Development of a New Welding Material for Arc Welding of As-cast Ferritic Ductile Iron

Using the nickel welding wire, the effect of alloying elements on the chilling tendency of partial fusion zone (PFZ) of arc-welded ferritic ductile iron and the mechanical properties of weld metal have been systematically studied. On this basis a new nickel electrode for arc welding of ferritic ductile iron (QT400-17) is developed for the first time. The mechanical properties of its weld metal can match those of ferritic ductile iron. The width of carbide layer (WCL) in PFZ is significantly decreased, the welded joint has excellent machinability and the weid metal has high hot cracking resistance     

Surface reactivity of thin wall ferritic ductile iron. The effect of nodule count and grinding variables

Thin wall ductile iron (TWDI) castings constitute an attractive alternative to several applications in which the strength to weight ratio becomes a key design variable. In TWDI, the nodule count for a given chemical composition is highly dependent on cooling rate during solidification, and hence on thickness.For mechanical parts, where accurate dimensional tolerance is mandatory, the most common machining process applied is grinding. This process induces significant temperature gradients and surface plastic deformations which could affect service performance, particularly in corrosion environments.In TWDI, surface properties become more relevant due to the high surface area to volume ratio. Therefore, the aim of this paper is to determine the effect of nodular count and grinding conditions on surface reactivity of ferritic TWDI. Electrochemical assays were carried out in a three-electrode cell in a 3.5 wt.% NaCl solution on both polished and ground samples. The results obtained indicate that surface reactivity increases with higher nodular counts and residual plastic strain.     

A comparison of the fatigue strength of machined and as-cast surfaces of SG iron

The fatigue strength of machined specimens of ferritic SG iron has been compared with the strength of specimens having an as-cast surface. Micropores, exposed by machining, initiated failure in the machined specimens, whereas surface irregularities or dross defects initiated failure from the as-cast surface. While the endurance of specimens in which failures initiated at surface irregularities was only marginally less than that of machined spec imens, dross defects reduced fatigue life by a factor of ten.It is demonstrated that the fatigue life of SG iron is dominated by micro-crack growth and that the effect of all types of SG-iron defects on fatigue endurance is related to defect size. Specimen endurances of less than 10⁵ cycles can be predicted using a fracture mechanics approach and integrating the Paris Law from the appropriate defect size.     

Growth morphology and mechanism of MC carbide under quasi-rapid solidification conditions

The carbide of group IVB and group VB elements, i.e. MC carbide, is an important constitution and strengthening phase for many alloy tool steels and cast nickel-base superalloys. Since the as-solidified growth morphology, size and distribution have an important influence on both the mechanical properties and hot workability, research on the solidification behavior of MC carbide is an important subject for cast superalloys and many high alloy tool steels. The growth morphology and mechanisms of MC carbide, under slow-cooling and rapid solidification conditions, has been studied intensively as functions of the solidification cooling rate. The solidification behavior of MC carbide under quasi-rapid solidification conditions has not been reported in open literature. In this paper, the growth morphology and mechanism of an MC carbide (TiC type) under quasi-rapid solidification conditions is studied in a laser surface alloyed coating on a titanium aluminide alloy Ti–48Al–2Cr–2Nb (at.%). The growth morphology of the quasi-rapidly solidified MC carbide with a cooling rate of 4×10²°C is found to be dendritic with strong faceted, double zigzag brick-stacking growth characteristics on the dendrite arms. The growth mechanism of the MC carbide is found to be a brick-stacking/double zigzag micro-branching lateral growth from steps on the intersecting {111} planes.     

Cooling rate measurements on pure iron rapidly solidified by piston quenching

Cooling curves have been monitored during rapid solidification of pure iron, using a rapid response thermocouple embedded in one of the quenching pistons. Cooling rates are found to be typically 10⁶ to 10⁷ K sec^–1 in the vicinity of the solidification point at 1500° C, falling to 2 × 10⁴ to 3 × 10⁵ K sec^–1 at 500° C. Heat-flow analysis shows that cooling conditions during rapid solidification are clearly non-Newtonian, with heat transfer coefficients of 3 × 10⁵ to 6 × 10⁵Wm^–2 K^–1 and Nusselt numbers of 0.5 to 1.0. Cooling rates, heat transfer coefficients and Nusselt numbers are higher for piston quenching than for other rapid solidification processes such as melt spinning. Piston-quenched iron microstructures can be ferritic or martensitic depending on the cooling rate during rapid solidification.     

