former to a 12% Cr base steel can produce a fully ferritic
microstructure with a marked improvement in the cold-forming and welding
behaviour. This effect is achieved in Type 409 (12% Cr-Ti), the titanium also
eliminating the problem of chromium carbide formation as well as promoting
the ferritic structure. This steel has found extensive application in automobile
exhausts in place of plain carbon or aluminized steel.
Type 446 (25% Cr) is the highest chromium grade in the traditional range
of ferritic stainless steels and provides the best corrosion and oxidation resis-
tance. Whereas ferritic stainless steels generally possess low toughness, a further
embrittling effect can be experienced in steels containing more than 12% Cr
when heated to temperatures in the range 400-550~ The most damaging effect
occurs at a temperature of about 475~ and, for this reason, the effect is known
as 475~ embrittlement. The loss of toughness is due to the precipitation of a
chromium-rich, ct prime phase which becomes more pronounced with increase in
chromium content. However, 475~ embrittlement can be removed by reheating
to a temperature of about 600oc and cooling rapidly to room temperature.
Austenitic stainless steels
As illustrated in Table 4.2, most of the steels in the AISI 300 series of austenitic
steels are based on 18% Cr but with relatively large additions of nickel in order to
preserve the austenitic structure. However, as illustrated below, various compo-
sitional modifications are employed in order to improve the corrosion resistance
of these steels.
Type 304 (18% Cr, 9% Ni) is the most popular grade in the series and is used
in a wide variety of applications which require a good combination of corrosion
resistance and formability. As discussed later, the stability and work-hardening
rate of austenitic stainless steels are related to composition, the leaner alloy
grades exhibiting the greater work hardening. Thus a steel such as Type 301
(17% Cr, 7% Ni) work hardens more rapidly than Type 304 (18% Cr, 9% Ni)
and, for this reason, Type 301 is often used in applications calling for high
abrasion resistance. Steels such as Type 316 (18% Cr, 12% Ni, 2.5% Mo) and
317 (18% Cr, 15% Ni, 3.5% Mo) have greater resistance to corrosion in chloride
environments than Type 304 and represent austenitic counterparts of Type 434
(17% Cr, 1% Mo) discussed in the previous section.
Reference was made earlier to the corrosion problems experienced in stain-
less steels with the formation of chromium carbides. It was indicated that the
problem can be overcome in ferritic stainless steels with the addition of titanium
and niobium and the same is true in the austenitic grades. This gives rise to
Types 321 (18% Cr, 10.5% Ni, Ti) and 347 (18% Cr, 11% Ni, Nb) and, because
of their freedom from chromium carbide precipitation and intergranular attack,
these grades are often referred to as stabilized stainless steels.