With unalloyed steels, the nitrogen is dissolved in the iron lattice. Due to the diminishing solubility of nitrogen in iron during slow cooling, _'-Fe4N nitrides are precipitated in the outer region of the diffusion layer, some in form of needles, which are visible in the structure under the microscope. If cooling is done quickly, the nitrogen remains in super-saturated solution. With alloyed steels which contain nitride-forming elements, the formation of stable nitrides or carbonitrides takes place in the diffusion layer independent of the cooling speed. With increasing alloy content of the steel, the diffusion layer is thinner for identical nitrocarburizing parameters. However, with their higher level of nitride-forming alloying elements these steels have a greater case hardness. Fig. 3 illustrates the influence of chromium on the hardness and depth of the diffusion layer in steels with a carbon content of 0.40 - 0.45% after 90 minutes treatment at 580°C (1075°F). Total nitrocarburizing depth shown in Fig. 4 is the distance to the point where the hardness of the nitride layer is equal to the core hardness. After a 90 minute treatment the total nitrided depth is about 1.0 mm (0.040") on unalloyed steel, but barely 0.2 mm (0.008") on a 12% Cr steel. (See Fig. 4.)