【作者】 王为；

【导师】 方洪渊；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2013， 硕士

【摘要】 低合金高强钢广泛运用于工业部门的各个领域，但是在焊接过程中，低合金高强钢具有较强的冷裂纹倾向，特别是含碳量较高的钢种。在拘束度较大时，焊接会产生很大的残余拉应力，且低合金高强钢在焊接的冷却过程中必然会产生淬硬组织，抵抗裂纹的能力较弱。所以冷裂纹是低合金高强钢焊接中常见且危害很大的问题。因此，对控制冷裂纹方法的研究十分迫切。本文以具有代表意义的40Cr钢为研究对象，采用平板I型坡口不填丝对接焊接。在实验室条件下，利用拘束垫板和散热铜片模拟工程实际中的强拘束度和散热条件。通过金相组织观察，扫描电镜断口分析和有限元模拟计算，对冷裂纹的具体开裂原因进行研究。引入随焊冲击旋转挤压法来控制40Cr的焊接冷裂纹，通过冲击头对焊缝的延展来降低残余拉应力。为了增强对焊缝的延展效果，优化了该方法使用的冲击头。通过新设计的冲击头端部曲面，增强切向力，以减少产生延展所需要的轴向力。利用该方法，在不同参数下作用于试件，通过小孔法测量残余应力和组织观察，研究对40Cr焊接冷裂纹的控制效果和机理。通过实验发现，在强拘束和较大散热速度的条件下，40Cr薄板会以很大的概率出现冷裂纹。冷裂纹出现的位置位于高应力区，且开裂处组织为上贝氏体和马氏体混合组织，裂纹主要断裂模式为沿晶脆断，裂纹源多为有原始缺陷的地方。所以40Cr薄板的冷裂纹是由于淬硬组织中的微小原始缺陷在残余应力的作用下扩展而成。通过冲击头对金属的冲击旋转挤压作用，一方面使焊缝延展，减小了焊缝在加热过程中产生的压缩塑性形变，从而降低了焊接残余拉应力，另一方面，冲击引起的振动对能够减少焊缝的柱状晶组织，并且使焊缝成分更加均匀。这些作用使得该方法对冷裂纹有较好的抑制作用。在马氏体相变点以上，及冲击头与焊枪相距80mm~110mm时，裂纹在进行冲击旋转挤压作用效果最佳。在冲击头与焊枪距离较近的情况下，会在高温状态下产生裂纹，这是由于冲击作用过强导致的。综上所述，随焊冲击旋转挤压法降低了残余拉应力，改善了焊缝组织，实现了对冷裂纹的控制。更多还原

【Abstract】 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,更多还原