Search

This research focuses on the development of carbide-free nanostructured bainitic steel through the process of austempering at 250°C. The selected steels are also patented at 550°C, to get fine pearlitic structure. In austempered steels, microscopic analyses encompassing optical, scanning, transmission electron microscopy and X-ray diffraction revealed the presence of nanoscale bainite, filmy and blocky austenite. In contrast, lamellar pearlite was observed in the patented steels. With increase in the austempering duration, the extent of bainite improved, while the volume percentage of blocky retained austenite (RA) decreased. The tribological performance of high-carbon bainitic steels is compared with patented one of same composition against a tungsten-carbide counter disc. The specific wear rate as well as coefficient of friction decreases with rise in load from 10 to 50 N. The hardening volume of B15VA-1 sample is greater than that of B15VA-2 and P15VA, and it is mainly due to the transformation of blocky RA to strain-induced martensite. The bainitic pins with higher hardness exhibited superior tribological response than the pearlitic ones. SEM analysis of worn surfaces confirmed that at lower load (10 N), abrasive wear occurs, but at higher load (50 N), wear mechanism changes to adhesive along with abrasive in bainitic steel and oxidative wear along with adhesive and abrasive wear in pearlitic steel. [ABSTRACT FROM AUTHOR]

The previous research on two-step austempering processes have been mainly focused on the bainitic transformation kinetics, with few details the relationships between the parameters of two-step austempering process and the wear properties and fatigue lifetime. In this paper, the effects of two-step austempering process on wear resistance and fatigue life of high carbon nano-bainite steel were investigated. The results show that the first-step bainite with high volume fraction prolongs the total bainite transformation time. The two-step austempering samples contain finer bainitic ferrite plate and higher volume fraction of retained austenite, as well as resent excellent hardnesses, tensile properties, wear resistance and fatigue lifetime. In particular, the wear resistance of the 50%-290 °C sample is better than that of the 210 °C sample, this is caused by the transformation of the high fraction of retained austenite in the two-step austempering samples into hard martensite and the microstructure refinement zone that forms on the surface of the sample during the wear process. The 50%-290 °C sample have better fatigue properties at low total strain amplitudes (0.7% and 0.8%). When the total strain amplitude is 0.7%, the fatigue life of the 50%-290 °C sample is 57.5% higher than that of the 210 °C sample, which is because of the higher yield strength and finer microstructure of the 50%-290 °C sample. These results support the later study of controlling two-step austempering process for high-carbon nanostructured steels.

In this study, modifying the austenitizing temperature offers a strategically viable way towards creating blocky retained austenite (BRA) with varying sizes and morphologies in nanostructured bainitic steel, in strikingly contrast to the conventional method of adjusting the austempering temperature. Dynamic tensile and compressive evaluation at different strain rates of 1 × 103 s−1 and 5 × 103 s−1 were carried out to gain insights into the effect of BRA stability on the dynamic mechanical properties and its intrinsic relationship with high strain rates. Additionally, the phase transformations and the triggered transformation-induced plasticity (TRIP) effects of BRA at different high strain rates were analyzed. The results reveal that during dynamic deformation, most of metastable BRA underwent a two-stage martensitic transformation of γ→εM and εM→α′M, while the filmy retained austenite exhibits minimal phase transformation. BRA dominates the TRIP effect under the extremely short deformation time and adiabatic heating conditioning. Consequently, samples containing more and larger-sized BRA demonstrated superior dynamic mechanical properties, including strength and fracture strain. As the strain rate increases, the TRIP effect induced by less stable BRA contributes to more pronounced enhancement of properties. Besides the deterioration effect originating from the early transformed brittle martensite also gradually intensifies with the strain rate, which is manifested by the appearance of quasi-cleavage facets at the fracture comparable to the BRA size. Nevertheless, during dynamic deformation, the metastable BRA induced TRIP effect significantly enhances the mechanical properties, outweighing its deteriorating effect. Moreover, less stable BRA should be favored at higher the strain rate.

This study aims to produce a cost-effective and low-density, high-strength nanostructured bainitic steel by optimizing the austempering process conditions. To do this, a steel containing 0.8C-1.5Mn-0.6 Cr-0.35Mo-3.5Al was selected and the effects of austempering time and temperature on the microstructure and mechanical properties of this steel were studied. Xrd technique was used to recognize the phases present in the microstructure and measure the retained austenite volume fraction and carbon content. Hardness measurements showed that the highest value was obtained from the specimen containing the highest volume fraction of the bainitic ferrite associated with the retained austenite with the highest carbon content, at the completion time of transformation. Such a combination of microstructure and hardness was applied to find the optimum time at different temperatures during the austempering process. The microstructures of the optimal specimens were evaluated through optical microscopy (OM) and field-emission scanning electron microscopy (FESEM), and the average thickness of the bainitic ferrite plates was calculated. Finally, tensile tests were performed on the optimal specimens to assess the fracture surface through FESEM. For high steel with 3.5 wt% Al, the results indicated that it is possible to obtain a microstructure consisting of bainitic ferrite plates with an average thickness of 53 nm surrounded by the carbon-rich retained austenite with a hardness of 561HV, a yield stress of 1614 MPa, an ultimate tensile strength of 1981 MPa, and a total elongation of 9 %.

