电脉冲对铸态高韧性球墨铸铁凝固组织及性能的影响

对铸态高韧性球墨铸铁QT400-18的凝固过程进行电脉冲处理,电脉冲参数为电压2 600 V,频率0.88 Hz,电容200μF,处理时间15min。通过对比试验可知:电脉冲处理可使铸铁的过冷度相比未处理的升高12K,改善了石墨形核和生长的动力学条件,从而使球状石墨的粒径减小,石墨球数量增加。铸铁经过电脉冲处理,球化率由未处理的平均80%提高到平均91%,石墨球数由平均172/mm~2提高到209/mm~2,石墨不易变态生长,圆整度得到改善。同时,球墨铸铁的凝固组织得到了改善,铁素体数量增加,达到83%,相比未电脉冲处理的试样提高了26.6%,铁素体晶粒小于未处理试样,基体中分布的珠光体数量和片层间距减少,在抗拉强度符合要求的条件下,伸长率提高了4.89%,-233K的冲击功Akv提高了3.56J。     

热浸镀铝球墨铸铁失效机理研究

采用VK-9710型激光共聚焦显微镜对热浸镀铝球墨铸铁试样的三点弯曲失效过程进行原位观察,分析镀层和基体的裂纹萌生和扩展机理。结果表明:对于纯Al浸镀球墨铸铁,在拉应力作用下,铁铝合金镀层率先萌生裂纹,诱导临近基体中铁素体撕裂与石墨球剥离,裂纹近似垂直于拉应力方向并沿着临近石墨球最短途径扩展;压应力导致表面纯Al层剥离和铁铝合金层破碎,镀层失效对球墨铸铁基体基本无影响。对于Al-3.7Si-1.0RE浸镀球墨铸铁,拉应力作用下的失效机理与纯Al浸镀相似;压应力作用下纯Al层和铁铝合金层与基体脱开,表现为铁素体基体失效。     

基于非线性超声技术的球墨铸铁石墨大小及表层硬度的表征

球墨铸铁具有优良的力学性能被广泛应用在各工业领域，其内部的石墨形态和分布情况会影响其各项力学性能。相比于石墨球化率，石墨大小的表征研究相对较少，对球墨铸铁的石墨大小及表层硬度进行无损评定具有重要的工程意义。浇注了具有不同石墨大小和基体组织的球墨铸铁试件，对试件进行了金相观察及硬度试验。基于临界折射纵(longitudinal critical refraction, LCR)波和Rayleigh波模型，利用超声波声速及超声非线性系数表征球墨铸铁石墨大小及表层硬度。结果表明：LCR波的超声非线性系数随着表面硬度的增大而增大，随着石墨平均直径的增大而减少，并且相比超声波声速具有更高的灵敏度；LCR波的超声非线性系数的增加与微观组织中的晶界数量和碳化物的增加有关。因此，可以利用LCR波的超声非线性系数表征球墨铸铁的石墨大小和表面硬度，建立微观组织、超声非线性系数及机械性能之间的关系。     

球墨铸铁表面电弧增材制造Fe‒Cr合金结合区组织和性能

目的 在球墨铸铁基体上电弧增材制造Fe-Cr合金,研究结合区组织和性能,以期获得具有良好冶金结合、满足冲裁模具性能要求的双金属构件。方法 采用GMAW工艺增材制造,用金相显微镜和扫描电子显微镜表征结合区的显微组织,并分析其形成机制。结果 Fe-Cr合金与球墨铸铁结合区无明显裂纹和气孔,其凝固组织为柱状晶和等轴晶,冷却后转变为马氏体和残余奥氏体,但其分布不均匀,在界面处有一富奥氏体层。结合区内球墨铸铁受热影响发生奥氏体化和部分熔化,熔化发生在临近结合界面的石墨球周围,其冷却后形成一层马氏体和一层莱氏体的双层壳型组织结构,未熔化部位的组织为马氏体和铁素体,珠光体球墨铸铁比铁素体球墨铸铁形成的马氏体多。结合区内硬度分布不均匀,球墨铸铁的硬度从基材到结合界面逐渐升高,最高达630HV,Fe-Cr合金平均硬度为510HV。结论 电弧增材制造Fe-Cr合金与球墨铸铁基体冶金结合良好,Fe-Cr合金组织为马氏体和残余奥氏体,有较高的硬度,能满足冲裁模具的性能要求。     

改性纳米SiC粉体强化球墨铸铁的组织和力学性能研究

在生产条件下用改性纳米SiC粉体对球墨铸铁进行了强韧化处理,研究了不同的纳米SiC粉体加入量对球墨铸铁组织和力学性能的影响.结果表明,经改性纳米SiC粉体强韧化后,球墨铸铁中的石墨球尺寸减小,圆整度提高,铁素体含量增多,球墨铸铁的韧性提高.断口分析结果表明,经过强韧化处理后,球墨铸铁的断裂方式为脆性韧性混合断裂.     

