Help with 304SS 1

[Gmat]

I have run into a problem with manufacturing a part and i am looking for some help to determine what went wrong.

First off we manufactured 2 perfect working parts about 6 months ago. We have now tried to duplicate the process and are having problems. The problem is that the 304SS material is not taking a texture from sandblasting the same as the first time. After the original time sandblasting the parts new parts we found they did not take the required texture we decided to anneal them and try again. We have just received a report saying the parts are still not taking the required texture. I have verified that the sandblasting medium is new and it is the same physical person doing the sandblasting and they are using the same pressure and gun as last time.

We only changed one thing we know of between the first set and this set we are having problems with. The newest set was subcontracted out to a different machine shop. I don't know if this could actually be causing the problem or not. And if it is is there anyway we can fix it.

Any help or suggestions would be much appreciated. Thanks

 

[Gmat]

It would be great to have a starting place. I really have no idea how thick the superficial hardness is. Are we talking 0.0001" or 0.001". The less i have to take off the better.

We deal with a few plating houses so i will talk to them about their process.

If you are willing to give me a hand with this that would be great.

 

[kenvlach]

Some further thoughts.

Nobody can really judge the thickness of the superficial hardness layer; it depends upon all the variables of the final machining passes, plus material removal and heat generated in the polishing. Heat causes a thicker oxide film on the surface.

Question: Which surfaces matter? Is it perhaps the sides only, as in a disk brake rotor?

Don't like the idea of etching SS (except to reveal grain boundaries for metallography) to remove metal. You might not like the appearance afterwards, anyhow. As alternatives, I suggest wet polishing or electropolishing or electrochemical machining ( A simpler process than EDM; you can even do by hand by brush plating process, with saltwater electrolyte and the workpiece as anode.).

First though, try a mild stress relief at or below 480^oC (900^oF), with slow cooling. Then, dip in the nitric acid/HF (or nitric/ammonium bifluoride) long enough to descale (remove when it starts to etch).

Besides platers, Al anodizers and anyone who passivates SS should have the nitric/HF solution available. Don't think anyone uses except at ambient temperature (nasty fumes when heated).
Ken