Isothermal Transformation Research Articles

Effect of microalloying and isothermal transformation on the microstructure and mechanical properties of SWRH82B steel

This study investigates the effects of microalloying elements vanadium (V) and niobium (Nb), along with varying isothermal transformation temperatures, on the microstructural evolution and mechanical properties of SWRH82B high-carbon pearlitic steel. Comprehensive microstructural characterization was conducted using optical microscopy, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results show that the addition of V alone or in combination with V and Nb refines the lamellar spacing, pearlite clusters and pearlite ball clusters. Compared with the matrix steel, the lamellar spacing was refined by 46% at lower isothermal transitions; the dimensions of pearlite clusters and pearlite globule clusters were reduced by up to 43% and 31%.The additions of V and Nb significantly increased the microhardness, tensile strength, and yield strength of the steels. The tensile and yield strengths increased by 272 MPa and 178 MPa to 1172 MPa and 657 MPa, respectively. This increase in strength was dominated by the precipitation strengthening of VC and NbC particles and the fine grain strengthening effect. The impact toughness of pearlite steels increases with the refinement of the microstructure, which is attributed to the increase in fracture initiation energy and fracture extension energy. The increase in fracture initiation energy is greater than the extension energy under the same isothermal conditions. The fracture mode is a mixture of deconvoluted and ductile fracture. This research provides a scientific foundation for optimizing the manufacturing process of SWRH82B steel and offers significant insights into the study and application of other microalloyed high-strength steels.

Effects of Heating Methods on Precipitation Behavior and Nucleation Activation Energy of γ′ Phase in Iron–Nickel-Based Alloy

Electromagnetic induction heating, which converts electromagnetic energy into thermal energy via electron-lattice collisions, and heat conduction heating, which transfers thermal energy through lattice vibrations, both have significant impacts on the solid-state precipitation behavior caused by atomic diffusion. This paper proposes a creep method based on heat conduction heating, which utilizes the turning point of negative creep to measure the isothermal transformation start curve of the γ′ phase in the alloy. The results are compared with the thermal expansion experiments under electromagnetic induction heating and simulations from the thermodynamic analysis software JMatPro. The results indicate that the nucleation incubation period of the γ′ phase in the creep experiment is longer, excluding the non-thermal effects of electricity, and more consistent with actual heat treatment conditions. The overlapping precipitation of other phases, such as M23C6 carbides at grain boundaries, reduces the γ′ phase’s fastest precipitation temperature determined by the creep and thermal expansion methods, thereby lowering the accuracy of the isothermal transformation curve. This study provides a reference for optimizing production processes and evaluating the service performance of precipitation-strengthened iron–nickel-based alloys.

Effect of PWHT process on carbide precipitation behavior and impact toughness of pressure vessel steel

The effects of the post-weld treatment on the impact performance, microstructure, and carbide precipitation behavior of pressure vessel steel were evaluated under simulated post-weld conditions. The continuous cooling transformation and isothermal transformation curves of undercooled austenite for the steel were constructed based on the expansion curve, serving as a guide for the potential heat treatment of the steel plates. A more detailed study was conducted on the simulated post-weld process with an insulation temperature of 690 ℃ and an insulation time of 24 h, based on the delivery status of the steel plate. The microstructure was characterized using transmission electron microscopy, field emission scanning electron microscopy combined with electron backscatter diffraction, and electron probe microanalysis. The Charpy V-notch impact test was used to assess the impact performance of the steel plates. The results showed that refining the microstructure to 50% bainite and 50% ferrite, along with a high proportion of large-angle grain boundaries and large-angle misorientation grains at half the thickness of the steel plate, contributed to enhanced low-temperature impact toughness in its delivered state. Additionally, the steel predominantly consists of chromium-containing carbides. In the as-delivered state, the carbide size was measured at 110 nm. However, after post-weld heat treatment (PWHT), the carbide size significantly increased to 360 nm, reflecting a 227% growth. This coarsening is observed along the grain boundaries and through intragranular aggregation. Additionally, there was a change in carbide type from Cr7C3 in the as-delivered state to Cr23C6 following the heat treatment. This transformation was accompanied by a significant reduction in impact toughness, as evidenced by the impact energy dropping from 116 J to an unacceptable 43 J.

