The Welding Process Research On The Heavy Thickness DH40 Steel

DH40 steel used in shipbuilding has the advantage of high strength, good ductility in low temperature, resistant to corrosion, resistance to fatigue and so on, so it is used widely in ship stem, basement of ship crane and stressed position such as ship body reinforced projecting beam and counterbalance projecting beam. Hence, it is meaning that the weldability of ultra-thick ship building DH40 steel is researched to improve the serviceability of ship stressed constructionIn this paper, the weldability experiment of DH40 Steel used in shipbuilding was conducted by using the method of SAW and CO₂ and the multi-pass welding technology, in the case of different thickness,different joint,different groove formation and different welding position. The effect rule of welding process parameter on joint quality was analyzed, and the interrelationship of structure in welded seam and heat-affected zone and joint impact ductility was discussed.The results showed that for all the samples, there are no defects of cracks, porosities, inclusions and insufficient melt in the welding seam. The tensile strength of joint was over it of base metal and the value of bend test was up to standard requirement. In addition, the impact ductility in the zone of 5mm distance to bond line was lower than other zones. The proportion of fiber zone and the shear lip zone is much bigger in the specimens with much higher impact strength. The reason was the effect of structure heredity which resulted in creating the coarse crystal grain in the last welding thermal cycle. But the basic reason that the impact ductility in the zone of 5mm distance to bond line of welded seam root decreased obviously was that the much granular binate was created. The impact ductility value was different with the different of shape, quantity, size and distribution of M-A component in the banite. The effective method to improve the impact ductility of this zone was to change welding process and conduct postweld heat treatment, which was order to divide M-A component into ferrite and carbide.The microhardness of base metal is 167²02 HV with mixed structure of ferrite, pearlite and some troostite, weld metal is 205²30 HV with mixed structure of ferrite and troostite and the mircohardness is 191²35 HV in the zone of 5 mm distance to bond line with the mixed structure of ferrite, pearlite and banite. DH40 hull steel has good characteristic of resistance to lamellar tearing. For butt joint or T-type joint, the area reduction of the Z-direction tensile specimens is as high as 71.7% which is much higher than the standard needed.