Abstract: Carbon partitioning in untransformed austenite during bainite transformation has been studied using high speed dilatometry. It was found that in specimens partially transformed to bainite, during subsequent quenching to ambient temperature two martensite start temperatures M s can be registered. Because M s depends directly on a carbon content in austenite, the obtained results may indicate that the carbon concentration trapped in films of austenite between parallel subunits of bainitic ferrite is much larger than in the blocks of austenite. It would indicate the necessity of a substantial modification of bainite and martensite regions on the time–temperature–transformation (continuous cooling) diagrams.

Abstract: The effects of heat-treatment conditions on mechanical properties are comprehensively investigated to optimise the industrial process of the 590 MPa grade TRIP steel sheet with the metallurgical understanding. The substantial effect of the thermal conditions are first clarified by laboratory investigation, which includes the effects of annealing conditions, cooling conditions from intercritical temperature to austempering temperature and austempering conditions. The results indicate that the optimum annealing temperature is between 800 and 850 °C and the mechanical properties are hardly influenced by the annealing time between 30 and 120 s at an annealing temperature of 825 °C. It is also suggested that the optimum quenching rate is 45 °C/s to obtain the stable properties of the products and the optimum austempering conditions are 425 °C with over 300 s in case of a constant temperature austempering. Based on the laboratory investigation, mill trial is performed using the NKK No.4-CAL in Fukuyama works. The heat treatment conditions are intentionally varied to examine minutely the stability of the production. The mechanical properties are sensitive to the austempering start temperature, when the austempering temperature is gradually decreased during austempering in the industrial conditions for the stable operation without meanders. Excellent mechanical properties can be obtained by controlling the austempering start temperature between 445 and 460 °C. On the contrary, the properties deteriorate in case of the austempering start temperature over 470 °C although the amount of retained austenite is the same or slightly larger than the material which exhibits excellent properties. This is because the retained austenite is less stable in the high-temperature austempered material caused by less bainite transformation.

Abstract: Austempered Ductile Iron (ADI) results from a specialty heat treatment of ductile cast iron. Strength improvements up to 100% (or more) in combination with excellent toughness can be realized by using this process. Successful production of ADI requires a cooperative effort between the foundry and heat treater. High quality ductile iron is the necessary raw material. The proper heat treatment will then yield the desired mechanical properties.

Abstract: The stress-failure (S-N) curves for ferritic irons, pearlitic irons, and austempered ductile irons (ADIs) have been determined under tension-tension loading with a stress ratio of 0.1. The effects of Ti contents of up to 0.10 wt pct (resulting from the deliberate use of Ti-containing steel scrap) on fatigue behavior were investigated. It was found that ferritic and pearlitic ductile irons can contain up to 0.10 wt pct Ti without any adverse effect on fatigue behavior. In ADIs, fatigue properties deteriorate at such high Ti contents. Tests were also conducted to investigate the effects of microstructural features on fatigue properties. It was found that the effect of the graphite nodule count (the number of graphite particles on a unit area of a polished surface) on the fatigue limit is significant only in ADIs. Scanning electron microscope (SEM) analysis has shown that cracks usually initiate from surface dross-type defects. However, in ADIs, fatigue cracks can also initiate at shrinkage cavities and at surface or subsurface locations. An offset bilinear S-N curve behavior (the linear S-N curve at higher stress levels is separated from the linear S-N curve at lower stress levels) has been observed in ADIs. This is attributed to surface residual compressive stresses, which prohibit fatigue crack initiation from surface positions at lower applied stress levels. In ferritic and pearlitic ductile irons, the offset bilinear S-N curve behavior is not observed because of the rapid relaxation of the residual compressive stresses.

Abstract: Short fatigue crack nuclei in austempered ductile cast iron have been studied using optical microscopy, scanning electron microscopy, atomic force microscopy and X-ray microtomography and by electron backscatter diffraction analysis. Fatigue cracks nucleate at graphite nodules and shrinkage microporosity. The crack nuclei are arrested and retarded by barriers in the microstructure, by either blocking of slip at boundaries or owing to the requirement for tilt and twist of the stage I crystallographic crack at grain boundaries. These observations indicate that both the size of the defects, such as graphite nodules and microporosity, and the size of the prior austenite grains control the largest crack nucleus that can develop, and hence determine the component fatigue limit.

