Machining & stress reliving question.

Tin Man · Apr 19, 2017

If you have a part that looks like say, a 4' tall horseshoe burned from 3" A-36 plate and you machine the inside of the "U" the legs will move out as material is "relived" from that area.

If the part was stress relived after the burning (cutting)process and before the machining process,would it still move as material is being removed from it?

DMF_TomB · Apr 19, 2017

Tin Man said:
If you have a part that looks like say, a 4' tall horseshoe burned from 3" A-36 plate and you machine the inside of the "U" the legs will move out as material is "relived" from that area.

If the part was stress relived after the burning (cutting)process and before the machining process,would it still move as material is being removed from it?

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usually stress relieving by heat (annealing or normalizing) will reduce warpage to less than 10% of what it would do with no stress relieving.
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often warpage is in such small amounts that a rechuck at lower torque and a light finish cut will remove even the small warpage if there was any.
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some parts not stress relieved will literally warp ever time even if only a small amount of material removed

Tin Man · Apr 19, 2017

So you are saying that stress reliving will greatly reduce the movement caused by material removal correct?

eKretz · Apr 19, 2017

Tom has given a good guideline. I'll add that the amount of stock you remove will also play a role, and whether you remove equal amounts of material from both sides of the "U".

DMF_TomB · Apr 19, 2017

Tin Man said:
So you are saying that stress reliving will greatly reduce the movement caused by material removal correct?

usually heat stress relieved parts warpage is very small or reduced considerably
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stress relieve by vibration i have no data that is is as nearly as effective or even if it does anything. vibration stress relieving basic part is in contact with a table or vibration producing device that is intense. like if not bolted down or clamped it will vibrate off the table.
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just saying you got to be specific on method of stress relieving

Tin Man · Apr 19, 2017

Stress reliving by heat.

I am asking because I have heard many different "opinions" on this. One source has said that even after stress reliving it can warp because machining puts stress back in. Is that so?
I am only talking about machining the inside of the U. I realize if I machined the outside it would move the opposite way. I always thought it moved because the outside still has stress from the burn process and when you take material out from the inside it allows the outside to pull it.
I would think stress reliving would help with that unless machining does put stress back in.

Matt_Maguire · Apr 19, 2017

Rolled plate has a grain direction to it & you'll see more movement cutting along the grain as opposed to across the grain.

I've never heard of anyone stress relieving A36? If you aren't going to weld on it later one of the resulfurized steels will play way better with machining.

Good Luck,
Matt

DMF_TomB · Apr 19, 2017

metal when cold rolled and if machined in certain ways the skin of metal has different properties. it acts bimetalic or like 2 different metals joined. they push and pull on each other. when you machine one side you weaken one side so other side is stronger and part can warp.
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normally machining will not create a lot of stress but some critical parts are stress relieved, rough machined, stress relieved again. finish machined like from milling, possible stress relieved again and then ground
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depends on tolerances. majority of parts i have seen are only stress relieved before machining. maybe only 2% get extra stress relieving

michiganbuck · Apr 19, 2017

Job causing too much stress.. walk away..go have a beer or two.. think about it or not and come back tomorrow.

Tin Man · Apr 20, 2017

So if I have a 8' long bar of 2" x 4" 304 stainless steel that has been stress relived,and machine it to a 2-1/4 x 2" x 8' bar all taken off 1 side,will it be expected to move? If it was stress relived and moved,machining must put stress in then correct? Is there a formula to calculate projected deviation?

DMF_TomB · Apr 20, 2017

Tin Man said:
So if I have a 8' long bar of 2" x 4" 304 stainless steel that has been stress relived,and machine it to a 2-1/4 x 2" x 8' bar all taken off 1 side,will it be expected to move? If it was stress relived and moved,machining must put stress in then correct? Is there a formula to calculate projected deviation?

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304 typically is cold rolled or cold finished and usually will warp when machined. stress relieved it will warp less. usually a lot less warpage. but even if warpage was reduce to 10% there still is that small amount of warpage.
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machining usually does not put a lot of cold working stress into metal if cutter is sharp. condition of cutter and how its used effects any induced stress.
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i am not aware of any formulas as how much cold working was done normally cannot be determined with any accuracy thus warpage cannot be determined with accuracy.
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normally a part is rough machined and rechucked lightly and not beaten down any way to distort its relaxed warpage shape. the finish cut typically less than .020" then takes the warpage out. when unchucked sometimes some warpage occurs again of a small amount. sometimes it is rechucked a 2nd time and a 2nd finish cut of often only .003" or less is taken to remove the slight warpage if any.
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from my experience if not stress relieve a 12" long 304 SS part can often warp over 0.100" depends on thickness. usually thinner stuff warps more.

charlie gary · Apr 20, 2017

If you have stress left behind by burning the part out of plate, can you find a waterjet to cut the part for you? No heat affected zone to cause grief later on, meaning you won't have that extra stress relief step if the torch cut is the source of your movement.

