griding cracks on nitrided tool steel

[cmkmechengr]

I am working on a grinding issue where our H13 nitrided parts are cracking and chipping on a 90 deg internal cylidrical edge. Only around 3-7% of parts will chip, and there are no signs of grinding burn, Any ideas?

 

[cmkmechengr]

I am not very experienced in grinding practices, from what i have read many grinding cracks are a result of excessive temperatures and often not from the stresses that occur at the grinding interface.

 

[saberblue]

Are your rejects randomly distributed through out the run or do they come in batches?

If they occur in batches check your wheel size - as you approach stub size your SFPM --you may be getting so slow that your wheel is acting too hard.

Alternatively if you are grinding the shoulder take a look at the dressing of the radius it maybe loading up.

 

[unclesyd]

One Of the big problems I've seen with certain nitriding processes is the presence of a "white Case", which can crack spontaneously and almost certain during grinding if the grinding parameters are not correct.
You might want to ascertain if you have a White Case.

What is your nitriding process and conditions?

When you nitride the part how much grinding is needed to bring the part to size?

 

[cmkmechengr]

The distribution is both "random" and in batches, the worst batch I have seen is about 30 parts chipping consecutively, but usually they are spread throughout all parts. The wheel diameter is smaller than 10mm and the so I would assume the SFPM does not change signifcantly, *correct me if im wrong, just learning the grinding process. Both the shoulder and the ID are ground in a ID, shoulder, ID order.

As for the white case, about 100 microns is removed from the parts on both the shoulder and ID which do not have the white layer removed before grinding, From what I have read is well below the white layer and into the diffusion zone. The nitriding process used is just regular gas nitriding with ammonia I believe, althogh I think i remember someone saying that a batch of plasma nitrided parts was tested once and they all cracked/chipped during machining so I should probably do more research on the difference between the two.

** Would it be advantageous to remove the white layer before grindng, is this the usual process for grinding nitrided parts?
The surface before grinding has cracks in the white layer which under magnification look terrible, I have done some tests and grinding away these cracks usually occurs so they must be superficial and not very deep at all. But I have to wonder if they contribute at all to the chipped parts.

One article i recently read said that nitrided parts should just simply not be ground after processing.

It seems like a complex problem so it must have a simple answer. Have definitely learned a lot about grinding though.

 

[tricky1975]

internal? how big is the face? wheel? speed? machine?