Induction vs. Furnace Harden - Residual Stress 2

[sodeen]

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?

 

[btrueblood]

Try a stress relief treatment prior to hardening?

 

[arunmrao]

As suggested, a prior stress relieving cycle will definitely be useful.You could also try oil quenching instead of polymer quenching,followed by flash tempering .

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[blacksmith37]

My thought is that this is a deeper subject than it appears :many decades ago, New Departure /GM , developed heat-treatment for rolling element bearings that made about a 10X fatigue life improvement. Basically,it left the surface layer residual stress in compression. I can't be more specific ,there must be literature available.

 

[EdStainless]

The residual stresses largely come from the quench operation.
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?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[redpicker]

sodeen,

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

 

[TVP]

I agree with redpicker. Especially if the regular process used oil quenching instead of the water + polymer quenchant from the induction process. The short tempering times at relatively low temperatures can lead to significant residual stresses.