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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.

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2023). = Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023, Carbide-free bainitic (CFB) steel has become a new forefront of advanced high-strength steels owing to their outstanding balance in mechanical properties. Due to a thermodynamic instability of austenite in low carbon CFB steels, formations of only primary phase bainitic ferrite and secondary carbon enriched retained austenite phase are impracticable. The untransformed austenite at high temperatures could partially transform into fresh martensite during cooling operation, depending on the local carbon concentration in the austenite. A general consequence is that an excessive formation of fresh martensite may deteriorate ductility, despite the enhanced strength of the steel. Thus, controlling the thermodynamic stability of austenite has been a challenging issue in developing low-carbon carbide-free bainitic (CFB) steels, besides increasing mean carbon content and chemical compositions. Ausforming as a thermomechanical heat treatment process is applied to compromise the formation of fresh martensite and to balance the phase constituent of the steels. This process combines plastic deformation of the untransformed austenite with the conventional process of isothermal heat treatment. Parameters of ausforming, such as deformation temperature, strain, and strain rate, are of significant importance in defining appropriate conditions for desirable microstructures and mechanical properties. The correlation between the ausforming conditions throughout the kinetics behavior of isothermal bainitic transformation, factors inherent in the martensite transformation, hardness, and tensile properties have been established. A unified physics-based model has been developed based on nucleation rate theory to provide a better understanding of how ausforming influences the variations of activation energy, corresponding driving energy, and the evolution of carbon enrichment in austenite. In addition, the impact of the chemical compositions has been conducted to reveal a limitation of ausforming with respect to the deformation strain on improving the thermodynamic stability of austenite against the formation of fresh martensite. Throughout the dissertation, a systematic investigation in heterogeneous microstructure and mechanical properties subjected to ausforming conditions allows for establishing advanced high-strength steels with reasonable hardness and improved strength and ductility., Published by RWTH Aachen University, Aachen

One of the main problems limiting further growth in the production of parts by the hot forging of porous performs (HFPP) is that the obtained materials are prone to brittle fracture due to the poor quality of interparticle jointing formed during hot deformation, as well as the presence of impurities in the composition of initial powders. The paper studies the possibility of increasing the mechanical properties and endurance performance of hot-deformed powder steels by doping them with sodium or calcium microadditives and using thermomechanical treatment. Sodium bicarbonate and calcium carbonate were used for microalloying. Carbon was added as pencil graphite powder. The temperature of heating porous preforms before hot forging and the carbon content in steels were varied; the content of microalloying additives was, wt.%: 0.2 for sodium, and 0.3 for calcium. Mechanical properties as well as contact and low-cycle fatigue life were tested on 5 × 10 × 55 mm and 10 × 10 × 55 mm prismatic specimens, as well as ∅ 26 × 6 mm cylindrical specimens. In comparison with carburizing and thermal treatment, thermomechanical treatment improves the impact strength and endurance performance of hot-deformed powder steels with Na or Ca microadditives under the contact and low-cycle fatigue loading, and the hot repressing temperature of porous preforms is reduced without compromising the mechanical properties of powder steels obtained. It may be associated with the formation of a more fine-grained structure and higher microstresses of the crystal lattice. The cooling down of preform surface layers during hot forging process operations creates conditions for ausforming in them.

In this study, the effect of ausforming temperature and strain on the bainite transformation in a model alloy containing 0.39 pct C, 2.0 pct Mn, 1.9 pct Si, and 0.5 pct Cr (mass percent) was investigated. Samples were deformed at 300 °C, followed by isothermal holding to monitor the phase transformation kinetics. Dilatometry results confirm the acceleration of transformation kinetics when the material is deformed to small strains (0.05 to 0.20). Tensile testing results show that, for long isothermal holding times, no improvement in strength or ductility is achieved as a result of ausforming in comparison with the non-deformed state. However, the benefit of ausforming lies in its ability to accelerate the bainite transformation kinetics and to reduce the isothermal holding time needed to avoid premature fracture due to the presence of a high volume-fracture of fresh martensite. Fracture analysis reveals ductile fracture, with cracks along martensitic regions within the microstructure. The distribution of martensite in the final microstructure was associated with bands of Mn segregation. This banding effect was less visible within the ausformed microstructure because bainite was able to nucleate within both the high- and low-Mn regions. A second advantage of ausforming is, therefore, to reduce the microstructure heterogeneity associated with compositional banding within the steel sheets and, by so doing, eliminate crack propagation within the bands. In spite of the fact that ausforming accelerates the bainite transformation kinetics, the required transformation times for the present steel are still too long for use within industrial continuous annealing lines.

