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: AISI 4340 and AISI 4140 are normally used for different structural applications in quenched and tempered state due to their high strength, high abrasion and fatigue resistance but they lack toughness. Presence of dual phase microstructure such as ferrite-bainite and ferrite-martensite in steels have been reported to improve toughness but at the cost of strength. It is believed that development of bainite-martensite dual phase microstructure may help in obtaining better combination of mechanical properties. In this research work, development of bainite-martensite dual phase microstructure has been carried out at different austempering temperatures and times to study microstructure evolution and its effect on mechanical properties. Microstructural characterization of samples was carried out by scanning electron microscope (SEM) whereas mechanical characterization was done by performing hardness and impact testing. Development of dual phase bainite-martensite microstructure has shown to impart better combination of strength and toughness. In comparison with AISI 4140 steel, AISI 4340 provided better combination of hardness and toughness in the austempered state. It has been observed that decrease in the austempering temperature helped in obtaining better combination of hardness and impact strength in both grades of steels due to formation of lower bainite martensite dual phase microstructure at lower temperature.

Abstract: This paper highlights some recent efforts to extend the use of medium-Mn steels for applications other than intercritically batch-annealed steels with exceptional ductility (and strengths in the range of about 1000 MPa). These steels are shown to enable a range of promising properties. In hot-stamping application concepts, elevated Mn concentration helps to stabilize austenite and to provide a range of attractive property combinations, and also reduces the processing temperatures and likely eliminates the need for press quenching. The “double soaking” concept also provides a wide range of attractive mechanical property combinations that may be applicable in cold-forming applications, and could be implemented in continuous annealing and/or continuous galvanizing processes where Zn-coating would typically represent an additional austempering step. Quenching and partitioning of steels with elevated Mn concentrations have exhibited very high strengths, with attractive tensile ductility; and medium-Mn steels have been successfully designed for quenching and partitioning using room temperature as the quench temperature, thereby effectively decoupling the quenching and partitioning steps.

Abstract: The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous cooling process (CCP) and isothermal transformation process (ITP). The results indicate that for the CCP treatment, Al addition significantly promoted the ferrite and bainite transformation; however, it did not significantly increase the product of tensile strength and total elongation (PSE). For the ITP treatment, Al addition significantly promoted the kinetics of bainite transformation, and thus, more bainite was formed with Al addition; however, it was found that Al addition resulted in a decrease in tensile strength and an increase in elongation of the tested bainitic steels. Moreover, the effects of Al addition on comprehensive property were profoundly dependent on austempering temperatures. When the austempering temperature was higher (430 °C), PSE significantly increased with Al addition, whereas it decreased at the lower austempering temperature (400 °C). Therefore, it can be concluded that the effects of Al on properties of bainitic steels were more significant at higher austempering temperatures.

Abstract: Austempered ductile iron (ADI) is a heat-treated nodular iron variant that has for a given strength level much higher elongation than conventional iron with spheroidized graphite. This paper considers the effect of niobium addition on the graphite microstructure, bainite microstructure, bainite transformation process and properties such as hardness and impact toughness under given heat-treatment conditions. The resulting properties are significant with regard to the wear resistance of the material. The effects of niobium addition on the graphite morphology can be understood based on a detailed precipitation analysis of the niobium precipitation behavior in the liquid phase. Niobium influences the temperature and kinetics of pearlite as well as bainite transformation. Furthermore, niobium refines the bainite microstructure generated during austempering treatment. It was found that niobium addition in the range of 0.2–0.5 wt percent allows obtaining an optimum combination of hardness, impact toughness and wear resistance.

