Steels_ Metallurgy and Applications, Third Edition

Stainless steels 323

Figure 4.15 lntergranular corrosion in Type 304 stainless steel

performance of the steel is then judged in terms of the presence or absence of
cracks or fissures after bending.
A third test, often designated the oxalic acid test, has been standardized as
ASTM A262-70 Practice A. In this test, a polished sample of austenitic stainless
steel is etched electrolytically in a 10% solution of oxalic acid at room temper-
ature. In the absence of chromium carbide precipitates, the grain boundaries of
the etched sample exhibit a step between adjacent grains compared with a ditch
in material which has experienced intergranular attack. The test also recognizes a
dual, intermediate condition in which some intergranular corrosion has occurred
but no single grain is surrounded completely by ditches.
At temperatures below about 850~ the solubility of carbon in an austenitic
stainless steel falls below 0.03% and exposure in the temperature range
450-800~ can result in the precipitation of M23C6 carbide. This can occur
during:

1. Slow cooling through the sensitization temperature range following solution
   treatment at 1050~ or after welding.


2. Stress relieving after welding or service exposure in the critical temperature
   range.

Slow cooling from welding and subsequent intergranular attack gave rise to the

term weld decay before the mechanism was fully understood. The weld decay

area is generally a band of material, some distance from the weld, which has

been exposed to the temperature range favouring the precipitation of chromium

carbide.