double induction & quenching 2

[kevinthompson]

Dear all
I would like to learn about the applicatin of double induction hardening in industry. As far as i know especially when a problem accurs during induction hardening of the shafts(for example the induction operation was interrupted very close to the ending of the cycle due to electricity fluctuation) these parts are reworked by applying second cycle of induction&quench. At this point i would like to learn that does double induction hardening has a drawback (in the condition of conforming all the pattern,hardness,microstructral and geometrical specs ).I need a supplementary literature about double induction hardening.
Waiting for your suggestions.
Regards

 

[redpicker]

You don't really give enough information for a good answer.

Electrical Induction is merely a method of heating and can be used in several different ways for heat treating.

In some applications, it is used to selectively heat the surface of a part to austenitizing temperatures for a surface hardening treatment.

In other applicatons, it is used to through heat a part to austenitizing temperatures for a through hardening treatment.

In still other applications, it is used to pre-heat a part so furnace times are shorter for through hardening.

Yet other applications use it to heat material to forge-welding temperatures for welding applications.

And the list goes on...

Whether or not a double cycle has a drawback depends on many things (it most likely has some drawbacks, at least on a cost basis, if a single cycle is acceptable). Off-hand, you can run the risk of cracking the parts, you can create a heating pattern that is too deep, you can run the risk of inadequate hardening. Or, it might not matter at all. It really depends on the set-up and what you are trying to accomplish.

rp

 

[Wrenchbender]

Kevin,
In your original post, if you were to change the words “induction hardening” to “heat treating” or even “hardening”, the same concerns and the same answers would apply. So I interpret your question as how many times can you heat treat a part, induction heating being incidental. Of course induction heating only takes a few seconds and only affects the outer surface of the shaft which becomes heated above the critical temperature. Before the surface cools, it is quenched resulting in a solid state phase change from austenite to martensite.

You can theoretically bounce back and forth between austenite and martensite forever; however there are practical limitations. Excessive grain growth is one, but this depends on the alloy and the time/temperature history. Quench cracks and dimensional stability could also be problems depending on part geometry.

Apparently your rework procedure is being done successfully so I guess there is little, if any, material degradation or significant microstructural change from a second heat treatment. The best way to be sure is thru experiment. Take shafts that have been heated treat once and twice; test for mechanical properties, microetch analysis, and full functional tests.

I once worked on thick-wall cylinderical pressure vessels made from quench and tempered 1551 steel. Production quantities were in the thousands. Process interruptions did cause a few lots to be re-HT’ed. Its been a long time but I believe the metallurgists were comfortable with up to 3 or 4 times if necessary (for this application).

 

[kevinthompson]

Dear redpicker/bestwrench thanks for your precious comments.As from your answers it seems to be that i need to give further information about the process. The axle half shafts in are induction hardened in our facility.The green shaft material is quenched&tempered condition BS150M36 (close to 1040).The parts are fixed with two centerings in induction machine. The coil begins to heat the part in flange and moves towards to the end of the shaft. During the coil austenize the parts, at the same time delay-quench oil is sprayed from the unit which is located just below the coils. By this way the parts is austenized and quenched at one induction harden cycle.The problem is; becouse of the coordinate set up problem of the coil, the induction pattern was shifted up, so that there is an unhardened area(discontonuity) accured in the pattern.For these defective parts, we fixed the problem in machine and set up the induction machine and re-run second cycle of induction hardening in original set parameters. As a result, the geometrical and metallurgical controls were all in specifications(as in your questions grain growth, deeper pattern etc. did not occur). I will appreciate if you share some literature or information about ''double induction hardening''.
With my best regards
KT

 

[dbooker630]

At our facility we make similar parts. If the induction pattern was interrupted or incomplete, or otherwise out of specification, we do not re-induction harden the parts without either retempering the shafts at least 800F or renormalizing to avoid the risk of cracking.

I understand that you are not seeing grain growth or excessive depth of hardening but I would be concerned about surface and subsurface cracking. I recommend that you at least perform magnetic particle inspection on your parts before release.

 

[CoryPad]

dbooker630,

I am eager to learn more of your requirement. What is the concern if the parts originally were induction tempered after induction austenitizing and quenching? The martensitic structure transforms easily to austenite if re-heat treated. Is the concern regarding retained austenite, untempered martensite, other? Thanks.

 

[dbooker630]

Cory,

I agree that induction tempering is permissible as a rework step; it did not appear that the OP's process included an induction temper. Although my machine has the capability, our approved process was for furnace temper only.

Even if the machine program included an induction temper, the operator would have to be certain of when did the cycle interrupt, or re program the machine to automatically perform an temper step in case of an austenitize fluctuation.

 

[kevinthompson]

Dear Cory/Booker,
Thanks for sharing your experiences.
Do you perform second cycle of induction hardening rework, after intermediate induction temper for austenizin/normalizing
Regards