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The mechanism of inclusions transformation during the isothermal heating at 1498 K was elucidated in the current study, that was, the CaO in inclusions diffused to the inclusions surface, reacting with the sulfur to form CaS, and the Al2O3 and MgO receded to the inclusions core, resulting in inclusions of MgO–Al2O3 core with a CaS shell. A comprehensive kinetic model coupling thermodynamics, mass transfer, and changes in inclusions diameters was established. Two diffusion layers for steel elements, named steel layer and CaS layer, and one diffusion region for inclusion components were presumed in the kinetic model. Four model parameters which determine the diffusion process were optimized using the sensitive analysis by comparing simulated results and experimental data. Employing the kinetic model, Time-Diameter-Transformation (TDT) contours were proposed, which revealed the effect of diameter and heating time on the transformation. Calculated results also show a little increase in inclusions diameter during the isothermal heating. Inclusions with an initial diameter less than 5 μm increased by 1.5 μm after the heating. An extreme thickness of the CaS layer in inclusions existed during the heating, which was 0.35 μm for inclusions with a diameter of 10 μm.

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.

A new Q&P processing route, i.e., applying austempering below Ms in normal two-step Q&P route (termed AQ&P), was proposed and applied to a medium-carbon steel. The effects of isothermal transformation at the quenching temperature on the microstructure evolution and mechanical properties of AQ&P steel were investigated. It was found that both the strength and the elongation of the steel were improved using AQ&P processing route. The yield strength and tensile strength were improved by ~ 100 and ~ 160 MPa, respectively, with similar elongation through austempering for 600 s compared to that of specimens treated by normal two-step Q&P route. Therefore, the product of strength and elongation was increased by 18.8% after AQ&P treatment. In addition, the volume fraction of retained austenite was first decreased and then increased with the extension of the quenching time. Compared to a normal two-step Q&P route, the retained austenite decreased about 6% after austempering for 600 s in AQ&P route. Moreover, the medium-carbon steel experienced a continuous martensitic transformation during isothermal holding at the quenching temperature. The phase transformation during isothermal holding includes two stages: quick martensitic transformation and slow bainitic transformation.

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.

The aim of the study was to develop a technology for welding non-weldable 42CrMo4 and NANOS-BA® steel grades in the process of hot rolling and two-stage heat treatment. As a result of physical experiments carried out in a line for semi-industrial simulation of the production of metals and their alloys (LPS) and additional heat treatment, a durable combination of 42CrMo4 and NANOS-BA® steels with high mechanical properties was obtained, including: Rp0.2 = 1036 MPa, Rm = 1504 MPa and A = 10.9%, without microscopically visible cracks and other discontinuities in the joined surface. The quality of the 42CrMo4/NANOS-BA® clad plates produced in this way was assessed on the basis of microstructure examination as well as bending, shear and tensile strength tests.

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...

In this research, hypereutectoid Si–Mn TRIP steel was prepared via casting method followed by thermomechanical processing and different heat treatments. Subsequently, fracture behavior and mechanical properties of the prepared steel were investigated. The time–temperature–transformation curves of the specimens were predicted by a simulation software. Their mechanical properties were studied through tensile, hardness, and impact tests. The microstructure and fracture behavior of the specimens were evaluated using an optical microscope, scanning electron microscope and fractographic analysis. Results indicated that the heat-treated sample containing a multi-phase microstructure of bainite, martensite, pearlite and retained austenite, had the highest ultimate tensile strength around 1,300 MPa. The software predictions of the heat treatments were in good accordance with the microstructural results. The samples have undergone thermomechanical process and heat treatment including isothermal transformation showed brittle behavior with intergranular and cleavage fracture surfaces. In addition, the two-stage quenching cycles resulted in dual-phase microstructure composed of martensite embedded in retained austenite with a combination of brittle and ductile fracture behavior.

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.

