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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 authors gratefully acknowledge the support for this work by the European Research Fund for Coal and Steel under the Contract RFCS-2015-709607. The authors also acknowledge the supports of the Metallography and Phase Transformations, the X-Ray and the Microscopy laboratories at CENIM. Adriana Eres-Castellanos acknowledges the Fonds de recherche du Québec – Nature et technologies (FRQNT) for the support received under the Merit scholarship program for foreign students, personal ref 290835.

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.

Strength and toughness limits of a newly developed low alloy martensitic steel are explored using impact toughness experiments and room temperature tensile testing. These steel samples are subjected to either a selected heat treatment, to obtain tempered martensite, or equal channel angular pressing (ECAP) performed between 950 °C and 1050 °C, to refine prior austenite grains. Martensite refinement through ECAP led to a significantly enhanced mechanical response, resulting in impact toughness above 55 J at -40 °C and ultimate tensile strengths above 2 GPa at room temperature. Consequently, this new low alloy steel can be an excellent candidate for high specific strength transportation applications.

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 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.

The effects of ausforming temperature on bainite transformation kinetic and plastic deformation mechanism were evaluated by thermal simulation method and warm rolling process. Results showed that entire process of bainite transformation austempered at 300 °C was notably accelerated by ausforming due to the increased nucleation sites, and by diminishing ausforming temperature as well. However, ausforming would increase undercooled austenite stability, leading to the decrease of maximum attainable volume fraction of bainitic ferrite. Compared with traditional isothermal transformation, ausforming process could effectively refine microstructure, as well as improve mechanical properties. And with decreasing ausforming temperature, strength, hardness and ductility were all increased, which attributed to the thinner thickness of bainite lath and smaller dimension and proportion of blocky retained austenite, by contrast, the change trend of impact toughness was virtually opposite resulted from variant selection. When ausforming at 300 °C, the ultra-fine bainitic steel exhibited the best comprehensive mechanical performance, which was almost 1850 MPa and with up to 23% for ultimate tensile strength and total elongation, respectively. Keywords: Ultra-fine bainitic steel, Ausforming, Retained austenite, Plastic deformation, Mechanical properties

The mechanisms contributing to the excellent mechanical properties of the ultrafine-grained (UFG) Fe-23 wt.%Mn alloy processed by heavy ausforming are unraveled based on detailed characterization analysis and modelling. The UFG microstructure is fully austenitic after the heavy ausforming step involving a 90% rolling reduction; while the material quenched from the coarse-grained (CG) austenite consists of epsilon (e)-martensite and austenite. The UFG Fe23Mn alloy shows a high strain-hardening capacity which leads to a much higher true uniform elongation (0.33) and true tensile strength (1330 MPa) than the CG counterpart (0.17 and 950 MPa, respectively). The high ductility of the heavily-ausformed microstructure with no subsequent annealing step contradicts the general trend of UFG alloys produced by severe plastic deformation. In addition, a ductile fracture mode with improved resistance to damage initiation in the UFG microstructure contrasts with the brittleness of the CG counterpart. Therefore, the UFG Fe23Mn alloy exhibits a high combination of strength, resistance to plastic localization and resistance to cracking. This superior mechanical performance is attributed to the gradual deformation-induced e-martensitic transformation and to the large plastic co-deformation of the UFG e-martensite, in which twinning is suppressed at the expense of the activation of non-basal slip. A mean-field micromechanical model is used to analyze the contribution of the phase transformation to plasticity, and to further rationalize the mechanical response of the UFG e-martensite. This finding provides new insight into the effects of microstructural scale on the mechanical behavior of this class of transformation-induced plasticity (TRIP)-assisted alloys.

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., The authors gratefully acknowledge the support for this work by the European Research Fund for Coal and Steel under the Contract RFCS-2015-709607. The authors also acknowledge the supports of the following laboratories at CENIM: X-Ray diffraction, Metallography and Phase Transformations, in addition to the Electron Microscopy Service facility (Polytechnic School of Valencia).

We studied effects of thermo-mechanical processing (TMP) on microstructure and mechanical properties of the Korea RAFM steel, ARAA, as a feasibility study for enhancement of creep-resistance. Austenitized ARAA plates were subjected to three different TMP schedules: i) 20% hot-rolling at 1000 °C, ii) 20% hot-rolling at 700 °C, and iii) 20% hot-rolling at 700 °C followed by 30% cold-rolling, all of which were then subjected to tempering at 750 °C. 20% hot-rolling resulted in strengthening effects on the as-tempered ARAA in both tensile and short-term creep tests, the extents being greater for that rolled at lower temperature. However, additional cold-rolling promoted recrystallization of tempered martensite structure, leading to a significant loss of strength. Enhanced tensile and creep strength of the TMPed ARAA can be attributed to the larger number of fine MX carbides and slightly higher density of dislocations. It is concluded that ausforming, which is simple and readily applicable to industry, is a method that can effectively improve the high-temperature creep strength of ferritic-martensitic steels without a loss of impact toughness.

