Isothermal Transformation Kinetics Research Articles

Effect of applied stress on bainite transformation, microstructure, and properties of 15CrMo steel

The effect of external stress on the isothermal bainite transformation kinetics and microstructure of 15CrMo steel was studied using dilatometer, optical microscopes (OM), electron back-scatter diffraction (EBSD), scanning electron microscopes (SEM) and X-ray diffraction (XRD). The results show that the additional mechanical driving force provided by the external stress promotes the transformation of bainite, resulting an increase volume fraction of bainite-ferrite (BF) and causing the BF laths to become slender. The application of external stress can also induce selective orientation during the bainite transformation process, and the orientation relationship between the prior austenite and the BF under stress is closer to the Kurdjumov-Sachs (K-S) relationship. There is also an incomplete transformation of bainite transformation under stress. At the same time, the Vickers hardness of 15CrMo steel would increase after applying external stress. The mechanical driving forces calculations demonstrate that when tensile stress and compressive stress are of similar magnitudes, they provide comparable mechanical driving forces. As a result, they exert similar effects on the kinetics of bainite transformation, leading to no discernible differences in the resulting bainite microstructure.

Effects of acicular ferrites on crystallography and incomplete transformation of low-carbon low-alloy steel

In this study, we investigated the effects of acicular ferrite (AF) on the crystallographic and incomplete transformation characteristics of low-carbon low-alloy steel. Intermediate annealing after austenitization induced a transformation yielding AFs, accelerating the isothermal transformation kinetics and regulating the microstructure. This process formed highly dense packet grain boundaries and improved the stability of untransformed austenite. AF formation increased the number of packet interfaces, increasing the crack propagation energy. The reduction and refinement of martensite/austenite (M/A) islands increased the crack nucleation energy. A displacive model was used to fit the transformation kinetics curve, indicating the significant effect of the nucleation rate on the isothermal transformation of the experimental steel at 450, 475, and 500 °C. The findings of this study indicate that nucleation regulation can accelerate transformation kinetics to strengthen and toughen bainitic structures.

Comparative Study of Quench and Partition Processes in High Si and High Al Steels

The quench and partition process is a means to develop third-generation high-strength steels using many possible process variants. In this work, two variants of quench and partitioning heat treatments, one-step and two-step, were carried out for high Si and high Al steel alloys. The kinetics of isothermal transformation occurring during the one-step quench and partition process were analysed using dilatometry. Experimental analysis revealed the swing-back phenomenon in high Si steel, and the transformation characteristics above and below the Ms temperature differed. The high Al alloy resulted in higher retained austenite (19%) compared to high Si steel (17%) during the one-step quench and partition process. Aluminium addition favoured bainite formation more than silicon addition. A comparison of two heat treatment variants shows the two-step quench and partition heat treatment seemed preferable as it produced more retained austenite (22%) in the high Si steel.

Uncovering the isothermal transformation dynamics and elemental diffusion behavior of reversed austenite in PH13-8Mo steel

The isothermal transformation kinetics and microstructural evolution of the reversed austenite significantly influence the properties of PH13-8Mo steel. However, the microscopic phase transformation behavior related to elemental diffusion during isothermal processes for PH13-8Mo steel needs further examination. Herein, we systematically investigate the isothermal transformation and elemental diffusion behavior of reversed austenite in PH13-8Mo steel. Phase transformation kinetic measurements unravel that the complete elimination of residual austenite can be achieved. Only hierarchical martensitic microstructure without austenite is detected after the solution-cooling treatment at 927 °C with a cooling rate of 10 °C/s, determining the critical martensite transformation start and finish temperature to be 176.6 °C, 11.6 °C. The isothermal kinetics of reverted austenite satisfy the Johnson-Mehl-Avrami relationship. The fraction of reversed austenite increases from 0.14 % at 510 °C to 8.73 % at 593 °C. Lath martensite boundaries with large kernel average misorientation values promote elemental diffusion at the interface. Further, the underlying behavior of elemental diffusion is elucidated during the isothermal aging process. The interfacial model indicates the growing width of the reversed austenite up to 100 nm and the compositional gradient formed by the elemental diffusion during aging for 0–5 h at 593 °C. The interdiffusion coefficient calculations interpret the diffusion direction of alloying element Ni from martensite to reversed austenite; Cr and Mo may form alloying carbides at the austenite-martensite interface; Al possesses the fastest diffusivity in martensite and austenite phases. The present work unveils the phase transformation kinetics of PH13-8Mo steel, achieving quantitative regulation for maraging stainless steel.

