QT400球墨铸铁的热疲劳行为

采用V形缺口试样,通过上限温度为500~900 ℃的热疲劳试验研究了QT400球墨铸铁的热疲劳行为,并对不同上限温度下材料热疲劳裂纹的萌生与扩展以及试样的显微组织与硬度等进行了分析。结果表明,热疲劳裂纹主要从试样的V形缺口底部萌生,上限温度越高,裂纹萌生越早,萌生后扩展速率也越快,其热疲劳寿命越低;随着上限温度的升高,试验后试样基体的硬度逐渐增加,上限温度由700℃上升到800℃时,试样硬度快速上升;热疲劳试验过程中的氧化腐蚀、球墨形态以及组织变化等为球墨铸铁QT400的热疲劳破坏的主要影响因素。     

Effect of shot peening process on fatigue behavior of an alloyed austempered ductile iron

Shot peening is one of the most common surface treatments to improve the fatigue behavior of metallic parts. In this study the effect of shot peening process on the fatigue behavior of an alloyed austempered ductile iron (ADI) has been studied. Austempering heat treatment consisted of austenitizing at 875℃ for 90 min followed by austempering at three different temperatures of 320, 365 and 400℃. Rotating-bending fatigue test was carried out on samples after shot peening by 0.4-0.6 mm shots. XRD and SEM analysis, micro hardness and roughness tests were carried out to study the fatigue behavior of the samples. Results indicate that the fatigue strengths of samples austempered at 320, 365 and 400℃ are increased by 27.3%, 33.3% and 48.4%, respectively, after shot peening process.     

A study of several factors governing the fatigue limits of austempered ductile cast iron with various microstructures

The effects the leading factors bearing on fatigue limits were investigated with three kinds of ductile iron specimens various microstructures. As a result of examination, the fatigue limits in relationship to hardness and tensile strength, the expected higher improvement for fatigue limits in the case of the high strength specimens that experienced austempering treatment are not observed in comparison with the specimens treated with stress relief treatment. The estimated maximum defect size is one of the important parameters in predicting and evaluating fatigue limits for three different heat-treated ductile cast irons. Also, a quantitative relationship can be established between the fatigue limit and maximum defect size. Moreover, it is possible to explain the difference in fatigue limits in the three ductile cast irons by application of the rates of non-propagating crack which connects the adjacent graphite nodules before it stops.     

Recent development of ductile cast iron production technology in China

Recent progress in the production and technology of ductile cast iron castings in China is reviewed. The manufacture and process control of as-cast ductile iron are discussed. The microstructure, properties and application of partial austenitization normalizing ductile iron and austempered ductile iron （ADI） are briefly depicted. The new development of ductile iron production techniques, such as cored-wire injection （wire-feeding nodularization） process, tundish cover ladle nodularizing process, horizontal continuous casting, and EPC process （lost foam） for ductile iron castings, etc., are summarized.     

Effects of slope plate variable and reheating on semi-solid structure of ductile cast iron

Semi-solid metal casting and forming are known as a promising process for a wide range of metal alloys production. In spite of growing application of semi-solid processed light alloys, a few works have been reported about semi-solid processing of iron and steel. In this research inclined plate was used to change dendritic structure of iron to globular one. The effects of length and slope of plate on the casting structure were examined. The results show that the process can effectively change the dendritic structure to globular. In the slope plate angle of 7.5° and length of 560 mm with cooling rate of 67K·s^-1 the optimum nodular graphite and solid globular particle were achieved. The results also show that by using slope plate inoculant fading can be prevented more easily since the total time of process is rather short.
In addition, the semi-solid ductile cast iron prepared by inclined plate method, was reheated to examine the effect of reheating conditions on the microstructure and coarsening kinetics of the alloy. Solid fraction at different reheating temperatures and holding time was obtained and based on these results the optimum reheating temperature range was determined.     

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.

     

QT-500球墨铸铁表面激光熔覆镍基合金的组织与性能

利用DL-T5000型二氧化碳激光器在QT-500球墨铸铁表面熔覆镍基合金,分析了激光熔覆层的显微组织,测试了其显微硬度及磨损性能。结果表明:所制得熔覆层组织致密、无裂纹,与基体形成了良好的冶金结合。从熔覆层表面到基体热影响区,组织依次为大量的树枝晶、等轴晶、树枝晶。熔覆层的硬度较基体提高了5倍,熔覆层的总磨损率大约为基体的1/6。熔覆层耐磨性能增强的主要原因是镍基合金与涂层元素镍、铬等固溶强化和碳化物等析出相的强化作用。     

The influence of chromium on mechanical properties of austempered ductile cast iron

