Abstract: The focus of this investigation was to examine the influence of intercritical austempering process on the microstructure and mechanical properties of low-alloyed austempered ductile cast iron (ADI). The investigation also examined the influence of intercritical austempering process on the plane strain fracture toughness of the material. The effect of both austenitization and austempering temperature on the microstructure and mechanical properties was examined. The microstructural analysis was carried out using optical microscopy, scanning electron microscopy and X-ray diffraction. The test results indicate that by intercritical austempering it is possible to produce proeutectoid ferrite in the matrix microstructure. Lower austenitizing temperature produces more proeutectoid ferrite in the matrix. Furthermore, the yield, tensile strength and the fracture toughness of the ADI decreases with decrease in austenitizing temperature. A considerable increase in ductility was observed in the samples with higher proeutectoid ferrite content. The fracture surfaces of the ADI samples revealed that dimple ductile fracture produced higher fracture toughness of 60±5 MPa√m in this intercritically austempered ADI.

Abstract: A medium carbon Mn-Si-Cr alloyed steel was treated by a novel bainite-based quenching and partitioning (BQP uniform elongation and total elongation: 26.2% and 31.8%; the reduction of area: 47.9%). Besides the transformation-induced plasticity effect of the retained austenite and the composite effect of the multiphase after BQ&P treatment, the formation of carbide free bainite also plays a significant role on the enhanced mechanical properties. The carbide-free bainite could improve the damage resistance of the multiphase due to the additional strain-hardening capacity within the local plasticity deformation zone near the tip of micro-cracks. In this case, the fraction and distribution of CFB should be controlled properly and the macrosegregation should be avoided.

Abstract: It is crucial to eliminate the thermally and mechanically less stable austenite blocks as much as possible if enhanced mechanical properties are demanded in high performance nanostructured bainitic steels. Step-austempering would be an efficient procedure in this regard. This article aims to investigate the strength-ductility- impact toughness combinations in nano-bainite after two-step austempering process comparing with those obtained after conventional isothermal bainite transformation. It has been shown that large austenite blocks further decomposed to bainite after step-austempering process which in turn decreased the average volume fraction of austenite, increased its mechanical stability and generally refined the final microstructure. Step-austempering increased the hardness value and enhanced the yield strength and ultimate tensile strength properties. Additionally, the higher mechanical stability of retained austenite in step-austempered samples increased the elongation level and improved the impact toughness at earlier stages of the second step of transformation. The important point is, it has been found that ductility and toughness properties were influenced by transformation time at the second stage of austempering which approved the idea that not only the mechanical stability and morphology of retained austenite but also its volume fraction must be taken into account when applying a multi-step austempering heat treatment in nanobain steels.

Abstract: A low-carbon Mn-Si-Cr-Mo alloyed steel was treated by two different heat treatment routes, namely bainite-based quenching plus tempering (BQ-T) and bainite-based quenching- partitioning- tempering (BQ-P-T). The strength, ductility and toughness were enhanced concurrently after BQ-P-T treatment (i.e., ultimate tensile strength: 1416 MPa, the PSE: ~ 25.5 GPa%, the CVN impact energy at 20 °C and − 40 °C: ~ 95 J cm −2 and ~ 45 J cm −2 , respectively). These enhanced mechanical properties were attributed to the refined ductile bainite/martensite and the filmy retained austenite multiphase microstructure. The microstructural characterization were carried out by conducting scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, transmission electron microscopy and dilatometry. The carbon partitioning between bainite/martensite and austenite can not only stabilize the austenite films but also hinder the coalescence of the bainitic plates and promote the formation of ultrafine bainitic plates. Besides, both the carbon partitioning and the enhanced tempering of martensite/bainite in the BQ-P-T condition contribute to generating the ductile bainite/martensite. Finally, the microstructural factors in controlling the strength, ductility as well as impact toughness were investigated through the analyses of the fracture surface morphology and the retained austenite evolution beneath the fracture surface.