Abstract: Fatigue crack growth (FCG) behavior has been investigated for two different grades of austempered ductile irons (ADIs). These ADIs were produced from an alloyed ductile iron (DI) and heat treated respectively at two austempering temperatures, 300 and 360°C, to generate two different ausferrite microstructures. FCG tests using compact tension (CT) specimens were conducted under load control with three load ratios, R = 0.1, 0.5 and 0.7. The fatigue crack growth rates (FCGRs) of the given ADIs were compared with those of the as-cast DI with a bull's eye microstructure to examine the influence of austempering treatment on the FCG behavior of DI. The FCG behavior for the given materials was found to be dependent on the matrix structure with a demonstration that the as-cast DI had a better FCG resistance than did the ADIs at low Δ K regime and vice versa at high Δ K regime. As for the comparison made between the two ADIs, the one austempered at 360°C exhibited a lower FCG rate as a result of its coarse ausferrite microstructure, higher volume fraction of retained austenite, and greater toughness. The ADIs also demonstrated a load ratio dependence of intrinsic FCGR; that is, the enhancement of the FCGR with an increase in R value could not be rationalized by the crack closure effects.

Abstract: Austempered ductile iron (ADI) has lower density and higher damping capacity compared to steel. To evaluate the applicability of ADI to electric car driving gears, rolling contact fatigue (RCF) tes...

Abstract: The Carbo-Austempering™ process is a high performance steel heat treatment that combines a high carbon bainitic case with either a bainitic or tempered Martensite core to produce a component with an exceptional combination of strength and toughness. This paper will review the Carbo-Austempering™ process and its commercial applications. The properties of Carbo-Austempered™ steel along with the benefits and limitations of the process will also be discussed.

Abstract: A nonisothermal annealing was applied to austempered Ni-Cu-Mo alloyed and unalloyed ductile irons to determine the thermal stability of the ausferritic structure. Differential thermal analysis (DTA) results were used to build the corresponding stability diagrams. The transformation starting temperature of the high carbon austenite was found to be strongly dependent on the austempering temperature, the heating rate, and the chemical composition of the iron. The Ni-Cu-Mo alloying elements and high austempering temperature increased the stability. The transformation of the austenite to ferrite and cementite is achieved via the precipitation of transition carbides identified as silico-carbides of triclinic structure.

Abstract: Impact properties of standard American Society for Testing Materials (ASTM) grades of austempered ductile iron (ADI) were evaluated at subzero temperatures in unnotched and V-notched conditions and compared with ferritic and pearlitic grades of ductile irons (DIs). It was determined that there is a decrease in impact toughness for all ADI grades when there is a decrease in content of retained austenite and a decrease in test temperature, from room temperature (RT) to −60 °C. However, the difference in impact toughness values was not so noticeable for low retained austenite containing grade 5 ADI at both room and subzero temperatures as it was for ADI grade 1. Furthermore, the difference in impact toughness values of V-notched specimens of ADI grades 1 and 5 tested at −40 °C was minimal. The impact behaviors of ADI grade 5 and ferritic DI were found to be more stable than those of ADI grades 1, 2, 3, and 4 and pearlitic DI when the testing temperature was decreased. The impact toughness of ferritic DI was higher than that of ADI grades 1 and 2 at both −40 °C and −60 °C. The impact properties of ADI grades 4 and 5 were found to be higher than that of pearlitic DI at both −40 °C and −60 °C. The scanning electron microscopy (SEM) study of fracture surfaces revealed mixed ductile and quasicleavage rupture morphology types in all ADI samples tested at both −40 °C and −60 °C. With decreasing content of retained austenite and ductility, the number of quasicleavage facets increased from ADI grade 1–5. It was also found that fracture morphology of ADI did not experience significant changes when the testing temperature decreased. Evaluation of the bending angle was used to support impact-testing data. Designers and users of ADI castings may use the data developed in this research as a reference.

