Abstract: The present study investigated the effect of austenitising temperature (850, 900, and 950°C) and austempering time (0–7 h) on the volume of retained austenite of a 0.3 wt-%Mn ductile iron containing two different levels of silicon, namely 2.02 wt-% and 3.31 wt-%, and austempered at 360°C. The volume fraction of retained austenite and austenite carbon content results were then correlated with microstructural changes and impact toughness results. It is shown that the austenite stability is of great significance with respect to impact toughness, and that ferrite, when present in acicular form, can increase the mechanical stability of the austenite. It is observed that decreasing the austenitising temperature increases the driving force for the stage I transformation reaction in which mother austenite transforms to high carbon austenite plus acicular ferrite. However, the austenitising temperature has only a small effect on the kinetics of the stage II reaction in which high carbon austenite transform...

Abstract: The effect of austenitizing conditions on the microstructure and impact properties of an austempered ductile iron (ADI) containing 1.6% Cu and 1.6% Ni as the main alloying elements was investigated. Impact tests were carried out on samples of initially ferritic matrix structure and which had been first austenitized at 850,900, 950, and 1000°C for 15 to 360 min and austempered at 360°C for 180 min.

Abstract: A ductile iron was austempered at 302 and 385°C for various times to get lower and upper ausferrite microstructures respectively. The microstructures were characterised hy y optical microscopy and X-ra y diffraction. Plane strain fracture toughness was determined under all heat treatment conditions. While the austempered ductile iron with lower ausferrite microstructure showed higher fracture toughness, the one with upper ausferrite microstructure exhibited higher tensile toughness and strain hardening coefficient. A model was developed relating fracture toughness to the yield strength (σ y ) volume fraction of retained austenite (X y ) and the carbon content of the retained austenite (C y ). Experimental results showed excellent agreement with the prediction of the model that K 2 1C is proportional to σ y (X γ C γ ) 1/2 .

Abstract: Manganese is an inexpensive element and a potent hardenability promoter in ductile iron, but in this role it has the inherent limitations of severe segregation and carbide formation tendencies. In order to use this material in an actual application, attempts must be made to overcome these limitations. In the present work, the effect of austempering time and austenitising temperature on the impact strength and abrasive wear resistance of 1.5Mn austempered ductile iron (ADI) after austempering at 270°C is investigated. The results show that impact resistance is affected adversely by untransformed austenite volume but this parameter improves the wear resistance of high Mn ADI.

Abstract: The effect of segregation of alloying elements on the phase transformation of ductile iron during austempering was investigated. Four heats, each containing 0.4%Mn, 1% Cu, 1.5% Ni, or 0.4% Mo (wt%) separately, were melted; then three different sizes of casting bars (3,15, and 75 mm diameter) were poured from each heat. The distribution and the degree of segregation of certain elements were quantitatively analyzed using an electron microprobe. A personal computer (PC)-controlled heat treating system was used to measure electrical resistivity, and the information on resistivity variations was used to analyze the effect of segregation on phase transformations during austempering. Also, Charpy impact and Rockwell hardness tests were performed to determine the effect of segregation on properties.

Abstract: Recent advances in metallurgy of hypereutectoid steels and cast irons show that unique properties, such ultrahigh hardness and strength, and superplasticity, are achievable. This book focuses on the mechanical properties of hypereutectoid steels and cast irons as influenced by thermomechanical processing and microstructure. Some topics covered are: (1) Hot workability of hypereutectoid tool steels; (2) Thermomechanical processing of austempered ductile iron: An overview; (3) Mechanical behavior of ultrahigh strength, ultrahigh carbon steel wire and rod; and (4) Tensile elongation behavior of fine-grained Fe-C alloys at elevated temperatures.