Abstract: In the present work, Quenching and Partitioning (Q&P) heat treatments were carried out in a quench dilatometer on a 0.2 wt% carbon steel. The microstructure evolution of the Q&P steels was characterized using dilatometry, SEM, EBSD and XRD. The martensitic transformation profile was analyzed in order to estimate the fraction of martensite formed at a given temperature below the martensite start temperature Ms. Q&P was shown to be an effective way to stabilize retained austenite at room temperature. However, the measured austenite fractions after Q&P treatments showed significant differences when compared to the calculated values considering ideal partitioning conditions. Indeed, the measured austenite fractions were found to be less sensitive to the quench temperature and were never larger than the ideal predicted maximum fraction. Competitive reactions such as austenite decomposition into bainite and carbide precipitation were found to occur in the present work. Furthermore, a broad range of mechanical properties was obtained when varying the quenching temperatures and partitioning times. The direct contributions between Q&P microstructural constituents -such as retained austenite as well as tempered/fresh martensite- and resulting mechanical properties were scrutinized. This was critically discussed and compared to quenching and austempering (QAT) which is a more conventional processing route of stabilizing retained austenite at room temperature. Finally, Q&P steels were shown to exhibit an interesting balance between strength and ductility. The achievement of this interesting combination of mechanical properties was reached for much shorter processing times compared to QAT steels.

Abstract: This paper presents two high-strength steels composed of granular bainite microstructure that have higher extrinsic resistance to hydrogen embrittlement (HE) than conventional tempered-martensite steel, because the granular bainite steel traps less hydrogen than tempered-martensite steel. However, the granular bainite steels had lower intrinsic HE resistance than tempered-martensite steel because hydrogen in granular bainite steels becomes concentrated at microstructures composed of martensite islands and retained austenite. The granular bainite steels with less sulfur showed better HE properties, because S degrades grain-boundary strength. Since the extrinsic properties correspond to the HE resistance that is relevant in industry, granular bainite steels are suitable for applications in hydrogen environment.

Abstract: The interest in austempered ductile irons (ADI) is continuously increasing due to their various advantageous properties over conventional ductile irons and some steels. This study aimed to determine the roles of alloying elements Ni, Cu, and Mo, on the austemperability of GGG-60 ductile cast iron. Two different sets of GGG-60 (EN-GJS-600-3) samples, one set alloyed with Ni and Cu and the other set alloyed with Mo, Ni, and Cu, were subjected to austempering treatments at 290 °C, 320 °C, and 350 °C. A custom design heat treatment setup, consisting of two units with the top unit (furnace) serving for austenitizing and the 200 L capacity bottom unit (stirred NaNO2-KNO3 salt bath) serving for isothermal treatment, was used for the experiments. It was found that austempering treatment at 290 °C increased the hardness of the Ni-Cu alloyed GGG-60 sample by about 44% without causing a loss in its ductility. In the case of the Mo-Ni-Cu alloyed sample, the increase in hardness due to austempering reached to almost 80% at the same temperature while some ductility was lost. Here, the microstructural investigation and mechanical testing results of the austempered samples are presented and the role of alloying elements (Mo, Ni, and Cu) on the austemperability of GGG-60 is discussed.

Abstract: Bearing steel 100CrMnSi6-4 and tool steel C105U were used to carry out this research with the steels being austempered to obtain a martensitic-bainitic structure. During the process quite a large number of acoustic emissions (AE) were observed. These signals were then analysed using neural networks resulting in the identification of three groups of events of: high, medium and low energy and in addition their spectral characteristics were plotted. The results were presented in the form of diagrams of AE incidence as a function of time. It was demonstrated that complex transformations of austenite into martensite and bainite occurred when austempering bearing steel at 160 °C and tool steel at 130 °C respectively. The selected temperatures of isothermal quenching of the tested steels were within the area near to MS temperature, which affected the complex course of phase transition. The high activity of AE is a typical occurrence for martensitic transformation and this is the transformation mechanism that induces the generation of AE signals of higher energy in the first stage of transition. In the second stage of transformation, the initially nucleated martensite accelerates the occurrence of the next bainitic transformation.