Tin Man · Apr 20, 2017

I am only using this part for reference so people can understand my question the side actually has a profile you could not water jet.

If a part was "stress relived" why would it warp if you did not put stress into it with the cutter?

MCritchley · Apr 20, 2017

Matt_Maguire said:
Rolled plate has a grain direction to it & you'll see more movement cutting along the grain as opposed to across the grain.

I've never heard of anyone stress relieving A36? If you aren't going to weld on it later one of the resulfurized steels will play way better with machining.

Good Luck,
Matt

Every A-36 plate i get stress relived. Common for me is a shape burned from a plate, stress relived, Blanchard ground and machined.

Mtndew · Apr 20, 2017

DMF_TomB said:
metal when cold rolled

He's not using CRS. He's using A36 plate. :rolleyes5:

TeflonDave · Apr 20, 2017

Tin Man said:
If you have a part that looks like say, a 4' tall horseshoe burned from 3" A-36 plate and you machine the inside of the "U" the legs will move out as material is "relived" from that area.

If the part was stress relived after the burning (cutting)process and before the machining process,would it still move as material is being removed from it?

I think you got a hella big piece there. I would suggest skin cutting most of the molten plasma cut surfaces and the 2 faces. Problem is usually that ya start figuring on movement by stresses after the part is rough cut and in the house. I do not know application or the thickness of the legs etc... but I do believe a good stress relieve cycle is found in the controlled rate of cooling. the cooling rate is very important. A cycle could take 2-3 days.

A good heat treating house can tell you what kind of cycles they can run and how good the ovens are etc, but they usually will shy away from recommending the best way to do the job because of well - I don't really know, just my experience.

It looks like you got alot of pieces of good advice here in this thread. And don't be surprised if you think about 2 stress reliving cycles a pre-cut and finish cut. Beats the hell out of straightening and bending and redesigning and welding add-ons I can tell you that for sure.

One more word on annealing or stress reliving - I worked with metals that were frozen to very low temperatures like cold liquid nitrogen temps and then brought up to ambient temperature very slowly and that was an annealing or stress relieving cycle also.

eKretz · Apr 20, 2017

TinMan, you don't seem to be understanding the actual mechanics of the issue. The stress is already in the steel, some remains even when the steel has been thermally stress relieved, it has just been significantly reduced. One could say that the steel is forced into an unnatural or "sprung" state during its manufacture. The movement due to heating and cooling and external forces like rolling as well as the forces produced internally by heating being uneven throughout the steel produce these sort of "locked in" or frozen stresses.

When you cut away some of the steel, you unbalance these forces that had been balanced against each other, and movement occurs. From what I understand, thermal stress relief brings the temperature of the steel up high enough that the steel molecules all push against each other enough to yield the steel somewhat, helping to equalize the internal forces. Then the steel is slowly cooled so that it evenly cools throughout such that forces aren't going to be locally higher in some areas than in other areas. There still may be stress/locked-in "spring" in the steel, it will just be much reduced.

DMF_TomB · Apr 20, 2017

A36 steel like other metals can warp immediately or have to think about it for some time. i have often seen metal warped and bent back straight and after a few days it warps back a bit.
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not sure the reason why but many metals warpage is not immediate and stops rather you might get a large change suddenly and some more change after some time has gone by
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A36 like any rolled product you got no ideal how much rolling done while still hot enough. if temperature too low it basically can have some rolling done in the cold rolled condition. i would have my doubts on how thoroughly A36 plate is stress relieved at a steel mill. where i work everything goes to get annealed even if supposedly it is already stress relieved

gbent · Apr 20, 2017

You don't say what type of tolerances you are trying to hold on the legs. Even 1/8" across two 4' legs can be a tight tolerance. If I was worried I would have them stress relieved before machining. Given A36 is often a scrap yard metallurgy, the stress relieving could also help with machining the cut edge.

Matt brings up a very good point on grain direction. Again, depending on tolerance, it may yield more predictable results if all pieces are cut with the same grain orientation, even if it is wasteful of the raw plate.

IME, yes the parts can move depending on machining stress. If you are using sharp corn cob roughers with a 10hp 40 taper spindle, you aren't going to put much stress in the part. Now if you have a 60 taper 50hp spindle with a 5" inserted carbide rougher and you are running .100 of wear land on the inserts, yes, you can put considerable compressive stresses into the part.

Tin Man · Apr 21, 2017

eKretz said:
TinMan, you don't seem to be understanding the actual mechanics of the issue.

From the answers I don't think some understood why I was asking the original question..

I guess I could rephrase the question..
Because what I am really trying to get at is:

If a part is "stress relived" does it still contain residual stress?

Ekretz says yes and that answers my question. So stress reliving is not stress eliminating. Steel will still have some after and will warp as material is removed.
Is there a way to 100% stress relive metal? If so and there was none it wouldn't warp then correct?

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