Abstract: Casting size affects the solidification cooling rate and microstructure of casting materials. Graphite nodules existing in the structure of ductile iron are an inherent and inert second phase that cannot be modified in subsequent heat-treatment processing. The matrix and the fineness of the second phase undoubtedly have some impact on the fracture toughness of the as-cast material, as does the subsequent heat treatment, as it alters the microstructure. This research applied austempering heat treatment to ductile iron of different section sizes and graphite nodule finenesses. The influence of these variables on the plane strain fracture toughness (KIC) of the castings so treated was compared to that of the as-cast state. Metallography, scanning electron microscopy (SEM), and X-ray diffraction analysis were performed to correlate the properties attained to the microstructural observation.

Abstract: Multiphase hot-rolled steel exhibiting transformation induced plasticity has a structure comprising ferrite and bainite or a mixture of bainite and martensite, with retained austenite. It contains by weight: 0.05-0.5% carbon, 0.50-2.5% manganese and 0.30-0.80% silicon. Also claimed is preparation of the steel from an ingot of this formula, which is heated at 1150-1300 degrees C for 135-200 minutes, roughly rolled while cooling to between 900 and 1150 degrees C, then finish rolled while cooling to or below the austenite transformation temperature. The resulting steel band is cooled slowly to just above the pearlite formation temperature, then rapidly to below that temperature. It is wound onto a spool below the temperature of bainite formation but above that of martensite formation, thus forming some bainite in the microstructure. Finally it is quenched to stop the bainite formation and prevent precipitation of iron carbide.

Abstract: A transmission electron microscope (TEM) equipped with an energy dispersive spectroscopy (EDS) system was used to study the bainitic reaction in a conventional and a successive austempering process for 1 wt pct Mn ductile iron. In the case of conventional austempering, the specimens were full austenitized at 900 °C and then austempered at 375 °C (high austempering temperature) and 315 °C (low austempering temperature) for different periods. In the case of the successive austempering process, following austempering at 375 °C for different periods, specimens were austempered at 315 °C, and subsequently quenched in ice water. The TEM-EDS study showed that carbide precipitation in the ferritic and retained austenitic component of bainite is a function of the local concentrations of the alloying elements, austempering time, and temperature. After a short time at high austempering temperature, carbide-free bainite forms near graphite nodules. Longer austempering time or lower austempering temperature encourages carbide precipitation in the bainitic ferrite. A long austempering time at high temperature leads to decomposition of retained austenite to ferrite and carbide. A rough inspection shows that the precipitated carbides in the ferritic component of specimens austempered at low temperature lie at an angle of about 40 to 50 deg to the sheaf axis.

Abstract: A method of complete bainite hardening of steel for use in bearings and other load carrying components, wherein bainite transformation is performed at a temperature just above the martensite formation temperature, transforming 25%-99% of the austenite into bainite at said temperature, and then increasing the temperature to speed up the transformation of the remaining austenite into bainite.

Abstract: Austempering kinetic measurements are reported for a compacted graphite iron of composition Fe-3.4C-3.5Si-0.25Mn- 0.5Mo-0.5Ni (wt-%) for an austempering temperature of 375°C and austenitising temperatures of 890, 900, and 940°C. The measurements show that increasing the Si content accelerates the stage I reaction and delays the stage II reaction. An explanation is offeredfor the previous suggestions that increasing the Si content delays the stage I reaction.

Abstract: In the present investigation the influence of the microstructure, obtained after an austempering treatment in a "process window", on the mechanical properties of austempered ductile iron has been investigated. These properties include tensile strength, elongation and hardness. Conversion electron Mossbauer spectra (CEMS) were measured, after heat treatment.

Abstract: : This study describes the resistance to penetration and the damage to austempered ductile iron (ADI) from ballistic impact. The resistance to penetration is determined with an average velocity with a 50% probability for complete penetration, the V-50 ballistic limit. The responses of the ADI material to impact are shown by observations of penetration modes, microstructural changes, and fracture topographies. Mechanical properties and ballistic limits are shown for two variations of the austemper process. ADI targets reveal a capability for multiple impacts without structural failures. Penetration modes include ductile hole growth, radial fracture, petaling, and scabbing. V-50 velocities of ADI with lower values of hardness and strength are equal or greater than the V-50 velocities of ADI with higher values of hardness and strength. Graphite spheroids of this ductile cast iron appear to affect plastic deformation and penetration modes by localizing stresses, microstructural changes, and fracture.

