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Herein, the potential of using the thermomechanical treatment ausforming to optimize the microstructure of PH 13‐8 Mo maraging steels is investigated. Of particular interest are the effects on the formation of β‐NiAl precipitates and reverted austenite during subsequent aging. Ausforming experiments at deformation temperatures of 400 and 600 °C and strains of 40% and 70% are conducted using a deformation dilatometer. Afterward, the samples are subjected to aging at temperatures varying from 482 to 593 °C. This is followed by a thorough microstructural characterization ranging from light optical microscopy to X‐Ray diffraction and atom probe tomography. The results demonstrate that ausforming effectively refines the microstructure, resulting in a significant increase in hardness. Moreover, it is revealed that the ausforming parameters have a noticeable influence on the number density of β‐NiAl precipitates and the amount of reverted austenite. A well‐balanced microstructure containing relatively fine precipitates and a high amount of reverted austenite can be achieved by ausforming at 600 °C with 40% strain. Overall, the results prove a promising potential of ausforming for PH 13‐8 Mo maraging steels with regard to tailoring optimized microstructures, which could ultimately benefit the aerospace industry where these alloys are used. [ABSTRACT FROM AUTHOR]

The refinement of bainitic ferrite laths is one of the most critical ways to improve the combination of strength and ductility. However, low-temperature bainitic transformations in the microstructural evolution of carbide-free bainite in high-carbon steels cause challenges for both the manufacture of coils and their application in car bodies. In this study, deformation-induced bainite refinement was proposed for refining bainite and retained austenite laths using a combination of ausforming and austempering in low-carbon low-alloy steel. The deformed austenite supplied more nucleation sites for bainitic transformation and suppressed the thickening of the bainitic ferrite laths by strengthening the surrounding untransformed austenite. A tensile strength of 1283 MPa and total elongation of 23.6% were obtained in the steel with an ausforming strain of 40% before austempering. The significant increase in strength was mainly attributed to the refinement of the bainitic ferrite laths and the high accumulation of dislocations within both the bainitic ferrite and retained austenite. The more stable retained austenite improved the sustainable work-hardening capability via the continuous transformation-induced plasticity effect.

There is a need worldwide to develop materials for advanced power plants with steam temperatures of 700°C and above that will achieve long-term creep-rupture strength and low CO2 emissions. The creep resistance of actual 9-12Cr steels is not enough to fulfil the engineering requirements above 600°C. In this paper, the authors report their advances in the improvement of creep properties of this type of steels by the microstructural optimization through nano-precipitation using two methodologies. 1) Applying a high temperature austenitization cycle followed by an ausforming step (thermomechanical treatment, TMT) to G91 steel, to increase the martensite dislocation density and, thus, the number density of MX precipitates (M = V,Nb; X = C,N) but at the expense of deteriorating the ductility. 2) Compositional adjustments, guided by computational thermodynamics, combined with a conventional heat treatment (no TMT), to design novel steels with a good ductility while still possessing a high number density of MX precipitates, similar to the one obtained after the TMT in G91. The microstructures have been characterized by optical, scanning and transmission electron microscopy, EBSD and atom probe tomography. The creep behaviour at 700°C has been evaluated under a load of 200 N using small punch creep tests.

Since flow stress is an important parameter in the processing and application of metallic materials, it is necessary to trace the flow stress during austenite deformation. Thermal compression deformation of austenite in a high-strength bainitic steel was conducted using a Gleeble-3500 thermo-mechanical simulator, within the deformation temperature range of 400 °C~900 °C. By analyzing the stress–strain curves and strain-hardening exponent, the effects of strain hardening and dynamic recovery on the dislocation density of the material during the thermal processing were considered in the present work. Based on the general form of the Kocks–Mecking–Estrin (KME) model, the effects of deformation temperature and strain on the key parameters of the model were clarified. Differing from other work which commonly terms m (strain rate sensitivity exponent) and k2 (dimensionless parameters for dynamic recovery) as constants, the current models consider the quantitative relationship between key parameters and deformation temperature and strain. The results show that m is an exponential function related to temperature and strain, which decreases with the increase in strain. Meanwhile, k2 is a temperature-dependent polynomial function that increases as the deformation temperature increases. Finally, a modified constitutive KME model was proposed to predict the austenitic plastic stress with strain. Using established m-ε and k2-T models, the predicted curves are in good agreement with the experimental measurements. [ABSTRACT FROM AUTHOR]

