Abstract: Microstructural observations and measurements of the retained austenite content, hardness, austenite carbon content and unreacted austenite content are reported as a function of austempering time at 400, 375, 350, and 300°C after austenitising at 920°C for a ductile iron of composition (wt-%) Fe–3·49C–2·33Si–0·42Mn–0·004P–0·016S–0·25Cu–0·23Mo–0·035Mg. Solute profiles are presented which show solute segregation during solidification and that austenitising for 8 h does not remove the segregation. The segregation of Mo and Mn to intercellular areas is shown to result in stage I austempering reactions in the eutectic cell and intercellular region occurring at different austempering times. Evidence is provided of the occurrence of the stage II austempering reaction before the completion of the stage I reaction. The consequence of this sequence of changes is that the processing window is predicted to be closed for austempering temperatures of 400 and 375°C and open for temperatures of 350 and 300°C.MST/3392

Abstract: Transformation behavior and compositional partitioning in TRIP (Transformation Induced Plasticity) steel was investigated by means of microstructural observation and computer modeling. Studies were made on each of three stages of the continuous annealing process applied to TRIP steel. Ortho-equilibrium partitioning of alloying elements of Si and Mn was attained even in short intercritical annealing time. A transformation model, in which transformation is controlled by carbon diffusion, well described the volume fractional change of ferrite and pearlite during the cooling to austempering temperature. Slower cooling rates significantly increased carbon concentration enriched in untransformed austenite and caused pearlite transformation. Ultimate bainite volume fraction obtained by austempering increased with austempering temperature. Analysis with computer modeling revealed that transformation kinetics above 350°C followed a model based on the diffusional mechanism, while it complied with a model based on the displacive mechanism below 350°C.

Abstract: Austempered ductile iron (ADI) finds wide application in the industry because of its high strength and toughness. The QB' process has been developed to produce a fine microstructure with high fracture toughness in ADI. This process involves reaustenitizing a prequenched ductile iron in the (α + γ) temperature range followed by an isothermal treatment in the bainitic transformation tem-perature range. In the present work, the effect of holding time in the (α + γ) temperature range on the structure and un-notched toughness of ADI has been studied. Prior to the austempering treatment, the as-cast ductile iron was heat treated to obtain martensitic, ferritic, and pearlitic matrix structures. In the case of prequenched material (martensitic matrix), the un-notched impact toughness increased as a function of holding time in the (α + γ) temperature range. The reaustenitization heat treatment also resulted in the precipitation of fine carbide particles, identified as (Fe,Cr,Mn)3C. It was shown that the increase in holding time in the (α + γ) temperature range leads to a reduction in the number of carbide particles. In the case of a ferritic prior structure, a long duration hold in the (α + γ) temperature range resulted in the coarsening of the structure with a marginal increase in the tough-ness. In the case of a pearlitic prior structure, the toughness increased with holding time. This was attributed to the decomposition of the relatively stable carbide around the eutectic cell boundary with longer holding times.

Abstract: Austempered ductile iron (ADI) has excellent mechanical properties, but its Young's modulus is low. Austempered spheroidal graphite cast steel (AGS) has been developed in order to obtain a new material with superior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI and twice the Charpy absorbed energy of ADI. The impact properties and the elongation are enhanced with increasing volume fraction of carbon-enriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minutes in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 pct Si alloy. The X-ray diffraction analysis shows that appropriate quantity of silicon retards the decomposition of the carbon-enriched retained austenite. For austempering at 570 K, the amount of the carbon-enriched austenite decreases and the ferrite is supersaturated with carbon, resulting in high tensile strength but low toughness.

Abstract: A process for austempering ductile iron includes austenitizing a ductile iron casting of low alloy content followed by quenching the workpiece for a controlled period of time in a quench medium such as water, an aqueous polymer solution or a medium speed quench oil. The workpiece is then austempered in an air tempering furnace, resulting in a ausferrite microstructure essentially free of pearlite and martensite, and with mechanical properties meeting ASTM designation A897-90 "Standard Specification for Austempered Ductile Iron Castings." The process eliminates the need for a molten salt bath for quenching and tempering.