The microstructure of nanostructured bainite formed by austempering in high-carbon silicon steel is characterized by high resolution electron microscopy. The results show that the bainitic sheaves consist of nanoscale subunits. The found kinds of crystal lattice distortions of each subunit in one bainite lath show clear difference. The latest generated subunit has most distortions. As the subunit grows up, the lattice distortion gradually gets relaxed. This indicates that the subunits form with high phase transformation strain energy, which causes severe lattice distortion. The results provide new experimental evidence on subjects relevant to the understanding of the mechanism of formation of bainitic ferrite. [ABSTRACT FROM AUTHOR]

In response to the growing interest of the industry in advanced high-strength steels subjected to extreme operating conditions, two novel grades BainTS and BainNiAlCu bainitic steels have been developed. Since the in-use properties of bainitic steel are directly related to the thermal stability of the bainitic structure, two different alloy design concepts to improve tempering resistance considering various strengthening mechanisms were proposed. BainTS steel follows the standard assumptions of nanocrystalline bainitic steel design, where the retained austenite is stabilized with silicon. A potential secondary hardening was also considered using Cr, Mo, and V alloying additives. On the other hand, BainNiAlCu steel was designed taking into account the precipitation strengthening with intermetallic compounds (nickel aluminide and copper particles). It was revealed that both steels after heat treatment consist of bainitic ferrite laths and a significant content of retained austenite (above 25%) with film and blocky morphologies. After the tempering process (350 °C–650 °C), there was significant structural evolution following the gradual decomposition of the metastable bainitic structure. It was proved that BainTS steel is characterized by the higher thermal stability of both bainitic ferrite and retained austenite compared to BainNiAlCu. Despite this, the hardening effect after tempering BainNiAlCu steel was higher (by about 140 HV compared to heat treatment, approx. 30% increase). The presented results suggest that bainitic steels strengthened with intermetallic phases are extremely promising in the context of future industrialization in applications where operating conditions are exposed to elevated temperatures.

In the present study, carbide-free nanostructured bainite is produced by austempering high carbon and silicon steels at 250 °C. Fine pearlitic structure is also made by patenting the steels of selected compositions at 550 °C. The formation of carbide-free nanostructured bainite and retained austenite in the austempered steels and ferrite and cementite in patented steels were confirmed through X-ray diffraction and TEM studies. The optical, scanning and transmission electron microscopies reveal the presence of nanoscale bainite, filmy and blocky austenite in austempered steels and the presence of lamellar pearlite in patented steels. Increasing austempering time enhances the amount of bainite but decreases the blocky retained austenite. The corrosion behaviour of the newly developed bainitic steels is compared with that of pearlitic steel of the same compositions. Electrochemical and immersion corrosion tests are conducted in an aqueous 3.5% NaCl solution. The corroded surfaces are analyzed with the scanning electron microscope (SEM) and X-ray photoelectron spectrometer (XPS). Corrosion resistance increases with increasing carbon percentage due to the formation of magnetite (Fe3O4), provided the content of other alloying elements remains the same. The presence of Ni decreases corrosion rate significantly, i.e., enhances corrosion resistance, charge transfer resistance (Rct) and polarization resistance (Rp). Reducing retained austenite in nanostructured steel decreases the corrosion rate due to the lesser area of Galvanic couples and the formation of uniform, compact, and non-porous passive layer. The bainitic steel samples demonstrated superior corrosion resistance to that of the pearlitic steel due to lesser cell formation. The corrosion mechanisms of high-carbon bainitic and pearlitic steel are discussed. [ABSTRACT FROM AUTHOR]

The influence of martensite on the transformation rate of bainite as well as enhancement of various properties is well established, yet the combined influence of both the primary martensite and low temperature ausforming below the martensite start temperature (MS) on the characteristics of microstructural evolution comprising ultrafine/nanostructured bainite, martensite and retained austenite (RA) in medium C steels has not been unambiguously clarified. A series of physical simulation experiments involving isothermal holding at various temperatures below and above the MS was conducted with or without prior ausforming (∼0.5 strain, 0.5 s−1 strain rate) in order to assess the influence of ausforming on subsequent transformation behaviour. The results suggest that when ausforming was applied above MS, it not only accelerated the bainite transformation kinetics, but also initiated strain-induced bainite/martensite formation. At temperatures below MS, prior ausforming did hinder the onset of bainite transformation, but the transformation rates were faster suggesting the strain-induced acceleration in kinetics. Due to a remarkable refinement of the bainitic structure achieved by prior ausforming in samples held below MS, the Vickers hardness improved appreciably unlike in the case of samples held above MS. Ausforming promoted significant refinement and uniformity of the bainitic plates, and also the orientations of the plates became more randomized. The dislocation density of RA decreased with the reduced content of secondary (fresh) martensite that formed during final cooling to room temperature.

