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。     

Damage mechanism of a nodular cast iron under the very high cycle fatigue regime

Fatigue behaviour in the very high cycle regime (VHCF) of 10¹⁰ cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = −1.

The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An advanced, high-speed, and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels caused by internal damping due to a very high frequency cycling. The temperature field on the surface specimen was determined by using a non-destructive measurement technique called infrared pyrometer. An infrared camera made up of a matrix of 320 × 240 detectors was used.

The S-N curves obtained show that fatigue failure occurred beyond 10⁹ cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Under the current test conditions, the high cycle fatigue (VHCF) behaviour of the cast iron exhibited a typical fatigue crack growth process, that is, fatigue initiation takes place always at the surface graphite or subsurface void; the distinctive stable fatigue crack growth zone can be found around the fatigue crack initiation site, the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition.

Under lower stress amplitude and high cycle condition, surface graphite fatigue initiation is predominantly depended on cyclic stress amplitude; subsurface void fatigue initiation is determined by maximum cyclic stress.

In the process of small crack propagation, the temperature in local plastic zone increase very sharply. The temperature field of ultrasonic fatigue specimen can be changed with the cooling condition; internal heating can accelerate surface fatigue crack initiation and propagation.

Fatigue properties in VHCF regime were studied for cast iron (GS51) at 20 kHz frequency and for the first time, crack initiation and propagation stages were analyzed using a high-sensitivity infrared camera. The new Paris's model for fish eye formation in the gigacycle fatigue were also confirmed by this study.     

Hydrogen embrittlement of nodular cast iron

Ferritic nodular cast iron, intended for use as the material for inserts of canisters for long-term geological disposal of spent nuclear fuel, was studied for hydrogen sensitivity. In the canisters, the insert provides the mechanical strength against external loads. Hydrogen was charged from 0.1 N H₂SO₄ solution in free-corrosion tests and under controlled cathodic potential. Hydrogen uptake and trapping were then measured using thermal desorption spectroscopy. The hydrogen desorption rate after hydrogen charging manifests two distinct peaks. Plastic deformation during hydrogen charging increases the hydrogen uptake considerably. Hydrogen reduces the elongation to fracture and time to fracture in slow strain rate testing and constant load testing (CLT), respectively. Especially, the strain rate in CLT is dramatically increased. The appearance of hydrogen-induced cracking in the ferrite phase changes from ductile dimple fracture to brittle cleavage fracture due to hydrogen charging, which initiates from the interphases of the graphite nodules. The results are discussed in terms of the role of hydrogen and the graphite nodules in hydrogen embrittlement of ductile cast iron.     

Crystallography of graphite spheroids in cast iron

To further understand graphite growth mechanisms in cast irons, this study focuses on the crystal structure of a graphite spheroid in the vicinity of its nucleus. A sample of a graphite spheroid from a commercial cast iron was characterised using transmission electron microscopy. The chemical composition of the nucleating particle was studied at the local scale. Crystal orientation maps of the graphite spheroid revealed misorientations and twist boundaries. High-resolution lattice fringe images showed that the basal planes of graphite were wavy and distorted close to the nucleus and very straight further away from it. These techniques were complementary and provided new insights on spheroidal graphite nucleation and growth.     

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.     

Cavitation-erosion failure of cast iron components

Moscow Institute of Instrument Engineering. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 20–24, April, 1989.     

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.     

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.     