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.

Both ausforming and austempering below the martensite start temperature (MS) have been shown to accelerate the bainitic transformation. This work investigates the combined effects of deformation and austempering below the MS on the isothermal transformation kinetics, microstructure, and hardness in a medium-carbon bainitic steel. The results show that, deformation below the MS can delay the onset of the baintic transformation compared to the case in which no deformation is applied. The larger the amount of martensite presents prior to the deformation step, the greater the delay in the onset of bainitic transformation. This behavior is attributed to the development of strong misorientation gradients within austenite during the co-deformation of the austenite-martensite microstructure. These large misorientation gradients make the pre-existing martensite a less effective nucleation site for bainite.

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.

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., This research was supported by the Ministry of Higher Education and Scientific Research in Libya with the collaboration with the Canadian Bureau for International Education (CBIE). The author also would like to thank Adriana Eres-Castellanos from CENIM-CSIC in Spain, for her informative scientific support.

The interplay between ausforming strain level applied prior to austempering step and bainite transformation kinetic as well as plastic deformation mechanism of nanostructured bainitic steel was essentially investigated. The results indicated that the bainite transformation proceeded rapidly with increasing ausforming strain owing to the added heterogeneous nucleation sites and overall storage energy. Nevertheless, when the ausforming strain further increased to 40%, the acceleration effect became less pronounced attributing to the counteraction mechanism by inhibition effect of retarded bainite growth. Increasing ausforming strain could effectively refine bainite sheaves and minimize the dimension of blocky retained austenite, yet strengthen undercooled austenite, leading to a decreased volume fraction of bainitic ferrite at the termination of transformation. Ausforming strain had a positive effect on mechanical properties, particularly, the sample ausformed at 300 °C for 40% strain achieved the largest tensile strength of 1900 MPa and the best total elongation of 24%. This enhanced strength–ductility synergy mainly ascribed to the benefit of the refined bainitic ferrite lath and the increased dislocation density together with transformation induced plasticity effect. Unfortunately, the apparent variant selection brought about by ausforming could offset the increased impact toughness resulting from microstructure refinement when ausforming strain was higher than 10%, which leaded to an intensive deterioration of impact property.

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 cha...

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

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., The authors would like to thank James Burns for assistance in performing APT sample preparation and running the APT experiments. The authors are grateful for the dilatometer tests by Phase Transformation laboratory and for the SEM microscopy by the Microscopy Lab at CENIM-CSIC. This work contributes to the Joint Programme on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA).

The comprehensive effect of strain and ausforming temperature on the martensite start temperature (MS) of a medium-carbon bainite steel was investigated by thermal simulation, optical microscope, scanning electron microscope, etc. It is already known that small strain increases the MS, while larger strain decreases the MS. However, the effect of ausforming temperature on the MS has not been reported and clarified. In this study, the concepts of critical strain (ec) and saturated strain (es) are proposed. The MS at the critical strain is equal to the MS of the nondeformed specimen. The saturation strain, which is first observed, is the strain value, and the MS does not further decrease with the increasing strain. The results show that the MS depends on the strain amount of ausforming but is not affected by the ausforming temperature. Moreover, with the increase of strain amount and ausforming temperature, the length of the martensite laths decreases. In addition, the hardness of the specimen increases with the increase of the ausforming strain amount, whereas the ausforming temperature has little effect on the hardness.

Due to high thermo-mechanical loads, tools used in hot forming operations need a high resistance to different damage phenomena, such as deformation, cracking and abrasion. They are exposed to cyclic thermo-mechanical stress conditions, which leads to tool failure and subsequent tool replacement during cost-intensive production interruptions. To increase wear resistance, forging tools can be produced in the metastable austenite area. Forming of steel below the recrystallisation temperature, also known as “ausforming”, offers the possibility to increase strength without affecting ductile properties. This is due to grain refinement during forming. In this study, the thermo-mechanical treatment ausforming will be used to form the final contour of forging dies. For this purpose, an analogy study was performed where a cup-preform is ausformed, which represents the inner contour of a highly mechanically loaded forging die. It is investigated to what extent a fine-grained microstructure generated in the last forming stage can be achieved and how it influences the tool’s performance. The hot-working tool steel X37CrMoV5-1 (AISI H11) was used as workpiece material. To achieve optimal properties, process routes with tempering temperatures from 300 °C to 500 °C and global true plastic strains of φ = 0.25 and φ = 0.45 were examined. The results were evaluated by pulsation tests, metallographic analysis and hardness measurements of the formed parts. Optimal ausforming parameters were derived to produce a high performance forging die.

