S7 heat treat - Can it cause problems in wheatstone bridge app?
S7 heat treat - Can it cause problems in wheatstone bridge app?
(OP)
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?
Thank you for your time
Qu1nn
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?
Thank you for your time
Qu1nn
Qu1nn





RE: S7 heat treat - Can it cause problems in wheatstone bridge app?
The only thing I would try is to cool the parts as low as possible after the 1st temper.
RE: S7 heat treat - Can it cause problems in wheatstone bridge app?
RE: S7 heat treat - Can it cause problems in wheatstone bridge app?
So ramp rates are not as important as the number of tempering cycles (except try to cool down further before repeating cycle ). Wow was I off course with that one!!!..... Several references that I have seen seem to suggest low ramp rates and controlled cycles.... I thought that drastic changes from one oven to another would be considered beyond this "ramp rate" ...and thus bad.
S7 was used because " that is what we have always used". Personally I would like to see the use of some stainless i.e. 316SS
Thanks again,
qu1nn
Qu1nn
RE: S7 heat treat - Can it cause problems in wheatstone bridge app?
When in doubt temper again. As “metalguy” always double temper especially tool steels.
If you are looking at stainless look at the precipitation hardening types like 17/4.
RE: S7 heat treat - Can it cause problems in wheatstone bridge app?
2) Ramp rate does matter. I suggest after your final temper, you place the part(s) in warm sand to achieve a slow cooling rate. This will furhter minimize residual stresses although as stated the double or triple tempers will achieve the greater part of removing stresses.
3) quinn, do you need hardness for this application. The material choice is an interesting question. Can you give us a hint as to the operating environment and material properties required?
Jesus is THE life,
Leonard