A combination of physical simulation and laboratory rolling experiments, including thermomechanical rolling and low-temperature ausforming, was conducted for designing a suitable processing route to enable phase transformation from austenite to ultrafine bainite in a medium-carbon steel. Following low-temperature ausforming at 500-550 °C, two different cooling and holding paths were tried in the study: (1) water cooling close to martensite start temperature (300 °C), followed by isothermal holding (route A), and (2) air cooling to 350 °C followed by isothermal holding (route B). For reference, a third sample was directly water-cooled to 300 °C after hot rolling without ausforming treatment, followed by isothermal holding (route C). Field emission scanning electron microscopy and electron backscatter diffraction, as well as x-ray diffraction, were employed for microstructural analysis and correlations with the mechanical properties evaluated in respect of hardness and tensile properties. The low-temperature ausforming and subsequent cooling schedules resulted in the decomposition of austenite into ultrafine bainite and some martensite, while stabilizing a fraction of finely divided, carbon-enriched interlath austenite. Results suggested the development of a novel, multiphase bainite-martensite-austenite microstructure, achieved via low-temperature ausforming and subsequent air-cooling (route B), was beneficial in respect of mechanical properties. Most of the bainitic plates were in the range of 50-200 nm with the occasional presence of coalesced plates as wide as 2000 nm. Despite the differences in the microstructure, the mechanical behavior of non-ausformed samples was not significantly different. The technique paves way for developing medium-carbon nanostructured/ultrafine bainitic steels with high mechanical properties achieved via innovative modification of processing routes including low-temperature ausforming. [ABSTRACT FROM AUTHOR]

Ausforming at 900 °C with 30% reduction in V and N micro-alloyed steel could induce VN precipitation, which has low lattice misfit (1.8%) with bainitic ferrite in a particular crystal orientation. The incubation period of bainitic transformation was shortened by introducing VN. A further ausforming at 300 °C with different strains prior to isothermal transformation was carried out, which provided more nucleation points and more distortion-energy storage for bainitic transformation. The incubation period was dramatically shortened and bainitic transformation was notably accelerated. There existed a competition between accelerating nucleation and suppressing growth after low temperature ausforming with different strains, and a small strain resulted in a more significant acceleration and a higher degree of bainitic transformation. In addition, thinner bainitic ferrite and finer blocky retained austenite were obtained after low temperature ausforming. The orientation relationship between bainitic ferrite and austenite in the ausformed steel were K–S and N–W relationships.

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.

Ausforming treatments are thermomechanical treatments that consist in plastically deforming a fully austenitized steel, usually leading to either a bainitic or a martensitic microstructure. The effects of the mentioned previous deformation have been mainly studied under compression conditions. However, as widely known, many large-scale processes do not only require the application of deformation under a single mode. For that reason, it is important to assess whether different deformation modes could induce similar effects on the final microstructure. In this work, the effect of the deformation mode was analyzed in a medium carbon high silicon steel. Samples were deformed under compressive and tensile stresses at different temperatures and subsequently subjected to isothermal holding steps in the bainitic range for prolonged times. The final microstructures consisted of bainitic ferrite and retained austenite, where the bainitic ferrite was formed at different stages: a certain fraction was dynamically formed during the deformation step, while the remaining fraction was formed during the isothermal holding. The study of the changes in length observed during these treatments enabled to understand both phase transformations, while the analysis of the final microstructures by different characterization techniques, such as X-Ray Diffraction and Scanning Electron Microscopy, enabled to better understand the effect of the deformation mode at different levels. It was proved that tensile ausforming led to more rapid and intense strain induced transformations than compression ausforming. Moreover, results showed that ausforming led to anisotropic structures, regardless of the deformation mode, where bainitic ferrite plates were aligned with respect to the deformation direction. The effect of the deformation mode on the bainitic ferrite plate thickness, the volume fractions of the different phases, several crystallographic parameters and hardness is also discussed.