An investigation was carried out to examine the influence of microstructure and chromium on the tensile properties and plane strain fracture toughness of austempered ductile cast iron (ADI). The investigation also examined the growth kinetics of ferrite in these alloys. Compact tension and round cylindrical tensile specimens were prepared from ductile cast iron with Cr as well as without Cr. These specimens were then given four different heat treatments to produce four different microstructures. Tensile tests and fracture toughness tests were carried out as per ASTM standards E-8 and E-399. The crack growth mechanism during fracture toughness tests was also determined. The test results indicate that yield strength, tensile strength, and fracture toughness of ADI increases with an increase in the volume fractions of ferrite, and the fracture toughness reaches a peak when the volume fractions of the ferrite are approximately 60% in these alloys. The Cr addition was found to reduce the fracture toughness of ADI at lower hardness levels (<40 HRC); at higher hardness levels (≥40 HRC), the effect of chromium on the fracture toughness was negligible. The crack growth mechanism was found to be a combination of quasi-cleavage and microvoid coalescences, and the crack trajectories connect the graphite nodules along the way.     

Effect of microstructure on resonant vibration fatigue fracture of ferritic spheroidal graphite cast iron

The effect of graphite nodule size, counts and eutectic cell wall inclusions on the fatigue fracture behaviour under resonant vibration of spheroidal graphite (SG) cast iron was investigated in this study. Experimental results indicated that the variations of graphite nodule size and counts slightly influence resonant frequency. The D-N curves (deflection amplitude vs. number of vibration cycles) of all test materials show similar manner. The deflection amplitude maintains a constant value for a certain period of vibration cycles (region I) and then decreases significantly with increasing the vibration cycles (region II). The feature of fatigue fracture behaviour under resonant vibration of ferritic SG cast iron can be concisely divided into four steps, namely (1) crack initiation, (2) crack linking, (3) the major cracks formation, and (4) deepening of the major cracks in the through-thickness direction. The initial three steps correspond to the region I period of the D-N curves, and the fourth step occurs in the region II period. Increasing the graphite nodule counts and refining the microstructure increase the region I period, and improve the resonant vibration fatigue life. During fatigue fracture process under resonant vibration, cracks mainly initiate from graphite nodules and some from eutectic cell walls. The eutectic cell wall inclusions are more crowded in SF specimen than that in PF specimen. So, the crack path of SF specimen is more tortuosity. The feature of fracture surface is mostly the brittle fracture.     

贝氏体球铁轧辊的工艺优化及使用效果

采用离心铸造 低温回火工艺并通过合金成分调整和工艺控制,生产出"改进型"贝氏体球铁轧辊。将其用于棒材连轧机精轧机架,可使轧辊裂纹和剥落缺陷明显减少,一次修磨单槽轧制量大于450t,且轧辊非正常报废比率由15%降至1%左右。     

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.     

新型含碳化物马氏体球墨铸铁的热处理工艺

设计了一种新型含碳化物马氏体球墨铸铁材料,并对该材料进行了不同奥氏体化温度的热处理。结果表明：经不同温度奥氏体化后,新型铸铁的组织均为细针状马氏体+孤立分布的硼-铬碳化物+球状石墨组织。奥氏体化温度在820～890 ℃范围变化时,随奥氏体化温度的升高,该铸铁的硬度增大,冲击韧度减小;奥氏体化温度高于890 ℃时,硬度和冲击韧度均减小。奥氏体化温度为890 ℃时,试验铸铁硬度和冲击性能达到良好的匹配。     

Effect of Bi on graphite morphology and mechanical properties of heavy section ductile cast iron

To improve the mechanical properties of heavy section ductile cast iron, bismuth （Bi） was introduced into the iron. Five castings with different Bi content from 0 to 0.014 wt.% were prepared; and four positions in the casting from the edge to the center, with different solidification cooling rates, were chosen for microstructure observation and mechanical properties test. The effect of the Bi content on the graphite morphology and mechanical properties of heavy section ductile cast iron were investigated. Results show that the tensile strength, elongation and impact toughness at different positions in the five castings decrease with a decrease in cooling rate. With an increase in Bi content, the graphite morphology and the mechanical properties at the same position are improved, and the improvement of mechanical properties is obvious when the Bi content is no higher than 0.011wt.%. But when the Bi content is further increased to 0.014wt.%, the improvement of mechanical properties is not obvious due to the increase of chunky graphite number and the aggregation of chunky graphite. With an increase in Bi content, the tensile fracture mechanism is changed from brittle to mixture ductile-brittle fracture.     

