Nodule influence on fatigue crack growth thresholds in nodular cast iron

The influence of microstructure i.e. nodules (voids) on the fatigue threshold value in nodular cast iron is numerically investigated. It is qualitatively shown that the fatigue threshold value is unaffected by the individual nodules and that the threshold value is proportional to the bulk Young's modulus. Furthermore it is shown that nodules (voids) do not influence the crack tip conditions (stress-intensity factor and crack tip opening displacement) if the voids do not lie very close to the tip for physically long cracks. It is concluded that linear elastic fracture mechanics is valid for these types of materials and no influence of the nodules need be taken into count for fatigue calculations when dealing with long cracks.     

球墨铸铁曲轴断裂失效分析

对柴油机早期断裂的球墨铸铁曲轴进行了分析研究。结果表明,轴颈圆角处的应力集中和表面加工缺陷容易使此处成为疲劳裂纹的起始源,断裂机制为准解理断裂,具有典型的脆性断裂特征。     

Micromechanical modelling of nodular cast iron,a composite material

This article aims to show that the overall characteristics of nodular cast iron can be modelled with rather straight-forward micromechanical models. The mechanical and short crack properties of this composite are analysed using finite element models with symmetrical patterns of holes or voids, representing the graphite nodules. Closed form expressions for the elastic-plastic properties are also applied and compared. The mechanical properties are well described. An approximate expression for the stress intensity factor (K) is derived for a crack emanating from a nodule in a nodule pattern. Introducing a fatigue limit condition based on the assumption of the existence of a short crack fatigue threshold, simulations are performed with variations in crack size, nodule size and spacing. The fatigue results are essentially qualitative, but are considered capable of capturing some important trends since the qualitative agreement with experiments is good.     

热疲劳作用下球墨铸铁组织变化规律

按球化率优化了球墨铸铁的化学成分,分析了热疲劳作用下球墨铸铁的组织变化.结果表明,球墨铸铁最佳热处理工艺为890℃淬火+400℃回火,可获得最佳的强韧性配比.890℃淬火次数对石墨级别基本没有影响.初次时,基体组织以马氏体为主;随淬火次数增加铁素体数量逐步增加,约25次左右铁素体量最多;随后铁素体数量减少,直至50次时组织几乎都是马氏体+石墨.     

511球铁曲轴断裂分析

对大功率柴油机球墨铸铁曲轴进行了宏观、微观全面的理化检验及分析；球墨铸铁曲轴未经过氮化处理，表面疲劳强度低是导致曲轴断裂的原因。提出了相应的改进建议。     

Influence of Graphite Nodules Geometrical Features on Fatigue Life of High-Strength Nodular Cast Iron

This paper is concerned with the evaluation of different fatigue strength theories to predict the fatigue life of high-strength nodular cast iron. There have been some studies on the effects of the shape and size of graphite nodules, and of microstructure, on the fatigue strength of nodular cast iron. However, there is not a consensus on how to correlate the fatigue limit with material intrinsic properties or with external features such as considering graphite nodules as defects. Some researchers found good correlations between fatigue strength, σ_w0, and the geometrical aspects of the graphite nodules, considering it as internal material defects. It will be shown in this study that geometrical features such as shape, size, and relative position seem to be adequate to be included in those predictions. In this article, a high-strength cast iron, with rupture strength of about 1300 MPa and Young’s modulus of about 160 GPa, has been used. Correlations both with intrinsic properties as well as with other geometrical effects have been made. A comparison of different theories has also been carried out. This article was presented at Materials Science & Technology 2007, Automotive and Ground Vehicles symposium held September 16–20, 2007, in Detroit, MI.     

风电机用低温球墨铸铁疲劳性能和断裂韧度试验的研究

对一种强度相近而低温性能优于QT400-18球铁的新型低温球墨铸铁进行了疲劳性能测试和断裂韧度测试,得到了旋转弯曲条件下的S-N曲线及断裂韧度KI C。通过经验公式转换得到了拉压状态下的S-N曲线及P-S-N曲线。将试验结果与G-L规范的设计曲线和GB／T 1348中规定的球铁性能相比较表明,该新型低温球铁的疲劳性能和断裂韧度均达到了QT400-18球铁的水平。     

球墨铸铁曲轴断裂原因分析

针对球墨铸铁发动机曲轴发生的典型断裂,检测了曲轴的化学成分、显微组织和力学性能,并对其宏观断口进行了分析,找出了导致曲轴断裂的原因和潜在因素,提出了球墨铸铁曲轴在生产过程中的质量控制重点.结果表明,采取针对性的工艺措施后,生产的曲轴未再出现断轴问题.     