In order to develop the application of the more cost-effective copper-based shape memory alloys (SMAs), rather than nickel&ndash, 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 &deg, C for 20 min, one-step hot rolling at 800 &deg, 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&ndash, 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 &deg, C) and the largest for the martensite finish temperature (42.0 &deg, 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 reason why variant selection phenomena occur in ausforming treatments is still not known. For that reason, in this work, the effect of compressive deformation on the macro and micro-texture of a bainitic microstructure was analyzed in a medium-carbon high-silicon steel subjected to ausforming treatments, where deformation was applied at 520 °C, 400 °C and 300 °C. The as-received material presented a very weak 〈331〉 fiber texture along the rod axis, due to prior thermomechanical processing. For the samples isothermally heat-treated, it was detected that the bainitic ferrite inherited a 〈100〉 fiber texture from the 〈110〉 fiber texture present in the prior austenite. The intensity of this transformation texture was more pronounced as the deformation temperature decreased. Also, variant selection was examined at different scales by combining Electron-Backscattered Diffraction and X-ray Diffraction. The quantification of the fraction of crystallographic variants under certain conventions for every condition revealed variant selection in samples subjected to ausforming treatments, where these phenomena were stronger as the deformation temperature was lower. Finally, some of the theories proposed so far to explain these variant selection phenomena were tested, showing that variants were not selected based on their Bain group and that their selection can be better described in terms of their belonging to packets, if these are defined according to a global reference frame. This suggests that the phenomena might have to do with the effect of deformation mechanisms on the prior austenite., The authors acknowledge Sidenor for providing them with the material, Irene Llorente (from the X-Ray Diffraction lab at CENIM) for her support to analyze the XRD results, in addition to the support of the Metallography and Phase Transformations labs (CENIM) and the Electron Microscopy Service facility (Polytechnic School of Valencia). Open Access funding enabled and organized by Projekt DEAL. This work was supported by the European Research Fund for Coal and Steel under the Contract RFCS-2019-899482; the German Research Foundation (DFG) under the project 411091845 and the German Academic Exchange Service (DAAD) under the funding program Research Grants—ShortTerm Grants (57440917), personal ref 91710971.

A combination of experimental and numerical approaches was applied for constructing a dynamic model for thermomechanical processing, which was used for simulating laboratory rolling and cooling, and for designing a cooling path to enable phase transformation from austenite to ultrafine ( ∼ 50–100 nm) bainitic laths. Physical thermomechanical simulation experiments were used for calibrating the numerical models. Hot rolling and water cooling experiments were conducted and they were numerically simulated. The calibrated numerical models were used for simulating the main processing stages affecting the final microstructure evolution during a laboratory scale processing, i.e. the low temperature (500 °C) ausforming and subsequent cooling schedules leading to the decomposition of austenite into bainite and martensite. The fitted model parameters and simulation results are presented for the laboratory rolling and two different cooling paths: (i) air cooling to 350 °C temperature with subsequent holding for 1–1.5 h, and (ii) water cooling close to martensite start temperature, and furnace holding for 1–1.5 h. Microstructural analysis was carried out using scanning electron microscopy combined with electron backscatter diffraction as well as X-ray diffraction and the structures were corroborated with mechanical properties evaluated in respect of hardness, tensile and impact toughness properties. The achieved mechanical properties and microstructures were further interpreted with the numerical simulation results. The results show that the calibrated numerical simulations provide an effective tool for designing suitable thermomechanical processing paths leading to desired microstructure.

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., This work was supported by the European Research Fund for Coal and Steel under the Contract RFCS-2019-899482 and the German Research Foundation (DFG) under the project 411091845.

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.

The effects of deformation temperature on phase transformation and microstructure in nanostructured bainite steel were studied. The results indicate that the deformed austenite with a strain of 0.3...

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.

