Stresses and stress relieving. How best to go about it.

[David ColwillParticipant @davidcolwill19261]

Hi,

Most of us are aware that if we take a piece of bright drawn mild steel, skim one side of it flat and leave it, when we come to check it, the machined side will no longer be flat.

So how do I avoid this?

Stress relieving by heating to bright red seems to be one way. I would be interested to find a reliable method and any formulas for time / thickness / rate of cooling.

I have also heard that hot rolled or black steel is better as there is less stress in the material due to the temperature that it is worked at. I would like to hear any comments about this.

Guage plate or ground flat stock is supposed to be good in this respect but is quite expensive and it is not always necessary to use this grade of material. Again any thoughts / experiences?

Next if all this is happening in flat bar what about round?

Whilst I realise that for most things that I make none of this is necessary, There are occasions that I would be prepared to go to some trouble and would like to know how best to go about it.

I do have facilities for heating materials, so it is perhaps not so difficult but I would welcome the voices of experience before embarking on any particular route.

Many thanks.

David.

[David ColwillParticipant @davidcolwill19261]

[AjohnwParticipant @ajohnw51620]

For turning black bar free cutting mild steel is better than bright drawn. I understand this is down to it cooling naturally so there is less stress. Bright drawn is then processed cold which will add stress and it varies. Machinists did have a term for the bad stuff – park railings. From WWII when they were melting anything that could be found. I have had a supplier wait for some "good stuff" to come in. He cut hexagon disks for me from plate. I expected cut bar. Only ever happened once and it turned rather well. Some doesn't.

I have no information on annealing it other than red heat and slow cooling. Castings are much the same except lathe manufacturers traditionally leave their castings lying around for a very long time before doing anything to them. This is why some people prefer to fabricate rather than buy castings especially for tooling. It also suggests that at least in this case heat treatment isn't the best way to go about it.

John

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[KWILParticipant @kwil]

Heat to red, hold for 1 hour per inch thickness (bit long and can be less) then cool slowly. I always do this on BMS section which I intend to change the shape, so as to avoid unrelieved stress bending referred to above.

Heat within a heat insulated area ie with a few insulating blocks, cover once you stop heating and allow to cool.

Spoils the shiny surface of BMS but you cannot have both ways!

[IanTParticipant @iant]

A simple solution to this issue that I use occasionally (originally not with raw material – but with a large work piece that needed to be "repaired" by welding) is to stick it in the bonfire – or in my case – my garden incinerator.

After I screwed up some milling work on a large piece of block steel, I had to fill an unwanted deep 'groove' with some built up weld, I was worried about distortion and wanted to relieve any stresses before I finished re-machining the work. My solution was to stick it my garden incinerator once I got a good bed of hot ashes going in the bottom of it. I then kept the fire going with the rest of the wooden garden waste through the evening and then let it burn all night with the lid on. The ashes (and steel part) were all still very hot the next morning. I was able to trim off the excess weld and re-machine the surface without any distortion later. It wasn't a perfect finish but it also happened to be in an area that wasn't normally visible once assembled.

My garden generates lots of wood the local council won't take away in our brown bin, so I have incinerator bonfires several times a year. Since the original welded steel piece, I've dumped several small brazed fabrications in there, as well as a few (part machined) steel bits from the scrap bin that I was worried might distort. Whether my fears were well founded (or not) I don't really know but I can say that they all behaved themselves perfectly afterwards. I most certainly have had work that has bent like a banana after machining, so better safe than sorry…

Regards,

 

IanT

Edited By IanT on 20/07/2015 12:25:12

[Brian WoodParticipant @brianwood45127]

David,

Some time ago I made a spare arbor for my Senior horizontal mill, with 1/4 inch keyway, so that I could use that to cut a keyway in the pukka version that belonged to the mill.

I had a section of 40 mm diameter EN1A from which to turn this out, the 40 diameter gave enough material to include the drive flange. Stress relieving was done in the domestic oven at 250 degrees C for 2 hours and left to cool down slowly overnight. It turned a pretty shade of dark blue

It did all I asked of it and when last seen it has not moved, neither did the pukka arbor after the keyway was cut in it; that had no such treatment.

Perhaps red heat is a council of perfection, my piece was a fairly hefty section to heat that far.

Regards

Brian

[David Clark 13Participant @davidclark13]

Machine a light skim off one side. Allow to bend. Turn over and machine a light skin of the other side. Now you can machine both sides and they should remain flat. If you do heat treat it do not bend it afterwards or you will introduce more stress.

 

Edited By David Clark 1 on 20/07/2015 13:05:02

[Anonymous]

For normalising low carbon steels I put the parts in an electric muffle furnace and let it heat up to around 850ºC and then soak for about an hour per inch of thickness. Then I turn the thermostat down and let the parts cool without opening the furnace.Once the parts are below 300ºC or so I let them cool in air out of the furnace. Then into vinegar to get rid of the scale.

I use a lot of hot rolled steel, as it is a darn sight cheaper than bright drawn. A disadvantage is that the standard length is 6 metres. That's why I use a lot of it! I don't normalise it, and it has never distorted during machining. However, it can be a bit 'gooey' and is more difficult to get a good finish compared to EN1A. It seems to be rather less prone to rusting compared to EN1A. For round bar I tend to use EN3 in preference to EN1A. Again more difficult to get a good finish but less prone to rust. If you are machining round bar symmetrically, ie, reducing the diameter then distortion is unlikely to be a problem.

I have made a number of parts and specialised tooling out of gauge plate and silver steel. I have never had a problem with distortion during machining. Apparently they can distort during quenching, but it is not a problem I have seen. Although I'm not looking to hold tolerances to tenths.

Andrew

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