Abstract: Ductile cast iron was austenitized at four different temperatures and subsequently austempered at six different temperatures. Plane strain fracture toughness was evaluated under all the heat treatment conditions and correlated with the microstructural features such as the austenite content and the carbon content of the austenite. Fracture mechanism was studied by scanning electron microscopy. It was found that the optimum austempering temperature for maximum fracture toughness decreased with increasing austenitizing temperature. This could be interpreted in terms of the microstructural features. A study of the fracture mechanism revealed that good fracture toughness is unlikely to be obtained when austempering temperature is less than half of the austenitizing temperature on the absolute scale.

Abstract: Carbidic Austempered Ductile Iron (CADI) describes a family of ductile cast irons, with given amounts of carbides that are subsequently austempered to exhibit adequate toughness and excellent wear resistance. The abrasion resistance of this new material is improved over that of ADI and increases with increasing carbide content. In a number of wear applications, it can compete favorably with high chromium abrasion resistant (AR) irons in addition to providing improved toughness.

Abstract: A machinable austempered cast iron article has improved strength, machinability, fatigue performance, and resistance to environmental cracking. A method of making the machinable austempered cast iron article includes austenitizing an iron composition having a substantially pearlitic microstructure in an intercritical temperature range of between 1380° F. and 1500° F. This produces a ferritic plus austenitic microstructure. The ferritic plus austenitic microstructure is quenched into an austempering temperature range of between 575° F. and 750° F. within 3 minutes to prevent formation of pearlite. The ferritic plus austenitic microstructure is then austempered in the austempering temperature range of between 575° F. and 750° F. to produce a microstructure of a continuous matrix of equiaxed ferrite with islands of austenite. Finally, the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is cooled to ambient temperature to produce the machinable austempered cast iron article.

Abstract: A ductile iron containing 0.6% copper as the main alloying element was austempered at a fixed austempering temperature of 330 °C for a fixed austempering time of 60 min after austenitization at 850 °C for different austenitization periods of 60, 90, and 120 min. The austempering process was repeated after changing austenitization temperature to 900 °C. The effect of austenitization temperature and time was studied on the carbon content and its distribution in the austenite after austenitization. The effect of austenitization parameters was also studied on austempered microstructure, structural parameters like volume fraction of austenite, X γ , carbon content C γ , and X γ C γ , and bainitic ferrite needle size, d α after austempering. The average carbon content of austenite increases linearly with austenitization time and reaches a saturation level. Higher austenitization temperature results in higher carbon content of austenite. As regards the austempered structure, the lowering austenitization temperature causes significant refinement and more uniform distribution of austempered structure, and a decrease in the volume fraction of retained austenite.

Abstract: To the design engineer, hardness often means an easily measured quantity which indicates something about the strength and heat treatment of a metal. Austempered ductile iron (ADI) is an alloyed and...

Abstract: The Influence of intercritical deformation, cooling rate and prior austenite grain size on bainite formation were investigated by dilatometry tests. Intercritical deformation (0‐40 %) performed in steels with a prior austenite grain size of 15‐28 mm leads to formation of more ferrite during the cooling and less bainite during the subsequent isothermal stage, and even almost no bainite is formed after 40 % strain. Fast cooling after deformation can suppress ferrite transformation. Relaxation following deformation can significantly, but not completely, reduce such effect of deformation due to the occurrence of recovery and recrystallization, particularly for the finer prior austenite grain size. When the prior austenite grain size was changed from 26.8 to 16.8 mm, bainite formation was suppressed. The mechanism for influence of deformation on bainite formation was discussed on the basis that deformation could refine the austenitic microstructure. Further, it is suggested that there is a critical size of austenite grains or subunits after deformation for the formation of bainite.

Abstract: A spheroidal cast iron, particularly for piston rings for pistons of engines, with a particular alloyed chemical composition and subjected to a heat treatment known as austempering, which comprises an austenitization treatment followed by an isothermal hardening treatment, has mechanical characteristics that are comparable with those of the steel currently used to produce piston rings, while maintening the best tribological and self-lubrification characteristics of spheroidal cast irons, and a process suitable to obtain said spheroidal cast iron.

