Abstract: The possibility of producing bainite at low temperatures by suppressing transformation using substitutional solutes has been investigated, as an alternative to using large carbon concentrations to achieve the same purpose. It is found that although transformation temperatures can indeed be suppressed in this way, the difference between the bainite and martensite start temperatures diminishes. This, combined with the relatively low carbon concentration of the steels studied, promotes the coarsening of the microstructure via a coalescence of fine bainite plates, which may have detrimental consequences on the properties although this remains to be demonstrated. The study also reveals the need for a better interpretation of the bainite start temperature to cover circumstances where the transformation times are unusually long.

Abstract: An extremely fine α single-phase nanocrystalline microstructure was induced in the dry sliding friction surface layer of 9SiCr steel austempered at low temperature. The mean size of nanograins in the top surface layer is about 3 nm, and such fine nanograins had never hitherto been achieved in near surface severe plastic deformation metals and alloys. Grains in the near surface layers coarsen linearly with increasing depth from the top surface. The retained austenite in the surface layer of the austempered sample was decomposed into α phase owing to the action of the shear strain during the dry sliding friction. In addition, the friction surface microstructure and the wear resistance were compared between samples with low-temperature austempering treatment and quenching plus tempering treatment.

Abstract: The present work has been taken up to study the influence of microstructure on the formation of martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel and 0.3 wt.% molybdenum was subjected to two types of austempering treatments. In the first, called as conventional austempering, the samples were austempered for 2 h at 300, 350 or 400 °C. In the second treatment, called as stepped austempering, the samples were initially austempered at 300 °C for 10, 20, 30, 45 or 60 min. These were subsequently austempered for 2 h at 400 °C. Tensile tests revealed considerable variation in the strain-hardening behaviour of the samples with different heat treatments. In the case of samples subjected to conventional austempering, it was found that strain-hardening exponent increased with increasing austempering temperature. In the case of samples subjected to stepped austempering, increased strain hardening was observed in samples subjected to short periods of first step austempering. Study of the microstructures revealed that increased strain hardening was associated with the formation of strain-induced martensite. There was a greater propensity for the formation of strain-induced martensite in the samples containing more of blocky austenite. Retained austenite in the form of fine films between sheaths of ferrite was relatively more stable. Studies revealed that the morphology, size and carbon content of the retained austenite were important parameters controlling their tendency to transform to martensite.

Abstract: The present work focuses in the study of the effect that a new two-step austempering heat treatment process, developed by Putatunda, has on the mechanical properties with emphasis on the response to the abrasive wear. A ductile cast iron melt was prepared in an industrial facility obtaining both, Y-blocks (ASTM A 897) to be used for the laboratory tests samples, and bucket tips to perform field tests by using a wheel loader. The results show that the two-step austempering process promotes an increase in the amount of retained austenite which in turn improves most of the mechanical properties, such as ultimate stress, yield stress, hardness and impact toughness. Two different abrasion tests were carried out in order to evaluate the material response to different tribosystems. The dry sand-rubber wheel abrasion test (ASTM G 65) was used at the lab, where the results show better wear resistance for the two-stepped ADI in comparison with conventional ADI. On the other hand, the bucket tips tested in a more severe environment, showed an opposite tendency.

Abstract: This study was conducted to compare the hydrogen embrittlement (HE) resistance of austempered 4340 steel with quenched and tempered (Q&T) 4340 steel with an identical yield strength (YS) of 1340 MPa (194 ksi). A baseline comparison showed that the austempered steel with a lower bainite microstructure exhibited higher hardness, tensile strengths, Charpy V-notch (CVN) impact toughness, and ductility at both low 233 K (−40 F) and ambient temperatures, as compared to the Q&T steel with a martensite microstructure. After machining and just prior to testing, subsized CVN specimens and notched bend specimens were immersed in hydrochloric acid-water baths. The HE resistance was higher for the austempered steel than the Q&T steel. No differences in room-temperature CVN energy resulted from hydrogen charging of the austempered and Q&T steels vs their unexposed counterparts. However, in the notched bend specimens, the hydrogen charging caused significant peak load decreases (40 pct) for the Q&T steel, while the austempered steel exhibited only small (6 pct) decreases in peak load. Intergranular (IG) fracture occurred solely in the charged Q&T bend samples, which is further evidence of their embrittlement.

