Transformation In Low Carbon Steel Research Articles

In situ observation of the austenite to ferrite transformation in low-carbon steels from different initial phases at defined cooling rates

An experimental investigation has been conducted with respect to the decomposition of austenite to various ferritic products from different initial phase states (molten-state of 1550 °C and solid-state of 1050 °C) and pre-defined cooling rates (1 °C/s and 10 °C/s) in a typical low-carbon steel using the confocal laser scanning microscope (CLSM). Surface morphology, microstructure, internal stress and dislocation density were analyzed. Changing the initial states from the molten to the solid state and lowering the cooling rate from 10 °C/s to 1 °C/s increased the phase transformation starting temperature. Compared with the uniform cooling condition to produce polygonal ferrite of the solid-state sample, unevenly distributed internal stresses were found from the initial molten-state sample. Additionally, high dislocation densities were observed in stress concentrated areas of the initial molten-state samples resulting in the formation of lath-like ferrite but coarse quasi-polygonal ferrite in relatively stress-free areas. Moreover, when the cooling rate was increased to 10 °C/s, the proportion of lath-like ferrite obviously increased in the molten-state and the size of the polygonal ferrite decreased in the solid-state sample due to the increasing of nucleation rate from austenite to ferrite.

Kinetic Model of Isothermal Bainitic Transformation of Low Carbon Steels under Ausforming Conditions

Carbide-free bainitic steels show attractive mechanical properties but are difficult to process because of the sluggish phase transformation kinetics. A macroscopic model based on the classical nucleation theory in conjunction with the modified Koistinen–Marburger relationship is proposed in this study to simulate the kinetics of incomplete bainitic and martensitic phase transformations with and without austenite deformation. A 0.26C-1Si-1.5Mn-1Cr-1Ni-0.003B-0.03Ti steel and a 0.18C-1Si-2.5Mn-0.2Cr-0.2Ni-0.02B-0.03Ti steel were investigated with different levels of ausforming. The concept of ausforming is expected to accelerate the onset of the bainitic transformation and to enhance the thermodynamic stability of austenite by increased dislocation density. The phase transformation kinetics of both steels is quantitatively analyzed in the study by dilatometry and X-ray diffraction so that the carbon concentration in the retained austenite and bainitic ferrite, as well as their volume fractions, is determined. A critical comparison of the numerical and experimental data demonstrates that the isothermal kinetics of bainite formation and the variation of driving energy can be satisfactorily described by the developed model. This model captures the incompleteness of the bainite phase transformation and the carbon enrichment in the austenite well. A fitting parameter can be used to elucidate the initial energy barrier caused by the ausforming. An increase in austenite stability can be described by the nucleation reaction and the thermodynamic energies associated with the change of dislocation density. The proposed model provides an in-depth understanding of the effect of ausforming on the transformation kinetics under different low-carbon steels and is a potential tool for the future design of heat treatment processes and alloys.

The Role of Parent Phase Topology in Double Young–Kurdjumow–Sachs Variant Selection during Phase Transformation in Low-Carbon Steels

The present paper investigates the role of parent phase topology on a crystallographic variant selection rule. This rule assumes that product phase nuclei appear at certain grain boundaries in the parent structure, such that a specific crystallographic orientation relationship is observed with both parent grains at either side of the grain boundary. The specific crystallographic orientation correspondence considered here is the Young–Kurdjumow–Sachs (YKS) orientation relationship <112>90° (which exhibits 24 symmetrical equivalents). The aforementioned relationship is characteristic of phase transformations in low-carbon steel grades. It is shown that, for different parent phase textures, ~20% of the grain boundaries comply with the double YKS condition allowing for a tolerance of 5°, ignoring the presence of topology in the parent phase microstructure. The presented model allows for connecting the presence of a specific parent phase topology with the condition of the double YKS variant selection rule in a number of practical cases: (i) for hot rolled Ti–Interstitial Free (IF) steel with and without Mn addition, (ii) for cold rolled IF steel exhibiting very strong texture memory after forward and reverse α ⇌ γ phase transformation and (iii) for a martensitic transformation in a Fe–8.5% Cr steel. It is shown that the double YKS variant selection criterion may explain several specific features of the observed transformation textures, while assuming a non-correlated arbitrary pair topology of the parent austenite structure (implying that for N parent orientations N/2 pairs are selected in an arbitrary manner).

