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Distortion due to nitriding

2005

I am facing a crisis. I am not able to find out the reason of why a shaft subjected to nitriding gets distorted after the heat treatment process. I am sure the distortion is occurring only after the nitriding process. Can someone please tell me what parameters can be responsible for the distortion. It is a machined shaft on which spline cutting and finishing ooperations are performed. I am baffled because inherently nitriding is a distortion free process.
shaft dia: 2 inches
shaft length: 18 inches

Shreyas PC
manufacturer of automobile components - Hosur, Tamil Nadu, India

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Possibility 1: Perhaps the shaft is not stress-relieved prior to machining and whatever the "finishing operations" are, then the moderate temperature of the nitriding process (that is, the temperature itself, not the nitriding action) permits stress relief to occur with associated distortion.

Possibility 2: Perhaps the machining and "finishing operations" are creating a stress pattern on what might have originally been a stress-relieved blank, and then the nitriding temperature allows stress relief.

You are correct in saying that nitriding is a distortion-free process, but be aware that the nitriding temperature is high enough to allow stress-relief to occur. Running some parts through at nitriding temperature but without the nitriding atmosphere, could provide a useful pointer to whether you should look for a prior internal stress pattern.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2005

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Hi Shreyas

Bill hit it on the head ... distortion due to improper or zero stress relieving beforehand.

Damn it, this can happen to plastics, too, which is why most sheet materials are 'pressed' before sale to plastic fabricators.

The old test is called the saccrimeter analysis ... you get two pieces of polarized glass ... you get some acrylic sheet, rod, whatever ... you place the polarized glass in front of and behind that acrylic ... nothing will happen UNTIL you try to bend it, weld it, glue it ... and the stresses will then be easily shown radiating outwards. This is one good method of proving welding integrity as the expert will show far fewer stresses.

Freeman Newton [deceased]
(It is our sad duty to advise that Freeman passed away
April 21, 2012. R.I.P. old friend).
2005