Precipitation behavior in bimodal ferrite grains in a low carbon Ti-V-bearing steel

With austenite under 50% compression using a dilatometer, ferrite transformation in a low-carbon Ti-V-bearing steel was sequenced through the isothermal transformation at 650 °C for various holding times (10 s, 20 s, 30 s). Bimodal grain size distribution was observed in the prolonged holding conditions. Corresponding measurements of Vickers and nano-indentation hardness were carried out; the tiny ferrite grains possessed lower hardness (303 HV and 4.8 GPa) as compared to larger ferrite grains (336 HV and 5.6 GPa). Transmission electron microscopy provided strong evidence attributing this discrepancy to interphase precipitation taking place in the larger-grained ferrite, instead of in the tiny-grained ferrite.