Abstract: Recent reports indicate that austempered ductile iron (ADI) is embrittled on contact with water This environmentally induced cracking (EIC) phenomenon is not clearly understood The objective of this investigation is to study the behavior of ADI on tensile testing in aqueous media under controlled electrochemical conditions, aiming to identify whether the embrittlement can be either inhibited or enhanced by stimulating or avoiding the reduction of protons on the sample's surface The results suggest that the EIC of ADI is not an electrochemical phenomenon, since neither cathodic nor anodic applied potentials have been able to inhibit embrittlement Local wetting of the tensile sample surface has been used to localise fracture Early stages of fracture propagation have been shown to take place by cleavage Later, the fracture mode changes to quasi-cleavage, a much more ductile mechanism, as it grows far from the fracture initiation site

Abstract: Mechanical properties of spheroidal graphite (SG)-iron subjected to variable and isothermal austempering temperatures heat treatment have been examined. Variable austempering temperature heat treatment is carried out by austenitizing at 1183 K then quenching into a salt bath held at 593 and 723 K, respectively. After quenching, the former is steadily heated to 723 K while the latter is allowed to cool progressively to 593 K. The tensile properties, impact toughness and hardness are determined and correlated with the microstructure. It has been found that, specimens quenched at 593 K and heated steadily to 723 K have higher 0.2% yield stress, ultimate tensile strength and hardness, but much less ductility than those quenched at 723 K and cooled progressively to 593 K. The elongation of specimens austempered under variable austempering temperature is significantly improved. It reached about three times of that of specimens austempered isothermally at 593 or 723 K.

Abstract: A multiple low alloyed ductile iron with 0.8 wt-%Ni and 0.25 wt-%Mo was austempered in single and two step processes at 300 and 400°C for 120 min. Specimens were used to study the effect of austempering conditions on the wear behaviour of this material. Sliding wear tests were carried out using a pin on disc apparatus, the tes tmaterials rubbing under dry atmospheric conditions against a surface of hardened steel (55 HRC) at speeds of 0.6, 0.7 and 1.0 m s-1 and normal loads of 15.82 and 22.84 N. Test durations were 30, 60, 90 and 120 min. Scanning electron microscopy was used to examine the worn surfaces of test specimens. It was found that two step austempered specimens exhibited wear resistance that was higher than that of specimens austempered at 400°C, and almost as high as that of specimens austempered at 300°C. These two step austempered specimens, moreover, gave the highest impact energy and showed the best combination of mechanical properties. During two step austempering, the first stage ...

Abstract: A spheroidal cast iron, particularly for piston rings for pistons of engines, with a particular alloyed chemical composition and subjected to a heat treatment known as austempering, which comprises an austenitization treatment followed by an isothermal hardening treatment, has mechanical characteristics that are comparable with those of the steel currently used to produce piston rings, while maintening the best tribological and self-lubrification characteristics of spheroidal cast irons, and a process suitable to obtain said spheroidal cast iron.

Abstract: The kinetics of bainitic transformation was studied in unalloyed and 1%Mn alloyed ductile irons. The samples were subjected to different austempering heat treatment cycles in dilatometry equipment. The results showed the effect of Mn on delaying the start and end of bainitic transformation, supposedly because of Mn segregation to the primary austenite grain boundaries. Decreasing austempering temperature led to precipitation of carbides inside the lower bainitic ferrite plates and therefore, more dilatation during the bainitic reaction. The dilatometric results were also used to calculate n and k in the Johnson-Mehl-Avrami equation and to derive the time-temperature-transformation diagrams. X-ray results were used to calculate the quantitative amount of precipitated carbides within lower bainitic ferrite and the dilatation during austempering.

Abstract: A model is developed for simultaneous prediction of the processing window and austemperability of austempered ductile iron (ADI). The processing window represents a frame of time and temperature in which ADI satisfies optimum mechanical properties defined by ASTM A897M:1990. Austemperability is the maximum section size of ductile iron that can be austempered without formation of pearlite during the austempering process. The outcome of the model presents the processing window and austemperability as a three dimensional diagram (processing - austemperability window). The processing window boundaries are estimated according to a model for prediction of the time for ausferritic reaction in ADI. The austemperability of ductile iron is predicted according to the estimated pearlite curve of the TTT diagram and a mathematical model that simulates conduction of heat in a solid cylinder. The heat transfer model is calibrated for a ductile iron of composition (wt-%) 3.63C, 2.4Si, 0.39Mn, 0.4Mo, 0.25Cu, 0.04...

