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The ability to use common computational thermodynamic and kinetic tools to study the microstructure evolution in Inconel 625 (IN625) manufactured using the additive manufacturing (AM) technique of laser powder-bed fusion is evaluated. Solidification simulations indicate that laser melting and re-melting during printing produce highly segregated interdendritic regions. Precipitation simulations for different degrees of segregation show that the larger the segregation, i.e., the richer the interdendritic regions are in Nb and Mo, the faster the δ-phase (Ni3Nb) precipitation. This is in accordance with the accelerated δ precipitation observed experimentally during post-build heat treatments of AM IN625 compared to wrought IN625. The δ-phase may be undesirable since it can lead to detrimental effects on the mechanical properties. The results are presented in the form of a TTT diagram and agreement between the simulated diagram and the experimental TTT diagram demonstrate how these computational tools can be used to guide and optimize post-build treatments of AM materials.

Quenching and partitioning (Q andP) heat treatments of high- and low-silicon hyper-eutectoid steels, 0.21% and 1.7% silicon grades, have been investigated using dilatometry. In the present work, the amount and stability of retained austenite were quantified by a magnetic measurement technique. Optical microscopy (OM), high-resolution scanning electron microscope techniques and electron backscattered diffraction (EBSD) were used to identify and characterise the constituent phases. The mechanical properties were evaluated by micro-Vickers hardness measurements and nano-indentation measurements and linked to microstructural features. The results illustrate that increasing the silicon content will not prohibit bainite formation. At partitioning temperatures of 300 °C and higher, most retained austenite (RA) transformed to bainite in the low-silicon steel, while carbon partitioning was the main phenomenon in the 1.7 silicon grade steel. However, 28% of the bainite still formed in the presence of 1.7% silicon. In the high-silicon steel, the hardness decreased by 120HV by a mere increase in partitioning temperature from 250 to 300 °C. The wear resistance of bainitic microstructures resulting from isothermal transformation at 200 °C was similar to those of martensite. These outcomes provide an improved understanding of microstructural development with a view to industrial applications. A combination of 20–30% pre-existing martensite with 20% stabilized retained austenite and untempered martensite or/and lower bainite is suggested as a means of achieving the required mechanical properties.

This paper investigates the influence of niobium on the thermal stability of ultra-fine bainite microstructures and their mechanical properties by means of an isothermal transformation at 340 °C, followed by tempering in the range of 200–700 °C for two medium-carbon, carbide-free bainitic steels with and without 0.018 wt. % Nb. The results reveal that ultra-fine bainite microstructures, especially film-like retained austenite, is refined, and the average carbon content of retained austenite is also improved as a result of the addition of Nb element. Both of the bainitic steels in this paper exhibit good thermal stability at low temperatures (≤400 °C) and precipitation hardening arising from carbide precipitation following tempering at 500 °C. The precipitation strengthening compensates for the decrease in strength induced by the decrement in the dislocation density and coarsening of the bainitic ferrite plates during tempering process. M23C6- and MC-type carbides are precipitated in Nb-free and Nb steels, respectively, when the tempering temperature exceeds 400 °C. The further coarsening of the carbides and bainitic ferrite plates, and the decrement in the dislocation density in the latter, relate to the decrease in the mechanical properties for both of the tested bainitic steels following tempering at 500 and 600 °C. Compared to those in the bainitic steel without Nb, the larger size and superior thermal stability of MC-type carbides results in greater strength and elongation of the bainitic steel containing 0.018 wt. % of the Nb element, but does harm to its impact toughness after tempering at 700 °C.

The effects of isothermal and pre-transformation temperatures on the microstructure and mechanical properties of ultrafine bainitic steels are investigated and theoretically analyzed by using optical microscopy, scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analyses in this study. The results show that the QBT (quenching followed by isothermal transformation of bainite) samples exhibit a multiphase microstructure containing lamellar ferrite, lenticular martensite, retained austenite. The isothermal and pre-transformation temperatures have an effect on the morphology, quantity of bainite and the stability of retained austenite. As the pre-transformation temperature is decreased from 245 °C to 210 °C, the content of premartensite increases significantly from 21% to 56%. The premartensite not only refines the bainite structure, but also provides nucleation points for the bainite transformation and accelerates the bainite transformation. Correspondingly, the pre-martensite has a good contribution to the strength of ultrafine bainitic steels, when premartensite transformation temperature is 210 °C, the martensite content is 56%, ultrafine bainitic steels reach the highest tensile strength of 1857 MPa. With the increase of isothermal temperature of bainite, the width of bainite lath increases accordingly, and the retained austenite changes from thin film to blocky, which results in the decrease of strength and improvement of plasticity of ultrafine bainitic steels. However, when the bainitic isothermal temperature is too high, the transformation of bainite gradually stops because of the enrichment of carbon in austenite.