Abstract: High manganese austenitic steel, popularly called “Hadfield steel” has dominated and played significant role in wear applications, especially in the mines and minerals industries since its invention over a century ago. A review on the researches on this steel revealed that its prominence in these fields is mainly due to its good combination of impact and abrasion wear resistance arising from its high toughness and high hardness respectively. Its strain hardening ability under impact loading is evidenced by increase in hardness as the material work hardens; this lowers the amount of wear in service. The work hardening property of the steel has been linked to governing mechanisms such as dislocation, deformation twinning, and dynamic strain ageing; also, it is enhanced by increase in carbon, ageing temperature and reduction in manganese content. Carbide precipitation along the grain boundaries and within the grains is the major cause of embrittlement of the steel. These carbides together with voids and porosities during casting solidification, improper heat treatment, overheating during welding, use of unsorted scrap metal and wrong wear application have been identified as the causes of premature failure in service. Hardfacing method has been proposed as a means of substituting the steel in wear applications, as alternative wear materials such as white cast iron and austempered ductile iron lack the combination of impact and abrasion resistance being offered by the Hadfield steel.

Abstract: Austempered ductile iron (ADI) is produced by an isothermal heat treatment. Tempering is an effective method to increase the toughness and decrease the hardness of ADI. In the present research, the transformation of ADI was investigated after applying various tempering temperatures. The hardness of ADI samples with and without tempering was measured and the microstructure of ADI samples was analyzed by using metallographic optical microscopy. It was found that the ausferrite decomposed into dispersive cementite particles above a tempering temperature of 538 °C. Thus, the tempering process for ADI must be carefully selected so that the excellent properties of ADI are not degraded.

Abstract: The relationship between the mechanical properties and the fatigue limit (FL) of medium-carbon steels with various microstructures and tensile properties was investigated through measurement of their hardness, tensile properties, and high-cycle fatigue resistance after subjecting them to heat treatment or prestraining. Carbon steels with 0.30 wt% C and 0.55 wt% C subjected to austempering and quenching-tempering treatments underwent microstructure variation from ferrite-pearlite to binate and to tempered martensite, respectively. Cold rolling of austenitic high-Mn steels with 0.57 wt% C caused an increase in their tensile strength owing to strain hardening. Fatigue tests of these materials showed that the FL increased linearly with increasing hardness (HV) of the material irrespective of the microstructure; this relationship can be expressed as FL=1.54·HV+189. In addition, the relationship between the FL and the ultimate tensile strength (UTS) can be expressed as FL=0.55·UTS+134. Application of additional fatigue test results of high-carbon (0.86 wt%) steel and 151 data points extracted from the Fatigue Data Handbook to these HV- or UTS-based FL prediction models confirmed the high reliability of these models, with good agreement between the experimental and predicted FL values.

Abstract: Some results of materials characterization activities, dedicated to classical and notch mechanics fatigue and elastoplastic properties, have already been published for some Ferritic–Pearlitic Ductile Iron, including the patented heat treated Isothermed (IDI) and Austempered Ductile Iron (ADI) grades. Others have not yet been published. The possible use of all of these results in new standards is discussed in this paper. It is proposed that new standards should provide a criterion that is able to measure the process quality that represents more accurately the actual market needs and manufacturing capabilities. Classification of grades, considered by existing standards, is based on minimum properties for strength and ductility parameters that are separately evaluated. A different approach that is based on a quality index, which considers strength and ductility all in one, is proposed. However, this new proposed approach may not be sufficient to provide a satisfactory classification for the ADIs. This is because their fracture mechanical behavior and machinability can be correlated with their austenite stability. It could also be insufficient for the classification of the recent High Silicon Solid Solution Strengthened Ductile Irons that exhibit a decreasing ultimate tensile strength/proof stress ratio with increasing Si. For construction steels, fracture mechanics properties are sometimes believed to be related to the Charpy impact energy. This paper introduces an innovative practical and inexpensive data analysis, performed on the tensile test curve, which appears to be a potential estimator of fracture mechanical properties, at least for ADIs, where said properties could be correlated with the austenite stability.

Abstract: This study investigates the effect of discrete spot laser surface hardening (LSH) on austempered ductile iron (ADI) specimens. ADI surfaces were irradiated with single laser pulses, having different beam powers and diameters with the intent of establishing the parameters that yield a surface with the best combination of maximum hardness and hardened depth, whilst exhibiting minimal distortion and no cracking. A process parameter map was plotted and showed that to produce a hardened microstructure without melting and cracking, one must operate within a very small range of parameters achieving surface hardness values between 700 and 800 HV and depths of approximately 150 μm. The resulting microstructure consisted of martensite with unaltered graphite nodules. After laser hardening, the mean surface roughness increased from 0.15 to 0.43 μm. When applying laser powers higher than 780 W, melting was observed within the laser spot. Laser surface melting (LSM) resulted in a severely distorted surface, with an increase in the mean surface roughness from 0.15 to 1.34 μm. Within the melted zone, transverse cracks and the dissolution of the graphite nodules could be observed. The rapid self-quenching associated with laser melting resulted in an austenitic microstructure having a surface hardness of approximately 400 HV. A martensitic structure with hardness values peaking around 700–800 HV, was observed at a depth 250–300 μm below the molten structure.

