Characterization of a CrMoB high strength, low alloy steel: Relationship between the microstructure and mechanical properties

Abstract

The present study was performed on a high strength, low alloy steel containing 0.11 wt.%C, 1.46 wt.% Mn, 1.04 wt.% Cr, 0.25 wt.%Mo and 0.003 wt.%B. The steel was subjected to isothermal transformation in the temperature range 350–600°C, normalizing and tempering, and quenching and tempering. The microstructure and mechanical properties of this steel after these heat treatments were compared with those produced after dual-phase treatment (intermediate quenching from three different temperatures corresponding to martensite volume fractions of 20%, 40% and 60%) and tempering. The changes in the microstructure were investigated by means of optical and transmission electron microscopy, while the variations in the mechanical properties were analysed using tensile and hardness measurements. Continuous yielding was found for all the steels except the fully annealed steel. On tempering at 400°C or higher, all the dual-phase steels exhibited a complete return of discontinuous yielding. As the volume fraction of martensite is reduced to about 20%, the yield strength is constant up to 200°C. Dual-phase steel with a martensite volume fraction of about 40% reveals better strength and ductility levels up to 400°C than does granular bainitic steel with a similar volume fraction of bainite granules.