Austenite Transformation Kinetics Research Articles

Unlocking the Mechanisms behind Austenite Formation of Cold‐Rolled Ferrite–Martensite Dual‐Phase Steels: The Role of Ferrite Recrystallization

In this study, the austenite formation behavior is investigated for the ferrite–pearlite initial microstructure, considering the influence of ferrite recrystallization and cementite distribution. Special attention is also given to the effect of recrystallization state on cementite distribution for varied heating routes. Austenite nucleation for varied recrystallization state is elucidated in terms of activation energy evaluation and post microstructural characterization, and the austenite transformation kinetics are modeled and experimentally validated. In the uncrystallized state, austenite forms on the original pearlite colonies quickly due to the short carbon diffusion path. The high activation energy leads to the exceptionally difficult nucleation at the ferrite–cementite interface inside ferrite matrix. Once recrystallization occurs partially, priority of austenite nucleation is on those ferrite boundaries (both recrystallized and unrecrystallized boundaries) decorated by cementite. With complete recrystallization, preferential nucleation sites are found to be junctions of ferrite–cementite interface on recrystallized ferrite boundaries. Austenite transformation kinetics is well predicted by considering the varied nucleation density due to different recrystallization state, showing a good agreement with experimental data. It is demonstrated directly by experimental evidence that proceeding of recrystallization postpones the austenite transformation kinetics, while getting rid of the interference of heating rate and nucleation location.

Study on Austenite Transformation and Growth Evolution of HSLA Steel.

HSLA steel is widely used in various applications for its excellent mechanical properties. The evolution of austenite transformation and growth has been systematically studied in HSLA steel Q960 during the heating process. A thermal expansion instrument and optical microscope were adopted to analyze the kinetics of austenite transformation, which is a nonlinear continuous process and was accurately calculated by the lever rule based on the dilatation curve at the holding time within 10 min. The austenite growth behavior at temperatures above Ac3 was explored using TEM and DSC. The main precipitates in austenite were Nb-rich and Ti-rich (Nb, Ti)(C, N), and the particle size increased and amount decreased with the increase in the heating temperature, which resulted in the rapid growth of austenite. With the increase in holding temperature and time, the growth of austenite progressed through three stages, and a heat treatment diagram was established to describe this evolution.

Effect of Heating Rate on Martensite to Austenite Transformation Kinetics in Supermartensitic Stainless Steel Weld Deposit

Effect of Heating Rate on Martensite to Austenite Transformation Kinetics in Supermartensitic Stainless Steel Weld Deposit

In Situ Synchrotron X-ray Diffraction and Microstructural Studies on Cold and Hot Stamping Combined with Quenching & Partitioning Processing for Development of Third-Generation Advanced High Strength Steels

A novel combined process of Cold Stamping (CS) and Hot Stamping (HS) with Quenching and Partitioning (Q&P) treatment applied to advanced TRIP-assisted steel has been conducted by thermomechanical simulation to evaluate the influence of CS or HS in the Q&P processing. With this purpose, Q&P, CSQ&P, and HSQ&P cycles were designed to obtain multiphase microstructures containing ferrite, martensite, bainitic-ferrite, and the maximum retained austenite (RA) fraction after the processes. The objective was to investigate the effects of the variables involving the heat treatments, such as the intercritical austenitization temperature, the isothermal and non-isothermal deformation, the amount of deformation, and the temperature and partitioning times, and to analyze their influence on the microstructural and mechanical responses. Time-resolved X-ray diffraction using synchrotron radiation was undertaken in a thermomechanical simulator coupled to the synchrotron light source to understand the influence of time, temperature, and strain on the level of carbon enrichment in austenite. In addition, the in situ austenite transformation kinetics and lattice parameter evolution were tracked, making it possible to optimize the RA fraction at room temperature after Q&P processing. The newly developed combined process is promising as the transformation-induced plasticity phenomenon during deformation can contribute to the formability and energy absorption. The results also indicate that the deformation of austenite promotes the ferrite transformation while suppressing the bainite transformation. It was possible to plot the results in an elongation-mechanical strength diagram, coupled to material property charts, also known as, ‘banana curve’, allowing us to identify and correlate the thermal or thermomechanical treatment conditions that led to an increase in ductility or strength according to the volume fractions of the resulting phases. Comparing the results for the HSQ&P treatments, it was observed that isothermal strains at higher temperatures (≥800 °C) are more advantageous to increase mechanical strength, while non-isothermal strains (starting at 750 °C) are suggested if the objective is the increase in ductility, with mechanical strength being slightly sacrificed.

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.

