304 Stainless vs 316

[ImanCarrot]

I got to make some lens holders that will go into a coating chamber under high vaccuum and temperatures to about 600 deg C.

I know nothing about material selection, but I note that 316S16 is rated up to about 300 deg C while 304S15 is rated up to 800 deg C.

304S15 would seem to be the chap I need. Is there a better option though please?

Thinking lateraly, is there any other material that would do the job? the largest holder I got to make is about 12" square and about 5mm thick. I've looked into ceramic like Shapal (too expensive) and Duraluminium etc (not tough enough) so I just assumed stainless.

Can I even mill this stuff using an HSS end mill?

I have searched on Google for the info, but all I got was stuff about Austentitic and Ferritic/Martenistic grain structure which left me more confused than when I started.

My Engineer's Black Book states that the US equivalent of UK 316S16 is "316", but gives no US equivalent for UK 304S15 and that both are Austentitic whatever that is... so you see I have tried! not being lazy here

[Karl_T]

Can I even mill this stuff using an HSS end mill?

Machinist SS mantra: "304 she's whore, 303 she's for me." I do know 316 machines just like 304.

To machine 304 you use unusually deep cuts, hi feed. And slower speeds. I prefer oil over coolant. The instant your tool starts to dull, STOP!, replace the cutter. 304 workhardens and you are SCREWED. DAMHIKT. I have successfully used HSS cutting tools with the above cautions.

Karl

[ImanCarrot]

Many thanks Karl, I would have gone with a shallow cut so you probably saved me a tool or two there.


Like the mantra lol, have written it up on my white board.

[Geof]

Thinking laterally:

High vacuum means not a lot of stuff in there; except what you are sputtering onto your lenses.

Not a lot of stuff means not much to cause oxidation.

Why not use mild steel?

One problem with stainless I think you are bound to have is distortion. Any sheet/plate is full of internal stresses and 600C is high enough to start them moving.

Whatever material you use I suggest you consider getting them laser cut roughly to shape and size in plate that is around 8 to 10 mm, and have them annealed before machining. It may even be necessary to rough machine, anneal again and then finish.

[ImanCarrot]

Thanks Geof, good advice as always!

I didn't think of annealing it. I'll do that. The last thing I want is for it to warp- I got a hole in the plate in which a window sits waiting to be coated and another steel plate on top of the glass (bolted down) to keep it in place. If the metal warps I can guarantee that the glass will not and it'll fracture.

I've taken into account the thermal expansion of the hole and the glass (the hole will expand as if it was a disk of the same material as the metal), but didn't think of annealing to stop warping.

I'll go with anneal, rough, anneal again and then final machine.

Will definately look into the mild steel option. As you say, there's nothing in there at 10E-6 Bar apart from the evaporating material so I might get away with it. Will check.

Cheers mate!

[madman]

Well i just had a reactor buuild project. (No not nuclear0 and i used 304 . i was intimidated at first by every one telling me how crappy to machine material it is,. I do prefer 303 but 304 machined very nice with 4 to 8 percent cobalt tooling and i utilized a aerospace?? coolant tremsol with a mist spray unit, heavy mixture. It worked extremely well on my antique manual equipment. Good Luck. Madman Mike

[dgapilot]

One shop I workin at did a lot of 303, 304, 316, and 17/4. #03 is a lot easier to work than 304. Like someone said before, 304 work hardens, take deep cuts and keep going. We used oil as a lubricant. Don't forget to passivate after machining, or corrosion can be a problem.

David

[pistolknight]

Hi everyone! It's nice to be able to try and help someone else out for a change. I work in an engineering/ fabrication shop and our main customer is the local cement plant. from time to time, we repair kiln inlet temperature probes which need to be able to withstand high temps. the spec for the stainless used in them is 316Ti which is a stainless with titanium added. This i think is for the sake of acidic gases which literally eat the stainless.
another high temperature stainless which we use a lot is grade 310. This is expensive when compared to 316 but machines like a sweet thing and is excellent at coping with extreme temperatures. It also does not suffer so much from distortion.
My personal preference for your situation would be to go with the 310. I'm sure i have a few offcuts of 6mm 310 lying around too!