Abstract: A carbide-free bainite steel of 0.29C–2.50Si-1.50Mn-1.0Mo-0.97Cr (wt.%) with no voids and cracks was fabricated via laser powder deposition (LPD). Different post-treatment plans were conducted and then microstructures and properties of the steel were studied through optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffractometry (XRD), transmission electron microscopy (TEM), hardness meter and tensile test machines. Obtained results show that the microstructure of the deposited specimen consisted of granular pearlite and retained austenite (RA). The yield strength, tensile strength and elongation were 662 ± 71 MPa, 1051 ± 65 MPa and 13.03 ± 1.33 %, respectively. After austempering at 578 K, the microstructure of the specimen changed to bainite ferrite (BF) and RA. Moreover, it was found that the yield strength and tensile strength significantly increased to 1156 ± 60 MPa and 1289 ± 110 MPa, respectively. However, it had poor strain hardening capacity and elongation remained unchanged. After hot rolling and austempering, the RA content of the specimen significantly increased and the strain hardening capacity improved. However, when the isothermal temperature was less than the Ms temperature (i.e. 553 K), the corresponding microstructure consisted of martensite, BF and 13.6 vol% of RA. In this case, the yield strength, tensile strength and elongation were 895 ± 74 MPa, 1157 ± 92 MPa and 14.59 ± 1.13 %, respectively, indicating slight improvement. Meanwhile, it was found that when the isothermal temperature exceeded Ms temperature, the mechanical properties improved significantly. The present study demonstrated that specimens with an isothermal temperature of 578 K had the best mechanical properties. In particular, the yield strength, tensile strength and elongation of the optimized alloy were 1177 ± 44 MPa, 1398 ± 57 MPa and 19.80 ± 0.79 %, respectively.

Abstract: In this work, effect of martensite pre-quenching on the subsequent bainite transformation kinetics of GCr15 bearing steel has been investigated by means of dilatometry. The results show that the pre-quenched martensite is significantly accelerate nucleation of subsequent bainite transformation in comparison with direct bainite austempering. Meanwhile, specimen of martensite pre-quenching at 200 °C has the shortest incubation period of bainite transformation at 240 °C. Moreover, the transformation rate of bainite transformation after martensite pre-quenching is faster than direct bainite austempering in the initial growth stage, and decrease with the decreasing of martensite pre-quenching temperature. Compared with the conventional quenched and tempered (QT) specimen, the tensile strength is slightly decrease from 2170 MPa to 2143 MPa, but the impact toughness is obviously increase from 43 J to 71 J in specimen heat-treated by martensite pre-quenching at 200 °C and subsequent bainite transformation at 240 °C. In addition, the mechanical stability of retained austenite in all martensite pre-quenching specimens are higher than that of QT specimen.

Abstract: A nanobainitic matrix composing of nanoscale α phase and carbon-riched γ phase was obtained for the ductile iron by austempering treatment The microstructure and properties of ductile iron were compared with GCr15 The microstructural characteristic gave rise to a good strength with an ultimate tensile strength higher than 1200 MPa, which was comparable and even superior to conventionally forged steels The friction coefficient of the ductile iron was approximate 1/3 compared with that of commercial GCr15 under dry sliding wear condition, which was related to the existence of homogeneously distributed spheroidal graphite in ductile iron The low friction coefficient and moderate strength resulted in a superior wear resistance of ductile iron with nanobainitic matrix by contrast to GCr15

Abstract: In this study, the effects of austempering treatment on the microstructure, mechanical properties, and fracture mechanisms of Fe-0.4C-1.5Si-1.5Mn transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel were investigated. When the austempering time increased from 0 to 3600 s, the volume fraction of retained austenite and its carbon concentration initially increased and then decreased. The increase in carbon concentration during austempering may be attributed to the formation of fine bainitic ferrite, representing the matrix and in turn the formation of retained austenite with a film-type morphology. When the austempering time was increased from 0 to 1000 s, the TBF steel showed excellent mechanical properties such as high yield and tensile strengths, and uniform and total elongations. Meanwhile, void density increased significantly in the initial stages and then slightly decreased; in contrast, the trend observed for mean crack length was opposite to that of void density leading to the change in the fracture mode of the TBF steels from intergranular to quasi-cleavage and dimple fractures. It could be inferred that the observed improvement in mechanical properties, especially the good ductility of TBF steels, at the optimal austempering time is due to the suppression of crack propagation and crack blunting to form voids owing to the effective deformation-induced transformation of metastable retained austenite.