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., Authors acknowledge financial support to Spanish Ministerio de Economia y Competitividad (MINECO) through in the form of a Coordinate Project (MAT2016–80875-C3-1-R). Authors also would like to acknowledge financial support to Comunidad de Madrid through DIMMAT-CM_S2013/MIT-2775 project. The authors are grateful for the dilatometer tests by Phase Transformation laboratory. J.Vivas acknowledges financial support in the form of a FPI Grant BES-2014–069863. This work contributes to the Joint Programme on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA).

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/cm2 at a temperature of 293 K, and KCV of 109 J/cm2 at liquid nitrogen temperature for impact direction perpendicular to rolling plane. In contrast, KCV of the ausformed steel was 180 and 10 J/cm2 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.

Crystallographic peculiarities of bainite were investigated in two low carbon microalloyed steels as a function of the parent austenite deformation and the cooling rate. The study was focused on the orientation relationship (OR) between parent austenite and bainitic ferrite and on the paring among probable orientation variants of the bainitic blocks. For the steels studied a common dependency of the OR on the parameter, characterizing the relative position of the transformation temperature within the bainite transformation temperature range, has been obtained. The same remains valid for the variant paring, though the dependency turns out to be more complex in case of ausforming.

The addition of boron (B) is frequently adopted to increase the hardenability of bainitic steels. Although it is well known that B can retard the bainitic transformation kinetics, it is still not clear how the B affects the bainitic transformation kinetics after ausforming. By systematic high-resolution dilatometry tests, the present work reveals that the bainitic transformation kinetics is accelerated in a low C steel with B addition after ausforming from all aspects including incubation time, transformation velocity and transformed volume fraction. In contrast, for the same steel without B addition, both transformation velocity and transformed volume fraction are retarded after ausforming. It is proposed that ausforming can reduce B segregation at prior austenite grain boundaries as some boron can interact with dislocations and therefore enhance bainite nucleation rate. Furthermore, auforming can refine the average volume of bainitic sheaf. Based on the competing mechanisms between increase of nucleation rate and refinement of bainitic sheaf, the effects of B and ausforming on the bainitic transformation kinetics are discussed.

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.

Ausforming treatments at different temperatures were applied to a medium C[sbnd]Si rich carbide free bainitic steel, and the role of transformation plasticity on the evolution of the microstructure is discussed. Three ausforming temperatures were selected prior to isothermal bainite transformation at 325 °C. Deformation was applied above the bainite start temperature, B, (600 °C), below B (400 °C), and at the isothermal transformation temperature (325 °C). A combination of high-resolution dilatometry, XRD, SEM, and EBSD was used to investigate phase changes, microstructure evolution, and variant selections for the different ausforming conditions. The results indicated that the transformation rate was enhanced for all deformation conditions compared with the pure isothermal condition. Ausforming above B did not modify the morphology of the carbide free microstructure, while ausforming below B led to an asymmetrical morphology in the specimen. The alignment of the bainite plates can be at a random or a specific angle, depending on the supercooled austenite condition prior to the bainitic transformation. The thickness of the bainitic ferrite plates was refined to about 100 nm for the ausforming cycle at 325 °C resulting in a hardness of values of 540 HV. Transformation plasticity strains increased with increasing bainite transformation and were more intense with increasing microstructure alignment at the two lower ausforming temperatures. Texture studies revealed that both pure isothermal and ausforming at temperatures above B resulted in an almost random texture, while a strong texture was obtained when ausforming below B, a which was attributed to variant selection., This research is part of a Ph.D. scholarship awarded by the Ministry of Higher Education and Scientific Research in Libya in cooperation with the Canadian Office for International Education (CBIE).

Different characterization techniques are jointly utilized to investigate the effect of ausforming and subsequent annealing treatments on microstructural evolution of cobalt (Co). Results show that the microstructure characteristics in the ausformed Co consist of slender laths (γ phase) and blocky laths (e phase). The low angle boundaries and ∑ 3 boundaries exist in the slender laths; in addition, the special boundaries with 70.5 ° / 11 2 ¯ 0 > misorientation exist in the blocky laths. Moreover, the final annealing treatment has remarkable influence on the ausformed Co, including the incomplete recrystallization and annealing-induced phase transformation.

