Parent Austenite Grains Research Articles

Strengthening mechanisms in vanadium-microalloyed medium-Mn steels

In this work, the impact of adding vanadium, ranging from 0 to 2 wt%, on the microstructure and mechanical properties of as-cast medium manganese steel was explored. A dual phase microstructure consisting of martensite and retained austenite was observed in the 0 V, 0.05 V and 0.8 V conditions. The 2 V condition contained retained austenite, lath martensite, and δ-ferrite bands. The retained austenite fraction and prior austenite grain size initially increased at lean vanadium concentrations but significantly dropped at the highest vanadium concentration. The element distribution in the constituent phases was investigated in detail. Mn, C and Si partitioning to austenite was observed in the 0 V, 0.05 V and 0.8 V conditions. V and C segregation to the grain boundaries and significant grain refinement were evident in the 2 V condition. The findings also revealed that increasing the vanadium content led to an increase in the hardness of the steel. This assessment was validated by tensile testing, which showed an improvement in yield and tensile strength of the steel with increasing vanadium content, and were supported by reconstruction of the parent austenite grains employing martensitic structures. Finally, the influence of different strengthening mechanisms on the yield strength of as-cast, microalloyed medium-manganese steels was also discussed in terms of simulated stacking fault energy, as well as the quantitative contributions from solid solution, grain boundary, and precipitation strengthening mechanisms.

The Effect of Static Recrystallisation of Parent Austenite on the Lath Martensite Intervariant Boundary Network

The current study investigated the effect of static recrystallisation (SRX) of parent austenite microstructure on the lath martensite intervariant boundary network. In general, the misorientation angle distribution of lath martensite exhibited a bimodal distribution in the range of 10–22 deg and 47–60 deg for all thermomechanical conditions, closely matching the misorientations expected from the ideal Kurdjumov-Sachs orientation relationship. Nevertheless, the population of 60 deg misorientation angle initially reduced with the extent of softening up to 50 pct beyond which it progressively increased to a maximum in the fully recrystallised condition. This was closely consistent with the trend noticed for the 60 deg/[110] intervariant boundaries having twist character. This was explained through the distinct variant selection mechanism occurring due to the change in the parent austenite grain characteristics (i.e., size and deformed state) upon static recrystallisation. The recrystallisation initially led to the formation of fine dislocation-free parent austenite grains up to 50 pct softening, promoting a 4-variant (V1V2V3V4) clustering arrangement to decrease the strain related to the martensite transformation. In turn, this promoted the formation of symmetric tilt 60 deg/111¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left[ {11\\overline{1}} \\right]$$\\end{document} intervariant boundaries, which ultimately reduced the 60 deg/110\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left[ {110} \\right]$$\\end{document} boundaries. Further softening, accompanied with the growth and/or coalescence of parent austenite grains, altered the variant clustering to a 3-variant arrangement (V1V3V5) and the increase in 60 deg/110\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\left[ {110} \\right]$$\\end{document} twist intervariant boundaries. Indeed, the martensite boundary network was largely controlled by the SRX parent austenite characteristics (i.e., size), which can be controlled to enhance the materials performance for a given application.

Process Parameter Optimization of Directed Energy Deposited QT17‐4+ Steel

AbstractThe feasibility of using argon‐atomized QT 17‐4+ stainless steel powder for directed energy deposition (DED) additive manufacturing is studied. The QT 17‐4+ steel is a novel martensitic steel designed based on the compositional modification of the standard 17‐4 precipitation‐hardened (PH) stainless steel. This modification aims to achieve better mechanical properties of as‐deposited components compared to the heat‐treated wrought 17‐4PH steel. In this study, QT 17‐4+ steel powder is used for DED, for the first time. The influence of laser power, laser scan speed, powder feed rate, and hatch overlap on the density is studied. The central composite design is used to determine the experimental matrix of these factors. The response surface methodology is used to obtain the empirical statistical prediction model. Both columnar and equiaxed parent austenite grain structures are observed. X‐ray diffraction analyses reveal a decrease in the percentage of retained austenite from 19% in the powder to 5% after DED. The microhardness of the DED processed sample in the as‐deposited state is slightly higher than that of wrought 17‐4PH steel either solution‐annealed or H900‐aged. A higher 0.2% yield strength, a lower ultimate tensile strength, and lower elongation are observed for the vertically printed test sample, when compared to the horizontal one.

