Re-Nitrding Gearing

[PGCTGS]

We currently have a Gear with a tire made from NiCrMoV steel that we had gas nitrided. The company performing the nitriding stacked two gears on top of each other in a pit furnace. There was plenty of space between the gears and gear teeth for gas flow. The pit furnacve is heated from the bottom and the ammonia gas is pushed in from the bottom with a fan that pulls it up the furnace. The gas is measured every hour for potential and showed no irregularities. The diameter of the gear is about 40 inches. The bottom gear nitrided perfectly. The top gear only sporatically nitrided and has a surface hardness range varying from 43 to 51HRC, with a minimum requirement of 50 HRC. The core hardness prior to entering the furnace was 40HRC. Metallurgical test coupons were run with the gear, and they show perfuect hardness and case depth. So, the question is, what are my possible issues as to why the top gear did not fully nitride?

 

[dinjin]

I think we would have to know how they were stacked and how the space between them was designed. When you say tire, I think you are talking about internal gears.

 

[PGCTGS]

These are welded gears with shaft and flanges on both ends that ar 15" long.The distance between the gear teeth would be 30". The tire of a welded gear is also called the rim, it is where the teeth are cut.

Another question is whether I can have this gear renitrided without fear of over nitriding at this point. I know we will have to blast the gears prior to renitriding.

 

[tbuelna]

PGCTGS,

There are two stage gas nitriding processes (AMS 2759/6 CL 1). The minimum second stage temperature limit is about 35degF higher than the first stage, but otherwise the procedure for each stage is similar.

Also, check for decarburization in your parts.

Good luck.
Terry

 

[arunmrao]

Blasting the gears will remove nitride layer. You might then renitride the gear. As tbuelna has suggested,please look for any decarb layer,which might have affected the nitriding process.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[PGCTGS]

Thanks for you help and information.