Charpy question related to nitrocarburizing thin walled 4340 tubing

[WillCole]

Hello, we planning on gas nitrocarburizing some short sections of thin walled (< 0.020”) 4340 tubing. The question has come up that since we are going to get basically through penetration with the nitrocarburizing, what is that going to do to the impact strength of the tubing? My guess it that it will lower it significantly, but I can't find any reference that even gives a sense of a magnitude. Will it reduce it by 10% or closer to 90%? If anyone has and experience with analogous materials (or knows of a reference) I would be interested in hearing from them.

 

[WillCole]

Corrected Version Not Typed on a Smartphone Keyboard.

Hello, we are planning on gas nitrocarburizing some short sections of thin walled (< 0.020”) 4340 tubing. The question has come up that since we are going to get basically through penetration with the nitrocarburizing, what is that going to do to the impact strength of the tubing? My guess is that it will lower it significantly; but I can't find any reference that even gives a sense of a magnitude. Will it reduce it by 10% or closer to 90%? If anyone has any experience with analogous materials (or knows of a reference) I would be interested in hearing from them.

 

[metengr]

Why are you performing ferritic nitrocarburizing (FNC) on a 4340 steel with this carbon content? FNC is usually performed on plain and low carbon/low alloy steels.

Excerpt from a David Pye publication;
Ferritic Nitrocarburizing (FNC) is a low temperature nitriding procedure that can be applied to low alloy and plain carbon steel’s. The procedure is conducted at a slightly higher temperature than would be for nitriding. The process temperatures are generally in the range of 975°F to 1075°F. The process is still based on the diffusion of nitrogen into the surface of the steel but with the addition of a hydrocarbon gas which will assist in the formation of the epsilon compound zone at the steel surface. The process is generally applied to low cost components that require a hard wearing surface. However the surface will not take any impact conditions, as it has no core strength to support any potential impact on the surface. But it does give a hard wearing surface.

 

[WillCole]

metengr - Thank you for the info! We have used FNC in the past (with good success) to improve wear and corrosion resistance on some smaller 4340 parts. Someone did not really think through how thin walled the parts were in this case, before the suggestion was made. I appreciate the reference.

 

[EdStainless]

How deep do you plan to go, 0.002"?
If you do it through you will have very brittle parts.

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Plymouth Tube

 

[WillCole]

Ed - The plan is to limit depth to under .003". The question is more one of magnitudes. There is relatively little information out there (at least that I have found) related to fnc and thin walled parts. But that is basically a guess as to where to start testing. It may be that .005" would be better because the change in toughness it graduated. I was just hoping to get some general magnitude feedback (or relative references like more than this less than that) from folks with experience.

I would be curious for example of the tensile strength change in a wire that was fnc'd, assuming it was already tempered to the point that no core change would take place. Would it go up in function tensile or would stress risers in the case cause it fail sooner? I can find vague references to charpy values or tensile increasing in thicker parts, but little on thinner parts. As I don't have much experience with the process, I don't really have a rule of thumb or gut feel for how it changes mechanicals on small parts, again assuming temper is accounted for.

 

[EdStainless]

I presume that your temper temperature is about the same as the NC temp?
So you know what your core properties will be.

Yes it is a balancing act. Too thin and you risk flaking the surface, too thick and the entire part will crack.

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Plymouth Tube

 

[TVP]

Will,

I have some experience with nitriding of thin-wall tubes, specifically, 36 mm ID x 1.3 mm thick. The grade is plain carbon steel St 37-2 DIN 2393 (1015-1020 ASTM A513). Brittleness is definitely an issue if the diffusion zone is excessive. I strongly recommend keeping it below 75 micrometers, and any white layer (compound layer) to 25 micrometers maximum. As long as you meet these parameters, the tube can actually be flattened and flared quite a bit.

 

[WillCole]

TVP, that is just the sort of info I was looking for. Thank you!