差压铸造薄壁铝硅合金铸件的位置效应

采用差压铸造工艺,研究垂直缝隙式浇注系统浇注的铝合金硅铸件不同位置的组织和力学性能变化.采用石英砂型、SiC砂型和冷铁,浇口处铸件的晶粒最细小,致密度高、力学性能最好;铸件冷端的组织和性能次之;位于两者之间的铸件的组织和性能最差.分析表明对于具有垂直缝隙式浇注系统,差压铸造凝固压力对金属的凝固作用具有位置效应,浇口处液态金属温度高,凝固时间长,凝固压力对浇口处金属的凝固作用显著;铸件冷端金属凝固时间短,凝固压力对该处金属的凝固作用不显著,铸型的冷却速度对铸件组织和性能的影响起显著作用.浇口处与冷端之间的金属液体的凝固受压力和冷却速度的影响小,铸件的晶粒尺寸最大、密度最小、性能最低.冷却速度提高,铸件的任意位置的组织和性能都相应得到提高.     

Volume change during the solidification of SG iron: comparison between experimental results and simulation

In order to understand the shrinkage behaviour of spheroidal graphite (SG) iron during solidification, a volume change kinetic model was set up to simulate the volume change during the eutectic solidification, which was presented in an earlier paper. Furthermore in the present work experiments were carried out for comparison with theoretical prediction. The microstructure of the mushy zone during the solidification of SG cast iron was obtained by the quenching method and analysed by normal metallography and image analysis. The results show that the mushy zone exists in front of the interface between the liquid and the solid. The study by quantitative stereology shows that the graphite fraction in the mushy zone has the same trend as that of the theoretical prediction and the silicon content in cast iron strongly influences graphitization during the solidification. A heat-transfer model to stimulate the heat transfer of the experimental apparatus was developed. A modified Rappaz’s model was used to simulate the eutectic growth under fully equilibrium conditions. The theoretical prediction has been compared with the experimental results, and found to be in good agreement with each other. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigating the Tool Life, Cutting Force Components, and Surface Roughness of AISI 302 Stainless Steel Material Under Oblique Machining

The aim of this work is to investigate the machinability of austenitic AISI 302 stainless steel under oblique cutting. This can be achieved by studying the cutting forces, analysis of tool life, and investigation of the surface roughness at different cutting conditions and nose radius. A factorial experiment and analysis of variance technique are used in which several factors are evaluated for their effects on each level. The machinability experiments are based on design of experiments to obtain empirical equations for machinability values for machining conditions such as speed, feed, depth of cut, and nose radius. The parameters considered in the experiments were optimized to attain maximum tool life using a response graph and a response table. Based on the response models, dual response contours (tool life and surface roughness as a response and metal removal rate) have been plotted in cutting speed-feed planes. Evaluating the effect of the predominant variables influencing the value of tool life is very important for improving the machined product quality.     

Growth morphology and mechanism of MC carbide under quasi-rapid solidification conditions

The carbide of group IVB and group VB elements, i.e. MC carbide, is an important constitution and strengthening phase for many alloy tool steels and cast nickel-base superalloys. Since the as-solidified growth morphology, size and distribution have an important influence on both the mechanical properties and hot workability, research on the solidification behavior of MC carbide is an important subject for cast superalloys and many high alloy tool steels. The growth morphology and mechanisms of MC carbide, under slow-cooling and rapid solidiication conditions, has been studied intensively as functions of the solidiication cooling rate. The solidiication behavior of MC carbide under quasi-rapid solidification conditions has not been reported in open literature. In this paper, the growth morphology and mechanism of an MC carbide (TiC type) under quasi-rapid solidification conditions is studied in a laser surface alloyed coating on a titanium aluminide alloy Ti–48Al–2Cr–2Nb (at.%). The growth morphology of the quasi-rapidly solidified MC carbide with a cooling rate of 4 × 10²°C is found to be dendritic with strong faceted, double zigzag brick-stacking growth characteristics on the dendrite arms. The growth mechanism of the MC carbide is found to be a brick-stacking/double zigzag micro-branching lateral growth from steps on the intersecting {111} planes.     

