**9.1. Carburizing and diffusion annealing at a coexistence of the** α **-** γ **diffusion couple**

During annealing at temperatures below α-γ transformation of the steel substrate, the coating already contains a significant amount of the γ phase, which is the solid solution of Ni in γFe (fcc). At the same time, the substrate during annealing remained fully pearlitic. Thus, at the annealing temperature, cementite coagulated, dissolved and acted as a source of carbon, diffusing towards coating. Due to a relatively low temperature, the transport of large amounts of carbon for long distances within coating was difficult. After 30 min of annealing at 710 oC, the mean square root displacement of the carbon in austenite is as low as 32 µm. However, the carbon concentration gradient within the coating caused substantial modifications of the microstructure formed after cooling from annealing temperatures.

An example of cross-sectional image of the microstructure formed after cooling from the two-phase (α-γ) range of coating, is shown in Fig. 23a. The coating cooled from the onephase region γ comprises continuously graded microstructures caused primarily by differences in carbon content at the coating-gas and coating substrate interface. Since the substrate was not transformed, the decarburization is seen as a thin ferritic layer, adjacent to fully pearlitic microstructure. The hardness depth profile for the treatment performed at 710 oC, is represented by the lower curve in Fig. 24. An increase in hardness is seen in the region adjacent to the substrate and to the outer surface, due to diffusion of carbon from these two directions.
