Correlation Between Secondary Precipitation and Tensile Ductility of High-Speed Steels

The aim of this study is to evaluate the effect of secondary carbide precipitation on strain hardening and tensile ductility of high-speed steels and to develop a novel pathway for ductility enhancement by engineering the annealing microstructure. The results demonstrate a strong correlation between secondary precipitation and austenitization. With decreasing austenitization temperature, secondary carbides exhibit a transformation from a rod-shaped Cr-rich M23C6 type to a granular Mo-rich M6C one, corresponding to a transition of the underlying eutectoid decomposition mechanism from a cooperative growth mode to a divorced eutectoid manner. Highly dispersed rod-shaped M23C6 precipitates contribute to an enhanced tensile strength but lead to a degraded work hardening rate in the late deformation stage and, therefore, a lower total elongation. In contrast, granular M6C precipitates exhibit an excellent capacity of accumulating dislocations and enhancing the work hardening rate especially at a high strain, which enables a significant increase of ductility. It is suggested that granular M6C precipitates embedded in fine ferritic grains with lean dislocations is a desirable annealing microstructure to produce a more ductile high-speed steel.