The microstructure and mechanical properties were investigated as a function of the austempering temperature in an Si-rich advanced high-strength steel that is expected to serve for automotive appl...

Kinetics of face-centered cubic (FCC) phase evolution in equiatomic AlCoCrFeNi alloy has been studied in this work, measuring the phase fraction from X-ray diffraction (XRD). As-cast alloy showed a body-centered cubic (BCC)+B2 structure. Heat treatments performed at different temperatures showed the formation up-to 30 to 35 pct FCC phase between 1073 K and 1373 K. A systematic decrease in hardness from 540 to 390 HV10 with increase in temperature suggested an increase in the FCC volume fraction. Kinetics of FCC evolution were analyzed using the Johnson–Mehl–Avrami–Kolmogorov equation and Arrhenius law to calculate the activation energy for the phase transformation. Furthermore, a time-temperature-transformation diagram was constructed from the isothermal transformation studies. Detailed microstructural investigation suggests faster kinetics of FCC phase formation near dendritic boundaries compared to interdendritic regions. The Kurdjumov–Sachs orientation relationship between FCC and BCC phases suggested a coherent interface between these phases. Results of the present study pave the way to decide on heat treatment practices in AlCoCrFeNi alloy.

Ametallographic study of advanced low-carbon martensitic steel with high resistance of the supercooled austenite to the transformations by pearlitic and bainitic mechanisms under continuous cooling is performed. The critical temperatures of the transformations occurring under heating and cooling are determined. The kinetics of the isothermal transformation of the austenite in the temperature range of 300 – 400°C is analyzed. The possibility of successful implementation of isothermal quenching of the steel after carburizing is demonstrated.

The authors have studied grade R260 rail steel after austenization in the temperature range from 800 to 930°C and isothermal soaking at 500, 550, and 600°C followed by quenching in water. The steel structure and abrasive wear resistance were characterized. Recommendations concerning an increase in steel wear resistance were provided.

The microstructure transformation and precipitation behavior of nano-carbides in Ti microalloyed steel during isothermal process were studied by a compression test on a Gleeble 3800 thermomechanical simulator and analyzed by optical microscopy, transmission electron microscopy and other methods. The results show that γ → α phase transformation and TiC precipitation take place in Ti microalloyed steel during the isothermal process, and time–temperature–transformation curve and precipitation–time–temperature (PTT) curve are all of “C”-type. During the isothermal process, the interphase precipitation of TiC mostly occurs at the period of the phase transformation, and the random precipitation of TiC mostly occurs on the ferrite after the phase transformation. The increment in yield strength at the initial stage of isothermal transformation mainly comes from phase transformation strengthening. With the increase in isothermal time, the precipitation hardening effect becomes more important for nucleation and growth of titanium carbides and eventually reaches the maximum value at the precipitation finished point of the PTT curve.

A simple mathematical model for establishing isothermal transformation kinetics from continuous cooling transformation data is presented. A new regression function of k, which is a reaction parameter of the Johnson–Mehl–Avrami equation, is proposed. The isothermal transformation kinetics of austenite-pearlite transformation in a eutectoid steel is converted from continuous cooling transformation data using the regression function in combination with the inverse additivity rule. The converted isothermal transformation kinetics excellently agrees with experimental results irrespective of temperature.

A transformation-induced plasticity steel was thermomechanically processed and then transformed to bainite at an isothermal transformation temperature of 723 K for 1800 seconds, which exceeds the time required for completion of the bainite transformation. The formation of lenticular-shaped carbides with a triclinic lattice and internal substructure was found after thermomechanical processing. After 16 years of storage at room temperature, the decomposition of retained austenite into pearlite was observed for the first time at this temperature.

