Proeutectoid Ferrite Transformation Research Articles

Modeling of the austenite decomposition kinetics in a low-alloyed steel enriched in carbon and nitrogen

A model predicting the effect of carbon (0.1–0.7%mC) and nitrogen (0.3–0.4%mN) enrichment of a low-alloyed steel (23MnCrMo5) on its austenite decomposition kinetics during cooling is introduced here. Its practical purpose concerns the simulation of microstructure evolutions after surface treatments of carburizing, nitriding or carbonitriding. Effects of nitrogen in solid solution in austenite are considered for the first time. Regarding the carbon, previous approaches from literature which estimate IT C curves displacements along time and temperature axes are adapted. Phase transformation kinetics are predicted from phenomenological rules (JMAK, Koistinen–Marburger). However, the number of empirical model parameters is reduced to a minimum thanks to thermodynamic calculations. JMAK kinetic parameters have to be established in one single reference steel composition. The model gathers, in one comprehensive framework, several phenomena which are generally considered separately, such as, e.g., the austenite stabilization after the proeutectoid ferrite transformation (by carbon enrichment) or after the bainite transformation. The kinetics simulations and the predicted final microstructures are critically compared to a large set of experiments carried out on dilatometry samples with different homogeneous C/N composition and samples with composition gradients. After carbonitriding, microstructure and hardness profiles differ strongly from the classical case of carburizing. This highlights the consequences of the previously reported, unexpected acceleration of the austenite decomposition when it has been enriched in nitrogen.

Effects of plastic deformation on austenite transformation in Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo ultra-high strength steel during continuous cooling

Dynamic continuous cooling-transformation (D-CCT) diagram is an essential research method to predict mechanical properties of ultra-high strength steel (UHSS) before material processing. In this study, D-CCT diagrams of Fe-1.93Mn-0.07Ni-1.96Cr-0.35Mo at different cooling rates of 0.5–15 °C s−1 were plotted by a combined method of dilatometry and metallography. Influences of deformation on microstructure, transformation behavior, and carbon diffusion were elucidated. Proeutectoid ferrite transformation behavior was discussed in detail based on strain induced nucleation kinetics. When the cooling rate increased above 1 °C s−1, the initial austenite grains were divided into blocks by newly forming complex microstructures, which were mainly consisted of martensite (M), bainite (B), deformation induced ferrite (DIF) and retained austenite (RA). Moreover, the decrease of proeutectoid ferrite and the increase of granular bainite (GB) resulted in the strength increased linearly at the cooling rate of 0.25–2 °C s−1. Increasing cooling rate has a remarkable effect on the tensile strength of the UHSS up to 1 °C s−1. Further increase in the cooling rate from 1 to 15 °C s−1, the results showed a relatively small effect on the intensity value. Besides, carbon diffusion from α-phase (face-centered-cubic microstructure) in DIF can enhance the stability of γ-phase (body-centered cubic microstructure) and delay the transformation of γ-phase to GB. Furthermore, attributed to the diffusion path of carbon decreases, the proeutectoid ferrite transformation was restrained, and γ-phases were transformed into GB as raising the cooling rate.

Super long-range diffusion of carbon during proeutectoid ferrite transformation

In order to explore the possible diffusion distance of carbon during proeutectoid ferrite transformation, a slow cooling test of low carbon steel was carried out under vacuum of the thermal simulator. The microstructure and thermal expansion curve were discussed and the carbon concentration inside the sample was measured. The ferrite layer of about 450 µm thickness was obtained without pearlite on the surface of the sample in the microstructure. The thermal expansion curve shows that the ferrite layer without pearlite is formed during the local phase transformation, which is followed by the global transformation. The carbon concentration in the core of the sample (0.061%) is significantly higher than that of the bulk material (0.054%). All results show that carbon has long-range diffusion from the outer layer to the inner layer of the sample. The transformation is predominantly interface-controlled mode during local transformation, and the interface migration rate is about 2.25 µm/s.

