STUDY OF THE EFFECT OF GRAPHITIZED ANNEALING ON MICROSTRUCTURE OF CHROMIUM-NICKEL CAST IRON

The technology of manufacture of dual-layered centrifugally cast rolling rolls includes the process of casting and crystallization of work layer metal and, some time later, of core metal. It supports non-monotonous features of cooling of work layer metal that is subjected to the secondary heating and holding at high temperature, or to annealing. At the same time graphite is extracted in cast iron structure of work layer; its amount can be varied both by holding time and by alloying. Free carbon can be extracted by dissolution of carbide phase and by depletion of solid solution, what should effect on quantitative parameters of microstructure of mottled cast iron and its mechanical properties. Revealing of regularity of forming the structure of mottled cast iron after its graphitized annealing depending on holding time is the aim of this study. Experimental cast samples were subjected to annealing in the protected medium at 900 T during 30-330 min. Regularities of forming the microstructure were examined using optical metallography. Examination of experimental samples was resulted in obtaining of relationships between graphite amount, carbide phase and residual austenite (from one side) and time of graphitized annealing (from other side). Graphite amount increases up to certain holding time, and then additional extraction in the structure was not observed. On the contrary, carbide phase amount is essentially decreased at the same time up to certain holding time, and then its amount leaves practically the same. Joint analysis of obtained relationships displays that graphite is partially extracted owing to dissolution of carbide phase, while quantitative relationship itself is characterized by power features based on the fact that graphite is also extracted due to carbon that is dissolved in metal base austenite. Alloying composition of austenite has effect on its resistance to decomposition during overcooling, thereby graphite amount in the structure of mottled chromium-nickel cast iron is considered as the main regulator of quantitative phase relationship in metallic base and of micro hardness of samples in general.