Research On Trailing Impact Rolling For Controlling Welding Cold Crack Of 30CrMnSi

As one kind of low alloy high strength steel, 30CrMnSi has been widely utilized in various industrial fields for its excellent strength characteristics, unfortunately, however, the weld-ability of this steel is not good enough. Such problem as cold welding crack is easily to be found in the joints of 30CrMnSi steel.In this thesis, the mechanism of the cold welding crack was studied for the butt joints welded under the high restraint condition by both experimental and finite element method, and a method called welding with trailing impact and rolling was applied to control the cold welding crack as well.Experimental results show that cracking were found in the butt joint along the weld in high-constraint conditions which were identified as cold cracking by fractural photograph analysis. Combined with the joint microstructure with or without fixture and finite element simulation, the cold cracking is fractured due to hardened tissue and micro-defects in high welding tensile stress state. The mechanism effecting of the method of welding with trailing impact and rolling on the cold welding crack was put into forward, i.e. Under the effect of front wheels, weld metal volume become shrinkage, while under the rear wheels , rolling on the weld-surface is directly on the weld metal which makes metal extended. So that in the weld center area the compressive strain was changed into extensive strain, and then the weld residual stress was reduced. So the weld cold cracking was controlled. A reasonable parameter of impact and rolling with welding was obtained. In the condition of the 50mm distance from the wheel to gun and the 12KN and 10KN shocking force of the front and rear wheel, weld cold cracking rate was reduced from the original 4/5 to 0, and was found that the horizontal and vertical high-tensile stress state of weld metal had been reduced and microstructure of the heat affecting zone improved significantly. Comparative analysis on tissues of specimens and properties of weld area before and after impacting and rolling showed that this method does not change the microstructure, but the weld zone micro-defects has been reduced significantly. Through above analysis, because the method of welding with trailing impact rolling changed the high weld stress in the low alloy high strength steel, cold cracking was controlled from the mechanical angle.