Abstract: 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: Four isothermal heat treatment schedules are designed to study the effects of transformation temperature below and above martensite starting temperature (MS) on bainitic transformation kinetics, microstructure, and mechanical properties of a low‐carbon bainitic steel. The results indicate that the product of tensile strength and elongation (PSE) of tested steel do not show any improvement when the samples are austempered at a temperature below MS. Finer bainite microstructure can be observed when the samples are austempered at a temperature below MS, but the highest PSE is obtained in the sample austempered at a temperature above MS (400 °C). Lower PSE of the samples transformed below MS is mainly attributed to less bainite and retained austenite (RA) amount and more amount of athermal martensite, as well as the existence of carbide. In addition, with the decrease of the isothermal transformation temperature, the PSE first increases and then decreases. Different from austempering above MS, when the sample is austempered below MS, PSE decreases with the decrease of transformation temperature. Moreover, whether the treatment below MS can improve mechanical property of bainitic steels depends on the composition of steels and transformation temperature.

Abstract: Austempered ductile iron alloys (ADI) are an interesting class of materials because of their unique microstructure and mechanical properties. When subjected to austempering treatment, ductile iron transforms to a microstructure consisting of ferrite and stabilized austenite rather than ferrite and carbide as in austempered steels. Because of the presence of stabilized austenite, austempered ductile iron (ADI) exhibits an excellent combination of strength and ductility together with good fatigue and wear properties. Accordingly, there is a growing interest in using ADI in several applications due to its mechanical properties. However, as with difficult-to-cut materials, the machinability rating of ADI is low and there is still a need to understand the impact of the cutting process parameters. Machinability of a material depends not only on its properties and microstructure, but also on the proper selection and control of process variables. The current work is focused on performing a machinability analysis of ADI in order to understand the effect of the cutting process parameters on the machined surface quality and tool life. It also studies the effect of different coolant strategies. Thus, the motivation of this study is to develop a better understanding of the influence of cutting parameters and cooling strategy to be able to machine ADI directly with acceptable tool life and cycle time. The design of experiments technique and response surface methodology is employed to analyze and model the measured responses. In addition, the cutting tool wear mechanisms are identified and discussed.

Abstract: Austempered Ductile Iron (ADI) represents an alternative solution for the manufacturing of the housing of small planetary gearboxes, with the gear teeth obtained directly on the housing itself: such solution combines a cost-effective process with the possibility of obtaining complex geometry of the case. With respect to most traditional solutions, by means of ADI the requirements of strength and accuracy of the gear teeth can be satisfied without an additional finishing step after the heat treatment: the teeth can be obtained by broaching and, thanks to the low distortion which can be granted by the austempering process, a subsequent finishing operation is not needed. For these reasons, ADI has been selected for the application to a family of small gearboxes for automation. Due to the limited experience and data available for such material, to improve the design and rating processes, a testing campaign has been performed. The aim was to obtain strength data for bending and contact fatigue, considering the specific manufacturing and heat treatment processes. The paper describes the test procedures adopted and the test results, which have been obtained on gears specimens by means of Single Tooth Fatigue (STF) and pitting tests on a FZG type bench respectively. The tests are supported by metallurgical investigations on the failed teeth, to describe and understand the failure mechanisms. The results are then compared with the data and the shape curves provided by the international standards.

Abstract: A low-carbon 18Mn3Si2CrMo steel was subjected to austempering at different temperatures below Ms, 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 (VM), bainite (VB) and retained austenite (Vγ) 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/cm2. 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: 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: 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.

Abstract: The 9%Ni low-carbon steel is applied to utilities and processes at temperatures as low as − 196 °C. However, the microstructural features play an important role on the mechanical properties. Notably, the cryogenic toughness and mechanical strength are strongly dependent on the final heat treatment. In this paper, the microstructure of a 9%Ni low-carbon steel was modified by different heat treatments. The hardness and cryogenic toughness were measured and correlated to microstructural features. The material shows a temper embrittlement with intergranular cracking and minimum cryogenic toughness after tempering around 400 °C. Austempering at 480 °C also produced very low toughness results. On the other hand, excellent cryogenic toughness was obtained with single tempering at 600 °C after quenching or normalizing. Even higher toughness was obtained with the double tempering at 670 °C/2 h plus 600 °C/2 h. The amount of reversed austenite and its morphology in the specimen quenched and tempered at 600 °C were shown in the paper.

Abstract: The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.

Abstract: The wear resistances of austempered ductile iron (ADI) were improved through introduction of a new phase (carbide) into the matrix by addition of chromium. In the present investigation, low-carbon-equivalent ductile iron (LCEDI) (CE = 3.06%, and CE represents carbon- equivalent) with 2.42% chromium was selected. LCEDI was austenitized at two different temperatures (900 and 975°C) and soaked for 1 h and then quenched in a salt bath at 325°C for 0 to 10 h. Samples were analyzed using optical microscopy and X-ray diffraction. Wear tests were carried out on a pin-on-disk-type machine. The effect of austenization temperature on the wear resistance, impact strength, and the microstructure was evaluated. A structure–property correlation based on the observations is established.