The mechanical properties of low carbon bainite steel are closely related to the microstructure and proportion after phase transformation. The microstructure of the deformed austenite of low carbon bainite steel after isothermal transformation and continuous cooling transformation was studied by thermal simulation test. The metallographic structure was observed by optical microscopy (OM) and scanning electron microscopy (SEM). The metallographic and microhardness were used to judge the microstructure type, and the CCT (continuous cooling transformation) curve and TTT (time-temperature-transformation) curve of the test steel were drawn. It was found that at 700-430 °C isothermal, undergo a variety of medium-temperature microstructure transformations appeared for the test steels, such as ferrite, pearlite, granular bainite and lath bainite. The cooling rate and final cooling temperature have great influence on the type and performance of the final microstructure. The final cooling temperature was controlled at about 515°C. The mixed microstructures of granular bainite (GB) and fine martensite-austenite (M-A) island, a small amount of acicular ferrite and lath bainite were obtained. The yield and tensile strengths of this type of microstructure reached 639 MPa and 750 MPa respectively, the shrinkage rate reached 17%, and the better low-temperature impact performance was realized.

The element chromium and molybdenum delays the γ→α phase transition, while the deformation energy storage and high temperature diffusion can promote the γ→α phase transition process. The phase change of alloy structural steel SCM435 under different deformation temperature and cooling path after heating and holding at 1050°C was measured by a thermal simulator. The microstructure and hardness of the steel under various rolling processes were studied by using SEM and micro-hardness tests. The low deformation temperature (800~720°C) and the slow post-rolling cooling speed (1~0.3°C/s) were favorable for the γ→α phase change, where the phase microstructure showed the low hardness. As the isothermal temperature decreases, the microstructure changes from Pearlite (P)+ Ferrite (F) to P+ F + Martensite(M). The lower the isothermal temperature, the higher the martensite content in the structure, and the higher the hardness was. Both the low temperature deformation and 660°C isothermal treatment could quickly accelerate the γ→α transformation of SCM435. The fastest transformation temperature in this case for pearlite of SCM435 was 660°C, and the phase transition ended within 300s, which significantly shortened the time compared with the isothermal time in the low temperature zone.

The aging of an Al-Mg-Si alloy with a high silicon excess and reinforced with ceramic particles was characterized between 373 K and 623 K to understand how the particles affect: (i) the precipitation kinetics of coherent and semi-coherent phases, (ii) the precipitation sequence of the alloy, and (iii) the loss of mechanical strength from a peak-aged microstructure obtained by a T6 treatment. Compared with the unreinforced alloy, heterogeneous precipitation of disordered semi-coherent phases occurs on dislocations and the precipitation kinetics were found to be accelerated in the composite leading to an acceleration of the loss of strength from the T6 condition, due to the precipitation of the Type-C phase. An experimental isothermal transformation curve was proposed for the composite and compared with that of the unreinforced alloy. Then, a study was performed on a deformed alloy to demonstrate that most of the differences observed between the unreinforced alloy and the composite can be explained by the high dislocation density generated in the matrix of the composite due to the presence of ceramic particles. Lastly, the decrease in mechanical strength occurring during isothermal treatments from the T6 state could be successfully modeled using the JMAK approach.

In this work, dedicated isothermal transformation studies were performed in a Fe–C–Mn–Nb alloy to obtain two types of precipitate morphology embedded in a similar ferritic matrix. Nano-hardness of ...

Cold stretch-formability and stretch-flangeability of 0.2%C-1.5%Si-5.0%Mn (in mass%) martensite-type medium Mn steel were investigated for automotive applications. High stretch-formability and stretch-flangeability were obtained in the steel subjected to an isothermal transformation process at temperatures between Ms and Mf − 100 °C. Both formabilities of the steel decreased compared with those of 0.2%C-1.5%Si-1.5Mn and -3Mn steels (equivalent to TRIP-aided martensitic steels), despite a larger or the same uniform and total elongations, especially in the stretch-flangeability. The decreases were mainly caused by the presence of a large amount of martensite/austenite phase, although a large amount of metastable retained austenite made a positive contribution to the formabilities. High Mn content contributed to increasing the stretch-formability.

The wear resistance of carbon-free bainitic steel obtained by isothermal transformation is studied and compared with untempered martensite in this paper. The experimental steel obtained carbide-fre...

The effects of aluminum addition on bainite transformation and properties of carbide-free bainitic steels containing 0.22 wt.% carbon were investigated by two different types of heat treatment processes: continuous cooling process (CCP) and isothermal transformation process (ITP). The results indicate that for the CCP treatment, Al addition significantly promoted the ferrite and bainite transformation; however, it did not significantly increase the product of tensile strength and total elongation (PSE). For the ITP treatment, Al addition significantly promoted the kinetics of bainite transformation, and thus, more bainite was formed with Al addition; however, it was found that Al addition resulted in a decrease in tensile strength and an increase in elongation of the tested bainitic steels. Moreover, the effects of Al addition on comprehensive property were profoundly dependent on austempering temperatures. When the austempering temperature was higher (430 °C), PSE significantly increased with Al addition, whereas it decreased at the lower austempering temperature (400 °C). Therefore, it can be concluded that the effects of Al on properties of bainitic steels were more significant at higher austempering temperatures.

