The Plasma nitriding process produces a hard outer skin on the material being nitrided. The hardness achieved on the surface decreases with depth until the core hardness is reached. The graph in Fig. 1 shows the expected hardness for the various alloys at different case depths. The slope is more gradual for low alloy steels and very sharp for highly alloyed steels. The nitride forming elements in the steel's composition are the primary factors controlling the hardness and the case depth. The low alloy steel will provide a deeper case depth but a lower overall hardness.

The micrograph in Fig. 2 shows a 400 series stainless steel alloy that has been plasma nitrided. The high alloy content of the stainless steel creates a high surface hardness and a sharp transition zone between the nitrided surface and the core material. Low alloy steel such as 4140 would have a lower overall surface hardness and a gradual transition zone between the nitrided layer and the core material; however, the overall penetration of the nitride layer would be deeper. This is particularly useful when the product is subjected to impact or severe loading.