How to minimize core hardness?

[gearcutter]

We have a regular job which recently went wrong after heat treatment and I'm after your thoughts.
The gear blank is 380mm OD, 100mm wide and the material is 4317 case hardening steel. Case hardness is around 58 - 60Rc at around 1.2mm depth.
We need to spline the bores after case hardening/quenching to eliminate the risk of distortion. The finished minor diameter of the splined bore is 90mm. The bores are turned, pre heat treatment, to 70mm and 5mm is left on both faces. After heat treatment the bores and faces are finish turned and then the blanks are spline cut.
After doing around 100 of these gears we've found that 1 in 10 or so will develop "hard spots" which are difficult to get through with HSS cutters but not impossible and so we have been able to get by.....until we changed heat treater.
We went from a Fluidized Bed furnace, where the blanks were stacked horizontally on top of each other with spacers between them for both carburizing and quenching, to a Vacuum Sealed Quench furnace where the blanks were hung vertically for both processes. This seems to have greatly reduced the level of distortion and consequently has reduced the gear grinding time by around 30%; a pretty good result we thought.
The problem though began when it came time to cut the splines. The HSS cutters would begin the cutting process but would very quickly become blunt. We went from a situation where 3 – 4 blanks were being spline cut between sharpening of the cutters to where we were lucky if the cutters could get through 4 of 5 teeth gaps of the 24 that needed to be cut in each blank.
We then had one of the blanks hardness tested on a spot near the bore, on a face which had had the hard carburized case removed and the result was 43 – 45 Rc.
We are now in the process of figuring out a way of annealing the bores without affecting the hardened & ground teeth. The best proposal has been to induce heat into the bores with electric coils and then ensure all the relevant post heating precautions are taken. One of the blanks has just returned from having this process done so wish me luck. We’re also waiting for results from a lab to determine the material’s composition.
Is it normal for case hardening steels of this type and mass to end up with such a hard core? Any thoughts on how we can stop it happening again?

 

[unclesyd]

If you want to prevent any case hardening of the bore area you can use one of many different Stop Off materials.

http://www.heatbath.com/Products-link-page.htm

 

[dbooker630]

I work with a lot of 4320H on gearing and it is quite possible to get core hardnesses in the mid to high 40's HRC on gear teeth so a surface hardness that high is not unusual after quench and temper.

My suggestion would be to try cutting the splines in the green again, since apparently your overall distortion has reduced with the new vacuum HT process.

It's possible to induction temper the bore but you must take due care with controlling the heat affected zone and thus affecting your ground surface. You would need to use a high frequency induction setup (~10kHz) to aid in minimizing the HAZ.

What are your final hardness requirements for the splines? You might have to heat treat again after such induction treatment.

 

[EdStainless]

My hunch is that the spacers in the old system limited the quench rate of the core and reduced the maximum hardenss that you could achieve. The new system is full hardening hte entire part. Maybe a dumby plug in the centers to limit the cooling rate would keep them from getting as hard.

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[TVP]

I agree with dbooker630 about reducing the grinding allowance since you are now using the vacuum process. The idea of spacers limiting the core hardness sounds like a good one to pursue as well. Trying to reduce the bore hardness after carburizing by induction or other means does not sound like a robust solution.

 

[EdDanzer]

I agree with EdStainless, use a heavy spacer to reduce the quench speed of the bore. We did an opposite concept on a sharp edged part, we needed the mass to keep the edge hot between the furnace and quench.