S7 Heat Treat - Can it cause wheatstone bridge anamolies

I am working with load cells made from S7 Tool Steel (T41907) that have already been machined and are heat treated in the following manner: 1) individual 1"x1"x3" are placed in a steel foil bag, and evacuated. 2) the pieces are placed in an oven that has been sitting at 1200-1300F and soaked for 1/2 hr 3) the pieces are transferred to a 1750F oven and held at 1hr. 4) the pieces are removed from the oven and are allowed to sit in an ambient air (typically opened garage door) for 2-3min and then cooled by forced air via fans to about 100F and the rockwell hardness is checked (59-61) 5) the parts are then placed into an oven that is at 1060F and held there for 2 hrs. Again the pieces are cooled as in step 4. 6) the parts are again subjected to step #5 - 2 more times (except at 1000F) for a final desired rockwell hardness of 50-51.
I am under the impression that ramp rates (hot to cold, cold to hot) if done at a controlled rate will help to eliminate stresses. Seeing that these parts are being used in a wheatstone bridge application I would believe that you would want to keep any residual stresses to a minimum. Could creep at full scale range / non linearity / non-repeatability be attributed to internal stresses caused by inconsistencies in heat-treat?
Thanks for your time Qu1nn

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I don't believe that's the case, Qu1nn. The ramping rates are intended to prevent *differential* expansion and contraction *during the heat treatment*. I don't believe they have any effect on the internal stress of the finished piece. They just keep it from cracking during the heat treatment.
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Please explain what you mean by "creep." Physical creep, in the sense that the word is used by materials scientists, is so slight in tool steel, and requires such very heavy tension or compression loads, that it's hard to imagine how it would be involved in a Wheatstone bridge.
If you mean a release of internal stress, that's one that may not have a known answer. How is this cell loaded? And how does it function in the bridge -- as a variable resistance?
Ed Huntress

EH> >> I am working with load cells made from S7 Tool Steel (T41907) that >> have already been machined and are heat treated in the following >> manner: >> 1) individual 1"x1"x3" are placed in a steel foil bag, and evacuated. >> 2) the pieces are placed in an oven that has been sitting at >> 1200-1300F and soaked for 1/2 hr >> 3) the pieces are transferred to a 1750F oven and held at 1hr. >> 4) the pieces are removed from the oven and are allowed to sit in an >> ambient air (typically opened garage door) for 2-3min and then cooled >> by forced air via fans to about 100F and the rockwell hardness is >> checked (59-61) >> 5) the parts are then placed into an oven that is at 1060F and held >> there for 2 hrs. Again the pieces are cooled as in step 4. >> 6) the parts are again subjected to step #5 - 2 more times (except at >> 1000F) for a final desired rockwell hardness of 50-51. >> >> I am under the impression that ramp rates (hot to cold, cold to hot) >> if done at a controlled rate will help to eliminate stresses.
EH> I don't believe that's the case, Qu1nn. The ramping rates are intended to EH> prevent *differential* expansion and contraction *during the heat EH> treatment*. I don't believe they have any effect on the internal stress of EH> the finished piece. They just keep it from cracking during the heat EH> treatment.
>> Seeing >> that these parts are being used in a wheatstone bridge application I >> would believe that you would want to keep any residual stresses to a >> minimum. Could creep at full scale range / non linearity / >> non-repeatability be attributed to internal stresses caused by >> inconsistencies in heat-treat?
EH> Please explain what you mean by "creep." Physical creep, in the sense that EH> the word is used by materials scientists, is so slight in tool steel, and EH> requires such very heavy tension or compression loads, that it's hard to EH> imagine how it would be involved in a Wheatstone bridge.
EH> If you mean a release of internal stress, that's one that may not have a EH> known answer. How is this cell loaded? And how does it function in the EH> bridge -- as a variable resistance?
EH> Ed Huntress
If it is part of a load cell, it will typically have a strain gauge glued to its side. The strain gauge will form one of the arms of the Wheatstone bridge.