Abstract: Given the transformation stasis after the incomplete transformation of carbide-free bainite, this paper aimed to study the effects of austempering time within transformation stasis on the bainite microstructure, monotonic deformation behavior, and especially cyclic deformation behavior using an ultrahigh silicon (2.59 wt%) steel. With increasing austempering time, the dislocation density of bainitic ferrite decreased, carbon content of retained austenite slight increased, and carbon distribution in retained austenite blocks gradually homogenized. The best combination of strength and ductility was obtained through a longer austempering time within transformation stasis. But the longer austempering time, indeed, did not result in longer fatigue life. The opposite trend could be explained by the fact that the primary factors affecting these mechanical properties were different. The lower density of mobile dislocation pre-existed in the starting microstructure of the samples austempered for a longer time was primarily responsible for its lower cyclic hardenability. Moreover, one retained austenite block with completely homogeneous carbon distribution was only transformed to one martensite grain, which increased the cyclic softening and degrade the fatigue life. By contrast, the deformation-induced martensite transformation from the retained austenite with higher mechanical stability in the samples austempered for a longer time enhanced strain hardenability at higher monotonic tensile strains and well delayed the necking, thus improving the combination of strength and ductility.

Abstract: This study investigates the effect of austempering temperature and time on the microstructural and mechanical properties of unalloyed Compacted Graphite Iron (CGI) with an initially ferritic matrix ...

Abstract: The effect of magnetic field on bainite microstructure and mechanical properties of 70Si3MnCr steel was studied. Microstructure observation shows that introducing magnetic field during the austempering process, the microstructure was significantly refined and the volume fraction of the bainitic ferrite was increased. The high nucleation rate for bainitic ferrite caused by the high magnetostatic energy should be responsible for this positive effect. As compared with samples without magnetic field treatment, the magnetic samples also show a simultaneously increased strength and ductility. The ultra-fine bainitic ferrite with higher dislocation density, in magnetic samples increased the tensile strength, and carbon-riched filmy retained austenite and small blocky retained austenite in magnetic samples have a tremendous effect on ductility. High magnetic strength in magnetic samples increases the strength of the bainitic ferrite, which acts as a stress shielding source to prevent the retained austenitic phase from higher stress and increase the plasticity. Moreover, high magnetic strength can alter the spins of the free radicals between the dislocations and obstacles from S state to T state which had weak bonding with dislocations. Hence, depinning of dislocations is facilitated which results in increased plasticity. Therefore, due to the magnetoplasicty, high magnetic properties, and ultra-fine microstructure, magnetic samples show a more excellent strength and ductility as compared with the normal samples.

Abstract: Experimental research of cutting force components during dry face milling operations are presented in the paper. The study was provided when milling of ductile cast iron alloyed with copper and its austempered ductile iron after the proper austempering process. In the study, virtual instrumentation designed for cutting forces components monitoring was used. During the research, orthogonal cutting forces components versus time were monitored and relationship of cutting forces components versus speed, feed and depth of cut were determined by artificial neural network and response surface methodology. An analysis was made regarding the consistency of the measured cutting forces and the values obtained from the model supported by an artificial neural network for the investigated interval of the cutting regime. Based on the results, an analysis of the feasibility of the application of austempered ductile iron in the industrial sector with the aspect of machinability as well as the application of the models based on artificial intelligence, was given. At the end of the presentation, the influence of the aforementioned cutting regimes on cutting force components is presented as well.

Abstract: An in situ neutron diffraction experiment during austempering of low-alloyed transformation-induced plasticity steel, Fe-1.48Si-1.52Mn-0.15C, in wt pct was conducted. In this study, time-of-flight neutron diffractometer with a large detector coverage, iMATERIA at J-PARC MLF, was employed. The phase fraction and carbon concentration in austenite could be quantitatively determined with a time resolution 1 minute although considerable textures existed for both ferrite and austenite. The carbon concentration in austenite during austempering was found to be inhomogeneous, resulting in a bimodal concentration distribution. The low-carbon region was consumed by bainite transformation whereas the high-carbon austenite slightly increased and even survived the final cooling to room temperature, forming a retained austenite. The rate of bainite transformation was affected by the state prior to the start of austempering. Consequently, different morphological features of the retained austenite were formed. More block-shaped austenite was observed in the case of slower bainite transformation, and it was determined that film-shaped austenite could also exist. The average carbon concentration was similar to that of high-carbon austenite during austempering. Hence, the film and block shapes of the retained austenite do not necessarily reflect the difference in carbon concentration.

