Nitrided Gears

[ecampos]

Anybody have any info about "White Layer" on nitrided gears. What are some standards that you use other than AGMA, ASME etc. I want to know what you in your industry think is acceptable?

Edson Campos
edsoncampos@earthlink.net

 

[diamondjim]

Ed,
This is a strange question in that you are questioning
an official standard. I assume you are talking about the
benefits of nitriding gears. What DP range are you
talking about. Are you nitriding for strength or for
durability? Most of the standards are probably too deep for the little guys and not deep enough for the larger
gears. There are several nitriding companies with web sites
and they should be able to guide you. Nitriding benefits in
two ways. It provides a compressive stress into the material thus allowing greater tensile strenths to be applied. It also provide a shallow skin providing greater
contact stresses to be applied and extend the durability life of gears. Can you be more specific?

 

[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.

 

[ecampos]

Thanks, Diamondjim and Ricko, I am aware of what the "White Layer" is and what the standards are, it is just simply that the standards vary and the "Experts" have differing opinions. I just wanted to know what other engineers in the industry call out for the white layer on their nitrided gears. I think that the greates problem is the 64 Diametral Pitch gears that we have and the excessive case depth form poor process control.

Edson Campos
edsoncampos@earthlink.net