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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