Abstract: The objective of this research was to investigate the bainitic formation in austempered SAE 52100 steel. The original microstructure of SAE 52100 steel consisted of spheroidized pearlite. In the formation of bainite, SAE 52100 steel samples were austenitized at a temperature of 849 °C for 20 min, and then quickly quenched to the isothermal temperatures between 232 °C and 427 °C for various holding times from 20 s to 120 min. The hardness and microstructure of austempered SAE 52100 samples were analyzed using a Rockwell C hardness tester and metallurgical optical microscope. In addition, the bainitic formation in SAE 52100 steel was characterized using the theory of kinetic phase transformation. The activation energies of lower bainite and upper bainite microstructure were found to be 4.72 × 104 J/mol and 6.07 × 104 J/mol with frequency factors of 23.03 (1/s) and 273.65 (1/s), respectively.

Abstract: In this paper, we investigated the effect of titanium modification on microstructures and properties of carbidic austempered ductile iron (CADI) containing 3.72 wt.% C, 2.77 wt.% Si, and 0.51 wt.% Mn. The results showed that with the addition of 0.1 wt.% Ti, TiC particles were formed, and distributed in carbides and matrix. Microstructural observation indicated that the maximum diameter of graphite nodules decreased from 58 to 40 μm, and the shape of carbides transformed from network to fragmentation, which was related to the nucleation of the eutectic structure (γ + (Fe, Cr)3C) where TiC particles act as the nucleus. After isothermal quenching at 300 °C, the hardness of Ti-bearing CADI was 3.5 HRC higher than ordinary CADI, and the impact toughness was increased by 14.3%. Additionally, due to the decrease in exfoliation, the wear loss of Ti-bearing CADI was 14.8% lower than ordinary CADI during the block-on-ring wear test at the surface load of 300 N.

Abstract: Carbide free nano bainitic steels have recently become a promising material for several applications such as rails, armour etc. because of their impressive combination of mechanical properties. Very high strengths up to 2500 MPa have been achieved because of the very fine nano-scale microstructure obtained through transformations at low austempering temperatures. However, the low ductility and impact toughness are the limiting factors which prevent their use in several applications. It is well known that alloying elements such as Nb and V are added in very small quantity in order to improve strength and ductility of low and medium carbon steels through Nb, V (C, N) precipitates. Influence of these microalloying elements on the mechanical properties of nano bainitic steel is not well understood. In the present study, mechanical properties of two high carbon nano bainitic steels with 0.037 wt% Nb and without Nb addition have been studied. Nb addition results in a substantial increase in ductility (total % elongation) from 16 % to 28 %. Effect of Nb addition on transformation kinetics, microstructure and mechanical properties is also discussed.

Abstract: Ultrahigh-strength low alloy steels transformed below the martensite start temperature (MS) with a mixed microstructure consisting of lath martensite, lower bainite, and minor amounts of retained austenite are of interest in a variety of potential commercial applications as they exhibit good combinations of strength, ductility, and toughness. However, their use requires a knowledge of the resistance to crack propagation. Analysis of the microstructure and fractography show that it is possible to distinguish the fracture behavior of steels austempered below MS from those quenched and tempered. The sharp-notch plane-strain fracture toughness KIC has been determined by the notched tensile testing and modeled with different expressions from literature. Based on pioneer works of Irwin (1962) [20] and Naylor (1979) [22], KIC associated with austempering below MS has been modeled with the aspect ratio of the microstructural subunits and monotonic plastic zone size ahead of the crack tip. A comparison between existing expressions and the new one is conducted. It is found that modeling with the new expression delivers the best agreement with the experimental values.

Abstract: This article presents the results of a set of innovative research to analyse a new structural material – steel like austempered ductile cast iron with mechanical properties similar to heat-treated steel, and with technological and exploitation characteristic close to high-strength cast iron with nodular graphite. Specific examples of its high efficiency for the manufacture of critical, mass and heavy-loaded components of modern machinery and equipment are presented. New structural material MoNiCa was tested on tension and was compared to commercial grades of austempered ductile cast irons and heat treated steels. Unconventional behaviour of examined material was observed: with tensile strength increase to almost 1600 MPa the metal became less brittle. Hardness test revealed another distinctive feature of new material that there is no linear relation between strength and hardness increase, different hardness values ~ 40 HRC or ~ 55 HRC can be achieved with the same tensile strength 1000 – 1200 MPa. Microscopic analysis demonstrated of sophisticated structure formed owing to different regimes of austempering treatment. Specific properties emerged from successful ratio of three elements Mo, Ni, and Cu (carbon equivalent), and properly chosen heat treatment modes. DOI: http://dx.doi.org/10.5755/j01.ms.25.3.23079