Microstructure and Properties of Low- and Medium-C Press-Hardened Steels During Hot Stamping with Intermediate Pre-cooling Stage Tailored Process

High-strength steels contribute to meeting green transition goals by enabling lightweighting of vehicles. However, sufficient ductility, which is typically compromised by high strength, is a prerequisite for the use of a material in structural components. Tailored processes address this challenge by improving ductility in critical areas by applying locally a softening heat treatment. Such processes are established based on detailed time-temperature transformation information that is often not available for newly developed grades. In this study, by coupling physical simulation informed by finite element modeling of process conditions with tensile testing and optical and scanning electron microscopy, we investigate the microstructural evolution and resulting mechanical properties of a recently introduced high-strength B-containing steel (37MnB4) for automotive applications. The investigation focuses on an advanced press-hardening process with an intermediate pre-cooling step prior to stamping and quenching operations. The results are compared with those obtained with the established grade 22MnB5, where the process has previously been shown to be successfully applicable, in terms of their response to this tailored process. The data indicate that the higher hardenability of 37MnB4 compared to 22MnB5 dictates the need for a process modification. Furthermore, it is shown that extending the pre-cooling phase, the simplest adaptation of the process to the new scenario, produces an unsuitable ferritic-martensitic microstructure, resulting in both reduced strength and reduced ductility.

Synthesis and characterization of Sb/P substituted Ge–Ga–Se–Te bulk glasses

Mixed Se–Te glasses are unique materials for infrared optical applications due to their exceptional IR transparency, molding and fiber-drawing capability, environmental stability, and tailored physical properties. In this work, the effect of the gradual substitution of Sb with P in bulk PxGa5Ge20Sb10−xSe45Te20 glasses is studied with structural methods, optical spectroscopy, and differential scanning calorimetry (DSC). Non-isothermal and isothermal DSC studies reveal crystallization of several phases, some of them following the non-conventional anti-Arrhenius behavior of crystallization kinetics. The thermal stability of the glass is shown to increase with P concentration, and the glasses with x = 7, 9, 10 do not fully crystallize during the regular DSC heating scans. Their time–temperature-transformation curves reveal a known “nose”-like shape, suggesting crystallization at close proximity to the melting points of corresponding crystalline phases. It is found that the substitution of Sb with P in glass composition leads to a slight decrease in the optical gap and shift of the main Raman signal at ~ 150 cm−1 to higher wavenumbers.

Folding Competition and Dynamic Transformation in DNA Origami: Parallel Versus Antiparallel Crossovers.

DNA is a versatile abiomaterial for constructing nanostructures with biomedical and biotechnological applications. Among the methods available, DNA origami is a robust and widely recognized technique. Traditionally, most origami designs adopt antiparallel crossovers in both scaffold and staple strands, with less emphasis on parallel crossovers, which offer advantages like enhanced nuclease resistance and single-strand routing potential. Here, a DNA origami nanostructure is designed, featuring two rotational panels that can be locked into configurations based on either antiparallel or parallel crossovers. By systematically varying the length and arrangement of these key staples, 36 pairs of antiparallel and parallel designs are studied in competitive folding tests, providing insights into the relative preference for each design. The 12 antiparallel and parallel designs are ranked, their folding pathways are examined, and nuclease resistance is assessed. The results reveal that the arrangement of staples near the central scaffold crossover is crucial for shifting betweenparallel and antiparallel conformations. Additionally, a two-way isothermal transformation between antiparallel and parallel origami driven by toehold-mediated displacement reactions is demonstrated, highlighting the potential of parallel designs as dynamic nanodevices for temperature-sensitive environments. This study offers valuable insights into - dynamics in antiparallel and parallel DNA origami, opening opportunities for designing nanodevices based on parallel crossovers.

Microstructure Evolution of Cold-Rolled Carbide-Free Bainite Steel Sheets During Continuous Annealing Process

A modified carbide-free bainite (CFB) steel has been developed, building on existing alloys for compatibility with commercial continuous annealing lines (CALs), featuring a low austenitization temperature and short overaging time. The microstructural features of such candidate CFB sheets are compared with those of conventional CFB steel sheets that require higher reheating temperatures and longer overaging times. The effects of annealing parameters such as reheating temperatures and overaging temperatures on phase transformation kinetics and microstructure evolution are presented. The annealing process was simulated in a Gleeble thermomechanical processing simulator, and the microstructural characterization was carried out using XRD, SEM, and EBSD. Reconstruction of parent austenite grains from EBSD data did not reveal any variant selection, regardless of changes in the austenitization temperature, overaging temperature, or carbon content. It was observed that the V1–V2 variant pairing is the most common at the lower overaging temperature for reheating at 950 °C; however, this pairing decreases as the isothermal overaging temperature increases, with variant pairings involving low misorientation boundaries—such as V1–V4 and V1–V8—becoming more frequent. Steels with higher carbon content exhibit no significant changes in their variant pairing, regardless of variations in the austenitizing or isothermal temperatures. The XRD results show that the retained austenite fraction is reduced by increasing the isothermal transformation temperature.