Abstract: This work studied the microstructural characterization and mechanical behavior of low manganese Austempered Ductile Iron (ADI), with a view to improve the properties of iron and to increase the areas of applications. Three sets of ductile iron of specified composition were machined from Y-blocks to tensile and hardness pieces. The samples were preheated at 350 0C for 1hr and austenitised at 900 0C for 1hr in salt bath furnace. The three sets of samples were immediately austempered in the austempering salt bath furnace at uniform austempering temperatures of 300 0C, 350 0C and 400 0C for 90, 120 and 150 minutes; each sample for each temperature window. All sets were prepared for metallographic examination; tensile and hardness tests were carried out. The results showed that maximum hardness, tensile and yield strength were obtained at austempering temperature of 350 0C and at 150 minutes. At 300 0C and 350 0C, it was noticed that the hardness and strength increase with austempering time. The optimum tensile strength was 1300 MPa at 350 0C after austempering for 150 minutes. In conclusion the austempering operation has a significant effect on the mechanical and microstructural properties of ADI.

Abstract: The aim of this investigation was to determine a procedure based on tensile testing to assess the critical range of austempering times for having the best ausferrite produced through austempering. The austempered ductile iron (ADI) 1050 was quenched at different times during austempering and the quenched samples were tested in tension. The dislocation-density-related constitutive equation proposed by Estrin for materials having high density of geometrical obstacles to dislocation motion, was used to model the flow curves of the tensile tested samples. On the basis of strain hardening theory, the equation parameters were related to the microstructure of the quenched samples and were used to assess the ADI microstructure evolution during austempering. The microstructure evolution was also analysed through conventional optical microscopy, electron back-scattered diffraction technique and transmission electron microscopy. The microstructure observations resulted to be consistent with the assessment based on tensile testing, so the dislocation-density-related constitutive equation was found to be a powerful tool to characterise the evolution of the solid state transformations of austempering.

Abstract: Austempered ductile iron (ADI) is known to have a good combination of mechanical properties due its unique ausferrite microstructure. The strength of ADI is mainly a function of the austempering temperature and the stability of ausferrite matrix. To increase the stability of the ausferritic matrix, two stage austempering processes was developed. During this investigation, in the Ist step, ductile iron specimens were austenitized at 900 °C for 60 min followed by quenching to 250 °C in salt bath. In the IInd step, after quenching at 250 °C, the salt bath was gradually heated to 350 °C, 400 °C and 450 °C respectively where specimen were soaked for 120 min. The tensile strength and impact strength were evaluated according to ASTM standards. The results were compared with that obtained by conventional austempering process by quenching directly into salt bath at 400 °C for 120 min. Both tensile and impact strength were found to have improved by two step austempering process. During Ist stage of austempering, martensite was observed while during IInd stage of austempering microstructures revealed acicular ferrite and carbon stabilized austenite. The fractographic examination revealed mixed type of fracture mode and intergranular fracture was seen under SEM. It was further observed that the tensile strength decreased whereas the impact strength increased with IInd stage of austempering temperature.

Abstract: Attractive properties of TRIP-type bainitic ferrite (TBF) steel ascribe to its unique microstructure of lath structure bainitic ferrite matrix and interlath retained austenite films. This work is concerned with obtaining ultra high-strength hot forged TBF steel with high elongation and excellent strength-elongation balance. The effect of austempering temperature on the microstructure along with its retained austenite characteristics and tensile properties of a hot forged TBF steel was studied. A detailed investigation correlating the steel structure and its tensile properties was carried out. Tensile strength ranging from 1058 to 1552 MPa was achieved when the hot forged steel was austempered at (325 - 475 °C). Ultra high tensile strength of 1058 MPa, large total elongation of 29% and excellent strength-elongation balance of 30 GPa % were attained when the steel was austempered at 425 °C. The large total elongation of this steel is mainly due to the uniform fine lath structure matrix and the pronounced TRIP effect of a large amount of retained austenite films which prevents a rapid decrease of strain hardening rate at low strain and leads to a relatively high strain hardening at high strain level. Rapid transformation of blocky retained austenite at low strain in the hot forged TBF steel austempered at higher temperatures results in a rapid increase of initial strain hardening. In addition, the coarse microstructure that contains large blocks of retained austenite / martensite and the insufficient numbers of bainitic ferrite lathes and retained austenite films deteriorate the total elongation and the strength-elongation balance of the TBF austempered at 475 °C.