The effect of deformation on the growth kinetics of bainitic ferrite plates was investigated under two types of conditions. In the first, deformation (ausforming) precedes the austenite-to-bainite transformation, while in the second case the phase transformation and deformation occur concurrently. The results clearly demonstrate that ausforming accelerates the nucleation of bainitic ferrite plates, leading to significant refinement of the final microstructure. When the bainite transformation takes place during deformation, the effect on nucleation was less obvious, but a clear increase in the growth rate of bainitic ferrite was observed. A modified version of the Zener-Hillert model for plate growth, with adjustments that allow for incorporating the effect of defects in the matrix and the effect of applied deformation on the movement of interfacial disconnections, was used to rationalize the effect of deformation on the growth kinetics. The calculations show that after ausforming, the stored dislocations act as a barrier to the growth of bainitic ferrite plates, resulting in a decreased lengthening rate. Conversely, when the transformation occurs concurrently with deformation, the lengthening rate of bainitic ferrite plates is accelerated due to the interaction between the applied stress and the disconnections at the tip of the bainitic ferrite plates. [Display omitted] [ABSTRACT FROM AUTHOR]

Dislocation engineering is a novel alloy design strategy to produce advanced high strength steels with both high strength and good ductility, which can be accomplished through an ausforming process. The present work studies the effect of ausforming strain on the evolution of lath martensite microstructure in low carbon steel by detailed transmission electron microscopy observation. Ausforming strain determines the length of the martensite blocks and the substructure in the lath martensite. The large ausforming strain (20%–45%) reduces the martensite blocks and leads to the development of dislocation cell structure, both of which are absent in the ausformed martensite with a small ausforming strain (5%–10%). The formation of the dislocation cell structure in the prior austenite grains after the large plastic deformation inhibits the propagation of martensite and thus reduces the length of martensite blocks. Irrespective of the ausforming strain, the plastic deformation of austenite grains does not lead to an obvious change of martensite lath width. The large ausforming strain slightly enhances the auto-tempering of martensite on the aspect of precipitates density. The coarse precipitates formed close to the lath boundaries after small ausforming strain of 5% is absent in the martensite with large ausforming strain (20%–45%), which is ascribed to the presence of intensive dislocations generated by elevated ausforming strain can facilitate the pipe diffusion of carbon. The present finding serves as the microstructural basis for the application of the dislocation engineering concept in the design of advanced high-strength steel with the martensite as the intrinsic component. [ABSTRACT FROM AUTHOR]

Since flow stress is an important parameter in the processing and application of metallic materials, it is necessary to trace the flow stress during austenite deformation. Thermal compression deformation of austenite in a high-strength bainitic steel was conducted using a Gleeble-3500 thermo-mechanical simulator, within the deformation temperature range of 400 °C~900 °C. By analyzing the stress–strain curves and strain-hardening exponent, the effects of strain hardening and dynamic recovery on the dislocation density of the material during the thermal processing were considered in the present work. Based on the general form of the Kocks–Mecking–Estrin (KME) model, the effects of deformation temperature and strain on the key parameters of the model were clarified. Differing from other work which commonly terms m (strain rate sensitivity exponent) and k2 (dimensionless parameters for dynamic recovery) as constants, the current models consider the quantitative relationship between key parameters and deformation temperature and strain. The results show that m is an exponential function related to temperature and strain, which decreases with the increase in strain. Meanwhile, k2 is a temperature-dependent polynomial function that increases as the deformation temperature increases. Finally, a modified constitutive KME model was proposed to predict the austenitic plastic stress with strain. Using established m-ε and k2-T models, the predicted curves are in good agreement with the experimental measurements.

The mechanisms driving variant selection phenomena in ausformed bainitic microstructures are not fully controlled yet. Previous results are not congruent among them, as they cannot be explained by the same rule. In this work, we aimed to understand the mechanisms governing variant selection in a medium carbon-high silicon steel subjected to ausforming treatments. To do so, the microstructural characterization in different regions of barreled samples has been combined with the simulation of the stress and strain state at the macro and micro level, by finite elements simulations and crystal plasticity simulations, respectively. The main conclusion is that the variant selection shown in these microstructures can be explained by understanding the distribution of the plastic strains at the macro and micro level. The selection of crystallographic variants is more pronounced in the regions which have been more deformed. Also, the deformation distributes along deformation microbands, where the most promoted variants seem to grow, in good agreement with previous results in ausformed lath martensite.