球墨铸铁组织与性能的分析

本文对QT40018、QT500—7两种球墨铸铁的组织差异进行了分析。并利用图像分析软件计算出石墨球数,并求出球化率。试验结果表明,随着珠光体含量的增多,球墨铸铁件的硬度增加了。利用图像分析软件可以判断石墨球化率及分布均匀性。并且可参照标准表比对法产生的人为误差。     

不同含量改性纳米粉体对球墨铸铁拉伸性能的研究

通过冲入法将改性纳米粉体M加入到球墨铸铁中,研究了不同含量的改性纳米粉体M对球墨铸铁组织和拉伸性能的影响。实验结果表明,加入0.01%(质量分数)改性纳米粉体M后,石墨球的综合性能提升最明显,其尺寸减小且变得均匀,数量增加,圆整度提高,晶粒得到细化,珠光体含量降低,铁素体含量增加,拉伸性能得到提升。抗拉强度提高到493.82 MPa,延伸率提高到19.09%。通过断口分析,确定其拉伸断裂的机理为混合式断裂。     

球墨铸铁和铝硅合金组织与机械性能关系比较——相似性与相异性

考察了混合基体球墨铸铁和铝硅合金组织与机械性的关系。球铁和铝硅合金基体共晶类组织与机械性能关系相似;球铁中球状石墨和铝硅合金初生硅对组织受力行为影响显著,使合金表现出一些相异的性能特征。基体塑性相的引入对两种合金机械性能有不同影响。如仅以组织与机械性能关系看,珠铁的组织特点更适于使合金获得高的机械性能。     

热等静压处理对球墨铸铁疲劳性能的影响

研究了球墨铸铁材料(EN-GJS500-7)热等静压处理前后疲劳性能的变化,并分析了珠光体、石墨、铁素体含量随热等静压工艺的变化及其对球墨铸铁疲劳性能的影响。结果表明,热等静压处理后如果珠光体含量增加而铁素体含量减少,则材料抗疲劳性能增强;反之,材料疲劳性能降低。     

Sb微合金化大断面球墨铸铁组织和性能的研究

目的 研究Sb微合金化对大断面球墨铸铁组织性能的影响,以提升大断面球墨铸铁件质量.方法 向铁液中加入质量分数约为0.02％的合金元素Sb,随后浇注成180 mm×180 mm×200 mm的球墨铸铁立方体试块,并对其进行部分奥氏体化正火(880℃×2 h)和回火(500℃×2.5 h)处理.借助光学显微镜(OM)、扫描电子显微镜(SEM)、拉伸试验机和布氏硬度计等仪器研究未合金化试块、热处理前后的Sb合金化试块表层和中心位置的微观组织和力学性能,分析拉伸断口形貌,并探讨形成机理.结果 未合金化铸态试块由铁素体和石墨组成,其表层石墨球数量较多、球径较细,中心部位出现了碎块状石墨等畸变组织,其综合力学性能较差;Sb合金化铸态试块由大部分珠光体、少部分牛眼状铁素体、细小球状石墨及少量的渗碳体组成,表层和中心位置的石墨球数量均得到增加,中心位置的增加较为明显,石墨球径细化明显,断口为脆性断裂,其强度和硬度偏高,塑性偏低;热处理后的试块强度和硬度得到适当降低,塑性得到明显改善,石墨球的数量、大小和分布变化不大,断裂模式为以解理断裂为主、伴有少量塑性变形的韧脆混合断裂模式.结论 经部分奥氏体正火(880℃×2 h)和回火(500℃×2.5 h)的Sb微合金化试样强度和塑性较佳.     