Abstract: This study produced austempered ductile iron (ADI) with ausferrite structure using forced-air cooling as the quenching method. ASTM A536 65-45-12 grade of ductile iron was produced, machined into flat samples of 5, 10, 15, 20 and 25 mm thickness, austenitised at a temperature of 820 °C for 1-h soaking period inside a muffle furnace, forced-air-cooled in an air quenching chamber to an austempering temperature (T A) of 300 °C and rapidly transferred into another muffle furnace maintained at 300 °C in order to austemper them for a fixed period of 2 h. Finally, the microstructural morphology and phase distribution of heat-treated samples were characterised using scanning electron microscopy (SEM) and X-ray diffraction (XRD) method. The SEM image with electron-dispersive X-ray analyses shows predominant carbon and iron peaks of high-carbon austenite and ferrite, respectively, while the XRD patterns predominantly consist of austenite (γ) and ferrite (α) phases which are the characteristic features of ADI. The study concluded that with the use of forced-air cooling as a quenching method, ADI of section thicknesses up to 25 mm with ausferrite structure is producible.

Abstract: In this study the as-cast macro and microstructures of medium C – high Si cast steels of three different levels of alloying are characterised. The application of a colour-etching reagent sensitive to Si segregation effectively revealed the solidification macrostructure, showing that the patterns of macrostructure and microsegregation are governed by the initial precipitation of δ-ferrite dendrites. A study of microsegregation carried out using advanced EDS techniques showed that, for the studied chemical compositions, Si, Mn, Cr, Ni, Mo and Al tend to concentrate at the last liquid to solidify. Accordingly, effective partition coefficients of values below unity were calculated for all alloying elements tested. It was verified that the minimum local Si contents measured on the steels investigated were greater than 1.7%, value above the minimum value (1.5%) necessary to obtain carbide-free bainite after austempering.

Abstract: Ultrahigh strength steel 56NiCrMoV7 was austempered at 270 °C for different durations in order to investigate the microstructure evolution, carbon partitioning behaviour and hardness property. Detailed microstructure has been characterised using optical microscopy and field emission gun scanning electron microscopy. A newly developed X-ray diffraction method has been employed to dissolve the bainitic/martensitic ferrite phase as two sub-phases of different tetragonal ratios, which provides quantitative analyses of the carbon partitioning between the resultant ferrites and the retained austenite. The results show that, a short-term austempering treatment was in the incubation period of the bainite transformation, which resulted in maximum hardness being equivalent to the oil-quenching treatment. The associated microstructure comprises fine carbide-free martensitic and bainitic ferrites of supersaturated carbon contents as well as carbon-rich retained austenite. In particular, the short-term austempering treatment helped prevent the formation of lengthy martensitic laths as those being found in the microstructure of oil-quenched sample. When the austempering time was increased from 20 to 80 min, progressive decrease of the hardness was associated with the evolution of the microstructure, including progressive coarsening of bainitic ferrite, carbide precipitating inside high-carbon bainitic ferrite and its subsequent decarbonisation.

Abstract: The microstructural evolution and consequent changes in strength and ductility of advanced NANOBAIN steel during prolonged isothermal heat-treatment stages were investigated. The microstructure and mechanical properties of nanostructured bainite were not expected to be influenced by extending the heat-treatment time beyond the optimum value because of the autotempering phenomenon and high tempering resistance. However, experimental results indicated that the microstructure was thermodynamically unstable and that prolonged austempering resulted in carbon depletion from high-carbon retained austenite and carbide precipitations. Therefore, austenite became thermally less stable and partially transformed into martensite during cooling to room temperature. Prolonged austempering did not lead to the typical tempering sequence of bainite, and the sizes of the microstructural constituents were independent of the extended heat-treatment times. This independence, in turn, resulted in almost constant ultimate tensile strength values. However, microstructural variations enhanced the yield strength and the hardness of the material at extended isothermal heat-treatment stages. Finally, although microstructural changes decreased the total elongation and impact toughness, considerable combinations of mechanical properties could still be achieved.

