Carbide precipitation in ferrite in Nb-V-bearing low-carbon steel during isothermal quenching process

The precipitation behavior of nanometer-sized carbides in ferrite in Nb-V-bearing low-carbon steel was studied by electron microscopy and nanoindentation hardness measurements. The results indicated that interphase precipitation and random precipitation could occur simultaneously for the specimen isothermally treated at 700 ° for 60 min, while in other specimens, only random precipitation was observed. This phenomenon might be explained by mass balance criterion during the diffusional phase transformation. Nanohardness result indicated that the average hardness of the specimens isothermally held at 600 ° for 20 min was 3.87 GPa. For the specimen isothermally holding at 650 ° for 20 min, the average hardness was 4.10 GPa and the distribution of the nanohardness was in a narrower range compared with that of the specimen isothermal holding at 600 ° for 20 min. These implied that the carbides in the specimens isothermal treated at 650 ° were more uniformly dispersed, and the number density of the carbides was greater than that treated at 600 °. Using Ashby-Orowan model, the contribution of precipitation strengthening to yield strength was estimated to be∗110 MPa for the specimen isothermally treated at the temperature of 650 ° for 20 min.