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Ok. I'm going to resist why you're using S7 for a spring, then, and just say that the only possible (though unlikely) issue I can see is the result of improper quenching and/or tempering. If any high-alloy steel isn't quenched right it *may* have retained austenite, which *may* convert to martensite over time (months or years), and cause slight growth as the austenite converts at room temperature. We're talking about one or two millionths of an inch per inch of length, except in cases where the heat treatment is *really* botched.
But I'm still wondering why you'd use S7 as a load spring. Its Young's Modulus is almost the same at that of mild steel, and it's no more or less linear in stress/strain (they're both quite linear up to near the yield point). If you're loading the cell near to or beyond the yield strength of mild steel, then S7 seems a strange choice. I assume you have a specific reason for using it.
Internal stresses in the S7, or any heat-treated steel, can be relieved through repeated cycling, which could be an issue if the load is applied at high frequency. Even then, though, the state of stress in a piece of metal won't have much effect on its Young's Modulus, unless the internal stress + applied stress approaches the yield point in one direction or the other. If that's the case, there could be some very small apparent decline in the Modulus if there are high stresses within the material. But S7 isn't prone to that problem as much as a piece of W2 might be.
That's all theory, of course. I've never heard of an application in which such a miniscule effect could be an issue. Growth from austenite/martensite conversion is only an issue for such things as gage blocks. Internal stresses are an issue for such things as positioning components of an assembly, where relief of stress over time can cause inaccuracy in the assembly. But I can't see how it could be an issue in a load cell.
-- Ed Huntress (remove "3" from email address for email reply)

Regarding "martensitic drift," since it is so small, and so slow, chances are that the entire external circuit would drift more. Most setups like this would be zeroed before each use, eliminating even the last little bit of drift from all sources.

Thank you for your responses.
S7 has been the material of choice. Why... I am not sure, I believe it is more historic than any other reason.
One test was perfomed on a sample many months ago where first the cell is cycled 5x at a 150% of Full scale (which is no where near the yield point)...thus exercising the cell. Then a full scale load is applied (100%) and and the output is monitored (0 min, 15min, 30 min, 4hrs). This test that was performed many months ago ... and there was no change in the output! No Creep!
When the test is performed today ... with the same conditions the output is creeping (slowly changing output) in a positive direction.
I am trying to narrow down some possibilities for this 'change in behavior' I am sure that I am only at the tip of the iceburg as far as possibilities; --> bonding technique, glue line, etc, etc...
Again Thank you for insight into this matter.
Qu1nn

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That's a strange one, Qu1nn. Excuse me if I'm telling you something you already know, but the creep properties of steel are so low that they usually can be ignored, especially at loads well below the elastic limit. Steel creeps less than any other material that's likely to be in your system.
It's most likely something else. One way you could isolate the steel itself as an issue is to give that piece of S7 a full anneal. Or make another one out of plain, low-carbon steel. The Young's Modulus of any of the three -- hardened S7, annealed S7, or low-carbon steel -- will be nearly identical. If you're well within the elastic limit of hardened S7, you're probably within the elastic limit of the other two.
The only physical things that heat treatment changes are the strength and hardness of the material, not its elasticity nor the linearity of its stress/strain. Indirectly, softening a piece of steel by annealing it, or using a lower-strength steel to begin with, actually produces lower resistance to creep. So testing with one of those other materials should clear up the physical properties of the steel itself.
Good luck!
Ed Huntress

qu1nn wrote:
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Just a guess- the strain gage bonding is faulty. Long term drift is one reason people choose sputtered over bonded. Other suspect is temperature effect but I assume this is not the case since the creep is in one direction and you probably have temp compensation.

what about skipping the oven for stress relief? i've seen equipment around that claims to do stress-relief/ redistribution of residual (machined) stress. something like 40 or 50% reductions with good success rates for tool steels like S7.
it wouldnt help with the hardening, but maybe give it a try pre- and post- heat treat and see if it helps.
i think size is an issue with the vibratory stress relief. so if its really small (light), it wouldnt be an option.
-tony