Thermal annealing of organic semiconductors is critical for optimization of their electronic properties. The selection of the optimal annealing temperature –often done on a trial-and-error basis– is essential for achieving the most desired micro/nanostructure. While classical materials science relies on time-temperature-transformation (TTT) diagrams to predict such processing-structure relationships, this type of approach is yet to find widespread application in the field of organic electronics. In this work, we constructed a TTT diagram for crystallization of the widely studied organic semiconductor 5,11-bis(triethylsilylethynyl)anthradithiophene (TES-ADT) from its melt. Thermal analysis in the form of isothermal crystallization experiments showed distinctly different types of behaviour depending on the annealing temperature, in agreement with classical crystal nucleation and growth theory. Hence, the TTT diagram correlates with the observed variation in the number of crystal domains, the crystal coverage and film texture as well as the obtained polymorph. As a result, we are able to rationalize the influence of the annealing temperature on the charge-carrier mobility extracted from field-effect transistor (FET) measurements. Evidently, the use of TTT diagrams is a powerful tool to describe structure formation of organic semiconductors and can be used to predict processing protocols that lead to optimal device performance.

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.

Phase transformations in high-carbon steel has been investigated by dilatometry during continuous cooling and isothermal transformations, following austenizing at temperatures of 900 °C–1200 °C (1173–1473 K). Optical microscopy and high-resolution scanning electron microscopy techniques revealed the presence of martensite, complex bainite, pearlite and metastable retained austenite under a variety of heat treatments. Magnetic measurements proved to be a useful technique to determine the fraction of retained austenite. Quantitative image processing with ImageJ software and magnetic techniques were used to calculate phase fractions. The rate of phase transformations increased by an increase in austenizing temperature for transformations occurring during continuous cooling and isothermal transformations. A mere increase in cooling rate from 3 °C (276 K) per second to 5 °C (278 K) per second changed the relative phase fractions and hardness significantly. The hardness of isothermally cooling specimens was a strong function of temperature and microstructure. Although isothermal holding at 500–525 °C (773–798 K) resulted in a mixture of bainite and pearlite, the hardness decreased significantly by decreasing the isothermal transformation temperature from 525 °C (798 K) to 500 °C (773 K). These outcomes provide valuable guidelines for the development of novel microstructure in high carbon steel in industrial practice.

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.

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.

We elucidate here the effects of annealing twins on phase transformation products based on electron back-scatter diffraction analysis and corresponding microstructure visualization and quantification methods. Martensite and bainite were obtained by rapid continuous cooling and isothermal processing at different temperatures, respectively, which were designed to study variants formed in austenite with Σ3 boundary. The isothermal transformation near martensite start (Ms) temperature was most conducive in obtaining the highest content of twin-related V1/V2 variant pair. Based on the classical nucleation theory of martensite and bainite, respectively, the role of austenite Σ3 boundary in martensite and bainite transformation is illustrated.

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.

There are overwhelming experimental observations indicating intermetallic (IM) phase formed in the intermediate temperatures in high entropy alloys. In this study, we proposed a model to show how kinetics could intervene the thermodynamic determination of phase formation. The model offers a good explanation for the prevalent formation of IM formation in the intermediate temperature range in high entropy alloys available in the literature. To further demonstrate the kinetic effect, we selected the equiatomic CrMnFeCoNi alloy (i.e., Cantor Alloy) as a surrogate material and employed differential scanning calorimetry (DSC) at different heating rates to investigate IM formation in this alloy. In this case, we found the presence of four IM compounds, BCC-Cr, L10-NiMn, B2-FeCo, and Cr-rich σ phases, in the temperature ranging from about 200 to 900°C, above which the alloy was a complete solid solution. From the DSC measurements, we were also able to build a quasi-equilibrium TTT diagram for Cantor Alloy. It was the first time that such a TTT diagram was established for a high entropy alloy.

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.

The inhomogeneous magnetic state arising from isothermal aging of the rapidly quenched U–6Nb alloy (6.3 wt % or 14 at % niobium) was studied for the first time using nuclear magnetic resonance of the 93Nb nucleus. In the process of phase transformation during isothermal annealing at Тan = 500°C, the fraction of niobium atoms in the bulk of the alloy increases in regions with a magnetic susceptibility corresponding to the alloys U1–хNbх (х > 0.14). It is shown that the process of isothermal transformation, accompanied by the formation of niobium-enriched structural precipitates, is fully completed after 60 hours.

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.

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

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 ...

