SAE 1050 modified Axle shaft temper time

[zunom]

We induction heat treat axle shafts made of SAE 1050 (mod.) and then furnace temper it. My question is does it really affect the microstructure or the hardness of the shafts if they have been tempered longer than they are supposed to be. In other owrds, if I am not sure whether the shafts were tempered or not and may have re tempered it. Is it going to affect the hardness of some other metallurgical change.

 

[Yallapragada1022]

Base on my experience, re-tempering at the same temperature should not change the hardness or microstructure. I am confused about your statement "whether the shafts were tempered or not". What is your hardness specification after induction hardening and after temper?

Rao Yallapragada

 

[zunom]

Thanks for your reply. I thought so the same but was confused. By the statement thats confusing you, I mean there are a few shafts on my shop floor that are not tagged, so we dont know if they have been tempered already or not. The hardness specifiction after induction heat treat and quench is arounf 64-65 HRC. After temper it comes down to 61-62 HRC.

 

[TVP]

My experience is the same as Mr. Yallapragada: tempering a second time at the same temperature and for the same duration will not further lower the hardness for a grade like SAE 1050. Only with highly alloyed tool steels that may contain significant amounts of retained austenite would one expect to see a change in hardness after a second tempering at the same temperature/duration.

 

[swall]

I would want to investigate those untagged shafts to make sure that they are normal production parts, rather than set-up parts, parts set aside for mag particle inspection or something like that.

 

[dbooker630]

My rule for unidentified induction hardened parts is to temper them within four hours of hardening, especially if the shafts are 65HRC as-quenched.

I would either rework (draw above 800F), mag particle after temper or scrap them alotogether if there is absolutely no idea when they were IH'd.

 

[Yallapragada1022]

I would recommend the following:

1) Perform mag particle inspection for cracks before re-temper.
2) Re-temper the parts above 800 deg. F, like dbooker630 has suggested.

You may also want to scrap the parts if the quantity is very small, instead of going through all the additional inspections and rework costs.

Hope this helps.



Rao Yallapragada

 

[zunom]

Thanks for all the inputs.
Just FYI we furnace temper shafts at 303 F. The customer print calls for stress relieving at 280 F for 75 minutes. From my experiments with the thermocouples located at several positions on the rack we came up with 303 F for three hours.
Also I want to ask dbooker630 about "tempering before the end of the four hour period" How important is that? And what are the consequences if it does not happen within that duration?
Thanks

 

[Yallapragada1022]

The time gap between the two operations is very critical for induction hardening and neutral hardening processes to prevent parts from cracking. The martensitic structure is highly stressed and unstable and will lead to cracking if not tempered in an established time frame.

In my opinion, 4 hours may be too long. I would only allow less than an hour.

Hope this helps.

Rao Yallapragada

 

[dbooker630]

Yallapragada1022 has explained the reasons. In my experience I have allowed anywhere from two to eight hours interval.

It depends (metallurgist talking!) on the part configuration, if there are not many severe transitions in diameter or fillets you can allow more time.

Ideally I would temper parts within an hour also but then realize things happen in manufacturing that you have to allow for or you will have an ISO nightmare.