超低温奥氏体球墨铸铁微观组织与低温冲击断裂行为的研究

发布时间：2018-08-13 14:37

【摘要】：近年来,在极低温条件下使用的工业装备越来越多,例如大型超低温BOG压缩机的工作温度一般在-160℃甚至更低,因而对超低温铸造材料有着较大的需求。通常情况下,以铁素体为基体的球墨铸铁材料能够承受的温度最低在-60℃左右,无法满足更低温度的应用需求。高镍奥氏体球墨铸铁随着温度的降低没有韧-脆转变现象,拥有着良好的低温力学性能,因而在超低温(-100℃以下)工业制造领域有着广泛的应用前景。目前,针对高镍奥氏体球墨铸铁的相关研究主要集中在高温性能方面,对于超低温奥氏体球墨铸铁的研究极少,对其微观组织、冲击断裂特征、示波冲击断裂过程以及冲击裂纹的萌生和亚稳扩展规律并未见报道,因此本课题开展了超低温奥氏体球墨铸铁微观组织与低温冲击断裂行为的研究。对超低温奥氏体球墨铸铁微观组织及其对摩擦磨损行为的研究表明:超低温奥氏体球墨铸铁的微观组织主要由奥氏体、石墨球以及分布在晶界处的碳化物构成,材料中的锰元素和铬元素会偏析分布至材料基体中的奥氏体晶界处形成M23C6(M=Fe、Mn、Cr)型碳化物,其微观硬度可达到1200HV以上,远高于奥氏体基体硬度值,因而使得材料的宏观硬度得以提升。铬元素有着比锰元素更强的碳化物形成能力,对材料的摩擦磨损性能影响更大。通过对不同铬元素含量下的材料摩擦磨损后的形貌进行分析发现,该材料表现为磨粒磨损机制,其中铬元素促进形成的晶界碳化物作为硬质颗粒使得材料的摩擦磨损性能显著提高。采用低温示波冲击手段,针对不同合金(镍、锰和铬)元素下的超低温奥氏体球墨铸铁低温冲击性能进行研究,结果表明:随着温度的降低,不同合金元素下的超低温奥氏体球墨铸铁冲击性能均存在着相似的特征,即呈现先上升后下降的变化趋势,且镍元素含量的变化对低温冲击性能存在着正相关的影响,而过多的锰元素和铬元素加入会导致低温冲击性能恶化。采用扫描电子显微镜对冲击断口形貌进行分析发现:材料在室温至-193℃的温度区间内均呈现了以石墨球或石墨球凹坑作为韧窝中心的韧性断裂形貌特征,并且其冲击断口中石墨球数量与冲击性能有着直接的因果关系,即石墨球越多则冲击性能越好;碳化物数量的改变在室温下对材料的冲击性能影响并不明显,而随着温度的降低其影响呈现增大的趋势,在-193℃的超低温条件下会导致冲击断口中出现纵向微裂纹,严重破坏材料冲击性能。在对超低温奥氏体球墨铸铁低温冲击性能规律的研究基础上,对不同温度下的示波冲击曲线进行了深入分析,进一步地揭示了材料的冲击断裂过程,结果表明:以斜率法与柔度变化率法相结合的方式对示波冲击曲线进行分段分析的方法,可以有效的定量表述材料的低温冲击断裂过程;其中,冲击裂纹的高载荷亚稳扩展能量的比重可以达到冲击总能量的60%以上,且两者的变化趋势相一致,即随温度的降低呈现先上升后下降的趋势,因而高载荷亚稳扩展能量是决定低温冲击性能的主要因素;而该材料的低温冲击性能之所以呈现先上升后下降的趋势(在-80℃时为极大值),是因为在室温至-80℃时,高载荷亚稳扩展段的平均载荷对低温冲击性能起到了主导作用,而当温度继续降低时,高载荷亚稳扩展段的位移则成为了主导因素,研究还发现,即便镍元素含量变化这一规律仍然存在。同时,采用三维激光共聚焦显微镜对冲击断口的几何形貌进行定量分析,对不同温度下高载荷亚稳扩展段的表面粗糙度指数进行统计,验证了上述分析结论的正确性。由于超低温奥氏体球墨铸铁实际示波冲击曲线中高载荷亚稳扩展段的能量正好对应着冲击裂纹萌生与亚稳扩展过程中所吸收的能量,因此进一步研究了冲击裂纹萌生与亚稳扩展过程,结果表明:随着温度的降低,超低温奥氏体球墨铸铁有着更好的抵抗冲击裂纹萌生的能力;而前期抵抗冲击裂纹亚稳扩展的能力受到温度的影响,后期则受到温度和材料中镍元素含量变化的共同影响;材料基体中的石墨球(特别是相邻的石墨球)和位于晶界处的碳化物是影响冲击裂纹亚稳扩展路径的最主要因素,而温度的降低和材料中碳化物数量的增加都会加剧材料的脆性断裂倾向;同时,采用Schindler方法对不同镍元素含量的超低温奥氏体球墨铸铁在动态载荷下的延性断裂韧度JBl0.2进行了计算后发现,随着温度的降低,材料在动态载荷下的延性断裂韧度呈现持续升高的趋势,而当温度达到-80℃以下时,这一趋势会发生明显的放缓。
[Abstract]:In recent years, more and more industrial equipments have been used under extremely low temperature conditions. For example, the working temperature of large-scale ultra-low temperature BOG compressor is generally - 160 C or even lower, so there is a great demand for ultra-low temperature casting materials. The high-nickel Austenitic Ductile iron has good mechanical properties at low temperature, so it has a broad application prospect in the field of ultra-low temperature (-100) industrial manufacturing. At present, the research on high-nickel Austenitic Ductile iron is mainly focused on. In the aspect of high temperature properties, there is little research on the microstructure, impact fracture characteristics, oscillographic impact fracture process and the law of initiation and metastable propagation of impact cracks of ultra-low temperature Austenitic Ductile iron. The microstructure and friction and wear behavior of ultra-low temperature Austenitic Ductile iron are studied. The results show that the microstructure of ultra-low temperature Austenitic Ductile iron is mainly composed of austenite, graphite nodules and carbides distributed at grain boundaries. Manganese and chromium elements in the material will segregate and distribute to austenite grain boundaries to form M. The micro-hardness of 23C6 (M=Fe, Mn, Cr) carbide can reach 1200HV, which is much higher than that of austenite matrix, so the macro-hardness of the material can be improved. The carbide forming ability of chromium element is stronger than that of manganese element, which has greater influence on the friction and wear properties of the material. The wear morphology analysis showed that the material exhibited abrasive wear mechanism, in which the grain boundary carbide promoted by chromium element was used as hard particles to improve the friction and wear properties of the material. The results show that the impact properties of ultra-low temperature Austenitic Ductile Iron with different alloying elements have similar characteristics as the temperature decreases, that is, the impact properties of ultra-low temperature Austenitic Ductile Iron increase first and then decrease, and the change of nickel