Gibbs energy minimisation model for the austenite-ferrite phase transformation in Fe-C-X-Y alloys

ABSTRACT A new model has been developed to predict austenite ferrite transformation kinetics in steels. For each alloying element, the concentration profile is computed solving a unique diffusion equation (including the 2 phases and the interface). The interface is described assuming linear variation of chemical potentials, saving thus computational time. Interface motion is driven by the minimisation of Gibbs energy. The model naturally reproduces the transition between thermodynamic equilibria (Para equilibrium, Local equilibrium with negligible partitioning, Local equilibrium) during heating. The validity of the model for reverse transformation has been validated on ternary and quaternary systems Fe-C-(Mn-Si-Mo) on decarburisation experiments.

Effect of Starting Microstructures on the Reverse Transformation Kinetics in Low-Carbon Steel

The effect of the initial starting microstructures on the austenite reverse transformation kinetics is thoroughly studied in low-carbon steel. The different initial starting microstructures including the ferrite + pearlite, bainite, and martensite are obtained through varied forward transformation. It is found that the bainite phase demonstrates highest reverse transformation rate while the ferrite + pearlite shows the lowest transformation rate. The above observation can be explained through the different grain size of the initial starting microstructures as the grain boundaries could act as the nucleation sites for austenite reverse transformation. The explanation is further substantiated based on the consideration of the reverse transformation kinetics from the martensite microstructure with different grain size.

Phase Transformation Kinetics of Austenite during a Combination of Laser-Hybrid and Multi-Arc Welding of High Pressure Pipes

This study investigated the phase transformation kinetics of austenite in the heat-affected zone of the joints welded by laser-hybrid and submerged multi-arc welding. Optimum cooling rates to form favorable structural-phase composition in the heat-affected zone were found.

Austenite Transformation Behavior and Mechanical Properties of Constructional V, Nb-Alloyed TRIP-Assisted Steel

The present article is aimed at studying the austenite transformation kinetics and tensile properties of constructional 0.2 wt%C-Si2Mn2CrMoVNb TRIP-assisted steel subjected to isothermal holding in the subcritical temperature range (350-650 °C with the step of 50 °C) after intercritical annealing at 770 °C. The study was fulfilled using optical microscopy (OLYMPUS GX-71), electron scanning microscopy (JEOL JSM-), dilatometric analysis, tensile testing, Vickers hardness measurements. The critical temperatures of the steel were found to be Ac1=750-760 °C and Ac3=930 °C. The results showed that austenite demonstrated increased stability to pearlite and bainite transformations with an incubation period of decades of seconds at any of the mentioned temperatures. The bainitizing treatment at 400 °C with holding of 300-600 s resulted in ferrite/bainite/retained austenite structure with precipitates of nanosized carbide (V,Nb)C providing an improved combination of mechanical properties as compared to direct quenching (YS=548-555 MPa, UTS=908-1000 MPa, total elongation=16-18 %, PSE index=14.6-18.0 GPa%, YS/UTS ratio=0.55-0.60). The contributions of different strengthening components were estimated in order to reveal the benefits of a multi-phase microstructure for constructional applications.

Metastable Austenite Transformation Kinetics of Medium-Carbon Silicon-Rich Steel during Partitioning in a Q & P Process

In the present study, quenching and partitioning (Q & P) treatment of a medium-carbon silicon-rich steel was processed by a dilatometer. The volume fraction of the retained austenite at different partitioning times was determined by X-ray diffraction (XRD). The metastable austenite transformation process after different partitioning times was studied by a combination of dilatometry, XRD, and transmission electron microscopy (TEM). Analysis of the transformation kinetics of metastable austenite during partitioning by means of dilatometry and the Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation was carried out. The results show that the volume fraction of retained austenite increases first and then decreases with the increase of the partitioning time. The transformation active energy Q = 141 kJ·mol−1 and the Avrami exponent n = 1. The transformation product of metastable austenite is needle-like bainite, which grows perpendicularly towards the boundary of the austenite. Finally, the blocky austenite was divided into lath-shaped forms.

Kinetics modeling for austenite transformation in AISI 1045 steel during rapid heating under high frequency electromagnetic field

To investigate the effect of alternating magnetic field on austenite transformation process in the case of rapid heating, the austenite kinetics model of AISI 1045 steel was built for spot continual induction hardening (SCIH) process. The results shows that the effect of alternating magnetic field on austenite transformation fraction reaches the maximum (about 3%) when heating rate is the lowest. Relatively low magnetic flux density still has a certain effect on the austenite transformation process during the SCIH process. Concave surface structure can reduce the influence scope of alternating magnetic field on surface in all cases and the minimum influence scope appears when the feed path of inductor is longitudinal. Convex surface structure can minimize the influence scope of alternating magnetic field in depth when the feed path of inductor is longitudinal. The austenite distribution of transitional region on surface for horizontal movement is more uniform than that for longitudinal movement. The austenite distribution of transitional region in depth for longitudinal movement is more uniform than that for horizontal movement. The simulated results are consistent with the experimental results and the austenite transformation kinetics model developed for SCIH process is valid.

