Shafts may be cold straightened prior to induction heating. The cold work strain is localized predominantly at the surface. Relaxation during induction heating can bring the materials nearly back to their original state prior to cold working.
There are a number of process variables that will affect warpage/distortion separate from the material stress state. All of the following will increase warpage, all other things being equal:
1. Decrease in quench temperature
2. Decrease in polymer concentration (assuming water + polymer quenchant)
3. Degradation of polymer in quenchant due to excessive usage, biodegradation, etc.
4. Increase in part temperature prior to quenching (slower scan speed while using the same power)
5. Increase in quench intensity (higher spray pressure or flow rate)
6. Irregular quench spray pattern due to clogged or obstructed nozzles, etc.
7. Poor thermal distribution in the part due to excessive scan speed.
My previous position was plant metallurgist at a Tier 1 axle shaft supplier. We found that the biggest variable effecting shaft warpage during induction hardening (single shot technique) was the length of the shaft. Shafts under 16" or so were not much of a problem. The longer shafts always experienced a high percentage of warpage. As a manufacturing expediency, a decision was made to straighten all shafts over 16" after induction hardening.
Thanks Swall. All our shafts are over 16 inches because they are rear axle shafts. We use a scan process for induction hardening. Straightening the shaft is part of our regular process. We have ran these shafts before but we havent seen this much warpage.
The straightener rejects the shaft even before it starts to straighten it and I do not want to change the starightner parameters. I m looking into the reasons the TVP suggested in the earlier reply. Can there be other reasons apart from the ones mentioned by TVP that I should be looking into as well?
I can't add anything to what TVP posted. But, I think I would look at power intensity, scan speed and quench intensity first. Quench intensity can get out of whack due to maintenance issues, but power intensity and scan speed you have to intentionally change--have these changed at some point in your process?
I agree with the previous posts but I would also check chemistry. Is the DI of this heat of material significantly different from the past? With axle shafts I have found that a tight DI range helps minimize distortion from heat to heat if the IH parameters have not changed. Since these are axle shafts there should be a heat code stamped or forged into the part. I would track distortion by heat code to see if there is a relationship.
I agree with everything that is said above. Here are my comments, I hope they help.
Are the parts as straight before heat treat as they have been in the past?
If these are induction hardened on centers vertically, make sure the top one is free floating. I have seen when these fail or are setup wrong cause a lot of warpage, because they push excessively as it is scanned.
Another thing to look for is if there are unusual stresses present. Stress relieve some in vacuum ~1100°F (rack vertical!) before induction hardening, then harden them to see if they stay straighter.
