Formation of secondary boundaries in cast steel

Purpose . Study of the formation of secondary grain boundaries during crystallization in centrifugally-cast billets (steel grade 40Х25Н20С2). Methodology . The microstructure of hollow billets (steel grade 40Х25Н20С2) produced by horizontal centrifugal casting machines was analyzed. Features of secondary grain boundaries in 40Х25Н20С2 steel were examined in the cast state after annealing at temperatures of 1 100°C and 1 200 °C and equalizing for one and five hours. Metallographic “Neophot−21” and electron microscopics JSM−35, «Tesla», EMW−100B studies were conducted. 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 . We discuss the main sources of polygonizational formation of secondary grain boundaries in centrifugally-cast steel . Practical value . The obtained results allow using the secondary boundaries to inhibit the development of intergranular cracks.