The presence of strain-induced phases formed in the course of ausforming step has been proven by thermodynamic calculation, microstructural observation and hardness examination, besides that, the effect of ausforming parameters on isothermally transformed bainite during austempering step was also essentially investigated. The results indicated that the types of strain-induced phases were determined by ausforming temperature and strain, for instance, the trace of strain-induced martensite could be detected in the sample subjected to ausforming at 300 °C for 20% strain. However, ausforming would introduce strain-induced bainite at a higher ausforming temperature (500 °C) instead of strain-induced martensite, which could gradually increase the average thickness of bainitic ferrite lath and impair bulk hardness with raising ausforming strain. Although ausforming process lessened the maximum attainable volume fraction of bainitic ferrite formed during austempering stage due to austenite mechanical stabilization, the existence of strain-induced martensite and the refined bainitic sheaves would make a contribution to hardness improvement when ausformed at 300 °C.

Carbide-free bainitic steels show attractive mechanical properties but are difficult to process because of the sluggish phase transformation kinetics. A macroscopic model based on the classical nucleation theory in conjunction with the modified Koistinen–Marburger relationship is proposed in this study to simulate the kinetics of incomplete bainitic and martensitic phase transformations with and without austenite deformation. A 0.26C-1Si-1.5Mn-1Cr-1Ni-0.003B-0.03Ti steel and a 0.18C-1Si-2.5Mn-0.2Cr-0.2Ni-0.02B-0.03Ti steel were investigated with different levels of ausforming. The concept of ausforming is expected to accelerate the onset of the bainitic transformation and to enhance the thermodynamic stability of austenite by increased dislocation density. The phase transformation kinetics of both steels is quantitatively analyzed in the study by dilatometry and X-ray diffraction so that the carbon concentration in the retained austenite and bainitic ferrite, as well as their volume fractions, is determined. A critical comparison of the numerical and experimental data demonstrates that the isothermal kinetics of bainite formation and the variation of driving energy can be satisfactorily described by the developed model. This model captures the incompleteness of the bainite phase transformation and the carbon enrichment in the austenite well. A fitting parameter can be used to elucidate the initial energy barrier caused by the ausforming. An increase in austenite stability can be described by the nucleation reaction and the thermodynamic energies associated with the change of dislocation density. The proposed model provides an in-depth understanding of the effect of ausforming on the transformation kinetics under different low-carbon steels and is a potential tool for the future design of heat treatment processes and alloys. [ABSTRACT FROM AUTHOR]

Effects of process parameters of the ausforming such as temperature, strain and strain rate on the martensitic start temperature, kinetics of isothermal bainitic transformation and microstructure refinement of a low carbon carbide-free bainitic steel were investigated. It was found that applying plastic deformation to untransformed austenite during intermediate temperatures decreased the martensite start temperature of steel and enabled isothermal bainitic transformation at low temperatures. Hereby, ausforming significantly generated heterogeneous nucleation sites, which accelerated the overall kinetics of bainitic transformation and thus increased bainitic phase fraction in steel. In addition, the ausforming enhanced the stability of austenite that led to reduced amount of martensite after cooling down to room temperature. Finally, the ausforming parameters and observed microstructure features were correlated and discussed along with the hardness of steel.

In this work, the influence of thermomechanical treatments, consisting of a combination of ausforming followed by isothermal holding and cooling to room temperature, on the stability of retained austenite of a carbide-free medium carbon-high silicon steel was investigated. The amount of retained austenite and its transformation to martensite for a range of ausforming treatments was determined by means of X-ray diffraction, dilatometry signal analysis, and metallographic investigation. Different amounts of deformations were applied at 600 °C in the austenite region prior to fast cooling into the bainitic transformation region. Four isothermal temperatures (325, 350, 375, and 400 °C) with a holding time of 1800 s were selected to estimate the extent of the stability of the retained austenite after the completion of the bainitic transformation. The results show that the deformation-free austenite was more stable for the samples isothermally treated at temperatures between 325 and 350 °C. However, the decomposition of retained austenite to martensite during cooling to room temperature increased for isothermal holding temperatures above 350 °C. It was found that the stability of retained austenite was a function of the temperature and percent deformation, and the critical temperature and percent deformations were determined. It was also found that ausforming enhanced the formation of blocky-shaped retained austenite for isothermal holdings above 350 °C, resulting in a decrease in retained austenite stability. The results were analyzed in terms of the influence of thermomechanical conditions on bainitic transformation and its impact on the transformation of retained austenite to martensite.