The impact and fracture toughness of a nanostructured, kinetically activated bainitic steel was determined using Standard methods. Prior to testing, the steel was quenched in oil and aged naturally for a period of 10 days in order to obtain a fully bainitic microstructure with a retained austenite content below 1%, resulting in a high hardness of 62HRC. The high hardness originated from the very fine microstructure of bainitic ferrite plates formed at low temperatures. It was determined that the impact toughness of the steel in the fully aged condition improved remarkably, whereas the fracture toughness was in line with expectations based on the extrapolated data available in the literature. This suggests that a very fine microstructure is most beneficial to rapid loading conditions, whereas material flaws such as coarse nitrides and non-metallic inclusions are the major limitation for obtaining a high fracture toughness. [ABSTRACT FROM AUTHOR]

Conducting the carburizing process on the surface layer of low-carbon steel and subsequent austempering heat treatment can be implemented to obtain nanobainite microstructure on the surface of steels. In this research, steel with 0.23 wt.% carbon was carburized for 3 h at 900 °C in a liquid salt bath containing sodium cyanide, sodium carbonate, and sodium chloride and immediately quenched to room temperature. The samples were then heated to 900 °C for 30 min and isothermally transformed at three different temperatures of 200, 250 and 300 °C for 72, 24 and 12 h, respectively. It was found that nanostructured bainite was formed on the surface layer and the subunits of bainitic ferrite and high-carbon austenite films were almost 60-300 nm thick depending on the heat treatment temperature. It was also found that the samples austempered at these temperatures contained 18, 21 and 28% volume fractions of retained austenite on the surfaces, respectively. Due to the comparable microstructural characteristics, similar friction coefficients were obtained as for ordinary nanostructured bulk bainitic steels with high-carbon content. [ABSTRACT FROM AUTHOR]

Nanostructured bainite (NB) bearing steel has excellent strength and ductility combinations, which can improve the fatigue life and wear resistance of bearing steel in harsh conditions. However, the phase transformations and the correlation between the microstructure and wear properties of NB bearing steel are still unclear. In this study, bearing steels with different Si contents (GCr15SiMo and GCr15Si1Mo) were prepared to have nano-bainitic structures, and their microstructure transformations and wear mechanisms were investigated. The results show that the Si element can inhibit the precipitation of carbides and can then promote the block-like retained austenite formation and refine the bainitic ferrite lamellar structure. The impact energy of GCr15Si1Mo is larger than that of GCr15SiMo because the nanostructured bainite and retained austenite are the main toughness phase in these steels. The wear results indicate that the steels which possess appropriate strength and toughness are helpful for improving wear resistance properties. Finally, the wear resistance performance of the GCr15Si1Mo austempered at 210 °C and GCr15SiMo austempered at 230 °C was good in this work. [ABSTRACT FROM AUTHOR]

The impact and fracture toughness of a nanostructured, kinetically activated bainitic steel was determined using Standard methods. Prior to testing, the steel was quenched in oil and aged naturally for a period of 10 days in order to obtain a fully bainitic microstructure with a retained austenite content below 1%, resulting in a high hardness of 62HRC. The high hardness originated from the very fine microstructure of bainitic ferrite plates formed at low temperatures. It was determined that the impact toughness of the steel in the fully aged condition improved remarkably, whereas the fracture toughness was in line with expectations based on the extrapolated data available in the literature. This suggests that a very fine microstructure is most beneficial to rapid loading conditions, whereas material flaws such as coarse nitrides and non-metallic inclusions are the major limitation for obtaining a high fracture toughness.

Excellent mechanical properties of nanostructured bainitic steels are primarily related to the ultra fine scale of their structure constituents. Typically, such structure is exhibited by suitably treated steels with high carbon and high silicon content. Regardless, the common application of this steel grade in the industry is limited by some in-used properties that have not yet been explained. This mainly concerns the possibility of joining, long-term operationing at elevated temperatures and ensuring corrosion resistance. The feature promoting the provision of these properties is the sufficiently high thermal stability of the retained austenite. Enhancing thermal stability would allow for the application of these steels in the automotive, energy and marine industries. In contrast to the well-known control of the mechanical stability of the retained austenite, thermal stability requires a different approach in the context of chemical composition design and heat treatment parameters. This work presents the concept of increasing the thermal stability of the retained austenite and its influence on the improvement of the in-used properties of nanostructured bainitic steels. Mechanisms ensuring higher stability of the microstructure at elevated temperatures were reviewed, such as controlling the morphology and carbon content of retained austenite, solution strengthening, precipitation of intermetallic phases, and the prospect of secondary hardening.

Nanostructured bainite (NB) bearing steel has excellent strength and ductility combinations, which can improve the fatigue life and wear resistance of bearing steel in harsh conditions. However, the phase transformations and the correlation between the microstructure and wear properties of NB bearing steel are still unclear. In this study, bearing steels with different Si contents (GCr15SiMo and GCr15Si1Mo) were prepared to have nano-bainitic structures, and their microstructure transformations and wear mechanisms were investigated. The results show that the Si element can inhibit the precipitation of carbides and can then promote the block-like retained austenite formation and refine the bainitic ferrite lamellar structure. The impact energy of GCr15Si1Mo is larger than that of GCr15SiMo because the nanostructured bainite and retained austenite are the main toughness phase in these steels. The wear results indicate that the steels which possess appropriate strength and toughness are helpful for improving wear resistance properties. Finally, the wear resistance performance of the GCr15Si1Mo austempered at 210 °C and GCr15SiMo austempered at 230 °C was good in this work.

