A Study on the Microstructures and Mechanical Properties of Ni-Cr-Mo-V Low Alloy Steels

Ni-Cr-Mo-V steel alloys for high-speed railway brake discs were prepared to investigate the effect of alloying element contents on the microstructure and mechanical properties. The Cr, Mo, Mn alloying elements were incorporated into the steel alloys, which contained low carbon content in the range 0.16 wt.% similar to 0.21 wt.% to provide sufficient hardenability. The steel alloys were austenitized at 940 degrees C for 1 hour and quenched, and tempered at 610 degrees C. Microstructural study showed a tempered martensitic microstructure with different sized austenite grains and packets. C-Mo alloy with high Mo content and the smallest prior austenite grain size showed the highest hardness and tensile strength. But, the alloy exhibited lower impact toughness than low Mo content alloys. The lowest tensile strength of the low Mo content Mn-Cr alloy, at room temperature and elevated temperature of 600 degrees C, was 1053.4 MPa and 667.2 MPa, respectively. The grain refinement in the C-Mo alloy was considered to be due to the solute drag effect of the Mo element. The absorbed impact energy increased with tempering temperatures, but the impact energy of the three alloys had lower values than the generally guaranteed impact energy of the currently used disk. The low impact toughness of the Mo containing alloys was attributed to the higher Si content and higher tempered hardness of the alloys. A higher thermal conductivity and lower thermal expansion coefficient were obtained in the high Mo content C-Mo alloy, which had a higher Ac-3 transformation temperature.