球墨铸铁行走轮的低温正火工艺

对一种Cu微合金化球墨铸铁行走轮进行了低温正火工艺试验,获得了850 ℃和860 ℃正火后行走轮轮缘和轮辐的显微组织和力学性能数据。分析了合金元素Cu对球墨铸铁组织和性能的影响,以及对奥氏体化及珠光体形成相变温度区间带来的变化。结果表明,试验含Cu的球墨铸铁行走轮850 ℃正火就可以获得95%珠光体,达到完全正火的状态,力学性能符合QT650-4牌号的要求。比较了850 ℃和860 ℃正火后组织和性能,虽然都能满足要求,但860 ℃正火后表面硬度较850 ℃正火的高,机械切削性能相对较差。说明试验行走轮正火工艺温度窗口较窄,因此,选择850 ℃正火工艺为佳。该热处理工艺已在工程上得到应用,生产的行走轮已批量出口。     

低牌号球墨铸铁风冷强化工艺的试验研究

研究了风冷热处理对低牌号球墨铸铁(QT450-10)组织和性能的影响.利用对比法和网格法对珠光体组织进行了定量分析,并测试了风冷前后球墨铸铁的力学性能,分析了球墨铸铁风冷强化的微观机制.结果表明,QT450-10球铁经风冷强化以后,其内部组织由原来85%的铁素体基体转变成90%以上的珠光体,且经测量的珠光体片层间距仅为0.45μm,这种细珠光体的片层结构是导致球墨铸铁的抗拉强度由原来的495 MPa增至905 MPa的主要原因.     

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.     

Occurrence and behaviour of Mo containing precipitates in nodular cast iron at high temperatures

In nodular cast iron, molybdenum is alloyed in the range of 0.5 to 1.5?wt.-% to increase the mechanical properties for high temperature application, especially for thick walled castings like housings of gas or steam turbines. The behaviour of Mo precipitates in the as cast state as well as under long exposure to heat was the object of this investigation. In the as cast state, Mo forms metastable carbidic grain boundary precipitates with iron, silicon and carbon. These transform into stable MoC precipitates during sustained temperature influence. Furthermore, there are finely dispersed Mo containing precipitates found in the matrix. These increase the high temperature strength of Mo alloyed cast iron due to the Orowan mechanism and the occurrence of additional grain boundaries. However, it has been shown that these Mo containing precipitates underlie Ostwald ripening under continuous heat exposure, and a precipitate free area occurs along the ferritic grain boundary.     

激光强化合金球墨铸铁轧辊的微观组织及性能

应用5 kW横流CO2激光器对合金球墨铸铁热轧辊进行了激光强化处理,用光学显微镜、洛氏硬度计及显微硬度计进行了显微组织分析和硬度测试.结果表明,激光处理后的剖面组织区域分为熔凝区、相变区和基体三部分,各区域的尺寸及显微硬度与激光功率、光斑大小、扫描速度等工艺参数有关.激光处理后轧辊表面的洛氏硬度在54.4～63.8 HRC之间,较基体硬度提高约29～39 HRC,经激光强化处理后热轧辊使用寿命比原轧辊提高了1.52～1.87倍.     

烧结机台车球墨铸铁梁断口分析及焊接修复

针对烧结机台车运行工况,结合球墨铸铁材料特性对台车侧梁断裂原因进行分析,并在此基础上制定合理可行的修复工艺进行焊接修复,效果理想。     

Complex（Mn, X）S compounds - major sites for graphite nucleation in grey cast iron

Despite the cubic system, the ability of sulphides to nucleate graphite can be enhanced by inoculating elements which transform them in complex compounds with a better lattice matching to graphite, a low coagulation capacity, good stability and adequate interracial energy. (Mn,X)S compounds, usually less than 5.0 μm in size, with an average 0.4-2.0 μm well defined core (nucleus), were found to be important sites for graphite nucleation in grey irons. A three-stage model for the nucleation of graphite in grey irons is proposed: (1) Very small micro-inclusions based on strong deoxidizing elements (Mn, Si, Al, Ti, Zr) are formed in the melt; (2) Nucleation of complex (Mn,X)S compounds at these previously formed micro-inclusions; (3) Graphite nucleates on the sides of the (Mn,X)S compounds with lower crystallographic misfit. Al appears to have a key role in this process, as Al contributes to the formation of oxides in the first stage and favors the presence of Sr and Ca in the sulphides, in the second stage. The 0.005-0.010% Al range was found to be beneficial for lower undercooling solidification, type-A graphite formation and carbides avoidance.