Abstract: Ultimate tensile strength, 0·2% proof strength, elongation, and impact energy measurements are reported for an alloyed ductile iron of composition (wt-%) Fe–3·49C–2·33Si–0·42Mn–0·25Cu–0·23Mo–0·035Mg for austempering temperatures of 400, 375, and 350°C and a range of austempering times after austenitising at 920°C for 120 min. The ADI ASTM A897M:1990 standard is satisfied for an austempering temperature of 350°C but not at 375 or 400°C. This behaviour is discussed in terms of the influence of the unreacted austenite volume from the stage I austenitising reaction and the carbide product of the stage II austenitising reaction on the ductility. The present findings are predicted by the processing windows determined from the austempering kinetics.MST/3393

Abstract: The effect of different contents of silicon on the transformation kinetics of austenite in a ductile cast iron austempered at 400°C has been investigated. The amount of silicon varied from 1·55–3·48 wt-%. The number of nodules/mm2 and the carbon content in the original austenite Cγ°, were similar for all of the chemical compositions studied. The effect of silicon on the rate of formation of bainitic ferrite, the product morphology, and the segregation was studied. The transformation of austenite occurs in two stages: in the first stage a bainitic ferrite and retained austenite microstructure forms; in the second stage austenite decomposes, increasing the volume fraction of ferrite and carbide. It was observed that increasing silicon content greatly increased the time for completion of the first stage; the volume fraction of bainitic ferrite decreased with increasing silicon content, whereas the volume fraction of retained austenite increased slightly. The morphology of bainitic ferrite did not cha...

Abstract: Crystallographic properties of an austempered ductile iron (ADI) were studied by using neutron diffraction. A quantitative phase analysis based on Rietveld refinements revealed three component phases, α-Fe (ferrite), γ-Fe (austenite), and graphite precipitate, with weight fractions of 66.0, 31.5, and 2.5 pct, respectively. The ferrite phases of the samples were found to be tetragonal,14/mmm, with ac/a ratio of about 0.993, which is very close to the body-centered cubic (bcc) structure. The austenite phase had C atoms occupying the octahedral site of the face-centered cubic (fcc) unit cell with about 8 pct occupancy ratio. A strong microstrain broadening was observed for the two Fe phases of the samples. The particle sizes of the acicular ferrite phase were studied by using small angle neutron scattering. The analysis suggested a mean rod diameter of 700 A. The scattering invariant predicts a ferrite volume fraction consistent with the powder diffraction analysis. A textbook case of nodular graphite segregation, with average diameters ranging from 10 to 20 μm, was observed by optical micrography.

Abstract: The microstructure of austempered ductile iron (ADI) was investigated using conventional light microscopy as well as scanning and transmission electron microscopy. The microstructural variations arising from variations of composition and austempering processing parameters are discussed. Specimens with a microstructure consisting of carbide-free bainitic ferrite and retained austenite show promising mechanical properties.

Abstract: SAE 9260 type steels have silicon and carbon contents similar to those of the ductile iron matrix, and present a bainitic transformation with the same characteristics as ADI (Austempered Ductile Iron). The hypothesis is that excellent mechanical properties can be obtained by means of austempering (in times so short as to be accessible from the industrial point of view), the same as in ADI and even better because it is a rolling material instead of a cast material. It will be compared with the mechanical properties obtained by quenching and tempering at different temperatures.

Abstract: Availability of two specific austempering treatments were tested for the improvement of drilling machinability of austempered ductile cast iron (ADI). The one is called successive austempering process, in which the upper bainite transformation was immediately followed by the lower bainite transformation by a successive isothermal heat treatment at two temperature levels. The other is the austempering with pre-heat treatment. The samples were firstly heated just below Al temperature and then austempered. With the successive austempering the volume of untransformed austenite (UAV) was reduced to 0 - 2.3% from 8.3% of ordinary austempered iron, and the borehole number was increased by 20 - 30 times in drilling test with high speed steel tool drills of 6mm dia. The second treatment increased the borehole number by about 6 times of ordinary austempered iron. The decrease in UAV raised the elongation and almost unchanged the strength of ADI.

Abstract: PURPOSE: To provide a spheroidal graphite case iron having excellent vibration attenuatability and a process for producing this cast iron. CONSTITUTION: The spheroidal graphite cast iron after casting is subjected to repetitively at least >=2 times annealing heat treatments, A1 to A4, for cooling the cast iron down to an eutectic transformation point or below after heating and to hold to and at 750 to 1100 deg.C. Base tissue adjusting heat treatments B1, B2, such as annealing and austempering, are executable after the final annealing heat treatment or in the cooling process thereof. The outer peripheral surfaces of the spheroidal graphite of the spheroidal graphite cast iron obtd. by such heat treatments are formed to rugged shapes; in addition, many fine gap parts are formed in the inner and outer peripheral parts of the graphite and the vibration attenuability is improved.

