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In this study, the decomposition of a martensite/austenite (M/A) microconstituent in bainitic steels was analyzed using differential scanning calorimetry (DSC) data in conjunction with Kissinger's and Johnson–Mehl–Avrami–Kolmogorov (JMAK)'s formulas. In bainitic steel subjected to austempering heat treatment, the presence of an M/A microstructure adversely affects the mechanical properties. According to the kinetic equations derived, it is observed that after tempering the sample at 600 °C for 4000 s, the generation of each phase reaches its maximum. The SEM images taken before and after tempering reveal extensive decomposition of the M/A constituent in the microstructure. The proportion of the M/A microstructure decreased significantly from about 10% before tempering to less than 1% after. Additionally, the content of residual austenite also reduced nearly to zero. These observations are consistent with the predictions of the kinetic equations. [ABSTRACT FROM AUTHOR]

High-silicon cast steel exhibits good ductility and mechanical strength after microstructure transformation through an austempering heat treatment. The combination of these properties is outstanding in other steels, leading to emerging applications for high-silicon cast steel, such as in the railway sector. The Si content above 1.5% is a crucial factor for the formation of refined bainite and a dispersion of carbides in the matrix. This work involves the evaluation of the machinability of a steel alloy with 0.77% C and 1.77% Si using ceramic metal and ceramic tools. The alloy was cast, homogenized, austempered, and tempered. Subsequently, the high-silicon cast steel was characterized using scanning electron microscopy (SEM), hardness, tensile strength, and elongation. Machinability tests were conducted by turning and varying the cutting speed under dry cutting conditions with a constant feed rate. The analyzed output parameters were surface roughness, tool wear, and wear mechanisms. The increase in cutting speed directly influenced tool wear for all tools. The predominant wear mechanisms observed during the tests were abrasion and diffusion. The hard metal tool exhibited chipping on the cutting edge. [ABSTRACT FROM AUTHOR]

Austempered ductile iron is an innovative material obtained by subjecting it to an austempering heat treatment. The wide application of this material is because its properties are comparable to those of steel, which has a much lower weight. However, the machinability of this material is always a difficult task owing to its high hardness. This problem becomes more complex when a considerable amount of Mn is added. In this study, a novel two-step austempering heat treatment was performed on spheroidal graphite iron containing different amounts of manganese. Although the novel heat treatment method helps to obtain a superior combination of hardness and impact, its effect on machinability must be determined. This study provides the results of machinability tests carried out on austempered ductile iron with various manganese contents produced using the novel method. Scanning electron microscope images of the microstructures revealed a typical ausferrite structure with no segregation of manganese at the grain boundary. This is evident from the good tool life obtained in the machinability tests. Regression equations are fitted to determine the tool life and surface roughness for machining parameters with a range of values considered in this study. The results obtained have shown that till 1 wt% of manganese alloyed austempered ductile iron can be successfully produced using the novel heat treatment method. This helps to obtain the optimum combination of strength and impact properties. [ABSTRACT FROM AUTHOR]

A newly developed medium-carbon carbide-free bainitic steel was fabricated for the first time utilizing the laser powder bed fusion (L-PBF) technique. Process parameters were optimized, and a high density of 99.8% was achieved. The impact of austempering heat treatment on the bainite morphology and transformation kinetics was investigated by high-resolution microstructural analysis (SEM, TEM, and EDS) and dilatometric analysis, and results were compared with conventionally produced counterparts. Faster kinetics and finer microstructures in the L-PBF specimens were found as a consequence of the as-built microstructure, characterized by fine grains and high dislocation density. However, a bimodal distribution of bainitic ferrite plate thickness (average value 60 nm and 200 nm, respectively) was found at prior melt pool boundaries resulting from carbon depletion at such sites. [ABSTRACT FROM AUTHOR]

Hot-work tool steels play a crucial role in applications exposed to extreme thermal, mechanical, and chemical stresses and require exceptional properties such as high strength, hardness, wear resistance, and toughness. The latter is crucial to prevent an unexpected tool failure due to the formation and propagation of fatigue cracks in demanding environments. In addition, high thermal conductivity is crucial to prevent overheating of the tool and the resulting degradation of the material. This study focuses on a new generation hot-work tool steel with increased Mo and W contents, which has excellent thermal conductivity but limited toughness, as it contains stable Mo-W carbides that remain stable up to 1100 °C. To improve toughness, an alternative heat-treatment method involving austempering at different temperatures was applied. The investigation begins with the characterisation of the chemical composition of the steel, followed by the determination of the martensite-start (MS) and martensite-finish (Mf) temperatures. Based on the results, the researchers established a set of samples for austempering heat treatment. They investigated the effects of different isothermal holding temperatures on the microstructure of the steel and its subsequent mechanical properties. The results show that reduced bainite formation, achieved by austempering at certain temperatures, led to significantly improved impact toughness and moderate hardness. This study also showed a correlation between the isothermal holding temperature and the extent of martensitic transformation, which affected the microstructure and mechanical properties of the steel. [ABSTRACT FROM AUTHOR]