硅含量对耐热球墨铸铁显微组织和力学性能的影响

采用光学显微镜、扫描电镜、电子万能拉伸试验机测试分析手段,研究硅含量对耐热球墨铸铁显微组织和室温、高温力学性能的影响规律。试验结果表明,随着硅含量的增加,石墨圆整度逐渐下降,硅含量(质量分数,下同)达到3.8%时,基体中开始出现碎块状石墨,球化级别为3级。随着硅含量从2.8%增加至4.8%,珠光体含量从51.06%减少至8.65%。随着硅含量的增加,室温抗拉强度先上升后下降,伸长率逐渐下降,当硅含量为3.8%时,抗拉强度为726 MPa,伸长率为1.6%。随着硅含量的增加,高温抗拉强度逐渐上升,伸长率逐渐下降,当硅含量为4.8%时,抗拉强度为532 MPa,伸长率为6%。室温拉伸断口出现大量的解理面和河流花样,表现形式为脆性断裂。高温拉伸断口出现韧窝和撕裂棱,拉伸断口表现形式为韧-脆混合型断裂。     

Different thermal fatigue behaviors between gray cast iron and vermicular graphite cast iron

The initiation and propagation of thermal fatigue cracks in gray cast iron and vemicular graphite cast iron were investigated by Uddeholm method to reveal the complex thermal fatigue behaviors of cast iron. Differences of thermal fatigue behaviors of gray cast iron and vemicular graphite cast iron were observed and analyzed. It is found that the observed differences are related to the combination of graphite morphology and the oxidization of matrix. More oxidized matrix is observed in gray cast iron due to its large specific surface area. The brittle oxidized matrix facilitates the propagation of microcracks along the oxidization layer. By contrast, the radial microcracks are formed in vermicular graphite at the edge of graphite due to fewer oxidization layers. It indicates that the thermal fatigue resistance of gray cast iron is dominated by graphite content and morphology while that of vermicular graphite cast iron strongly relates to the strength of the matrix.

     

提高6110车用柴油机球铁曲轴的疲劳性能

由球铁拉伸性能和疲劳试验结果可见，球铁通过合金化可大幅度提高抗拉强度，但疲劳强度提高不多，而通过二次孕育处理可在提高球铁抗拉强度的同时，提高球铁疲劳强度。     

水平连铸铸态铁素体球墨铸铁型材的疲劳性能

研究了不同硅量的水平连铸铸态铁素体球墨铸铁型材的弯曲疲劳性能．结果指出：这种铸铁的σ－１高达２６０～２７０ＭＰａ，而对比的砂铸试样σ－１只有１８０ＭＰａ．细小、圆整的石墨球、致密的组织，较少的非金属夹杂物是水平连铸球墨铸铁型材疲劳极限性能高的主要原因     

Fatigue behaviour and life estimation of Mg/Al ultrasonic spot weld bonding welds

Mg/Al dissimilar joints with favourable static and dynamic mechanical properties were obtained utilising ultrasonic spot welding and ultrasonic spot weld bonding technology. The fatigue behaviour, fatigue failure mode, and typical fatigue fracture surface morphology of Mg/Al ultrasonic spot welding and ultrasonic spot weld bonding joints were compared and observed to get a better understanding of the fatigue failure mechanism. In general, the Mg/Al ultrasonic spot weld bonding joint exhibited better fatigue performance than the ultrasonic spot welding joint. The two kinds of joints presented different fatigue failure modes with decreasing applied load level. A fatigue life prediction model was established, and the fatigue life prediction results were in good agreement with the experimental fatigue results.     

