Grain Growth in HAZ of Ultra-fine Grain Steels

In this paper,the grain growth in the HAZ of three kinds of ultra-fine grain steels such as low carbon steel,commercial X65 steel and high purity X65 steel were evaluated. Experimental results show that these ultra-fine grain steels are prone to serious grain coarsening. The low carbon ultra-fine grain steel has more serious grain growth in its HAZ than the ultra-fine grain X65 steels. It is because that the low carbon ultra-fine grain steel doesnt contain niobium and titanium, while ultra-fine grain X65 steels contain these elements. The high purity ultra-fine X65 steel has less grain growth tan commercial ultra-fine grain X65 steel. It is because that the higher the ultra-fine gain X65 steel's purity is,the higher the phase transformation temperature from α to γ is,that is to say, the lower A_c3 is. As a result, under the same condition, the shorter the time of the γ phase in the commercial X65 steel staying in high temperature is,the bigger the grain growth in its HAZ is.