合金球墨铸铁的单向淬火试验和淬透性测量

采用一种新的端淬方法，研究了球墨铸铁的淬透性能。试验结果表明．该方法可有效地测定合金球铁的淬透性；合金成分不同其淬透性也不同；从淬火端其组织变化依次为马氏体和贝氏体(加铁素体)、贝氏体(加铁素体)、贝氏体和珠光体(加铁素体)、珠光体(加铁素体)。     

Evaluation of water embrittlement on ‘dual phase’ ADI

Austempered ductile iron (ADI) suffers an embrittlement phenomenon when loaded in contact with water and other liquids. This phenomenon causes noticeable drops in elongation, ultimate tensile strength and fracture toughness of ADI of different strength grades. This paper studies the susceptibility to embrittlement of a new kind of austempered ductile iron called dual phase ADI (DPADI), which shows a matrix composed by the typical ausferrite present in ADI mixed with allotriomorphic ferrite. The new material is obtained by means of specific heat treatments that involve a partial austenitising stage. The susceptibility of DPADI to this type of embrittlement was evaluated by carrying out tensile tests in dry and wet conditions. Fracture surfaces were observed using scanning electron microscopy. The results showed a gradual decrease of the degree of embrittlement caused by contact with water as the ferrite content in the matrix increases.     

Investigation of the Variations in Microstructure and Mechanical Properties of Dual-Matrix Ductile Iron by Magnetic Barkhausen Noise Analysis

The variations in the microstructure and tensile properties of dual-matrix ductile irons have been investigated non-destructively by Magnetic Barkhausen Noise (MBN) method. Specimens have been intercritically austenitised at 795°C and 815°C for 20 minutes, and then oil-quenched to obtain different martensite volume fractions. Two specimens, namely as-cast and oil-quenched from 900°C, were prepared for comparison purpose. To investigate the effect of tempering, some specimens were tempered at 500°C for 1 h and 3 h. The results showed that there is a good correlation between MBN response and variations in microstructure and mechanical properties. The volume fraction of martensite can be controlled to modify the mechanical properties, and all changes in the microstructure can be nondestructively monitored by MBN.     

Development of a New Electrode for Arc Wclding of Austempercd Ductile Iron(ADI)

The effect of Ca,Ba,Bi and Al on the amountof carbide in ductile iron weld metal,themicrostructural characteristics of ADI weld metaland the effect of heat treatment process on themicrostructure and mechanical properties of ADIweld metal have been studied.On this basis theoptimum composition of weld and the optimumheat treatment process of ADI weld metal were de-termined and a new electrode for arc cold-welding(i.e.,without preheat) of ADI was developed.Theductile iron welded joint free from eutectic carbidecan be produced by using this electrode beforeaustempering and the weld metal obtained afteraustempering has a microstructure and mechanicalproperties similar to those of ADI.The mechanicalproperties of welded joints can match the require-ment of ADI.     

Analysis of mechanical properties and its associated fracture surfaces in dual‐phase austempered ductile iron

