Characterization Of Microstructure Of Low Temperature Chromized Layers On Steels

Low-temperature composite chromizing is the two-step process, which is first nitriding or nitrocarburizing at 540℃～580℃and following chromizing in a salt-bath below 700℃. In this paper, the microstructure, the distribution of the Cr and N, the crystalline of phases and the morphology of nitrides or carbides in composite chromized layers, which had formed during low-temperature composite chromizing, were investigated using X-ray diffractometer, optical microscopy, scanning electron microscopy, transmission electron microscopy. In order to decrease the chromizing temperature and improve the quality of chromized layer, the processing parameters, such as the ingredients of salt bath, were studied.The results show that the main phases in the low-temperature chromized layers are CrN,β,α-(Cr,Fe) and Cr7C3. When the chromium atoms from the salt bath diffuse into the surface of sample and then combine with the nitrogen atoms in original nitrided compounds layer, the chromized layer is gradually formed. The amount of nitrogen in original nitrided layer plays an important role in the quality and thickness of chromized layer, and the carbon of the substrate was no significant contribution to increase the thickness of chromized layer at 500℃. The nitrogen atoms of original nitrided layer had two actions in the chromizing process:on the one hand, they combine with the active chromium atoms in the surface layer to form chromized layer; on the other hand, they diffuse into the inside of the specimen, resulting in the ineffective loss. At the lower temperature, the diffusion rate of nitrogen atom is very slow, as reduce the loss of the ineffective loss, so it is helpful to increase the thickness of chromized layer. The chromium carbides in the outer layer are formed when the chromium atoms on the surface combine with the carbon atoms from the substrate after the consuming of nitrogen atoms of the original nitride layer. Owing to the concentration of chemical composition and thermodynamic effect, CrN will be formed prior toβphase. When the chemical potential of nitrogen in the sample surface is decreased to a certain extent, the nitrogen atoms can combine with the chromium atoms to formβ. With increasing of temperature and the concentration of carbon in the matrix, the concentration of CrN phase is gradually increasing while that ofα-phase is gradually decreasing. There are some sub-micron CrN clustered areas and nano--crystalline CrN clustered areas in the low-temperature composite chromized layers, and fewer defects exist in the CrN grains. But many of Cr7C3 compounds with micro-twins or stacking faults appear at the later time of low-temperature composite chromizing.The technology of chromizing was used to the foam iron. It was found that the process could effectively apply to the foam iron, and the content of chromium on the surface of workpiece was more uniform. And more, chromizing could enhance the high temperature oxidation resistance of the foam iron, so it enhanced the application range of the foam iron.