Effect of part size on microstructure of medium carbon high silicon steel during austempering

This study explores the effect of sample diameter on the microstructure and hardness gradient in a medium-carbon high-silicon steel that was subjected to austempering treatment. Cylindrical steel samples of varying diameters (5 mm, 8 mm, 11 mm, 14 mm, and 16 mm) were austenitized at 1000°C and austempered at 350°C for 10 min. Microstructural analysis via X-ray diffraction and scanning electron microscopy revealed that 16 mm diameter samples have 21 % retained austenite , 76.3 % bainitic ferrite with 44 μm lath size while 5 mm diameter samples have 13 % retained austenite , 81.4 % bainitic ferrite with 32.3 μm lath size, suggesting a direct relationship between part size and microstructural attributes. Larger samples experienced lower cooling rates, leading to a smaller driving force and a lower nucleation rate of bainitic ferrite laths. At low cooling rates, the transformation took place at higher temperature, which limited carbon partitioning into untransformed austenite and reduced the extent of the austenite-to-bainite transformation thereby resulting in higher fraction of retained austenite . Small-diameter samples experienced higher cooling rates at the core, leading to a homogeneous microstructure across entire section, consisting of fine bainitic ferrite sheaves separated by film-type retained austenite. Microstructure variation in the various sized samples also led to variations in the hardness profiles measured radially across the samples. Empirical relations demonstrate that hardness variation is influenced by bainitic ferrite lath size, dislocation density , and the fraction of retained austenite. Results of this study could be useful for designing austempering treatment routes of parts made of third-generation advanced high-strength steels. The size of the part could be considered as an important aspect influencing microstructural attributes and hence the mechanical properties of these steels.