Prior austenite grain measurement: A direct comparison of EBSD reconstruction, thermal etching and chemical etching

It is well known that the prior austenite grain (PAG) microstructure of steels has a significant impact on their microstructure evolution and mechanical properties. Many PAG measuring techniques have been developed over many decades, with the effectiveness of each varying alloy-to-alloy. Some of the more common techniques, specifically for bainitic and martensitic grades, are thermal etching and picric acid etching, which both involve the preferential etching of PAG boundaries in order to reveal the austenitic microstructure. More recently, parent grain reconstruction techniques using EBSD (electron backscatter diffraction) mapping have shown good promise in recreating PAG microstructures from BCC-FCC orientation relationships, and can now be deployed at speed with the advent of rapid detectors (1000s of pixels per second). This study aims to compare the accuracy and relative advantages/disadvantages of picric acid etching, thermal etching and EBSD reconstruction methods. A TESCAN and NewTec In-Situ Testing (TANIST) capability was used to directly observe and measure the true high-temperature PAG structure during the austenitisation of SA-540 B24 low alloy steel. These high-temperature PAG measurements were then compared to measurements from the same area obtained using thermal etching, picric acid etching and EBSD parent grain reconstructions after quenching to room temperature. Reconstructing the parent austenite grains from EBSD data resulted in the closest measurement of PAG size. PAG boundaries were delineated well by thermal etching but surface effects (such as surface relief and ghost traces) created complexities when identifying the exact position of boundaries. Picric acid etching, which produced the least accurate measurement, was found to reveal PAG boundaries well, however it was limited by its ability to sufficiently etch annealing twin boundaries and its susceptibility to microsegregational effects. EBSD reconstruction and thermal etching were more consistent at reconstructing/revealing these boundaries, although inaccuracies with the techniques were still observed.

Effects of above- or below-AC3 austenitization on bainite transformation behavior, microstructure and mechanical properties of carbide-free bainitic steel

The isothermal bainite transformation kinetics, microstructure, crystallography and mechanical properties of a carbide-free bainitic steel, subjected to pre-quenching treatment, followed by austenitization at various temperatures above or below AC3 and then by above-MS austempering, have been investigated via dilatometric measurements, microstructural characterization and mechanical tests. The results show that, as the austenitization temperature decreases from 980 °C to 860 °C (above AC3), bainite transformation first decelerates and then accelerates; this is primarily a result of the competition between the availability of parent austenite grain (PAG) boundaries as nucleation sites for bainite transformation and the shear resistance of bainite growth, both of which increase with decreasing the PAG size but play a reverse role in bainite transformation rate. Further decreasing the austenitization temperature down to 820 °C and 800 °C (below AC3) even more notably accelerates bainite transformation and shortens the transformation completion time, since the intercritical ferrite (or tempered martensite), which is formed in the intercritical region, favor the nucleation of initial bainitic laths. The resulting microstructure after austempering is composed of bainite, retained austenite and intercritical ferrite and/or fresh martensite, the feature of which depends on the austenitizing temperature below or above AC3. Below-AC3 austenitization largely refines the microstructure and decreases the amount of large, brittle fresh martensite/austenite blocks, mainly attributed to the synergetic role of the refinement of austenite grains and pre-existence of intercritical ferrite prior to austempering. In addition, the intercritical ferrite and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. As compared with above-AC3 austenitization, below-AC3 austenitization shows large increases in elongation, especially for specimens subjected to shorter time austempering, despite some decreases in yield and tensile strengths.

