Effect of Heat Input on Microstructure and Performance of Welded Joint in 1 000 MPa Grade Steel for Construction Machinery

Abstract

1000MPa high-strengthsteelplate(thermo mechanical control process(TMCP) steel) for construction machinery is welded by metal active gas(MAG) welding method with GHS90 welding wire as filler material under three heat inputs, and effects of heat input on microstructure and properties of the weld joint are investigated by means of optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the microstructure of weld metal consist prodominantly of lath martensite and lath bainite, a small amount of granular bainite and a few retained austenite for three kinds of heat input. The microstructure of the coarse grained heat affected zone(CGHAZ) consists of lath martensite, lath bainite, a small amount of granular bainite. When the heat input is increased, the lath of martensite and bainite is coarsened, the amount of martensite decreases graduslly, while the munber of granular bainite increases, and topographies of retained austenite is changed from thin film to block. With the increased of heat input, the low temperature impact toughness and the hardness of weld metal is reduced gradually, but the impact toughness of heat affect zone initially increases and then decreases. Whenthe heatinput is15 k J/cm, the welded joints with the combination of high strength and good toughness could be achieved.