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.

Acceleration of the bainitic transformation and modifying the microstructural characteristics are believed to be of major concerns in production of Low-Temperature Bainite. Ausforming prior to isothermal bainitic transformation seems to be an effective procedure in this regard. This article aims to study the influence of different ausforming strains on transformation kinetics and microstructural features of low temperature nano bainite obtained after austempering at 300 °C. Results indicated that bainitic transformation tended to be accelerated by plastic deformation of primary austenite depending on applied strain level. In addition, the volume fraction and morphology of the microstructural constituents have been shown to be affected by ausforming after which higher volume fraction of bainite, finer austenite micro blocks and thinner bainitic ferrites could be achieved. Finally, quantitative analysis demonstrated that ausforming favoured the distribution of bainitic sheaves in limited crystallographic directions whereas they spread randomly in different crystallographic orientations in non-ausformed specimens. This was more evident at higher ausforming strain levels at which bainitic sheaves' orientation distributions were narrower relative to the rolling direction regardless of the isothermal bainitic heat treatment time.

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.

The selection of crystallographic variants in bainitic microstructures is a deeply studied topic which can be attributed to several reasons. In the cases where the prior austenite is textured, stressed or plastically deformed, the final ferrite texture and the dilatometry signal may change, being anisotropic. Although the causality between variant selection and the variation of the dilatometric signal has been previously assumed, it has not been thoroughly proved. In this work, the phenomenological theory of martensite was combined with a continuum mechanics analysis to calculate the relative length changes associated to the bainitic transformation during different bainitic transformations in order to prove the connection between the selection of crystallographic variants and the distortion of the dilatometry signal. To do so, four different bainitic microstructures were analyzed: one of them obtained by a pure isothermal treatment and the rest of them obtained by ausforming treatments. Some of the mentioned microstructures had shown to lead to negative longitudinal dilatometry signals and to present a strong variant selection. Results showed that the experimental trends could be explained by theoretical means, which enables to further understand the bainitic transformation., The authors gratefully acknowledge the support for this work by the European Research Fund for Coal and Steel under the Contract RFCS-2015-709607. This work was also partially supported by the European Research Fund for Coal and Steel under the Contract RFCS-2019-899251. The authors acknowledge Sidenor for providing them with the material, the support of the X-Ray Diffraction lab (CENIM), the Metallography and Phase Transformations labs (CENIM) and the Electron Microscopy Service facility (Polytechnic School of Valencia). Lucia Morales-Rivas acknowledges funding support from the German Research Foundation (DFG), project number: 411091845. Adriana Eres-Castellanos acknowledges the German Academic Exchange Service (DAAD) for the support received under the funding program Research Grants - Short-Term Grants (57440917), personal ref 91710971.

This study elucidates the process conditions of ausforming low-alloy medium-carbon steel that can be used to make a fully martensitic microstructure. In the process, the austenitized steel at a high temperature is cooled to warm temperature kept in the supercooled austenite state, then deformed and quenched. The result of this study indicates that the duration time needs to be shortened sufficiently, because supercooled austenite of low-alloy steel only exists for a few seconds. The change of martensitic microstructure is also investigated. Originally, microstructure of low-alloy medium-carbon steel martensite consists of fine lath and coarse butterfly martensite. The frequency of coarse butterfly martensite is decreased by ausforming but is not zero for lower strain levels; coarse butterfly martensite does not form at higher strain levels. This change is presumed to be caused by lattice defects introduced into the austenite matrix by ausforming. In this research, changes in the martensite block width and hardness are investigated. The hardness increases as the block width decreases.

Recently, many steel researchers have been attempting to develop superfine low-carbon bainitic microstructure; however, the results are not satisfactory. In this paper, we applied a new thermo-mechanical process for achieving low-carbon superfine bainitic structure. By applying the new process, i.e. ausforming at temperatures above Ms followed by austempering at 355 °C, which is below Ms , we successfully obtained a superfine, ladder-like bainitic structure, with bainitic laths as thin as ∼100 nm and Vickers hardness as high as ∼500 HV, being of the same level of medium-carbon bainitic steels. This new process shows a potential for wide industrial applications.