Ausforming is being widely used to overcome the kinetics barrier in nanostructured bainitic steels and to enhance its practical industrial applications. It would also affect the microstructural characteristics and consequently the resultant mechanical performance. This article aims to investigate the tensile behaviour of nano bainite obtained from plastically deformed primary austenite at three different heat treatment stages. 10% of ausforming has been implemented and nanostructured bainite obtained after austempering at 300°C for 2, 4 and 6 h in salt bath furnaces. Results indicated that ausforming could successfully increase the volume fraction of bainite within the microstructure and refine the bainite packets sizes. Moreover, it has been demonstrated that higher mechanical stability of retained austenite and therefore more effective TRIP effect could be attained during the tensile test which consequently resulted in elongation improvement without deteriorating the strength properties. This claimed to be beneficial for materials performance during practical applications. [ABSTRACT FROM AUTHOR]

Presence of nanoscale bainitic ferrites and high carbon retained austenites that are stable at ambient temperature within the microstructures of super strong bainitic steels makes it possible to achieve exceptional strengths and ductility properties in these groups of nanostructured steels. This article aims to study the effect of the dislocation density variations during tensile testing in ambient temperature on deformation behavior of nanostructured low temperature bainitic steels. Results indicate that dislocation absorption from bainitic ferrite subunits by surrounding retained austenite reduces the work hardening and therefore increases the formability of bainitic ferrite during deformation, which in turn results in a suitable combination of strength and ductility.

Abstract The deformation behaviour of two nanostructured bainitic steels designed specifically to ensure the thermal stability of the retained austenite present alongside the bainitic ferrite, has been studied as a function of test temperature. One of the alloys is especially rich in silicon whereas the other in nickel. Surprisingly, the alloys are found to have greater ultimate tensile strengths and ductility when tested at 200 °C, compared with corresponding tests at ambient temperature. This is demonstrated to be a consequence of the more gradual deformation-induced transformation of the retained austenite at 200 °C. In contrast, there is a dramatic reduction in both strength and uniform ductility during testing at 450 °C. Some carbides precipitate during testing of the high-silicon alloy, thus making the austenite less stable to both thermally and mechanically. The high-nickel alloy suffers from the same fate but for different reasons, that the austenite actually grows at the 450 °C test temperature, leading to a reduction in its thermodynamic stability. The experimental data on the stability of the retained austenite both at the test temperature and during cooling from that temperature have been modelled. [ABSTRACT FROM AUTHOR]

Nanostructured bainitic steel has an extraordinary ultrahigh strength of about 2.0 GPa along with good toughness of 30 MPa m1/2. The finer thickness of plate 20-80 nm is largely responsible for achieving such a large hardness of 690-720 HV. In this work, a multilayer perceptron-based artificial neural network (ANN) model has been developed to predict the thickness of bainite plate pertaining to nanostructured bainitic steels. The inputs of the ANN model are, namely, Gibbs free energy for bainitic transformation, austenite strength, transformation temperature for bainite and carbon concentration in the steel. The model prediction revealed that the bainite plate thickness critically depends on austenite strength and Gibbs free energy. From the neural prediction, it is concluded that formation of nanostructured bainitic steel is feasible only if sufficient austenite strength (above 165 MPa) has been achieved. This can be accomplished with a minimum carbon content of 0.5 wt.% in steel and transformation temperature below 300 °C. Higher driving force (Gibbs free energy) below − 1800 J/mol is another prerequisite condition of formation of nanostructured bainite steel. The network-learning architecture has been optimized using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm to minimize the network training error within eight training cycles. The algorithm facilitates a faster convergence of network training and testing errors. [ABSTRACT FROM AUTHOR]

One of the main limitations in application of nanostructured carbide-free bainite as a construction material is the difficulty of joining. This research presents a structural characterization of welded joints of medium carbon 55Si7 grade steel after the welding process with a regeneration technique as well as post welding heat treatment (PWHT). The hardness distribution of the welded joint with regeneration exhibit an overall decrease in hardness when compared to the base material and a significant decrease in hardness was observed in the heat-affected zone (HAZ). Unfavorable hardness distribution was caused by the presence of diffusion-type transformations products (pearlite) in the HAZ and bainite degradation processes. On the other hand, welding with the PWHT promotes the achievement of a comparable level of hardness and structure as in the base material. However, a slight decrease in hardness was observed in the weld zone due to the micro-segregation of the chemical composition caused by the indissoluble solidification structure. Based on the structural analysis, it was found that steel with relatively low hardenability (55Si7) should be welded using PWHT rather than a regeneration technique.

