Abstract: The contribution of multi-phase microstructure and retained austenite on mechanical properties of austempered and intercritical annealed Fe–0.23C–1.8Mn–1.35Si (wt%) steel was studied. The multi-phase microstructure comprised of intercritical ferrite (IF), bainite/martensite, and retained austenite. During austempering, the retained austenite was stabilized, which was studied using a combination of experimental (XRD, TEM) and thermodynamic analysis. The termination of bainitic transformation combined with carbon rejection into residual austenite during the second step austempering treatment is believed to be the underlying basis for stabilization of retained austenite. This led to significant increase in uniform and total elongation (25% and 36%, respectively) and the product of tensile strength and % elongation was 33 GPa%. The work hardening behavior of retained austenite exhibited a three-stage process such that necking was delayed. The increased work hardening rate is attributed to the multi-phase microstructure and TRIP effect.

Abstract: The microstructures and the mechanical properties of 30MnSiCrAlNiMo low-carbon steel were systematically optimized by a series of heat-treatment processes, and the heat-treatment process of low-temperature bainite in low-carbon steel was explored. Results showed that the microstructure of low-temperature bainite in the low-carbon steel, containing a fine plate of carbide-free bainitic ferrite and a thin film of retained austenite, could be produced by continuous cooling transformation around the Ms temperature from Ms+10 °C to Ms−20 °C at a cooling rate of 0.5 °C min −1 . A new model was proposed to evaluate the comprehensive mechanical properties of steel, which found that the low-temperature bainite had the best comprehensive mechanical properties compared to any other microstructures for the low-carbon steel. The higher dislocation density and finer bainitic ferrite plate in the low-temperature bainite resulted in the higher yield strength and the higher toughness, but relatively lower ultimate tensile strength owing to the lower work-hardening rate caused by the higher initial dislocation density. There were some very fine particles in the bainitic ferrite of the steel after isothermal treatment at higher temperature. The ultimate tensile strength and the low-temperature impact toughness of the steel decreased with the volume fraction of the retained austenite increasing, while the elongation initially increased with an increase in the volume fraction of the retained austenite (

Abstract: Bainite microstructure in alloy steels yields materials with an excellent combination of mechanical properties such as high strength and toughness, resistance to creep and fatigue, and hydrogen embrittlement.1,2) During cooling, bainite transformation is initiated at the bainite start (Bs) temperature, which is the highest isothermal temperature at which upper bainite is observed, and ceases when the temperature reaches the martensite start (Ms) temperature. Addition of alloying elements mostly lowers the Bs temperature. Especially, carbon and manganese known as strong austenite stabilizing elements effectively decrease the Bs temperature. Several quantitative investigations have been performed to determine the relationship between the addition of alloying elements and Bs temperature variations in alloy steels. Some empirical equations have been proposed to predict the effect of adding alloying elements to carbon and alloy steels on Bs temperature.3–12) Prior austenite grain size (PAGS) can also affect the Bs temperature. Lee et al.13) reported that Bs temperature decreased with decreasing PAGS. Bs temperature variations due to the decrease in PAGS simultaneously influenced bainite transformation kinetics.13–15) However, few researchers have attempted to develop the equations for Bs temperature by considering both chemical composition and PAGS effects. Therefore, in the present study, we developed a simple empirical equation for Bs temperature prediction that includes both the alloying element effect and the PAGS effect based on experimental data obtained from the literature. We compared the accuracy of our empirical equation with existing equations and verified the performance of our equation using experimental data. Experimental Bs temperature data for carbon and alloy steels were extracted from published time-temperaturetransformation (TTT) diagrams.16) Both chemical composition and PAGS were used to evaluate existing equations and to derive a new equation. We selected the following alloying elements in the present work: C, Mn, Si, Ni, Cr, and Mo. Data for other alloying elements were excluded. The chemical composition range of the selected steels was limited to that of low alloy steels, thus high alloyed steels such as a stainless steel was excluded. Ranges of chemical composition, PAGS, and Bs temperature used for equation derivation are summarized in Table 1. The total number of Bs data points used in the present work was 97. The effects of alloying elements on Bs temperature prediction are generally expressed by a linear type relationship as follows:3–6,8,10)

Abstract: Novel alloys with high aluminum addition, so-called δ-transformation-induced plasticity (TRIP), have been developed recently for the third generation of advanced high strength steels for automotive applications, which are promising owing to the potential weldability as well as the combination of strength and ductility. In addition, the high aluminum addition results in a density reduction of approximately 5% in these δ-TRIP alloys without sacrificing the Young’s modulus in uniaxial tensile tests. The origin of δ-TRIP concept is introduced first with a review of the published work on δ-TRIP alloys. This review will include methodology for retention of δ-ferrite in casting, rolling and welding conditions, microstructure evolution by austempering, as well as microstructures–properties relationship involving the roles of blocky and lath retained austenite. In addition, currently unresolved problems will be discussed regarding the fundamentals of materials design, automotive application, and industrial manufacturing.