Abstract: A computer model has been developed to predict the processing window (austempering window) for austempered ductile iron (ADI). The model is a modification of an existing model for the isothermal decomposition of austenite in bainitic steels. It was calibrated using experimental data from the literature. In order to validate the model, the processing window corresponding to a ductile iron of composition 3.41%C, 2.46%Si, 0.36%Mn, 0.18%Mo, and 0.25%Cu is predicted and compared to experimental data. Computer assisted image analysis was used to investigate the volume fraction of martensite at the lower boundary of the processing window. X-ray diffraction was used to calculate the normalised volume fraction of austenite at the upper boundary of the processing window. The results show that the model successfully predicts the processing window corresponding to the iron investigated in this study.

Abstract: A high strength cold-rolled steel sheet, which simultaneously provides a tensile strength of 70 to 90 kgf/mm2 grade and is easily formed, thereby increasing an impact energy absorption in case of collision and enhancing the safety of automobiles, and its manufacturing method. The high strength cold-rolled steel sheet with excellent formability and weldability, comprises, in terms of percent by weight, 0.15 to 0.25% C, 0.5 to 1.5% Si, 1.0 to 2.0% Mn, 0.25% or less P, 0.020% or less S, 0.015 to 0.050% Al, 0.008 to 0.026% N, balance Fe and incidental impurities while satisfying a condition of 1.2≤ Si[%]+50/8P[%]≤ 2.0. The method comprises the steps of hot rolling a steel having the above composition using a conventional method, cold rolling the hot-rolled steel sheet using a conventional method to form a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature range satisfying a condition of 563+651C[%]+42Si[%]+18Mn[%]≤ annealing temperature [°C]≤ 850; and quenching the continuous annealed steel sheet at a rate of 20 to 100°C/s to an austempering initiating temperature ranging from 400 to 450°C and then cooling to a temperature ranging from 350 to 400°C .

Abstract: The Effects of austempering duration and silicon content on the particle erosion wear resistance of high strength upper bainitic ADI were characterized. The results showed that the amount of retained austenite dominantly affects tensile strength and variation of wear resistance. For a sample with higher silicon content, the austempering duration should be prolonged to ensure complete bainitic transformation. In addition, a tiny increase in carbide formation can certainly play an important role in debasing tensile strength and erosion wear resistance. This results in an alternation in erosion behavior. However, hardness cannot be correlated to wear resistance. Erosion-induced phase transformation of retained austenite phase will eventually produce e-carbide, which thus promotes the erosion wear rate.

Abstract: In this study we deal with the determination of crystallite-size distribution and microstrain measurement in austempered ductile irons (ADI) subjected to cold deformation, by means of x-ray diffraction line broadening. The deformation process imposed on the material yields the formation of microstrain and crystallite size domains within each grain, which are somehow related to the mechanical behavior of the alloy. Three series of samples were cold-worked from 2.5% to 20.0% of thickness reduction in order to determine the domain size and microstrain induced in the material, in terms of the original thickness of the castings and the percentage of cold work. The x-ray diffraction line-broadening effects were analyzed by means of the Warren–Averbach method, which allowed the separation of size and strain parameters. The particle size distribution resulted in an average column length in the range of 15.7–24.9 nm in the ferrite phase, while the austenite phase showed values varying between 13.4 and 36.3 nm. On the other side, the overall root mean square strain varied from 0.000 85 to 0.003 93 for ferrite and from 0.000 65 to 0.004 38 for austenite. In all of the studied cases the average column length decreased with increasing deformation, while the initial thickness of the cast samples did not show any clear correlation with increasing deformation.

Abstract: Studies on austempered nodular cast irons were carried out to establish the optimum isothermic heat treatment at a given chemical composition that rendered the highest fatigue crack propagation resistance. Seven nodular iron chemical compositions with different concentrations of copper, nickel, and or molybdenum were tested at three austempering temperatures achieving ausferritic microstructures. Three-point bend tests for crack growth rates were performed at room temperature in a close loop servo hydraulic machine. Crack opening displacement measurements were performed using a controlled displacement telescope. A simple linear statistical analysis indicated that the lower the austempering isothermal temperature, the higher the fatigue strength of the alloys. Cu and Mo additions along with a good spheroidicity of graphite nodules in the iron favored this effect.