Analysis of Carbon Partitioning at an Early Stage of Proeutectoid Ferrite Transformation in a Low Carbon Mn–Si Steel by High Accuracy FE-EPMA

Analysis of Carbon Partitioning at an Early Stage of Proeutectoid Ferrite Transformation in a Low Carbon Mn–Si Steel by High Accuracy FE-EPMA

Control of Carbon Content in Steel by Introducing Proeutectoid Ferrite Transformation into Hot-Rolled Q&P Process

A processing strategy involving primary and secondary carbon partitioning is proposed for the hot-rolled quenching and partitioning process through the introduction of proeutectoid ferrite transformation after rolling. The microstructures of the steels were characterized using scanning electron microscopy, transmission electron microscopy, electron probe microanalysis, and x-ray diffraction, and the mechanical properties were evaluated using a universal tensile machine. The blocky retained austenite that distributed along the ferrite grain boundaries was promoted based on the coupling action of the primary and secondary carbon partitioning, which enhanced the transformation-induced plasticity effect during deformation despite the high carbon concentration. A ferrite formation temperature range of ~ 760 to 800 °C was proposed. In addition, from the perspective of industrialization, the observed ‘plateau trends’ for the retained austenite fraction and product of strength and elongation suggest the availability of a wide processing window of 215-362 °C for controlling the coiling temperature.

高精度FE-EPMAによる低炭素鋼の初析フェライト変態初期における炭素の分配

高精度FE-EPMAによる低炭素鋼の初析フェライト変態初期における炭素の分配

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.

Effect of direct quenching and partitioning treatment on mechanical properties of a hot rolled strip steel

Three different online heat treatment processes were designed to study the effects on the mechanical properties of a 0.19C-1.6Si-1.6Mn (wt%) hot rolled strip steel. The microstructures were characterized by means of SEM, TEM, EPMA, and XRD. The mechanical properties were estimated by tensile tests. Results showed that a satisfying combination of strength and ductility could be obtained through the ferrite relaxation and direct quenching and partitioning process. Analysis was also focused on this process. The microstructure contained proeutectoid ferrite grains, martensite packets and blocky or interlath retained austenite, and also contained carbide-free bainite in the case of relatively high quench temperatures. The retained austenite fraction was increased through proeutectoid ferrite and partial bainite transformation, while the tensile strength was also consequently decreased. The most of retained austenite transformed to ferrite under deformation and the elongation was obviously improved.

Alloy optimisation of bainitic steel for large plastic mould

Based on the phase transformation theories, especially the T0 concept of bainite transformation, alloy optimisation of bainitic steel with carbides has been carried out aiming at the produce of plastic mould with large cross-section. The effect of manganese and silicon on proeutectoid ferrite and bainite transformation is explored by dilatometric analysis, XRD and different microscopy techniques. The results show that after the alloy optimisation, the transformation of proeutectoid ferrite is suppressed and when the cooling rate is lower than 0·1°C s− 1, the new lower bainite transformation appears by decreasing carbon capacity of austenite and promoting carbide precipitation. Industrial production proves that the optimised alloy SDP1 can meet the demand for the plastic mould with the thickness of 1050 mm.

Effects of Mo on Carbon Enrichment During Proeutectoid Ferrite Transformation in Hypoeutectoid Fe-C-Mn Alloys

Carbon enrichment in austenite during proeutectoid ferrite transformation in Fe-0.11C-1.5Mn and Fe-0.12C-1.5Mn-0.5Mo (mass pct) alloys was measured by Electron Probe Microanalysis to study the effects of Mo addition on the local condition of ferrite/austenite interface. Mo addition reduces carbon enrichment in austenite, resulting in retardation of transformation kinetics, while carbon concentrations in austenite on entering the steady stage of transformation agree well with negligible partition local equilibrium prediction and are hardly affected by Mo addition.

Effect of Cooling Rates on the Second‐Phase Precipitation and Proeutectoid Phase Transformation of a Nb–Ti Microalloyed Steel Slab

AbstractDuring the continuous casting of low‐carbon Nb–Ti microalloyed steel, control of the slab surface microstructure and the behavior of the second‐phase precipitation are significantly influenced by the cooling rate. Through confocal laser scanning microscopy, the effect of the cooling rate on the behavior of ferrite precipitation both at the grain boundary and within the austenite was observed in situ and analyzed. The relationship between the cooling rate and precipitation of the microalloying elements on the slab surface microstructure was further analyzed by transmission electron microscopy. The results showed that the effect of microalloying element precipitation on proeutectoid ferrite phase transformation is mainly manifested in two aspects: (i) the carbonitrides of microalloying elements act as inoculant particles to promote nucleation of the proeutectoid ferrite and (ii) the carbon near the grain boundary is depleted when the microalloying elements precipitate into carbonitrides, inducing a decrease in the local carbon concentration and promoting ferrite precipitation.

