La coltre bianca negli strati nitrurati in acciai a vario contenuto di carbonio.

Nitriding and carbonitriding treatments generate on the surface of the steel workpieces a combined double layer with an external compound layer of nitrides and carbonitrides and an inner layer with the microstructure of the base steel modified by nitrogen enrichment by diffusion. Tlte composition and constitution of the compound layer depend both on the treatment conditions and on the composition and microstructure of the base material. Off course, to different constitution correspond also differences in properties and technological behaviour: hardness, porosity, wear and corrosion resistance, etc. In the paper the compound layers obtained by the same "hard nitriding" treatment on steels of different carbon content have been examined. The interaction of nitrogen with the steel surface can produce too different nitrides: the γ-Fe4N and the ε-Fe2N. Only the latter can dissolve significant amount of carbon, up to about 3%. Tγe nitriding of pure iron (or ferrite) usually produce at first the γ′-nitride, and subsequently the C, superimposed to the first γ′layer. But the progressive growth of the C layer generate some porosity, coming from the partial decomposition in the inner zone of the layer of this nitride, not more in equilibrium with the external nitriding atmosfere, Therefore the nitriding compound layer on ferrite, or pure iron, is a double layer, with a thick external layer of C nitride over an inner layer of γ′-nitride. The nitriding mechanism of the steels is more complex because, as contemporary to the previously described phenomena, also occur the interaction of the nitrogen with the cementite, the iron carbide, which produce the c-carbonitride. Therefore the compound layer assume a more complex constitution: a nitrogen rich ε-carbonitride as surface layer, an intermediate γ′ layer, and a inner, carbon rich, carbonitridc layer. The compound layer obtained on steel samples shown in Table 1 have been examined by usual metallographic techniques (OM and SEM, backscattered electron images), X-ray diffraction and hardness measurements. In the same Table 1 hardness and thickness of compound layer obtained are also reported. XRD results in Fig. 1, enlighten the different constitution of surface compound layers obtained on various steels: the different ratios between the C and γ′ nitrides and the progressive formation of carbonitrides, increasing the carbon content of the treated steel. The microstructures of the compound layers are reported in the Fig. 2-11. The double layer C and γ′' are evident in steels at lower carbon content and the triple layer ε-γ′-E in other steels at higher carbon. The thickness values (table 1) are very similar, but higher in the alloyed steel. The surface hardness increase with the carbon content, but the hardness of the compound layer is not too much different, with a little increase with the carbon content. The porosity appear rather limited in steels up to 0,4 % of carbon, but more evident at higher carbon.