不同状态590车轮钢的疲劳裂纹原位分析

In Situ Fatigue Crack Analysis of 590 Wheel Steel in Different States

疲劳开裂是导致车轮钢失效的主要原因,但是其疲劳开裂与微观组织、织构、焊接过程、变形程度之间的关系尚不明确。本文结合车轮钢复杂的成形、焊接制备过程,利用激光共聚焦显微镜原位拉伸装置及实时观测系统分别研究了590 MPa强度车轮钢热轧态、冷变形态、焊接态疲劳裂纹萌生及扩展的过程,并利用背散射电子衍射(EBSD)技术分析了疲劳裂纹萌生及扩展过程与组织类型、晶体取向、织构强度之间的关系,用于揭示疲劳失效的微观机制。研究发现:冷变形10%板的裂纹萌生最早,热轧板介于中间,焊接接头起裂最晚。冷变形态的样品裂纹扩展速度最高,焊接态样品裂纹扩展速度居中,而热轧态最低。热轧态的疲劳裂纹扩展速度呈现由低到高逐渐变化的过程,即:开始阶段扩展速率较低,然后进入缓慢增长区;随着裂纹长度的增加,应力强度因子也随之增大,裂纹扩展速度迅速提高。通过背散射电子衍射分析发现,疲劳裂纹的萌生与位错滑移过程密不可分。铁素体组织中裂纹萌生与{001}<110>织构强度直接相关。通过上述研究可知,减少车轮钢成形过程中冷变形组织或降低变形量,降低热轧板{001}<110>织构强度可提高车轮钢疲劳裂纹萌生寿命。

Fatigue crack is the main failure course of wheel steels,but its relationship with microstructure,texture,welding process and deformation remains unclear.Herein,a laser confocal microscope integrated with a tensile testing apparatus was adopted to in situ investigate the crack initiation and propagation of a 590 MPa strength wheel steel during hot-rolled,cold-deformed,and welded states based on the complex fabrication and welding processes.Further,by utilizing Electron Backscattered Diffraction(EBSD)technology,the relationship between the crack initiation and propagation process and the structure type,crystal orientation and texture strength were analyzed to reveal the underlying mechanism of the fatigue failure.It was found that the earliest crack initiation occurred in the 10%cold-deformed plate,followed by the hot-rolled plate,while the welded joint had the latest cracking onset.The crack propagation speed was highest in the cold-deformed sample,intermediate in the welded one,and lowest in the hot-rolled one.The crack propagation rate increased gradually in hot-rolled state.At the beginning stage,the crack propagation rate was low,and then increased slowly.With the extension of crack length,the stress intensity factor increased,accelerating the crack propagation.The EBSD results confirmed that the crack initiation was closely related to the dislocation slip process and the initiation in ferrite depended on the strength of{001}<110>texture.Accordingly,it was concluded that the resistance of wheel steels to fatigue crack initiation can be improved by reducing the cold-deformed structure or deformation degree during the fabrication process,as well as lowering{001}<110>texture strength of the hot-rolled one.