Induction Bend and Subsequent HT

[Lescombes]

Hello learned forum members,

I've inherited a process for forming and heat treating some thick walled spring steel tube. The tube is 5160 steel with 0.55-0.65% C and 0.7-0.9% Cr. There is no record of how this process was developed, and it was long before my time at this company.

The process is:
- Induction heat to 850-880C and bend
- Air cool to room temp
- Anneal at 700-720C for 45 mins
- Furnace cool to 680C, then air cool to room temp
- Heat soak at 650-700C for 45 mins
- Heat to 840C for 1.5hrs
- Quench in warm oil
- Temper at 430C
to achieve a hardness of 44-48 HRC (which is hardness tested on a sample that was heat treated alongside the tubes, not the bent tubes themselves).

Now I've been given a concession request to allow some tubes that have been induction "cooked" to around 915C during the bending process. Subsequent heat treatment processes were unchanged. The temperature of 915C is above the temperatures I see suggested for normalizing steel of this carbon content (850C-900C) - does this departure just ensure the region is "extra" normalised before bending? Or are there some other adverse affects from going above the suggested range that a non-metallurgist (like me) can't see?

Any information that anyone might be able to share would be much appreciated.
Cheers,
Greg

 

[metengr]

No. The 915 deg C temperature upon heating this material is well within the normalization heat treatment range given the stated chemical composition. The key is to ensure that prior to air cooling for normalization, you achieve 100% austenite formation at or above 900 deg C. So, the added 15 deg C should be acceptable.

 

[Lescombes]

Hi metengr,

Thank you. From a review of (what I believe to be) an applicable phase diagram I thought that may have been the case.

Cheers,
Greg