Abstract: In this study, the effect of the austempering times on the microstructures and mechanical properties of dual-matrix structure austempered ductile iron (DMS-ADI) was investigated. With this aim, unalloyed as-cast ductile iron tensile samples were austenitized at 810 °C for 30 min to intercritical austenitizing followed by austempering at 350°C for various austempering times (45 min to 180 min). Experimental results showed that dual-matrix structures consisting of proeutectoid ferrite + ausferrite were obtained in austempered ductile iron from intercritical austenitizing temperatures. It was determined that as the austempering time increased, the ausferritic structure became considerably clear and its volume fraction was almost constant (45–47%) after 90 min of austempering time. The yield and tensile strength of the samples decreased and the total elongation and breaking energy increased with increasing austempering times, but after the 120-min austempering time, both the total elongation and breaking energy of the samples decreased. It was established that the austempering times had no significant effect on the morphology of ausferrite. The best mechanical properties were obtained between 70- and 130-minute austempering times that can be defined as the processing window. In addition, it was determined that the DMS-ADI had similar austempering kinetics with the ADI.

Abstract: Austempered ductile iron castings (ADI) are characterized by the high strength and resistance to fatigue, impact, and wear. ADI mechanical properties are obtained by performing a heat treatment on ...

Abstract: Owing to their exceptional physical, mechanical, and technological properties, ductile cast irons have been increasingly used, especially in automotive industries. Alloying elements and heat treatment routes can increase the strength of ductile irons. In this work, the effects of austempering temperature and time on the microstructure and mechanical properties of a ductile cast iron modified by 0.35% Nb have been studied. Two austempering temperatures of 320 °C and 360 °C for a holding time of 15, 30, 60, and 90 min were applied. The niobium addition was found to be effective in homogenizing the size and distribution of graphite nodules, maintaining the degree of nodularization above 90%, favoring pearlite refining, increasing the volume fraction of pearlite, forming niobium carbides in dispersed blocks in the matrix and graphite nodules, increasing both the yield and tensile strength, and reducing the elongation. Both the niobium addition and austempering temperature improved the yield and tensile strength but decreased the elongation compared to those of the reference ductile iron. The best combination of strength and ductility was observed at an austempering time of 60 min. The effect of austempering temperature on the microstructure and tensile mechanical properties of tested irons was found to be crucial.

Abstract: In engineering applications, to increase productivity and to decrease production costs, the selection of the proper engineering material is essential. At that point, machining operations directly affect the production costs. Therefore, determination of the material with the desired mechanical properties and easy-to-cut characteristics has a critical importance. This situation is currently gaining more importance in especially defense industry applications in which high strength engineering materials are heavily employed. In addition, tool performance and final product quality are directly influenced by the cooling and/or lubrication conditions in particularly interrupted cutting operations. In this study, machinability characteristics of G18NiMoCr3-6+QT1 cast steel (CS) and 1050-6 austempered ductile iron (ADI) with similar mechanical properties during milling operations were investigated. The tests were performed using TiAlN coated cemented carbide (WC-Co) end mills under dry, conventional cutting fluid (CCF), and minimum quantity lubrication (MQL) conditions. Under each condition, the variations of cutting forces, tool wear, average surface roughness (Ra), and subsurface microstructure and microhardness were analyzed for both materials and then compared to one another. Test results showed that 1050-6 ADI led to further tool wear in comparison to G18NiMoCr3-6+QT1 CS. According to obtained results, dry condition is more favorable than CCF and MQL conditions in terms of cutting forces, surface roughness, and tool wear for both types of material. In addition, examinations on subsurface microstructures showed that MQL conditions provided an effective cutting environment to maintain microstructural stability of workpiece materials.