Effect of Temperature on Kerogen Transformation and Hydrocarbon Generation in Bazhenov Formation (Western Siberia, Russia) Rocks During Hydrous Pyrolysis

The aim of this investigation is to determine the role of temperature in hydrocarbon generation in Bazhenov formation source rocks, as hydrothermal processes in situ are supposed to be the main contributor to kerogen transformation in Western Siberia. Laboratory modeling of organic matter isothermal transformation in powdered and extracted samples by hydrous pyrolysis at 300 °C, 325 °C, 350 °C, 375 °C, and 400 °C for 24 h was performed. Heated samples were analyzed via pyrolysis, and hydrocarbons were investigated by GC and GC-MS. Results show that more than 90% of the kerogen generation potential was utilized. The amounts of liquid and gaseous products reached 82 and 126 mg of hydrocarbon/g TOC, respectively. The volume of generated gas increased while the maximum production of liquid decreased with temperatures over 350 °C. The biomarker composition of liquid products at different temperatures was similar. The characteristics of kerogen and the product composition had a high correlation with the parameters for samples naturally formed in situ under an elevated heat flow. More subtle chemical analyses are required to determine the effects of kerogen’s chemical structure and the rock mineral composition on oil and gas generation.

Analysis of Fatigue Crack Propagation in AISI 4140 Steel with Multi-Austempering Treatment

This study examines the fatigue crack propagation behaviour of AISI 4140 steel subjected to multi-austempering heat treatments. Tensile test and fatigue crack propagation (FCP) specimens were prepared in accordance with ASTM E8 and ASTM E647, respectively. Multi-austempering was carried out by heating the specimen to an austenite temperature for 10 min using an induction heating coil. The specimen was then immersed in a salt bath for each isothermal transformation time of 60 min at three austempering temperature levels from 312°C to 412°C with a temperature increase of 50°C. Tensile and fatigue crack growth tests were performed on both annealed and multi-austempered specimens. It is observed that the multi-austempering heat treatment significantly improves the tensile properties and the FCP properties of AISI 4140 steel. Microstructural observations indicate that the bainitic phase and the retained austenite increase the tensile strength and reduce the fatigue crack propagation rate (da/dN). It is found that the bainitic structures are an effective barrier in reducing fatigue crack propagation as the fatigue loading cycle increases

Chemical Composition Optimization and Isothermal Transformation of δ‐Transformation‐Induced Plasticity Steel for the Third‐Generation Advanced High‐Strength Steel Grade

A new low‐manganese transformation‐induced plasticity steel is designed with optimized nickel content to achieve superior strength and ductility while minimizing the use of expensive nickel. The steel is optimized using JMatPro software, then cast, and hot rolled. To assess the effect of intercritical annealing on austenite (martensite at room temperature) volume fraction and carbon content, hot‐rolled steel samples quenched from different annealing temperatures (680–1100 °C) are used. Additionally, hot‐rolled steel coupons are intercritically annealed at about 50% austenite formation temperature (740 °C) and then subjected to isothermal treatments at 300–425 °C for varying times (10–90 min). After optimizing these treatments to maximize retained austenite (RA), tensile specimens are heat‐treated first at 740 °C and then isothermally at 325 °C. Thermodynamic calculations suggest that aluminum combined with silicon may lead to the δ ferrite formation, and even minimal nickel content can stabilize a considerable amount of austenite. In the experimental studies, it is shown that lower‐temperature bainitic holding enhances austenite stability by enriching the carbon content. Optimized two‐stage heat treatments yield up to 25.8% RA, with a tensile strength of 867.2 MPa and elongation of 40.6%, achieving a strength‐elongation product of 35.2 GPa×%, surpassing the third‐generation advanced high‐strength steel grades minimum requirement of 30 GPa×%.

The effect of the increased strain per pass during the hot-rolling and its effect on the tensile properties of V-Mo and Cr-V-Mo microalloyed dual-phase steels

It is well known that the thermomechanical processing parameters affect the mechanical properties of dual-phase (DP) steels, but optimization still remains a key challenge. This work studied two interphase precipitation-strengthened DP steels based on V, Mo, and V, Mo, Cr microalloy additions. Hot-rolling was performed with a strain per pass of 0.2 and 0.4 before isothermal transformation at temperatures between 600 and 690 °C to determine the effect of these process parameters on microstructure, particularly interphase precipitation. The microstructure was carefully correlated with the mechanical properties. It was found that a higher strain during hot rolling increases the transformation kinetics of austenite to ferrite and also increases the volume fraction of the interphase precipitation within it, leading to higher strength values. The reaustenitization temperature before the isothermal transformation also plays an important role, with increased temperature reducing the segregation banding effect and increasing the amount of ferrite. The optimal reaustenitization temperature depends on the composition, being 1250 °C for the microalloyed steel with Cr additions and 1150 °C for the Cr-free microalloyed steel. A new method was used to calculate the yield strength of these DP microalloyed steels, including the strengthening contributors of each phase and the banding effect brought into one single expression that matches the experimental results.

