高强X90管线钢焊接热影响区脆化及软化行为

Embrittlement and soften behaviors of heat affected zone of high strength X90 pipeline steel

采用热模拟方法研究X90管线钢热影响区（Heat affected zone,HAZ）组织和性能,并结合峰值温度对相变点（Ar3）的影响分析了热循环对组织和性能的影响。结果表明：X90钢热影响区存在脆化区和软化区,脆化区出现在峰值温度为750℃的两相临界区和1200℃以上的粗晶区;而软化区则出现在900～1000℃的两相临界区与细晶区的交界区。随着峰值温度的升高,Ar_3降低。峰值温度影响奥氏体化程度及晶粒尺寸,从而影响冷却过程中的相变。750℃两相临界区脆化是由于加热过程中的少量重结晶奥氏体在冷却过程中形成的脆性贝氏体或马氏体/奥氏体（M/A）岛所致;而粗晶区脆化则是因为奥氏体晶粒粗大及冷却后形成的粗大贝氏体;软化区的形成则归因于非均匀细小奥氏体在冷却过程中形成的多边形铁素体及粒状铁素体组织。

Microstructure and mechanical properties of welding heat affected zone（ HAZ） of X90 pipeline steel were studied by using thermal simulation,and the influence of weld thermal cycle on microstructure and mechanical properties was discussed by combining the effect of peak temperature on the phase transition point（ Ar3）. The results show that there are embrittlement and softening zones existing in heat affected zone of the X90 pipeline steel. The embrittlement zone appears in the critical heat affected zone with a peak temperature of 750 ℃and in the coarse grain heat affected zone with a peak temperature above 1200 ℃,while the softening area appears at the junction of the critical heat affected zone and in the fine grain heat affected zone when the peak temperature is in the range of 900-1000 ℃. Ar3 decreases along with the peak temperature increasing. The peak temperature greatly affects the austenitizing degree and the grain size,and further affects phase transitions during cooling process. The embrittlement in the critical heat affected zone at 750 ℃ is caused by brittle bainite and M/A islands formed during cooling from a little recrystallization austenite which formed during heating,the embrittlement in coarse grain heat affected zone is caused by coarse austenite grains and its coarse bainite transformation product during cooling,while the formation of softening zone is attributed to the polygonal ferrite and granular ferrite formed during cooling from those nonuniform fine austenite.