回火温度对工程机械用超高强钢组织及回火脆性的影响

Effect of tempering temperature on microstructure and tempering embrittlement of ultra-high strength steel for construction machinery

通过SEM、TEM、-20℃夏比V型冲击试验等分析手段研究了回火温度对工程机械用超高强钢微观组织及回火脆性的影响,并结合断口特征及微观组织分析裂纹扩展路径。结果表明,试验钢在200~500℃回火时,随着回火温度的升高,马氏体分解后形成的碳化物的析出位置从马氏体板条内逐步过渡到原始奥氏体晶界和马氏体板条界,其形状由针状变为粒状,并不断粗化。回火温度为200℃和500℃时,冲击试样断口的不稳定断裂区为韧性断裂。300℃回火时,出现了回火脆性,其冲击试样断口的不稳定断裂区为准解理断裂,裂纹扩展路径相对平直。微观组织分析发现,在原始奥氏体晶界及马氏体板条界析出大量的针状碳化物,这些碳化物提供了裂纹形核位置,促进了裂纹扩展,导致了回火脆性的产生。

Effect of tempering temperature on microstructure and tempering embrittlement of ultra-high strength steel for construction machinery was investigated by means of SEM,TEM and Charpy V-notch impact test at-20℃.And based on fracture characterization and microstructure,the crack propagation path was analyzed.The results show that carbides formed after martensite decomposition are precipitated gradually from martensite lath to the boundaries of prior austenite grain and martensite lath,the shape of which is changed from needle-like to granular and tends to coarsen with the increase of tempering temperature in the range of 200-500℃.Both tempering temperatures of 200℃ and 500℃result in the ductile fracture in the unstable fracture zone of impact specimens.When the tempering temperature is 300℃,the tempered embrittlement is occurred,the crack propagation path is relatively straight in the unstable fracture zone of impact specimens,which is in accord with quasi-cleavage fracture.By microstrucrure analysis,a large number of needle-like carbides are precipitated along the boundaries of prior austenite grain and martensite lath.These carbides provide crack nucleation sites and promote crack propagation,which can result in the occurrence of tempered embrittlement.