Abstract: The kinetics of reaction occurring during the austempering treatment of ductile iron (DI) containing different additions of Cu and Ni was investigated in this work. DI bars were heat treated in an instrumented dilatometer in order to follow the exhibited transformation kinetics. The dilatometric results indicated that the addition of Cu alone did not have a significant effect on the incubation times for the austempering transformation. Also, the addition of both, Cu and Ni resulted in a significant effect on reducing the transformation rates. It was found that the austempering process is characterized by two clearly distinguished transformation stages. In the initial stage, the addition of Cu, and to a greater extent, additions of both Cu and Ni led to reductions in the transformation rates shifting the maximum transformation rate values toward longer times. The outcome of this work indicates that during the first stage of austempering, nucleation of the ferrite plates occurs via a diffusionless mechanism while their growth is diffusion controlled. Moreover, after the maximum in the transformation rate has been reached, the growth of ferrite plates becomes dominant with the rate-limiting step becoming the diffusion of C into the surrounding austenite. A qualitative model for the austempering transformation is proposed in this work to account for the experimental observations.

Abstract: Steels with compositions that are hot rolled and cooled to exhibit high strength and good toughness often require a bainitic microstructure. This is especially true for plate steels for linepipe applications where strengths in excess of 690 MPa (100 ksi) are needed in thicknesses between approximately 6 and 30 mm. To ensure adequate strength and toughness, the steels should have adequate hardenability (C. E. >0.50 and Pcm >0.20), and are thermomechanically controlled processed, i.e., controlled rolled, followed by interrupted direct quenching to below the Bs temperature of the pancaked austenite. Bainite formed in this way can be defined as a polyphase mixture comprised a matrix phase of bainitic ferrite plus a higher carbon second phase or micro-constituent which can be martensite, retained austenite, or cementite, depending on circumstances. This second feature is predominately martensite in IDQ steels. Unlike pearlite, where the ferrite and cementite form cooperatively at the same moving interface, the bainitic ferrite and MA form in sequence with falling temperature below the Bs temperature or with increasing isothermal holding time. Several studies have found that the mechanical properties may vary strongly for different types of bainite, i.e., different forms of bainitic ferrite and/or MA. Thermomechanical controlled processing (TMCP) has been shown to be an important way to control the microstructure and mechanical properties in low carbon, high strength steel. This is especially true in the case of bainite formation, where the complexity of the austenite-bainite transformation makes its control through disciplined processing especially important. In this study, a low carbon, high manganese steel containing niobium was investigated to better understand the effects of austenite conditioning and cooling rates on the bainitic phase transformation, i.e., the formation of bainitic ferrite plus MA. Specimens were compared after transformation from recrystallized, equiaxed austenite to deformed, pancaked austenite, which were followed by seven different cooling rates ranging between 0.5 K/s (0.5 °C/s) and 40 K/s (40 °C/s). The CCT curves showed that the transformation behaviors and temperatures varied with starting austenite microstructure and cooling rate, resulting in different final microstructures. The EBSD results and the thermodynamics and kinetics analyses show that in low carbon bainite, the nucleation rate is the key factor that affects the bainitic ferrite morphology, size, and orientation. However, the growth of bainite is also quite important since the bainitic ferrite laths apparently can coalesce or coarsen into larger units with slower cooling rates or longer isothermal holding time, causing a deterioration in toughness. This paper reviews the formation of bainite in this steel and describes and rationalizes the final microstructures observed, both in terms of not only formation but also for the expected influence on mechanical properties.

Abstract: A low-temperature nanobainitic steel was obtained through one-step and two-step austempering. The effect of austempering temperature, time, and route on bainitic lath width, volume fraction of retained austenite, carbon concentration in retained austenite, and nanohardness of bainitic lath and retained austenite was studied. Results showed that the transformation kinetics was slowed down and the bainitic lath was refined as the austempering temperature decreased from 300 to 250 °C. Both coarser and finer bainitic laths were obtained with the two-step austempering, which was consistent with the lath size at 300 and 250 °C austempering, respectively. X-ray diffraction analysis showed that both volume fraction of retained austenite and its carbon concentration decreased with the decrease of austempering temperature for the one-step austempering, and especially the carbon concentration is obviously increased when the two-step austempering is adopted. The nanohardness of the bainitic lath in the sample after two-step austempering treated lies between that of the samples after 300 and 250 °C austempering treated. The product of tensile strength and total elongation of the two-step austempered sample is the highest, which increases monotonously with the product of retained austenite fraction and its carbon concentration. Higher strength–ductility balance may be resulted by the fine bainitic lath, high volume fraction, and high stability of retained austenite in the sample after two-step austempered.

