Q700D热影响粗晶区疲劳寿命与小裂纹扩展分析

摘要：为提高焊接构件的动载疲劳寿命，以热模拟为试验手段，对Q700D高强钢进行了焊接热模拟，研究了粗晶热影响区的疲劳寿命、小裂纹扩展行为以及组织软化特征。利用Paris方程和轴向拉伸疲劳试验数据，建立了ΔKth值与模拟粗晶区疲劳寿命的对应关系，利用ΔKth值实现了快速预估粗晶区疲劳寿命。研究表明：相同应力幅值下的lgN值与ΔKth值存在一定的线性拟合关系，即ΔKth值越大，则疲劳寿命N越长。小裂纹扩展微观机理在于所形成的大角度晶界（不小于15°）对小裂纹尖端的止裂性较强，可迫使小裂纹尖端转向耗能。CGHAZ的软化与第二相粒子回熔与粗化有关，粗化的第二相粒子易萌生小裂纹，可通过提高大角度晶界抑制裂纹扩展。

Prediction of fatigue life and behavior analysis of small crack propagation in CGHAZ of Q700D

In order to improve the dynamic fatigue life of welded structures，the welding thermal simulation of high strength steel Q700D was carried out by means of thermal simulation. The fatigue life，small crack propagation behavior and microstructure softening characteristics of coarse grain heat affected zone (CGHAZ) were studied. The approximate linear relationship between fatigue crack propagation threshold value (ΔKth) was obtained by Paris equation under different welding thermal simulation techniques and fatigue life under different fluctuating loadings. ΔKth could predict the fatigue life of CGHAZ quickly. The results showed that there was a linear fitting relationship between lgN value and Kth value under the same stress amplitude，that is to say，the larger the Kth value，the longer the fatigue life N. The microcosmic mechanism of small crack propagation was revealed that the large angle grain boundary (≥15°) has strong crack arrestability at the tip of small crack，which could force the tip of small crack to turn to energy dissipation. The softening of CGHAZ was related to the remelting and coarsening of the second phase particles. The coarsened second phase particles were liable to initiate small cracks，and crack propagation could be inhibited by increasing the large angle grain boundary