Influence of Part Size on Microstructure during Austempering of Medium Carbon Low-Alloy Steels

In this work, the influence of part dimensions on the microstructure of low-alloy steel during its austempering has been studied. 0.41C-0.24Si-0.63Mn-0.93Cr steel of different diameters ranging from 5 to 16 mm has been austempered at 350 degrees C for 10 min. Despite similar austempering conditions, an increase in the size and fraction of acicular/polygonal ferrite has been noted with an increase in the diameter of the samples. From the experimental observations and analytical calculations based on multi-dimensional transient heat transfer analysis, the cooling rate during austempering has been observed to increase with a decrease in the sample diameter. Variation in ferrite grain size and fraction is due to different cooling rates encountered by different sized specimens during austempering. Close agreement between the experimental and theoretically estimated ferrite grain size due to variation in cooling rate during austempering has been noted. This study examines how part size affect low-alloy steel microstructure during austempering. Across 5 to 16 mm diameters, increased sizes correlate with higher fractions of acicular/polygonal ferrite. Analytical and experimental findings reveal that smaller diameters experience faster cooling rates during austempering, resulting in small ferrite grain sizes and fractions while large diameter samples experience slower cooling rate during austempering and large ferrite grains and higher fraction.image (c) 2024 WILEY-VCH GmbH