The present study reveals the microstructural evolution and corresponding mechanisms occurring during different stages of quenching and partitioning (Q&P) conducted on 0.6C-1.5Si steel using in-situ High Energy X-Ray Diffraction (HEXRD) and high-resolution dilatometry methods. The results support that the symmetry of ferrite is not cubic when first formed since it is fully supersaturated with carbon at the early stages of partitioning. Moreover, by increasing partitioning temperature, the dominant carbon source for austenite enrichment changes from ongoing bainitic ferrite transformation during the partitioning stage to initial martensite formed in the quenching stage. At low partitioning temperatures, a bimodal distribution of low- and high-carbon austenite, 0.6 and 1.9 wt.% carbon, is detected. At higher temperatures, a better distribution of carbon occurs, approaching full homogenization. An initial martensite content of around 11.5 wt.% after partitioning at 280 °C via bainitic ferrite transformation results in higher carbon enrichment of austenite and increased retained austenite amount by approximately 4% in comparison with partitioning at 500 °C. In comparison with austempering heat treatment with no prior martensite, the presence of initial martensite in the Q&P microstructure accelerates the subsequent low-temperature bainitic transformation. [ABSTRACT FROM AUTHOR]

Arbomex S.A. de C. V. is one of the largest worldwide manufacturers of ductile cast iron camshafts, produced by means of the phenolic urethane no-bake sand mold casting method and cold box by stack molding technology. As a result of the development of high-strength ADIs, low alloyed with vanadium, for camshaft manufacturing, previous results were published on the as-cast process and the austempering heat treatments applied to the camshafts. In the present work, camshafts of ADIs, low alloyed with 0.2 and 0.3 wt.% V, were produced at austempering temperatures of 265 and 305 °C. The performance of the new camshafts was evaluated by wear testing to ensure the function and durability of the camshafts by means of the block-on-ring wear test and a valve train system to evaluate the volume loss of material removed and the geometrical changes of the camshaft, respectively. The ADIs heat treated to 265 °C showed a microstructure constituted of fine ausferrite that aided in obtaining the highest wear resistance in the block-on ring wear test. No wear or pitting evidence was detected on the camshaft lobes and roller surfaces after the OEM test protocol during the electric spin ring test at low and high conditions for the ADI alloyed with 0.2 wt.% V heat treated at 265 °C. [ABSTRACT FROM AUTHOR]

Conducting the carburizing process on the surface layer of low-carbon steel and subsequent austempering heat treatment can be implemented to obtain nanobainite microstructure on the surface of steels. In this research, steel with 0.23 wt.% carbon was carburized for 3 h at 900 °C in a liquid salt bath containing sodium cyanide, sodium carbonate, and sodium chloride and immediately quenched to room temperature. The samples were then heated to 900 °C for 30 min and isothermally transformed at three different temperatures of 200, 250 and 300 °C for 72, 24 and 12 h, respectively. It was found that nanostructured bainite was formed on the surface layer and the subunits of bainitic ferrite and high-carbon austenite films were almost 60-300 nm thick depending on the heat treatment temperature. It was also found that the samples austempered at these temperatures contained 18, 21 and 28% volume fractions of retained austenite on the surfaces, respectively. Due to the comparable microstructural characteristics, similar friction coefficients were obtained as for ordinary nanostructured bulk bainitic steels with high-carbon content. [ABSTRACT FROM AUTHOR]

The temperature and time of austenitization are of great practical importance during the austempering heat treatment of ductile iron, as these factors influence the transformations in the solid state, modifying the matrix microstructure and the resulting mechanical properties of the material obtained after subsequent austempering. The objective of this research is to investigate the influence of austenitization time on the microstructural characteristics, wear, and corrosion rate of ductile iron subjected to an austenitization heat treatment at 1050 °C for 120, 180, and 240 min, followed by austempering at 350 °C for 60 min. To assess the corrosion and wear resistance of the samples, potentiodynamic tests in a 3.5% NaCl solution and pin-on-disk tribological tests were, respectively, conducted. The results demonstrated that increasing the austenitization time results in ausferritic microstructures with a higher content of carbon-rich austenite accompanied by a higher content of carbon, and lower microstructural heterogeneities due to the last to freeze zones in ductile iron. The material's corrosion and wear resistance improved as the austenitization time was increased. [ABSTRACT FROM AUTHOR]

In this research, ultrasonic melt treatment (UST) was used to produce a new ultrafine grade of spheroidal graphite cast iron (SG iron) and austempered ductile iron (ADI) alloys. Ultrasonic treatment was numerically simulated and evaluated based on acoustic wave streaming. The simulation results revealed that the streaming of the acoustic waves propagated as a stream jet in the molten SG iron along the centerline of the ultrasonic source (sonotrode) with a maximum speed of 0.7 m/s and gradually decreased to zero at the bottom of the mold. The metallographic analysis of the newly developed SG iron alloy showed an extremely ultrafine graphite structure. The graphite nodules' diameter ranging between 6 and 9 µm with total nodule count ranging between 900 to more than 2000 nodules per mm2, this nodule count has never been mentioned in the literature for castings of the same diameter, i.e., 40 mm. In addition, fully ferritic matrix was observed in all UST SG irons. Further austempering heat treatments were performed to produce different austempered ductile iron (ADI) grades with different ausferrite morphologies. The dilatometry studies for the developed ADI alloys showed that the time required for the completion of the ausferrite formation in UST alloys was four times shorter than that required for statically solidified SG irons. SEM micrographs for the ADI alloys showed an extremely fine and short ausferrite structure together with small austenite blocks in the matrix. A dual-phase intercritically austempered ductile iron (IADI) alloy was also produced by applying partial austenitization heat treatment in the intercritical temperature range, where austenite + ferrite + graphite phases coexist. In dual-phase IADI alloy, it was established that introducing free ferrite in the matrix would provide additional refinement for the ausferrite. [ABSTRACT FROM AUTHOR]

