Why does prehardened 4140 (and similar) distort when rested between machining ops? 3

[Nereth1]

Hi,

It's a fairly common occurance in my work that 4140 quenched and tempered shaft, when machined to reasonably precise tolerances (e.g. 0.1mm TIR over 500mm, for a Ø120mm shaft), needs to be 'rested' for 24 hours or more between the last few passes on the lathe, to let it distort, so that there is no distortion left to happen after the final pass.

We are talking about shafts with say 20-30% diameter removal. There is no elevated temperature annealing in the process.

Now, I understand why it would move during annealing (yielding of the areas of highest internal stress as the temperature weakens them), and I understand why it would move during machining (uneven removal of internally stressed bits of steel, and maybe even microstructural changes from the temperatures/stresses at the cutting tip), but what I don't understand, is how it is still moving hours after machining is done?

What's happening? Ongoing phase changes at room temperature? Ongoing yielding or stress relief?

I would have thought all of those would happen basically instantly or in the first few seconds after machining?

 

[WKTaylor]

N1... RE 4140 shaft...

What specification 4140 material are You using?

What heat-treat [HT] temper per what HT specification?

What size/diameter shafting?

Are You flooding the machining with coolant-lubricant.. or only using cooling air? Any temper discoloration emerging?

Why are You 'resting' the part as-opposed-to intermediate stress relieve HT?

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[EdStainless]

Size is critical, 4140 is shallow hardening. Have you taken a cross section and looked at the hardness across it?
You are seeing the effect of residual stresses, and perhaps some phase transformation.
You might do better by rough machining and then re-tempering at 50F below the original temper.


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[weldstan]

In addition to Ed's statements, the mechanics of heating, quenching and tempering can affect the depth of hardening both about the shaft's circumference and length, further affecting residual stress distribution and phase changes. We have also seen this in het treated 1045 shafts.

 

[WKTaylor]

Ed... wink-wink...

AMS2759/11 Stress Relief of Steel Parts
Table 1
Carbon & Low Alloy Steels... Quenched and Tempered... 50 °F (28 °C) below tempering temperature

Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[EdStainless]

Will, amazing how that works isn't it?

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P.E. Metallurgy, consulting work welcomed

 

[Nereth1]

Hi all,

I understand residual strains and uneven hardening at a non-metallurgists level (i.e. what I'm missing may be key to the answer of this question), but that understanding indicates that when you remove some of the steel, the residual strains it was imposing would redistribute basically at the speed of sound in the steel, and any phase transformations likely wouldn't be occurring at room temperature, which the bar would be at within minutes of machining ceasing.

So that implies all the movement should happen almost instantly, as you machine the bar. So the question is, why does the bar keep moving over the next days/weeks post machining? By what mechanism is the movement delayed.

Ahmad

 

[EdStainless]

No, No, No, they aren't that fast at RT. Both the distortion from residual stress and delayed phase transformation (some of which may be driven by the forces from machining and some may by the changes in stress) can both take a few hours to resolve at RT. Even though they are both diffusionless and at even slightly elevated temps would happen very fast it isn't so at RT.
We used to straighten shafts and required a 2hr rest after straighten before final verification (these were only 1.375" diameter).
Some people will claim that vibration will assist in speeding the process, we never had consistent results. Hitting the critical frequency and intensity is very particular. We also experimented with slight heating (200F, 300F, 400F, our temper was about 1000F) and it helped some but wasn't worth the trouble. It did help if we could hang the shafts but at up to 32' this was a pain.

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P.E. Metallurgy, consulting work welcomed

 

[Nereth1]

OK so that's interesting. A star for you for now teasing me with hopefully imminent good discussion

1) I can 100% believe that phase transformation can take a while - mainly because I don't understand the mechanisms so I have no reason to believe it wouldn't take a while, except that I thought it didn't proceed at all at RT. I guess that it does?
2a) I'm confused as to how residual forces could take more than a fraction of a second to resolve - mechanistically that doesn't make sence
2b) Unless, the problem is the interactions between the two - the residual forces resolve, the new stress field causes new phase transformations, which causes new residual stresses that need to resolve, etc?
3) Would an elevated temperature stress relief basically force the resolution of all these delayed processes?

 

[EdStainless]

When you quench from austenitizing temp you may not get get full conversion to martensite. So the retained austenite wants to convert, but once cold the kinetics are very slow. When it does transform you get the slight size change and pieces of un-tempered martensite. Now as you start machining the energy input from cutting and changes in residual stresses provide additional energy input that may cause some additional transformation. Any energy input can do this, it doesn't have to be elevated temperature.
In alloys that are difficult to quench this is why you see cryo treatment (depending on the alloy -50 to -100F) being used, the greater change in temp creates more driving force for transformation. And then you follow this with a second temper.
In alloys that quench cleanly often a second temper works well for some stress relief.

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P.E. Metallurgy, consulting work welcomed