EFFECT OF Cu ON MICROSTRUCTURE FORMING AND REFINING OF WELD METAL IN 690 MPa GRADE HSLA STEEL

The effect of Cu on microstructure transformation and microstructure refilling of the weld metal of 690 MPa grade HSLA steel was investigated by OM, TEM, EBSD and thermal expansion instrument, and the mechanism of microstructure refining was discussed. Experimental results indicate that microstructure of weld metal is composed of granular bainite, lath bainite and residual austenite. The addition of Cu content from 0.24% to 0.53% in weld metal can decrease phase transition temperature, which induces the reduction of martensite-austenite (M/A) amount from 0.62% to 0.31%, the variation of M/A shape from small bulk and bar to granular shape, the increase of residual austenite amount, and the remarkable refining of microstructure. The increase of Cu content from 0.24% to 0.53% results in the decrease of the mean size of lath block from 2.18 to 1.99 mu m, the decrease of the width of lath from 0.39 to 0.36 mu m, and the increase of the amount of large angle boundary, which can inhibit crack propagation, from 68.5% to 71.0%. Analysis indicates that Cu can decrease phase transition temperature, increase the stability of austenite, raise the pontential difference between ferrite free energy (G(alpha)) and austenite free energy (G(gamma)), reduce the critical size of crystal nucleation. Meanwhile, low phase transition temperature can retard the diffusion rate of carbon atom and slower the growth rate of crystal grains. These factors result in the refining of substructure.