The volume fraction and morphology of the microstructural constituents are extremely important when discussing the impact toughness of nanostructured bainitic steels. On the other hand, microstructural features can be influenced by plastic deformation of austenite prior to isothermal bainitic transformation. This paper aims to investigate the effect of 10% ausforming process before austempering heat treatment on impact toughness of nanostructured low temperature bainitic steels austempered at 300 °C. Results indicate that the volume fraction and morphology of high carbon retained austenite within the microstructure are important factors affecting the impact toughness energy. However results showed that the role of bainitic sheaves cannot be ignored due to their valuable effect of arresting/deflecting the cracks. It has been found that bainitic sheaves grow at more limited crystallographic variants in ausformed samples which can influence the value of the impact energy absorbed during straining the samples.

In recent years, the high tensile steels have got much attention in steel structural fields. Especially, the steel plate which has bainitic structure manufactured by TMCP (thermo-mechanical control process) technology becomes widely used because it has good balance of strength, ductility and toughness. Microstructure of those material is basically bainite structures, which are consisted of bainitic ferrite, cementite and MA (Martensite-Austenite constituent). Furthermore bainite microstructure manufactured by modern TMCP has extremely fine structures due to its transformation process from heavily deformed band structure including sub-grain structures which were generated during large amount of rolling in unrecrystallized temperature range called “ausforming”. However, the theoretical mechanism of brittle fracture initiation of bainite is still not clear because of its complexity of its structure. According to previous research including the observation of the fracture surfaces or cross sections of fractured specimens, the prior phenomenon of brittle fracture may occur at either the boundary area between MA and matrix or inside of MA. However, the details of the prior event for the macroscopic fracture mechanism are still unknown. The purpose of this study is to clarify the precursor phenomenon and estimate the quantitative criterion of brittle fracture initiation of bainite steels, which have fine microstructures, by observing the process of it directly by the in-situ observation methods.

For improved performance of the components in a fusion reactor, an increased application temperature for structural materials such as 9%Cr reduced activation steels is crucial. The improvement of the current generation of 9%Cr steels (i.e. EUROFER) is one of the aims of the current EUROfusion programme for advanced steels. The goal of this work is to determine the most effective thermo-mechanical treatment of reduced activation ferritic martensitic steels with respect to high-temperature strength. Compatibility of these treatments with industrial production processes is essential. In the present study, two different batches of EUROFER-2 were prepared with a thermo-mechanical treatment. The materials were solution annealed at 1250 °C and then slowly cooled to the rolling temperature, which was varied between 600 and 900 °C. Hot-rolling was performed in the austenite regime with a subsequent rapid cooling to form the ferritic-martensitic structure. The characterization of the materials was done in as-rolled state and after a subsequent tempering at 750 °C. The materials characterization was performed by tensile and Charpy impact tests using miniaturized specimens. The microstructure was characterized by scanning electron microscopy (SEM) backscatter images and electron backscatter diffraction (EBSD) maps. All the results were compared to those of conventionally processed EUROFER-2 alloys. The first results show a gain in tensile strength of approximately 50 MPa at temperatures above 600 °C compared to conventionally treated EUROFER alloys. Microstructural investigations reveal a fine and homogeneous distribution of the martensitic laths, while the prior austenite grains are about one order of magnitude larger. This can be explained by the exceptionally high austenitization temperature compared to the as-received state.

This study encompasses a comprehensive account of the abrasive wear properties of carbide-free, ultrahigh-strength bainitic steels processed through ausforming at three different temperatures well below the recrystallization stop temperature followed by bainitic transformation at temperatures close to the Ms temperature. Five medium-carbon, high-silicon compositions were designed for the study by suitably varying the alloying levels of carbon, vanadium, niobium, molybdenum, and aluminum. While ausforming at lower temperatures enabled a large number of nucleation sites leading to significant refinement of bainitic laths, the decomposition of austenite at relatively low transformation temperatures was accelerated due to the presence of a high dislocation density, thus enabling completion of bainitic transformation in a reasonable length of time. The steels were characterized in respect of microstructural features and mechanical properties, besides evaluation of wear resistance through a high-stress abrasive wear testing method with natural granite abrasives. The microstructures comprised different fractions of bainitic ferrite and/or granular bainite (56–68%), martensite (0–25%), besides a significant fraction of retained austenite (20–34%) manifesting as pools and also interlath films, depending on the ausforming conditions and subsequent cooling paths. A tensile strength of 1900 MPa level was achieved with hardness exceeding 500 HV for the medium-temperature ausformed steel containing a high carbon content that also showed lowest mass loss in the wear test. The hardness-to-mass loss ratio appeared highly promising with some of the carbide-free bainitic steels on par with or better than the reference martensitic steel. The high work-hardening capability as a consequence of the strain-induced austenite to martensite transformation was considered as the main factor for the superior abrasive wear resistance of the carbide-free bainitic steels. publishedVersion