球墨铸铁加热过程中渗碳体石墨化的原位观察

采用激光共聚焦扫描显微镜直接观察了球墨铸铁在加热过程中渗碳体的分解和石墨形态的演化。结果表明：球墨铸铁在加热过程中渗碳体逐渐溶解,加热速率越快,渗碳体的分解速率越快。加热过程中石墨形态的演变行为与加热速率有关。加热速率较慢,组织中出现了小石墨,原有石墨长大。加热速率较快,渗碳体周围的小石墨逐渐溶解变小。     

热浸渗铝球墨铸铁合金层组织的演变规律

为了探索热浸渗铝球墨铸铁合金层的组织演变规律以及基体中Ni含量对渗铝合金层组织的影响,利用激光共聚焦显微镜(LCM)、场发射扫描电子显微镜(FESEM)以及能谱仪(EDS)对不同渗铝时间制备的球墨铸铁渗铝试样以及不同Ni含量的球墨铸铁渗铝试样的合金层组织进行观察和定量分析。结果表明:在本实验条件下,热浸渗铝球墨铸铁合金层组织由厚度较大的"舌状"Fe2Al5和厚度很小的FeAl3组成,合金层和表面纯铝层中存在石墨球分布;在0-15 min内,渗铝试样合金层经过短暂的过渡阶段后随渗铝时间的延长按抛物线规律扩散生长,15 min后合金层厚度达65μm以上;基体Ni含量由0增加至1.5%(质量分数)后,合金层厚度由88μm减小至49μm,合金层与基体的"舌状"界面程度变小。此外,对热浸渗铝球墨铸铁合金层组织演变机理进行了探讨。     

Effect of nodule count and austempering heat treatment on segregation behavior of alloying elements in ductile cast iron

The equilibrium partition ratio, k, has been measured for Mn, Mo, Si, Ni and Cu in a ductile iron with composition(wt.%): 3.45 C, 0.25 Mn, 0.25 Mo, 2.45 Si, 0.5Ni and 0.5Cu with different nodule counts obtained from different section sizes of13, 25, 75 mm in the as cast, austenitized(at 870 °C for times 1, 4 and 6 hours) and austempered(at 375 °C for times 1 to 1,440 min) samples. Results show that Mn and Mo segregate positively at cell boundaries, but Si, Ni and Cu concentrate in an inverse manner in the vicinity ofgraphite nodules and there is a depletion ofthese elements at cell boundaries. Segregation curves for Ni and Cu are more smooth than for Si. Carbide formation has been observed at cell boundaries. Based on the results, the partition ratios for all elements decrease with increasing the nodule count. More carbide with coarser morphology has been observed in the microstructure with a lower nodule count. Austenitization for a longer time can decrease partition ratio, but cannot eliminate it entirely. Increasing the austenitization temperature has the same effect. Austenitizing parameters have no significant effect on carbides volume fraction. The kinetics ofaustempering is faster in higher nodule counts and subsequently better mechanical properties including higher ductility, strength and toughness have been observed for all austempering conditions studied.     

球墨铸铁螺旋孔型斜轧贴皮现象的力学研究

球墨铸铁的螺旋孔型斜轧的生产中出现了贴皮等在轧制钢类材料时不会出现的问题。本文建立了发生贴皮现象部位的力学模型 ,并提出了相应的金属表面切应力的算法 ,作为球铁磨球斜轧中防止贴皮现象的设计准则之一。从力学角度建立起解决贴皮现象的理论基础 ,一方面 ,充实了球墨铸铁轧制工艺的理论基础 ,另一方面 ,也为工程人员提供了理论设计依据。     

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.     

Study of crack propagation behavior during low cycle fatigue in spheroidal graphite cast iron based on observation of surface and fracture sections

In this paper, in order to clarify the effect of crack coalescence for crack growth rate, low cycle fatigue tests were carried out using two kinds of spheroidal graphite cast iron (SGI). Crack propagation behavior from crack initiation to fracture was investigated by observing the surface of specimens and their fracture section. The main results obtained are as follows: (1) The variation range in the crack growth curves in SGI is larger than that in its matrix material. This tendency is caused by the occurrence of crack coalescence and the difference in crack initiation length. (2) In most of the cases where the crack growth rate accelerated, there was microshrinkage on the neighboring surface. (3) Crack coalescence is not the main factor in accelerating crack growth rate.     

Formation mechanism of spheroidal carbide in ultra-low carbon ductile cast iron

The formation mechanism of the spheroidal carbide in the ultra-low carbon ductile cast iron fabricated by the metal mold casting technique was systematically investigated. The results demonstrated that the spheroidal carbide belonged to eutectic carbide and crystallized in the isolated eutectic liquid phase area. The formation process of the spheroidal carbide was related to the contact and the intersection between the primary dendrite and the secondary dendrite of austenite. The oxides of magnesium, rare earths and other elements can act as heterogeneous nucleation sites for the spheroidal carbide. It was also found that the amount of the spheroidal carbide would increase with an increase in carbon content. The cooling rate has an important influence on the spheroidal carbide under the same chemical composition condition.