Effect of electropulsing on austenite to ferrite transformation in low-carbon steel

We explore the application of electropulsing for the control of phase transformation in a low-carbon steel. The effect of electropulsing on the transformation from austenite to ferrite in continuous casting low-carbon steel slab has been studied through experimental and thermodynamic investigations. The results reveal that electropulsing promotes the precipitation of ferrite within austenite grain in the low-carbon steel. When the electropulsing intensity increases, the precipitation of ferrite increases within austenite grains, and ferrite omentum precipitates become thinner along the austenite grain boundary. The results provide a basis for controlling the austenite to ferrite transformation in low-carbon steel.

Three-dimensional EBSD Analysis and TEM Observation for Interface Microstructure during Reverse Phase Transformation in Low Carbon Steels

Three-dimensional EBSD Analysis and TEM Observation for Interface Microstructure during Reverse Phase Transformation in Low Carbon Steels

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.

Phase field modelling allotropic transformation of solid solution

Based on multiphase field conception and integrated with the idea of vectorvalued phase field, a phase field model for typical allotropic transformation of solid solution is proposed. The model takes the non-uniform distribution of grain boundaries of parent phase and crystal orientation into account in proper way, as being illustrated by the simulation of austenite to ferrite transformation in low carbon steel. It is found that the misorientation dependent grain boundary mobility shows strong influence on the formation of ferrite morphology comparing with the weak effect exerted by misorientation dependent grain boundary energy. The evolution of various types of grain boundaries are quantitatively characterized in terms of its respective grain boundary energy dissipation. The simulated ferrite fraction agrees well with the expectation from phase diagram, which verifies this model. (Less)

Hardness testing and why it matters

In this work we concentrate on the in situ dynamics of interphase boundary motion during transformations. In-situ high temperature electron-back scatter diffraction (HT EBSD) was employed to study the ferrite-austenite-ferrite transformation in low carbon steel. A novel method was designed to derive the velocity of the interphase boundaries from the EBSD phase maps. It is concluded that the motion of the transformation front occurs in a jerky-type motion, i.e. not continuous in time on a microscale, as the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation would predict on the macro-scale. It is shown that HT EBSD is capable of providing relevant supplementary insight in the ferrite-austenite phase transformations with adequate spatiotemporal resolution, which would remain hidden in volume averaging experimental techniques such as X-Ray diffraction. In particular, ferrite-austenite phase boundary velocities between 1.4 ± 0.3 and 4.0 ± 0.2 nm/s were detected during isochronal heating with 0.5 °C/min. The mean interphase boundary velocity was ranging between 5.0 ± 0.2 and 6.4 ± 0.2 nm/s for austenite-ferrite transformation with a cooling rate of 1 °C/min. Ledge growth at isothermal conditions, resulted in velocity of 23 nm/s along phase boundary. A strong dependency of interphase boundary mobility and parent-daughter phase boundary misorientation angles was not found.

The role of the austenite grain size in the martensitic transformation in low carbon steels

There is sufficient experimental evidence to propose that the formation kinetics of athermal martensite directly depends on the austenite grain structure from which the martensite forms. Yet, this dependence is frequently ignored. The present study investigates the role of the prior austenite grain size (PAGS) in the martensitic transformation in low-carbon steels. The transformation kinetics was experimentally studied for PAGS in the range from 6 to 185 μm and theoretically analysed based on the nucleation rate and the thermodynamic balance between the chemical driving force and the resistance exerted by the austenite against the progress of the transformation. It is observed that grain refinement shifts the martensite start temperature (MS) to lower values and accelerates the transformation rate at initial stages. At a later stage, when approximately 30% martensite has formed, the transformation rate decreases rapidly for small PAGS, whereas higher rates are maintained in coarse-grained microstructures. The change in martensite formation rate with the grain size depends on the nuclei density and on the austenite strength. This research enables an optimised selection of processing parameters for the design of ultra-high strength steels that require the formation of a controlled fraction of martensite.