Analysis of Nodular Cast Iron Microstructures for Micromechanical Model Development

Abstract:  One of the main factors in determining the different grades of ductile iron is the matrix structure. In the as-cast condition, the matrix will consist of varying proportions of pearlite and ferrite, and as the amount of pearlite increases, the strength and hardness of the iron also increase. Three different nodular cast irons are here considered and their microstructure characterised in detail using metallographic methods. Then micromechanics models based on the unit cell approach and the finite element method are introduced to describe the actual constitutive response of the materials and the predicted behaviours are compared with experiments.     

Influence of chunky graphite on mechanical and fatigue properties of heavy-section cast iron

This research work aimed to find out the influences of the different amounts of chunky graphite on mechanical and fatigue properties of GJS 800 ductile cast iron. Chunky graphite has been a problem of heavy section thick-walled ductile cast iron components. Chunky graphite is branched and interconnected as a network within eutectic cells and has been observed to form in thermal centres of heavy ductile cast iron sections during solidification. This research work proved that chunky graphite in the microstructure decreases the ultimate tensile strength, the elongation to fracture and fatigue life significantly, but does not influence on the yield stress of ductile cast iron GJS 800 substantially. Low nodular count and nodularity rate also decrease the fatigue life of ductile cast iron, and the difference of fatigue life of specimens containing chunky graphite or having low nodular count and nodularity rate is not large. Influence of the amount of chunky graphite on fatigue life increases until 20% chunky graphite content, and above that the fatigue life does not decrease substantially.     

Correlation of the acoustic emission and the fracture toughness of ductile nodular cast iron

Salzbrenner has recently determined the fracture toughness of a series of ductile cast iron samples which were heat treated to produce a fully ferritic matrix. His results indicated that the fracture toughness is strongly dependent upon the average spacing between (or equally the diameter of) the spherical graphite nodules in the ferrite matrix. The acoustic emission generated during the uniaxial compressive deformation of nodular cast iron also depends strongly on the average diameter of the graphite nodules in the test sample. The present investigation was carried out to determine the correlations, if any, between the fracture toughness and the acoustic emission generated during compression of ductile cast iron. The acoustic emission generated during compression was determined using sample materials identical to those used by Salzbrenner. Excellent correlations between certain features of the measured acoustic emission and the fracture toughness were obtained. Data indicate that it should be possible to determine both the fracture toughness and the average size of the graphite nodules from the acoustic emission and load curve generated during a compression test of ductile cast iron.     

Effect of specimen preparation on evaluation of cast iron microstructures

Preparation of cast iron specimens is difficult due to the need to properly retain the graphite phase. Three procedures using automated grinding/polishing processes were used to study the effect on graphite retention of such factors as, the quality of SiC grit paper, the kind of polishing cloth and diamond paste, as well as the number of preparation steps. The best preparation method for different types of graphite consisted of four steps, with the polishing process carried out on a napless cloth with diamond in paste form. A procedure for specimen preparation of white, high-chromium cast iron is also given.

Examples of the use of different etchants for revealing unalloyed cast iron microstructures, such as: pearlite, ferrite and cementite, are presented. The phosphorous eutectic (Steadite) in gray iron can be observed after etching with 4% nital, but selective color etching helps to differentiate the revealing constituents. This method is also helpful in phase differentiation of the ADI matrix as well as revealing silicon segregation in ductile iron and microsegregation in the austenitic matrix of mottled iron. Color etching of chromium carbides in alloyed white irons facilitates the use of the color image analyzer for measurements of the amount and morphology of this phase.     

Effect of matrix structure on ultrasonic attenuation of ductile cast iron

The ultrasonic attenuation of ductile cast irons with different matrices was investigated by means of ultrasonic echo waves. In the ductile cast irons with ferritic and pearlitic matrix structures, both of the ultrasonic attenuation increased with frequency. For similar frequencies, the ultrasonic attenuation of the pearlitic matrix was larger than that of the ferritic matrix. Based on the theory of ultrasonic attenuation in the solid, the mechanisms of ultrasonic attenuation in the ductile cast irons with different matrices were analyzed. It indicated that in the ductile cast irons with transformation of matrix from the ferrite to the pearlite, the mechanism of ultrasonic attenuation varied in the range of present frequencies. In the ferritic matrix, the total ultrasonic attenuation was mainly attributed to the scattering loss which included the stochastic scattering and the Rayleigh scattering. On the contrary, the absorption loss predominated in the total ultrasonic attenuation of the ductile cast iron with pearlitic matrix.