The design of metastable retained austenite is the key issue to obtain nano bainitic steel with high strength and toughness. In this study, nanostructured Fe-based bainitic coatings were fabricated using laser cladding and following isothermal heat treatment. The microstructures and mechanical properties of the laser cladded coating were investigated. The results show that the Mn, Cr, Co, and Al segregated at the solidified prior grain boundaries. The micro-segregation of the solutes strongly influenced the stability of the austenite. As the isothermal temperature decreases, the interface of the bainite and blocky retained austenite approach to the prior interdendritic regions with the decreasing isothermal temperature, and the final volume fraction also decreases. The volume fractions of each phase and microstructure morphology of the coatings were determined by the interdendritic micro-segregation and isothermal temperatures. The stability of the blocky retained austenite distributed at the interdendritic area was lower than that of film and island-like morphology. This phenomenon contributed to the ductile and tough nano bainitic coatings with tunable mechanical properties.

Nanostructured bainitic steels have tremendous potential to be used in structural applications e.g. as rail materials. Strength and toughness in nanostructured bainite can be easily tailored by controlling the lath thickness of bainitic ferrite plates (BF) along with the morphology and volume fraction of retained austenite (RA). However, the wear response under repeated sliding depends on the microstructural evolution with the number of passes and thus is difficult to predict without careful experiments. In the current study, we have made nano-bainitic steels with a range of lath thicknesses from a steel of composition Fe-0.89C-1.59Si-1.65Mn-0.37Mo-1Co-0.56Al-0.19Cr (wt%). Austenitization followed by isothermal holding at 250 °C, 300 °C and 350 °C has been used to make specimens with average bainitic lath thicknesses of about 45, 54 and 89 nm respectively. Depth sensing instrumented indenter with a conical tip has been used to make multiple scratches on all the specimens and the damage has been characterized in the form of depth of scratch, scratch thickness and groove volume using a combination of profilometer and scanning electron microscopy. Specimens transformed at lower isothermal temperature show less wear damage evident in the form of thinner scratch profile, shallower scratch depth and hence smaller wear volume. Moreover, the friction coefficient is found to decrease with a reduction in the austempering temperature. However, the dynamic friction coefficient for all the specimens saturates after around 20 passes of sliding. Cross-sectional imaging of the scratches shows loss of blocky RA, refinement and orientation of the RA and BF laths along the shape of the bottom of groove after repeated sliding. Moreover, the sub-surface hardness decreases for all the specimens after sliding fatigue. However, the specimen austempered at 250 °C shows the lowest friction coefficient, the least material removal and smallest sub-surface deformation zone under conditions of repeated frictional sliding. [ABSTRACT FROM AUTHOR]

The deformation mechanisms operating in nanostructured bainite, leading to its excellent combination of strength and ductility, are far from being understood. Its nanocrystalline nature and its multiphase-evolving structure underlie the plastic flow and the strain-hardening behaviour. In this work, the microstructural and crystallographic bulk changes of a high-C nanostructured bainite under tensile testing have been evaluated. The influence of the mechanically-induced transformation of the C-enriched retained austenite into α martensite and other deformation mechanisms on the texture evolution has been analysed by electron backscatter diffraction (EBSD). Additionally, the undeformed and the deformed conditions have been examined by electron channelling contrast imaging (ECCI) and transmission electron microscopy (TEM). Results reveal the presence of plate martensite and suggest a strong variant selection during the transformation, mainly responsible for the texture observed. Mechanical twinning in austenite seems to be basically the mechanism of accommodation of the displacive bainitic transformation, while some direct interaction with the applied stress also appears. [ABSTRACT FROM AUTHOR]

The nanostructured bainitic (NB) bearing steel exhibits excellent rolling contact fatigue (RCF) resistance, which is critical to the service of bearing. However, the detailed microstructural evolution of the steel during the RCF process is still unclear. In this paper, a carburized NB bearing steel was studied and a detailed process investigation on microstructural evolution of the steel during the RCF progress was carried out. Results show that the nano-sized bainitic ferrite is gradually refined, and the undissolved carbide s gradually flake away and some of them remain in solution during the RCF process. It is interesting to find that there are three stages for the martensitic transformation of retained austenite. During the first stage of 0–0.48 × 10 7 cycles, nearly all of the block-like retained austenite in the top surface transforms into martensite rapidly, and during the second stage of about 0.48–1.92 × 10 7 cycles, nearly no retained austenite further transforms into martensite, but when the fatigue exceeds the 1.92 × 10 7 cycles, the film-like retained austenite gradually transforms into martensite at a slow rate. The transformed martensite not only increases the surface hardness but further brings additional residual compressive stress in surface layer. It is analyzed from the microstructure characterization along the longitudinal section of the fatigue specimen that the fatigue crakes are initiated at subsurface of specimen, which propagate up to the surface and finally result in fatigue flake. [ABSTRACT FROM AUTHOR]

The high-cycle, push–pull fatigue fracture behavior of high-C, Si–Al-rich nanostructured bainitic steel was studied through the measurement of fatigue limits, a morphology examination and phase composition analysis of the fatigue fracture surface, as well as fractography of the fatigue crack propagation. The results demonstrated that the push–pull fatigue limits at 107 cycles were estimated as 710–889 MPa, for the samples isothermally transformed at the temperature range of 220–260 °C through data extrapolation, measured under the maximum cycle number of 105. Both the interior inclusion and the sample surface constituted the fatigue crack origins. During the fatigue crack propagation, a high amount of secondary cracks were formed in almost parallel arrangements. The apparent plastic deformation occurred in the fracture surface layer, which induced approximately all retained austenite to transform into martensite. [ABSTRACT FROM AUTHOR]