Relation between microstructural heterogeneities and damage mechanisms of a ferritic spheroidal graphite cast iron during tensile loading

In this work, the influence of the metallic matrix heterogeneities and the spheroidal graphite nodules distribution on both crack initiation and propagation and damage evolution during tensile loading of ferritic spheroidal graphite cast iron is examined. The experimental methodology involves specialized metallographic techniques, step by step tensile loading, microscopic observation by using optical and scanning electron microscopy and three‐dimensional (3D) reconstruction of the graphite nodules distribution. The results show that the graphite nodules are the major heterogeneities responsible for inducing the development of cracks in the metallic matrix. Crack initiation is preferentially located at the irregular contour of graphite nodule cavities, ferritic grain boundaries and internodular areas highly strained. The final fracture involves cracks mainly propagating through the internodular ligaments of matrix‐nodule debonded areas belonging to the first‐to‐freeze zones resulting from the solidification process.     

Novel method for the fatigue strength assessment of heavy sections made by ductile cast iron in presence of solidification defects

The fatigue strength of ferritic, pearlitic, and solution strengthened ferritic ductile irons taken from heavy sections and characterized by long solidification times has been assessed. Starting from the idea of Murakami and co‐workers, a new model for the prediction of the fatigue strength is proposed. It allows a sound fatigue assessment of the fatigue strength of as‐cast ductile irons containing solidification defects, such as low nodule count, exploded, chunky and spiky graphite, or microshrinkage porosities. The newly developed equation validated by means of an extensive benchmarking with data taken from the literature has shown a very high potential for applications to thick‐walled components.     

Effect of cryogenic minimum quantity lubrication on machinability of diamond tool in ultraprecision turning of 3Cr2NiMo steel

This work presents a series of experimental investigations and corresponding theoretical analyses to research on the effect of cryogenic minimum quantity lubrication (MQL) on machinability of diamond tool in ultraprecision turning of typical die steel. The tool wear and machined surface quality were determined as experimental indexes, which were measured using the scanning electron microscope and surface profiler, respectively. Besides, the maximum temperatures of diamond tool surfaces acquired by infrared thermal imager were used to indirectly evaluate the cutting process. The experimental results revealed that cryogenic MQL had obvious advantages in improving diamond tool durability and machined surface quality by comparison with flood cooling, cryogenic gas cooling, and MQL, and its essential function mechanisms were thoroughly understood. On the basis of this, carbon nanofluid was found to achieve optimal tool life in diamond turning compared with polyethylene glycol, castor oil, synthetic ester, and emulsified liquid. Ultimately, the combined machining method of ultrasonic vibration-assisted turning and cryogenic minimal quantity lubrication was proposed in this work. The results showed that this technique could observably improve the machinability of diamond tool and also provide a new direction for exploring a suitable processing method for ultraprecision machining of ferrous materials.     

Effects of applied pressure on microstructure and mechanical properties of squeeze cast ductile iron

In this study, the effects of applied pressure during solidification on the microstructure and mechanical properties of cylindrical shaped ductile iron castings were investigated. Magnesium treated cast iron melts were solidified under atmospheric pressure as well as 25, 50 and 75 MPa external pressures. Microstructure features of the castings were characterized using image analysis, optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) techniques. Tensile properties, toughness and hardness of the castings were also measured. The results showed that average graphite nodule size, free graphite content and ferrite content of the castings decreased and pearlite and eutectic cementite contents increased as the applied pressure was raised from 0 to 75 Mpa. Graphite nodule count was first increased by raising the applied pressure up to 50 MPa and then decreased. The highest graphite nodule count was obtained at 50 MPa applied pressure. The microstructural changes were associated with the improved cooling rate and the expected changes in the corresponding phase diagram of the alloy under pressure. The ultimate tensile strength (UTS), yield point strength (0.2% offset) and fracture toughness of the castings were improved when the applied pressure was raised from 0 to 50 MPa. Further increase of the applied pressure resulted in slight decrease of these properties due to the formation of more cementite phase in structures as well as reduced graphite nodule count. Hardness of the castings continuously increased with increasing the applied pressure.     

Modelling microstructural and mechanical properties of ferritic ductile cast iron

Abstract

It is well known that the mechanical properties of ductile cast iron (DCI) depend on its microstructure, and that the microstructure depends on the properties of the melt and the cooling conditions during casting. There have been many studies of the individual elements of the process of casting DCI, but as yet there have been very few examples of modelling the entire process to predict cooling rates, microstructure, and mechanical properties, particularly for large castings. The present paper describes a method of modelling the microstructural and mechanical properties of ferritic DCI, and applies the methods to the case of a large (13 t) thick walled (300 mm thickness) casting. The microstructure calculated includes nodule count, nodularity, ferrite grain size, and percentage ferrite. The mechanical properties calculated include yield stress, tensile strength, elongation, and static upper shelf fracture toughness (J _1C and K _JC). The calculated results compare well with those of a test casting.     

