Nitride and Heat Treat Help

I've been doing this for 20+ years and have had 2 crazy things happen in heat treat.

1. Core hardened 4140 steel slugs Ø3" x 1" thick. Finish machined them holding +/-.0003 on 2 critical bores and 1 outer diameter. Parts were supposedly nitrided for 24 hours at a .026 case depth. Parts came back with the OD oversized by .007" and the ID's about .004" oversized. How can this possibly be? Maybe my material guy sent me the wrong material? Could the heat treater have carburized the parts instead of nitride? Could they have re-heat treated the parts? I doubt my customer will accept these parts and therefore we will have to start over unless these is a way to magically shrink the parts.

2. Sent 4340 steel Ø4"OD x 3"ID x .625" thick rings for heat treat to 42-45HRC. Heat treater says they had to re-process the parts as they did not initially meet hardness. Parts came back .010" smaller in diameter. How is this possible?? All of my experience and previous runs of this part have shown that the part grows outwards approx .004".

Any insight would be much appreciated. Thank you

While I haven't been heat treating for anywhere near 20 years, something smells fishy to me. Even when carburizing things in my home shop, I've never had anywhere close to that much movement. Usually not even enough to tell. And as far as nitriding goes, many AR and other rifle barrels are nitrided and still can have incredible accuracy. It seems like most barrel makers try and hold at most .0003 over then length of the bore, so the process can clearly be done with minimal movement. I would really thing about contacting another heat treating company and see what they have to say about it.

I hope you can get it figured out.

One of the worst aspects of being a machinist - getting totally screwed by heat treaters, platers, and others that take our hard work, muck it up, and hand it back with a "them's the breaks" "apology".

Maybe you can hand both sets of parts back, and tell them to do the others process to those parts, and they'll grow/shrink back to spec...

I try to get customers to handle HT or other post-machining work, it's about the only way to not be the screwee.

fernjul said:

Parts were supposedly nitrided for 24 hours at a .026 case depth.

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.026" case ? Are you sure they aren't carbo-nitrided ? It's very seldom to see nitride that deep. In fact most places can't give you more than a few thou. I only know of one place that says they can go deeper and all they did was mess up a batch of parts for me.

If you must nitride, 4130 works better than 4140.

Heat treater says they had to re-process the parts as they did not initially meet hardness. Parts came back .010" smaller in diameter. How is this possible??

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Have had similar experiences. That "re-process" thing seems to mess up dimensions. Similar situation, I had a batch of carburized 8620 bores grow by .030 when it ALWAYS shrinks. "Reprocessing" seems like a bad idea

Sorry to say it but I think you are screwed. The Heat Treater's Bible says they are only responsible for material loss. Better than nothing but not much

Heat treaters bible says always blame the steel supplier........something like....O. yeah.few gone like that lately ........reckon it Chinese steel is the problem..........Definitely not one of the setup laborers stuck the stuff in the wrong bay,so it went on the wrong process ride.

Its always a good idea to anneal through hardened parts after roughing them out and normalize 8620 after roughing. This will minimize (not eliminate) movement after hardening and tempering in my experience.

I agree that .026 depth is beyond the range for straight nitriding. Iirc, the practical limit is under .010. As for your current dilemma, the parts will need to be re-made. The heat-treaters we used limited their liability to the value of the service provided. Small consolation, but there are just too many variables before they receive the parts.

You've been doing this for 20+ years, so you already know that unless you normalize and stress relieve before machining and Heat Treat, that things ARE GOING to move. Period. How much, and in which direction is the fine detail stuff.

The problem is that most heat houses do not pay attention, much less care to "how" they quench or the orientation of the part during quench. Have to agree with EmanuelGoldstein about Carbo vs. plain Nitriding. That much growth sure sounds like it, to me.

We Heat Treat 90% of our work in-house due simply to making sure that we control as much as possible the processes that are carried out. That remaining 10% that goes to a vendor is done by someone that was found with some trial and error, has a long record of quality, and is open and willing to listen and ensure that parts are processed in the manner that I prescribe. That means that they get quenched in specific orientations and handled in specific ways. We also go to some troubles when Heat Treating to make fixtures for both handling and orienting parts. Some times, that even includes restricting part movements.

Good luck. In the end, most HT houses are like custom painters and ano houses. They're less like craftsman and more like crackheads...

Zahnrad Kopf said:

Good luck. In the end, most HT houses are like custom painters and ano houses. They're less like craftsman and more like crackheads...

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Kind of funny ... the most reliable (in 25 years I don't think they messed up on me even once) heat treater in the Bay area recently sold all his equipment. He was talking about renting the building out for a marijuana growing facility Good guy, he also had a Sherman tank for a weekend driver ... maybe too many decarb fumes over the years ?