This work was aimed at investigating the formation of nitrided layers during the isothermal transformation (austempering) and at describing the formed nitrided layer properties. The tested steels were characterized by a different content of carbon and alloying elements. In the case of the isothermal transformation, 4 variants of heat treatment parameters of nitrided layers were applied. The heat treatment differed in the austenitizing temperature (750°C–860°C) and the isothermal transformation temperature (390°C- 420°C). The microstructure and the mechanical properties (hardness) of the nitrided layers formed after the heat treatment processes were determined. After the nitriding process, during 30 hours in the nitriding atmosphere consisting only of ammonia, the high nitrogen saturation in the surface zone of the layers was obtained. The nitrided layers, after the heat treatment processes, were char-acterized by the diversified thickness, as evidenced by the hardness distributions at their cross-sections.

The present work discusses the comparative corrosion behavior of five microstructures of steels, namely, pearlite, bainite, spheroidized, martensite, and tempered martensite, which have been processed, respectively, by air cooling, isothermal transformation, spheroidizing, quenching, and tempering of a steel with composition 0.70C, 0.24Si, 1.12Mn, 0.026P, 0.021S, 0.013Nb, 0.0725Ta, and 97.7Fe (all are in wt pct). Dynamic polarization and alternating current (AC) impedance spectroscopic tests in freely aerated 3.5 pct NaCl solution show that the corrosion resistance of the steel specimens consisting of the preceding five microstructures decreases in the following sequence: pearlitic – bainitic – spheroidized – martensitic – tempered martensitic steels. The variation in the corrosion rate has been attributed to the shape, size, and distribution of the ferrite and cementite.

Stellite 6 alloy coatings were produced by the plasma arc cladding process, and the as-cladded samples were treated with isothermal aging at 700 °C for up to 1000 hours to evaluate the effect of long-term isothermal aging on microstructural evolution. The results show that the microstructure of the cladding consists of γ-Co solid solution and carbide-based eutectics. During aging, γ-Co gradually transformed to e-Co, and the volume fraction of e-Co reached to about 75 pct as the aging time was 1000 hours. On other hand, the M23C6 fine particles were found to gradually precipitate in the solid solution during aging. Approximately pairwise proportional relationships were found among the fraction of precipitated M23C6 carbides, the volume fraction of martensite transformation, and the stacking fault energy, and they were verified by the statistical results of phase fraction and change of SF possibility calculated through X-ray diffraction patterns. A kinetic model for martensite transformation was also established to explain the change of martensite volume fraction during the aging period and to predict the transformation degree during high-temperature aging.

Ultra-fine carbide-free bainitic (UCFB) steel, also known as nano-bainite (NB) steel, is composed of bainitic ferrite laths with nanoscale thickness and carbon-rich film-like retained austenite located between laths. The bainite transformation kinetic model can accurately describe the bainite transformation kinetics in conventional austempering (CA) processes based on the shear mechanism combined with the dilatometer test. UCFB steels with medium and high carbon composition are designed in this work to systematically study the transformation kinetics of bainite, and the evolution of its microstructure and properties, and reveal the influence of heat treatment processes on the microstructure and properties the UCFB steels. The results show that the activation energy for BF nucleation decreases during the CA process and isothermal transformation temperature decreases. The bainite transformation is first nucleated at the grain boundaries, and then nucleated at the newly formed bainitic ferrite/austenite interface.

Further development of high chromium cast irons (HCCI) is based on tailoring the microstructure, necessitating an accurate control over the phase transformation and carbide precipitation temperatures and can be achieved by thermal treatments (TT). To understand the underlying mechanisms controlling the transformation kinetics during the different stages of the TT, it is imperative to adjust the TT parameters to have information of the transformations occurring during non-thermal and isothermal heating cycles, since proper selection of the TT parameters ensures the optimum use of the alloying elements. In this work, the boundaries of the phase transformations for a HCCI containing 26 wt pct Cr for different cooling rates (continuous cooling transformation, CCT, diagram) were established by applying dilatometric measurements. Based on the CCT diagram, a temperature-time-transformation (TTT) diagram was constructed by isothermally holding the samples until complete phase transformation. For determining the initiation and finishing of the transformation, the lever rule assisted by derivatives was applied. The phases present after transformation were determined by combining X-ray diffraction (XRD) and metallographic characterization using optical microscopy (OM) and scanning electron microscopy (SEM). Finally, the data obtained from the dilatometer was experimentally verified by isothermally heat treating some samples using laboratory furnaces. The transformed phase fraction from OM and SEM images was then correlated to the fraction obtained from the TTT diagram.

