Microstructure of Ferritic Nitrocarburized 4140 Steel

[BenMet]

As part of a cost reduction effort, my company is in the process of evaluating various suppliers. The part in question is a 4140 grade steel piston. My understanding is that the part is quenched and tempered to optimize the strength, then given a ferritic nitrocarburizing (FNC) treatment to help improve corrosion resistance and wear resistance. The parameters for heat treatment are as follows:

"Parts shall be nitrocarbide processed in an atmosphere or fluidized bed furnace for 1 to 4 hours at 1058 - 1292F (570 - 700C) depending upon hardness and depth requirements. They are then processed in an oxidizing atmosphere for 15 to 45 minutes. Parts are then quenched from a temperature above 1022F (550C) in a suitable medium."

Our microstructure requirements are as follows:
"The process produces a duplex case consisting of a compound (white) surface zone and a nitrogen diffusion zone. An iron oxide film is deposited over the compound zone."
Compound zone (white layer) - minimum 0.0004 inches deep
Total case (Compound and diffusion zones) - minimum 0.015 inches deep


Two vendors have submitted parts which they claim have been heat treated according to our specification. Upon examining etched transverse cross-sections through the parts, I noticed that the structures do not resemble the typical ferritic nitrocarburized structures with which I am familiar. There appears to be an extremely thin light-colored layer (~0.00004 inches thick) on the outer diameter. Below that layer, I can only discern tempered martensite plus some carbides. I do not see the typical "white layer" or diffusion zone which are characteristic of FNC structures. All micros were etched in a solution of 5% nital. After initial examination, the samples were re-ground, polished, and etched again, but still displayed the same microstructure.

ASM and Internet references which we have checked do not present any photos of ferritic nitrocarburized treated HSLA steels. I believe that the reason for this is, most likely, because FNC is most commonly performed on carbon steels. My question is - does FNC affect alloy steels differently such that the white layer and diffusion zone are not noticeable? Or - are the parts from both vendors processed incorrectly? Hardness profiles would be helpful here, but unfortunately our microhardness tester is severely out of calibration and will not be serviced until Monday 6/23/03.

I have a number of scanned photos which clearly illustrate this trend. However, I have not figured out a way to attach them to this system. If anyone is seriously interested in taking a stab at viewing these micros, please contact me and I can e-mail them to your address.

Again, thank you very much for your assistance. Your comments are most welcome!

 

[Metalguy]

The temp. range doesn't make sense to me. There is a large difference between the strength of 4140 tempered at ~1058 deg. vs 1292 deg. At the latter temp. you lose nearly all the benefit of Q&T.

 

[TVP]

I agree with Metalguy that the maximum temperature specified appears to be too high-- this treatment is usually performed around 570-580 C. And the second oxidizing step is usually performed around 400 C, not 550 C, so as to minimize distortion.

Alloy steels respond the same way that carbon steels repsond to this type of treatment-- compound zone on the surface, and a diffusion zone that extends further below the surface. Alloys that feature nitride formers (Cr, Mo) will also have alloy nitrides within the case.

Regarding photos, click on the Process TGML link listed under Step 2 Options...

 

[diamondjim]

You might want to check with the vendor,
the white layer is often removed by polishing
as most people do not want this layer and
only the lower layers.

 

[diamondjim]

This was posted to the Gear Forum by Ricko:

White layer is not normally tolerated on gears due to its brittle nature and high line contact loads to which many gears are subjected. There are exceptions. The benefit of nitriding gears is that extremely good dimensional control is possible because the gear is finish machined (with some allowance for growth during nitriding) and does not go through austenite transformation thereafter -- nitriding is done in the vicinity of 1000 F which is below the transformation temperature (at or above 1300 F). The benefit to most gears, as previously stated, is from the case that is formed, not from the white layer.

The white layer does have its benefits. I worked on an application where a shaft on which the spider gears of a differential ran steel on steel. Diametral clearance was critical, but without a white layer, seizure failure was assured. The white layer has an inherently low coefficient of friction, i.e. good lubricity, making it very beneficial when in a high speed but low load application.

The following is a brief and simplified explanation of the difference between the white layer, sometimes referred to as the compound layer and the nitrided case. The white layer is a boundary layer ranging between minimal to upward to .002" in thickness. Typical thicknesses are .0005 - .0015" when produced by conventional gas nitriding. It is called a compound layer because it is made up of a number of different iron alloy nitrides, FE2-3N (epsilon), Fe4N (gamma) being the more common. The white layer is formed by direct contact with the nitriding atmosphere and is where the ammonia dissociates into its component, the most important being nascent nitrogen, which reacts with the steel surface to form the white layer. Diffusion then occurs from the white layer into the steel. As the nitrogen content of the steel increases, reactions occur between certain alloying elements (e.g. Cr, Al) to form extremely hard nitrides. But more importantly, the nitrogen remains in solid solution in the steel. The nitrogen atom's much smaller size permits it to nest within the steel alloy atoms, strenghtening the surface thereby increasing its hardness dramatically.