Analysis of the Kinetics of Isothermal Bainitic Transformation in Alloy Steels

Analysis of the Kinetics of Isothermal Bainitic Transformation in Alloy Steels

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.

Roles of cooling rate of undercooled austenite on isothermal transformation kinetics, microstructure, and impact toughness of bainitic steel

At present, the application of extra thick super bainite steel has attracted considerable attention. In this paper, medium carbon bainite steel is taken as the research object, and the effects of the cooling rate of undercooled austenite on the transformation kinetics, microstructure and mechanical properties of bainite steel are studied systematically. The results show that the incubation period of bainite shortens and the transformation kinetics accelerates with the decrease of the cooling rate of undercooled austenite. The segregation of solute atoms at a slow cooling rate shortens the incubation period. When bainite nucleates and grows by an autocatalytic method, the size of the untransformed austenite is large and the strain compatibility is strong, leading to low activation energy of its autocatalytic nucleation and accelerating the kinetics of bainite transformation. The size of block structure and the length of bainite sheaves decrease with the increase of cooling rate. The impact toughness of the sample with a cooling rate of 30 °C/s is twice as high as that of the sample with a cooling rate of 0.3 °C/s. As the smallest unit to control the toughness of bainite steel, block structure affects the impact toughness. The impact toughness of bainite increases with the decrease of its block size. The content of film-like retained austenite increases with the increase of cooling rate, which is also one of the factors of excellent impact toughness of bainite at high cooling rate.

Influence of Partial Replacement of Co with Cu on Isothermal Transformation Kinetics in Ferritic/Martensitic Heat‐Resistant Steel

This study examines the influence of partial replacement of Co with Cu on isothermal transformation kinetics in 10Cr‐based heat‐resistant steels. Two kinds of heat‐resistant steels, having similar chemical compositions except for Co and Cu contents, are prepared and isothermally heat‐treated at 710–790 °C up to 240 h after austenitization. During isothermal heat treatment, austenite is decomposed via two different reactions, γ → α + M23C6 and γ → α; the cellular precipitation (γ → α + M23C6) nucleates at prior austenite grain boundaries or the interior of prior austenite grains and grows toward the interior of austenite, followed by γ → α transformation originating from C depletion in the remaining austenite. A time–temperature–transformation diagram exhibits that the partial replacement of Co with Cu not only decelerates the isothermal transformation kinetics but also lowers the nose temperature of transformation. The calculation of the driving forces for the successive reactions reveals that the increase of Cu content predominantly reduces the driving force for both γ → α + M23C6 and γ → α reactions, contributing to the lower nose temperature and sluggish transformation kinetics as Co is partially replaced with Cu.

Revisiting high-temperature phase transition and magnetocaloric effect of LaFe11.6Si1.4 alloy

Sufficient understanding on the formation mechanism of the La(Fe,Si)13 phase (1:13 or τ1) in La–Fe–Si alloys would be helpful in optimizing the parameters for high-temperature annealing. In-situ annealing experiments of a centrifugally as-cast LaFe11.6Si1.4 plate were performed by using a differential scanning calorimeter (DSC) in the present study. We confirmed that the τ1 phase is resulted from a peritectic reaction between La-rich liquid phase and α(Fe) solid phase when isothermal annealing at 1423 K. A new finding of a eutectoid decomposition that may occur within the residual La-rich liquid phase upon cooling was revealed. The isothermal transformation kinetics of the τ1 phase in the as-cast plate followed a simplified model of diffusion-controlled continuous network growth. The isothermal entropy change (|ΔSM|) was calculated using the Maxwell equation on M−H data. The maximum |ΔSM| value reached 26.89 J/(kg·K) under 30 kOe in the LaFe11.6Si1.4 sample annealed at 1423 K for 120 min.

Isothermal crystallization behavior of undercooled liquid Pd40Cu30Ni10P20 in terms of crystal growth, overall volume crystallization kinetics and their relation to the viscosity temperature dependence

Abstract The crystallization behavior of undercooled liquid Pd40Cu30Ni10P20 metallic alloy has been studied with the aid of differential scanning calorimetry, optical metallography, and electron microscopy methods. The crystallization occurs as a single-stage process starting preferably on the surface of casting voids in the sample volume and on the outer sample cutting planes. The linear crystal growth velocity of crystallizing cells was measured at several constant temperatures in the range 590 –620 K. It is constant at constant temperatures and its temperature dependence is in good agreement with the theoretical concepts of Uhlmann using the temperature dependence of the viscosity of the alloy studied. A differential scanning calorimetry study of isothermal overall volume transformation kinetics of samples pre-annealed at 600 K for 600 s was carried out at several constant temperatures. The experimental data are interpreted on the basis of the equation of Kolmogorov-Avrami-Johnson-Mehl for the case of three-dimensional growth of a fixed number of pre-existing nuclei. The possibility to revitrify the crystallized samples by their melting in differential scanning calorimeter, followed by quick cooling with cooling rate of 5 K τ– 1 is shown. The volume transformation kinetics of revitrified samples has been also studied.