This research work aims to study the effect of 2wt%Mo on bainitic transformation kinetics and bainite morphology in 0.6wt%C-1.2wt%Si-1.0wt%Mn-0.2wt%Cr steel. Specimens are austenitized at 950 degrees C and rapidly cooled in salt bath and isothermally treated between 200 degrees C-300 degrees C for different time intervals. Another set of specimens are rapidly cooled in oil after an austenitization treatment and then tempered in the temperature range of 200 degrees C-550 degrees C. In C-Mn-Si specimens, the amount of retained austenite increases with increasing the amount of bainite but no retained austenite is observed in bainitic C-Mn-Si-Mo specimens. The tempering behavior of C-Mn-Si-Mo alloy is considerably different than that of the Mo free alloy. Mo in a martensitic microstructure of C-Mn-Si-Mo alloy shows the secondary hardening effect peak upon tempering at 500 degrees C. The examination of the secondary hardening would also contribute to the behavior of Mo in tempered bainitic steels. The bainitic specimens do not show a peak hardness but softening is retarded upon tempering at the same temperature range.

Quenched and tempered Fe-9Ni (wt. %) steels have been widely used in cryogenic applications. For the first time, the formation of nodular bainite in an Fe-9.10Ni-0.06C (wt. %) steel during isothermal transformation at 550 and 500°C for 24 h is reported. Electron microscopy revealed that proeutectoid ferrite forms predominantly by a massive transformation at prior austenite grain boundaries or at intragranular inclusions as the first stage of austenite decomposition. Then, nodular bainite composed of a ferritic matrix and rows of fine cementite precipitates, forms by interphase boundary precipitation during decomposition of adjacent C-enriched austenite. Both the Bagarayatskii and Isaichev orientation relationships were observed between ferrite and cementite in the nodular bainite. Microhardness of nodular bainite in the sample transformed at 500°C (214 ± 4 VHN) was higher than that in the sample transformed at 550°C (195 ± 5 VHN), due to smaller sizes of cementite precipitates in the former.

Carbo-austempered steels with a nanobainitic microstructure in the case are established as an exciting alternative in applications demanding high stability and reliability against wear and fatigue. In this work, the effect of heat treatment conditions, i.e. austenitizing and isothermal transformation temperatures, on the wear behavior under rolling/sliding conditions of a carburized cast steel with a nanobainitic structure in the surface was examined. The results show that wear damage is mainly due to contact fatigue, with a small extent of oxidative wear, abrasion and adhesion. The results were compared with the conventional tempered martensite plus retained austenite microstructure obtained by means of carburizing followed by quenching and tempering. The nanobainitic microstructures show a better wear resistance than the quenched and tempered microstructures. Also, the specific wear rate in carbo-austempered steels was lower as the isothermal heat treatment temperature decreased. Results are explained in terms of the microstructural differences, hardening behavior and plasticity of the materials under study., The authors want to thank the CODI-Universidad de Antioquia for the financial support for the development of this research through the project PRG2017-15869, the National Center for Metallurgical Research - CENIM - and the Tribology and Surfaces Laboratory at the Universidad Nacional de Colombia, Medellin, for the support provided for the experimental development of this work.

AISI 1018 mild steels are widely used for engineering applications in machine components and for structural purposes. These materials suffer mechanical damages especially when used under critical conditions of extreme load. In this study, the effect of austempering heat-treatment on the hardness, tensile strength, impact energy and the microstructure of AISI 1018 steels were evaluated. The steel specimens were subjected to austempering heat-treatment by austenitizing at a temperature of 830°C, maintained at this temperature for a period of 1 hour 30 minutes, before rapidly cooled down in a NaNO3 salt bath maintained at 300°C for isothermal transformation for a further 50 minutes before finally cooled down to room temperature. Microstructural analysis using Scanning Electron Microscope (SEM) shows transformation from ferrite/pearlite to bainite microstructure. The tensile strengths of the specimen increased from 400 MPa to 500 Mpa; hardness increased from an average value of 140Rc to 162Rc; while impact energy increased from 15.6 Joule to 30.6 Joule by the austempering heat-treatment. Keywords—Austempering, hardness, tensile strength, impact energy, microstructure

