S7 heat treat - Can it cause problems in wheatstone bridge app?

[qu1nn]

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


Qu1nn

 

[Metalguy]

After a double temper, I don't think you have much residual stress left in the parts. But you still could have some residual austenite (retained austenite) that will cause dimensional changes as it reverts to martensite with time.

The only thing I would try is to cool the parts as low as possible after the 1st temper.

 

[unclesyd]

Adding to "Metalguy's" post if there are any doubts about stress or movement add another tempering cycle or better yet cryogenic treatment. I have seen specs with 9 tempers to stop growth in D2 tool steel operating at extremely close clearances. I'm curious as to why you picked S7? We mainly used S7(for old folks Bearcat by Bethlehem Steel) for it’s impact resistance.

 

[qu1nn]

Thank you for your quick responses.

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

 

[unclesyd]

You are right in saying there are different rates in heat treating. The first rate you have to control is the heat up rate which you did. Then the time at austenitinizing temperature which you accomplished. Now is where the cooling rates can be quite variable depending on the metal and what you want to accomplish. You hear will austempering, martempering, interrupted quench, and others. When you cooled (quenched) your metal you had an interrupted quench because you changed the cooling rate. This is alright for your steel. Other steels have different requirements for cooling rates. A point to remember is after quenching some steels you have to immediately tempered while others it doesn’t matter. Follow the suppliers directions for heat treating. The information concerning the cooling rates for heat treatment are contained in what are called TTT curves. The TTT stands for time, temperature and transformation.
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.

 

[metman]

1) Does someone know the Mf temperature for S7? If so then we can tell quinn whether he needs to use dry ice or LN2 before the second temper.

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