现代球墨铸铁的诞生、应用及技术发展趋势--20世纪材料科学最重大的技术进展之一

通过对现代球铁诞生及发展历程的回顾,总结人类探索创新的经验,并指出球铁研完和应用的趋势。     

Fatigue crack growth mechanism in cast hybrid metal matrix composite reinforced with SiC particles and Al2O3 whiskers

The fatigue crack growth (FCG) mechanism of a cast hybrid metal matrix composite (MMC) reinforced with SiC particles and Al₂O₃ whiskers was investigated. For comparison, the FCG mechanisms of a cast MMC with Al₂O₃ whiskers and a cast Al alloy were also investigated. The results show that the FCG mechanism is observed in the near-threshold and stable-crack-growth regions. The hybrid MMC shows a higher threshold stress intensity factor range, ΔK_th, than the MMC with Al₂O₃ and Al alloy, indicating better resistance to crack growth in a lower stress intensity factor range, ΔK. In the near-threshold region with decreasing ΔK, the two composite materials exhibit similar FCG mechanism that is dominated by debonding of the reinforcement–matrix interface, and followed by void nucleation and coalescence in the Al matrix. At higher ΔK in the stable- or mid-crack-growth region, in addition to the debonding of the particle–matrix and whisker–matrix interface caused by cycle-by-cycle crack growth at the interface, the FCG is affected predominantly by striation formation in the Al matrix. Moreover, void nucleation and coalescence in the Al matrix and transgranular fracture of SiC particles and Al₂O₃ whiskers at high ΔK are also observed as the local unstable fracture mechanisms. However, the FCG of the monolithic Al alloy is dominated by void nucleation and coalescence at lower ΔK, whereas the FCG at higher ΔK is controlled mainly by striation formation in the Al grains, and followed by void nucleation and coalescence in the Si clusters.     

Fracture analysis of chilled cast iron camshaft

The fracture of a camshaft made of chilled cast iron, installed in a home-made Fukang car, happened only after running over a distance of 6,200 km. The fractured camshaft was received to conduct a series of failure analyses using visual inspection, SEM observation of fracture section, microstructure analysis, chemical composition analysis and hardness examination and so on, while those of CKD camshaft made by Citroen Company in France was also simultaneously analyzed to compare the difference between them. The results showed that the fracture of the camshaft mainly results from white section in macrostructure and Ledeburite in microstructure; the crack in the fractured camshaft should be recognized to initiate at the boundary of coarser needle-like carbide and matrix, and then propagate through the transverse section. At the same time, the casting defects such as dendritic shrinkage, accumulated inclusion and initiated crack and abnormal external force might stimulate the fracture of camshaft as well. Based on failure analysis, some measures have been employed, and as a result, the fracture of home-made camshafts has been effectively prevented.     

Nb对灰铸铁热疲劳性能的影响

采用Uddeholm热疲劳方法测定了Nb对灰铸铁热疲劳性能的影响,试验结果表明：（1）Nb的加入能够提高灰铸铁的抗热疲劳性能,随着w（Nb）的增加,试样最大裂纹深度和宽度都逐渐减小;（2）由于Nb细化石墨使裂纹源减少和裂纹扩展途径变细,Nb元素能够改善材料表面经热疲劳处理后恶化的性能。     

球铁基体组织对QPQ盐浴渗氮层深度和耐磨性的影响

对不同铁素体含量基体组织的球墨铸铁进行了QPQ盐浴渗氮试验,对渗层进行了显微组织观察和物相分析.试验结果表明,渗层组织中主要为Fe_3O_4和Fe_2N相,且基体组织中铁素体含量越高,渗层越厚.通过对不同基体组织的球墨铸铁进行QPQ处理前后磨损速率的比较,结果表明经QPQ处理后的球墨铸铁试样磨损速率为QPQ处理前的4.3%～19%.     

Strength distribution at interface of rotary-friction-welded aluminum to nodular cast iron

The morphology, size and composition of intermetallic compound at the interface of AI 1050 and nodular cast iron were studied by electron microprobe analysis（EMPA） and scan electron microscopy （SEM）, respectively. The bond strength of the interface was measured by the tensile tests and the morphology of the fracture surface was observed by SEM. The observation of the interface reveals that there are two distinct morphologies： no interrnetallic compound exists in the central area at the interface; while numbers of intermetallic compounds （FexAly） are formed in the peripheral area due to the overfull heat input. The tensile tests indicate that the distribution of strength in radial direction at the interface is irdaomogeneous, and the central area of the interface performs greater bond strength than the peripheral area, which proves directly that the FexAly intermetallic compounds have a negative effect on the integration of interface. The morphology on the fracture surface shows that the facture in the central area at the interface has characteristic of the ductile micro-void facture. So it is important to restrain the form of the intermetallic compound to increase the bond strength of the AI 1050 and nodular cast iron by optimizing welding parameters and the geometry of components.