The trapping of hydrogen in nanostructured bainitic steel has been investigated using thermal desorption analysis, in order to determine the potency of the ferrite-retained austenite (α/γ) interfaces and retained austenite as trapping sites. Thermal desorption data showed that the volume of retained austenite is more effective in trapping hydrogen than the interfaces. There is a close correlation between the quantity of hydrogen and the retained austenite content rather than the density of interfaces. A local equilibrium model was able to reproduce the hydrogen desorption behaviour of saturated and unsaturated samples considering both retained austenite and α/γ interfaces as the trapping sites. A trap binding energy ranging from 47 to 52 kJ/mol was estimated for retained austenite, suggesting that the observed trapping capacity originates from the austenite lattice sites. [ABSTRACT FROM AUTHOR]

Nanostructured bainite has been developed to meet the stringent demands of ballistic impacts. In a previous report [Y.T. Tsai et al., Scripta Mater. 2016;115:46], the deformation behavior of nanostructured bainite under high strain rate (2×10 3 s -1 ) was studied using a split Hopkinson pressure bar. Deformation twinning was found in both blocky austenite and film austenite, and it was concluded that twinning occurred first in the blocky austenite, enhancing the initial work-hardening, followed by twinning in film austenite, promoting further straining. In the present work, the deformation structure has been further studied using a severely deformed sample subjected to a high strain rate (3×10 3 s -1 ). It is found that blocky austenite, due to the inherent size inhomogeneity, can produce various deformation twinning structures; i.e., one variant or two variants of dominating mechanical twins, including twin kinking. The dominating twins are so-called primary twins, the apparent twinning planes of which deflect from the ideal {111} γ , resulting from the fact that the apparent twinning planes are stepped, presumably due to dislocation locking during deformation. Small twins are also found to be dispersed in parent austenite matrix, and they are categorized as secondary twins. Strong evidence suggests that the formation of α’-martensite usually occurs in the intersection of two primary twins. For film austenite, the deformation structure is one-variant twinning, and the twinning planes are all ideal. [ABSTRACT FROM AUTHOR]

The study investigates the microstructure and crystallographic orientation relationship in a high carbon nanostructured bainitic steel that is austempered at 250 °C for different durations. The resulting microstructures are examined using optical, scanning, and transmission electron microscopy (TEM). Quantitative and qualitative analyses of dislocation density are performed using X-ray diffraction (XRD) and electron-backscattered diffraction (EBSD) respectively. The study finds that longer austempering time reduces the dislocation density because of slowing down transformation kinetics due to the enrichment of carbon concentration to a level corresponding to T 0 ′ temperature in the retained austenite phase. The bainite plate thickness in the longer-duration sample shows abnormal growth rate at the edges of the sheaf. The abnormal thickening rate is attributed to the annihilation of dislocations after prolonged austempering because of slow kinetics. The thickness of the bainitic plate is predicted using an available empirical relation and critically compared with experimentally measured values. It is found that the empirical equation used to compute thickness overestimates the plate thickness due to the omission of factors such as dislocation density and austenite block size. The crystallographic orientation relationship between bainite and austenite is analyzed locally by TEM and over a broader area by EBSD. The results reveal that both the Kurdjumov–Sachs and Nishiyama–Wasserman relationships are present in the material in equal fractions and some specific variants are formed in the sample subjected to longer austempering. • Long austempering time saturates bainite transformation kinetics and partial annihilation of dislocations. • At a later stage, abnormal thickening of bainite is due to lack in hard impingement and partial annihilation of dislocations • Orientation relationship of bainite/austenite in high carbon steel maintains nearly equal proportion of K S and N W type. • Some variants (V19, V13 and V2) related to K S relationship dominates with progress of transformation. [ABSTRACT FROM AUTHOR]

The two-step austempering process has been reported to be an effective method to accelerate the bainitic transformation process by introducing martensite (Q-M-B). However, in this study, it was found that the Q-M-B process reduced the incubation time, but the transformation duration remained nearly unchanged. The notably reduced activation energy barrier for nucleation of bainitic ferrite on the preformed martensite should be responsible for the reduced duration time of the Q-M-B process. A process that both of the two steps were above, Ms (Q-B-B), has been demonstrated to increase transformation rate and improve the amount of bainitic ferrite, which probably results from the additional hysteresis free energy provided by the first quenching process.

