The influence of dislocation and grain structure on the formation of intergranular cracks in centrifugal-cast billets (steel grade 40Х25Н20С2)

Purpose. Investigating the causes of intergranular brittleness and determining the optimal conditions for the structure formation in centrifugal-cast billets (steel grade 40Х25Н20С2) to eliminate the possibility of hot cracking. Methodology. The structure and properties of hollow billets were analyzed using castings (steel grade 40Х25Н20С2) produced by true centrifugal casting (horizontal centrifugal casting machines). Rotation speed of the mold was varied between 900, 1200 and 1500 rev/min. Ad hoc methods of electrolytic, chemical and thermal etching were used to identify zonal/dendritic chemical and substructure inhomogeneity of the dislocation structure. For qualitative and quantitative assessment the X-ray electron probe analysis was applied. Features of secondary grain boundaries in 40Х25Н20С2 steel were examined in the cast state following thermal processing. Findings. It was found that changes in technological parameters of centrifugal casting leading to a reduction of the transcrystallization area and increased surface areas of equiaxed crystals in the structure of centrifugal-cast hollow billets (steel grade 40Х25Н20С2) allow reducing susceptibility to brittle intercrystalline fracture. Hot cracks of crystallization origin and polygonizational cracks form in castings. It is shown that formation of crystallization cracks is significantly affected by chemical heterogeneity, while formation of polygonizational cracks is affected by substructural heterogeneity caused by formation of secondary boundaries. A significant means to reduce susceptibility of cast steel to generation of microfraction is to control the scale and nature of chemical and substructural heterogeneity. Originality. A relation between the primary dendrite form of austenite and the secondary grain structure was detected. These results confirm the polygonizational nature of secondary grain boundaries. Practical value. Application of the obtained results allows substantiate use of casting process parameters, providing high quality of high-alloy steel castings (steel grade 40Х25Н20С2).