Abstract: The present work was taken up to study the influence of austenitising temperature on the formation of strain-induced martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel, 0.3 wt.% molybdenum and 0.5 wt.% copper was subjected to austempering treatments which consisted of three austenitising temperatures, namely 850, 900 and 950 °C, and three austempering temperatures, namely 300, 350 and 400 °C. Tensile tests were carried out under all the heat-treatment conditions and strain-hardening behaviour was studied by applying Hollomon equation. Microstructures were studied by optical microscopy and X-ray diffraction. It was found that increasing austenitising temperature increased the tendency for the formation of strain-induced martensite at all the austempering temperatures.

Abstract: In the current study, an unalloyed ductile iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.%, and 0.047 Mg wt.% was intercritically austenitized (partially austenitized) in two-phase regions (α + γ) at different temperatures for 20 min and then was quenched into salt bath held at austempering temperature of 365 °C for various times to obtain different ausferrite plus proeutectoid ferrite volume fractions. Fine and coarse dual matrix structures (DMS) were obtained from two different starting conditions. Some specimens were also conventionally austempered from 900 °C for comparison. The results showed that a structure having proeutectoid ferrite plus ausferrite (bainitic ferrite + high-carbon austenite (retained or stabilized austenite)) has been developed. Both of the specimens with ∼75% ausferrite volume fraction (coarse structure) and the specimen with ∼82% ausferrite volume fraction (fine structure) exhibited the best combination of high strength and ductility compared to the pearlitic grades, but their ductility is slightly lower than the ferritic grades. These materials also satisfy the requirements for the strength of the quenched and tempered grades and their ductility is superior to this grade. The correlation between the strain-hardening rates of the various austempered ductile iron (ADI) with DMS and conventionally heat-treated ADI microstructures as a function of strain was conducted by inspection of the respective tensile curves. For this purpose, the Crussard-Jaoul (C-J) analysis was employed. The test results also indicate that strain-hardening behavior of ADI with dual matrix is influenced by the variations in the volume fractions of the phases, and their morphologies, the degree of ausferrite connectivity and the interaction intensities between the carbon atoms and the dislocations in the matrix. The ADI with DMS generally exhibited low strain-hardening rates compared to the conventionally ADI.

Abstract: During conventional austempering austempered ductile iron showed a decrease in fracture toughness with increasing austempering temperature, while the tensile toughness increased. Thus high fracture toughness was associated with low tensile toughness. A two step austempering treatment was then adopted where the samples were first au stem p ered at 3000e for sh ort peri 0 ds varyi ng from 10m in utes to 60 minutes, and then subsequently transferred to a second furnace at 4000e for further austempering for 2 hours. It was found that this resulted in fine ferrite grain size, high carbon content of the retained austenite together with increased stability of the austenite. Under such conditions it was possible to achieve an excellent combination of high fracture toughness and high tensile toughness.