Abstract: The aim of the paper was to investigate the effect of austempering parameters (time and temperature) on the microstructure and mechanical properties of ADI alloyed with 1.5% Cu and 1.6% Ni (in wt.%) in order to establish the optimal processing window. It was shown that the strength, elongation and impact energy strongly depend on the amounts of ausferritic ferrite and retained austenite. A processing window was established according to the results of the kinetics of the isothermal transformation. The results show that the processing window for ADI alloyed with Cu and Ni at 350 °C was relatively wide, while the processing window for the isothermal transformation at 400 °C becomes narrower and shifted to the left. The processing window of ADI austempered at 300 °C is also narrower, but shifted to the right towards the longer times compared to the processing window of ADI austempered at 350 °C.

Abstract: In this article, an attempt was made to determine the effect of dynamic load on the austempered ductile iron resistance obtained under different conditions of heat treatment. Tests were carried out on six types of cylindrical ductile iron samples austempered at 320, 370 and 400 ◦ C for 30 and 180 minutes. For each type of material, two samples were collected. As a next step in the investigations, the samples were subjected to a Taylor impact test. The samples after striking a non-deformable, rigid target were deformed on their front face. After Taylor test, a series of material tests was performed on these samples, noting a significant increase of hardness in the deformed part. This was particularly well visible in the ductile iron isothermally quenched at higher temperatures of 370 and 400 ◦ C. In the zone of sample deformation, an increase in the content of ferromagnetic phase was also reported, thus indicating the occurrence of martensitic transformation in the microstructure containing mechanically unstable austenite. A significant amount of deformed graphite was also observed, which was a symptom of the deformation process taking place in samples. The ductile iron was characterized by high toughness and high resistance to the effect of dynamic loads, especially as regards the grade treated at a temperature of 370 ◦ C.

Abstract: The work compares the abrasive wear resistance of cast iron containing vermicular graphite, measured in the as-cast state and after austempering carried out at 290◦C, 340◦C, or 390◦C. The examinations were performed by means of the T-01M tribological tester using the pin-on-disc configuration. Specimens used for examinations were taken from the end tabs of the tensile specimens, these being cut out of the test walls of the double-leg keel block test castings. Examinations proved that the austempering process increases the abrasive wear resistance of vermicular cast iron by several times as compared with the as-cast material. A tendency for a slight decrease in abrasive wear with an increase in austempering temperature can be stated. The coefficient of friction took a little higher values for cast iron after thermal treatment than for the as-cast material. The work was completed with roughness examination by means of electron scanning microscopy.

Abstract: In recent years, there has been significant interest in the development of high silicon bainitic steels. This has been due to their excellent mechanical properties, including very high strength, good ductility, high fracture toughness, and high wear resistance. Recently, a high-strength, high-toughness bainitic steel with a high silicon content has been developed by two of the authors. In this paper, the mechanical properties as a function of austempering are presented. A low-carbon, low-alloy (LCLA) bainitic steel with a high (2%) silicon content was quenched and isothermally austempered at temperatures from 260 °C to 343 °C for 2 h. The heat treatment produced a microstructure consisting of tempered martensite with austenite located between the martensite laths. The mechanical properties of these austempered samples, including plane-strain fracture toughness, were characterized and compared with as-cast samples. All of the austempering treatments produced high strength (>1500 MPa UTS) and high fracture toughness (>105 MPa) with reasonable ductility (>4%). A promising potential application of this steel is a military vehicle armor plate; therefore, a small number of impact tests were performed to qualitatively assess the improvement in resistance. Impact testing was performed using projectiles propelled by a light gas gun; targets of mild steel and LCLA austempered at 316 °C were evaluated. Qualitative results reveal that, in comparison to the mild steel sample, the austempered LCLA samples suffered reduced damage and no penetration.

Abstract: The conditions of formation of what is known as “hard bainite” in the form of packets containing long thin ferrite crystals of a nanosize thickness are determined for a high-carbon low-alloy steel. The effect of the temperature of austenitizing before isothermal holds on the conditions of formation of such structure and on the hardness of the steel is studied. The austenitizing temperature corresponding to the highest hardening effect is determined.

Abstract: Research on austempering of high-Si steels has increased over the last years due to their attractive combination of strength and toughness. The presence of over 1.5% of silicon prevents the precipi ...

Abstract: The paper discusses the reasons behind current trends for substituting cast iron castings by aluminum alloys. In particular it is shown that it is possible to produce thin wall castings (control arms, cantilevers and rotors) made of ductile iron without the development of chills, cold laps or misruns, and with a strength to weight ratio of up 87 MPa cm3/g. In addition, austenitizing at 900 oC for 20 minutes and then austempering in a salt bath at 350 oC for 15 minutes promotes the development of a fully ausferritic matrix in thin wall castings with a the strength to weight ratio increase of up to 154 MPa cm3/g. Finally, it is shown that thin wall castings made of ductile or austemperded cast iron can be lighter and with superior mechanical properties then their substitutes made of aluminum alloy.