Abstract: This paper describes the machining and forming process of a Pitman arm, proposed to be used in the automotive industry, from austempered ductile cast iron (ADI) as an alternative to the commonly used material, AISI 1045 steel. Samples were austenitized at 900 °C for 180 min and subsequently austempered in a salt bath over a range of temperatures, from 340 to 360 °C, for 60 min, to obtain favorable mechanical properties. The results of metallographic testing show that the pieces formed with ADI have an average nodularity of 92.5%, and the average nodule count is 159 nodules/mm2. Mechanical tests results of strength, elongation, hardness, and impact energy are comparable to that of ASTM standard A897-15 grade 1. A finite element simulation and an experimental analysis were performed to determine the forming force and the material strength. The deformation limits for ADI were identified. These limits and the simulation results show that ADI is an attractive alternative high-strength material for the Pitman arm application.

Abstract: A commercial ductile iron is treated by a novel austempering process to obtain a good combination of strength and ductility. The samples are austenitised at 890°C for 10 min, then quenched into patented quenching liquid, and austempered in an electric furnace at 220°C for 5, 10, 30, 60, 240 and 600 min, respectively, finally air cooled. The bending test and the tensile test are conducted and microstructural features are analysed on the austempered ductile iron. The optimum mechanical property is achieved at 220°C for 240 min. Main reason for high strength and ductility is the formation of a fine structure consisting of multiple phases of pre-formed martensite and lath bainitic ferrite with film retained austenite.

Abstract: In previous studies, the process parameters to obtain ausferritic ductile iron in as-cast conditions by means of engineered cooling were defined, that is, without an austempering heat treatment. This material was fundamentally characterized, and its mechanical properties were determined. It was demonstrated that obtaining fully ausferritic microstructures by means of engineered cooling was feasible and that the properties met the requirements of the conventionally produced austempered ductile iron. Additionally, an experimental model was developed to define the optimal processing parameters of castings presenting different thermal moduli, in terms of chemical composition, temperatures and time parameters. The aim of the present work is to go into detail about the physical properties of the ausferritic as-cast materials. The chemical composition of the samples was defined by means of the experimental model. The isothermal transformation temperature was changed from 300 to 400 °C, while the other process parameters (shakeout temperature and isothermal transformation time) remained constant. Due to the excellent strength/toughness ratio of these materials, they are prone to being used on different applications such as automotive suspension components, rail components in low temperature environments and pumps and engines exposed to corrosive marine conditions among others. With the aim of responding to this demand, an advanced characterization of the material’s low temperature, corrosion or dynamic properties was performed on this work. These results were compared to the conventionally heat-treated austempered ductile iron as well as other nodular iron ferritic–pearlitic grades found in the literature.

Abstract: 6th Congress and Exhibition on International Advances in Applied Physics and Materials Science (APMAS) -- JUN 01-03, 2016 -- Istanbul, TURKEY

Abstract: The paper presents the results of wear tests obtained for 4 groups of materials: surface-hardened alloy steels and alloy cast steels for structural applications, hard-wearing surface-hardened alloy cast steels, and austempered alloy cast irons. The wear tests have been performed on a specially designed test rig that allows reproducing the real operating conditions of chain wheels, including the rolling and sliding form of contact between elements. The chain wheels subjected to tests were operated with the use of loose quartz abrasive. This study presents results of measurements of material parameters, micro-structure of a surface subject to wear, as well as the linear wear determined for the materials considered. Based on the results, the following was found: the best wear properties were obtained for surface-hardened alloy steels and wear surface; strengthening of the ADI surface took place – most probably as a result of transformation of austenite into martensite; the uniformity of the structure of the materials affects the surface wear process. The study also indicated a significant degree of graphite deformation in ADI characterized by the upper ausferritic structure and its oblique orientation in relation to the surface, which resulted in a facilitated degradation of the surface caused by the quartz abrasive.