准动态模型预估的Q960E大梁钢SR-CGHAZ疲劳裂纹扩展机理

Fatigue crack extension mechanism of SR-CGHAZ in Q960E girder steel based on quasi-dynamic model prediction

为研究半挂车大梁在使役过程中Q960E亚临界粗晶热影响区的疲劳裂纹扩展机理,通过动态应力测试耦合有限元仿真方法,研究者建立了一种准动态应力分析模型,以获取半挂车大量的动载疲劳应力,通过二次热模拟获得Q960E的亚临界粗晶热影响区(simulated reheat-coarse grained heat affected zone,SR-CGHAZ)并测试了疲劳裂纹扩展速率和冲击韧性,采用激光共聚焦显微镜(laser scanning confocal microscope,LSCM)分析了显微组织和马奥岛(martensitic-austenitic islands,M-A)形貌,通过电子背散射衍射分析(electron back-scattered diffraction,EBSD)分析了亚结构和取向,利用透射电子显微镜(transmission electron microscope,TEM)研究了疲劳裂纹扩展机理.结果表明,基于动态应力实测的准动态仿真模型具有较高精度,在峰值应力为300 MPa,应力比R=0.05的条件下,SR-CGHAZ随着第1次热模拟冷却时间t_(8/5)的延长,M-A由点状向针状和短棒状转变,冲击韧性逐渐降低,伴随疲劳裂纹扩展速率随之增加,主要与马氏体板条束宽化,基体亚结构取向变化,大角度晶界密度降低,以及马氏体板条束间M-A转变为链状析出的VCr_(2)C_(2)型碳化物有关.

In order to study the fatigue crack extension mechanism of Q960E SR-CGHAZ of semi-trailer beams in service,this paper establishes a quasi-dynamic stress analysis model and obtains the fatigue stresses by means of dynamic stress test coupled with finite element simulation.The SR-CGHAZ of Q960E was obtained by secondary thermal simulation and the fatigue crack extension rate and impact toughness were tested;the microstructure and M-A morphology were analyzed by LSCM;the substructure and orientation were analyzed by EBSD;and the fatigue crack extension was investigated by TEM.The fatigue crack extension mechanism was investigated by TEM.The results show that the quasi-dynamic simulation model based on dynamic stress measurement has high accuracy;under the condition of peak stress of 300 MPa and stress ratio R of 0.05,the M-A of SR-CGHAZ transforms from point-like to needle-like and short bar-like with the prolongation of the cooling time of t8/5 in the primary thermal simulation,the impact toughness decreases,and the accompanying fatigue crack extension rate increases.This is mainly related to the broadening of the martensite lath bundles,the change of substructure orientation of the matrix,the reduction of the density of large-angle grain boundaries,and the transformation of M-A into chain precipitated VCr2C2-type carbides between the martensite lath bundles.