The aim of this work was to characterize the microstructure and properties of cast steel after austempering heat treatment, which would make it possible to produce a carbide-free nanobainite. In order to design heat treatment parameters, dilatometric measurements of phase transformation kinetics were performed on cast steel samples. This study made it possible to construct a time-temperature-transformation diagram and to design the heat treatment parameters. Following this, the most suitable heat treatment parameters were chosen. The heat treatment consisted of austenitizing, followed by quenching with an isothermal stop at the temperature at which bainitic transformation occurred, and final quenching to room temperature. Four different durations of austempering were used at the same temperature of 250 °C to obtain a 55, 75, 85 and 100 pct degree of advancement of bainitic transformation. After 55 and 75 pct bainitic transformation, a martensitic transformation occurred in the remaining austenite during quenching to room temperature. In these cases, a partitioning process consisting of additional annealing at 250 °C, was then applied This kind of heat treatment has not been used in cast steel earlier All heat-treated samples were subjected to microscopic studies at various scales of observation with the use of light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The observations revealed that the heat treatment produced a strongly refined multiphase microstructure and revealed its dependence on the austempering duration. The obtained microstructures provided high tensile strength, however toughness strongly depended on applied heat treatment parameters. [ABSTRACT FROM AUTHOR]

Austempered ductile iron (ADI) offers a good combination of mechanical properties with low cost and successfully replaces cast and forged steel in many applications. Manual metal arc welding (MMAW) of such iron causes the formation of martensite in the heat-affected zone and an undesirable growth of iron carbide in the partial fusion zone. Although avoidance of hard products is accomplished at present by using Ni alloys as electrodes, they fail to satisfy the mechanical requirements apart from the high cost of Ni-based electrodes. Furthermore, such electrodes do not respond to austempering heat treatment. In the present study, an attempt has been made to develop a coated electrode with cost-effectiveness and response to austempering heat treatment by adjusting the alloying elements through flux coating on their ability to eliminate free carbides and response austemperability. The microstructures of different weld deposits and at different layers of each weld deposit have been evaluated in as-welded and after austempering treatment using an optical microscope, SEM with EDX. However, it has been possible to develop coated electrodes with shallow Ni content (0.71%), which responded to austempering resulting in a microstructure consisting of graphite nodules in a matrix of lower bainite. Microhardness of weldment (WM) was also carried out, which showed good agreement with microstructure characteristics. [ABSTRACT FROM AUTHOR]

The development of uniform tensile properties in cold-rolled BCC metals is generally achieved by annealing. The final microstructure of these components is typically characterized as having a rather coarse microstructure and therefore lower tensile properties. Cross-rolling offers a great potential of homogenizing the microstructure without losing the benefits of the rolled microstructure and crystallographic texture. The research summarized below examines the effect of cross-rolling and an austempering heat treatment on the mechanical properties of the AISI 8670 steel sheets. Comparison with the conventional production methods was performed to quantify the efficiency of the new deformation and heat treatment approach. The results clearly indicate that the combination of cross-rolling and low temperature austempering generates favorable isotropic mechanical properties by creating uniform strength of α and ε fiber textures and homogeneous lower bainitic microstructure. Increases in UTS and toughness of 40% and 57% respectively were systematically obtained with the new combination of deformation path and heat treatment. [ABSTRACT FROM AUTHOR]

This research evaluates the effect of temperature and time austempering on microstructural characteristics and hardness of ductile iron, validating the results by means of a statistical method for hardness prediction. Ductile iron was subjected to austenitization at 950 °C for 120 min and then to austempering heat treatment in a salt bath at temperatures of 290, 320, 350 and 380 °C for 30, 60, 90 and 120 min. By increasing austempering temperature, a higher content of carbon-rich austenite was obtained, and the morphology of the thin acicular ferrite needles produced at 290 °C turned completely feathery at 350 and 380 °C. A thickening of acicular ferrite needles was also observed as austempering time increased. An inversely proportional behavior of hardness values was thus obtained, which was validated through data analysis, statistical tools and a regression model taking temperature and time austempering as input variables and hardness as the output variable, which achieved a correlation among variables of about 97%. The proposal of a mathematical model for the prediction of hardness in austempered ductile iron represents a numerical approximation which validates the experimental results at 95.20%. [ABSTRACT FROM AUTHOR]

Austempered ductile iron (ADI) has numerous applications because it is lightweight and has excellent mechanical properties. However, although the ADI obtained by the traditional austempering process possesses high strength and hardness, it has low toughness. To obtain superior mechanical properties for ADI, a modified austempering process is adopted. In this study, ADI is produced using a novel modified method with multi-stage austempering heat treatments. Various mechanical properties are determined to assess the relationship between heat treatment parameters and properties. Microstructural analysis is performed using scanning electron microscopy. The ADI produced using the aforementioned austempering process involving heat treatment at 420 °C is found to have the optimum combination of ultimate tensile strength and impact energy. Moreover, a higher nucleation rate is observed for samples supercooled at 260 °C over a short period. [ABSTRACT FROM AUTHOR]