Precipitation strengthened, fully ferritic microstructures in low-carbon, microalloyed steels are used in applications requiring enhanced stretch-flange formability. This work assesses the influence of thermomechanical processing on the evolution of austenite and the associated final ferritic microstructures. Hot strip mill processing simulations were performed on a low-carbon, titanium-molybdenum microalloyed steel using hot torsion testing to investigate the effects of extensive differences in austenite strain accumulation on austenite morphology and microstructural development after isothermal transformation. The gradient of imposed shear strain with respect to radial position inherent to torsion testing was utilized to explore the influence of strain on microstructural development for a given simulation, and a tangential cross-section technique was employed to quantify the amount of shear strain that accumulated within the austenite during testing. Greater austenite shear strain accumulation resulted in greater refinement of both the prior austenite and polygonal ferrite grain sizes. Further, polygonal ferrite grain diameter distributions were narrowed, and the presence of hard, secondary phase constituents was minimized, with greater amounts of austenite strain accumulation. The results indicate that extensive austenite strain accumulation before decomposition is required to achieve desirable, ferritic microstructures.

Heat treatment modifies the structure of steel to obtain desired properties. The material is heated to some temperature at or above the critical range in order to form austenite. Different heat treatments are based on the subsequent cooling and reheating of the austenitized material. Phase diagrams are important tools for understanding the heat treatment process. Isothermal Transformation (ITh) or Time temperature transformation (TTT) diagrams have been developed to predict non-equilibrium structures.

Carburising followed by martensitic hardening is state of the art in the gear industry today. Carburisation followed by isothermal transformation into a partial or fully bainitic microstructure has not been systematically examined and evaluated yet. Especially isothermal transformation of carburised parts into lower bainite is promising in terms of the characteristic mechanical values of treated parts, due to the increased toughness of the bainite compared to a carbon rich martensite and finely distributed retained austenite. Using this procedure for case hardening requires a quenching technology for isothermal bainitic transformation and additional changes to the process. Examinations of the transformation characteristics of carburised samples in the dilatometer were discussed in the first part of the publication already. This second part examines the microstructures and residual stresses caused by “bainitic case hardening”. The 3rd part will follow in HTM 1/2019.

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.

In this study, isothermal bainite transformation kinetics in hypo-eutectoid steel 60Si2CrA was investigated. Isothermal transformation at temperature ranging from [528 K (255 °C)] to [653 K (380 °C)] after austenitization was performed on a quenching dilatometer. Based on the dilation data, a shear bainite kinetics model was established using the internal state variable (ISV) approach. The model is coupling with bainite phase nucleation and growth, and the incubation time is characterized by the proposed normalized bainite nucleus radius and normalized radius growth rate. Material constants within the unified kinetics model were calibrated with genetic algorithm (GA)-based optimization methodology using GA toolbox in MATLAB. Furthermore, fairly close agreement between model predicted and tested data was achieved, and predicted evolution of ISVs in the model was illustrated. The proposed shear-type bainite kinetics model may be applied to other hypo-eutectoid steels.

Graphite formation in the solid state is both in ductile cast irons and in steels strongly promoted by high silicon contents above 3 wt.% Si. The matrix microstructure in austempered ductile iron can be further refined by secondary graphite if the austenitization, quench, and isothermal transformation into ausferrite are preceded by an austenitization at a slightly higher temperature followed by quench to martensite, resulting in higher carbon content than being soluble at the second austenitization temperature. Hypoeutectoid steels with high silicon contents can be rapidly graphitized, causing recrystallization of surrounding ferrite due to plastic deformation making room for less dense graphite. In rolled steels, the interface between manganese sulfide and steel matrix is the most common nucleation site. Voids are formed when graphite is partly or completely dissolved during austenitization in succeeding hardening heat treatments, but the mechanical properties can still be good if the graphite particles dissolved into voids are below 20 µm. Graphitized Si-solution strengthened ferritic steels may perform similar to free-cutting steels but with improved mechanical properties.

The paper discusses the influence of retained austenite formed in a structure after the phase transition in the bainitic temperature range on the properties of structural low-alloy steels. It is shown that the bainite transformation proceeds through two stages. The first stage is characterized by the formation of completely carbide-free bainite containing up to 45% retained austenite, which is stable under deep cooling. The second stage develops with increasing holding time and is accompanied by the release of carbides and a reduction in the amount of retained austenite. The mechanical properties of steels are given in terms of the morphology and chemical composition of the phases forming in the course of isothermal transformation in the bainitic temperature range.