The coupling effects of ausforming and quenching process on isothermal bainite transformation behaviors, microstructures and mechanical properties of ultrafine bainitic steel have been analyzed. The synergism of ausforming and prior martensite transformation not only significantly accelerates subsequent bainite transformation, but also effectively refines bainite laths and greatly reduces the size of harmful blocky retained austenite, providing an improved transformation induced plasticity effect. Ultimate tensile strength of 2000 MPa and total elongation of 24% can be achieved in ultrafine bainitic steel by ausforming-quenching process and austempering at 300 °C for 1 h.

The influence of tempering at 400 and 500 °C on two bainitic ferrite microstructures obtained via pure isothermal transformation and ausforming followed by the isothermal transformation was studied in a medium C-Si rich bainitic steel. To understand the dilatometric behaviour and mechanisms of carbide-free bainite decomposition, extensive microstructural characterization using scanning electron microscopy, X-ray diffraction, and microhardness measurements were conducted. The softening due to tempering at 400 °C was negligible for both microstructures. Tempering at 500 °C led to a remarkable contraction in dilatometric signal in an ausformed sample, while a pure isothermal sample showed slight contraction. The main mechanism governing microstructural changes during tempering at 500 °C was the decomposition of as-ausformed highly dislocated bainitic ferrite plates and/or film-like retained austenite into cementite and ferrite as a result of enhanced carbon diffusion. While tempering at 400 °C did not result in changes in the size of the bainitic plates, the higher tempering temperature of 500 °C led to a thickening of the bainitic ferrite plates for both testing conditions. However, the thickening was restricted in the ausformed bainite, which was related to the pinning effect of very small cementite precipitates, which moderate the tempering effects on hardness reduction.

The bainitic ferrite plate thickness is the main parameter that controls the strength of this type of microstructures. Such thickness has been proved to mainly depend on the austenite yield strength, the driving force for the transformation and the transformation temperature. However, no research has focused on how these parameters evolve throughout the transformation and how this evolution can affect the outcome. In this study, thermal and thermomechanical treatments have been performed in three selected steels. The treatments have been designed in such a way that all the mentioned parameters are comparable, aiming to obtain similar microstructures in terms of bainitic ferrite plate thickness. However, significant differences have been found among the microstructures, with variations in plate thickness larger than 100 nm. These results indicate that there might be other factors that take part in the scale of bainitic microstructures. To explain these differences and based on the kinetics of the transformation and on the carbon content of austenite at the end of the transformation, a possible explanation has been proposed.

The contribution of ENEA together with Rina-CSM to the Eurofusion programme “WPMAT-Advanced Seels” deals with the development of innovative RAFM steels able to withstand the critical temperatures typical of the different operational environments foreseen for the blanket of the first DEMO reactor. The optimization of the chemical composition and the Thermo Mechanical Treatment for these materials should be done according to the blanket operating temperatures that are related to two possible working conditions: the WCLL-BB (Water Cooled Lead Lithium Breeding Blanket) or the H(D)CLL-BB (Helium (Dual) Cooled Lead Lithium Breeding Blanket). On the one hand the “water-cooling” option implies a minimum irradiation temperature for the blanket material in the range of 280–350 °C. On the other hand, the “helium-cooled” and the “dual-coolant” solutions imply an operating temperature for the blanket material in the range of 650 °C. Therefore in the first case the target is the improvement of the toughness of the martensitic alloys; whilst concerning the second scenario the target is the development of more creep resistant martensitic steels, suitable to tolerate such a high operating temperature. In both the cases the Tungsten content plays a key role, both in terms of solid solution hardening and influence on the DBTT. Two alloys aimed at fulfilling the specifications for the two DEMO operating conditions, both with increased Tungsten content respect to Eurofer, have been produced and characterized. The mechanical properties of these two alloys are hereby reported and discussed.