Current Status of Carbidic Austempered Ductile Iron

Grinding balls in wet ball mill are important consumables in mine grinding equipment, which have poor wear resistance and large consumption. It is imperative to find excellent wear-resistant materials for the grinding balls. Carbidic Austempered Ductile Iron (referred to as CADI) was used as small and medium-sized wet ball mills. This grinding ball has the advantages of less wear, low crushing rate, power saving and low noise. However, the CADI grain boundaries are distributed with net-like eutectic carbides, which seriously damage the continuity of the matrix. In addition, the mechanism of corrosion wear and impact fatigue is lack of research due to complex phase composition and unclear mechanism of phase properties on improving performance. So CADI can’t be applied to the grinding balls in large wet ball mill. Based on the above problems, this paper first analyzed the heteronucleation mechanism and adsorption mechanism of M3C type carbides by using the first principle of microalloying elements, and then verified it by combining with experimental results. Then the thermodynamics and kinetics of austenite homogenization and isothermal transformation of ductile iron containing carbides were analyzed by means of modeling calculation and experiment. On this basis, a new type heat treatment process comprising super-high temperature pretreatment and austempering treatment (S&A treatment) was used to process CADI, which provides a new idea for further improving toughness of CADI. Finally, the CADI corrosion wear and impact fatigue failure mechanism were revealed by analyzing the change rule of the sample surface and cross section after corrosion wear and impact fatigue.

Investigation of α phase evolution and tensile fracture behavior of Ti5Al2Sn2Zr4Mo4Cr alloy fabricated by directed energy deposition

α phase evolution and tensile fracture behavior of Ti5Al2Sn2Zr4Mo4Cr alloy fabricated by directed energy deposition (DED) were studied. Samples with a different number of layers were fabricated by the same processing parameters. Microstructure evolution was analyzed based on the thermal history calculated through finite element analysis and isothermal transformation kinetics. The results suggested that the samples exhibit various α phase characteristics. Two kinds of starting microstructures at different layers, single β phase and mixed α/β phases can be obtained depending on the deposited layer’s temperature and cooling rate. Further evolution of α phase is closely related to the consecutive thermal cycles with different durations and different temperature amplitudes. The in-situ scanning electron microscopy tensile tests were performed in the horizontal (perpendicular to the build direction) and vertical (parallel to the build direction) directions under the as-deposited and heat treatment conditions. The results showed that the cracks and micro-voids usually initiate on the grain boundaries or α/β interface. α phase characteristic plays an important role in deformation behavior. The sample containing fine α laths usually fractured quickly after the first surface crack was observed. The coarse α laths resulted in a larger crack tip plastic zone and delayed the crack propagation.

Isothermal transformation kinetics, microstructure and mechanical properties of a carbide free bainitic steel

The isothermal transformation kinetics, microstructure, and mechanical properties of a 0.23C high silicon steel, which is isothermally treated both below and above the martensite start temperature (Ms) are investigated. Results show that the small amount of athermal martensite (AM) in below Ms treated sample gives the additional nucleation sites for bainitic transformation, which accelerates the kinetics of bainitic transformation. The nucleation rate accelerates by two orders of magnitude at below Ms compared to above Ms. The microstructural analysis identifies the isothermal decomposition product formed in below and above Ms treated sample as bainitic ferrite. Furthermore, as compared to above Ms, below Ms isothermal treatment shows higher stability of austenite, giving rise to much improved ductility. Moreover, the below Ms isothermal treatment refines the bainitic lath more effectively.

Controlling the Content and Morphology of Phase Constituents in Nanobainitic Steel Containing 0.6%C to Obtain the Required Ratio of Strength to Plasticity

The phase composition of nanobainitic steel 0.56–0.60%C, 1.68–1.95%Mn, 1.58–1.80%Si, 1.30–1.47%Cr, 0.57–0.75%Mo is described in this paper. The phase composition is controlled in order to obtain diversified mechanical properties for specific applications, such as armor plates. The effect of temperature and time of isothermal heat treatment on both the microstructure and the mechanical properties of the steel were determined. Dilatometric studies, as well as measurements of volume fraction and size distribution of retained austenite were carried out. Analysis of the kinetics of isothermal transformation in the temperature range of 200–225 °C for times of up to 144 h were also carried out, and the parameters of the production process of the steel were determined. A microstructure consisting of nanolathy carbideless bainite and blocky and lathy retained austenite, providing tensile strength of at least 2000 MPa, yield strength of at least 1300 MPa, and total elongation of at least 10% has been found.