In order to impart superior mechanical properties to medium carbon carbide-free bainitic steels, an innovative approach has been adopted to extensively refine the bainitic ferrite plate thickness. Unlike controlled deformation in the no-recrystallization regime above the Ar3 temperature, an attempt has been made in this study to carry out low temperature ausforming in the bay between ferrite and bainite C-curves at 500 °C in order to impart high dislocation densities in the austenite prior to phase transformation. Two experimental high-silicon, medium carbon steels were suitably designed and processed for this study, with one steel containing small additions of 0.3Mo and 0.03Nb. Flow stress measurements were made using single-hit compression tests in the temperature range 300–900 °C in steps of 100 °C at different strain rates in the range 0.1–10 s − 1 on a Gleeble simulator. Samples ausformed at 500 °C were isothermally held for 1 h at different transformation temperatures in the range of 300–400 °C to complete the bainitic transformation. Influence of strain induced bainite transformation on flow stress was obvious at 0.01 s-1, particularly at 300 and 400 °C. Despite enhanced nucleation in fine-grained steel B containing Nb + Mo, growth of bainite sheaves was much slower. Dilatation behaviour was comparable for the two steels at

The effect of multi-step ausforming with small strain per pass (

A method to determine the orientation relationship (OR) in bainitic and martensitic transformations has been employed that does not use reconstruction of prior austenite grains. It is shown that the method keeps accuracy regardless of orientation non-uniformity induced inside austenite grains by hot deformation. The revealed OR behavior for bainite formed under continuous cooling agrees with its temperature dependence previously found in case of isothermal transformation. Ausforming effect on the OR detected in martensite is presumably due to deformation induced subgrains separated by low-angle dislocation boundaries.

The effects of prior deformation conditions on the bainitic transformation are investigated using dilatometry and optical metallography. The influences of both deformation temperature and strain on bainite transformation kinetics and morphology are examined in superbainitic carbide-free steel. The transformation is retarded by deformation at high temperature due to the retardation of bainite growth by dislocation debris in deformed austenite. At low deformation temperatures, the initial transformation rate is accelerated due to the presence of prolific nucleation sites in deformed austenite. The growth rate is, again, retarded by deformation. The combined effect of accelerated nucleation and retarded growth results in a complex dependence of the phase transformation kinetics on applied strain. For small deformation strains at 300 °C, the overall transformation kinetics is faster than that in the non-deformed material. From a practical point of view, this provides an important opportunity to reduce the processing times for carbide-free bainitic steels. Interestingly, both the final transformed bainite and retained austenite fraction under these conditions are much higher than the non-deformed material. The retardation of growth dominates the overall transformation kinetics at large strains and low temperature leading to a high volume fraction of martensite in the final microstructure.

The effects of ausforming temperature, strain and strain rate on isothermal bainite transformation behaviour and microstructural refinement in medium-carbon Si–Al-rich alloy steel were determined by thermomechanical simulation. Results show that ausforming decreased the martensite start temperature and enabled isothermal transformation at low temperatures, resulting in the formation of nanostructured bainite. Decreasing the ausforming temperature and increasing the ausforming strain did not only reduce the incubation of isothermal transformation but also refined bainite lath and enhanced the hardness of the alloy steel. Strain rate showed a weak influence on the transformation behaviour and microstructure of the obtained nanostructured bainite.

Deformation behavior of austenite/supercooled austenite in a medium-carbon Si–Al-rich alloy steel is investigated on a thermomechanical simulator and an optical microscope. Effects of ausforming temperature, strain rate and strain on martensite start temperature ( M S ) and ausformed martensite microstructure are studied in detail. Results show that power-law-type work hardening is caused during supercooled austenite deformation at 300 °C; and the work hardening followed by steady-state flow is caused during the deformation at 600 and 900 °C. The ausforming can decrease M S , refined lath martensitic microstructure and slightly enhanced hardness. With decreasing the ausforming temperature the M S and martensite lath thickness decrease markedly. The strain and strain rate do not have strong effects on the M S and lath thickness. In addition, the ausforming can decrease the tetragonality of martensite and facilitate the formation of dislocation substructured martensite.