Abstract: Shortening the austempering time for bainitic transformation is beneficial for the application of high-performance bainitic steel, which has attracted lots of attentions from materials researchers. It is interesting to find that adding nitrogen, ∼210 ppm, in a medium carbon steel can effectively accelerate the bainitic transformation process. Analysis results reveal that the heterogeneous nucleation sites provided by AlN particles should be mainly responsible for the accelerated bainitic transformation.

Abstract: The presence of vanadium, as an alloying element, has various effects on the properties of cast irons. In this research, the effects of adding different amounts of vanadium, including 0, 0.87, and 1.45 wt % V, on the microstructure, the formation of different phases, and the mechanical properties of the as-cast and austempered ductile iron have been investigated. After the casting of samples, preparation of the samples, and determining of their chemical composition, it is austenitization heat treatment at 900°C for 45 minutes and austempering heat treatment at 350°C for 60 minutes that were carried out on the samples. Tensile and impact tests, as well as X-ray diffraction (XRD) analysis and metallography, were conducted to study the mechanical properties and the structure. The microstructures of the samples included carbides in the ausferrite matrix. The results show that with increasing the vanadium, tensile strength and impact energy decrease in as-cast ductile iron, whereas heat treatment can improve them.

Abstract: The development of multiphase steels to obtain an optimum balance between strength and ductility is a very active topic of research. In particular, carbide-free bainitic steels have shown promising mechanical properties, making them good candidates for replacing well-established first-generation steels in the automotive industry. In this work, a detailed analysis of the microstructures attainable through overaging treatments is tackled in two bainitic steels with different Si contents. The focus has been put onto the mechanical characterization, via nanoindentation, of the phases that are generated as a consequence of the change in the bainitic treatment conditions and the final cooling to room temperature. The results show the suitability of the nanoindentation technique for gaining knowledge about the underlying transformation-related phenomena and for measuring the relative difference in hardness of the various micro-constituents. The latter is a key factor in understanding the origin of the damage in this kind of steels.

Abstract: In this study, to investigate the friction and wear behaviours of cutting tools made of YG8-cemented carbide in the process of cutting austempered ductile iron, friction and wear tests at different...

Abstract: Hot forming combined with austempering and quenching and partitioning (QP) processes have been used to shape two cold rolled high silicon steel sheets into hat profiles. Thermal simulation on a Gleeble instrument was employed to optimize processing variables to achieve an optimum combination of strength and ductility in the final parts. Microstructures were characterized using optical and scanning electron microscopy and X-ray diffraction. Tensile strengths (Rm) of 1190 and 1350 MPa and elongations to fracture (A50mm) of 8.5 and 7.4%, were achieved for the two high-silicon steels having 0.15 and 0.26 wt % C, respectively. Preliminary results show that press hardening together with a QP heat treatment is an effective method of producing components with high strength and reasonable tensile ductility from low carbon containing steels that have the potential for carbide free bainite formation. The QP treatment resulted in faster austenite decomposition during partitioning in the steels in comparison with an austempering treatment.

Abstract: In-situ synchrotron diffraction and atom probe tomography (APT) have been used to study the carbon diffusion and redistribution process in austempered ductile iron (ADI) during austempering. The process of carbon content change in bainitic ferrite during different austempering temperatures has been determined quantitatively. The transformation in ADI is controlled by decarburization of supersaturated ferrite and carbide precipitation and has been found to be divided into three stages based on a model developed for bainitic steels by Takahashi and Bhadeshia. The formation, morphology and composition of carbides and carbon clusters in ferrite after austempering have been identified unequivocally by APT. Finally, the relationships of carbon content in ferrite, carbon gap values, and austempering temperatures in the ADI alloy were expressed using empirical equations.

Abstract: In this paper, the effects of copper addition (0–2.0 wt.%) on the microstructure and properties of carbidic austempered ductile iron were studied using an optical microscope, a scanning el...