Abstract: A model for the strain-induced martensitic transformation in austempered ductile iron (ADI) has been developed based on neutron diffraction studies. Quantitative phase analysis was carried out using the Rietveld method including texture analysis. The key parameters applied in this model that influence the strain-induced martensitic transformation are temperature, strain state, and loading type. An empirical relation was derived for the martensite start temperature M s in austempered ductile iron, which takes into account the Ni and carbon content. The M s temperature was used as a scaling parameter for the stability of austenite in the model to describe the strain-induced phase transformation in austempered ductile iron.

Abstract: The effect of Cr content on the microstructure and mechanical properties of carbidic austempered ductile iron (CADI) was studied by means of optical microscopy (OM), X-ray diffractometry (XRD), scanning electron microscopy (SEM), Rockwell hardness testing, pendulum impact testing machine and ring-on-block wear tester. The results show that with the increase of Cr content the volume fraction of graphite and nodularity rate of as-cast ductile iron decrease gradually, but carbide content increases, and the diameter of graphite reaches the maximum followed by decrease. Then, the specimens were austenitized at 900 °C for 100 min and austempered at 300 °C for 100 min. The hardness and wear resistance of CADI reach maximum when Cr content is 1.42 wt.-% and then decrease with the increase of Cr content, and the impact toughness decreases gradually. Taking the impact toughness into consideration, the comprehensive mechanical property is best when Cr content is 0.96 wt.-%.

Abstract: Graphite formation in the solid state is both in ductile cast irons and in steels strongly promoted by high silicon contents above 3 wt.% Si. The matrix microstructure in austempered ductile iron can be further refined by secondary graphite if the austenitization, quench, and isothermal transformation into ausferrite are preceded by an austenitization at a slightly higher temperature followed by quench to martensite, resulting in higher carbon content than being soluble at the second austenitization temperature. Hypoeutectoid steels with high silicon contents can be rapidly graphitized, causing recrystallization of surrounding ferrite due to plastic deformation making room for less dense graphite. In rolled steels, the interface between manganese sulfide and steel matrix is the most common nucleation site. Voids are formed when graphite is partly or completely dissolved during austenitization in succeeding hardening heat treatments, but the mechanical properties can still be good if the graphite particles dissolved into voids are below 20 µm. Graphitized Si-solution strengthened ferritic steels may perform similar to free-cutting steels but with improved mechanical properties.

Abstract: 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 Al2O3 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: The aim of this study is to investigate the effect of aluminizing process on structural, mechanical and tribological properties of GJS 600-10 solution strengthened ferritic (SSF) ductile iron before and after austempering heat treatment. The as-cast samples were coated by hot-dip aluminizing (HDA) at 750 °C for 5 min. For austempering heat treatment, the as-cast and aluminized samples were austenitized at 975 °C for 2 h and austempered (ADI) at 330 °C for 1 h, which were referred as ADI and HDA + ADI samples, respectively. Structural characterizations made by scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDX) and x-ray diffraction (XRD) analysis revealed that single HDA process produced a coating of FeAl3 and Fe2Al5 layers with an Al topcoat on the surface. By the application of austempering after HDA, as-aluminized coating layer transformed into Fe2Al5 + FeAl2, FeAl and α-Fe with Al2O3 scale on the surface. Tensile tests conducted at room temperature indicated that single HDA does not significantly affect the mechanical properties of the as-cast ductile irons, whereas application of austempering after HDA decreases strength and increases ductility in comparison to ADI samples. Dry sliding wear tests performed against Si3N4 ball at room temperature showed that HDA samples exhibited much worse wear resistance than the as-cast samples. However, wear resistance was increased about 30% when austempering is applied after HDA. Formation of various oxides on the worn surfaces of the samples was studied by Raman spectroscopy and EDX analysis. Test results were comparatively evaluated on the basis of surface and subsurface characteristics of the investigated ductile irons, and showed that increased wear resistance can be obtained when HDA is followed by austempering for the ductile iron used in this investigation.

Abstract: Changes in microstructure and mechanical properties of medium-carbon spring steel during austempering were investigated. After austempering for 1 h at 290 °C or 330 °C, the bainite transformation stabilized austenite, and microstructure consisting of bainitic ferrite and austenite could be obtained after final cooling; the retained austenite fraction was smaller in the alloy austempered at 290 °C because carbon redistribution between bainitic ferrite and austenite slowed as the temperature decreased, and thereby gave persistent driving force for the bainite transformation. The products of tensile strength and reduction of area in the austempered alloy were much larger in the austempered steel than in quenched and tempered alloy, mainly because of significant increase in reduction of area in austempered alloy.

