Diametric Growth in Case Hardened and Tempered Steels

heat

Case hardening is achieved by deposition of a hard alloy over a parent metal together with a nitration of the outer layer of the parent metal. This deposition results in the growth you are experiencing.

This site has a pretty good explanation of the deposition layer that can be expected.

Oh,

During process, not after?

Dunno.

After process, the OD will increase as deposition is (presumably) uniform. I don't know a lot about fabricating piston pins. The Wiki article about them does not indicate case hardening as a typical treatment. In fact, it sort of looks as if case hardening is contraindicated in most cases.

We will have to wait for our OP to help us out a little.

Thanks.

But the expansion is causing increase in dia which is not required .Has raw material got anything to do with it.

"...the expansion is causing increase in dia ..." Yes. This is expected in case hardening. Is case hardening the process you are using? If yes, there will be an increase in the diameter of your pin.

If case hardening is what you are going to do, start with a pin of smaller diameter, refer to data from the site in comment #2 (or data you feel may be more applicable to this project) for the expected deposition you need to arrive at your final size and material specification.

Once again, it appears there may be better ways to accomplish this.

I'll agree with all the previous comments.

The proper procedure is that after any heat treatment the material should be grinded to the appropriate dimension.

There is not any heat treatment that will not alter the dimension, apart from simple nitriding, to my knowledge.

Depending on the dimension of the part the alteration varies. i.e. for a part of OD 20mm the alteration may be in the area of .03-.04 while in bigger parts ( OD 150mm) it exceeds easily .05mm . As it depends on the morphology of the part, i.e. a shaft isn't distorted as much as a pipe-type part etc.

Case hardening is a process that adds carbon to the surface of low-carbon steel. The hardened layer is very thin and not normally ground to final dimension because grinding would remove the hardened layer. In the case of piston pins, the hole in the connecting rod is normally honed to match the hardened pin diameter. Dimensional changes occur during heat treating because of the change in crystal structure of the hardened material.

I'll have to disagree. Grinding would not remove hardening as most types are definitely more than 1mm deep.We are talking about grinding in the range of 0.3mm

For proper and uniform fit between the pin and the connecting rod the pin should be grinded.

It would help if you could specify the material you used for the pin. Was it something in the range of 15CrNi6 or similar?

You are free to disagree, but OP has stated that his minimum hardening depth is 6 microns, not the minimum 1000 microns you suggest. If you start grinding a pin with 6 microns of case hardening, it won't take long to go through it.

Can you explain when this is occurring? Is it during use, or when you are trying to grind them, or during some other operation that you are doing? As mentioned heat will cause them to expand.

Dear All

Thanks for the overwhelming response.

Pins are subjected to grinding and laaping after carb/hard and tempering here and the required sized is achieved. However during volumetric test which states that pins to be tempered at 220degreec C for 4 hours with max. dia increase of 6 microns so that during engine running there is no signifiant rise in dia.Here we are getting dia growth in excess of 6 microns( to the tune of 14 microns)

A Good Answer vote for lyn, because he was able to correctly interpret a Type 75 post.

Now if he had used henways instead of microns I might have understood it better!

I am hoping our OP doesn't want us to help with the math. My gazinta skills are kind of rusty.

You know; two gazinta eight 4 times, two gazinta ten 5 times, two gazinta five... well, I guess two won't gazinta five.