Copyright of Materials & Technologies / Materiali in Tehnologije is the property of Institute of Metals & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Carbo-austempering is a heat treatment in which the surface of a steel is carburized and then quenched and hold at a temperature above the martensite start temperature, Ms, to obtain a bainitic structure in the carburized layer. In this research, two low carbon steels susceptible to carburizing were designed, the steels were named as A1 and A2. As part of the implementation of the carbo-austempering process, the optimal conditions for homogenization, austenization and austempering were established. The developed steels were characterized in each of the stages by optical emission spectrometry (OES), optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers microhardness test (HV). The results allow to understand the influence of chemistry on the control of the microstructures after the isothermal transformation, since often at an industrial level the fabrication of these kind of steels involves continuous cooling transformations and only by means of a correct balance with the heat treatment parameters, optimal microstructures and mechanical properties can be achieved. [ABSTRACT FROM AUTHOR]

In the present study, austempering heat treatment was performed on compacted graphite aluminum cast iron with the chemical composition of 4.8 wt% Al, 3.2 wt% C, 0.81 wt% Ni, 0.37 wt% Mn, and 0.02 wt% Mg. This study aims to investigate the effect of aluminum additions and removal of silicon on the kinetics of austempering transformation of Fe-3.2%C alloy. The cast samples were austenitized at 900 °C for 120 min and the isothermal austempering heat treatment was performed at 200 °C, 300 °C, and 400 °C for 5, 30, 60, 120, and 180 minutes, respectively. The kinetics of this transformation was studied by X-ray diffraction (XRD) analysis. The effect of temperature and time on the microstructure and hardness of the austempered samples was investigated and discussed. The presence of Al was seen to the prolonged form of the carbides from high carbon austenite, and that expanded the process window in the austempering transformation. Besides, the lower bainitic ferrite phase was observed in the austempered samples at 200 °C and 300 °C. Increasing austempering temperature to 400 °C changed the lower bainite to the upper bainite structure. The volume fraction of austenite reached its maximum level (34.6 %) after austempering the samples at 400 °C for 30 minutes. [ABSTRACT FROM AUTHOR]

A V-Nb microalloyed spring steel was investigated through a series of austempering heat treatments. Optimal properties were obtained after austempering at 240 °C for 2 hours because of the good coordination of multiphase martensitic/bainite, filmy retained austenite and fine carbides. The tensile strength was 2239 MPa and the total elongation was 16.1 pct. With an increase in temperature, the matrix changed from martensite to bainite. The volume fraction of retained austenite decreased from 13.7 pct at 240 °C to 9.6 pct at 280 °C and then increased to a maximum of 19.2 pct at 360 °C. The tensile strength remained above 1500 MPa and the impact energy increased to 30 J. MC carbides formed in martensite lathes and ɛ- or θ-type carbides with acicular nanoscale were precipitated in bainite lathes at a low temperature. MC- and M3C-type precipitated in bainite lathes at high temperature. Samples that were subjected to a low isothermal temperature had a high strain-hardening rate before necking, whereas samples at a high isothermal temperature could exert a TRIP effect for a long time. After stretching to fracture, a hierarchical nanotwinned structure and many shear cells formed, which enhanced the austempered sample ductility and toughness at 360 °C. [ABSTRACT FROM AUTHOR]

The current research studied the effects of laser surface hardening treatment on the phase transformation and wear properties of gray cast irons heat treated by austempering or quench-tempering, respectively. Three austempering temperatures of 232 °C, 288 °C, and 343 °C with a constant holding duration of 120 min and three tempering temperatures of 316 °C, 399 °C, and 482 °C with a constant holding duration of 60 min were utilized to prepare austempered and quench-tempered gray cast iron specimens with equivalent macro-hardness values. A ball-on-flat reciprocating wear test configuration was used to investigate the wear resistance of austempered and quench-tempered gray cast iron specimens before and after applying laser surface-hardening treatment. The phase transformation, hardness, mass loss, and worn surfaces were evaluated. There were four zones in the matrix of the laser-hardened austempered gray cast iron. Zone 1 contained ledeburite without the presence of graphite flakes. Zone 2 contained martensite and had a high hardness, which was greater than 67 HRC. Zone 4 was the substrate containing the acicular ferrite and carbon-saturated austenite with a hardness of 41–27 HRC. In Zone 3, the substrate was tempered by the low thermal radiation. For the laser-hardened quench-tempered gray cast iron specimens, three zones were observed beneath the laser-hardened surface. Zone 1 also contained ledeburite, and Zone 2 was full martensite. Zone 3 was the substrate containing the tempered martensite. The tempered martensite became coarse with increasing tempering temperature due to the decomposition of the as-quenched martensite and precipitation of cementite particles. In the wear tests, the gray cast iron specimens without heat treatment had the highest wear loss. The wear performance was improved by applying quench-tempering heat treatment and further enhanced by applying austempering heat treatment. Austempered gray cast iron specimens had lower mass loss than the quench-tempered gray cast iron specimens, which was attributed to the high fracture toughness of acicular ferrite and stable austenite. After utilizing the laser surface hardening treatment, both austempered and quench-tempered gray cast iron specimens had decreased wear loss due to the high surface protection provided by the ledeburitic and martensitic structures with high hardness. In the worn surfaces, it was found that cracks were the dominant wear mechanism. The results of this work have significant value in the future applications of gray cast iron engineering components and provide valuable references for future studies on laser-hardened gray cast iron. [ABSTRACT FROM AUTHOR]