This work aims at evaluating the fracture surfaces of tensile samples taken from a new kind of ductile iron referred to as ‘dual‐phase Austempered Ductile Iron (ADI)’, a material composed of ausferrite (regular ADI microstructure) and free (or allotriomorphic) ferrite. The tensile fracture surface characteristics and tensile properties of eight dual‐phase ADI microstructures, containing different relative quantities of ferrite and ausferrite, were studied in an alloyed ductile cast iron. Additionally, samples with fully ferritic and fully ausferritic (ADI) matrices were produced to be used as reference. Ferritic–pearlitic ductile irons (DI) were evaluated as well. For dual‐phase ADI microstructures, when the amount of ausferrite increases, tensile strength, yield stress and hardness do so too. Interesting combinations of strength and elongation until failure were found. The mechanisms of fracture that characterise DI under static uniaxial loading at room temperature are nucleation, growth and coalescence of microvoids. The fracture surface of fully ferritic DI exhibited an irregular topography with dimples and large deformation of the nodular cavities, characteristic of ductile fracture. Microstructures with small percentages of ausferrite (less than 20%) yielded better mechanical properties in relation to fully ferritic matrices. These microstructures presented regions of quasi‐cleavage fracture around last‐to‐freeze zones, related to the presence of ausferrite in those areas. As the amount of ausferrite increased, a decrease in nodular cavities deformation and a flatter fracture surface topography were noticed, which were ascribed to a higher amount of quasi‐cleavage zones. By means of a special thermal cycle, microstructures with pearlitic matrices containing a continuous and well‐defined net of allotriomorphic ferrite, located at the grain boundaries of recrystallised austenite, were obtained. The results of the mechanical tests leading to these microstructures revealed a significant enhancement of mechanical properties with respect to completely pearlitic matrices. The topographies of the fracture surfaces revealed a flat aspect and slightly or undeformed nodular cavities, as a result of high amount of pearlite. Still isolated dimple patterns associated to ferritic regions were observed.     

Nanoausferritic matrix of ductile iron

Abstract

Austempered ductile iron is known for its excellent mechanical properties resulting from special phase composition and austempering heat treatment. Typical microstructure consists of ferrite plates of micrometre size submerged in untransformed austenite matrix. It has been recently shown that by use of appropriate chemical composition of cast iron and well targeted heat treatment parameters, it is possible to reduce ferrite plates width to submicron or even nanometric size. This creates the potential to achieve even higher mechanical properties of austempered ductile iron. The paper describes the influence of applied heat treatment parameters on microstructure of selected austempered ductile iron grades. Conditions necessary to reduce size of phases to a nanometric scale by heat treatment in austempered ductile iron are discussed.     

奥－贝球铁断裂与疲劳研究的新近进展

奥 -贝球铁具有很强的强度、塑性、韧性、疲劳强度和耐磨性等综合性能 ,应用日益广泛 ,已成为一种重要的工程材料。近年来对其断裂与疲劳的研究发展很快 ,得到不断的深入 ,本文详细介绍了其新近进展。     

Affects of graphite morphology and matrix structure on mechanical properties of cast ions

A comparative study of the affects of graphite morphology and matrix structure on mechanical properties was carried out on spheroidal, compacted and flake graphite irons by a short austenitizing plus austempering treatment. Transformation kinetic data showed that compacted graphite iron had the fastest, spheroidal graphite iron the second and flake graphite iron the slowest austenitizing rate. In spheroidal and compacted graphite irons the strength increased, while ductility decreased, with increasing the amount of bainite in matrix due to prolonging austenitizing time. On the other hand, the increase of bainite structure in the matrix had no significant affect on the mechanical properties of flake graphite iron. Fractographic examinations showed that the fracture surface of spheroidal graphite iron changed from a ductile mode to a brittle mode when its matrix changed from ferrite to bainite dominant. The flake graphite iron ruptured with brittle mode no matter what matrix it had. Compacted graphite iron exhibited an intermediate type of fracture surface. With a short austenitizing plus austempering treatment, the mechanical properties of spheroidal and compacted graphite irons could be improved and extended to a very wide range.     

A study on microstructure and toughness of copper alloyed and austempered ductile irons

In this study, ductile irons with and without 1 wt% copper alloy were austempered to become austempered ductile irons (ADIs). Microstructure, impact toughness, and fracture toughness were evaluated to determine how both the copper alloying and austempering treatments influenced the toughness properties of ductile irons. The results show that, because copper increases the retained austenite content in ADI, the Cu-alloyed ADI has better impact toughness and fracture toughness (K_IC value) than does the unalloyed one. In particular, the impact toughness and the fracture toughness of ADI could be efficiently improved by treating the Cu-alloyed ductile iron at a higher austempering temperature (360 °C) to obtain more retained austenite in its microstructure.     