New insights into the effects of deformation below-MS on isothermal kinetics of bainitic transformation

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 Features of Bainite Transformation in Alloyed Steels

The main factors affecting the kinetics of isothermal bainite transformation in alloyed steels were determined using the algorithms of the computer modelling of phase transformations in the solid state. The experimental and calculated transformation kinetics was described and compared using well known Kolmogorov-Johnson-Mehl-Avrami (KJMA) equation and Austin-Rickett (AR) equation. The effect of the initial configuration and the nucleation rate of the new phase nuclei on the parameters of the bainite transformation was analysed. The calculated kinetics of phase transformation at different nucleation rates was compared with the experimentally observed for HY-TUF and D6AC steels. The mechanism of the bainite transformation in alloyed steels was explained.

Computer Simulation and Experimental Study of Isothermal Bainitic Transformation in Alloy Steels

The main factors responsible for the kinetics of isothermal bainitic transformation in alloy steels are determined using computer simulation of the solid-state phase transformation. The effect of the initial configuration of nuclei of the new phase in the volume of the metal on the parameters of the process of bainitic transformation is determined. Comparative analysis of the computed kinetics of phase transformation at different rates of nucleation of the new phase and of the experimentally observed bainite kinetics in steels 25G2S2N2MA, 300M and 50KhMFA is performed.

Transformation kinetics of carbide-free bainitic steels during isothermal holding above and below MS

The transformation kinetics of a low carbon bainitic steel was investigated using in situ confocal microscopy and dilatometry for isothermal treatments above and below the martensite start (MS) temperature. Both martensite and bainite are formed during isothermal holding below the MS, but bainite was the dominant transformation product. The prior martensite formed during cooling to the holding temperature strongly influenced the bainite precipitation kinetics. A critical volume fraction of prior martensite, fcPM, was identified. In order to accelerate the bainite transformation kinetics by austempering below the MS, the prior martensite fraction should be less than fcPM. The best combinations of strength and ductility were obtained in steels by austempering below the MS temperature with an optimum martensite fraction (∼5%). Finally, the isothermal bainite transformation kinetics is analyzed by Johnson-Mehl-Avrami-Kolgomorov (JMAK) kinetics model. The analysis suggests that the nucleation of bainite approaches site saturation when the isothermal holding temperature is below the MS.

Effect of ausforming on microstructure and hardness characteristics of bainitic steel

Effects of process parameters of the ausforming such as temperature, strain and strain rate on the martensitic start temperature, kinetics of isothermal bainitic transformation and microstructure refinement of a low carbon carbide-free bainitic steel were investigated. It was found that applying plastic deformation to untransformed austenite during intermediate temperatures decreased the martensite start temperature of steel and enabled isothermal bainitic transformation at low temperatures. Hereby, ausforming significantly generated heterogeneous nucleation sites, which accelerated the overall kinetics of bainitic transformation and thus increased bainitic phase fraction in steel. In addition, the ausforming enhanced the stability of austenite that led to reduced amount of martensite after cooling down to room temperature. Finally, the ausforming parameters and observed microstructure features were correlated and discussed along with the hardness of steel.

Thermodynamic analysis and isothermal bainitic transformation kinetics in lean medium-Mn steels

The work presents the results of thermodynamic analysis of two medium manganese steels with different Mn contents. The steels containing 3.1 and 3.6% of manganese were subjected to theoretical thermodynamic calculations using MUCG83 software and dilatometric experiments. The steels were heat-treated in two different isothermal holding temperatures of 400 and 350 °C for 15 min. The bainite transformation kinetics at different temperatures for different manganese contents was investigated. In the steel including 3.1% Mn, a complete transformation was obtained. The results indicated a strong influence of the holding temperature on the kinetics of bainitic transformation. It was related to the driving force of this process. When the manganese content was increased by 0.5%, an incomplete bainite transformation occurred. The microstructure investigations after heat treatment were performed using light and scanning electron microscopy. The XRD analysis to determine retained austenite amount and its carbon enrichment was performed. The microstructure of 3MnNb steel consisted of bainite and retained austenite with filmlike and blocky morphologies. The steel with the higher Mn content contained also fresh martensite for both isothermal holding temperatures.