27SiMn钢窄间隙GMAW环焊缝开裂原因分析

Cracking Causes of 27SiMn Girth Weld by Narrow Gap GMAW

目的 以发生开裂的27SiMn钢窄间隙GMAW环焊缝为研究对象,研究其裂纹形成原因及扩展过程。方法 对裂纹附近的母材及焊缝进行化学成分分析、裂纹宏观形貌分析、金相组织分析、扫描电镜分析、结构受力分析等,探明裂纹的形貌特征、形成原因及扩展路径。结果 试样化学成分和金相组织无明显异常,满足使用要求。裂纹属于明显的张开型(Ⅰ型)裂纹,从焊缝根部向缸体表面呈碗状扩展,形成缸体内部与外部的连接通道,导致液压油渗漏。裂纹起源于焊缝根部的热影响区,焊缝根部熔深不稳定,部分拼接处存在未熔透缺陷,未熔透处存在的氧化铝、碳酸钙等夹杂物是形成裂纹的主要诱因。在焊接残余应力的作用下,沿焊缝根部的夹杂物形成轴向裂纹和周向裂纹;在工作载荷的作用下,焊缝根部周向拉应力和轴向拉应力达到最大值,其中周向拉应力约为工作载荷的5.5倍,在周向拉应力的作用下,裂纹沿径向和轴向发生扩展,最后形成碗状的裂纹扩展面。结论 焊缝根部的未熔透缺陷和非金属夹杂物是产生裂纹的主要原因,在工作载荷的作用下,裂纹沿轴向和径向发生扩展,最后扩展至缸体表面呈碗状。

The work aims to study the causes of crack formation and propagation process,in order to provide technical sup-port for the actual production of 27SiMn steel’s narrow gap girth weld structure.The crack morphology,crack formation reason and crack propagation path were studied by means of chemical composition analysis,macroscopic morphology analysis,weld metallographic analysis,crack scanning electron microscope analysis and structural stress analysis.There was no obvious ab-normality in the chemical composition and metallographic structure of the sample,which met the using requirements.Cracks belonged to obvious open type(type I)cracks,expanding from the root of weld to surface of the cylinder block in a bowl shape,forming a connection channel between internal and external of the cylinder block,resulting in hydraulic oil leakage;Cracks originated from the heat affected zone at the root of weld.The penetration depth at the root of weld was unstable,and some of joints were not penetrated to form incomplete fusion defects.The inclusions such as alumina and calcium carbonate at incom-plete fusion were the main causes of cracks.Under the action of welding residual stress,inclusions along the weld root formed axial cracks and circumferential cracks.Under the action of working load,the circumferential tensile stress and the axial tensile stress at the root of weld reached the maximum,among which the circumferential tensile stress was the largest,which was about 5.5 times of the working load.Under the action of circumferential tensile stress,axial crack was more likely to expand along ra-dial and axial directions,finally form a bowl shaped crack propagation surface.The incomplete penetration defects and non-metallic inclusions at the root of weld are the main causes of cracks.Under the action of working load,cracks expand along axial and radial directions,and finally expand to surface of the cylinder block in a bowl shape.