Investigation On Strengthening And Toughening Factors Of Deposited Metals In HSLA Welds

At present there is a certain gap between the domestic high-strength weldingconsumables and the foreign, especially the welding consumables used in Q960steel, sothe welding consumables in some ways can not meet the request of high steel. The issuethat the welding consumables continue to adapt to the requirements of the developmentof high strength low alloy steel (HSLA) steels has to be solved urgently. The gasshielded welding wires were studied in this paper. The relationship among chemicalcompositions, microstructures and mechanical properties was investigated by means ofimpact test machine, tensile test machine, optical microscope, scanning electronmicroscope (SEM), transmission electron microscope (TEM) and X-ray diffraction(XRD) etc. In addition, microstructures and properties of Q960steel joints were studied.Through the optimization of the alloying elements ratio, the type ofMn-Ni-Cr-Mo-Ti solid wires had been successfully developed, and the yield and tensilestrength of deposited metals were higher than890MPa and940MPa,-60℃impactenergy greater than47J. With the increase of concentration of alloying elements inwelding wires, the hardenability of deposited metals increased and the microstructures ofdeposited metals transformed from bainite (B) to bainite and martensite (B+M).With the shielding gas Ar+5%CO2, the formation of a large number of fine parallellath bainite (LB) played a vital role in enhancing strength and impact toughness indeposited metals. The results of TEM showed high density dislocations existed inside ofBF laths. The lath martensite-austenite constituent (M-A) between the bainitic ferrite(BF) laths in granular bainite (GB) deteriorated the impact toughness. In order to ensurethe high strength and toughness, the formation of LB was crucial.With the increase of welding heat input, the strength of deposited metals decreasedgradually. The microstructures of deposited metals were B and M. The optimummechanical properties of deposited metals were obtained with13.7kJ/cm heat input,because the fine and staggered BF lath occurred. There was a certain amount of residualaustenite inside between the BF laths, which were beneficial to impact toughness.Relatively small inclusions of Mn2TiO4in deposited metals were much more effective inpromoting the nucleation of BF and refining microstructure.Using the solid wires, the Q960steel joints obtained high strength and toughness.Under the different welding heat input the tensile strength reached the requirements ofbase metal and the impact toughness was better. With the increase of welding heat input, the impact toughness of the heat affected zone (HAZ) decreased gradually. The increaseof welding heat input led to the formation of GB which was adverse to impact toughness.Meanwhile, it caused the increase of austenite grain size, the decrease of nucleationparticles and the coarsening of BF.