Questions about Gas nitriding 4140 steel parts

I am asked to quote some parts made from 4140 steel. With gas nitride heat treat to 50 to 54 RC.

These are small parts 1/2" square and about 2" long.
Customer is asking for gas nitride to get its anti galling properties and corrosion protection.

Does the gas nitride thru harden or just surface harden?
Do in need to use prehard 4140 material. Or will the nitride work on soft 4140?
Do the part dimensions "grow" or "shrink" during heat treat?

Thanks in advance.

trademarkmachine said:

I am asked to quote some parts made from 4140 steel. With gas nitride heat treat to 50 to 54 RC.

These are small parts 1/2" square and about 2" long.
Customer is asking for gas nitride to get its anti galling properties and corrosion protection.

Does the gas nitride thru harden or just surface harden?
Do in need to use prehard 4140 material. Or will the nitride work on soft 4140?
Do the part dimensions "grow" or "shrink" during heat treat?

Thanks in advance.

Click to expand...

It's been a very long time since I worked with it, so take this with a grain of salt...

It does not thru harden, it will be case hardened. IIRC the parts we did grew few tenths, or so. They were 1.5-2.0" dia mandrels, not sure what will happen since your parts are considerably smaller. I *think* we used 4140ph (28-32Rc), but not 100% sure...

Gas nitriding in that hardness range is very doable on 4140. You do not need pre-hard. The nitriding will be a surface hardening only - not through. Part change with gas nitriding is minimal but still best to finish critical features after nitriding if you're trying to hold microns. If you're only trying to hold plus/minus .001" on that size I would finish to size.

trademarkmachine said:

I am asked to quote some parts made from 4140 steel. With gas nitride heat treat to 50 to 54 RC.

These are small parts 1/2" square and about 2" long.
Customer is asking for gas nitride to get its anti galling properties and corrosion protection.

Does the gas nitride thru harden or just surface harden?
Do in need to use prehard 4140 material. Or will the nitride work on soft 4140?
Do the part dimensions "grow" or "shrink" during heat treat?

Thanks in advance.

Click to expand...

1) surface harden, depending on the size of the part you can expect .02-.04” of case depth so small,parts are effectively thru hardened.

2) soft, they’re going to take the parts up to nearly 1000*F so whatever you stick in there is going to thoroughly annealed anyways. PH stock maybe yield better machining properties and surface finish.

3) because of the “low” temperature distortion should be limited but maybe someone has more experience than I do.

My first issue is with your query. Gas nitriding creates surface hardening only and you specified Rockwell "C". The "C" test requires a 45 degree diamond cone with a 150kg weight applied. That will puncture most surface hardening results. The "N" test is more realistic.

trademarkmachine said:

I am asked to quote some parts made from 4140 steel. With gas nitride heat treat to 50 to 54 RC.

These are small parts 1/2" square and about 2" long.
Customer is asking for gas nitride to get its anti galling properties and corrosion protection.

Does the gas nitride thru harden or just surface harden?
Do in need to use prehard 4140 material. Or will the nitride work on soft 4140?
Do the part dimensions "grow" or "shrink" during heat treat?

Click to expand...

There is no quenching involved and temperatures are too low to get 50 to 54 Rockwell on the core from just nitriding. Nitriding temperature is usually only around 1000 F so softening of the base material is not a huge problem. Nitrided case is very thin, only a few thousandths (altho Thermofusion claims to get more, but I've never used them for this.)

I've made nitrided gears - they were blanked, quench-and-tempered to the desired core hardness, usually in the low forties. Then finish machined and teeth cut, then nitrided. The nitrided case is in the high sixties to low seventies Rockwell (N scale ? or just an "equivalent to C scale ? been a while.) Distortion was nil.

The lack of distortion was the only benefit I saw over carburizing. Just about everything else about carburizing seems better for what I was doing.

There is also carbo-nitriding, which I have not tried. Maybe look into that.

steve-l said:

My first issue is with your query. Gas nitriding creates surface hardening only and you specified Rockwell "C". The "C" test requires a 45 degree diamond cone with a 150kg weight applied. That will puncture most surface hardening results. The "N" test is more realistic.

Click to expand...

Steve:

This is what is on the print.
I don't think the customer needs thru hard part. But want the surface hardened for wear reasons.

trademarkmachine said:

This is what is on the print.
I don't think the customer needs thru hard part. But want the surface hardened for wear reasons.

Click to expand...

As is often the case, your customer has not got a clue. Good luck teaching him.