The major challenge in a heat-resistant steel is to generate thermally stable microstructures that allow increasing the operating temperature, which will improve the thermal efficiency of the power plant without diminishing strength or time to rupture. The strengthening mechanism in tempered martensitic 9Cr steels comes mainly from the combination of solid solution effect and of precipitation hardening by fine MX carbo-nitrides, which enhance the sub-boundary hardening. This work is focused on the effect of ausforming processing on MX nanoprecipitation, on both their distribution and number density, during the subsequent tempering heat treatment. The creep strength at 700 oC was evaluated by small punch creep tests. The creep results after ausforming were compared to those obtained after conventional heat treatment concluding, in general, that ausforming boosts the creep strength of the steel at 700 oC. Therefore, conventional ausforming thermomechanical treatment is a promising processing route to raise the operating temperature of 9Cr heat-resistant steels.

In the present study, the effects of ausforming on the bainitic transformation, microstructure and mechanical properties of a low-carbon rich-silicon carbide-free bainitic steel have been investigated. Results show that prior ausforming shortens both the incubation period and finishing time of bainitic transformation during isothermal treatment at a temperature slightly above the Mspoint. The thicknesses of bainitic ferrite laths are reduced appreciably by ausforming; however, ausforming increases the amount of large blocks of retained austenite/martenisite and decreases the volume fraction of retained austenite. And accordingly, ausforming gives rise to significant increases in both yield and tensile strengths, but causes noticeable decreases in ductility and impact toughness.

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).

Three ductile irons with different aluminum-and manganese-content were subjected to two thermo-mechanical schedules. In the first schedule, a total deformation of φt= 0.3 is applied on the ductile irons in the austenitic region before the austempering process. In the second schedule, the materials are subjected to deformation of 0.2 in the austenitic region and deformation of 0.1 during austempering (ausforming). Mechanical deformation of austenite prior to the transformation “stage I” pronouncedly accelerated the transformation due to increasing the nucleation sites of ausferrite. This increase has its impact on enhancing the microstructural uniformity and refining the ausferrite platelets. On the other hand, the retained austenite content was not significantly affected by the applied ausforming. Remarkable increase in hardness, strength and ductility of the ausformed ductile iron due to the latter effects is observed.

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 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.

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 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.

Ductile iron was subjected to a total true strain (φt) of 0.3 either by applying φtin 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 φAand 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 procedure and subsequent annealing treatments on microstructures of cobalt (Co) samples are investigated by electron channeling contrast and electron backscatter diffraction techniques. Results show that the ausformed Co samples consist of coarsen blocky laths (single e phase) with the irregular morphology and the ultra-fine acicular laths (dual phase: γ and e) with the slender and rectangular morphologies. As compared to the slight reduction sample, the much denser acicular laths are observed in the heavily ausformed sample. In addition, recrystallization behavior and annealing-induced γ → e transformation have occurred in ausformed Co samples during the annealing treatment.

The effects of ausforming strain on bainite transformation in high-carbon low-alloy nanobainite steel were investigated using a Gleeble 3500 thermomechanical simulator machine. The bainite transformation speed at 300°C was found to be accelerated by ausforming at 300, 600, and 700°C under applied strains ranging from 10% to 50% followed by isothermal transformation at 300°C. The ausformed bainite volume fraction varied with the ausforming strain because of the mechanical stabilization of the deformed austenite. Ausforming at low temperatures not only enhanced the bainite ferrite volume fraction but also refined the microstructure substantially. Although the amount of bainite ferrite might have been reduced with increasing strain, the microstructures were refined by ausforming.

The varying trends of the amount and rate of bainitic transformation with strains at low temperature were investigated through metallography, X-ray diffraction and dilatometry. The results show that deformation at 573 K promotes bainitic transformation, whereas the promotion degree on bainite transformation by ausforming is nonlinear with strains. The amount of bainite in deformed austenite first increases and then decreases with the increase of strains. There exists a maximum value of the promotion effect corresponding to a critical small strain at a low temperature. Bainitic transformation rate can be increased by ausforming at low temperature, whereas a large strain weakens the acceleration effect. The amount of bainite in deformed materials is synthetically depended on the effect of enhanced nucleation and repressed growth. In addition, the volume fraction of retained austenite is not completely consistent with carbon content, indicating that ausforming plays a important role in determining the amount of austenite.

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.

