324 Steels: Metallurgy and ApplicationsFrom the foregoing remarks, it will be obvious that a reduction in the carbon
content will reduce the susceptibility to intergranular corrosion but, at one time,
major expense was incurred in reducing the carbon content of an austenitic
stainless steel to below 0.06%. However, as indicated earlier, the problem was
controlled initially by adding elements such as titanium or niobium, which are
stronger carbide formers than chromium, and therefore TiC or NbC are formed
rather than the damaging chromium-rich M23C6. The addition of these elements is
said to stabilize stainless steels against intergranular attack and gives rise to the
standard grades-Type 321 (Ti-stabilized) and Type 347 (Nb-stabilized). These
elements also form nitrides and the additions made to stainless steels are slightly
in excess of those required by stoichiometry for complete precipitation of carbon
and nitrogen, namely, Ti = 5 x (C -t- N)% and Nb = 10 x (C + N)%.
Although the problem of intergranular corrosion is controlled by the addition
of stabilizing elements, Types 321 and 347 are not completely immune to this
form of corrosion since they are susceptible to knife-line attack. During welding,
the heat-affected zone is raised to temperatures above 1150~ and this can result
in the partial dissolution of TiC and NbC. Carbon is therefore taken into solution
in a narrow region adjacent to the weld and can be available for the formation
of chromium carbide on cooling through the sensitization range of 450-8000C.
The susceptible region may only be a few grains wide but can give rise to a thin
line of intergranular attack. Hence the term knife-line attack.
The problem of intergranular corrosion in austenitic stainless steels can also
be overcome by reducing the carbon content to low levels. Although steels
containing 0.03% C max. (L grades) have been available since the 1950s,
their use was restricted very severely in the early days due to high production
costs. This involved the use of low-carbon ferro-chrome, which was expensive.
However, the introduction of AOD refining has cheapened the production of
the L-grades very considerably and these steels are now used extensively in
applications formerly satisfied by the stabilized grades. However, the presence of
TiC and NbC particles gives rise to some dispersion strengthening and Types 321
and 347 are still used in applications where advantage can be taken of their higher
strength.
According to Sedriks, s molybdenum has an adverse effect on intergranular
corrosion as assessed in the Huey test and a beneficial effect in the Strauss test.
It is suggested that the adverse effect in the former may be concerned with
the fact that nitric acid attacks areas other than chromium-depleted regions, e.g.
areas associated with solute segregation or the early formation of sigma phase.
At one time, small additions of boron were made to austenitic stainless steels
in order to improve hot workability and the high-temperature creep properties.
However, such additions are very deleterious to the performance in the Huey
test. It has been suggested that boron additions lead to the formation of M2B
and M23(CB)6 precipitates at the grain boundaries, both of which give rise to
chromium-depletion effects. Performance in the Huey test is also improved by
reducing the phosphorus content of austenitic stainless steels to low levels, i.e.
less than 0.01% P.