Abstract: In this study, the effect of isothermal temperature on microstructure and mechanical properties of a high Al–low Si TRIP steel was investigated using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, electron back scattered diffraction, and tensile test. The results show that typical microstructure containing ferrite, bainite, and retained austenite can be obtained when two-stage heat treatment process was utilized. When annealing temperature is 840 °C and austempering temperature is 400 °C, the tensile strength is 542 MPa and the product of strength and elongation is 17,685 MPa%. The morphologies and stability of the retained austenite in low silicon/high aluminum TRIP steel were finally discussed.

Abstract: In the present study a high-boron high speed steel (HSS) roll material was designed. Many expensive alloy elements have been substituted by cheap boron alloy, and high-boron high speed steel roll has been manufactured by centrifugal casting method. The microstructures, mechanical properties and wear resistance of centrifugal casting high-boron high speed steel roll have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and X-ray diffraction (XRD) analysis, hardness test, impact test and wear test. The results indicated that the solidification microstructures of high-boron high speed steel roll consisted of M2(B,C), (W,Mo)2(B,C), M3(B,C), M23(B,C)6 type borocarbides and martensite, a small amount of retained austenite. Borocarbides were continuously distributed over the grain boundary. After quenching from 1050 °C, local broken network appeared in partial borocarbides, and fine secondary borocarbide precipitated from the matrix. After tempering from 525 °C, the amount of precipitated borocarbide increased significantly. After heat treatment, the hardness of high-boron high speed steel roll excelled 60 HRC, and its impact toughness excelled 8.0 J/cm2. The single groove steel rolling amount of high-boron high speed steel rolls increases by 500% than that of bainite cast iron roll, when the rolls are used in K1 mill housing of bar mill.

Abstract: Although alloying and heat treatments are common industrial practices to obtain ductile irons with desired mechanical properties, related information on how the two practices affect corrosion behavior is scarce. In this study, two ductile irons—with and without 1 wt pct copper addition—were austempered to obtain austempered ductile irons (ADIs). Polarization tests and salt spray tests were conducted to explore how both copper-alloying and austempering heat treatments influenced the corrosion behavior of ductile irons. The results showed that the corrosion resistance of 1 wt pct copper-alloyed ductile iron was better than that of the unalloyed one, while ADI had improved corrosion resistance compared with the as-cast. In particular, the ductile iron combined with the copper-alloying and austempering treatments increased the corrosion inhibition efficiency up to 84 pct as tested in 3.5 wt pct NaCl solution.

Abstract: The results presented in this paper are a continuation of the previously published studies. The results of hest treatment of ductile iron with content 3,66%Si and 3,80% Si were produced. The experimental castings were subjected to austempering process for time 30, 60 and 90 minutes at temperature 300C. The mechanical properties of heat treated specimens were studied using tensile testing and hardness measurement, while microstructures were evaluated with conventional metallographic observations. It was again stated that austempering of high silicone ferritic matrix ductile iron allowed producing ADI-type cast iron with mechanical properties comparable with standard ADI.

Abstract: The paper presents a study about the influence of some heat treatment parameters of austempering and annealing with tempering over the structure and values of hadness, tensile strength, elongation and impact strength of a low alloyed S.G. cast iron. It is pointed out the influence of some factors (the temperature and the holding time at the isothermal level) on the phase transformation and properties in the studied cast iron.

Abstract: Present investigations involve a systematic study on the machinability of austempered ductile irons (ADI) developed from four commercially viable ductile irons alloyed with different contents of 0, 0.1, 0.3 and 0.6 wt.% of Ni. The influence of Ni content, amount of retained austenite and hardness of ADI on machining behavior has been conducted systematically. Austempering heat treatment was carried out for 120 minutes at four temperatures270C, 320C, 370C or 420C, after austenitization at 900C for 120 min. Milling tests were performed and machinability index, cutting forces and surface roughness measurements were used to evaluate the machinability. Higher cutting forces, lower machinability index and the poorer surface roughness of the samples austempered at lower temperatures indicated that austempering at higher temperatures resulted in better machinability. The machinability of samples austempered at 420C, which contained higher fractions of retained austenite, was superior to that of samples austempered at lower temperatures, indicating that hardness is an important factor in assessing machinability in addition to high carbon austenite content. The ADI with 0.6% Ni, austempered at 420°C for 120 minutes, demonstrated best machinability. Keywords—Austempering, machinability, machining index, cutting force, surface finish.