Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels

The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels (UHSSs) with different levels of C, Cr and Ni has been evaluated for the as-cooled and untempered condition. One UHSS had higher contents of C and Cr, while the other one had a higher Ni content. On the basis of dilatation curves, microstructures, macrohardness and microhardness, continuous cooling transformation diagrams were constructed as a guide to heat treatment possibilities. Cooling rates (CRs) of 60, 1 and 0.01 °C/s were selected for more detailed investigations. Microstructural characterization was made by laser scanning confocal microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, electron probe microanalysis and X-ray diffraction. Mechanical properties were characterized using macrohardness, tensile and Charpy V-notch impact tests. UHSS with the higher C and Cr contents showed lower transformation temperatures and slower bainite formation kinetics than that with the higher Ni content. Higher cooling rates led to lower volume fractions and carbon contents of retained austenite together with finer prior austenite grain size, as well as effective final grain size and lath size. These changes were accompanied by higher yield and tensile strengths. The best combinations of strength and toughness were obtained with martensitic microstructures and by avoiding the formation of granular bainite accompanied by proeutectoid carbides at low CR. For the cooling rates studied, UHSS with the higher C and Cr contents showed the higher hardness and strength but at the cost of toughness.