Stress relieve in castings.

[Eric CoxParticipant @ericcox50497]

[Eric CoxParticipant @ericcox50497]

Does anyone put their cast iron outside to stress relieve or do you machine it as soon as you get it.

[Martin WParticipant @martinw]

Eric

 

This might be a very naive question but why would putting a lump of cast iron outside relieve stresses in the material? I can understand that heating a lump of cast iron to a relatively high temperature, holding there for a period of time and then letting it cool very slowly could reduce stresses but not how by just putting it outside would work.

 

Cheers

 

Martin

 

PS

 

Just read an old US Naval document that looked at stress relief in grey cast iron and it showed that heat treatment could relief almost all of the stresses. However if the casting was exposed to varying atmospheric conditions for at least 4 months then it this could reduce stress by 15%. That still leaves a lot of stress in the material.

 

The pdf file can be viewed here.

Edited By Martin W on 25/01/2012 10:49:43

Edited By Martin W on 25/01/2012 11:04:15

[Gordon WParticipant @gordonw]

Depends how long since the casting was in the furnace, and of course what material they are. Used to be quite common for a buyer to specify “weathered castings”, but what good it did I do not know. Rough machining a casting, to remove the skin, and then leaving for a week or two, is probably worthwhile.

[Tony JeffreeParticipant @tonyjeffree56510]

I find that I generally need to leave the materials and kits for potential projects under the bench to “mature” for a few years before I start building them, but that is more to do with the world shortage of round tuits than reducing stresses

 

Regards,

Tony

[Martin WParticipant @martinw]

Tony

 

If you do find some round tuits I would be very grateful if you would let me know where you found them, as you say they seem to be as rare as rocking horse droppings.

 

 

Martin

[Donald MitchellParticipant @donaldmitchell68891]

I bought an iron casting from Blackgates in 1979 for an Alan Timmings dividing head, it’s been under the bench ever since – does the team think I should leave it for another couple of months or so, or will I get started into it now?

Donald Mitchell

Castle Douglas

Bonne Scotland

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Ageing and stress relief are strictly speaking not the same thing .

 

The apparent tendency for castings to spring when first machined or change shape in storage has two main causes :

 

(1) Trapped stresses caused by different cooling rates for different sections of the casting . These stresses will be there for a long time unless deliberately relieved by heat treatment or less well by mechanical treatment such as tumbling with other castings in a barrel .

 

(2) Metallurgical changes . The common description of cast iron is that it simply consists of flakes of carbon in an iron matrix . This is not entirely true in an actual casting -especially in ones done without much process control . The metal as cast is acually made up of several different components . The bulk is simple cast iron but the remainder consists of several different types of iron/carbon compounds each with different properties . The actual cast iron bulk and these other materials are in an active chemical state when at casting temperature – mutating from one form to another as temperatures change . The problem is that these mutating changes are quite slow acting and when the casting is cooling down these mutations may not be complete before it solidifies . Some of the changes will continue very slowly for a long time after casting has cooled . Since each compound has different mechanical properties small shape changes are continually happening until stability is finally reached .

 

This problem can be reduced by a post casting heat treatment but leaving castings just to age in a field works just as well – but be aware it can take years for a big complex casting like a machine bed .

 

If any of you have access to big freshly cooled complex castings try listening to them with an engineers stethoscope – at intervals you will hear tiny clicks as the metal changes . Also with a search coil and a sensitive oscilloscope you can sometimes detect tiny magnetic pulses coincident with each click .

 

MW

 

 

Edited By MICHAEL WILLIAMS on 25/01/2012 12:51:06

[Ian S CParticipant @iansc]

I think the only reason the castings are put outside for stress relief is that there is no room inside. When I bought my lathe, the owner of the company I got it from had just returned from Taiwan, where he toured the factory, and out side was a large stack of lathe bed castings with Colchester England written on them (same bed as my lathe), the bloke showing him round said oh we make all their beds, they make their own headstocks and othe bits. Think he may have said they stayed there 3yrs, before machining and hardening. Ian S C

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Martin W on 25/01/2012 11:50:00:

Tony

 

If you do find some round tuits I would be very grateful if you would let me know where you found them, as you say they seem to be as rare as rocking horse droppings.