Abstract: The low-cycle fatigue properties, microstructural evolution and crack growth behaviors of a low-carbon carbide-free bainitic steel subjected to austempering at 300, 320 and 350 °C have been investigated. The results indicated that the fatigue lifetimes of the specimens increase with decreasing the austempering temperature at lower total strain amplitudes, but a comparable fatigue lifetime or even an opposite result is observed at higher total strain amplitudes. During fatigue loading, the blocky retained austenite with inhomogeneous carbon distribution partially transforms into martensite, and the transformation amount increases with the total strain amplitude, especially for the specimens austempered at high temperatures. The transformed martensite and the untransformed austenite satisfy a Kurdjumov-Sachs (K–S) orientation relationship, which can increase the cooperative deformation capacity between the both phases. However, with continuous cycling, more martensite is formed, and these newly formed martensite grains are more likely to form micro-voids/cracks due to their brittleness, which reduces the crack initiation lifetime, and become the fast path for crack propagation, which deteriorates the crack propagation lifetime. Alternatively, more crack branching, crack deflection and severely twisted bainitic ferrite laths are observed for the specimens austempered at high temperatures, which helps to delay/hinder crack growth and increase the crack propagation lifetime, especially at high total strain amplitudes. This suggests that the fatigue lifetime can be closely associated with the interactions of the crack initiation and propagation caused by mechanically-induced martensite from retained austenite.

Abstract: In this research work, heat-treatment processes have been utilized to obtain multiphase microstructure in the silicon rich steel samples, silicon in the steel helps in the development of multiphase microstructure and to keep away from carbide precipitation development through the austempering. The desired multiphase microstructure (Retained austenite-RA, Ferrite-F, Bainite-B and Pearlite-P) consisting of continuous cooling (CC) for 0, 20 and 40 s respectively after austenization followed by austempering at (300, 350 and 400 °C) to form a high wear resistance multiphase steels with microstructure varies amount of F,B, P and RA during continuous cooling. Steels with varies retained austenite up to (5±1.1 to 18±1.9%) along with excellent specific wear rate (2.038 × 10−9-1.061 × 10−8 m3/N-m) were obtained. Further, the rolling/sliding wear rate has been obtained through the disc-on-disc experimental setup. The effect of continuous cooling on retained austenite, carbon content of retained austenite on specific rolling/sliding SWR (specific wear rate) has been studied; phase fraction (P, B and F) and the materials characterization with the help of XRD, AFM and FE-SEM.

Abstract: The present work has been carried out to study the different heat-treatment processes to obtain carbide-free bainite in the high silicon spring steel. Silicon content helps to develop carbide-free bainite (compositions of steel is 0.551% C, 1.756% Si, 0.825% Mn, and 0.13% Cr) and also, to avoid carbide precipitation formation during austempering. The desired microstructure of treated samples has been achieved through heat treatment process at different austempering temperatures such as 300 °C, 350 °C, and 400 °C at holding time of 10, 20 and 30 min. Further, the wear rate of the base and treated samples has been analysed through the pin-on-disc testing machine. The retained austenite helps to resist the crack initiation and propagation through transformation during deformation, and it also improves wear resistant and hardness of the treated sample. The effect of fraction of retained austenite and its carbon content on the specific wear rate has been systematically studied. The phase fraction and retained austenite stability have been critically analysed with the help of X-ray diffraction, atomic force microscopy and scanning electron microscopy.

Abstract: The effects of partitioning treatment on microstructure and mechanical properties of austempered ductile iron (ADI) were investigated in the present study. The microstructure of the ADI sample was composed of spheroidal graphite, nano-scale α phase, and high-carbon γ phase. During the partitioning process, the carbon atoms transported from α phase to γ phase, which resulted in the coarsening of α phase and the increase of carbon concentration in γ phase. The carbon concentration in γ phase was increased from 1.6 wt% for ADI to about 2.1 wt% for the sample after partitioning treatment. These microstructure variations resulted in an increase in plasticity by about 50% without sacrificing strength.

Abstract: A study of surface hardening of Ductile Iron (DI) with and without austempering heat treatment was developed. The chemical composition of the material contains alloying elements such as Cu and Ni, that allow to obtain a Ductile Iron Grade 120-90-02, based on ASTM A536, which was heat treated to be transformed to Austempered Ductile Iron (ADI). Specimens of 10 × 10 × 5 mm3 were obtained for application of surface hardening by Nd:YAG UR laser of 150 W maximum power. The parameters used were advance speed of 0.2 and 0.3 mm/s and power at 105, 120, 135 and 144 W; the departure microstructures were fully pearlitic in the samples without heat treatment, and ausferrite for austempered samples. Microstructural characterization of hardened samples was performed were analyzed and martensite and undissolved carbides were identified in the pearlitic samples, while in ausferrite samples it was found finer martensite without carbides. The depth of hardened surface to different conditions and their respective microhardness were measured. The results indicate that the surface hardening via laser is a suitable method for improving wear resistance by means of hardness increment in critical areas without compromising the core ductility of DI components, but the surface ductility is enhanced when the DI is austempered before the laser hardening, by the reduction of surface microcracks.