The influence of the hold time of the austempering heat treatment at 280°C on the microstructure and corrosion resistance in NaCl-based media of austempered ductile iron was investigated using X-ray diffraction, micro-hardness measurements, corrosion tests and surface observations. Martensite was only found in the sample which was heat treated for a short period (10 minutes). Corrosion tests revealed that this phase does not play any role in the anodic processes. Numerous small pits were observed in the a-phase which is the precursor sites in all samples (whatever the value of the hold time of the austempering heat treatment). [ABSTRACT FROM AUTHOR]

Copyright of Materials & Technologies / Materiali in Tehnologije is the property of Institute of Metals & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Austempered ductile cast iron (ADI) offers a good combination of high tensile and fatigue strength, good ductility, toughness, wear resistance and damping characteristics, lower density in an economical way. This excellent combination of properties is due to the specific microstructure of ADI; which is composed of spheroidal graphite particles on an ausferritic matrix. The ausferrite consists of acicular ferrite and high carbon retained austenite; which is produced via austempering heat treatment after casting. The alloying additions of Cu or Cu + Ni increases austemperability, which means completely ausferritic structures can be produced on larger cross-sections. In the present study the effect of the alloying additions of Cu and Cu + Ni on mechanical properties and microstructure of ADI was studied. For that purpose, Y-block specimens having a lean composition, 0.8% Cu and 0.8% Cu + 0.4% Ni alloying additions were cast. After austempering treatment, mechanical tests, fractographic and metallographic examinations were performed. The results show that the Cu + Ni alloyed specimen has higher strength and elongation. The lean alloy on the other hand, has the highest nodularity and matrix hardness but the lowest strength and ductility. Those differences in mechanical properties were attributed to the fraction and morphology of the retained austenite regions of the matrix. [ABSTRACT FROM AUTHOR]

The aim of this study is to investigate the effects of austempering and induction hardening on the tensile properties of GGG70 ductile cast iron for camshaft production. For this purpose, camshafts have been produced by sand mould casting method. For nodulizing process, Fe-Si-Mg alloy has been used and Fe-Si-Ba-Ca-Al alloy has been for inoculation process. The casting has been done between 1420-1440 °C and the pouring time was in between 11-13 s. The casted camshafts have been austenitized at two different temperatures and time under controlled furnace atmosphere. The austenitized camshafts have been quenched into the molten salt bath at 320 °C temperature and held 90 min and then cooled in air. By this way, austempering heat treatment has been applied. After that, surface hardening process was conducted by induction hardening machine with medium frequency. Microstructures of camshafts have been examined by optical and mechanical tests (hardness and tensile tests) have been performed. The fracture surfaces of tensile specimens were examined by SEM analysis. Results show that austempering heat treatment increases the tensile strength of camshaft as-cast condition. Tensile strength of the cam shaft increases with increasing austenitizing temperature, time and induction hardening. The highest tensile strength 1285 MPa, has been obtained from the induction hardened camshaft austenitized at 900 °C and 90 min time. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the effect of the austempering treatment applied on ductile graphite cast iron with different chemical composition; microstructure, hardness, tensile strength and wear resistance. For this purpose sand mold casting models of camshafts were prepared and casting simulation was carried out with the NOVACAST program. The casting quality of camshafts and production process was optimized. For nodulizing process, Fe-Si-Mg alloy has been used and Fe-Si-Ba-Ca-Al alloy has been used for inoculation process. The casting has been done 1430 °C and the pouring time in 15 s. In different analyzes, a cast camshaft subjected to a 90 min austenitizing process at 900 °C under a mill vacuum furnace. The austenitized cam shafts have been quenched into the molten salt bath (50% KNO3 + 50% NaNO3) at 260 °C temperature, held for 30, 60, 90, 120, 150, 180 and 210 min and then cooled in air. Microstructures analysis of cam shafts have been examined by optical microscope and scanning electron microscopy (SEM), mechanical tests (hardness, tensile tests and wear tests) have been performed. Results show that austempering heat treatment increases the tensile strength of cam shaft as-cast condition. In this study, the tensile strength values of the austempered structure, camshaft core which has been formed in 120 minute duration which has the combination of excellent strength and ductility, have been brought from the level of 711.08 MPa to 1424.28 MPa. [ABSTRACT FROM AUTHOR]

Austempered ductile iron castings have a wide range of application fields including defense and heavy-duty industries due to their promising mechanical properties. Austempered ductile iron castings could provide excellent combination of high strength, toughness, and wear resistance at the same time. Unlike other spheroidal ductile cast irons, austempered ductile iron castings need to be processed by a special austempering heat treatment which enables the verification of the conformity of the nodularity level after the casting operations. Previous studies have shown that wide area mappings for determining nodularity are crucial to eliminate the representativeness problems. In this sense, some precautions should be taken for the accuracy of results in large scale nodularity examination. In this study, the effect of auto-thresholding and auto-focusing algorithms on the advanced nodularity analysis is investigated. The results suggest that auto-thresholding and auto-focusing methods have significant contributions for determining average nodularity levels and average graphite sizes. [ABSTRACT FROM AUTHOR]

