Microstructure and hardness evolution for LB/M microstructures in a hot working die steel

Hot working die steels require high hardness, strength and toughness, and there is a strong industrial drive to improve mechanical property combinations through microstructure design and optimization. A study was carried out on a commercial hot working die steel plate (Uddeholm Dievar) using austempering, isothermal holding at 340°C followed by cooling, to understand LB/M microstructure and hardness development, considering spatial alloying element distributions. Local substitutional alloying element compositions between solute-rich and solute-depleted regions led to an around 30°C difference in the local undercooling for isothermal bainite formation. Inhomogeneous distributions of LB/M microstructures were evident when the lower bainite phase fraction was less than 30 vol.%. As the isothermal holding time extended, carbon partitioning resulted in a progressive refinement of bainitic plates and martensite laths. Needle-like bainitic cementite grew to plate-like morphology under the paraequilibrium condition with carbon diffusion. Carbon-enriched sandwiched LB/M interface microstructures, consisting of coplanar thin-plate martensite and adjacent ultrafine martensite laths, formed at the onset of final cooling to room temperature. The decrease of micro-hardness for LB/M microstructures followed the rule of mixture, with micro-hardness remaining relatively unaffected for lower bainite phase fraction up to ∼30 vol.%. Narrow nano-hardness distributions for LB/M microstructures with ≤ 10vol.% lower bainite changed to broad ones for LB/M microstructures with ≥30 vol.% lower bainite. This work has shown that substitutional alloying element micro-segregation and carbon redistribution on isothermal holding contribute to spatial heterogeneous LB/M phase distributions and need to be carefully dealt with for the LB/M microstructure design in hot working die steels.