Abstract: The influence of the austempering temperatures on the microstructure and mechanical properties of austempered ductile cast iron (ADI) was investigated. ADI is nodular graphite cast iron, which owing to higher strength and elongation, exceeds mechanical properties of conventional spheroidal graphite cast iron. Such a combination of properties is achieved by the heat treatment through austenitization, followed by austempering at different temperatures. The austenitization conditions were the same for all the samples: temperature 890 °C, duration 30 min, and quenching in a salt bath. The main focus of this research was on the influence of the austempering temperatures (270 °C, 300 °C, and 330 °C) on the microstructure evolution, elongation, toughness, and fatigue resistance of ADI modified by certain amounts of Ni, Cu, and Mo. The Vickers and Rockwell hardness decreased from 535.7 to 405.3 HV/1 (55.7 to 44.5 HRC) as the austempering temperature increased. Optical images showed the formation of graphite nodules and a matrix composed of ausferrite; the presence of these phases was confirmed by an XRD diffraction pattern. A fracture surface analysis revealed several types of the mechanisms: cleavage ductile, transgranular, and ductile dimple fracture. The stress-controlled mechanical fatigue experiments revealed that a 330 °C austempering temperature ensures the highest fatigue life of ADI.

Abstract: The effect of the austenitization route on the bainitic reaction kinetics of an alloyed (3.2C–2.8Si–1.8Ni–1.4Cu–0.4Mn–0.2Mo–0.1Cr) austempered ductile iron was studied. Two-step, conventional and rapid austenitization heat treatments were employed to produce different austenite grains sizes (94, 39, and 15 μm, respectively) and, in one case, secondary graphite precipitation. The overall bainitic transformation kinetic at 350 °C was described using the Johnson-Mehl-Avrami-Kolmogorov equation, and the values of the Avrami's adjustable parameters were discussed. The austempering reaction of the coarser austenite microstructure with secondary graphite precipitation featured the fastest kinetics, while the one derived from the medium austenite grain showed the slowest reaction rate. The increase in the graphite/austenite interfacial area reduced the half-transformation time (t50-value) by one magnitude compared to the austenite grain boundary area. The austempered samples from the rapid austenitization route comparatively featured the best tensile properties and the highest ISO and ASTM standards grades despite the considerable proportion of grain boundary allotriomorphic ferrite.

Abstract: steel was investigated in the present contribution. The double soaking heat treatment is a two step intercritical annealing heat treatment, which generates microstructures of athermal martensite, retained austenite and ferrite when applied to medium manganese steels. Microstructures following double soaking and (aus)tempering contained a combination of retained austenite, athermal or tempered martensite, and blocky or bainitic ferrite. X-ray diffraction, dilatometry and transmission Kikuchi diffraction were utilized to investigate microstructural changes which occurred during tempering or austempering. The resulting mechanical properties were measured using uniaxial tensile testing. The double soaking plus tempering heat treatment was shown to generate an ultimate tensile strength of 1340 MPa in combination with 28 pct total elongation while the double soaking plus austempering heat treatment resulted in an ultimate tensile strength of 1675 MPa and total elongation of 22 pct. Overall, both novel heat treatments produced a combination of strength and ductility desired for the third generation of advanced high strength steels.