Exploring the Difference in Bainite Transformation with Varying the Prior Austenite Grain Size in Low Carbon Steel

The simulation welding thermal cycle technique was employed to generate different sizes of prior austenite grains. Dilatometry tests, in situ laser scanning confocal microscopy, and transmission electron microscopy were used to investigate the role of prior austenite grain size on bainite transformation in low carbon steel. The bainite start transformation (Bs) temperature was reduced by fine austenite grains (lowered by about 30 °C under the experimental conditions). Through careful microstructural observation, it can be found that, besides the Hall–Petch strengthening effect, the carbon segregation at the fine austenite grain boundaries is probably another factor that decreases the Bs temperature as a result of the increase in interfacial energy of nucleation. At the early stage of the transformation, the bainite laths nucleate near to the grain boundaries and grow in a “side-by-side” mode in fine austenite grains, whereas in coarse austenite grains, the sympathetic nucleation at the broad side of the pre-existing laths causes the distribution of bainitic ferrite packets to be interlocked.

Predicting the cooperative effect of Mn–Si and Mn–Mo on the incomplete bainite formation in quaternary Fe–C alloys

ABSTRACTPredicting the effect of alloying elements on the degree of incomplete austenite to bainite transformation in low carbon steels is of great industrial importance. This study introduces an extended Gibbs energy balance model which makes use of an additive approach to calculate the coupled effect of Mn, Si and Mo on the fraction of bainitic ferrite after the incomplete transformation in multicomponent steels. The model predicts significant effects of Mn and Mo and the negligible effect of Si levels on the fraction of bainitic ferrite. This is attributed to the high value of dissipation of Gibbs energy caused by interfacial diffusion of Mn and Mo and low values caused by Si diffusion. The model predictions for quaternary Fe–C–Mn–Si system are comparable with the experimentally measured values of bainite fraction. For the Fe–C–Mn–Mo system, the agreement is less accurate and the accuracy decreases with increasing Mo content, which is attributed a substantial carbide formation but interaction effects between Mn and Mo or a temperature dependent binding energy cannot be ruled out.

Three-dimensional EBSD Analysis and TEM Observation for Interface Microstructure during Reverse Phase Transformation in Low Carbon Steels

Three-dimensional EBSD Analysis and TEM Observation for Interface Microstructure during Reverse Phase Transformation in Low Carbon Steels

Interphase boundary motion elucidated through in-situ high temperature electron back-scatter diffraction

In this work we concentrate on the in situ dynamics of interphase boundary motion during transformations. In-situ high temperature electron-back scatter diffraction (HT EBSD) was employed to study the ferrite-austenite-ferrite transformation in low carbon steel. A novel method was designed to derive the velocity of the interphase boundaries from the EBSD phase maps. It is concluded that the motion of the transformation front occurs in a jerky-type motion, i.e. not continuous in time on a microscale, as the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation would predict on the macro-scale. It is shown that HT EBSD is capable of providing relevant supplementary insight in the ferrite-austenite phase transformations with adequate spatiotemporal resolution, which would remain hidden in volume averaging experimental techniques such as X-Ray diffraction. In particular, ferrite-austenite phase boundary velocities between 1.4±0.3 and 4.0±0.2nm/s were detected during isochronal heating with 0.5°C/min. The mean interphase boundary velocity was ranging between 5.0±0.2 and 6.4±0.2nm/s for austenite-ferrite transformation with a cooling rate of 1°C/min. Ledge growth at isothermal conditions, resulted in velocity of 23nm/s along phase boundary. A strong dependency of interphase boundary mobility and parent-daughter phase boundary misorientation angles was not found.

Microstructural evolution during ultra-rapid annealing of severely deformed low-carbon steel: strain, temperature, and heating rate effects

An interaction between ferrite recrystallization and austenite transformation in low-carbon steel occurs when recrystallization is delayed until the intercritical temperature range by employing high heating rate. The kinetics of recrystallization and transformation is affected by high heating rate and such an interaction. In this study, different levels of strain are applied to low-carbon steel using a severe plastic deformation method. Then, ultra-rapid annealing is performed at different heating rates of 200–1100°C/s and peak temperatures of near critical temperature. Five regimes are proposed to investigate the effects of heating rate, strain, and temperature on the interaction between recrystallization and transformation. The microstructural evolution of severely deformed low-carbon steel after ultra-rapid annealing is investigated based on the proposed regimes. Regarding the intensity and start temperature of the interaction, different microstructures consisting of ferrite and pearlite/martensite are formed. It is found that when the interaction is strong, the microstructure is refined because of the high kinetics of transformation and recrystallization. Moreover, strain shifts an interaction zone to a relatively higher heating rate. Therefore, severely deformed steel should be heated at relatively higher heating rates for it to undergo a strong interaction.