A triplex microstructure of bainite, martensite and retained austenite in the 0.3C-1.4Si-1.8Mn (wt.%) steel was obtained by the bainitic isothermal transformation combined with Q&P process (B-QP process). The effects of bainite morphology and content on the mechanical properties have been studied by controlling the isothermal holding temperature and time. Results show that with isothermal holding temperature increasing from 320 to 400 °C, the bainite changes from lower bainite to upper bainite and the ultimate tensile strength and toughness decrease. Compared to the Q&T, 360 °C B-QP and 400 °C B-QP processes, the microstructure of the 320 °C B-QP-treated sample consists of lower bainite, martensite and retained austenite, and it exhibits the best combination of strength (1617 MPa), elongation (18.6%), the product of strength and elongation (PSE, 30.1 GPa%) and maximum impact toughness (103 J) when the lower bainite content is about 50%. In addition, the work hardening behaviors indicate that both the 400 °C B-QP sample with upper bainite morphology and the 320 °C B-QP sample containing 75% lower bainite exhibit the higher work hardening rate at the high strain level and larger uniform elongation.

International audience; The effects of boron segregation and austenite grain size on the bainitic isothermal transformation of a high-strength steel were studied independently. Dilatometry, microstructural observations and previous atom probe tomography analyses showed that bainitic transformation rate decreases with increasing boron excess at austenite grain boundaries. We also found that boron segregation causes an unexpected grain size effect: large austenite grains transform faster than small ones. A kinetic model assuming slow nucleation at austenite triple junctions and rapid growth of bainitic nuclei successfully described our experimental data. These results confirm that bainite nucleation is inhibited by segregated boron at grain boundaries. On this basis, austenitic grain size could be optimized to make the most of boron addition in advanced high-strength steels.

A time–temperature–transformation–viscosity (TTT-η) diagram can reflect changes in the physical states of a resin, which take on significance for the study of the curing process of polyurethane resin lenses. Coupling the differential scanning calorimetry (DSC) test, the curing kinetic parameters of 1,4-bis(isocyanatomethyl)cyclohexane (H6XDI)/2,3-bis((2-mercaptoethyl)thio)-1-propanethiol (BES) polyurethane system were obtained. By phenomenological modeling, the relationships between degree, temperature, and time were obtained. An isothermal DSC test was carried out at 423 K. Based on the DiBenedetto equation, the relationships between glass transition temperature, degree of cure, and time were obtained, and the glass transition temperature was thus correlated with temperature and time. The gelation time at different temperatures was measured by rotary rheometry, and the relationship between gelation time and gelation temperature was established. The time–temperature–transformation (TTT) diagram of H6XDI/BES system was constructed accordingly. Subsequently, a six-parameter double Arrhenius equation was used as the basis for the rheological study. The viscosity was examined during the curing process. The TTT-η diagram was obtained, which laid the theoretical foundation for the optimization and setting of processing parameters.

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.

Bainitic ferrite plate thickness evolution during isothermal transformation was followed at the same holding temperatures in two nanostructured steels containing (in wt.%) 1C‐2Si and 0.4C‐ 3Si. A dynamic picture of how the bainitic transformation evolves was obtained from the character-ization of the microstructure present at room temperature after full and partial transformation at 300 and 350 °C. The continuous change during transformation of relevant parameters influencing the final scale of the microstructure, YS of austenite, driving force of the transformation and evolution of the transformation rate has been tracked, and these variations have been correlated to the evolution of the bainitic ferrite plate. Instead of the expected refinement of the plate predicted by existing theory and models, this study revealed a thickening of the bainitic ferrite plate thickness as the transformation progresses, which is partially explained by changes in the transformation rate through the whole decomposition of austenite into bainitic ferrite., This research was partially funded by the Research Fund for Coal and Steel under grant agreements (RFCS) (2019) No. 899482 and No. 899251.

To apply the duplex type low-carbon medium-manganese steel to the hot/warm-forging and -stamping products, the influence of cooling process routes immediately after intercritical annealing such as air-cooling (AC) and isothermal transformation (IT) processes on the impact toughness of 0.2%C-1.5%Si-5%Mn (in mass %) duplex type medium-Mn (D-MMn) steel was investigated. Moreover the microstructural and tensile properties were also investigated. The AC process increased the volume fraction of reverted austenite but decreased the thermal and mechanical stability in the D-MMn steel, compared to the IT process. The AC process increased the tensile strength but decreased the total elongation. The Charpy V-notch impact value and ductile-brittle transition temperature were deteriorated by the AC process, compared to the IT process. This deterioration of the impact toughness was mainly related to the reverted austenite characteristics and fracture mode.

