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Castings with 316L
2

Castings with 316L

Castings with 316L

(OP)
Are there shortcomings when casting parts in 316L, as opposed to machining the parts from 316L bars (extruded profiles)?

I'm investigating the possibility, of casting our parts rather than obtaining them from CNC machining. Our product are luxury accessories made out of 316L SS: the parts are mirror polished (Ra=0.008 micrometers). One requirement is low nickel release, to minimize allergic reactions on the skin of the wearer. Corrosion is also a concern.

I believe that long time ago the industry singled out 316L, because it is corrosion resistant and malleable enough to allow for mirror polishing (cloth wheel). Cold-work is normally necessary to impart extra hardness (150HV before cold work, 270HV after); without it, micro-scratches caused by handling would cause too much rework (in some case even a 1-micrometer-wide scratch would be cause for extra buffing).

Recently I started testing a carburization treatment that imparts  extra hardness to the first 10-20 micrometers of skin, which may render the cold work unnecessary. The cost benefits should be sizeable, but I do not know if there are side effects: would the nickel release increase for instance, or would there be any other unwanted behavior?

RE: Castings with 316L

Investment casting can give you the finish and intricacy that you might be looking at. There is a patented process for increasing surface hardness of stainless steels. This has been discussed in some of the earlier threads. But possible effects on skin after treatment, some of the experts may sooon come in and discuss.

RE: Castings with 316L

I have not been able to find any data on the influence of carbon supersaturation on corrosion and, therefore, nickel release. I suspect however that carbon may, like nitrogen, be beneficial if it doesn't get to form Cr23C6. This is based on thermodynamic considerations only.
You could use low temperature nitrogen supersaturation and get the same effect of hardening. Then you would be assured of greatly lowering the risk of nickel release because of the demostrated benefit of nitrogen on such corrosion.

RE: Castings with 316L

Cast parts are more likely to have minor imperfections that will show when polished. Using investment castings and following with a good anneal will minimize this, but it still will be an issue.
I have used ion-nitriding for surface hardening.  With mixed results.  I never had to polish the surfaces.
Right now we polish a lot of 316 tubing.  It is in the soft annealed condition.  We mechanicaly polish to better than 10 micro-inch Ra and then electro polish.  There are often small pits and other imperfections revieled at EP.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

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