Small-angle neutron scattering (SANS) and neutron diffraction have been utilized for micro-structural characterization of Eurofer97/2 heats submitted to thermo-mechanical treatments, with the aim to investigate macroscopic material volumes and to complete the local information provided by scanning electron microscopy. The measurements were carried out at MLZ in Garching, utilizing for SANS also a polarized neutron beam. The investigated samples had been submitted to austenitization and tempering at different temperatures, as well as to double austenitization and to “ausforming”, that is austenitization followed by hot rolling. For most of the examined treatments, nearly identical SANS cross-sections were measured, close to the one of Eurofer97/1. In the ausformed sample the nuclear SANS cross-section is also nearly identical to the other samples but the magnetic one is one order of magnitude higher. Furthermore, over a wide experimental interval its nuclear-magnetic interference, measured by polarized SANS, is of opposite sign with respect to the non-ausformed samples. Neutron diffraction measurements provided strong evidence that the origin of such effects is due to the presence of the non-magnetic austenite phase in the ausformed sample, with an estimated volume fraction of 0.17 ± 0.02; within the experimental resolution, it disappears after subsequent tempering.

Abstract The effects of deformation on transformation kinetics during continuous cooling and microstructure evolution of a medium carbon high strength steel were investigated by metallographic method, dilatometry and X-ray diffraction etc. The results show that ausforming accelerated the ferrite and martensite transformation, but significantly retarded the bainite transformation. In addition, the amount of retained austenite decreased first and then increased, while the carbon content in retained austenite increased with the accumulation of strain. The change trend of the average dislocation density in martensite with strain showed the opposite trend as that of the amount of retained austenite. Moreover, the martensite laths were refined by ausforming. The work provides the theoretical reference for the design of parameters of thermalmechanical controlled process (TMCP) in the production of medium-carbon high strength steels.

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]

The so-called ausforming treatment consists in plastically deforming a fully austenitized steel below the recrystallization stop temperature, prior to either a martensitic or a bainitic transformation. Although this procedure is envisioned as a way to improve the mechanical response of the attained microstructure, it is not without its drawbacks, as the possibility of phase transformations occurring during the deformation step. When such step is applied at relatively higher temperatures than those of the aimed microstructure, the presence of those softer phases could be rather detrimental for the final microstructure. The current study aims to further study those phase transformations to analyze whether they are induced via either stress or strain and to identify the temperature ranges in which they tend to occur, so that they can be avoided or used to improve the final microstructure properties in the future. A final evaluation on the impact of these induced transformations on the final microstructure when deformation is followed by either cooling or an isothermal treatment is also performed. Image, graphical abstract [ABSTRACT FROM AUTHOR]

A high-intensity and high-resolution neutron diffractometer with a thermomechanically controlled processing simulator was employed in-situ to investigate martensite transformation behavior with and without ausforming for a medium-carbon low-alloy steel. Serial neutron diffraction profiles have revealed that the transformation behavior could be successfully monitored during quenching with and without the ausforming process. The fresh martensite exhibits a body-centered tetragonal structure when it forms immediately below the martensite start (Ms) temperature, and its c/a ratio decreases rapidly as time elapses. The lattice parameter and the full width at half maximum of austenite peaks significantly decreases and increases upon martensite transformation, respectively. After ausforming, the data reveal that lattice parameters are larger in austenite whereas smaller in martensite compared with those in the non-ausformed case, which is ascribed to the introduced dislocations. Thus, the lattice defects affect the lattice parameter during martensite transformation. Ausforming also slightly raises the Ms temperature and increases the amount of retained austenite at room temperature as a result of different dislocation densities. The cutting-edge operant quantitative measurements with neutron diffraction for steel production is demonstrated. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Ductile iron was subjected to a total true strain (φt) of 0.3 either by applying φt in the austenite region or by apportioning it through applying a true strain of 0.2 in the austenite region before quenching to austempering temperature (TA) of 375°C, where a true strain of 0.1 is applied (ausforming). Additionally, two types of matrices were produced in the ductile iron, namely ausferritic and ferritic-ausferritic matrices. The ferrite is introduced to the matrix by intercritical annealing after austenitization. Dilatometric measurements as well as microstructure examination showed a fast ausferrite transformation directly after applying φA and that the introduction of ferrite to the matrix resulted in a remarkable acceleration of the ausferrite formation. The transformation kinetics, microstructure evolution, hardness and compression properties are studied.

The effects of ausforming in an Fe30Mn10Cr10Co high-entropy alloy on the microstructure, hardness, and plastic anisotropy were investigated. The alloy showed a dual-phase microstructure consisting of face-centered cubic (FCC) austenite and hexagonal close-packed (HCP) martensite in the as-solution-treated condition, and the finish temperature for the reverse transformation was below 200 °C. Therefore, low-temperature ausforming at 200 °C was achieved, which resulted in microstructure refinement and significantly increased the hardness. Furthermore, plasticity anisotropy, a common problem in HCP structures, was suppressed by the ausforming treatment. This, in turn, reduced the scatter of the hardness.