Abstract: Austempering is an important thermal processing operation, where strong and tough bainitic steel is produced in a single heat treatment [1, 2]. During the austempering process, the steel is first austenitized and then cooled rapidly just above the martensite start temperature until bainite nucleates and grows, usually until the transformation stops and then it is cooled to room temperature. Due to the sluggish solid-state transformation kinetics, industrial austempering necessitates isothermal holds of 2–24 h, depending on the size and composition of steel. In contrast to conventional isothermal processing, cyclic thermal processing has been shown to accelerate the kinetics of several phase transformations [3–5], with a significant beneficial impact on productivity and energy consumption of these energy intensive operations. This was attributed to the non-isothermal effects resulting from cyclic treatment. The non-isothermal effect on phase transformations has also been utilized to enhance the productivity of a modern batch annealing operation [6]. In the present work, the effect of cyclic processing on austempering kinetics has been compared with conventional isothermal processing for 1080 steel. Furthermore, the effect of cyclic frequency on the austempering kinetics has been studied. Austempering kinetics experiments were performed on 6 mm diameter cylindrical samples of a 1080 steel using a Gleeble 3500 thermo-mechanical simulator (DSI Poestenkill, NY). A diametrical dilatometer was mounted on the specimen to measure the diameter change during the thermal processing. The austempering experiments were performed in two cycles, where the first cycle provides the same initial microstructure prior to each experiment. After completing the first cycle, the cylindrical specimens were heated to the austenitizing temperature (850 C), held for 5 min and then cooled to different austempering temperatures, where the bainite transformation was monitored for the desired period of time, followed by cooling to room temperature. The cooling rate was sufficiently fast to avoid any transformation occurring before reaching the austempering temperature (Fig. 1). The isothermal experiments were carried out at austempering temperatures of 260 and 300 C, whereas the cyclic experiments were carried out between 260 C and 300 C at two different heating/ cooling rates of 1 and 5 C/min. The percentage of bainitic transformation as a function of time was computed from the dilation curve [2] in conjunction with the microstructural examination by optical microscopy and SEM. The normalized dilatation curves for isothermal austempering at 260, 300 C, and cyclic austempering between 260 C and 300 C for 1 C/min and 5 C/min are shown in Fig. 2. In this figure, only the portion of the curve corresponding to austempering is shown and the dilatation curve has been offset to start at zero. The plateau of the dilatation curve with respect to the time represents the end of the transformation to bainite. It must be noted that the end of the bainite transformation does not necessarily correspond to 100% volume fraction of bainite [1]. Previously, using the same type of Gleeble experiments, it was shown that the end of the transformation corresponds closely to the austenite carbon content corresponding to a T0 equal to the austempering temperature [2, 7]. Furthermore the transformation dilatation at the end of the V. Sista P. Nash (&) Thermal Processing Technology Center, IIT, 10 W 32nd St., Chicago, IL 60616, USA e-mail: philip.nash@iit.edu

Abstract: The temperature for the start of martensite formation (M S ) for three Al-alloyed ductile cast irons containing 0.48, 4.88 and 6.16 wt.%Al were determined by high-speed dilatometry. Microscopic observations were also used to indicate the effects of aluminium addition on the martensite transformation during quenching. Various heat treatment cycles, with different continuous cooling and isothermal heat treatments, were used for the experimental irons to indicate the phase transformation in the solid state. The examination confirmed that an austenite/martensite transformation occurs for irons containing 0.48 and 4.88 wt.%Al. The results indicate that martensitic transformation occurs at about 200 °C for 0.48 wt.%Al and about 190 °C for 4.88 wt.%Al. In contrast, there was no evidence of the martensitic transformation for 6.16 wt.%Al. It was further found that for the 4.88 wt.%Al experimental alloy used, isothermal transformation at 350 °C for different soaking times gave a typical bainitic microstructure that increased with increasing time of austempering.

Abstract: In the present work, an unalloyed ductile cast iron containing 3.50 C wt.%, 2.63 Si wt.%, 0.318 Mn wt.% and 0.047 Mg wt.% was austempered for various times at both the conventional austenitizing temperature (single phase austenitic region) and the different intercritical austenitizing (partially austenitizing) temperature ranges to investigate the effect of parent austenite dispersion at the austenitizing temperature on the nature of austenite decomposition during austempering. A microstructure map was created to illustrate the transformation of products quantitatively as a function of austempering time for a particular austenitizing temperature and austempering time. It was concluded that in addition to proeutectoid and bainitic ferrite, the new ferrite (also called epitaxial ferrite) introduced into the partially austenitized structure during austempering and new ferrite formation and its content was dependent on the scale of the parent austenite dispersion at the austenitizing temperature, var...

