Heat treating a reworked part -necessary?

[Knuts02]

I have a pin (4" length, 1.5" diameter) of 4340 that was heat treated to RC40 which requires rework: turning down the diameter about .004" to meet the drawing's spec. Does the part require heat treatment again (RC40 specified on drawing)? More importantly, can someone explain why heat treatment would be required again after rework is done? I don't think the machinist intends on annealing it before reworking. I'm not a manufacturing engineer and just trying to understand. Thanks!

[Kelly_Bramble]

I'm not convinced you're using the right words here.. One might case harden to a RC40 (Rockwell Hardness) I'm not sure one can heat treat the material (4340) to a Rockwell hardness of RC40.

Heat treating can create a higher hardness the difference being that the core of the material is hardened as well. If the surface being re-worked is case-hardened by QPQ, nitriding, etc.. and needs a RC40 hardness then yes a re-case harden process would need be done.

Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface.

So, the big question is how or what process was utilized the harden the material?

[Hudson]

It is quite common to harden shafts before final grinding. Often this is done to correct any change in shape (warping) resulting from heat treatment. Finishing after heat treatment removes scale aand such.

Your turning operation would be similar to grinding. 4340 through hardens very well, even in large sections.

That said, we have no history of the heat treatment cycle or application data to give you a definitive answer.

[pktryq]

Why don't you just test the hardness after the rework? It is not a laborious intervention, and you should do this anyway, after optional heat treatment.