Abstract: Austempered ductile iron with its unique ausferritic structure is produced by an isothermal heat treatment process. Austempered ductile iron is a potential material to substitute for traditional steel castings and forgings in current industry due to its excellent mechanical properties. The tempering process is frequently used to enhance the ductility and toughness of a material and reduce residual stress. In this research, the phase transformation of austempered ductile iron was studied by applying various tempering temperatures with constant holding duration. It was found that the ausferritic structure was decomposed into dispersive cementite particles after receiving a tempering temperature of 538 °C or higher. The specific amount of retained austenite was analyzed by X-ray diffraction. The wear resistance of tempered austempered ductile iron was investigated by using a ball-on-disk sliding test configuration. The results were compared with conventional quenched and tempered ductile iron under equivalent hardness. Both austempered ductile iron and tempered austempered ductile iron samples had better wear resistance than quenched and tempered ductile iron. The results presented in this research can be utilized as a reference in the tempering treatment of austempered ductile iron material for future applications.

Abstract: Strength of carbide-free bainitic steels primarily depends on the thickness of bainitic ferrite and its volume fraction in the microstructure. The morphology and volume fraction of phases formed in bainitic transformation are governed by the steel's composition and transformation temperature. Higher carbon concentration in bainitic steels is expected to refine the bainitic microstructure even when the austempering temperature is kept to be the around 300 °C due to the enhanced strength of austenite. However, there is a concomitant loss in the volume fraction of bainite formed. Therefore, predicting the optimal composition for getting the highest strength and hardness in nanostructured carbide free bainitic steels is not possible without systematic experimental studies. We have made steels with different carbon concentrations and have demonstrated that after austempering at 300 °C, maximum hardness as well as tensile strength are attained in a wide range of carbon concentration ranging from 0.5–0.9 wt% C but drops significantly outside this range. The experimental results have been compared with the empirical strength in terms of lath thickness and volume fraction formulated from data extracted from several past studies with different carbon concentrations.

Abstract: This work aims to evaluate the use of two different zinc–tin and zinc–aluminum molten metal baths on austempering heat treatments performed in ductile cast iron. Samples were extracted from as-cast...

Abstract: Samples of ductile cast irons alloyed with Cu, Cu-Ni and Cu-Ni-Mo were austempered, borided and boro-austempered and characterized for hardness and micro-adhesive wear behavior. The kinetics of layer formation was also studied. The boriding method used was molten borax bath, in periods of 2 and 4 hours at temperatures of 850, 900 and 950 oC. The direct austempering treatment was performed immediately following the boriding treatment (from 850, 900 and 950 oC) using molten salt baths at temperatures of 240, 300 and 360 oC for 4 hours (boro-austempering treatment). For comparative purposes, the conventional austempering treatment was also conducted. Optical microscopy, scanning electron microscopy, Brinell hardness measurements (on the substrate) and Vickers (on the layers) were performed, along with micro-adhesive wear tests. The boriding treatment resulted in the formation of high hardness layers, in the range of 1450 to 1750 HV, with high wear resistance. The wear resistance of borided and boro-austempered samples were increased by 40 times when compared to as cast or austempered samples, indicating the high efficiency of this type of treatment in increasing the wear resistance of this material.

Abstract: Austempered ductile iron (ADI) is frequently obtained by means of a three-step austempering heat treatment. The parameters of this process play a crucial role on the microstructure of the final product. This paper considers the influence of some process parameters (i.e., the initial microstructure of ductile iron and the thermal cycle) on key features of the heat treatment (such as minimum required time for austenitization and austempering and microstructure of the final product). A computational simulation of the austempering heat treatment is reported in this work, which accounts for a coupled thermo-metallurgical behavior in terms of the evolution of temperature at the scale of the part being investigated (the macroscale) and the evolution of phases at the scale of microconstituents (the microscale). The paper focuses on the sensitivity of the process by looking at a sensitivity index and scatter plots. The sensitivity indices are determined by using a technique based on the variance of the output. The results of this study indicate that both the initial microstructure and the thermal cycle parameters play a key role in the production of ADI. This work also provides a guideline to help selecting values of the appropriate process parameters to obtain parts with a required microstructural characteristic.

Abstract: The austemperability of seven high silicon cast steels with different alloy contents was characteri sed. The maximum round bar diameter that can be fully austempered changed from about 10 mm for an unalloyed steel to more than 70 mm for a low-alloy steel. The austemperability was calculated by applying a procedure based on a standard Jominy test and the characterisation of the microstructure along the Jominy sample. The method, which was validated experimentally, creates a relatively simple procedure to measure austemperability. Processing factors such as the ability of the salt bath to extract heat and the austempering temperature are accounted for the method. The metallographic study revealed the influence of microsegregation on hardenability, which is particularly important for cast steels.