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Ok, so a machinist gave me the tip that charcoal can be used to stress relieve weldments, as it burns around 1100F, which is also a suitable temp for stress relief. Then I got thinking I could essentially build a large campfire, taking care to keep the piece in coals rather than the full heat, and do it that way. Any tips or words of warning?
Thanks, and I realize this may not be exactly the correct sub-forum but hope it's close enough.
Cheers
Brian
(Almost ready to start a build thread!)
Hi Brian - thermal stress relief (TSR) of steel is around dull to cherry red 650-700degC. Its the min working temp of a blacksmiths forge which is fueled with charcoal. We have one at the local mens shed and I help them hammer occasionally. If the piece is small TSR is practical as you described if large it's not as its difficult to get all parts up to same temp. Thats why its usually "soaked" in an oven. The ideal process is to bring the entire item to cherry red then let it cool slowly. At cherry red steel is like butter. It will have very little strength and barely supports its own weight. So the piece has to be free standing or supported so it won't sag. If its a light frame you can also TSR using oxy and do it joint by joint. Steel aircraft frames are done this way. If the piece can be brought to red and let cooled and it does not warp you have succeeded :) Here's a picture of a shaft I designed coming out of tempering which is same temp as TSR to give you an idea of the colour. Peter
I think he was mistaken by using charcoal you would add carbon to the steel and case harden the part, if you can't control the temperature, that is what they use charcoal for, the temperature has to be carefully controlled or you will mess your parts upQuote:
Originally Posted by catahoula
Mild steel should not be heated above this temperature 450C to 650C, for stress relieving 450C/842F being the ideal temperature for stress reliving mild steel
Tool steels to 700c not mild steel that is too highQuote:
Originally Posted by peteeng
Hi Mactec - typical mild steel weldment TSR is 550-680degC I'll be more precise in future. But 700C wouldn't hurt it I oxybraze frames with bronze and its 800-900C melting range..... Charcoal is used as fuel in forges as it provides more heat than coal or timber. I also checked cherry red and different charts show it from 700-900C so use a pyrometer! . So in terms of colour its red but not dark red. Good to have an umpire around... Peter
Heat Treatment Australia - offers a variety of treatments including Bristuff ® Nitriding, hardening & tempering, carbo nitriding, cryotemp, aluminium treatments, annealing, carburising. Brisbane, Sydney, Melbourne, Australia
Oxy-brazing temperature has nothing to do with stress relievingQuote:
Originally Posted by peteeng
Any good blacksmiths would be using coke, not charcoal, 450C to 550C I never let them get above 525C, this is the best temperature to stress relieve steel weldments especially if you have to do any machining after heat treatment
I do it all the time on our welded machine frames and Ground plates
Yeah I definitely don't want to go too hot for fear of distortion. He definitely wasn't talking about adding carbon for hardening, just that that filling a tube with charcoal would be an easy way to get some stress relief at a fairly controlled temperature. But if case hardening is a byproduct I would want to avoid that.
Frame is about 800x850mm, so not outrageously large, but large enough that finding an oven for it is not easy.
Using Oxyacetylene torch joint by joint is a possibility, though I'd be worried about consistency
mactec, do you use the coke in a furnace? can you describe your process a little?
Coke has been the preferred heat source for hundreds of years for blacksmithsQuote:
Originally Posted by catahoula
When you want to case harden something you pack it in a container with charcoal, there are other ways to case harden a part, but using charcoal is cheap and is used a lot for this process
I don't use coke for stress relieving the oven we use is electric or gas fired depending on the size of the part as to which oven would be used
Oxyacetylene torch is no good to use unless you could do every welded joint at the same time, this would only relieve the joint area the rest of the steel would still have stress in it, so you would not achieve very much by using a oxyacetylene torch and the cost would be more than having it done in a oven
It does not cost very much to have it done if you can find an oven big enough for your parts, this is the oven I use it can do 8' x 8' x 14', you want them to sand blast it after stress relieving to clean it up then it is ready for paint / power coating and then machining
Thank you. That confirms many suspicions. I'll just find an oven. Might have to drive about 3 hours as I'm in a smallish town.
Yes we all do that at some time to find what we need, so some that do the stress reliving, can also do any machining at the same time, but make sure they blast it before machining, it's not fun to sand blast it clean after it has been machinedQuote:
Originally Posted by catahoula
I called a shop in the neighboring city and they'll do it for $0.40 USD per pound, so about $200 for the 500 pound (225 kg) frame. Seems like money well spent for peace of mind.Quote:
Originally Posted by mactec54
Their standard heat treatment is 1100 degrees F/600 C for two hours soak, with additional hour per 25mm of part thickness.
that temperature is a little higher than normal but within the max rangeQuote:
Originally Posted by catahoula
Make sure they are able to sit it flat so it does not twist to the surface it is sitting on