Influence of zirconium on mechanical properties and phase transformation in low carbon steel

The influence of zirconium on the mechanical properties and phase transformation was investigated in low carbon steel. First, the steels are subjected to a special thermomechanical regime, and the hot rolled plates were used to characterise the tensile properties and impact toughness. Second, the phase transformation behaviour of the steels with various Zr contents was evaluated by both dilatometry and metallography. Finally, to confirm the existence of Zr containing precipitates in the Zr added steels, transmission electron microscopy and energy dispersive spectroscopy were used. It was verified that plenty of fine spherical (Nb,Ti,Zr)C, which is identified to be nearly 10 nm, can be formed when the concentration of Zr is in the range of 0.015–0.030%. The effects of zirconium on the phase transformation, including proeutectoid ferrite and pearlite transformation, and mechanical properties evolution were also identified and discussed.

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

Characterization of coarse bainite transformation in low carbon steel during simulated welding thermal cycles

Coarse austenite to bainite transformation in low carbon steel under simulated welding thermal cycles was morphologically and crystallographically characterized by means of optical microscope, transmission electron microscope and electron backscattered diffraction technology. The results showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to granular bainite with the increase in cooling time. The width of bainitic laths also increases gradually with the cooling time. For a welding thermal cycle with relatively short cooling time (e.g. t8/5 is 30s), the main mode of variant grouping at the scale of individual prior austenite grains changes from Bain grouping to close-packed plane grouping with the progress of phase transformation, which results in inhomogeneous distribution of high angle boundaries. As the cooling time is increased, the Bain grouping of variants becomes predominant mode, which enlarges the effective grain size of product phase.

Cellular automaton simulation of ferrite-austenite transformation in low-carbon steels

A two-dimensional (2-D) cellular automaton (CA) model is adopted to simulate ferrite (α)-austenite (γ) transformation in low-carbon steels. In the model, the preferential nucleation sites of austenite, the driving force of phase transformation coupled with thermodynamic parameters, solute partition at the α/γ interface, and carbon diffusion in both the α and γ phases are taken into consideration. The model is able to simulate the ferrite-to- austenite transformation during isothermal heating in the ferrite-austenite two-phase region, and the austenite-to-ferrite transformation during continuous cooling. The influences of cooling rate and α grain size on the final carbon concentration field are investigated. The results show that after isothermal heating at 815°C for 300 s, the carbon concentration in both the α and γ phases reach the respective equilibrium values. The simulated microstructures compare well with the SEM images. After cooling to room temperature, the carbon distribution is more uniform when cooled at 1.2°C/s than at 7°C/s. The carbon distributions for different α grain sizes cooled at 1.2°C/s are similar. The simulation results are used to understand the mechanisms of the experimental phenomena of an enameling steel.

Phase-field Simulation of Habit Plane Formation during Martensitic Transformation in Low-carbon Steels

The origin of the habit plane of the martensite phase (α′) in low-carbon steels is elucidated by three-dimensional phase-field simulations. The cubic → tetragonal martensitic transformation and the evolution of dislocations with Burgers vector aα′/2〈111〉α′ in the evolving α′ phase are modeled simultaneously. By assuming a static defect in the undercooled parent phase (γ), we simulate the heterogeneous nucleation in the martensitic transformation. The transformation progresses with the formation of the stress-accommodating cluster composed of the three tetragonal domains of the α′ phase. With the growth of the α′ phase, the habit plane of the martensitic cluster emerges near the (111)γ plane, whereas it is not observed in the simulation in which the slip in the α′ phase is not considered. We observed that the formation of the (111)γ habit plane, which is characteristic of the lath martensite that contains a high dislocation density, is attributable to the slip in the α′ phase during the martensitic transformation.

The morphology of lath martensite: a new perspective

A mathematical framework is proposed to predict the features of the (5 5 7) lath transformation in low-carbon steels based on energy minimisation. This theory generates a one-parameter family of possible habit planes and a selection mechanism then identifies the (5 5 7) normals as those arising from a deformation with small atomic movement and maximal compatibility. While the calculations bear some resemblance to those of double shear theories, the assumptions and conclusions are different. Interestingly, the predicted microstructure morphology resembles that of plate martensite, in the sense that a type of twinning mechanism is involved.