Research On Impacting Rotation Compression During Welding For Controlling Welding Cold Cracking Of40Cr Steel

HSlA steel is widely used in various field of industry, but in the welding process,HSLA steel has a strong tendency to cold cracking, especially those which containhigher carbon content. In large degree of restraint, welding residual stresses will belarge. Moreover, HSLA steel will inevitably produce quenched microstructure afterwelding cooling process, which makes it weak to resist cracking. So cold cracking inHSLA steel welding is a common and serious problem. Therefore, the control methodof cold cracking is needed urgently.As a typical kind of HSLA steel,40Cr steel was chosen as base material in thisresearch. Autogenous welding method was used to obtain the I-groove butt joint of40Cr steel. The restraining and radiating conditions of engineering practice weresimulated by using binding plate and copper sheet. By microstructure observation, theSEM analysis and finite element simulation, the concrete reason for cold cracking wasgiven. Impacting rotation compression (IRC) during welding was introduced to controlthe cold cracking of40Cr steel sheet, for it can decrease residual stress by extendingweld metal. In order to enhance the effect of the extension, impact head was optimized.Tangential force was enhanced by using new designed surfaces, for reducing the axialforce which was needed to extend weld metal. The effect and mechanism ofcontrolling cold cracking in40Cr sheet was studied by measuring residual stress andmicrostructure observation.Experimental result shew that, the cold cracking in40Cr sheet appeared withgreat probability under the experimental condition. Cold cracking occurred in hightensile stress areas, surrounded by the mixed microstructure of bainite and martensite.Cracking grew mainly in the mode of intergranular fracture. And some original defectsin the weld metal were found at the source of cracking. Therefore, the conclusion thatthe cold cracking of40Cr sheet grow under the effect of residual stress, quenchedmicrostructure and original defects was drawn. With the method IRC during welding,welding residual stress was decreased, and the vibration caused by impacting reducedcolumnar crystals and uniformed ingredient in weld metal. All effect made IRC a goodway to control cold cracking. Simultaneously, crack appeared at high temperatureswhen the distance between welding torch and impact head was too short, which was caused by excessive impacting effect. Using IRC above the martensitic phasetransformation point would get the best results, the distance between the impact headand arc was80mm~110mm.In summary, as a good way to control cold cracking, IRC during welding canreduced residual stress and refine the weld microstructure,...