Composition and processing design of medium-Mn steels based on CALPHAD, SFE modeling, and genetic optimization

Medium-Mn steels are being thoroughly investigated as potential candidates for the 3rd generation of advanced high strength steels. Despite the wide experimental work, limited attempts have been presented to systematically optimize the chemical composition and heat treatment process to obtain desired microstructural features. In the present work, CALPHAD-based thermodynamic and kinetic modeling, coupled with multi-objective genetic optimization was adopted for the development of δ-ferrite containing medium-Mn steels with optimized microstructure, meeting set design requirements associated with retained austenite fraction and stability. A new sub-regular solution model for the prediction of the austenite stacking fault energy (SFE) was developed and compared to experimental literature data. A MATLAB implementation of the SFE model is provided as supplementary material. Pareto optimal compositions and associated process windows were identified via thermodynamic modeling coupled with the NSGA-II algorithm. A single optimized steel was selected for further consideration through kinetic simulation of the entire process chain including solidification, hot-rolling, accelerated cooling, quenching, and intercritical annealing, considering the effect of δ-ferrite on retained austenite stability and the martensite to austenite transformation kinetics. Temporal optimization resulted in the selection of an optimal intercritical annealing time. Model predictions were validated with metallographic observations on two different δ-ferrite containing medium-Mn steels, revealing excellent agreement between predicted and observed phase fractions.

Influence of initial microstructures on intercritical annealing behaviour in a medium Mn steel

ABSTRACTThe present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 900°C. The specimen austenitised at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitisation temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to a difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at an early stage of intercritical annealing but retards the transformation kinetics afterwards.This paper is part of a Thematic Issue on Medium Manganese Steels.

Structure-properties relationship of ultra-fine grained V-microalloyed dual phase steels

The effect of vanadium microalloying on ultra-high strength dual phase (DP) ferrite-martensite steel microstructure and properties was studied. It was found that the addition of 0.14wt% V to a Fe-0.18C-1.5Mn-0.3Si-0.008N reference alloy introduced very significant ferrite grain size refinement in the cold rolled and annealed state. During continuous annealing the initial ferrite to austenite transformation kinetics were strongly retarded, however under slow cooling both pearlite and bainite transformations were suppressed indicating increased hardenability. After cold rolling and intercritical annealing at 750⁰C intense V(C,N) precipitates (mean radius 3.7nm) were observed in the ferrite phase whereas precipitates were scarce in martensite (austenite) and much larger (mean radius 6.7nm). Significant gains in YS, UTS and work hardening rate were observed at low martensite fractions due to a combination of selective precipitation strengthening and grain refinement of ferrite. However, at higher martensite fractions (> 45%) the YS, UTS and work hardening rate became lower than the reference, primarily due to softening of the martensite. The latter was attributed to the fixing of solute carbon by V(C,N). The net increase in tensile strength with martensite content of the vanadium alloy was ~ 4MPa/%α’ compared to ~ 16MPa/%α’ for the reference alloy. A recently developed size-sensitive mean field structure-properties model was extended to capture these microalloying effects. At iso-tensile strength both the fracture strain and hole expansion behaviour of the new microalloyed steel showed improved performance over the reference.

A simulation study of pearlite-to-austenite transformation kinetics in rapidly heated hot-rolled low carbon steel

The main aim of the present research was to obtain an optimized microstructure with adequate mechanical properties in a low carbon steel. The effect of microstructure on kinetics of austenite transformation was simulated. A 3.2mm hot rolled steel was subjected to continuous annealing to obtain properties of Dual Phase 590 grade. Kinetics of austenite transformation was studied with respect to the condition of just pearlite dissolution to form austenite under rapid heating. Annealing parameters were based on process conditions of dual phase steel production in a continuous annealing line. DICTRA was used to simulate heating rates of the order 10–500°C/s with peak temperatures in the range 750–850°C to predict isothermal annealing time required for complete dissolution of pearlite into austenite under different temperature-heating rate conditions. Simulation results showed dependency of temperature and heating rate on austenite transformation time. Interestingly, no significant effect of heating rate on complete pearlite dissolution into austenite was evident. Results were validated with limited experimentation on Gleeble. Microstructure analysis validated the simulation results to be accurate. The observations have pertinent inputs while designing industrial continuous annealing line parameters where rapid heating rates are generally encountered (10–20°C/s).

