铁素体球墨铸铁低温冲击性能影响因素及断裂行为研究

发布时间：2018-06-18 19:11

本文选题：铁素体球墨铸铁 + 冲击吸收功　；参考：《沈阳工业大学》2015年博士论文

【摘要】：铁素体球墨铸铁具有良好的韧性和不容易发生脆性断裂等优点,满足大型风力发电机组轮毂、变速箱、底座等部件在常温和较低温度(-20℃左右)工作环境下的使用要求,但在更低温条件下(-40℃以下),其力学性能发生显著变化容易发生脆性破坏。目前关于铁素体球墨铸铁低温冲击断裂机理的研究很少,对冲击过程中材料断裂特征、断裂行为、石墨球和晶界在断裂过程中的作用及温度变化对冲击断裂的影响机理均不掌握。基于上述原因,本课题开展了铁素体球墨铸铁低温冲击性能影响因素及断裂行为的研究。对铁素体球墨铸铁进行系列示波冲击试验结果表明：温度对冲击过程中裂纹形成和裂纹扩展能力均有明显影响,在韧脆转变温度以上时,球墨铸铁冲击断裂所需的裂纹形成功变化不大,裂纹扩展功有所下降,而在韧脆转变温度以下时,裂纹形成功与裂纹扩展功均存在剧烈的下降。采用激光共聚焦显微分析系统对QT400-18L球墨铸铁冲击断口进行定量研究发现,随着温度的降低球墨铸铁的断面粗糙度指数减小且断面粗糙度指数随温度的变化与冲击吸收功的变化趋势一致。空穴扩张比和分形维数的变化反映出材料冲击断裂行为随温度的变化,空穴扩张比随温度的下降而减小,冲击吸收功的对数随分形维数的增加呈单调下降的直线关系。通过扫描电镜及透射电镜等手段观察分析了铁素体晶粒及石墨球对低温冲击断裂的影响,QT400-18L球墨铸铁的有效解理断裂单元尺寸比QT350-22L球墨铸铁的有效解理断裂单元尺寸大,低温冲击作用下解理裂纹启动和扩展遇到粗大晶粒的概率也相应较高,低温下位错在晶界处的塞积导致微裂纹更容易在晶界处萌生。晶界处夹杂物的存在易引起显微孔洞的连接导致材料最终断裂。石墨对球墨铸铁冲击断裂行为的影响首先是通过石墨与基体界面的断裂行为引起的,石墨与基体的弹性模量之差越大越容易在石墨与基体界面处萌生裂纹,二者之差越小则越容易在石墨内部开裂。采用冲击试样断口金相原位观察方法分析了不同温度下QT400-18L铁素体球墨铸铁裂纹萌生与扩展路径以及断口附近组织的演变规律,韧脆转变温度以上,冲击后断口附近大量石墨-基体界面发生开裂；韧脆转变温度区间,铁素体球墨铸铁冲击试样表现为韧窝和解理混合断口形貌,两种断裂模式裂纹形核均与石墨球有关；韧脆转变温度以下,垂直交叉孪晶成核进而导致微裂纹扩展,解理断裂主要是孪晶起裂。这种形变孪晶引起的裂纹形核、扩展方式造成了裂纹形成功与裂纹扩展功的剧烈下降。利用高分辨透射电子显微镜,分析了不同温度下冲击断口附近位错组态由自由态到胞状结构再到位错缠结的变化规律,采用XRD衍射方法计算了不同温度下冲击断口附近位错密度,随着温度的降低位错密度有所降低可动位错数目减少。根据位错理论及微观断口转变机制,对铁素体球铁韧脆转变温度表达式进行理论推导,表明铁素体晶粒越大、石墨球尺寸越大、石墨球间距越小、韧脆转变温度越高。基于位错动力学理论及Johnson-Cook本构模型建立了描述QT400-18L球墨铸铁塑性变形行为的数学模型,模型计算值与实验结果有着相同的变化规律,发现在230K出现一临界温度即韧脆转变温度。在韧脆转变温度以上相对于非热效应而言,热激活应力的作用可以忽略不计,而在韧脆转变温度以下表现出明显的热激活效应,需要借助热激活作用来克服位错滑移过程中的短程障碍。
[Abstract]:Ferrite spheroidal graphite cast iron has the advantages of good toughness and brittle fracture, which can meet the requirements of large wind turbine hub, gearbox, base and other components in working environment at normal temperature and lower temperature (about -20 C), but the mechanical properties of the large wind turbine are easily changed at a lower temperature (below -40 C). Brittle failure. At present, there are few studies on the mechanism of low temperature impact fracture of ferrite spheroidal graphite cast iron. The fracture characteristics, fracture behavior, the role of graphite ball and grain boundary during the fracture process and the influence mechanism of temperature on the impact fracture are not grasped. Based on the above reasons, this subject has carried out the ferrite spheroidal graphite cast iron. A series of oscilloscope impact tests on ferrite spheroidal graphite cast iron show that temperature has an obvious effect on the formation of cracks and the ability of crack propagation in the process of impact, and the crack shape needed for the impact of ductile iron at the transition temperature above the ductile brittle transition is not very successful. The expansion work decreased, while the crack growth and the crack propagation work were greatly reduced at the toughened and brittle transition temperature. The quantitative study on the impact fracture of QT400-18L ductile iron by laser confocal microscopy found that the roughness index of the ductile iron decreased with the decrease of the temperature and the roughness of the cross section. The variation of the number with the temperature is consistent with the change trend of the impact absorption work. The variation of the hole dilation ratio and the fractal dimension reflects the change of the impact fracture behavior of the material with the temperature, and the void expansion ratio decreases with the decrease of the temperature. The logarithm of the impact absorption work is linear with the increase of the fractal dimension. The effect of ferrite grain and graphite ball on low temperature impact fracture is observed by means of electron