Monitoring the Microstructural Evolution in Spheroidized Steels by Magnetic Barkhausen Noise Measurements

The aim of this study is to monitor nondestructively the degree of spheroidization in steels by Magnetic Barkhausen Noise (MBN) method. Various series of specimens consisting of either lamellar pearlite or partially/completely spheroidized carbides were produced from AISI 1060 steel by appropriate heat treatments. All specimens were characterized by metallographic examinations, hardness and MBN measurements. The results show that MBN signals are very sensitive to the variations in the microstructure caused by the spheroidizing heat treatment. The change of microstructure, from coarse lamellar carbides to uniformly dispersed spherical carbides in ferrite matrix, is reflected as higher Barkhausen activity due to less effective pinning of domain walls.     

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.     

Austempered ductile iron heat treatment for machine hammer peening of milled tool surfaces

Austempered ductile iron (ADI) has become a competitive material to conventional steels. In addition to its favorable price the main reasons are its mechanical properties can be adjusted over a wide range via heat treatment. Austempered ductile iron consists of ferrite, graphite and metastable austenite. Tailoring its microstructure (phase fractions, stability) with regard to the application is an important challenge. A cast iron used for forming dies is EN‐JS2070. In earlier studies it could be shown that EN‐JS2070 can be transformed into austempered ductile iron [1]. Machine hammer peening, causes martensitic transformation of the metastable austenite and leads to hard and smooth surfaces. Focus of this study is to optimize the microstructure with regard to machine hammer peening process. Before and after machine hammer peening the sample surfaces were characterized using optical and laser microscopy, X‐ray diffraction and hardness measurement. It could be shown that a combination of high amount of metastable austenite with a high carbon content leads to the best results in surface roughness and hardness.     

Alloy design of ductile phosphoric iron: Ideas from archaeometallurgy

Alloy design criteria to produce ductile phosphoric irons have been proposed based on a detailed microstructural study of ancient Indian irons. The alloy design aims at avoiding phosphorus segregation to the grain boundaries by (a) soaking the phosphoric iron at high temperatures within the ferrite + austenite region to precipitate austenite allotriomorphs, (b) utilizing a critical amount of carbon to segregate to grain boundaries, and (c) precipitation of some of the phosphorus in solid solution in the ferrite matrix as fine coherent phosphide precipitates.     

Influence of alloy element distributions on austempered ductile irons

The influence of alloy element distributions on austempered ductile iron microstructure and austempering treatment was analysed by a cellular automaton model that considers the ausferritic and martensitic transformations. The initial microstructure is modelled as spherical graphite nodules inserted in an austenitic matrix, in which the alloy elements are distributed in a uniform or non-uniform way. The study is performed for different chemical compositions and graphite nodule sizes. Delays in the development of ausferritic transformation are produced by the increment of graphite nodule size and the presence of alloy element microsegregations. Moreover, microsegregation reduces the final volume fraction of ferrite platelets. The predicted retained austenite volume fraction is in good agreement with the experimental measurements reported in the literature.     

等温淬火对奥贝球铁上贝氏体转变动力学的影响

本文研究了等温淬火过程中奥氏体化温度和等温淬火温度对奥贝球铁上贝氏体第一阶段转变速度的影响及其机制；观察与测定了不同等温转变时奥贝球铁的组织与性能。结果表明，升高奥氏体化温度和等温淬火温度，使上贝氏体第一阶段转变速度减慢，在该转变结束时，奥贝球铁组织与性能最稳定。     

A Dilatometry Study of the Austenitization and Cooling Behavior of Ductile Iron Meant for the Production of Austempered Ductile Iron (ADI)