Variant selection of lath martensite and bainite transformed in an ausformed low-alloy low-carbon steel is analyzed quantitatively by applying reconstruction method of austenite orientation. It was found that transformation texture having {1 1 1} α and {0 1 1} α along compression axis is developed by ausforming both in martensite and bainite transformation. Development of such texture can be explained qualitatively from deformation texture in austenite without taking variant selection into account. However, their intensities cannot be reproduced quantitatively, indicating that specific variants are favorably selected by ausforming. Quantitative analyses of variant fractions revealed that martensite variants whose habit planes are nearly parallel to the primary and secondary slip planes in austenite are formed preferentially. Similar variant selection rule is observed in ausformed bainite while it is not as strong as in ausformed martensite. A preferential nucleation model is proposed for variant selection by ausforming in which variants with habit planes nearly parallel to the primary or secondary slip planes in austenite are nucleated on the microband boundaries in austenite introduced by compression.

The bainite transformation behavior after plastic deformation of austenite, i.e., ausforming was studied by in situ neutron diffraction and ex situ experiments, and the effects of ausforming temperature was made clear. Ausforming, at a low temperature (573 K) was found to accelerate bainite transformation and produce a characteristic microstructure, whereas at a high temperature (873 K), ausforming had little influence. The reason for the different results stems from the dislocation structure introduced in austenite; planar dislocations remaining on the active slip planes are believed to assist bainite transformation, accompanied by strong variant selection. The variant selection rule that focuses on Shockley partial dislocation was verified from electron backscatter diffraction results.

The variant selection of lenticular martensite transformed from austenite in an Fe–25Ni–0.5C alloy uniaxially compressed at 973 K was investigated quantitatively. It was found that four variants with habit planes closely parallel to the compression plane were favorably formed. Preferential growth along elongated austenite grains parallel to the compression plane was proposed for the selection of variants since it involves fewer disturbances at the austenite grain boundaries.

The effect of ausforming on kinetics, morphology and crystallography of nanobainite steel was examined by electron backscattered diffraction and transmission electron microscopy. Ausforming has been found to accelerate bainite transformation at 573 K. A characteristic microstructure consisting of blocky bainitic laths and retained austenite is observed in the ausformed bainite steel, where strong variant selection takes place due to the operated slip systems.

Phase transformations associated with shape memory effect in nickel–titanium (NiTi) alloys can be one-stage, B19′ (martensite) ↔ B2 (austenite), two-stage including an intermediate R-phase stage, or multiple-stage depending on the thermal and/or mechanical history of the alloy. In the present paper, we highlight the effect of (i) deformation by cold-rolling (from 10% to 40% thickness reduction) and (ii) final annealing on the transformation characteristics of a Ti-rich NiTi shape memory alloy. For this purpose, one set of samples initially heat treated at 773 K followed by cold-rolling (10–40% thickness reduction), has been further heat treated at various temperatures between 673 and 1073 K. Another sample was subjected to heat treatment at 1040 K for 300 s followed by hot rolling (50%) after cooling in air to 773 K and water quenching to room temperature ( T room ). Phase transformations were studied using differential scanning calorimetry, electrical resistivity measurements and in situ X-ray diffraction. A specific pattern of transformation sequences is found as a result of combination of the competing effects due to mechanical-working and annealing.

The effects of severe plastic deformation via equal channel angular extrusion (ECAE) and cold drawing followed by low temperature annealing on the shape memory behavior of the Ti 50.27 Ni 49.73 alloy were comparatively investigated. The ECAE billets were processed at 300 °C using a 90° angle ECAE tool while 30% cold drawing was performed at room temperature followed by a low temperature annealing treatment. Transformation temperatures, and transformation and irrecoverable strain levels were revealed using thermal cyclic experiments under various constant tensile stress levels. Considerable improvement in the thermal cyclic stability of transformation temperatures and transformation strains and reduction in irrecoverable strains under high stress levels were achieved in the ECAE processed material. More interestingly, the thermal hysteresis was notably lower in the ECAE case. This was attributed to the formation of favorable dislocation substructure and microstructural refinement due to the deformation twinning in austenite. The results showed that the severe ausforming via ECAE has certain advantageous over cold deformation followed by annealing process in enhancing the shape memory properties of NiTi alloys.

The effect of ausforming on formation of upper bainite structure was investigated in Fe–9Ni–(0.15–0.5)C (mass%) alloys, focusing on the crystallographic feature of bainitic ferrite. The specimens were ausformed at 1073 or 873 K by 50% compressive reduction after austenitizing, and transformed at 623 K. Lath-shaped upper bainite structures similar to lath martensite are formed at 623 K. A bainite packet is partitioned into blocks of all the six variants of the K–S orientation relationship satisfying the same parallel close-packed plane relationship. In both of the non-ausformed and ausformed specimens, bainite blocks are coarsened with increasing carbon content. In the Fe–9Ni–0.15C alloy, the effect of ausforming on the bainite block width is small. For higher carbon alloys (Fe–9Ni–0.3C and 0.5C), bainite blocks are remarkably coarsened by the ausforming. In contrast, the ausforming refines lath martensite blocks in the Fe–9Ni–0.15C alloy but has a small effect in higher carbon alloys.

