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Hardening of 316 St/St

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BVMan

Mechanical
Jun 20, 2003
27
A couple of years ago I saw some components (Valve Ball and Seats) which had undergone a solution treatment which Increased the Surface Hardness of the components and turned the parts almost black in colour.

Whatever the treatment was it did not involve too high a temperature as it was carried out on finished components. The process only changes a few microns from the surface and allows 316 to run against 316 without picking up.

If it helps I seem to recall the process had a German sounding name....although I might be wrong here.

Can anyone point me in the right direction here as to what the process was?
 
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I think you are talking about one of the salt bath nitriding processes that work on Austenitic SS. Do a search on thsi board for the thread as this has been discussed several times.

(I think it was a Kolene process or similar. Possibly QPQ?)

nick
 
It sounds like you are thinking of the Kolsterizing process, which does give you a very hard case about 25 microns deep on austenitic stainlesses, but the black color is not necessarily a result of Kolsterizing. Another possibility is burnished-in Molybdenum Disulfide, which will result in a black color and exceptional lubricity.

Maybe use the latter over the former??
 
Kolsterizing is but one of a handful of processes known as low-temperature carburizing (between 400 and 600 Celsius). I have tried 5 different companies for my parts, and they all do it in different ways. The blackening comes from the surface activation, which is done prior to carburizing. Some process is patented, others are only proprietary, but very well guarded.
 
the downside of these processes is that the corrosion resistance is severely affected. QPQ imparts nitrogen and carbon into the surface. carbon ties up the chromium such that it cannot form a protective chromium oxide surface layer. the bulk corrosion resistance of stainless is not much better than carbon steel. you will be relying almost exclusively on the corrosion resistance of the resulting case which will be better than carbon steel but not as good as stainless.
 
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