I've searched here and all over without much luck.
I'm looking for a little insight about how this surface flame hardening is accomplished.
The job call's for flame hardening the surface of some 4" thick 4140 parts.
Spec's are "surface flame hardening to Rc 50 for a depth of at least .100"
This surface to be hardened is also machined, with differences in surface height to 1.875"
Can anyone tell me something about how is this done ?
TIA, Cheers, YOOO VINNY
You'd probably be way ahead dollar-wise to have the parts induction hardened and tempered back to Rc50. Heat treaters who do induction hardening are pretty good at getting the depth they're shooting for.
A couple problems I can foresee with flame hardening.... By the time you get the surface up to the austenitizing temp with a rosebud, you'll likely also get some decarb on the corners of the part due to overheating and exposure to air. The decarbed areas won't harden at the surface. Also, if you get a full transformation in 4140, its going to be harder than Rc50. Probably more in the 55 range if the transformation is truly complete. So, once its hardened, it also would have to be tempered to get it back to the desired hardness. If you end up with retained austenite because the quench wasn't fast enough, then the part is going to be highly stressed in the hardened zone due to the different cubic structure of austenite versus martensite. Depending on the geometry of the part, this can start screwing around with your dimensions.
Perhaps you can put some sorta goop on the surface to prevent decarb and scaling. Don't know. What I do know is that I'd hate for someone to hand me a 4" thick chunk of steel and a rosebud and expect to receive a decently heat treated part in return. Just offhand, I'd imagine those guys who laid torch to ways on things with names like Monarch and Cincinnati had a lot of practice before they got turned loose on an actual machine tool.
Actually, the way that HT spec is written (assuming its a direct quote), a thru hardened part would meet the spec. At least .100" doesn't have an implied "but less than" depth associated with it.
The flame hardening of 4140 that I've done, was
on round items, gear teath and sprocket hubs,
wheels and rollers. It's not to difficult, but
messy. Plus you require alot of set up to do it.
If you possibly can "send it out". It will cost
less and you'll know it's done right.
Takes lots of practice to do it right and the
P.s. How it's done is you heat the part while it's
rotating up to 1675F on the face, then hold for
the required lenght of time, for the depth of
hardening. Quench immediately in 150F deg oil.
Best if still rotating, or moving, to prevent gas
Temper to required hardness, then quench again
in 150F oil.
If I take the job, I will end up farming out the flame hardening.
I've spoken with the customer about the hardening spec and they've reiterated the parts must be to spec and surface flame hardened only.
The flame hardening outfit I've spoke with told me that meeting the spec was no problem, so I'll let them handle it and worry about the cert.
I'm just very curious how they can accomplish this, especially on a 400lb part with almost 2" of differences in surface hight on the side to be hardened ?