The kinetic behavior of highly undercooled austenite and its effects on the microstructural evolution and mechanical properties of high-carbon steel are studied. The undercooling degree is increased through a temporary undercooling treatment at the bainite transformation temperature of 380 °C to 450 °C before the pearlite isothermal transformation at 500 °C to 620 °C. The transformation kinetics reveals an increased nose temperature and a rightward shift of the transformation curve under high undercooling conditions. In addition, the undercooling treatment leads to an increased driving force during pearlite transformation, which is responsible for the refined hierarchical structures of pearlite. As a result, a 20 to 40 pct size reduction is achieved for pearlite colonies and lamellae. Such refinement is in turn attributed to an ~ 26 pct increase in reduction in area. This work provides both a new understanding of high-performance fully pearlitic steels for practical applications and new perspectives for potential technological applications in drawing processes for hypoeutectoid steels.

The work presents the results on the structure of CuNi2Si1 copper alloy. The alloy was treated in two variants: supersaturation - aging (variant I) and supersaturation - cold rolling - aging (Variant II).The structure of the CuNi2Si1 alloyed copper were analyzed by high resolution transmission electron microscopy (HRTEM). The TEM investigation showed in the Cu matrix after applying cold rolling after solution heat treatment, during aging at 600°C, causes the Ni2Si phase occurrence immediately after the begin of aging. Cold rolling (50% reduction) of the CuNi2Si1 alloy after supersaturation changes the mechanism and kinetics of precipitation and provides possibilities for production of broader sets of functional properties.

The effects of Nickle (Ni) addition on bainitic transformation and property of ultrahigh strength bainitic steels are investigated by three austempering processes. The results indicate that Ni addition hinders the isothermal bainite transformation kinetics, and decreases the volume fraction of bainite due to the decrease of chemical driving force for nucleation and growth of bainite transformation. Moreover, the product of tensile strength and total elongation (PSE) of high carbon bainitic steels decreases with Ni addition at higher austempering temperatures (220 and 250 °C), while it shows no significant difference at lower austempering temperature (200 °C). For the same steel (Ni-free or Ni-added steel), the amounts of bainite and RA firstly increase and then decrease with the increase of the austempering temperature, resulting in the highest PSE in the sample austempered at temperature of 220 °C. In addition, the effects of austempering time on bainite amount and property of high carbon bainitic steels are also analyzed. It indicates that in a given transformation time range of 30 h, more volume of bainite and better mechanical property in high carbon bainitic steels can be obtained by increasing the isothermal transformation time.

International audience; The industrial importance of carbonitriding is owed to the exceptional wear and fatigue resistance it imparts to treated steel parts. This resistance is related to microstructural changes occurring during the enrichment treatment and upon cooling. Here, the effects of interstitial contents, in particular nitrogen, and transformation temperature were investigated in 23MnCrMo5 steel. Samples were homogeneously enriched in the austenitic phase and the isothermal transformation of the enriched austenite between 750 °C and 600 °C was studied. CrN was found to precipitate during the enrichment treatment. During subsequent isothermal holding, CrN precipitate as fine platelets in nitrogen containing samples. At equal carbon content, ferrite formed faster and in finer grains in presence of nitrogen. Preexisting CrN facilitate ferrite nucleation resulting in more numerous ferrite grains. The intense nitride precipitation is the main origin for enhanced hardness in nitrogen-enriched alloys. The exact mechanism leading to the observed microstructures could not be determined and remains under investigation. In particular, the high nitrogen supersaturation of ferrite required to produce the observed fraction of CrN has to be explained.

Thermomechanical controlled processing of 560-MPa (X90) linepipe steel was simulated in the laboratory using a thermomechanical simulator to study the microstructural evolution and precipitation behavior during isothermal holding. The results indicated that martensite was obtained when the steels were isothermally held for 5 s at 700 °C. Subsequently, granular bainite and acicular ferrite transformation occurred with increased holding time. Different amount of polygonal ferrite formed after isothermally holding for 600-3600 s. Pearlite nucleated after isothermally holding for 3600 s. Precipitation occurred after isothermal holding for 5 s and continuous precipitation occurred at grain boundaries after isothermally holding for 600 s. After isothermally holding for 3600 s, large Nb/Ti carbide precipitated. The presence of MX-type precipitates was confirmed by diffraction pattern. The interphase precipitation (IP) occurred between 5 and 30 s. Maximum hardness was obtained after isothermally holding for 600 s when IP occurred and rapidly decreased to a low value, mainly because polygonal ferrite dominated the microstructure after isothermally holding for 3600 s.

