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Stress Relieving Rolled Mild Steel

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Stewart Hart05/04/2019 08:17:16
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I want to machine a part from cold rolled mild steel bar from experience I know that the part will warp out of square and this would be a no no, also for what I have in mind hot rolled bar would not be suitable

So has any one tried stress relieving rolled mild steel bar?. I thought about sticking it in the oven but our ovens max temp is 240 C I don't think this will be hot enough as far as I can find out you stress relieve at around 600 - 700 C I,ve also considered putting it into a barbeque but that would be a bit hit and miss.and I don't want to scale the bar

My other option would be to rough the part out and let it sit for a week or two to let the stresses work on to part, then to finish machine it but I'm unsure if this will be effective enough.

Any ideas views enlightened

Stew

Nicholas Farr05/04/2019 08:55:59
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Hi Stewart, this graph shows heat treatment ranges if it is of any help.

graph01.jpg

Regards Nick.

Tony Pratt 105/04/2019 09:04:54
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600 deg C is about the right temp for stress relieving mild steel so the oven is out, barbeque will do it but not sure how you monitor the actual temperature?

To prevent scaling the part can be placed in an airtight tin or wrapped in stainless steel foil with something combustible to burn off the oxygen.

Tony

Stewart Hart05/04/2019 09:05:58
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Hi Nick

Thanks for the useful graph, as I thought it shows the temp for stress relieving to be around 600 C our oven won't be hot enough.

Cheers

Stew

Stewart Hart05/04/2019 09:11:24
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Thanks Tony

The trick with the air tight tin/foil would do the trick to prevent scale, thanks for the tip

I have enough bar to have a couple of tries so I think I'll try rough machining first to see how much it warps.

Stew

JohnF05/04/2019 09:40:30
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Stewart, take a skim off all round the bar and you will find warpage will be minimised, stress reliving is of course better but you do need to achieve the correct temperature. Depends on size of the job and the tolerance you wish or need to achieve.

There is heaps of info on the net if you look but 600 to 650 deg C is about right for steel.

How big is the piece and where are you?

John

IanT05/04/2019 09:41:31
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Stewart,

Mentioned this recently on another thread - but don't be tempted to use kitchen foil - it will melt.

I know!

IanT

Hopper05/04/2019 09:47:41
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Ask your steel supplier about buying normalized cold rolled bar. The makers do the faffing about with torches/furnaces for you.

IanT05/04/2019 10:00:17
1266 forum posts
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There was a thread about cast iron 'chill' - and it's treatment recently Stewart. Not the same problem I know but perhaps a similar solution.

https://www.model-engineer.co.uk/forums/postings.asp?th=138364

I've placed both suspect castings and mild steel in my incinerator overnight to help with these things. I won't repeat everything again (as it's all in the previous thread) but I do feel it helps.... but I would also do a wee bit of roughing first too

Regards,

IanT

Stewart Hart05/04/2019 10:00:30
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Thanks for your input chaps

the part is 1”x5/8x21/2” with the middle bit removed so it’ll bend like a banana putting the ends out of square which I want to avoid

I’ll see what roughing it out first does to it I may be able to live with the results

cheers

Stew

John Olsen05/04/2019 10:33:49
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I would get out the propane torch and heat it up to a good red heat, keep it there for a while, and then let it cool as slowly as possible. A few chunks of firebricks around it to make a rudimentary furnace would help keep the drafts off. There will be some scale but not as much as with the old way of putting it in the embers of the fire overnight. There is very little danger of accidently getting it too hot, you won't melt it with anything you are likely to have at home.

If you don't have a propane torch, it is a really worthwhile thing to acquire, since you can also use it to harden bits of silver steel, or drill rod if you like. (or gauge plate.)

John

Ian S C06/04/2019 12:36:35
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After heating, the quench should be quick and for a rod or bar it should go in vertically, put it in sideways and you have a fair chance of getting a bend in the article.

Ian S C

Andrew Tinsley06/04/2019 12:44:20
890 forum posts

Hello Ian, I thought that treatment was for hardening? Do you not want a slow cooling for stress relief? Quite happy to be shot down over this as the memory is not too sharp these days.

Andrew.

IanT06/04/2019 13:12:36
1266 forum posts
128 photos

That's right as far as I know Andrew (other Ian here)

My incinerator treatment builds up a deep ash bed and the bits inside are still very hot even 12-15 hours after the night before. I leave them there until I tip the whole lot out. So fairly prolonged high heat, followed by a slow cool. Not too scientifically controlled in my case perhaps - but I don't have the equipment to do anything else...

Regards,

IanT

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