My article in the November issue of Industrial Heating generated some interest and may have caused some unintended confusion. Here’s an edited comment received and my edited reply.  

Comment: I read with interest your article "Microstructure of Nitrided Steels" and would like to suggest some clarification regarding the references to salt bath nitriding. Unfortunately, the phrase “salt bath nitriding” is commonly used to describe salt bath processes that are actually salt bath nitrocarburizing (e.g., Melonite®, Sursulf®, Tufftride®, etc.), and I believe this to be the case here. But nitriding and nitrocarburizing produce significant differences.  

One example is your statement that compound layers are generally deleterious. This is true for gaseous nitriding processes, as they tend to produce a brittle, high-strength structure of high-nitrogen Fe2N. However, compound layers developed from salt bath nitrocarburizing are composed of primarily carbon-containing, lower-strength, lower-nitrogen content Fex (N,C) and retain a certain amount of ductility despite a significant increase in hardness.  

With salt bath nitrocarburizing, we are dealing with much shorter treatment times and, therefore, much shallower depths. Depending on material, a hardness gradient is typically present to about 0.25-0.50 mm. This hardness is the direct result of solid-solution strengthening and can contribute to a significant increase in fatigue strength, depending on material.  

To summarize, it appears to me that the phrase “salt bath nitriding” is being treated as synonymous with salt bath nitrocarburizing in your article, resulting in some confusion. Identification of the actual salt bath process used to generate the data would help make it clear. Thanks.

Response: Thanks for your letter. The only specimen that I showed that was "salt bath nitrided" was the 1215 carbon steel. That was what I was told by the people who gave me the specimen. I have no way to know exactly what process was used. I have to believe what I am told about that. None of my specimens that I showed were nitrocarburized. The gas nitrided 41B50 specimen broke immediately upon being put into service. That is where I stated that the compound layer was brittle. I have heard people claim that the compound layer on the carbon steel, as for the 1215 but perhaps a better treatment, gives improved wear resistance (as I said at the end of paragraph one).  

But you can see from the hardness profile that there was virtually no real hardness increase. Diffused nitrogen into the steel does not produce solid solution strengthening. That is only produced by alloying elements added to the steel in melting. The diffused nitrogen strengthens the case by forming nitrides, but not all nitrides are created equally! Fe4N does not give much strengthening nor does epsilon phase. You need very fine, extremely small (sub-micron) nitrides of Al, Cr and V to get a high case hardness. You can see this when you compare the hardness profiles in Figures 2, 6 and 11. You can see from Figure 4 that there is no case hardening at all in the 1215 specimen, only a compound layer, and that was not very hard – but harder than the steel below it.  

I do not have any steel specimens that were salt bath nitrocarburized using any of the processes that you mentioned. I would love to have some specimens to do microscopy upon. My article is about the microstructure of nitrided specimens. I am not a heat treater, per se, and I have no heat-treating equipment, let along carburizing, carbonitriding, nitriding, etc. furnaces. I have to rely upon the good graces of people, such as you, to give me good specimens to study.