High Magnetic Field Influence on the Widmanstätten Transformation in High Purity Fe-0.36 wt% C Alloy

The influence of high magnetic field with different strength on the proeutectoid ferrite transformation in high purity Fe-0.36 wt% C during diffusional γ→α phase transformation was studied. It was found that the formation of acicular ferrite ( i.e. Widmanstatten ferrite) was obviously suppressed by the applied high magnetic field. The stronger the magnetic field is, the more the ferrite grains elongating and aligning along the field direction will be. This is attributed to additional driving force for phase transformation and demagnetization effect introduced by the applied magnetic field. This is also considered to be related to the preferential growth of proeutectoid ferrite nuclei along field direction caused by magnetic dipolar interaction.

Research on the Effect of Microalloy Elements Mo and Nb on CCT Curve of Q&P Steel

A novel microalloy Q&P steel with Nb and Mo is designed. The dilatometric curves of low-carton microalloyed Q&P steel with and without Nb and Mo are detected at different cooling rates on DIL-805 thermal dilatometer．The CCT curves are determined using thermal dilation measurement, microstructure observation and hardness measurement. In the Q&P process, the niobium carbide precipitate plays a precipitation strengthening effect, so the tensile strength of Q&P steel is enhanced. The results show that: The element Nb decreases the phase-transformation temperatures of ferrite and pearlite, which leads the proeutectoid ferrite and pearlite phase transformation finish lines to move to lower-right. For the low-carton Q&P steel microalloyed with Nb and Mo, when cooling rate is higher than 5°C•s－1, the microstructures is only martensite. Its excellent hardenability is propitious to improve the strength of the Q&P steel. While, for the traditional C-Si-Mn Q&P steel, the martensite can be only obtained until the cooling rate higher than 20°C•s－1.

Calculation on the incubation period of proeutectoid ferrite transformation for Si-Mn TRIP steel

The incubation period of proeutectoid ferrite transformation for Si-Mn transformation induced plasticity (TRIP) steel has been calculated by the Aaronson’s incubation period model for transformation. The influences of chemical compositions and hot deformation of austenite on the incubation period have been taken into consideration in the calculation, and some parameters have been proposed and validated with the measured time temperature transformation (TTT) curves from dilation tests. The calculation results show that it is essential to take into account of the effect of solute atoms on the interfacial energy in the austenite grain boundaries. For hypoeutectoid steel, the incubation period of ferrite transformation increases with the increase of C and Mn contents, and C has a greater impact than that of Mn, while the incubation period of ferrite transformation decreases with the increase of Si content. Hot deformation shortens the incubation time and promotes austenite to ferrite transformation.

A calculation of partitional local equilibrium/negligible partitional local equilibrium boundaries in Fe–C–X systems

Local equilibrium theory has long been employed to explain the thermodynamics and kinetics of proeutectoid ferrite transformation in Fe–C–X systems. The growth kinetics of proeutectoid ferrite is decided by the diffusion behavior of the alloy element X during transformation. One of the key issues in application of this theory is the theoretical calculation and experiment determination of the partitional local equilibrium/negligible partitional local equilibrium boundary (PLE/NPLE transition temperature). In this study, we developed a simple analytic method to calculate the PLE/NPLE boundary. With this method, the PLE/NPLE boundaries in Fe–C–Mn and Fe–C–Ni systems are successfully computed and the results agree well with the formerly reported experimental results.

Study of the phase diagram and continuous cooling transformation of 12%Cr ultra-super-critical rotor steel

The pseudo-equilibrium phase diagram of a 12%Cr ultra-super-critical rotor steel was calculated by the use of Thermo-Calc software. The equilibrium microstructures at room temperature consist of ferrite ( α) and carbides of M 23C 6 type, and the critical points A 1 and A 3 of the steel are 811 °C and 928 °C, respectively. The critical points Ac 1, Ac 3 and Ms determined by a dilatometry method are 806 °C, 930 °C and 395 °C, respectively, which are very similar to the calculated values. The continuous cooling transformation (CCT) curve of the steel was also determined, and the CCT products and microstructural morphologies were analyzed. The critical cooling rate without proeutectoid ferrite transformation is about 0.1 °C/s, and the quantitative expression between microhardness ( Hv) of the CCT structure and cooling rate ( v) for the steel is as follows: H v = 482 − 35 exp ⁡ ( − v / 0.89 ) − 30 exp ⁡ ( − v / 3.80 ) .