Duplex aging is one of the common heat treatments in titanium alloys. The microstructure introduced in the first-step aging has an effect on the growth/dissolution of α in the second-step aging. In the present work, a β + αacicular microstructure is preset in Ti–55531 (Ti-5Al-5Mo-5V-3Cr-1Zr wt%) alloy. The isochronal and isothermal phase transformation kinetics in the second-step aging is studied by combining the dilatometer test with microstructure characterization and local composition mapping. The phase transformations and corresponding temperature ranges are determined as β → αacicular [643–845 K] and αacicular → β [845–1130 K] by isochronal annealing. A TTT diagram for isothermal transformation kinetics is plotted based on the transformed phase fraction and reproduced by Johnson–Mehl–Avrami theory. The calculated kinetic curves are in good agreement with experiment ones. The C-shaped TTT curves verify the classical nucleation and growth of α in the second-step aging. In comparison with Ti–55531 alloy with preset β + αlath microstructure (in authors’ previous work), the α precipitation exhibits prolonged incubation period and slowed average transformation rate, which is evidenced by a right shift of C-curves for the α precipitation portion along the time axis. However, the C-curves of α dissolution show a left shift on the TTT diagram. The precipitation kinetics of α aciculae from dilatometry is synchronous with that obtained from the diffusion of Al detected in STEM mapping, while the diffusion of slow-diffusion elements lags behind the structural transformation. The TTT diagram and the dataset of microstructure features obtained in the present work can be employed to optimize processing in duplex aging.

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 important property of a sinter is its ability to withstand the load in a blast furnace (BF). During the cooling process, many compounds among CaO(C), FeO(W), and Fe2O3(F) precipitate, thereby affecting the quality of the sinter. In a previous study, the compounds that precipitated at a high quenching rate (7600 K/s) were clarified. In this study, a 10 pct CaO-90 pct Fe2O3 sample was melted and held at the desired temperatures from 1 minute to 168 hours using the hot thermocouple (HTC) method. The precipitated compounds were clarified by X-ray diffraction (XRD), while a time-temperature-transformation (TTT) diagram for the 10 pct CaO sample was determined below 1499 K. Unstable compounds (C2W4F9 and WFSS) were precipitated together with CF and F in the low-temperature region below 1428 K (1155 °C). However, the unstable compounds disappeared when the holding time was long. In the temperature region from 1428 K to 1499 K (1155 °C to 1226 °C), the rate of formation of CF2 was low; over a period longer than 168 hours, CF disappeared. Therefore, the reaction, CF + F = CF2, is very slow because the locations of CF and F are separated during precipitation from the melt.

In this work, bainite steel thin wall was prepared by additive manufacturing (AM) with an in situ isothermal transformation at 300 °C. The microstructures were analyzed by scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The mechanical properties of the AM samples were measured by microhardness testing and tensile testing. The results show that the bainite steel thin wall is free from cracks and each layer has a good bond with preceding one. The bainite steel thin wall displays consistent mechanical properties. The formation of the bainite microstructure improves the toughness of the thin wall and reduces the embrittlement caused by dendritic segregation.

Blocky alpha can be systematically generated during heat treatment of certain as-welded Zircaloy-4 microstructures. The volume fraction depends upon the exposure time and temperature of the postweld heat treatment. It is necessary to understand the transformation kinetics of the fast-cooled as-welded structure to determine the microstructures present after extended periods of high-temperature operation. An experimental test campaign was devised to observe the evolution of blocky alpha and to understand its transformation kinetics. A number of postweld heat treatments were performed at between 610°C and 760°C to form microstructures with various volume fractions of blocky alpha. Following microstructural analysis, the data were subsequently used to construct a time–temperature transformation (TTT) diagram, capable of predicting blocky alpha formation. The TTT diagram shows that blocky alpha is not expected to form below 600°C in any realistic timeframe and is therefore not an issue at typical civil pressurized water reactor operational conditions.

