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Cast Iron stress relieving

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Pete.13/03/2019 00:32:02
222 forum posts
36 photos

A few weeks back those who read it might remember a thread about straight edges, one of the members on here (thank you Brian) made a small batch, I just received mine, from the info I can find, views on stress relieving seem to vary, anyone have any experience on this subject?

Will doing several cycles in a kitchen oven do? would it be better to have it done properly? and if so, does anyone know where? all I can find is places that seem to deal with big industrial jobs, not sure how receptive they'd be if approached by some random for a single small job.


Paul Kemp13/03/2019 01:37:48
548 forum posts
18 photos

I think heat treatment to stress relieve is a fairly modern idea to speed up the job. Back when Stephenson was a lad (well maybe when his grandson was a lad anyway) new castings as you probably know we're chucked out in a field to weather, brought in and part machined and stuck back out to grass before final machining and finishing. All of this you are probably well aware of! This was still considered to be the way to do it when I worked for a large marine diesel manufacturer in the late 90's (probably though because there were few facilities big enough to take the castings anyway!) Obviously you want to use your camel back before you are too old to pick it up though, so that approach is probably not going to fly! I doubt though that the domestic oven will do the trick, better bet might be an aquaintance with a pottery kiln or similar. Best bet if you want a professional job is to find someone on the inside somewhere that can slip it in with something else. As you say most big companies view the likes of us as a pain in the backside so quote telephone number prices to discourage us!


Hopper13/03/2019 03:04:23
4778 forum posts
104 photos

There's a school of thought that throwing it in the bottom of a camp fire or wood stove overnight to soak up heat in the glowing embers then slowly cool off in the ashes overnight would do the job. Or you could heat it up with two large propane torches until red hot and bury in a bucket of lime to cool slowly overnight. I did that with a welded-together fabricated steel dividing head body and it worked well. Machined beautifully and lack of binding of any moving parts some years later indicates that the body has not moved or warped etc.

Martin Johnson 113/03/2019 08:30:53
129 forum posts
1 photos

The last time I got something stress relieved (about 30 years ago) I was able to go to a local heat treatment company (in Loughborough) and get it put through with a batch of something else. If you can find a company, it costs nothing to ask.


Pete.13/03/2019 23:46:44
222 forum posts
36 photos

Paul, after doing a little research, there was a discussion on a well known Machinist site, apparently the US navy did research on the leaving outside to weather theory, and the conclusion was it made little to no noticeable difference, there used to be a lot of placed here in Leicester that I could have popped in to to ask years ago, but these days they seem to be fewer and further between, and make themselves very unapproachable to Joe Public.

Hopper, I like the BBQ idea a lot, a couple of bags of charcoal sounds like my kinda budget, it could be left at heat for an hour or so, then periodically add a few bits of charcoal to give it a nice slow descent back to cool, and doing this at home I could repeat 2 or 3 times over 4 weeks or so, thanks for idea, I think that's the route I'm going to take.

Hopper14/03/2019 05:22:34
4778 forum posts
104 photos

Not sure if BBQ charcoal would get up to the required temp to get the casting up to cherry red heat, unless you add some kind of blower like a hair dryer etc to up the ante a bit. But I have never tried it so there is only one way to find out...

pgk pgk14/03/2019 05:29:05
1890 forum posts
288 photos

Right now if you can keep the rain off it then a blower isn't needed

Mad March days can be useful...

HOWARDT14/03/2019 08:23:55
583 forum posts
15 photos

Where I worked, in Leicester, castings were kept outside. But this was in the days of reasonable volumes when buying a 100 or so of a part was no big deal. Move on twenty years and small volume and single castings, heat treatment of castings and fabrications. Lastly over the last years some vibrational stress relieving of fabrications and machining from solid rather than casting.

Nigel McBurney 114/03/2019 09:39:00
727 forum posts
3 photos

Its the slow cooling thats important,to avoid cracking, Suggest machine the mounting feet,then place the casting feet down on a surface plate check that the feet are flat and the casting does not rock,to get a good job scrape it flat with scrape rand engineers blue.Then rough machine the long top surface and then put the casting aside ,store it the garden shed for at least a month so daily temp range will help to remove any further internal stress.Check the feet again with blue and surface plate to ensure the feet are still flat,then machine the top surface and sides if required, then check flatness against a good surface place and scrape in if necessary or take somewhere to get it surface ground.

This is the process that I learned during my time (1950s) making optical benches in lengths from 250 mm to two metres,Very few true fish belly types were made and these usually stood for over a year at the cold end of the factory,they were not annealed by heat treatment.

Pete Rimmer15/03/2019 14:59:39
773 forum posts
50 photos

There's no point scraping the feet THEN stress-relieveing it because the scraped faced will need doing again afterwards.

If you're going to do it - rough-machine it, stress-relieve, machine again then either scrape it or do another round of stress relieving and machining then scraping. Forget fire pits and barbeque tricks for a reference surface like this, take it to a professional and have them heat soak it in an oven so it gets even heat all over then controlled cooling. Anything else is just rolling the dice and hoping.

Personally I would have liked to see a lot more thickness on the sole of the one in the pic above. It's going to be very thin once it's machined past the skin.

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