A Mathematical Model to Predict Microstructure of Heat-Treated Steel

The dual effects of chemical composition and cooling rate on microstructural evolution were theoretically simulated extending the Johnson-Mehl equation for non-isothermal pro-eutectoid ferrite transformation kinetics incorporating the effect of supercooling (ΔT) as well as for isothermal pearlite transformation kinetics assuming the exponent on time ‘n’ to vary inversely with time as, n = p/tm, for case carburized steel quenched in oil. Carbon concentration profile of steel carburized at 930 °C for 10 h was theoretically computed by solving the Fick’s diffusion equation. The cooling curves from surface to core were generated for typical 20.32 and 15.6 mm diameter steels using FEM package (ANSYS) for oil quenching, water quenching, and air cooling. The effect of varying carbon concentration from surface to core was incorporated in the heat transfer equations while generating cooling curves, at different case depths (surface, 0.31, 0.558, 1.239, and 3.469 mm). The cooling curves for oil quenching were superimposed on the published TTT diagrams of the steels of corresponding carbon content and using the empirical equations the evolution of different microconstituents, e.g., ferrite, pearlite, and martensite from the parent austenite phase were computed for the carburized 20.32 and 15.6 mm diameter steel samples. These steel samples were also case carburized experimentally at 930 °C for 10 h followed by oil quenching. The theoretically predicted case depth of 3.469 mm matched closely with the experimentally observed value. Microstructural studies were done on inverted microscope and quantitative image analyzer at different case depths/nodal points. Microhardnesses were also measured at case depths from 0.1 mm to center of the samples at selected areas to identify the different phases. The experimentally observed microstructures matched well with the theoretically predicted evolution of microconstituents.

Local conditions at moving α/γ boundaries of proeutectoid ferrite transformation in iron alloys

The local conditions at moving α/γ boundaries in iron alloys are examined from the data on growth kinetics, solute partitioning, and critical limit of transformation. In Fe-C alloys, local equilibrium of carbon is likely to be sustained at the majority of α/γ boundaries during the growth of allotriomorphic ferrite except at some boundaries containing immobile low-energy facets. In Fe-C-X alloys, there is experimental evidence that local equilibrium of the substitutional alloying element is established at higher temperatures. However, growth under near paraequilibrium conditions may be prevalent at lower temperatures and at early growth stages. The diffusion of alloying elements in ferrite and along the austenite grain boundary may have a significant influence on the growth of ferrite near the boundary between fast and slow growth. The growth of Widmanstatten and bainitic ferrite is likely controlled by carbon diffusion, that is, without a supersaturation of carbon, while the chemical condition of carbon near the plate edge may not be identical to that of a planar disordered α/γ boundary.

New insights into the widmanstätten proeutectoid ferrite transformation: Integration of crystallographic and three-dimensional morphological observations

The crystallography and three-dimensional (3-D) morphology of Widmanstatten proeutectoid ferrite precipitates are examined in an Fe-0.12 wt pct C-3.28 wt pct Ni steel isothermally reacted at 650 °C, 600 °C, and 550 °C. This article integrates new orientation mapping (OM) results with the findings of a companion article to this one on the 3-D morphology of proeutectoid ferrit[1] and an earlier transmission electron microscopy (TEM) study which is reanalyzed here in light of the new OM and 3-D results. All of these studies were performed for the same alloy and heat treatments. The 3-D morphologies and distributions of proeutectoid ferrite precipitates are now known to often be quite different from those deduced by conventional two-dimensional (2-D) microscopy techniques. The present crystallographic studies indicate that “primary” ferrite (nucleated directly on prior austenite grain boundaries) forms monolithic single crystals and can be approximated as elongated triangular pyramids. “Secondary” ferrite morphologies can be described as laths and plates branching into the austenite from a thick and/or broad allotriomorphic ferrite base. These secondary Widmanstatten branches are composed of many misoriented crystals with ferrite: ferrite boundaries between them and appear to approach a common orientation as they extend into the austenite grain. Implications of the current findings on existing growth and crystallography models are discussed, and a preliminary hypothesis or mechanism of ferrite formation has been proposed to account for the present observations.

Influence of magnetic field on the kinetics of proeutectoid ferrite transformation in iron alloys

The kinetics of proeutectoid ferrite transformation in Fe-C base alloys in a strong magnetic field (7.5 T) were studied. The transformation kinetics were accelerated at temperatures not only below but also significantly above the Curie temperature. The free energy and equilibrium ferrite/austenite phase boundaries in applied magnetic fields were calculated using the reported experimental magnetic susceptibility and Weiss molecular field theory. The persistence of magnetic field effects above the Curie temperature can be attributed to the rapidly diminishing difference in relative stability between ferrite and austenite toward the Ae 3 temperature of iron.