Sound welded joints of ductile iron were first made by developing coated electrodes as well as weld procedure. The weldments were then austenitized at 900 °C for 2-h holding time followed by austempering at 300 and 350 °C for three different holding times (1.5, 2 and 2.5 h) at each austempering temperature. The influence of austempering temperature and time on the microstructural and mechanical properties was studied. Microstructural characterization, phase analysis, microhardness and mechanical properties were performed to understand the effect of austempering heat treatment on microstructure, austempering kinetics and mechanical properties of welded joints. In spite of significant variation in microstructures among all the three zones of DI weldment before austempering, the response of both weld metal and HAZ to austempering heat treatment similar to a base metal was noted. However, a significant variation in matrix structures (bainitic ferrite, retained austenite) as well as graphite nodules among the three zones was observed with changing the austempering temperature and holding time. Also, 100% joint efficiency of the welded joints was achieved at both 300 and 350 °C for 2-h holding time. Hardness and charpy impact toughness mostly depended on the volume fraction of retained austenite and carbon content. Based on the results of tensile and charpy impact testing, 350 °C and 2-h holding time could be considered as an optimum austempering condition for ADI weldment. [ABSTRACT FROM AUTHOR]

Abstract This study determined the influence of discrete laser spot hardening on the scuffing wear and rolling contact fatigue resistance of Cu-Ni austempered ductile iron (ADI) specimens. The frequency of the single laser pulses were such so as to produce patterns of separated spots, adjacent spots or overlapping spots. Lubricated sliding wear tests were carried out to determine the behaviour of ADI under starved lubrication conditions, usually occurring during the start-up of gears. ADI specimens hardened with adjacent and separated laser spots displayed higher resistance to scuffing than the surfaces treated with overlapping laser spots and the control samples (those tested in the as-austempered condition). Results obtained also show that laser hardening increased the average rolling contact fatigue life over the as-austempered ductile iron specimens. This improved performance was attributed to the higher volume of martensite formed and hence higher surface hardness, and also the residual compressive stresses induced in the laser-treated surfaces. This benefit outweighed the negative effect of the slight roughness induced by the laser treatment. Highlights • Overlapping laser spots caused back-tempering. • Scuffing tests showed that adjacent laser spots exhibited the best performance. • Laser hardening improves the rolling contact fatigue resistance of ADI. [ABSTRACT FROM AUTHOR]

A novel isothermal bainitic transformation combined with quenching and partitioning (i.e. 'disturbed' bainitic austempering) process was employed in an Mn–Si–Cr–C low-alloyed steel to reduce the fraction and size of the blocky martensite/austenite (M/A) islands; the M/A islands that are inevitably formed during the conventional bainitic austempering process are coarse and undesirable for property optimization. Owing to a novel heat treatment, an enhanced combination of strength and ductility was achieved (e.g. ultimate strength, ∼1445 MPa; total elongation, ∼28%). A pronounced TRIP effect of the refined retained austenite contributes a lot to the overall improvement. [ABSTRACT FROM AUTHOR]

This study investigates the effect of chemical composition and microsegregation on the bainitic transformation of high Si cast steels at two different austempering temperatures. The advance of the bainite precipitation as a function of time and temperature was monitored through measurements of the amount of retained austenite by X-ray diffraction, the evolution of hardness and microstructural characterization. The results showed that the bainitic transformation starts preferentially at zones with lower contents of alloying elements but later covers the entire matrix. The austempering heat treatments resulted in an apparently homogeneous bainitic microstructure throughout the entire matrix after a short period of time. The results encourage the continuation of the development of high Si steels suitable to produce carbide-free austempered cast parts. [ABSTRACT FROM AUTHOR]

Abstract High tensile strength of ~ 1550 MPa, uniform elongation of ~ 30%, and impact energy of ~ 80 J were achieved in a high-C high-Si bainitic steel, by austempering around the nose-tip temperature of the time–temperature–transformation (TTT) diagram. This bainitic microstructure revealed multiple lengths of nanobainite plates and various sizes and shapes of retained austenite. Its bainite transformation during austempering and microstructure evolution during tensile deformation were discussed in depth. Results demonstrated that large blocks of retained austenite (with inhomogeneous carbon distribution) transformed partly and gradually to multiple martensite grains as strain increases, due to the strong effect of carbon on mechanical stability of retained austenite. Ductile long nanobainite plates (~ 0.13 at% C) suppressed the propagation of microcracks, because of stress relief effect ahead of the current microcracks. [ABSTRACT FROM AUTHOR]