Analysis of Isothermal Bainite Transformation in Alloyed Steels by Means of Generalized Kinetic Equation

Analysis of Isothermal Bainite Transformation in Alloyed Steels by Means of Generalized Kinetic Equation

Effects of Nb Solute and Precipitates on the Continuous Cooling and Isothermal Transformation of Pearlite in High‐Carbon Steel Wire Rods

To investigate the influence mechanism of Nb solute and precipitates on pearlite transformation, the effects of Nb content and heat‐treatment process on pearlite transformation and microstructure in high‐carbon steel wire rods are analyzed in this article. During the austenite deformation stage, the increase in solute Nb and strain‐induced precipitates (SIPs) suppresses austenite recrystallization, while the decrease in prior austenite grain size promotes the reduction of refined pearlite colonies size. During the continuous cooling transformation process, an increase in Nb content and cooling rate increases the supercooling of pearlite transformation and refines the interlamellar spacing (ILS) of pearlite. During the isothermal transformation process, the increase in isothermal temperature increases the incubation period of pearlite transformation and slows down the rate of pearlite transformation. With the increase in Nb content, the incubation period of pearlite transformation lengthens. But uneven distribution of C in austenite is caused by the SIP, which promotes interface migration, and shorts the pearlite transformation time. In high‐carbon steel wire rods, the refinement of pearlite colonies and pearlite ILS is achieved by increasing the Nb content, raising the cooling rate, and lowering the isothermal temperature, thereby increasing the content of precipitates and improving their mechanical properties.

Exploring the impact of intercritical annealing on microstructural evolution and mechanical performance in low alloy multiphase TRIP-assisted steels

Exploring the impact of intercritical annealing on microstructural evolution and mechanical performance in low alloy multiphase TRIP-assisted steels

Application of crystallization additivity principle in continuous casting mould flux

Testing the isothermal crystallization of mould fluxes poses challenges due to their high critical cooling rates. As a result, isothermal crystallization can be estimated from non-isothermal crystallization using the principle of crystalline additivity. However, the relationship between the TTT (Temperature Time Transformation) and CCT (Continuous Cooling Transformation) curves for volatile mould fluxes remains chaotic. This research introduces precise measurements of both isothermal and non-isothermal crystallization of volatile mould fluxes under conditions that suppressed fluoride volatilization for the first time. The CCT curves derived using the additive principle closely match the experimentally observed patterns, with only minor discrepancies, thereby validating the effectiveness of the additive principle in slag studies. Mould flux BA3 displays a double C-shaped TTT curve, where the high-temperature and low-temperature C-shapes represent distinct crystalline phases. If the computed CCT curve, generated by combining the incubation times of the two C-shaped crystallizations from the TTT curve, significantly diverges from the measured value at temperatures below the partition temperature, segmenting the CCT curve calculations based on high-temperature and low-temperature C-shape data from the TTT curve leads to calculated values that align with experimental results. The study enhances understanding of mould flux crystallization kinetics, crucial for improving the surface quality of continuous casting slab.

High-silicon spring steel transformation to carbide-free bainitic (CFB) steel: a mechanical and tribological approach under rolling/sliding conditions

The present study concentrates on the development of carbide-free bainite steel and explores its tribological and mechanical characteristics using a newly devised disk-on-disk tribometer. The microstructure of the treated samples was examined through heat treatment at various holding temperatures (300, 350, and 400 °C) and durations (10, 20, and 30 min), along with isothermal transformation. The wear rate demonstrated significant sensitivity to holding duration, transformation temperature, phase fraction, and the stability of retained austenite. The microstructural analysis encompassed atomic force microscopy (AFM), FE-SEM, x-ray diffraction (XRD), and energy-dispersive x-ray spectroscopy (EDS). The study extensively investigated the robust correlation among mechanical properties, microstructure, wear behavior, and the presence of retained austenite (RA) and bainite volume fraction. The findings indicate that carbide-free bainite steel holds promise for replacing conventional railway wheel track steel in the steel industry. The structural property correlations have developed of high silicon steel, hardness reduced with increasing temperature and isothermal preserving time; resulting in a gradual increase in the wear rate of CFB steel. The salt bath soaking temperature of 350 °C with a period of 30 min was found to be the preeminent wear properties condition for CBF steel.