microscope. The effective cleavage unit size of QT400-18L ductile iron is larger than that of QT350-22L ductile iron. The probability of the crack initiation and expansion under the impact of low temperature is higher than that of the coarse grain. The deposition of microcracks at the grain boundary causes the microcracks to occur more easily at the grain boundary. The existence of inclusions at the grain boundary causes the connection of the micropores to lead to the final fracture of the material. The effect of graphite on the impact fracture behavior of ductile iron is first caused by the fracture behavior of the graphite and the matrix interface, and the elastic model of the graphite and the matrix. The bigger the difference is, the more easily the cracks occur at the interface between the graphite and the matrix. The smaller the difference between the two is the easier to crack in the graphite. The crack initiation and propagation path of the QT400-18L ferrite spheroidal graphite cast iron at different temperatures and the evolution of the microstructure near the fracture surface, and the ductile and brittle transition at different temperatures are analyzed. A large number of graphite matrix interfaces are cracked near the fracture surface after the impact, and the toughness and brittleness transition temperature range, the impact specimen of ferrite spheroidal graphite cast iron is characterized by dimple and cleavage fracture surface, and the two fracture modes are related to the graphite ball. Below the toughening and brittle transition temperature, the vertical twinning twins are nucleated and then lead to micro cracks. The expansion, cleavage fracture is mainly the twin fracture. The crack nucleation caused by this deformation twin causes the crack shape success and the sharp decrease of crack growth work. By using high resolution transmission electron microscope, the change rules of dislocation configuration near the impact fracture in different temperatures are analyzed from self from state to cellular structure and then wrong tangles in place. The XRD diffraction method is used to calculate the dislocation density near the fracture surface at different temperatures. With the decrease of the dislocation density, the number of movable dislocation decreases with the decrease of the dislocation density. According to the dislocation theory and the microscopic fracture transition mechanism, the expression of the ductile brittle transition temperature of Ferritic Ductile iron is theoretically derived, which indicates that the larger the ferrite grain is, Shi Moqiu The larger the size, the smaller the graphite ball space, the higher the toughening and brittle transition temperature. Based on the dislocation dynamics theory and the Johnson-Cook constitutive model, a mathematical model is established to describe the plastic deformation behavior of QT400-18L nodular cast iron. The calculated values of the model have the same changing rules as the experimental results, and the critical temperature, that is, the toughened and brittle transition temperature appears at present of 230K. At the toughening and brittle transition temperature above the non thermal effect, the effect of the thermal activation stress can be ignored, while the thermal activation effect is obvious below the toughened and brittle transition temperature, and the short-range obstacle in the dislocation slip process needs to be overcome by the use of thermal activation.
【学位授予单位】：沈阳工业大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG143.5

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