There is an increasing drive toward the monitoring of the structural health of costly and safety critical infrastructure. Quite often, however, this does not require the cost and nuisance of active monitoring, and a simple passive system will suffice. One of the most robust, cost effective, and neat methods of passive peak monitoring involves the use of strain memory alloys; which are ferrous alloys that display paramagnetism in the unstrained state, but transform proportionally to display varying degrees of ferromagnetism depending on the level of peak strain induced in the material. This effect is achieved using a transformation in crystal structure from a metastable austenitic structure to a stable strain-induced martensitic structure. Because of the physical properties and the relatively low cost of strain memory alloys it is actually possible to manufacture complete components from this family of materials, and the materials can be tailored in terms of physical as well as transformation properties using alloying chemistry and prior deformation of austenite in the ausforming temperature range. Several dual purpose components have been developed for a variety of different applications, using strain memory alloys: a smart composite laminate, a smart rock anchor, and a smart aircraft bolt among others. This paper presents the general principles involved in using strain memory alloys for structural health monitoring, as well as specific application designs and results.

By investigating the effects of the main elements such as Si, Mn, Mo and ausforming on the microstructure and properties, a new type ductile iron with the composition of 3.0–3.8%C, 2.9–3.4%Si, 1.6–2.1%Mn, 0.02–0.05%RE and 0.02–0.05%Mg, which has a microstructure composed of bainite, small amount of martensite, residual austenite and carbide, was developed. Contrasting to the traditional ductile iron, this new type ductile iron does not contain precious alloys, which reduces the manufacturing cost. It is found that the ratio of Si/Mn is an important parameter to the microstructure and the mechanical properties of the new type ductile iron. The favorable value for the ratio of Si/Mn is in 1.6–2.2. The ausforming is an important method to greatly improve the properties of this material. Favorable ausforming parameters with a deformation content of 20–40% and a tempering temperature of 200–300 °C were provided for this material. Performance measurements indicate that this material is of excellent general mechanical properties. The surface hardness exceeds HRC54 when the impact energy is reaching 30 J/cm2.

Ausforming implies plastic deformation of austenite (β) at temperatures T AF > M d , at which no stress- or strain-induced transformation can occur. It introduces a variety of extrinsic lattice defects, which in turn modify the course of transformation, the structure of martensite, and increase the conventional strength of the alloys. The temperature range of ausforming has to be subdivided into three subranges, depending on whether the β-phase is (1), disordered; (2), ordered; or (3) capable of precipitation of a second phase or massive transformation. For the Cu-base alloys the ranges 1 and 3, and for Ni–Ti 2 and 3 may apply. This causes a different hot-deformation behavior of the two types of alloys: the formation of dislocation groupings (2-d, 3-d-nets, and Moires) in the brass-type alloys which undergo ordering during cooling from T AF. In addition a particular twinning mechanism and the formation of a rolling texture are found in ordered Ni–Ti. In Ni–Ti-alloys premartensitic R-phase formation can be caused by ausforming. Ausforming leads to lower temperatures but not to suppression of martensitic transformation cycles. Conventional strength is increased in both types of alloys.

Microstructural observations of prior-austenite grain boundary cementite in tempered martensite of an Fe–2%Mn–0.36%C alloy were performed. Grain boundary cementite could be markedly refined by applying ausforming. Unlike the case of the non-ausformed specimen, no coarse cementite films were observed and fine cementite particles with various variants tended to form at a serrated prior-austenite grain boundary in the ausformed specimen.