Nanostructured bainite was obtained in high-carbon Si-Al-rich steel by low-temperature (220-260 °C) isothermal transformation after austenitisation at different temperatures (900 °C, 1000 °C, and 1150 °C). Improved strength-ductility-toughness balance was achieved in the nanostructured bainitic steel austenitised at low temperatures (900 °C and 1000 °C). Increasing the austenitising temperature not only coarsened prior austenite grains and bainite packets, but also increased the size and fraction of blocky retained austenite. High austenitising temperature (1150 °C) remarkably decreased ductility and impact toughness, but had a small effect on strength and hardness. [ABSTRACT FROM AUTHOR]

The growth and propagation behavior of austenite-to-bainite isothermal transformation in laser-cladded, Si-rich, and Fe-based coatings is investigated. The crystallographic features, orientation relationship at different isothermal temperatures, and the morphology of the nanostructured bainite are determined. The Nishiyama-Wassermann type orientation relationship is observed at a high temperature and at a low temperature, and mixed Nishiyama-Wassermann and Kurdjumov-Sach mechanisms are seen. The growth direction is investigated by the partial dislocation theory and an extrapolated model based on the repeated formation of lenticular-shaped subunits and pile-up along the close-packed directions of the close-packed planes. The variants of the bainite growth directions would be more selective at the high transformation temperature. [ABSTRACT FROM AUTHOR]

Large-strain ausforming and low-temperature bainite transformation were carried out in a medium C - Si rich alloy steel on a thermomechanical simulator. Then, temperature–time–transformation curves of the bainite transformation after ausforming at different temperatures were obtained by dilatometry. The effects of ausforming on transformation kinetics, microstructure, and hardness of samples were studied. Results show that the entire process of bainite transformation can be accelerated by ausforming. With decreasing ausforming temperature, the transformation rates greatly increased at the initial transformation stage but slightly decreased at the final stage. Compared with non-ausformed sample, not only was the maximum growth rate in the ausformed samples larger, but the maximum growth rate also appeared earlier. In addition, the maximum rate of the ausformed bainite transformation was increased and the time of reaching the maximum rate was shortened with decreasing the ausforming temperature. The effect of ausforming on bainite transformation kinetics depended not only on the ausforming temperature and strain but also on the isothermal transformation temperature. The reduction in transformation time was increased under decreased isothermal transformation temperature. Ausforming refined the bainite plates and increased the fraction of retained austenite formed in the subsequent low-temperature isothermal transformation. Consequently, nanostructured bainite with enhanced hardness was prepared in the medium C - Si rich alloy steel. [ABSTRACT FROM AUTHOR]

Presence of nanoscale bainitic ferrites and high carbon retained austenites that are stable at ambient temperature within the microstructures of super strong bainitic steels makes it possible to achieve exceptional strengths and ductility properties in these groups of nanostructured steels. This article aims to study the effect of the dislocation density variations during tensile testing in ambient temperature on deformation behavior of nanostructured low temperature bainitic steels. Results indicate that dislocation absorption from bainitic ferrite subunits by surrounding retained austenite reduces the work hardening and therefore increases the formability of bainitic ferrite during deformation, which in turn results in a suitable combination of strength and ductility. [ABSTRACT FROM AUTHOR]

In the present work, the nanostructured bainitic microstructures were obtained at the surfaces of a carburized steel and a high-C steel. The rolling contact fatigue (RCF) performances of the two alloy steels with the same volume fraction of undissolved carbide were studied under lubrication. Results show that the RCF life of the carburized nanostructured bainitic steel is superior to that of the high-C nanostructured bainitic steel in spite of the chemical composition, phase constituent, plate thickness of bainitic ferrite, hardness, and residual compressive stress value of the contact surfaces of the two steels under roughly similar conditions. The excellent RCF performance of the carburized nanostructured bainitic steel is mainly attributed to the following reasons: finer carbide dispersion distribution in the top surface, the higher residual compressive stress values in the carburized layer, the deeper residual compressive stress layer, the higher work hardening ability, the larger amount of retained austenite transforming into martensite at the surface and the more stable untransformed retained austenite left in the top surface of the steel. [ABSTRACT FROM AUTHOR]

Nanostructured bainite is a novel ultra-high-strength steel-concept under intensive current research, in which the optimization of its mechanical properties can only come from a clear understanding of the parameters that control its ductility. This work reviews first the nature of this composite-like material as a product of heat treatment conditions. Subsequently, the premises of ductility behavior are presented, taking as a reference related microstructures: conventional bainitic steels, and TRIP-aided steels. The ductility of nanostructured bainite is then discussed in terms of work-hardening and fracture mechanisms, leading to an analysis of the three-fold correlation between ductility, mechanically-induced martensitic transformation, and mechanical partitioning between the phases. Results suggest that a highly stable/hard retained austenite, with mechanical properties close to the matrix of bainitic ferrite, is advantageous in order to enhance ductility. [ABSTRACT FROM AUTHOR]

Highlights • Low-temperature bainite transformation was accelerated by high magnetic field. • The model of bainite ferrite was built according to composition ratio of Fe:C. • The magnetic moment of bainitic ferrite decreased with increasing temperature. • The acceleration is in essence related to the magnetic Gibbs free energy reduction. Abstract We have elucidated the determining role of high magnetic field on low-temperature nanostructured bainite transformation at an isothermal temperature of 300 °C. The application of 12 Tesla magnetic field increased the bainite volume percentage by 193% and 106% after isothermal treatment at 300 °C for 1 and 5 h, respectively. It indicated that low-temperature nanostructured bainite transformation is significantly accelerated by a high magnetic field. The magnetic moment of bainitic ferrite was 1.99 μ B under the condition of high magnetic field and 300 °C. Thus, the magnetic Gibbs free energy of bainite under 12 Tesla is −133.90 J/mol, leading to increase in the driving force for transformation of austenite to bainite transformation. [ABSTRACT FROM AUTHOR]