Abstract An inverted multi-step bainitic austempering process is proposed, where a low austempering temperature step is followed by a high temperature counterpart: after austenitization, the specimens were cooled to the first austempering step at 260 °C for 600 s, then were raised to the second step at 360 °C and held for 1800 s. As comparison, the conventional multi-step bainitic austempering processes were carried out: the first step is at 360 °C for 1800 s, while the second step is at 260 °C for 7200 s. Through inverted multi-step bainitic austempering process, the efficacy of reducing blocky austenite is also achieved and is comparable to that obtained by conventional multi-step bainitic austempering process. But the total time used for inverted multi-step bainitic austempering treatment is reduced markedly due to accelerating bainitic ferrite formation. Furthermore, the novel heat treatment can enhance the strength of a Mn-Si-Cr-C low-alloyed steel without sacrificing ductility, which is attributed to refining austenite and bainitic ferrite blocks. Highlights • Inverted multi-step BAT process can reduce the amount of blocky austenite. • The total time used for inverted multi-step BAT treatment is shortened. • Yield strength increases without sacrificing ductility after IBAT treatment. • The increment of yield strength is attributed to refinement of bainitic ferrite block. [ABSTRACT FROM AUTHOR]

The high carbon steels with bainitic constituents present very interesting mechanical characteristics, such as good ductility, high mechanical resistance to fatigue and high fracture toughness. In this work, two cast and hot-formed alloys were studied, varying the silicon contents (1.5% and 2.0%), to verify their effect on the morphology and kinetics of the bainite transformation during austempering heat treatment at 280 °C wth times varying between 1 min and 839h. Temperatures Bs and Ms were determined from thermodynamic simulations. For the morphological and kinetic characterization, the techniques of optical microscopy (OM) and scanning electron (SEM) were used, the mechanical properties were determined by tensile and hardness tests. A higher bainitic ferrite volume fraction was detected for the alloy with higher Si content. The average ultimate tensile stress (UTS) ranged from 1200 to 1600 MPa and the yield stress ranged from 1000 to 1200 MPa, with a mean hardness of 50 HRC for the samples in the bainitic reaction stage. [ABSTRACT FROM AUTHOR]

The multi-step isothermal austempering heat treatment to achieve an ultra-fine bainitic microstructure and maximum volume fraction of bainite was conducted on a steel containing 0.26 wt% carbon. The microstructural and crystallographic characteristics, as well as the mechanical properties and fracture behavior were studied. The results showed that the subsequent austempering heat treatment at a lower temperature, immediately after partial bainite formation at a higher temperature, would replace the coarse austenite/martensite areas with much refined bainite consisting nanoscale plates of bainitic ferrite and filmy austenite which ultimately leads to the refinement of the bainitic microstructure. This microstructural modification, in addition to the increased yield strength, causes a significant increase in the impact fracture toughness of the multi-step austempered steels. The EBSD analysis also showed that the subsequent austempering heat treatment at a lower temperature results in a finer structure of Bain groups and increase in the fraction of high angle grain boundaries leading to higher resistance against crack propagation and subsequently higher impact energy absorption. [ABSTRACT FROM AUTHOR]

In this study, GJS-500-14 solution-strengthened ferritic ductile iron was subjected to successive hot-dip aluminizing (HDA) and austempering. The combination of these processes resulted in the formation of a thin Al 2 O 3 layer on the iron surface, an intermetallic layer beneath the surface, and an ausferritic microstructure in the interior. Scanning electron microscopy-energy dispersive X-ray spectrometry and X-ray diffraction analyses allowed the qualitative identification of the intermetallics formed due to the aluminizing and their transformation after austempering. Corrosion properties of the final product were also evaluated by potentiodynamic polarization and salt spray corrosion tests, and the surfaces and subsurfaces of the coating layer were examined. The results revealed that this combined process improved the corrosion resistance of ductile iron and enhanced its mechanical properties. [ABSTRACT FROM AUTHOR]

Yield strength of nano-bainitic steels is enhanced while retaining significant ductility with a refinement in the bainitic lath thickness and increasing its volume fraction. This study makes an effort to improve toughness of nano-bainite by investigating the effect of content, size and morphology of constituent phases when austempered at 250 °C, 300 °C and 350 °C. Improved impact and fracture toughness at higher austempering temperature has been observed primarily due to higher content of ductile phase austenite in spite of its coarser morphology. Micrographs of the fracture surface show quasi-cleavage fracture for all the conditions. [ABSTRACT FROM AUTHOR]

The effects of austempering temperature on microstructure and surface residual stress of carbidic austempered ductile iron (CADI) grinding balls were systematically investigated in this work. The microstructures were oberserved by optical metallography and analyized by X-ray diffraction. The surface residual stress measured by the cutting method is mainly composed of thermal stress and phase transformation stress. The thermal stress in grinding balls was determined by ANSYS simulation technique, and the surface phase transformation stress was obtained by subtracting the simulated surface thermal stress from the measured surface residual stress. Results show that all microstructures consist of ausferrite, white-bright zones (mixture of martensite and austenite), nodular graphite, and carbides. The distribution of ausferrite shows uniform. With the increase of austempering temperature, the volume fraction and carbon content of austenite increase, whereas the amount of white-bright zone decreases. In addition, the surface residual stress increases with the increase of austempering temperature. Only the tension exists at the austempering temperature of 200 °C, and the pressure exists at the austempering temperature of 220-260 °C. The thermal stress changes from the tension on the inside with the radius of 0-35 mm to the pressure on the outside with the radius of 35-62.5 mm, and the stress balance state presents at the radius of 35 mm. It is also found that the transformation stress is related to the content of carbon-rich austenite, and will reduce by 5.03 MPa accompanied with 1vol.% increase of the austenite. The thermal compressive stress and the transformation tensile stress on the surface both decrease with the increase of the austempering temperature. [ABSTRACT FROM AUTHOR]

