Abstract:
In this research, investigations were performed on development of nanostructured dual phase steel via thermal-cycling with nanoquenching from inter critical temperature range. A total of 150 standard samples of low carbon steel were prepared for mechanical tests and wear test specimens by a conventional lathe machine. All the samples were made to undergo a full Normalizing, Lamellarizing, Quenching and Tempering (NLQT) process. The steel samples prepared were first normalized in a muffle furnace by heat treating them at 830°C for 30minutes followed by cooling in air in order to eliminate the previous thermal and mechanical history of the steel. 15 samples each represent, tensile, fracture toughness, hardness, impact and wear for 10 experimental runs which were normalized, heat treated within the dual-phase region of 730oC, 750oC and 770oC for a holding time of 20, 40 and 60 minutes respectively. Respective samples were then quenched in a prepared nanofluid (ZnO) followed by tempering at 480oC for a soaking time of 30 minutes, then allowed to air cool. Microstructure, mechanical properties and wear resistance were examined to determine the performance of the developed dual phase steel. The microstructure examination showed ferrites dispersed in pearlite matrix for normalized specimen while homogenously dispersed duplex ferrite – tempered martensite and a fraction of bainite with very little retained austenite was obtained for the developed heat treated dual phase steel samples. The tensile properties showed good improvement due to good synergy between ferrites and martensite grains. However, NLQT770 at 60 minutes displayed better strength with 530.13 MPa and E-modulus while good combination of strength and ductility is obtained for NLQT730 series. Carbon redistribution and grain refinement upon tempering increases hardness in some samples. However, this is more pronounced at NLQT770/40 with 72.15. The improvement observed in the properties tested as compared with literature justify the efficacy of the heat treatment procedure viz a viz the use of the nanoquenchant.