Effect of Low-Carbon Steel 20GFL Heat Treatment Regime on Structure and Mechanical Properties

Results are given for a study of the effect of heat treatment parameters on supercooled austenite transformation kinetics in steel 20GFL. Specimen microstructure is studied in specimens 10 × 10 mm in cross section with the following regimes: soaking at 860 and 940°C, cooling in an air stream 3–8 m/sec through a channel 100 mm in diameter. The change in microstructure features in the course of high-temperature soaking at 940°C for 30 and 60 min after specimen cooling in salt water is determined. It is established that in cast steel there is chemical composition microliquation inhomogeneity with respect to carbon and manganese, which decreases during soaking (60 min) and is accompanied by austenite grain growth. Critical cooling rates are determined with which a structure forms containing upper and lower bainite. A dependence is obtained for impact strength KCV−60 on soaking temperature and time in the γ-region. The effect is demonstrated of alloying with vanadium on the possibility of stimulating the formation of globular lower bainite components.

Effect of metastable austenite on fracture resistance of quenched and partitioned (Q&P) sheet steels

The fracture resistance was comparatively characterized on two commercial sheet steel, quenched and partitioned (Q&P) steel and dual phase (DP) steel, with the same strength levels but differing by the phase constituents and by the ductility. In Q&P steel, transformation of martensite as well as its effect on fracture resistance by tension of double edge notched (DEN) plate specimens was investigated. The map of the distribution of transformation ratios measured locally around the notch was compared with the map of the plastic deformation zone computed by finite element simulations. The influence of stress triaxiality on austenite transformation kinetics was analyzed. Essential work of fracture of Q&P steel (290.9kJm−2) was relatively lower than that of DP steel (544.8kJm−2), however, it was found that the fracture resistance (work of fracture) of Q&P steel is effectively enhanced by the transformation of metastable austenite as stress triaxiality decreases. Through deduction, transformation of 8vol% of austenite (ΔUT) was proved to absorb 22%–35% of the increments of the work of fracture (ΔW).

Special features of transformation kinetics of austenite in heat treatment of stamped and welded components made of 10G2FBYu steel

The results of investigations of the special features of austenite transformation kinetics during heat treatment of welded joints in components made of 10G2FBYu steel are used to determine the conditions for producing stamped and welded connecting components with strengths similar to that of the parent metal.

AUSTENITE TRANSFORMING IN CONTINUOUS COOLING PROCESS UNDER DIFFUSION CONTROL MODEL

Austenite-ferrite transformation in low carbon steels has a fundamental role in phase transforma- tion and is industrial importance. The kinetics of austenite transformation can be described by the kinetics of aus- tenite-ferrite interface migration. Two classical models, the diffusion-controlled growth model and the interface- controlled model, can be used to describe the growth of proeutectoid ferrite during g→a isothermal transformation. The austenite transformation in continuous cooling process is more common in production. In continuous cooling process, the equilibrium carbon concentrations in austenite and ferrite change with temperature and the kinetics of austenite transformation is different from that in isothermal process. Based on the models for g→a isothermal trans- formation, a diffusion control model is established for the growth of proeutectoid ferrite during the decomposition of supersaturated austenite in continuous cooling process. The interface position of proeutectoid ferrite varying with temperature is described with the model. The soft impingement effect at the later stage of transformation is considered. The carbon concentration at the austenite side of interface is difficult to reach the equilibrium carbon concentration when the cooling rate is high. A parameter as the function of cooling rate is proposed to modify the carbon concentration at the austenite side of interface. The polynomial diffusion field approximation is assumed in front of the interface. Simulation is done by utilizing the model to analyze the growth of proeutectoid ferrite in con- tinuous cooling process with different bulk concentrations, austenite grain sizes and cooling rates. The interface po- sition of proeutectoid ferrite as a function of temperature or time is obtained under different cooling conditions. Al- so, carbon diffusion length at the austenite side of interface as a function of time and carbon profile as a function of

Decomposition modes of austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels

The present paper presents the results of an extensive electron microscopy investigation on the decomposition modes of high temperature austenite in 9Cr–W–V–Ta reduced activation ferritic–martensitic steels. Although the displacive martensitic transformation is predominant on austenitisation, low volume fraction of Fe rich M3C or M23C6 precipitates formed, when the tungsten content exceeded 1 wt-%. The compositional inhomogeneity introduced in the austenite by the nature, chemistry and kinetics of dissolution of the pre-existing carbides is dependent on the steel composition and austenitisation conditions. The extent of repartitioning of tungsten between M23C6 and ferrite largely influences the kinetics of austenite and martensite transformation, for the same austenitisation conditions. Supporting evidence from calorimetry analysis is also presented.