Thermodynamic–kinetic calculations and dilatometric verification of the martensitic and bainitic transformations in 4% medium-Mn steel

The present study investigated the theoretical and experimental phase transitions phenomena during continuous cooling and isothermal holding above and below Ms temperature in 4 mass. % Mn medium-Mn steel. The thermodynamic–kinetic calculations were performed using JMatPro software, and phase transformations were recorded using a BÄHR high-resolution DIL805A/D dilatometer. The research covered continuous cooling rates from 60 to 0.05 °C s–1 and isothermal holding temperatures in a range between 420 °C and 230 °C. The issues related to both modelling and dilatometric methodology were discussed. The CCT and TTT diagrams were prepared on the basis of dilatometry and compared with the results of light microscopy and hardness tests. The alloy containing about 4 mass.% Mn and 0.22% Mo exhibited very high hardenability as only continuous cooling with rates lower than 1 °C s–1 allowed the bainitic transformation to be initiated. The bainitic transformation is accelerated after passing the Ms temperature. Both the incubation time and the time needed to complete the transformation were significantly reduced (the incubation time from 100 s to below 1 s and the completion time from over 4000 s to below 1000 s). The obtained microstructures were homogeneous and refined.

Thermodynamic prediction and experimental verification of phase transformation kinetics in 3Mn steel with Ti and V microadditions

In this work, the phase transformation kinetics and precipitation processes were studied using thermodynamic calculations and a dilatometric method to determine the optimal chemical composition of medium-Mn steel and its initial parameters of heat treatment. The investigated steel is intended for forgings with the microstructure composed of bainitic matrix and retained austenite (RA). An influence of Al and Si on the Ms temperature was characterized to obtain the best chemical composition in terms of RA stabilization. Theoretical continuous cooling transformation (CCT) and time–temperature transformation (TTT) diagrams were determined and compared with dilatometric results. Microstructural observations were compared with the dilatometric results. The hardenability of steel was high due to increased Mn and Mo additions. A very small fraction of cementite is expected in the microstructure due to Al and Si additions. The shortest time to start and finish the bainitic transformation was noted for the isothermal heat treatment at 420 °C. The completion of the bainitic transformation took about 25 min and is acceptable from the industrial point of view. The obtained results constitute a good basis for designing thermomechanical processing routes of bainitic steels with RA.

Effect of Initial Intergranular Ferrite Size on Induction Hardening Microstructure of Microalloyed Steel 38MnVS6

In this study, we investigated the effect of grain size of an initial microstructure (pearlite + ferrite) on a resulting microstructure of induction-hardened microalloyed steel 38MnVS6, which is one topical medium carbon vanadium microalloyed non-quenched and tempered steel used in manufacturing crankshafts for high-power engines. The results show that a coarse initial microstructure could contribute to the incomplete transformation of pearlite + ferrite into austenite in reaustenitization transformation by rapid heating, and the undissolved ferrite remains and locates between the neighboring prior austenite grains after the induction-hardening process. As the coarseness level of the initial microstructure increases from 102 μm to 156 μm, the morphology of undissolved ferrite varies as granule, film, semi-network, and network, in sequence. The undissolved ferrite structures have a thickness of 250–500 nm and appear dark under an optical metallographic view field. To achieve better engineering applications, it is not recommended to eliminate the undissolved ferrite by increasing much heating time for samples with coarser initial microstructures. It is better to achieve a fine original microstructure before the induction-hardening process. For example, microalloying addition of vanadium and titanium plays a role of metallurgical grain refinement via intragranular ferrite nucleation on more sites, and the heating temperature and time of the forging process should be strictly controlled to ensure the existence of fine prior austenite grains before subsequent isothermal phase transformation to pearlite + ferrite.

Monitoring of Isothermal Crystallization and Time–Temperature Transformation of Amorphous Felodipine: The Time-Domain Nuclear Magnetic Resonance Method

The isothermal crystallization process of felodipine has been investigated using the time-domain Nuclear Magnetic Resonance (NMR) method for amorphous bulk and ground samples. The obtained induction and crystallization times were then used to construct the time–temperature-transformation (TTT) diagram, both above and below the glass transition temperature (Tg). The Nose temperature was found equal to 363 K. Furthermore, the dynamics of crystalline and amorphous felodipine were compared across varying temperatures. Molecular dynamics simulations were also employed to explore the hydrogen-bond interactions and dynamic properties of both systems.Graphical