Abstract: The outstanding mechanical properties of austempered ductile irons (ADI) are linked to the microstructure of the matrix obtained by subjecting a ductile iron with an appropriate composition to a heat treatment called austempering. Then the microstructure of the matrix consists of bainitic ferrite with different volume fractions of retained austenite. The aim of this work is to use the magnetic Barkhausen noise (MBN) as a nondestructive method for characterizing the microstructure of ADI. First, it is shown that the amplitude and position of the peak-shaped MBN response is quite sensitive to the microstructure of the matrix of ductile irons. Thus each type of constituent (equiaxial ferrite, pearlite, martensite or bainite) exhibits a typical response and, in turn, it can be identified from the MBN response. Furthermore, a good correlation is found between MBN signal parameters and ADI heat treatment parameters, indicating that MBN is also quite sensitive to fine evolutions of the microstructure of ADI. MBN peak position is especially sensitive to the type of bainite, whereas peak amplitude is linked to the progress of the bainite reaction. Hence MBN measurements appear to be a powerful tool to assess some important microstructural features of ADI castings.

Abstract: TRIP-assisted steels show enhanced mechanical properties thanks to the strain-induced transformation of retained austenite to martensite (TRIP effect). Stabilisation of austenite is made possible by the combination of appropriate chemical composition (containing classically high levels of silicon, or, as developed more recently, aluminium) and of a two-step heat treatment comprising an intercritical annealing followed by an isothermal holding in the bainite formation range. A detailed understanding of the effects of Si and Al on the bainite transformation kinetics is thus of primary importance for the optimisation of austenite retention. The present study aims at a better knowledge of the influence of aluminium on the kinetics of the bainite transformation of intercritical austenite, as compared to the effect of silicon. The bainite transformation kinetics was followed by dilatometry in a set of 6 steel grades with varying aluminium and silicon contents. These dilatometry experiments were coupled with a characterisation of the microstructures by X-ray diffraction, SEM and image analysis. It was shown that aluminium has a less pronounced effect than silicon in retarding the bainite transformation.

Abstract: Predicted global amounts of bainite within austempered ductile iron correspond well to hardness, toughness and some aspects of damage tolerance. However, local microstructural fatigue events (e.g. fatigue initiation) do not correlate simply to overall predictions of phase distributions within the microstructure, appearing to be dependent on a complex combination of size and morphology of microstructural features. Fatigue evaluation of high hardness austempered ductile iron (ADI) has established the role of graphite nodule clustering and size in crack initiation. Adaptive numerical modelling, the SUpport vector Parsimonious ANalysis Of VAriance (SUPANOVA) approach, has been applied to image analysis data to identify those features which distinguish between the graphite nodules which initiate fatigue and those that do not. Critical crack initiating features are identified to be a combination of graphite nodule size and local clustering within a mesoscopic region containing a lower volume fraction of graphite nodules. The interpretable features introduced by the SUPANOVA technique allow visualisation of these complicated relationships, whilst maintaining a 72% successful classification rate.

Abstract: This paper aims to highlight developments in a generic computer model, which is capable of predicting the final microstructure, transformation kinetics and mechanical properties of ADI alloys. The overall aim is to enable a reduction in research and development costs and times, hence increasing the speed of new alloy development.

Abstract: This work focuses on the study of the kinetics of solid state transformations of thin wall ductile iron (TWDI). The aim is to identify the effect of the nodule count and the matrix homogeneity on the rate of transformation. The study is carried out on samples taken from sand cast plates of thickness ranging from 1.5 to 4mm, which show nodule counts up to 1700 per square millimetre. The results are compared to those obtained on samples taken from 12.5 mm thick Y-blocks, having approximately 300 nodules per square millimetre. Kinetics of ferritising, austenitising, austempering and cementite dissolution are studied.The results show that solid state transformations involving carbon transport from the matrix to the nodules or vice versa, such as subcritical ferritising and austenitising, show faster kinetics as the nodule count increases. The kinetics of the austempering transformation, which does not involve changes in the carbon content of the matrix, is not affected by the nodule count. The dissolution of ...

Abstract: This study deals with the influence of the austenite grain size and deformation state on the mechanism and kinetics of bainite transformation in several low-alloy steels differing by their C content. Nucleation and growth conditions are thoroughly modified when the austenite grain size becomes of the same order as the length of the individual bainitic ferrite platelets. As a consequence, the kinetics show some peculiarities. The austenite prior deformation also influences in a large way the morphology of the bainite - retained austenitc, microstructure.