Abstract It is well known the effectiveness of ausforming on increasing the dislocation density of fresh martensite. This work demonstrates, using atom probe tomography, that this increase in the dislocation density has a strong influence on the nucleation of nanoprecipitates during tempering. Therefore, ausforming might be a powerful processing route for the two-fold improvement of ferritic/martensitic steels where creep resistance is of paramount importance. Firstly, ausforming refines the martensitic microstructure (high strength and toughness), and secondly, it increases the number density of thermally stable nanoprecipitates (creep resistance). Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The microstructural degradation and the creep fracture behavior of conventionally and thermomechanically treated Grade 91 steel were investigated after performing small punch creep tests. A remarkable reduction in creep ductility was observed for the samples thermomechanically treated in comparison to those conventionally treated under the tested conditions of load (200 N) and temperature (700 °C). A change in the fracture mechanism from a ductile transgranular fracture to a brittle intergranular fracture was observed when changing from the conventionally treated to the thermomechanically treated processing condition, leading to this drop in creep ductility. The change in the fracture mechanism was associated to the localized concentration of creep deformation, close to coarse M23C6 carbides, at the vicinity of prior austenite grain boundaries (PAGB) in the thermomechanically treated samples. The preferential recovery experienced at the vicinity of PAGB produced the loss of the lath structure and the coarsening of the M23C6 precipitates. The electron microscopy images provided suggest that the creep cavities nucleate in these weak recovered areas, associated to the presence of coarse M23C6. After the coalescence of the cavities the propagation of the cracks was facilitated by the large prior austenite grain size produced during the austenitization which favors the propagation of the cracks along grain boundaries triggering the intergranular brittle fracture. This fracture mechanism limits the potential use of the proposed thermomechanical processing routes. [ABSTRACT FROM AUTHOR]

In order to develop the application of the more cost-effective copper-based shape memory alloys (SMAs), rather than nickel–titanium as earthquake energy dissipaters, the influence of ausforming-induced plastic deformation on phase transformations, microstructure, super elasticity and mechanical properties of the shape memory alloy Cu-26Zn-4Al was examined. These specific SMA properties were targeted by applying appropriate parameters of the thermomechanical (the so-called ausforming) process: beta-phase homogenization at 800 °C for 20 min, one-step hot rolling at 800 °C and water quenching. The results showed significant microstructural changes, increased mechanical resistance and change in the phase transformation behavior. The SMA treated by ausforming retained the reversible austenitic–martensitic transformation ability, with the appearance of the super-elastic effect up to 6% of strain recovery. Although some strengthening occurred after hot rolling (an increase in true yield strength of 125 MPa was detected), all phase transformation temperatures were decreased. The smallest decrease was detected for the austenite finish temperature (32.8 °C) and the largest for the martensite finish temperature (42.0 °C), allowing both the expansion and the lowering of the temperature range of super elasticity, which is favorable for construction applications. It is concluded that it is possible to achieve an optimal combination of adequate strength and improved transformation behavior of Cu-Zn-Al alloy by applying the ausforming treatment.

The microstructural mechanisms providing delamination toughness in high-strength low-alloyed steels are briefly reviewed. Thermo-mechanical processing methods improving both the strength and impact toughness are described, with a close relation to the microstructures and textures developed. The effect of processing conditions on the microstructure evolution in steels with different carbon content is discussed. Particular attention is paid to tempforming treatment, which has been recently introduced as a promising processing method for high-strength low-alloyed steel semi-products with beneficial combination of strength and impact toughness. Tempforming consists of large strain warm rolling following tempering. In contrast to ausforming, the steels subjected to tempforming may exhibit an unusual increase in the impact toughness with a decrease in test temperature below room temperature. This phenomenon is attributed to the notch blunting owing to easy splitting (delamination) crosswise to the principle crack propagation. The relationships between the crack propagation mode, the delamination fracture, and the load-displacement curve are presented and discussed. Further perspectives of tempforming applications and promising research directions are outlined.