The role of parent austenite grain size on the variant selection and intervariant boundary network in a lath martensitic steel

This study investigated the influence of parent austenite grain refinement on the intervariant boundary network (population and connectivity) in a lath martensitic steel. Parent austenite grain refinement revealed a progressive reduction in the fraction of the 60° misorientation boundaries in martensite, which was linked to a decrease in the 60°/[110] intervariant boundary population. The phenomenological theory of martensite crystallography demonstrated that the variant selection mechanism altered from the 3-variant clustering (V1V3V5) in the coarse parent austenite towards the 4-variant clustering (V1V2V3V4) in the fine parent austenite grain, due to the change in the lattice parameter of the parent and daughter phase in which the martensite transformation occurs, as measured using in situ neutron diffraction. The change in the variant clustering arrangement with the parent austenite grain refinement led to a progressive promotion of 60°/[111] and 10.5°/[011] intervariant boundaries at the expense of 60°/[110] martensite intervariant boundaries. Subsequently, the connectivity of low energy {110} tilt intervariant boundaries gradually increased through the refinement of parent austenite grain size, eventually reducing the high energy {110} twist boundary connectivity. This change improved the impact toughness of martensite produced from the fine-grained austenite as the weak connectivity of high energy boundaries delays the coalescence of voids, promoting ductile fracture.

Effects of additive manufacturing process parameters and heat treatment on texture evolution and variant selection during austenite-martensite transformation in 18%Ni-M350 maraging steel

Variant selection and texture evolution of 18%Ni M350 maraging steel produced by laser-based directed energy deposition (DED) additive manufacturing (AM) technology are investigated in this study. The effects of heat treatment and AM processing parameters, including energy area density (EAD), powder feed rate, and laser power, on variant selection and texture evolution are discussed. Theoretically, 24 martensite variants tend to be formed during a cubic-to-cubic transformation based on the Kurdjumov–Sachs orientation relationship. Identifying the preference of variants in a thermo-mechanical process can pave the way for tailoring the desired product texture. Grain graph and local neighbor voting algorithms using measured electron backscatter diffraction (EBSD) were employed to reconstruct the parent austenite grain structure in this study. Even though no evidence of global variant selection is found, the variant selection tends to take place locally in small parent grains of heat-treated samples. Strong rotated Copper 112110γ and Z 111110γ components are observed in the parent austenite phase of the low EAD sample after heat treatment due to a high degree of recrystallization and second-order twins. A Goss texture 110001α in child martensite transitioned from these strong components is also observed to be strongly derived from packet 3 of this sample. This finding indicates that some low-misorientation variants of packet 3 are more energetically favorable to develop in a heat-treated M350 alloy produced by a low EAD.

In Situ Observation of the Grain Growth Behavior and Martensitic Transformation of Supercooled Austenite in NM500 Wear-Resistant Steel at Different Quenching Temperatures.

In situ observations of the austenite grain growth and martensite transformations in developed NM500 wear-resistant steel were conducted via confocal laser scanning high-temperature microscopy. The results indicated that the size of the austenite grains increased with the quenching temperature (37.41 μm at 860 °C → 119.46 μm at 1160 °C) and austenite grains coarsened at ~3 min at a higher quenching temperature of 1160 °C. Furthermore, a large amount of finely dispersed (Fe, Cr, Mn)3C particles redissolved and broke apart at 1160 °C, resulting in many large and visible carbonitrides. The transformation kinetics of martensite were accelerated at a higher quenching temperature (13 s at 860 °C → 2.25 s at 1160 °C). In addition, selective prenucleation dominated, which divided untransformed austenite into several regions and resulted in larger-sized fresh martensite. Martensite can not only nucleate at the parent austenite grain boundaries, but also nucleate in the preformed lath martensite and twins. Moreover, the martensitic laths presented as parallel laths (0~2°) based on the preformed laths or were distributed in triangles, parallelograms, or hexagons with angles of 60° or 120°.

Effects of rapid heating on the phase transformation and grain refinement of a low-carbon mciroalloyed steel

The present study aims to clarify the possibility of rapid heating in the preparation of low-carbon ultrafine (UFG) steel. The effects of heating rate on the phase transformation, microstructure and precipitation of a low-carbon microalloyed steel were elaborately investigated. The results manifest that rapid heating potentially provides a simple and innovative process for the preparation of UFG steels just by increasing the heating rate. The parent austenite grains were refined due to the more reserved (Nb, Ti, V)C precipitates and dislocations within a short heating process by rapid heating. However, an effective holding time was proposed in the rapid heating to ensure the refined austenite grains. Moreover, the synthetic influences of initial increased grain boundaries and redissolved (Nb, Ti, V)C carbides provoked the abnormal grains with weakened thermostability. Redissolution of (Nb, Ti, V)C carbides were apparent during austenite nucleation at a slow heating rate, and it was verified during the isothermal holding stage by rapid heating. In addition, the austenitic phase transition occurred at higher temperature with the increase of heating rate, by which the transformation rate was accelerated.

