A study of the microstructure and mechanical properties of an lost wax casting of steel 4340 was carried out. Compared to a part made from hot-rolled steel, a cast part has a significantly lower level of ductility, toughness and fatigue strength. The characteristic defects were revealed in the cast part that lead to a deterioration in the viscosity and plasticity: dendritic segregation, pores, cracks, surface decarburization.