The ausforming process for 17-4 precipitation hardening (17-4PH) stainless steel is compared with the conventional warm-forging process, from the point of both the forgeability and the properties of the forged material. The forgeability is evaluated by upsetting, forward rod extrusion and backward can extrusion. The forging force required in ausforming is about half that required in conventional warm-forging: it especially keeps quite a low level in the temperature range of 200–400°C. Ausforming brings out the excellent upsettability of the material at the warm-working temperature, whilst conventional warm-forging leads to a drastic decrease of workability at 400°C. Forward extrusion and backward extrusion with a reduction of area of 30–70% are also achieved without cracks and seizures. The properties of the forged material are examined with the hardness test, tensile test, impact test, delayed cracking test and the corrosion resistance test. The mechanical properties of the ausformed material are slightly superior compared with those of the solution treated and aged (ST-AG) material. The Charpy impact value of the ausformed and directly aged (AU-AG) material is superior to that of the ST-AG material, keeping nearly the same level from room temperature to −100°C. The corrosion resistance of materials forged by the two processes are almost the same. The surface quality of the ausformed product is good compared with that of the cold-forged product. The ausforming process is applied successfully to the production of various 17-4PH stainless steel parts using a multi-stage forging machine.

The shape memory effect (SME) in Fe-Mn-Si based alloys results from the {gamma} {yields} {var_epsilon} transformation and its reverse transformation on heating, however, intersection of {var_epsilon} variants is detrimental to the improvement of the SME in these alloys. In the present study, improvement of SME in Fe-Mn-Si-Cr-Ni alloy by ausforming was investigated and the mechanism for the improvement of SME was proposed according to an analysis of the microstructure.

Rolling-contact fatigue is a common failure mechanism for the class of machine elements that includes gears, cams, bearings and sprockets. Improved endurance through the fabrication of more durable cases in carburized steels was undertaken in the present study, with surface ausforming the fabrication technique of interest. Surface ausforming, in a rolling-deformation scheme, was applied to AISI 9310 steel, carburized to 1.0 wt% carbon to a case depth of 1 mm. Ausrolling was applied in two distinct manners, either by a line contact or a point contact die, after quenching the alloy to 235°C. Ausformed alloy evidenced the lath martensite expected for this thermomechanical processing: its depth beneath the surface depended on the forming stress, with the point contact die inducing the most extensively formed surface. In general, surface hardnesses in ausrolled samples were greater by ∼ 5.0 HRC than in control samples conventionally marquenched. Rolling-contact fatigue endurance for ausrolled microstructures was also superior to that of conventionally marquenched samples, although subsurface oxides introduced during gas carbunization degraded the performance of the more lightly ausrolled examples. Light surface grinding, which removed these artefacts, led to ausrolled surfaces that were over an order of magnitude more durable in rolling-contact fatigue loading than were those of marquenched-only materials. Ultimately, rolling-contact fatigue endurance could be correlated with surface hardness distributions for all forms of thermomechanical processing.

Shape memory effect in two representative FeNiC alloys, 31% Ni-0.4% C and 27% Ni-0.8% C, with low Ms temperatures has been studied in detail. The shape memory test was conducted not only on as-austenitized specimens but also on ausformed specimens. The effect of an external load during reverse transformation was also examined. The reverse martensitic transformation behavior related with the shape memory effect was observed in situ, using a high temperature optical microscope. The results are (1) about 50% shape recovery is observed for up to 5% initial tensile strain, while 75–95% shape recovery is obtained for 1–2% initial bending strain. (2) An ausformed specimen shows a better shape memory effect probably owing to the increased austenite strength. (3) The austenite-martensite interface moves backward on heating not only for a plate-type martensite but also for a curved or irregular shaped martensite in a specimen strengthened by ausforming. (4) The shape recovery decreases with increasing an applied load, but even in this case the reversible interface movement occurs.

The relationship between the flow stress of austenite at ausforming temperature and the hardness of the ausformed martensite (or austenite) at room temperature was studied using martensitic steel 18CrNiW and austenitic steel 18-8. Ausforming was performed with a torsion test machine at various temperatures between 723K and 923K and at strain rates between 2.5×10−3 and 2.6×10−1. After deformation, specimens were rapidly quenched by water spray. It was found that the increase in hardness of the ausformed austenite at room temperature (ΔHvγ) is uniquely determined only by the increase in the flow stress (i.e., the amount of work-hardening) of austenite (δσγ) by ausforming. On the other hand, the increase in hardness of ausformed martensite at room temperature (ΔHvγ) is determined b/y both Δσγ and Z (Zener-Hollomon parameter). When Δσgg is the same, ΔHvα is increased with an increase in ausforming temperature or decrease in strain rate (i.e., with a decrease in Z). Furthermore, when the ausforming temperature is fixed, the contribution of Δσgg on ΔHvγ is small at smaller Δσγ range and becomes large at larger Δσgg range. Present results suggest that the dislocation cell structure in austenite is very effective for the strengthening of ausformed martensite, and uniformly distributed (pile-up) dislocations in austenite have little effect on the strengthening of the ausformed martensite.