Apr 22, 2003

Point of Interest: Metallurgy

Is 316ss Soft or Hard?

Why is stainless steel so good against cavitation? If it
were soft, wouldn't the imploding bubbles erode the material away in no
time?

The reason for such excellent resistance of stainless steel
to cavitation is its work-hardening property. You easily can make an
indentation on a surface of an unworked stainless steel strip. However, after
hitting the strip with a hammer for some time, it becomes very difficult to
make such indentation because the surface work-hardens. (Ironically, this is
why stainless steel, as soft as it is, is difficult to machine; it work-hardens
as a cutting tool goes over it, resulting in a chunky, chisel-like chip instead
of a smooth clean cut).

The same happens when cavitation bubbles bombard a
stainless-steel surface. It work-hardens and begins to resist any further
cavitation very quickly. 316ss (CF8M) resists cavitation about 10-15
times better than cast iron because of the work-hardening characteristics. Of
course, further metallurgical modifications can make stainless steel even more
resistant to cavitation. For example, CA6NM is roughly two to three times more
resistant to cavitation as compared to 316ss.

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