Applications of neutron diffraction to microstructure evaluation of steel investigated by a project commissioned by the Innovative Structural Materials Association are summarized. The volume fraction of austenite (γ) for a 1.5Mn-1.5Si-0.2C steel was measured by various techniques including backscatter electron diffraction (EBSD) and X-ray diffraction. It is recommended to measure volume fraction and texture simultaneously using neutron diffraction. The γ reverse transformation was in situ monitored using dilatometry, EBSD, X-ray diffraction and neutron diffraction. The γ reversion kinetics showed excellent agreements between dilatometry and neutron diffraction, whereas the γ formation started at higher temperatures in EBSD and X-ray diffraction measurements. Such discrepancy is attributed to the change in chemical compositions at the specimen surface by heating; Mn and C concentrations were decreased with heating. Phase transformations from γ upon cooling were monitored, which enabled us to elucidate the changes in lattice parameters of ferrite (α) and γ affected by not only thermal contraction but also transformation strains, thermal misfit strains and carbon enrichment in γ in the above hypoeutectoid steel. Pearlitic transformation started after the carbon enrichment reached approximately 0.76 mass% and contributed to diffraction line broadening. Martensitic transformation with or without ausforming at 700°C was monitored for a medium carbon low alloyed steel. Dislocation density after ausforming was determined using the convolutional multiple whole profile fitting method for 10 s time-sliced data. The changes in γ and martensite lattice parameters upon quenching were tracked and new insights on internal stresses and the axial ratio of martensite were obtained., GRAPHIAL ABSTRACT

There is a worldwide need to develop materials for advanced power plants with steam temperatures of 700°C and above which have the capacity to achieve high efficiency and low CO2 emissions. This request involves the development of new grades of 9-12Cr heat-resistant steels, with a nanostructured martensite, mainly focusing on the long-term creep rupture strength of base metal and welded joints, creep-fatigue properties, and microstructure evolution during exposure at such elevated temperatures. The main shortcomings of actual 9-12Cr high-chromium steels are that the creep resistance is not enough to fulfill the engineering requirements at temperatures higher than 600°C and the material undergoes a cyclic softening. Creep strength at high temperature could be improved by a microstructural optimization through nano-precipitation, guided by computational thermodynamics, and thermomechanical control process optimization., We acknowledge support for the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

Journal of materials research and technology 9(6), 13365-13374 (2020). doi:10.1016/j.jmrt.2020.09.016, Published by Elsevier, Rio de Janeiro

The project aimed at exploring the industrial feasibility of new super-high strength steels (SHSS) made using innovative processing methods and based on medium carbon nanostructured bainitic microstructures. It was envisaged that the steels could be applied in engineering applications in the transport industry and wear-resistant applications for example in the mining industry. The microstructural concept that was exploited, with the purpose of attaining UTS > 1600MPa, is the ausformed bainite in medium carbon steels (0.4-0.5 wt.%). In this process, austenite is deformed and during subsequent cooling and holding at lower temperatures (than that in regular practice) the unrecrystallized austenite transform to refined bainite with enhanced strength and toughness properties. The industrialization of this new steel design concept, involving innovative process development and novel design of relatively inexpensive compositions, is accompanied by new manufacturing risks and uncertainties in terms of achieving advanced mechanical and in-use properties (tensile, impact and fracture toughness, wear, bendability, etc.). Therefore, this project intended to gather information in terms of chemical composition design, alternative TMCP routes, microstructural evolution, mechanical properties and in-use performance to assess the potential of novel bainitic steel grades in order to develop recommendations for viable industrial processes., European Commission RFCS/RPJ/2015-709607.

The isothermal transformation kinetics, microstructure and mechanical properties of a low-carbon bainitic steel, subjected to below-Ms/above-Ms austempering with or without prior ausforming, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that for all austempered samples, prior ausforming largely refines the bainitic laths and enhances the mechanical stability of supercooled austenite, the latter leaving more austenite untransformed at the finish of isothermal treatment. Nevertheless, divergent consequences on the final microstructure arise: for the above-Ms austempered samples, prior ausforming increases the amount of large, unwanted brittle martensite/austenite blocks and decreases the volume fraction of retained austenite at room temperature; however, for the below-Ms austempered samples, prior ausforming decreases the size and amount of martensite/austenite blocks and increases the volume fraction of retained austenite. Accordingly, the below-Ms austempered samples with prior ausforming exhibit a product of strength and ductility of ∼ 43 GPa% and impact toughness of ∼180 J/cm2, in sharp contrast with those of the above-Ms austempered samples with prior ausforming, ∼33 GPa% and ∼80 J/cm2, respectively. The present results clearly demonstrate a favorable effect of the combined process of ausforming and below-Ms austempering against the adverse combining effect of ausforming and above-Ms austempering. Keywords: Mechanical properties, Carbide-free bainite, Transformation kinetics, Isothermal treatment, Ausforming

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).