Atomic‐Scale Study on the Interaction Mechanism between Cracking and Typical Grain Boundaries in 15‐5 PH Stainless Steel

In order to illuminate the hindering effects induced by grain boundaries on the propagation behavior of crack with respect to 15‐5 PH stainless steel, interactions between propagated crack and typical grain boundaries, including the parent austenite grain boundary (PAGB), coincidence site lattice Σ3 grain boundary (CSL Σ3), packet grain boundary (PGB), and block grain boundary (BGB) are investigated at the atomic scale. Results indicate that the higher stress in the crack initiation (CI) stage and crack and grain boundary interaction (CGB) stage is required to drive the propagating crack in terms of the PAGB model compared to the other models, and the longest interaction time between propagated crack and established grain boundary is needed for the crack from the crack propagation (CP) stage to the CGB stage in the case of the PGB model. The PAGB and PGB are determined to be the most effective in hindering crack propagation in 15‐5 PH stainless steel.

Void Formation and Crack Propagation in a Cr–Mn–N Metastable Austenitic Stainless Steel During Bending

Microstructure evolution via deformation‐induced martensitic transformation, void formation, and crack propagation is investigated in a metastable austenitic Cr–Mn–N stainless steel for up to 90° bending using a combination of electron backscattering diffraction and parent grain reconstruction. Stress–strain heterogeneity and stress triaxiality studied using finite‐element analysis reveal that the inner and outer radii are in approximately uniaxial compressive and tensile stress states, respectively, with the outer radius showing higher values for von Mises and principal stresses and equivalent strain. Voids are observed at both austenite/α′‐martensite and α′/α′‐martensite interfaces. Parent grain reconstruction applied to the 90° bending sample reveal that the cracks at α′/α′‐martensite interfaces tend to propagate predominantly along intergranular parent austenite grain boundaries. It is also found that both intragranular and intergranular cracks in parent austenite tend to propagate between α′‐martensite child–child grains comprising the same crystallographic packets. This is the first study of its kind to comprehensively show this phenomenon. A representative example of intergranular crack propagation within a ‖normal direction (ND)‐oriented parent austenite grain is also demonstrated.

Developing of containing Ta, Zr reduced activation ferritic/martensitic (RAFM) steel with excellent creep property

A reduced activation ferritic/martensitic (RAFM) steel with perfect creep property at elevated temperature (600 °C) was designed by implementing adding trace amount of Ta, Zr elements. In order to reveal the mechanism of creep property enhancing through adding Ta, Zr elements, the microstructural and mechanical evolution behaviors of RAFM steels, with and without containing Ta, Zr elements, were analyzed. It was found that Ta, Zr can not only refine the M23C6 carbide particles, but also can increase the mean size of MX particles in RAFM steels, especially for Zr element. Furthermore, both of Ta and Zr can contribute to increasing the amount of MX particles. Through combining with experimental results and excluding the influence behavior of parent austenite grain (PAG) size on the mechanical property of RAFM steel, it is not the M23C6 particles, but is the precipitation amount of MX carbide particles that promoted by adding Ta, Zr, which can have a significant effect on enhancing the strength and decreasing the creep rate of RAFM steel at 600 °C condition. In addition, the synergistic effect of Ta and Zr elements can significantly increase the creep-resistance breaking time of RAFM steel under the same conditions comparing with other RAFM steels, and then the mechanism of it was revealed in this work.

