不同热输入下F460钢焊接粗晶热影响区韧脆转变的内在机理

使用Gleeble 3800热模拟试验机模拟F460钢单道次焊接条件下焊接粗晶热影响区的热循环过程,通过光镜(OM)、扫描电镜(SEM)分析热影响区的显微组织、确定临界事件,通过ABAQUS软件计算临界解理断裂应力σf,进而系统分析不同焊接热输入E下韧脆转变温度变化的内在机理。结果表明:随着E的提高,焊接粗晶热影响区显微组织依次为少量板条马氏体和大量细密的板条贝氏体,板条贝氏体较多的板条/粒状贝氏体,粒状贝氏体较多的板条/粒状贝氏体,粗大的粒状贝氏体。原始奥氏体晶粒、贝氏体团的最大尺寸随着E的提高而变大。在完全解理断裂的冲击断口上,寻找停留在缺口尖端附近的残留裂纹,通过对比残留裂纹长度、原始奥氏体晶粒大小、贝氏体团尺寸,发现不同E下解理断裂的临界事件尺寸都是贝氏体团大小,而临界事件尺寸越小,韧脆转变温度越低。此外,通过有限元模拟缺口尖端的应力分布得到σf,σf越大冲击韧度越好,随着E的提高σf降低,故进一步说明随着E的提高韧脆转变温度Tk上升的内在机理。

Intrinsic M echanisms of Ductile-brittle T ransition for F460 Steel Welding Coarse Grained Heat Affected Zones with Different Heat Inputs

Coarse grain heat affected zone (HAZ) of F460 steel was simulated by a Gleeble 3800 ther‐mo‐mechanical simulator .The microstructure ,critical event of the HAZ formed at various heat inputs (E) were characterized and determined by optical microscopy (OM ) and scanning electronic microsco‐py (SEM ) ,and cleavage fracture stress σf was also calculated by ABAQUS software .Based on above systematic analysis ,the intrinsic mechanism of ductile‐brittle transition for F460 steel heat affected zones with different heat inputs were revealed .The results indicate that :with the improvement of heat input ,the microstructures in sequence are a minority of lath martensite and massive fine lath bainite ,more lath bainite with less granular bainite ,more granular bainite with less lath bainite , bulky of granular bainite;and the maximum size of the original austenite grain and bainite packet be‐comes bigger with the improvement of heat input .The size of bainite packet is critical event of the cleavage fracture for coarse grain heat affected zone specimens with various heat inputs by comparing the relationships among residual crack length ,original austenite grain size and bainite packet size . With the decreasing of the bainitic packet ,the ductile to brittle transition temperature decreases .In addition ,cleavage fracture stress σf is also calculated by ABAQUS software ,σf gradually decreases with the increase of the heat input ,which can explain the intrinsic mechanism of ductile to brittle transition temperature Tk with the change of the heat input .