Abstract Ausforming is the process by which austenite is plastically deformed prior to bainitic or martensitic transformation. The advantages of such a process on the kinetics of the bainitic transformations have already been reported, but little is found about the effect on the morphology and crystallography of the final microstructure, and in most cases the studies deal with high C steels. Since ausforming has been suggested to be an alternative to transfer the concept of nanostructured bainite to medium carbon contents, the necessity to study the effect of prior plastic deformation on the subsequent bainitic transformation in those steels arises. By means of techniques such as high-resolution dilatometry, scanning electron microscopy, electron backscatter diffraction and X-Ray diffraction, it has been proven that, in a medium carbon steel (0.4 wt%), the macroscopic isotropy that characterizes the bainitic transformation can be altered, under certain conditions, by plastically deforming austenite prior to transformation. The alteration is such that, at low temperatures, the interpretation of the dilatometric response or the plate thickness measurements can be hindered. In addition, the final microstructure is also affected, presenting highly ordered bainitic ferrite plates, most of them correspondent to specific crystallographic variants. Graphical Abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Small Punch Creep technique was used as a screening procedure to evaluate the creep properties of different microstructures developed in a thermomechanical simulator. The goal seek was to generate alternative microstructures in a conventional ferritic-martensitic G91 steel grade which boost thermal stability at temperatures as high as 700 °C. The developed microstructures allow studying the effect of the austenitization temperature optimized by thermodynamic calculations and the ausforming on the creep strength and ductility. The improvement in creep strength recorded was attributable to a higher number density of MX precipitates. By contrast, these microstructures showed an important reduction in creep ductility. [ABSTRACT FROM AUTHOR]

The stability of the martensitic microstructure in ferritic/martensitic G91 steel at operating temperatures up to 700 °C might be improved by means of ausforming thermomechanical treatments. The goal sought is to promote the formation of a high number density of MX nanoprecipitates in the martensitic microstructure obtained after a subsequent tempering. This work is focused on the effect of the ausforming temperature. The results show that the lower the ausforming temperature is the higher is the dislocation density obtained in ausformed fresh martensite and the larger the number density of MX carbonitrides after tempering is. Creep strength, evaluated by Small Punch Creep Tests has allowed us to conclude that the best creep resistance was obtained for the steel ausformed at the lower temperature due to the higher pinning force for dislocation motion triggered by the distribution of MX. The creep results obtained on the ausformed samples were compared with those after the conventional heat treatment, showing that the high density of MX carbonitrides after an ausforming thermomechanical treatment is a promising processing to raise the operation temperature of this steel. [ABSTRACT FROM AUTHOR]

The effect of different thermomechanical treatments, i.e., ausforming and tempforming, on the microstructure and mechanical properties including impact fracture behavior of high-strength low-carbon S700MC-type steel was investigated. The tempforming resulted in the development of ultrafine grained elongated grain microstructure with the transverse grain size of 530 nm, whereas the ausforming led to the formation of typical tempered martensite structure with a distance between high-angle boundaries of 1.5 µm. The tempered microstructures involved the formation of carbides with an average particle size about 50 nm and M(C, N)-type carbonitrides with average size of 10 nm. The tempformed steel exhibited the ultimate tensile strength of 1110 MPa and the impact toughness, KCV, above 450 J/cm 2 at a temperature of 293 K, and KCV of 109 J/cm 2 at liquid nitrogen temperature for impact direction perpendicular to rolling plane. In contrast, KCV of the ausformed steel was 180 and 10 J/cm 2 at 293 and 77 K, respectively. The enhancement of toughness was associated with delamination cracks which occurred in the ultrafine elongated grain structure with anisotropic properties. [ABSTRACT FROM AUTHOR]

Both ausforming and transformation temperature affect the successive bainitic transformation and microstructure. The individual influence of each case is clear, whereas the combined effects are still unknown. Thermomechanical simulation and metallography were used to investigate the combined effects of ausforming and transformation temperature on bainitic transformation and microstructure. The kinetics of isothermal bainitic transformation in non-deformed and deformed materials was analyzed. A lower transformation temperature can lead to more bainite formation without deformation. However, ausforming with small strains can partially compensate for the decrease of bainite amount caused by the decreased undercooling. The larger the applied strain is, the smaller the difference between the final amounts of bainite with different undercooling. Ausforming at a relatively higher temperature is more favorable to the acceleration of subsequent isothermal bainitic transformation. The results in the present work provide reference for optimizing the fabrication technology of medium-carbon nanobainite steels.