Influence of initial quenching on the microstructure and mechanical properties of quenched and partitioned ferritic stainless steels

Modern steel industry has great interest in developing new advanced high-strength steels, especially for the automotive industry. The need for stronger and more ductile sheet steels has led to development of novel heat treatments such as quenching and partitioning. The Q&P heat treatment provides an opportunity of manufacturing strong steels without sacrificing their formability. However, there is limited research conducted on the microstructure evolution of many alloys potential for Q&P such as stainless steels.This study evaluates the selection for the optimal quench interruption temperature during Q&P of ferritic stainless steels. The paper compares different simulation models for optimizing the Q&P-process. Q&P was applied to two AISI 420-type stainless steels EN 1.4021 and EN 1.4034 to assess the simulation results. Microstructure analyses with X-ray diffraction and electron microscopy revealed that simulated values overestimate the retained austenite fractions after Q&P due to formation of Cr-rich carbides. Mechanical tests showed that Q&P is applicable to grade EN 1.4021 stainless steel, whereas EN 1.4034 fractured in a brittle manner under tensile load. Electron microscopy revealed intergranular fracture type and concentration of Cr-rich carbides at parent austenite grain boundaries in EN 1.4034. These results suggest that impurities may expose stainless steels to temper embrittlement during partition.

Cerium-alloyed ultra-high strength maraging steel with good ductility: Experiments, first-principles calculations and phase-field simulations

Improving ductility while achieving ultra-high strength has always been a challenge in developing of high performance maraging steels. To this end, the study of the influence of Ce on the mechanical performance of B2–NiAl precipitation strengthened maraging steel was performed via experimental characterization combined with first-principles calculations-based phase-field simulations and crystal plasticity simulations. The tensile test revealed that the Ce-bearing maraging steel possesses a remarkable combination of ultimate tensile strength of ∼2000 MPa and tensile ductility of ∼10%. The microstructures at various scales were characterized by EBSD, HRTEM and APT. It was found that Ce exhibits a notable effect on the precipitation behavior of B2–NiAl particles because the presence of Ce decreases the precipitation driving force of B2–NiAl phase. EBSD characterization revealed that Ce-bearing steel possesses a higher fraction of HAGBs which facilitates the improvement the ductility of the experimental steel. Reconstruction of parent austenite grains and analysis of martensite variants selection were performed, and the results indicated that the lowering of the Ms temperature of the steel caused by Ce leads to more length fractions of V1/V2 and V1/V5 inter-variant boundaries, which contribute to the formation of higher fraction of HAGBs.

Roles of pre-formed martensite in below-Ms bainite formation, microstructure, strain partitioning and impact absorption energies of low-carbon bainitic steel

The roles of pre-formed martensite (PM) in below-Ms bainite formation, microstructure, crystallography, strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction, transmission electron microscopy, micro digital image correlation technique and mechanical tests. It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms, indicative of its acceleration effect at the early stage of transformation. This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite, with initial bainite tending to form around the PM. However, the finishing time for below-Ms bainite transformation, especially at even lower temperatures, is retarded, owing to the dividing effect of PM on parent austenite grains, the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite. PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite. The specimens with PM show relatively uniform strain partitioning among various phases, contrasting with the specimens without PM, for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite (M/A) blocks or between adjacent M/A blocks. The impact absorption energies of the specimens with PM, when austempered at 30–60 °C below Ms, are more than twice higher than those of the specimens without PM, at no expense of tensile properties.

Yield strength insensitivity in a dual-phase high entropy alloy after prolonged high temperature annealing

Recent studies of FeMnCoCr-based high entropy alloys have demonstrated uncommon deformation behaviors such as transformation-induced plasticity, which were largely believed to be restricted to select families of steels. Coupled with the potential for entropy stabilization of high symmetry phases at high temperatures, this system represents a promising class of materials for structural applications in extreme environments. Yet, transformation-induced plasticity mechanisms are notably sensitive to microstructure parameters and the literature offers examples of deleterious decomposition of high entropy alloys under heat treatment, which raises concerns of resiliency in mechanical performance. Here, we evaluate the evolution of microstructure and mechanical properties of a FeMnCoCr high entropy alloy after prolonged heat treatment at high temperature. Microstructures are found to retain their characteristic austenite/martensite features, with parent face-centered cubic grains partitioned by hexagonal close-packed laths after heat treatment at 1200 °C for up to 48 hours. Results of mechanical testing reveal an unusual insensitivity of this alloy to grain growth-induced weakening effects. Namely, the yield strengths of FeMnCoCr samples are observed to remain constant across all heat treatment conditions, despite a near four-fold increase in the grain size. Close examination of post-heat treatment microstructures reveals a dramatic decrease in the inter-lath spacing at longer durations, which segments parent austenite grains. This crystal partitioning counteracts conventional grain growth-induced weakening by introducing additional barriers for dislocation pile-up. These results offer new insights into the mechanical resiliency of this transformation-induced plasticity high entropy alloy under prolonged high temperature heat treatment.