Abstract: A numerical, non steady state microsegregation model is used to predict the extent of solute segregation in a ductile cast iron. The microsegregation of Mn and Mo to intercellular boundaries is sho...

Abstract: The present research focuses on carbon migration during the processes of carburisation (carbon diffusion from graphite particles to the matrix) and graphitisation (carbon diffusion from the matrix to graphite particles) in cast iron with spheroidal (or nodular) and flake graphite, and its effect on the kinetics of solid state transformations.The experimental methodology is based on metallographic observation of a large number of samples corresponding to different stages of the heat treatment cycles of ferritising, austenitising and austempering. This was carried out on pieces of three different sizes of both graphite morphologies. Quantitative metallographic techniques were used to measure the amount of phases present.The transformation kinetics are related to the distances for carbon diffusion, and to the graphite-matrix interface area. Hypotheses on the probable mechanisms of carbon migration, and on the way in which carbon atoms are incorporated into the primary graphite particles are discussed.The res...

Abstract: In order to reduce both manufacturing process and manufacturing cost, a thermomechanical process for plastic die AISI P20 steel has been developed. The P20 steel treated by the thermomechanical process can be used to manufacture plastic dies directly without any subsequent heat treatments. In the present work, the dependence of microstructure upon cooling rate and deformation condition has been investigated for P20 steel with the aim of optimizing the thermomechanical process. The results show that the hardness decreases and eventually reaches a plateau, meanwhile the microstructure changes from sheaf-like bainite to granular bainite with decreasing the cooling rate in both of the recrystallized and unrecrystallized austenite. Austenite deformed at 850°C exhibited mechanical stabilization against sheaf-like bainite transformation during continuous cooling, resulting in the microstructure refinement and higher hardness in P20 steel.

Abstract: Austempering kinetic measurements and mechanical property measurements are presented for a compacted graphite iron of composition Fe-3.4C-3.5Si-0.25Mn-0.50Mo-0.50Ni (wt-%) austempered at 375°C after austenitising at 890 and 940°C. Analysis of the austempering kinetics shows that alloying elements have a similar effect on the processing window as in ductile irons. The mechanical properties show optimum values at austempering times within the processing window. However, the graphite morphology limits the mechanical property enhancement achieved by austempering. Nevertheless, it is possible to double the strength of the as cast compacted graphite iron without loss in ductility.

Abstract: The present work continues research on the special features of formation of granular bainite in steel 20Kh2NACh. A thermokinetic diagram of decomposition of austenite is plotted, and the range of cooling rates in which granular bainite is formed after rolling and after preliminary heating from various temperatures is determined.

Abstract: Mechanical properties of austempered ductile iron (ADI) are mainly controlled by its unique microstructure. The objectives of this paper are to characterize the microstructural morphology and the phase distribution of ADI using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and to determine the mechanism of strengthening and toughening of ADI. The experimental results show that, in the microstructure of ADI composing of upper bainite, retained austenite, graphitic nodule, and a small amount of martensite, the upper bainite is composed of sub-units of ferrite in the shape of "wheat ears" on which the "wheat grains" grow at an angle of about 60 degrees to the long axis of the "wheat ears." The retained austenite is connected with each other in the shape of a continuous net. The wheat-ear like bainite with a homogeneous distribution in the continuous austenite net plays an important role to the strengthening and toughening of ADI. The metastable austenite appears in the shape of a large plate in which the martensite is preferentially formed. The appearance of martensite can be suppressed at the time when retained austenite remains stable, which is of benefit to the continuity and homogeneity of austenite net.