The role of Ti, Zr and Ce in shaping of the Cr-Ni-Nb cast steel resistance to carburising

From the centrifugally cast Cr-Ni-Nb steel pipes, specimens were cut out and subjected to carburising for 100 hours in a mixture of charcoal (90%) and Na2CO3 (10%) at the temperatures of 950 and 1150°C. The specimens were cut in direction normal to the pipe axis and were examined by optical and scanning microscopy. As a parameter describing the resistance of the examined alloy to the carburising effect, the thickness of a carburised layer was accepted. It has been observed that additions of Ti and Zr, and of Ti+Zr+Ce introduced jointly, increase the thickness of the carburised layer, while the addition of Ce improves the alloy resistance to carburising. On the alloy surface, a layer composed of oxides, mainly of chromium, silicon and iron, has been formed. Changes in the chemical composition of the surface layer were examined by scanning microscopy.