Effect of cathodic polarisation on stress corrosion cracking behaviour of a Ni(Fe, Al)-maraging steel in artificial seawater

The effect of cathodic polarisation on stress corrosion cracking (SCC) behaviour of peak-aged Ni(Fe, Al)-maraging steel in artificial seawater was investigated. The steel shows better compatibility of strength and hydrogen embrittlement (HE) resistance than reported 18Ni-type maraging steels when potential is above −850 mV. This advantage stems from the abundant and evenly dispersed Ni(Fe, Al) nanoprecipitates. SCC occurs above −850 mV in a transgranular/intergranular-mixed manner involving a mechanics-dominant-anodic-dissolution-promoted mechanism and below −850 mV in an intergranular manner along parent austenite grain boundaries involving HE. For interpreting the HE mechanism, a novel model is proposed based on hydrogen-weakening interatomic bonds.

Simulation and experimental study on microstructure evolution of 5CrNiMoV steel during multi-directional non-isothermal forging

Microstructure evolution during the hot forming shows a significant impact on material’s mechanical properties. To explore the deformation characteristics of 5CrNiMoV steel, numerical simulation and microscopic phase-field simulation of the multi-direction forgings were carried out. The strain distribution at each pass was investigated and the evolution of temperature, effective strain, effective strain rate, and grain size was acquired. The hot forging trials were carried out and three typical regions of forgings were taken to study the microstructure evolution. Detailed microstructure characterizations showed that the constructed parent austenite grain size of the forging in typical regions was slightly larger than the simulation results due to the grain coarsening during the air cooling. There were large amounts of high angle grain boundaries (HAGBs) for the occurrence of complete dynamic recrystallization and many bulging grain boundaries showed that discontinuous dynamic recrystallization (DDRX) could be the governing mechanism of nucleation and growth of dynamic recrystallization (DRX). Besides, the hot deformation texture changed significantly during the non-isothermal forging and the texture component differed remarkably at different regions of the forging. The main hot deformation texture components were Cube {001}<100> and Goss{011}<100>.

Discontinuous lath martensite transformation and its relationship with annealing twin of parent austenite and cooling rate in low carbon RAFM steel

Lath martensite transformation in low carbon steel relates to the discontinuity of the transformation rate, which exhibits a series of martensite transformation rate maxima (peaks), especially in low cooling rates. In the present work, we put an emphasis on the influence of annealing twins in parent austenite grains (PAGs) and cooling rates on the discontinuity of lath martensite transformation rate in a low carbon reduced activation ferritic/martensitic (RAFM) steel. Lath martensite microstructure is hierarchical and the lath martensite transformation rate peaks are hierarchical during the cooling process. If the cooling rate was not high enough, the lath martensite transformation rate peaks will be divided into two groups by annealing twins in parent austenite. These transformation rate peaks in the first group were mainly caused by lath martensite transformation in the non–twin zones in the PAGs. Then, the second group mainly consisted of a single transformation rate peak, which was primarily caused by the martensite transformation in the twin zones in the PAGs. In the present work, a new “localized stress field” theory is proposed and successfully used for interpreting the classification mechanism of lath martensite transformation rate peaks and the evolution of these peaks with changing cooling rate.

The role of grain-boundary cementite in bainite formation in high-carbon steels

Low temperature bainite formation in high-carbon steels leads to highly refined microstructures. However, the rate of bainite formation becomes impractically low as the bainite formation temperature decreases. Thus, it is important to create strategies to accelerate the low-temperature bainite formation kinetics in high-carbon steels. In this work, it is observed that decorating the parent austenite grain boundaries with grain-boundary cementite prior to bainite formation leads to an appreciable acceleration of subsequent bainite formation kinetics due to decrease in activation energy for bainite nucleation at cemenite/austenite interfaces.