Abstract: A series of studies were carried out regarding ausferrite transformation, the growth of ferrite from austenite, during isothermal holding. The mechanical properties associated with microstructures ...

Abstract: A heat treating technology for improving the antiwear nature and toughness of steel 30CrMnSi includes such steps as carburizing, austempering, cooling to ordinary temp and low-temp tempering.

Abstract: Ausferritic and mixed ausferritic plus tempered martensitic microstructures were produced, by the application of austempering heat treatments to a CuMn ductile iron, in order to evaluate the behaviour of these multi-phase microstructures under contact fatigue service. The mechanical properties of these microstructures were characterized, a set of heat treatments being selected to be applied on the contact fatigue trials. Contact fatigue tests were performed in a twin-disc machine, together with auxiliary techniques, such as video imaging for the contact surfaces observation and ferrographic analysis to study the wear particles released from the same surfaces. High contact pressures were used and artificial indentation defects were generated on the discs surfaces in order to accelerate the contact fatigue damage. Results from this work include the determination of a good correlation between the austempering temperature and the respective contact fatigue life, the identification of two methods to observe the fatigue damage progress with the number of cycles imposed, and finally the comparison between different grades of CuMn austempered ductile iron.

Abstract: This paper introduces the study for austempering of high carbon chromium bearing steels home and abroad. The microstructure and mechanical properties after quenching are analyzed comparably, the advantage and application of austempering in production are summarized. The equipment for austempering in China is introduced.

Abstract: A series of austempering process are used to the clover leaf specimens of the nodular graphite cast iron obtained from general casting conditions The metallographic structures are observed, the mechanical properties are examined, and the influences of different austempering processes on the mechanical properties are analyzed Finally a feasible heat treatment process of austempering is determined

Abstract: This chapter considers a general classification of the products of austenite decomposition and an understanding of the mechanisms that are involved in the conclusion of the brief account of bainite. Bainite is the name given to the microstructural constituent formed in steels by the decomposition of austenite at temperatures above those of martensite formation but below those at which fine pearlite is formed. This transformation is not easy to understand and has given rise to much controversy, of which only an outline account has been given in the chapter. Bainite consists of ferrite plates or laths (subunits) and a dispersion of carbide precipitates. The ferrite plates are in aggregates called sheaves and are separated from each other by regions of retained austenite or, after cooling, by transformed austenite—that is, martensite. The carbide (also called as cementite) is precipitated among the ferrite plates. In lower bainite, the carbide is precipitated inside the ferrite, it takes the form of plate-shaped particles orientated at about 60 to the axis of the ferrite plates. Two micrographs in the chapter are depicted to present the upper and lower bainite, respectively.

Abstract: The kinetics of the austempering transformation in vermicular cast iron with 0.11 wt% Mn has been studied in the temperature range 573K–673K by Mossbauer spectroscopy. The Johnson-Mehl-Avrami approximation was used to analyse the transformation kinetics in stage I. The fitted value of the exponent n (≈ 1.6) indicates a localised nucleation and a phase transformation controlled by an interface reaction. The values of the reaction rate constant k don’t display an Arrhenius-type dependence on austempering temperature, behaviour which proves independent of the Mn content.

Abstract: PURPOSE: A heat treatment method of an engine drive plate is provided to achieve a fine tissue of high durability, and to improve fatigue strength via the generation of high compressive residual stress. CONSTITUTION: In the heat treatment method of a drive plate comprised of an austempering process, a jigtempering process, and an after-treatment process, the jigtempering process maintains a temperature range of 440 to 460deg.C for 3 to 7minutes by heating the drive plate passed the austempering process. The after-treatment process is a shot peening process of the surface of the drive plate.

Abstract: Austempering is a high performance heat treat process that, when applied to ferrous materials, produces components that have properties superior to those processed by conventional means. Austempered materials include: Austempered Ductile Iron (ADI), Austempered Steels, Carbo-Austempered™ Steels, Carbidic Austempered Ductile Iron (CADI). Off-Highway applications of each of these materials will be discussed.