A metallographic method, dilatometry, and X-ray diffraction were applied to investigate the effects of undercooling and holding time on bainitic transformation, microstructure, and strength of Fe–C–Mn–Si superbainitic steel. The results indicate that the strength of the samples decreases and the elongation increases with the isothermal transformation time, resulting in an increase in the product of the tensile strength and the total elongation. Therefore, the prolongation of the transformation time can improve the comprehensive property of the sample. In addition, the morphology of bainite changes from granular bainite to lath-like one with a decline in the isothermal transformation temperature, leading to an increase in strength. The elongation first increases and then decreases with a decrease in the isothermal transformation temperature. Finally, the sample shows the best comprehensive property when it is austempered at an intermediate temperature (350°C). The data presented here are instrumental in optimizing the technology of austempering treatment in industrial production.

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.

The quenching temperatures below Ms are difficult to uniformly control during large-scale processing of quenched and partitioned (Q&P) steels, which results in significant variation in tensile properties for the entire coil. The present study is aimed at obtaining a stable elongation for a wide range of quenching temperatures in hot rolled Q&P steels through microstructural design. Instead of cold rolling and annealing processes, a novel process involving hot rolling and air cooling followed by cooling in furnace was applied to a newly designed 0.25C-3Mn-2Al (wt.%) steel. The results indicated that a stable amount of retained austenite (RA) ∼ 19–23% was obtained during air-cooling to finish temperatures in the range of ∼280–360 °C, resulting in no loss in elongation at low quenching temperatures. The RA that was promoted by the transfer of carbon from ferrite/bainite reached up to 1.346 μm, whereas the RA embedded in martensitic laths was ∼40 nm. The two types of RA provided continuous TRIP effect during tensile deformation and the ultra-fine martensite lath of ∼100 nm ensured tensile strength greater than 1091 MPa. Finally, a stable total elongation of ∼19.2–20.5% in combination with tensile strength of ∼1091–1196 MPa was obtained over a wide quenching temperature range of 80 °C, which broadened the processing window for the production of hot rolled Q&P steels. In addition, the isothermal transformation during the initial partitioning stage was the underlying reason to obtain stable amount of RA. The CCE (Constrained Carbon Equilibrium) model combined with isothermal transformation was used to quantitatively calculate the volume fraction of RA.

Pearlitic steels with three different inter-lamellar spacing ranging from 79 to 120 nm have been developed in bulk by austenitization and subsequent isothermal transformation with varying degree of under-cooling. The colony size was found to decrease with reduction of transformation temperature. However, the pearlitic nodule size had a similar range for all the specimens. Charpy impact energy of the specimens has been determined for all the transformation conditions at different test temperatures. At 25 °C, the variation of impact energy was negligible with the variation of inter-lamellar spacing and the fracture was completely brittle with clear cleavage facets. The facet size had a strong correspondence with the pearlitic nodule size. However, as the test temperature was raised to 125 °C and 250 °C, the impact energy was enhanced and the pearlite with the finest inter-lamellar spacing fractured after absorbing the highest energy. At higher temperatures, a region of ductile fracture was observed in the fracture initiation region which expanded with an increase in test temperature and decreasing lamellar spacing. At lower testing temperatures, the crack seemed to be cutting through the colonies without any noticeable deformation of cementite lamellae. However, the cementite for all the specimens bent significantly before breaking at higher temperatures. The bent in the direction of crack propagation increased with refinement of inter-lamellar spacing. The study shows that although nodule size determines the impact toughness at lower testing temperatures, inter-lamellar spacing influences it at higher temperatures.

This work aims at broadening water quenching window for production of hot rolled quenched and partitioned (Q&P) steels. Comparing with conventional researches, a low density steel 0.25C–3Mn–2Al (wt%) was designed to increase the amount of retained austenite (RA) through quenching and non-isothermal partitioning processes. The results indicated that stable amount of RA greater than 19.3% was obtained in the quenching temperature range of ∼280–360 °C, resulting in excellent plasticity ∼19.2–20.5% in combination with high tensile strength of ∼1091–1196 MPa. The isothermal transformation was responsible for the stable amount of RA and the CCE model combining with isothermal transformation was used to quantitatively calculate RA.

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.