Abstract: Austempered Ductile Iron (ADI) is a candidate material to replace case-hardened steel in many applications. The mechanical properties of ADI can be tailored by the chemical composition and heat treatment conditions. In this study, different nickel content Ductile Irons (DI) were cast and heat treated. In order to design the austempering process, time–temperature–transformation diagrams were generated with the Calculation of Phase Diagrams method. For each chemical composition, one group was tested in its as-cast state while the second and third groups were austempered. For austempering, the casting is reheated to austenitization temperature (900 °C) and then quenched in a salt bath at a temperature of 300 °C and held at this temperature for 2 h. The third group was cryo-treated (−196 °C for 6 h) and tempered (200 °C for 2 h) after the austempering process. Microstructural examination was performed using an optical microscope and X-ray diffraction technique. The effect of heat treatment on the hardness, toughness, and tribological behaviors of samples was investigated. The results showed that austempering with correct parameters significantly improved the hardness, toughness, and wear resistance of DI. The nickel content of DI plays a significant role in determining the properties of the alloy, and the optimum Ni amount among the tested compositions was found to be 1.64%. It was observed that cryogenic treatment facilitates some of the austenite to martensite transformations and improves wear resistance (20%); however, it has a limited effect on hardness (2–3 HRc) and toughness (±3 J).

Abstract: The plastic deformation and failure initiation of super bainitic steels containing different amount of bainite were analyzed using a real microstructure-based micromechanical finite element method (FEM). Real micrograph of such phase steels obtained by microscope were utilized as the representative volume elements (RVEs) in the micromechanical approach. Ductile mode of failure throughout selected RVE was simulated in term of strain localization for the duration of deformation. Calculations were carried out on the representative volume to numerically analyze the effects of characteristics of every individual constituents phase on the macroscopic mechanical properties of steels regarding their quantitative volumetric amount. The results obtained from simulation were assessed against experimental records and it was determined that the real microstructure-based FEM can predict strength and also ductility of the studied bainitic steels. Furthermore, shear failure mode occurs in all heat treated state of the studied steel which correlate quite well with experimental findings.

Abstract: Nano-bainitic steels have attracted great attention for good wear resistance. In this research, a medium-carbon low-alloyed steel was austempered at a low temperature close to its martensite-start temperature for various times to obtain mixed microstructure of nano-bainite, martensite and retained austenite. The austempered samples were characterised comprehensively by field-emission SEM and quantitative XRD. Its two-body abrasive wear property was evaluated by sliding on a SiC abrasive disc. The results revealed the formation of initial nano-width carbide-free bainitic ferrite (BF) after austempering by 10 min, whereas the BF size and amount both increased with the austempering time. The austempered samples exhibited wear coefficients lower than the quenched martensitic sample by up to 50%. SEM and TEM observations showed wear mechanisms of micro-cutting and ploughing deformation, including the formation of a nano-laminate top layer and bending deformation in the subsurface multiphase microstructure. The decreased wear loss was attributed to the role of retained austenite in the increased plasticity.

Abstract: The austempering above and below martensite transition temperature (Ms) was employed in a medium-carbon low-alloy 40CrNiMo steel, and the bainite and martensite multiphase microstructures with different volume fractions were obtained. Here, the effect of pre-existing martensite on subsequent transformation of bainite microstructure and mechanical properties is focused and researched. The microstructure with a volume fraction of pre-existing martensite (VPM), bainite (VB), and martensite/austenite (VM/A) constituents of approximately 28%, 46%, and 26%, respectively could be obtained by austenitizing below Ms (280 °C) for 1 h, and an optimum combination of strength, ductility, and impact toughness (yield strength of 1420 MPa, ultimate tensile strength of 1795 MPa, total elongation of 7.9%, and V-notch impact value of 37 J) was achieved. The considerable enhancement of mechanical properties in the sample austenitized below Ms is mainly ascribed to the formation of the pre-existing martensite, resulting in an effective reduction in the size of the bainite plates and martensite/austenite constituents.

Abstract: The microstructural evolution and properties for varied austempering routes are investigated in a cold-rolled bainitic steel. Special attention is given to the effect of retained austenite (RA) in terms of its fraction, carbon concentration, and morphology resulting from different austempering routes on mechanical properties and stretch flangeability. Bimodal sized bainitic laths are provided, and the carbon concentration of RA maintains the highest value through the two-step austempering. Total elongation (TEL) is remarkably enhanced for the two-step austempering, deviating from the exponential relationship between tensile strength (TS) and TEL as maintained by the one-step austempering. Considering the two plateaus of the strain-hardening exponent, it is considered that the hierarchical stability of RA is provided by the two-step austempering, leading to the postponed necking point so as to improve the uniform elongation. Two-step austempering could provide more complete bainitic transformation as well as more stable film-like RA, supplying a promising way to improve the combination of strength, ductility, and stretch flangeability.