 

 

Martin

Martin –

 

I suspect you will find them the same place as hens’ teeth and flying pigs

 

Regards,

Tony

Edited By Tony Jeffree on 25/01/2012 12:50:13

[Ady1Participant @ady1]

I just drop my castings into a bucket of guiness

 

Al stress is relieved within 24 hours

[Tony JeffreeParticipant @tonyjeffree56510]

Posted by Ady1 on 25/01/2012 12:51:09:

I just drop my castings into a bucket of guiness

 

Al stress is relieved within 24 hours

 

Is that before or after the guinness has been recycled?

 

Regards,

Tony

[colin hawesParticipant @colinhawes85982]

I can remember castings being left outside to rust .I was told that this was to stress relieve them by virtue of the rust eating away at the hard skin.Not much use to us though; they had to be left for several years for a good result! It seems that embedded sand was also removed by this process.

On second thoughts I have taken so long on one project it might have been a good idea! 

Edited By colin hawes on 25/01/2012 13:26:50

[Clive HartlandParticipant @clivehartland94829]

A better method is to pre-machine the castings with a given tolerance for later work and then put them away to age.

Hammering and vibration would work as well.

Time of aging is up to six months.

Remember welded components need to be stress relieved also by hammering.

Thats my theory anyway!

 

Clive

[mickParticipant @mick65121]

For what its worth, I was always told that back in the day Rolls Royce left their castings outside for twelve months prior to machining, the process was known as weathering, but as my old dad would have said, “it won’t make the price of woodbines any cheaper!”

[mgjParticipant @mgj]

Armour castings like tank turrets and hulls are left outside for 6 months or so. Its a lot cheaper to leave them outside than it is to reheat and cool slowly in a furnace.

 

Outside is better than indoors because of the greater thermal range.

[Ex contributorParticipant @mgnbuk]

The bed castings for the older 3 Vee bed Boxford lathes were “weathered” for 6 months before finish machining. There were pallets of bed castings around the works yard, and they were copiously watered daily witha hosepipe (when it wasn’ t raining or snowing !).

It was not unknown for some of the workforce to “water” them as need arose, either !

 

After 3 months the castings were shot blasted & “topped and tailed” – rough planed top and bottom – then put oustside for another 3 months of “weathering”. After another sholblasting, they were filled and painted before finish machining – bottom planed & top “gang-milled” to produce all the vees, flats & sides at one pass on a very robust Russian-built duplex head horizontal mill.

 

The later hardened bed machines were just being developed & put into production when I was there. The bed castings for those came in “stress-relieved” from the foundry – no “weathering”, they went straight from the delivery to production. There were some “issues” with the beds moving during machining. The beds had the top formation gang-milled as with the older machines, then they went on to a fixture on a new Snow grinder with a Radyne induction hardeneing plant mounted at one end. The bed passed slowly under the induction hardening device, which locally heated a small area to bright red heat, which was immediately quenched with coolant. After hardening, the grinder table was moved under the wheel for grinding the top formation in one pass, the wheel being dressed to shape by a special diamond dresser.

 

I seem to recall that the trial beds were not straight & the “old hands” were keen to blame the stress relieved casting – no substitute for weathering ! The answer, if I recall correctly , was to pre-stress the bed by tightening a clamp in the middle of the bed (with a torque wrench to apply a repeatable load) before hardening. This clamp was released before grinding. Result – straight beds without weathering.

 

I have – somewhere – photos of this hardening & grinding process. I got a Praktica MTL3 SLR for my 21st birthday & one of the first films I exposed though it was around the Boxford works. This was April – May-ish 1981. Seems a long time ago, now !

 

Regards,

 

Nigel B.

[Sub MandrelParticipant @submandrel]

I read somewhere that it’s the diurnal cycles of heating and cooling that imprve the stability of the casting, and that sub-zero temperatures rising to above zero are most effective. the process can be simulated and speeded up by cycles of freezing and warming to room temperature, but as many cycles are needed a winter outside is the simplest and most efficient way of doing this.

 

Neil

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