Induction vs. Furnace Harden - Residual Stress
Induction vs. Furnace Harden - Residual Stress
(OP)
So I work for a small bearing company that recently (~6 months ago) got an induction hardener to through harden stainless steel bearing races. Predictably, the faster production rate has the production manager trying to cram anything and everything that they can run into this machine. (The machine also uses a cone-die and polymer quenchant to preserve roundness during cooling.)
One of the parts in question is showing quite a bit of growth and movement while being machined post-induction harden. Because dimensional tolerance is so important in these parts, this is a bad sign.
The point of all that background info was to ask this:
Am I right in assuming that parts that are induction thru-hardened are likely to have more residual stress than their counterparts that are run in a regular old controlled-atmosphere furnace?
The stories that I'm getting indicate that the problems started about the time these parts were no longer being run in the furnace. I have a feeling that the growth is due to these residual stresses in the part, and that the induction hardener is playing a big part in introducing those stresses into the material.
(it is tempered twice post-austenitization before any machining takes place.)
Any thoughts?
One of the parts in question is showing quite a bit of growth and movement while being machined post-induction harden. Because dimensional tolerance is so important in these parts, this is a bad sign.
The point of all that background info was to ask this:
Am I right in assuming that parts that are induction thru-hardened are likely to have more residual stress than their counterparts that are run in a regular old controlled-atmosphere furnace?
The stories that I'm getting indicate that the problems started about the time these parts were no longer being run in the furnace. I have a feeling that the growth is due to these residual stresses in the part, and that the induction hardener is playing a big part in introducing those stresses into the material.
(it is tempered twice post-austenitization before any machining takes place.)
Any thoughts?





RE: Induction vs. Furnace Harden - Residual Stress
RE: Induction vs. Furnace Harden - Residual Stress
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RE: Induction vs. Furnace Harden - Residual Stress
RE: Induction vs. Furnace Harden - Residual Stress
My hunch is that the thinner case layer and more severe quench are causing the residual stress gradient to be steeper, as you grind you are causing higher stresses to come into play.
In general the compressive residual stress that you can get with this heat treatment is very helpful.
back off on the quench or perhaps leave less stock for finishing?
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Plymouth Tube
RE: Induction vs. Furnace Harden - Residual Stress
In my opinion, you are right on the money. The increased distortion is a direct result of using the induction equipment. The rapid heating creates a non-uniform temperature distribution and the residual stresses can be condiserably higher than furnace hardened parts. Furthermore, the tempering times are often shorter and can also have a non-uniform temperature distribution, so the residual stress pattern can be very different depending on many parameters, such as time, temperature, and section size. What this means is that one part may not give you any problems while another one that is given exactly the same cycle will.
Bear in mind that the heat treating process is an integral part of the overall manufacturing process (not just an add-on process) and changes in the heat treating process will necessitate changes in the overall manufacuring process. Also bear in mind, as has been mentioned already, that some residual stress distributions can be very beneficial to the life of the bearing, so you may achieve a better result by learing to work with the residual stresses than you would be just a high temperature stress rellieve would give you.
rp
RE: Induction vs. Furnace Harden - Residual Stress