How to reduce core hardness in cased steels?

[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?

 

[diamondjim]

Try plugging the bores and
add a spacer top and bottom
to keep the hardness from
penetrating the surface on
each side of the bore.

 

[tbuelna]

Use a higher tempering temperature after you quench.

 

[lbmakem]

you dont say what spec your cutters are except to say they are HSS,look at changing the spec of your cutter to S390,ASP60 or M42 all of these will cut gears of 43-45Rc you will have to have them coated, you'll not gain any time in the cutting but you will extended the number of components before sharpening.

hope this is of some help

 

[Pressed]

Induction hardening the OD will leave the ID un-altered. The depth of heat can be adjusted. I will look up a treater for you if you need one.

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[Philrock]

Look into selectively plating (before heat treat) the areas you don't want hardened. This will effectively mask these areas and prevent them from getting carburized.

 

[gearman1234]

Hello,

1)Just a curiosity. The gear is hardened and spline is kept soft . The spline is also usually supposed to carry the torque and should be case hardened too.

2) 4317 is a high hardenability steel. Carburising the OD and then Induction hardening OD only is the best bet. We use it in one of our applications but we do not cut internal splines.
But even with this process the core hardness could be around 40 RC.

But you could optimize the broach material, coating and use lower cutting speed with plenty of coolant. Which coolant oil do you use for broaching? Usually Castrol ILOBroach coolants are very effective in controlling wear beacuse they have high EP values.