Abstract: The study deals with the machinability properties of austempered ductile iron using cubic boron nitride cutting tools. To emphasize the role of the austempering process, ductile iron specimens were first austenitized in salt bath at 900°C for 60 min, after which they were quenched in a salt bath at 250°C and 325°C for 60 min. Machining tests were carried out at various cutting speeds under the constant depth of cut and the feed rate. Tool performance was evaluated based on the workpiece surface roughness and flank wear. The influence of the austempering temperature and cutting speed on the chip form was also studied. The results point out that the lower austempering temperature results in the increase in the cutting forces, while better surface roughness is attained.

Abstract: This study utilized electroless nickel (EN) and cathodic arc deposition (CAD) technologies, with the known advantage of low processing temperature, to treat the austempered ductile iron (ADI) substrate. The eligibility of applying the EN and CrN duplex coatings on ADI, along with the coating properties, such as surface roughness, surface hardness, and coating adhesion were evaluated and analyzed. Moreover, the microstructures of ADI were observed before and after the surface treatment. In addition, polarization tests were carried out to further understand the effect of the coatings on the corrosion resistance of ADI. The results showed that the unique microstructure of ADI did not deteriorate after EN and CAD treatments. Although the use of EN interlayer resulted in the increase of surface roughness, the interlayer evidently improved adhesion and hardness for the CrN arc-coated substrate. With regards to the corrosion resistance, the coated specimens performed better than that of the uncoated one in 3.5 wt.% NaCl aqueous solution. Moreover, the coating with EN interlayer was superior to that without EN interlayer.

Abstract: Austempered Ductile Iron (ADI) can be as twice as strong as standard spheroidal iron at the same level of toughness. It responds to work-hardening surface treatments and exhibits excellent fatigue and wear property. There is extensive work done on the fracture of steel with ferrite or/and austenite structure, but little on fracture behaviour of ADI whose microstructure also comprises austenite and ferrite but with graphite nodules in the matrix. The present work is aimed in this direction. The fracture behavior of Ni-Mo ADI is studied. It is found that the crack always originates from graphite nodules and the matrix affects the propagation path.

Abstract: Summary Theductiletobrittletransitiontemperatureisaveryimportantcriterion that is used for selection of materials in someapplications, especially in low-temperature conditions. Forthat reason, in this paper transition temperature of as-castand austempered copper and copper–nickel alloyed ductileiron (DI) in the temperature interval from −196 to +150 ◦ Chave been investigated. The microstructures of DIs and ADIswere examined by light microscope, whereas the fracturedsurfaceswereobservedbyscanningelectronmicroscope.TheADI materials have higher impact energies compared withDIsinanas−castcondition.Inaddition,thetransitioncurvesforADIsareshiftedtowardslowertemperatures.Thefracturemode of Dls is influenced by a dominantly pearlitic matrix,exhibiting mostly brittle fracture through all temperatures oftesting.Bycontrast,withdecreaseoftemperature,thefracturemode for ADI materials changes gradually from fully ductiletofullybrittle. Introduction Ductile iron (DI) is a type of cast iron with spheroidalgraphite embedded in the metal matrix. After heat treatment(austempering), the metal matrix of DI is transformed intoan ausferrite, a mixture of ausferritic ferrite and carbon-enriched retained austenite (Sidjanin & Smallman, 1992;Elliot, 1997). Materials with this unique microstructure arereferred to as austempered ductile iron (ADI). The ADIpossessesaremarkablecombinationofhighstrength,ductilityandtoughnesstogetherwithgoodwearandfatigueresistance(Harding, 2007). The as–cast DI and ADI are widely used forlarge parts of machinery that work in all weather conditions.Inviewofthat,itisofgreatimportancetoknowthebehaviourof DI and ADI at low temperatures (Riabov

Abstract: This study aims to clarify the influence of additive elements of Ni and Mn on tensile and impact properties of three kinds of spheroidal graphite cast irons (SG irons), which are as cast, annealed and austempered samples. Spheroidal graphite cast irons with Ni (0–4˙5 mass-%) and Mn (0–0˙5 mass-%) melted by a high frequency induction furnace and cast into a Y block CO2 mould with 30 mm in thickness. From the viewpoint of heat treatment, tensile strength and hardness of SG irons become larger in the order of ferritised