High-temperature properties of ferritic spheroidal graphite cast iron

The behaviour of fracture mode and intermediate temperature embrittlement of ferritic spheroidal graphite cast iron is influenced by many factors. From the experimental results, intermediate temperature embrittlement can be considered to be dominated by dynamic strain ageing and the triaxial stress field developed in the ferrite matrix amongst the graphite particles. In order to understand the effect of dynamic strain ageing on high-temperature properties, tensile properties, push-pull low-cycle fatigue properties, rotary bending fatigue properties and creep-rupture properties were investigated from room temperature to 500° C. It was found that all the properties investigated were influenced by dynamic strain ageing. The intermediate temperature embrittlement of ferritic spheroidal graphite cast iron found in different load conditions is reported.     

Microsegregation of manganese and silicon in high manganese ductile iron

Abstract

Manganese is known as an inexpensive element and a potent promoter of hardenabilty in ductile iron but it also segregates severely and encourages the formation of carbides in the matrix. Thus, it is suggested that when the Mn content is high, attempts should be made to minimise the Mn segregation. In the present work the effect of solidification rate and homogenisation treatment on the severity of segregation of Mn and Si in several types of wear resistant high Mn ductile iron was investigated. It is demonstrated that increasing the solidification rate, leading to a high nodule count, decreases not only the heterogeneity of these elements but also the carbide content.     

Influence of modification, solidification conditions and heat treatment on the microstructure and mechanical properties of A356 aluminum alloy

The effects of casting thickness, modification and heat treatment on the microstructure and mechanical properties of A356.2 alloy have been investigated. Experiments were conducted with unmodified, Sr-modified (0.02% Sr) and Sb-modified (0.2% Sb) on both sand cast test bars with various thicknesses (from 3 to 9 mm) and permanent mold cast test bars.The microstructural changes associated with these treatments have been studied by optical metallography, scanning electron microscopy (SEM) and image analysis.The tensile properties of all samples were determined and the relationship between cooling rate, modification and heat treatment has been investigated.The results show that modification has a beneficial effect on microstructure and improves the mechanical properties of the alloy. Modification has a major role in controlling the kinetics of the spheroidisation of silicon particles during heat treatment. Tensile properties improved more with heat treatment than with modification or cooling rate.Antimony is effective on mechanical properties at higher solidification rates, while Strontium is more effective at lower solidification rates.     

Mechanical and fatigue properties of pearlitic ductile iron castings characterized by long solidification times

In this paper, the fatigue behaviour of heavy section pearlitic ductile iron castings has been investigated. The inoculation treatment has been changed for each casting in order to investigate its influence on the mechanical and fatigue properties of the materials.Tensile tests and axial fatigue tests under nominal ratio R = 0.01 have been performed on specimens taken from the core of casting components characterized by long solidification times. Scanning Electron Microscopy has been used to investigate the fracture surface of the broken samples in order to identify crack initiation points and fracture mechanisms. Metallographic analyses have been carried out to measure nodule count and nodules dimensions and to identify matrices structures.It has been found that fatigue behaviour is strongly influenced by defects, such as microshrinkages or degenerated graphite particles near to specimens' surface. It has been also found that inoculation process influences the microstructure and the fatigue resistance of heavy section pearlitic ductile iron castings.     

Fracture surfaces and the associated failure mechanisms in ductile iron with different matrices and load bearing

Ductile iron (DI) is a family of cast alloys that covers a wide range of mechanical properties, depending on its matrix microstructure. For instance, ferritic matrices used in parts, such as automotive suspension components, demand high impact properties and ductility among some of their main requirements. On the other hand, pearlitic and martensitic matrices are used when hardness, strength and wear resistance are of particular concern. When it comes to very high strength parts, ausferritic matrices, typically austempered ductile iron (ADI), are widely used.DI has been employed to replace cast and forged steels in a large number of applications and its production has shown a sustained rate of growth over the last decades.Knowing about failure modes and fracture mechanisms associated to materials with the properties mentioned above is crucial, since they can be of great value for designers of mechanical components.This paper deals with the analysis of fracture surfaces of ductile cast iron generated under different conditions of load application, temperature and environments.The studies include the examination of fracture surfaces obtained by means of tensile tests, impact tests and by samples used to determine fracture toughness properties, where the zones of fatigue pre-crack and monotonic load condition were evaluated. A special case of ductile iron fracture is also examined.The study of the different surfaces permitted to establish patterns that contributed to unveil the fracture mechanisms of ductile iron with different matrices, nodule count, etc.     

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。