The Fatigue Performance of High Temperature Vacuum Carburized Nb Modified 8620 Steel
The Fatigue Performance of High Temperature Vacuum Carburized Nb Modified 8620 Steel
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高温真空渗碳铌变质8620钢的疲劳性能
DOI：
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
J. Speer
中科院分区：
文献类型：
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作者：
Richard E. Thompson;D. Matlock;J. Speer
采用改进的Brugger试件几何形状评价了含Nb量分别为0.02、0.06和0.1wt%的高温(1050℃)真空渗碳Nb变质8620钢的弯曲疲劳性能。样品以两种不同的加热速率(20℃和114℃min)加热到渗碳温度，导致不同的原始奥氏体晶相组织，这取决于特定的Nb加入量和加热速度。在较低的升温速率下，0.06Nb和0.1Nb钢形成了均匀细晶的优先奥氏体晶组织，而0.02Nb钢则形成了含有大(>；200μm)晶粒的复晶组织。在较高升温速率下，0.02Nb钢的异常晶粒长大倾向最大，只有0.1Nb钢才能完全抑制异常晶粒长大。疲劳性能与奥氏体晶粒度和晶粒度分布、显微硬度分布、碳分布以及由不同的晶粒生长方式导致的断裂行为的扫描电子显微镜测量结果相关联。加工成细小、均匀的原始奥氏体晶组织的高Nb合金表现出最好的疲劳性能，其持久极限和低周疲劳寿命都较高。异常长大样品中粗大的原始奥氏体晶的存在促进了疲劳裂纹的形核，导致疲劳性能降低。结果表明，通过添加Nb来控制渗碳过程中奥氏体晶的粗化特征，有效地抑制了奥氏体晶长大，消除了奥氏体晶长大对升温速率的敏感性，从而提高了疲劳性能。
The bending fatigue performance of high temperature (1050 °C) vacuum carburized Nb modified 8620 steel, with niobium additions of 0.02, 0.06 and 0.1 wt pct, was evaluated utilizing a modified Brugger specimen geometry. Samples were heated at two different rates (20 and 114 °C min) to the carburizing temperature resulting in different prior austenite grain structures that depended on the specific Nb addition and heating rate employed. At the lower heating rate, uniform fine grained prior austenite grain structures developed in the 0.06 and 0.1 Nb steels while a duplex grain structure with the presence of large (>200 μm grains) developed in the 0.02 Nb steel. At the higher heating rate the propensity for abnormal grain growth was highest in the 0.02 Nb steel and complete suppression of abnormal grain growth was achieved only with the 0.1 Nb steel. The fatigue properties were correlated with measurements of austenite grain size and grain size distributions, microhardness profiles, carbon profiles, and fracture behavior as evaluated with scanning electron microscopy resulting from the different modes of grain growth. The higher Nb alloys, processed to produce fine, uniform prior austenite grain structures exhibited the best fatigue performance as evidenced by higher endurance limits and higher low cycle fatigue life. The presence of large prior austenite grains in samples that exhibited abnormal grain growth enhanced fatigue crack nucleation leading to lower fatigue performance. The results indicate that controlling austenite grain coarsening characteristics during carburizing, through additions of niobium effectively limits grain growth and removes the susceptibility of austenite grain growth to heating rate, and subsequently increases fatigue performance.