Abstract: PURPOSE: To produce a spheroidal graphite cast iron having high strength and high toughness. CONSTITUTION: A mixed structure of ausferritic structure and retained austenite is selectively formed by 10-60vol.% around spheroidal graphite and also this mixed structure of the ausferritic structure and the retained austenite is formed by <=5vol.% in the intermediate region of the mutually separated spheroidal graphite, and the balance consists essentially of ferritic structure. The formation of crack originated from graphite can be practically prevented, and also the propagation of crack in the intermediate region of the mutually separated spheroidal graphite cast iron can be practically prevented. This structure can be obtained by subjecting a spheroidal graphite cast iron having a matrix structure containing ferritic structure and <=5vol.% pearlitic structure to temp. rise at a rate of <=16.7K/sec and to holding at 1153-1373K to selectively austenitize the vicinity of the spheroidal graphite and then subjecting the above 10-60vol.% austenitic structure around the spheroidal graphite cast iron to isothermal transformation by austempering.

Abstract: PURPOSE: To provide a weather resistant iron casting with which the hand intrinsic to the iron casting is exposed and maintained, generation of red rust is suppressed and easy maintenance is possible and a process for producing the casting CONSTITUTION: Pig iron and steel scrap are used as main raw materials An Fe-Si alloy, graphite, etc, are added thereto and the mixture is melted by heating The molten metal is subjected to a spheroidization treatment and is then subjected to an inoculation treatment with an Fe-Si alloy just prior to casting The molten metal is cast into a casting mold for a bolt of M20×90 to be used for fastening of cast iron products to be exposed outdoors, by which the molten metal is molded to the spheroidal graphite cast iron Such bolt is heated up to an austenitization temp of 1130K by using a batch furnace and is held for 05 to 1 hour to austenitize the entire part The bolt is thereafter immersed for 05 to 1 hour in a salt bath furnace consisting of 50% potassium nitrate and 50% sodium nitrate kept at 620K temp and is pulled up The bolt is then cooled with air and is subjected to an austempering treatment The bolt is immersed for about two months in an aq 5% iron sulfate soln so as to be forcibly corroded, by which the mixed layer of the product of corrosion and acicular metal is formed on the surface of the bainite structure

Abstract: Austempered ductile iron (ADI) possesses a superior toughness than ordinary cast iron because the residual phase associated with bainitic ferrite is retained aust-enite instead of martensite or carbide. The desired micro- structure (devoid of pearlite, martensite or carbide) can be obtained provided the effective cooling rates at diff-erent cross-sections are accurately determinable for components with varying shape, composition and heat treatment requirements. In this study, a finite element model (FEM) has been developed to predict the temperature profile along different cross-sections following austeni-tizing at 870°C and austempering at 370°C. Solid bodies of different shapes have been meshed into hexahedron ele-ments by the present software. The o verall global matrix equation has been numerically solved by the Gaussian elimination method. Finally, it is found that the results predicted by the present model is in well accordance with the relevant experimental data on austemperability.

Abstract: A method for producing a brake component involves providing a cast gray iron rotatable brake component where the gray iron has a carbon content between 3.4 % and 4.0 %. The brake component is subjected to an austempering heat treatment process. Then it is subjected to a re-tempering process to provide a microstructure which consists of spheroidized pearlite carbon in a matrix of bainitic and austenitic ferrite.

Abstract: X-ray diffraction, optical microscopy, and hardness measurements were used to determine the austempering kinetics of an alloyed ductile iron of composition (wt-%) Fe-3·49C-2·33Si-0·42Mn-0·25Cu-0·23Mo-0·035Mg at austempering temperatures of 300, 350, 375, and 400°C and austenitising temperatures of 870 and 920°C. The stage I reaction during austempering occurs in two steps, the first in the eutectic cell and the second in the intercellular area. Decreasing the austenitising temperature is shown to increase the driving force for the stage I reaction but to have a lesser effect on the stage II reaction. Decreasing the austenitising temperature produces a more uniform austempered microstructure and reduces the amount of martensite in this structure. These changes move the processing window to shorter austempering times and increase the temperature at which the processing window closes.MST/3390