Abstract: The present article investigates the stability of the retained austenite, present in austempered ductile iron (ADI) after cooling at sub-zero temperatures, considering that the austenite could transform into martensite when austempered parts are exposed to low temperatures or stresses or strains. Optical microscopy with oblique illumination, X-ray diffraction techniques and microhardness tests were used to analyse the transformation of the austenite on samples with different austempering thermal cycles. The results indicated that the martensitic transformation took place mainly at the unreacted austenite present at the last to freeze areas of samples austenitised and austempered at the highest temperatures. On the other hand, the reacted austenite, present in the bulk of all the investigated samples, remains unchanged after cooling. Tensile tests were performed in order to evaluate the influence of the martensitic transformation, promoted by the sub-zero cooling, on strength and ductility.

Abstract: This work investigates the thermal stability of ausferritic microstructures such as those found in NiCuMo austempered ductile iron (ADI). Typical ADI microstructures consisting of acicular ferrite and high carbon austenite were produced by heat treating NiCuMo ductile iron for 2 h at 315 °C and 370 °C. It was found that low temperature austempering led to the precipitation of a hexagonal epsilon-carbide phase. Hence, differential thermal analysis was used to closely follow the active phase transformations found in the experimental ADI during non-isothermal annealing. From these tests, it was apparent that the ausferrite stability is strongly influenced by the austempering conditions and heating rates. In particular, the experimental outcome shows that the ADI microstructures decompose at temperatures of 428°C and 459°C for irons austempered at 315°C and 370°C, respectively. Further evidence for the decomposition reactions involving acicular ferrite and high carbon austenite was provided by TEM observation...

Abstract: A machinable austempered cast iron article has improved strength, machinability, fatigue performance, and resistance to environmental cracking. A method of making the machinable austempered cast iron article includes austenitizing an iron composition having a substantially pearlitic microstructure in an intercritical temperature range of between 1380°F and 1500 °F. This produces a ferritic plus austenitic microstructure. The ferritic plus austenitic microstructure is quenched into an austempering temperature range of between 575 °F and 750 °F within 3 minutes to prevent formation of pearlite. The ferritic plus austenitic microstructure is then austempered in the austempering temperature range of between 575 °F and 750 °F to produce a microstructure of a continuous matrix of equiaxed ferrite with islands of austenite. Finally, the microstructure of the continuous matrix of equiaxed ferrite with islands of austenite is cooled to ambient temperature to produce the machinable austempered cast iron article.

Abstract: The bonding of an S45C steel inserted into copper during cast welding and heat treatment was examined. The interface shear strength was made with a push-out test. After cast welding, a cast-welding layer formed between steel and copper. After marquenching, martempering and austempering heat treatment, there was a cast-welding layer near the steel matrix, an irregular layer near the copper matrix and between of them was a middle layer. Through X-ray diffraction analysis was used to determine that the interface layer consisted of carbon and CuFeO2. Through electron probe X-ray microanalysis (EPMA), it was showed that iron atoms and carbons, mostly, diffused into the copper matrix. The interface shear strengths of the compound casting while with marquenching, martempering, and austempering were 6.29 MPa, 8.23 MPa, and 8.33 MPa, respectively; The fractured region was all happened near S45C steel matrix in the cast welding layer.

Abstract: Recent advances in the development of high performance steels presenting higher properties of strength and ductility rely on the TRIP effect, i.e. on the mech.-induced martensitic transformation of retained austenite dispersed in a soft ferrite-based matrix. As a consequence, the stabilization and retention of austenite at room temp. have become of primary importance, leading to specifically designed steel grades and thermal or thermomech. treatments. Particularly, carbon enrichment of the austenite during intercrit. annealing and bainite transformation was effective in retaining austenite. This study deals with the influence of chem., geometrical and mech. parameters of the microstructure of low alloy steels on the retention of austenite at room temp. The influence of different alloying elements on the inhibition of cementite pptn. and on the resulting austenite stabilization during the bainite transformation is scrutinised. The influence of the redn. in the austenite grain size on the mechanisms of nucleation and growth of bainite is demonstrated. Finally, the change in bainite morphol. resulting from prior deformation of the austenite is clearly exemplified and explained

Abstract: The influence of intercritical rolling on the microstructural development during the processing of hot rolled multiphase steels is characterised. Intercritical rolling leads to dynamic recovery of ferrite and to strain-induced transformation of austenite into ferrite. The prior deformation of austenite considerably influences the bainite transformation. It is shown that a smaller amount of austenite transforms into bainite when bainite forms from deformed austenite. This is explained by the physics of the bainite transformation.