The high-cycle, push–pull fatigue fracture behavior of high-C, Si–Al-rich nanostructured bainitic steel was studied through the measurement of fatigue limits, a morphology examination and phase composition analysis of the fatigue fracture surface, as well as fractography of the fatigue crack propagation. The results demonstrated that the push–pull fatigue limits at 107 cycles were estimated as 710–889 MPa, for the samples isothermally transformed at the temperature range of 220–260 °C through data extrapolation, measured under the maximum cycle number of 105. Both the interior inclusion and the sample surface constituted the fatigue crack origins. During the fatigue crack propagation, a high amount of secondary cracks were formed in almost parallel arrangements. The apparent plastic deformation occurred in the fracture surface layer, which induced approximately all retained austenite to transform into martensite.

The recently developed nanobainitic steels show high strength as well as high ductility. Although this combination seems to be promising for fatigue design, fatigue properties of nanostructured bainitic steels are often surprisingly low. To improve the fatigue behavior, an understanding of the correlation between the nanobainitic microstructure and the fatigue limit is fundamental. Therefore, our hypothesis to predict the fatigue limit was that the main function of the microstructure is not necessarily totally avoiding the initiation of a fatigue crack, but the microstructure has to increase the ability to decelerate or to stop a growing fatigue crack. Thus, the key to understanding the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features that could act as barriers for growing fatigue cracks. To prove this hypothesis, we carried out fatigue tests, crack growth experiments, and correlated these results to the size of microstructural features gained from microstructural analysis by light optical microscope and EBSD-measurements. Finally, we were able to identify microstructural features that influence the fatigue crack growth and the fatigue limit of nanostructured bainitic steels. [ABSTRACT FROM AUTHOR]

Mo containing high-C-Cr bearing steel was modified with Si (0.8-1.5 wt.%) and 0.8Si-1.0Al to prepare nanostructured bainite by low-temperature isothermal heat treatment. The modified steels were isothermal held at 220 to 240 °C after partial austenitization in an intercritical gamma+carbide region, and the resultant microstructure and mechanical properties were studied. Carbide-free nanostructured bainite with plate thickness below 100 nm and film retained austenite, as well as a small amount of undissolved carbide particles, was obtained in the modified steels except in 0.8Si steel, in which carbides precipitated in bainitic ferrite. As Si content increased, the mean thickness of bainitic ferrite plates modestly decreased, whereas the fraction of retained austenite markedly increased. The thickness of bainitic ferrite plate and the fraction of retained austenite in Si-Al-modified steel were smaller than those in Si-modified steels. The hardness and elongation of the Si-Al-modified steel were lower than those of Si-modified steels. The yield strength of Si-Al-modified steel was superior to that of Si-modified steels. Mid-level ultimate tensile strength and impact toughness were achieved in Si-Al-modified steel. For bearing applications, Si-modified steels could provide higher hardness and toughness but lower dimensional stability. Meanwhile, Si-Al-modified steel could offer higher dimensional stability but lower hardness and toughness. [ABSTRACT FROM AUTHOR]

Retained austenite in nanostructured bainite is able to undergo mechanically induced martensitic transformation. However, the link between transformation and deformation mechanisms involved makes difficult the understanding of the process. In this work, a model has been developed to assess the effect of the external stress itself on the martensite phase transformation. In addition, after a detailed initial microstructural characterization, the martensite fraction evolution during tensile deformation has been obtained by means of X-ray diffraction analyses after interrupted tensile tests in several nanostructured bainitic steels. Experimental results have been compared to the outputs of the model, as a reference. They suggests that stress partitioning between phases upon tensile deformation is promoted by isothermal transformation at lower temperatures. [ABSTRACT FROM AUTHOR]

Laser cladding and subsequent isothermal holding were employed to fabricate the Fe-based nanobainitic coatings. The microstructures of the isothermal heat treated coatings were observed and characterized by using the optical microscope (OM), filed-emission scanning electron microscope (FE-SEM), field-emission transmission electron microscope (FE-TEM), energy-disperse X-ray spectroscopy (EDAX), and X-ray diffraction (XRD). The results showed that the elements of Mn, Cr, and Si segregated at the prior austenite interdendritic regions. The segregation of the solutes significantly affected on the kinetics of the nanobainitic transformation and the final microstructures obtained at the different isothermal temperatures. The bainitic transformation incubation time was reduced by the increasing of available driving force for nucleation. The bainitic sheaves grow across the interdendritic regions of the prior austenitic dendrites. The continuity and thickness of blocky austenite distributed at the prior interdendritic regions were sensitive to the transformation temperature, as a result of the threshold of the bainitic transformation varied with the chemical gradients near the segregated interdendritic regions. [ABSTRACT FROM AUTHOR]