Application of a fluidized sand bed to control the structure of steel castings

This study investigates the influence of fluidized silica sand on the microstructure formation of 40KhL steel during lost foam casting. The aim of the research was to experimentally assess the possibility of controlling the microstructure of castings by modifying the cooling conditions within the mold, without applying conventional heat treatment. For this purpose, single-use stepped patters made of polystyrene foam were fabricated and poured with molten steel at a temperature of 1560 °C. The castings were cooled in molds filled with dry silica sand: the first series under natural conditions, the second with a fluidized sand bed created by supplying compressed air from the bottom of the mold after the casting reached a temperature of approximately 1070 °C. Microstructural analysis and microhardness measurements were carried out on samples extracted from walls of different thicknesses (5–20 mm). In the samples cooled in fluidized sand, the formation of a highly dispersed martensitic structure was observed in the thin walls (5 mm), along with bainitic structures in the thicker zones. The microhardness of martensite reached up to 900 HV200, which significantly exceeded the values observed in samples cooled under conventional conditions (~260–380 HV200). In castings with thicker walls, a gradient structure was formed with increased surface hardness. Additionally, the formation of an oxidized porous layer on the sample surface due to rapid cooling was detected, with a thickness of 209 ± 32 µm. The obtained results confirm the effectiveness of the fluidized sand method in controlling the cooling rate and, consequently, the phase composition of steel castings. This method enables the production of castings with tailored properties without additional thermal processing, aligning with current requirements for energy saving and resource efficiency in foundry operations. The proposed approach can be used to produce thin-walled or surface-hardened castings of complex geometry.