Article
#6: Metallurgy - is 316ss Soft or Hard?
What
sort of question is that?! Everyone "knows" that 316ss is very soft -
as any austenitic stainless steel would be!
Right? Mmmmm...
Why
is stainless steel so good against cavitation? - if it were soft, wouldn't the
imploding bubbles (see Pump Magazine Article on that subject) erode the
material away at no time?
The
reason for such excellent resistance of stainless steel to cavitation is it
work-hardening property. You can easily 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 much difficult to make such indentation - 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 the surface of a stainless steel -
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 this work-hardening characteristics. Of course, further metallurgical
modifications can make stainless steel even more resistant to cavitation - for
example, CA6NM is roughly 2-3 times more resistant to cavitation as compared to
316ss.
Things
are not always as they first appear!
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