content has a positive correlation with the impact properties at low temperature, while excessive manganese and chromium elements have a positive correlation. Scanning electron microscopy (SEM) was used to analyze the impact fracture morphology. It was found that the ductile fracture morphology with graphite sphere or graphite sphere pit as the dimple center was observed in the temperature range from room temperature to - 193 C. The number of graphite spheres and impact properties of the impact fracture surface were also observed. There is a direct causal relationship, that is, the more graphite spheres, the better the impact performance; the change of carbide number at room temperature has no obvious impact on the impact performance of the material, but with the decrease of temperature its impact shows an increasing trend, in the ultra-low temperature of - 193 C conditions will lead to the occurrence of longitudinal microcracks in the impact fracture, seriously destroying the impact of materials. On the basis of the study on the low temperature impact property of ultra-low temperature Austenitic Ductile iron, the oscillographic impact curves at different temperatures are analyzed in depth, and the impact fracture process of the material is further revealed. The results show that the oscillographic impact curves are segmented by slope method and flexibility change rate method. The analytical method can be used to quantitatively describe the low temperature impact fracture process of materials, in which the proportion of metastable propagation energy under high load can reach more than 60% of the total impact energy, and the change trend of the two is consistent, that is, the metastable propagation energy under high load increases first and then decreases with the decrease of temperature. The main factor determining the low temperature impact property is that the low temperature impact property of the material first increases and then decreases (the maximum value is at - 80 C), because the average load of the high load metastable extension plays a leading role in the low temperature impact property from room temperature to - 80 C, and the high load metastable property when the temperature continues to decrease. At the same time, the geometric morphology of impact fracture was quantitatively analyzed by using three-dimensional laser confocal microscopy, and the surface roughness index of metastable extended section with high load at different temperatures was calculated to verify the above results. The results show that the energy absorbed in the metastable growth stage of high load in the oscillographic shock curve of ultra-low temperature Austenitic Ductile Iron corresponds to the energy absorbed in the process of crack initiation and metastable growth. Therefore, the process of impact crack initiation and metastable growth is further studied. The results show that with the decrease of temperature, the energy absorbed in the metastable growth stage corresponds to the energy absorbed in the process of impact crack initiation and meta Low-temperature Austenitic Ductile iron has better resistance to impact crack initiation; the resistance to metastable propagation of impact crack in early stage is affected by temperature, while in later stage is affected by both temperature and nickel content in the material; graphite spheres in the matrix (especially adjacent graphite spheres) and carbonization at grain boundary Material is the most important factor affecting the metastable propagation path of impact crack, and the brittle fracture tendency is aggravated by the decrease of temperature and the increase of carbide content in the material. It is found that the ductile fracture toughness of the material increases continuously with the decrease of temperature under dynamic loading, and the trend slows down obviously when the temperature is below - 80%.
【学位授予单位】：沈阳工业大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG143.5

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