With austenite under 50% compression using a dilatometer, ferrite transformation in a low-carbon Ti-V-bearing steel was sequenced through the isothermal transformation at 650 °C for various holding times (10 s, 20 s, 30 s). Bimodal grain size distribution was observed in the prolonged holding conditions. Corresponding measurements of Vickers and nano-indentation hardness were carried out; the tiny ferrite grains possessed lower hardness (303 HV and 4.8 GPa) as compared to larger ferrite grains (336 HV and 5.6 GPa). Transmission electron microscopy provided strong evidence attributing this discrepancy to interphase precipitation taking place in the larger-grained ferrite, instead of in the tiny-grained ferrite.

The kinetics of sigma phase precipitation at the temperature range of 750–950 °C in a rapidly solidified 15Cr-2Ni-2Al-11Mn resource-saving duplex stainless steel was investigated. After fitting the experimental results with the Avrami equation, the TTT diagram of sigma phase was established. It is found that the precipitation rate of sigma phase maximizes at about 850 °C and that the precipitation hardening effect sharply peaks at about 1.5 vol% sigma phase content, which is obtained by aging at 850 °C for 180 min. With the further increase of sigma phase content from 1.5 vol%, the strength reduces and the ductility increases again.

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.

Analytical models of the diffusion controlled pearlite-to-austenite transformation during isothermal and uniform heating processes are presented. The solute elements diffusing through the volume of austenite and the interfaces between austenite and pearlite were both considered synchronously. The austenite formation from pearlite of a Fe-1C-1.44Cr steel was calculated with the present model and the results were compared to the experimental results. Calculation results show that the transformation is Cr diffusion-controlled and C diffusion-controlled below and above the transition temperature of austenite growth (so called PNTT), respectively. The theory about PNTT is quantitatively proved to be right.

A mathematical model of austenite - bainite transformation in austempered ductile cast iron has been presented. The model is based on a model developed by Bhadeshia [1, 2] for modelling the bainitic transformation in high-silicon steels with inhibited carbide precipitation. A computer program has been developed that calculates the incubation time, the transformation time at a preset temperature, the TTT diagram and carbon content in unreacted austenite as a function of temperature. Additionally, the program has been provided with a module calculating the free energy of austenite and ferrite as well as the maximum driving force of transformation. Model validation was based on the experimental research and literature data. Experimental studies included the determination of austenite grain size, plotting the TTT diagram and analysis of the effect of heat treatment parameters on the microstructure of ductile iron. The obtained results show a relatively good compatibility between the theoretical calculations and experimental studies. Using the developed program it was possible to examine the effect of austenite grain size on the rate of transformation.

The interlamellar spacings of pearlitic steels gradually increase as the isothermal transformation temperatures, correspondingly, the tensile strengths decrease, satisfying the Hall–Petch relationship. Si virtually completes its partitioning process during the eutectoid transformation. A depletion of Mn and an accumulation of Si are found in the concentration profiles of 0.95%-Si steel transformed at 793 K in the ferrite, but not in 0.22%-Si steel. It may attribute them to the Si–Mn diffusional flux couplings, which due to the strong interaction between Si and Mn atoms in the α-Fe. Transformation temperature increases to 873 K, a large amount of Mn atoms coming from ferrite cannot timely diffusion to cementite lamella core, forming a localised retention of Mn in the cementite phase side nearby the interface.This is part of a thematic issue on Pearlitic Steel Wires.

Medium-carbon, silicon-rich steels are commonly suggested to obtain a very fine bainitic microstructure at a low temperature slightly above Ms. Thereby, the resulted microstructure consists of slender bainitic-ferritic plates interwoven with retained austenite. The advanced strength and ductility package of this steel is much dependent on the fineness of bainitic ferrite, as well as the retained austenite phase. In this article, the aluminum to silicon ratio, and the isothermal transformation temperature have been adopted to obtain ultra-high strength high carbon steel. Optical and SEM investigation of the produced steels have been performed. XRD has been used to track the retained austenite development as a result of the change in the chemical composition of developed steels and heat treatment process. Mechanical properties in terms of hardness and microhardness of obtained phases and structure were investigated. Results show that the increment of aluminum to silicon ratio has a great effect in promoting the bainitic transformation, in tandem with improving the stability and the fineness of retained austenite. Such an advanced structure leads to enhancement in the whole mechanical properties of the high carbon steel.