An understanding of the kinetics of transformation during austenitization, cooling, and austempering of ductile iron is critical to achieving the desired microstructures and ultimately mechanical properties in austempered ductile iron (ADI). To this end, dilatometry experiments have been carried out to study the austenitization and cooling behavior of an unalloyed ductile iron. When a typical austenitization temperature of 900°C is used, unlike in steels, there is an initial expansion of the specimen, which levels off as the soaking time is increased. This occurs despite the fact that the temperature remains constant. This phenomenon, hitherto unreported, highlights the subtle differences between the austenitization of ductile irons and steels. The initial expansion is attributed to the increase in austenite lattice parameter, arising from the diffusion of carbon from the graphite nodules. The levelling off signals the saturation of austenite with carbon and can therefore be used as an indicator of the appropriate austenitization time. Studies of the cooling behavior of unalloyed ductile iron have also shown that the dilatometer can be used as a tool for determining the minimum cooling rates, which guarantee the formation of ausferrite during austempering. When ductile iron is appropriately heat-treated based on results from dilatometry studies, the mechanical properties obtained are typically superior and consistent.     

The influence of depth of cut on the machinability of an alloyed austempered ductile iron

The volume fraction of high carbon austenite present in the microstructure of austempered ductile iron (ADI) is one of the important factors that influence the mechanical and physical properties of the alloy. Formation of martensite by TRIP (transformation induced plasticity) mechanism during the machining operation in which a large amount of stress is applied to the microstructure results in a decrease in machinability of austempered ductile iron which has affected the expansion of ADI in industry. In this article, the effect of depth of cut as a machining variable is assessed in an alloyed austempered ductile iron containing Cu, Ni and Mo. The measurements of mechanical properties including impact energy, tensile strength, hardness and microhardness along the cross-section of samples are reported for samples austenitized at 870 °C followed by austempering at 375, 340 and 300 °C. Results indicate that contrary to the behavior of many alloys, in austempered ductile iron, reducing the depth of cut will not improve the machinability. In the case of studied composition, cutting with depths of 0.5 and 0.1 mm had the best and worst results, respectively.     

Morphology and constitution of the phases in as-welded microstructure of austempered ductile iron

Abstract

It was found by optical and electron microscopic examination of the microstructure of as-weld austempered ductile iron that the weld matrix is composed of austenite and bainite, the volume fractions of which were determined. In addition, the carbon content of austenite was measured and therefore the average carbon content of the matrix was calculated. In the matrix of the weld metal two types of bainite, bainite ferrite and lower bainite, were found. According to the morphology and distribution of the bainite plates, the nucleation and growth modes of bainite was inferred.     

Low-cycle fatigue of austempered ductile irons

The effects of austempering temperature and isothermal transformation time on the low-cycle fatigue (LCF) behaviour in ductile irons have been studied. The fracture surfaces were observed by a scanning electron microscope in order to understand the fracture mechanism of LCF. From the results, it can be concluded that the best LCF behaviour is for the irons austenitized at 950 °C and there is very good cyclic stability at the lower strain amplitude irrespective of the austempering condition. However, there is a little cyclic softening at higher strain amplitudes for all the austempering conditions. Under a larger strain amplitude, the best LCF behaviour is for the specimen that has undergone austempering at a higher temperature, but under a smaller strain amplitude, the best LCF behaviour is for the specimen austempered at 350 °C.     

Experimental and numerical assessment of fracture toughness of dual-phase austempered ductile iron

The effects of the microstructure topology on the fracture toughness of dual-phase austempered ductile iron are studied in this paper by means of finite element modelling and experimental testing. To this end, specimens with matrix microstructures ranging from fully ferrite to fully ausferrite were studied and the preferential zones and phases for crack propagation were identified in every case. The effectiveness of the ausferrite phase as a reinforcement of the ferritic matrix via the encapsulation of the brittle and weak last-to-freeze (LTF) zones was confirmed. The toughening mechanism is consequence of the increment in the crack path longitude as it avoids the encapsulated LTF zones. Besides, the presence of small pools of allotriomorphic ferrite increase the crack propagation resistance of the ausferrite-ferrite matrices.