A low-carbon 18Mn3Si2CrMo steel was subjected to austempering at different temperatures below M s , and low-temperature bainite was introduced. The multiphase microstructures of martensite, bainite and retained austenite with different volume fractions were obtained, and the relationship among their phase contents and mechanical properties was studied. The 18Mn3Si2CrMo steel with a volume fraction of martensite ( V M ), bainite ( V B ) and retained austenite ( V γ ) of approximately 80%, 16% and 4%, respectively was obtained by austenitizing at 900 °C followed by austempering at 205 °C for 2 h, and then exhibited the optimal mechanical properties with a yield strength of 944 MPa, a tensile strength of 1512 MPa, a total elongation of 15.4% and an impact toughness of 119 J/cm 2 . When the austempering temperature was increased from 205 °C to 330 °C, the yield strength, tensile strength and impact toughness of the steel decreased. The poorest comprehensive mechanical properties were observed after 330 °C of austempering. The introduction of low-temperature bainite enhanced the ductility and toughness of the 18Mn3Si2CrMo steel. [ABSTRACT FROM AUTHOR]

Since the machinability data on grade 3 austempered ductile iron is scarce, this experimental work mainly focuses on the impact of machining parameters on cutting force and surface roughness while turning the above work material with cubic boron nitride and tungsten carbide inserts. Parameters like depth of cut, cutting speed and feed were considered in this study when analyzing the machinability of austempered ductile iron. Austempered ductile iron was turned with CBN and coated WC inserts. The response surface methodology was utilized to design the experiments and optimize the cutting parameters for the work material by each of the above inserts. The cubic boron nitride insert performs well as compared to the coated tungsten carbide for turning the austempered ductile iron and it has been concluded by taking lower force and higher surface finish in to consideration. The optimum parameters for turning austempered ductile iron with the cubic boron nitride insert is as follows: 174 meter/minute cutting speed, 0.102 millimeter/revolution feed and depth of cut of 0.5 millimeter. [ABSTRACT FROM PUBLISHER]

Bilayer Ti/TiN and TiAl/TiAlN coatings were deposited onto austempered ductile iron (ADI) substrates by cathodic arc deposition in an industrial device. Structure and mechanical properties of the coated samples were comparatively examined. Wear behavior of the coated samples was investigated in comparison with uncoated ADI by means of rolling contact fatigue (RCF) tests, performed in a flat washer type testing rig and using lubricated pure rolling conditions. RCF tests results were analyzed using the two-parameter Weibull distribution and the Weibayes method. The results indicate that TiN and TiAlN coatings grew with a cubic-NaCl type structure. The arithmetic average roughness of the coated samples is similar for both coating variants. The surface hardness and residual stresses are higher for the TiAl/TiAlN coated samples. The coating hardness and elastic modulus are also higher for TiAl/TIAlN. The critical loads at massive delamination and the evolution of the friction coefficients are quite similar for both coating variants. Regarding RCF, failures in coated samples were characterized by substrate spalling. No massive delamination was observed in Ti/TiN and TiAl/TiAlN coatings. The statistical analysis indicates that the deposition of Ti/TiN improves noticeably the RCF resistance of ADI while the deposition of TiAl/TiAlN does not produce significant changes. The properties mismatch between substrate and coating seems to play an important role in the RCF behavior of coated samples, since Ti/TiN coatings possess a lower mismatch with respect to ADI substrates as compared to TiAl/TiAlN. [ABSTRACT FROM AUTHOR]

An investigation was carried out to examine the effect of austempering on the microstructure and mechanical properties of nodular cast iron GGG 50 (DIN EN 1563) alloyed with different amount of copper. Optical, scanning electron microscopy and energy dispersive spectroscopy analyses were performed for microstructural characterization. In addition, hardness and tensile tests were carried out for mechanical properties determination. Specimens were austenitized at 900 °C for an hour, then austempered for an hour at 330 °C in salt bath and cooled at a room temperature in air. The results indicated that the addition of Cu to GGG 50 encouraged pearlite formation in the matrix structure. In addition, with the austempering heat treatment, the structure was transformed from ferrite + pearlite into ausferrite and retain austenite. Furthermore, for the alloy with 2 wt% Cu addition, it was noted that the graphite nodules diverged from sphericity and Cu was concentrated around the graphite. After austempering, mechanical properties were significantly improved and the highest mechanical properties were found at 1.5 wt% Cu. [ABSTRACT FROM AUTHOR]

Ductile iron was subjected to a total true strain (φt) of 0.3 either by applying φt in the austenite region or by apportioning it through applying a true strain of 0.2 in the austenite region before quenching to austempering temperature (TA) of 375°C, where a true strain of 0.1 is applied (ausforming). Additionally, two types of matrices were produced in the ductile iron, namely ausferritic and ferritic-ausferritic matrices. The ferrite is introduced to the matrix by intercritical annealing after austenitization. Dilatometric measurements as well as microstructure examination showed a fast ausferrite transformation directly after applying φA and that the introduction of ferrite to the matrix resulted in a remarkable acceleration of the ausferrite formation. The transformation kinetics, microstructure evolution, hardness and compression properties are studied. [ABSTRACT FROM AUTHOR]