Growth Of Cementite Research Articles

Impact of carbon segregation on transition carbides and cementite precipitation during tempering of low carbon steels: Experiments and modeling

Tempering of low-carbon steels (less than about 0.3 wt.% C) may differ from the traditional precipitation sequence, which includes transition carbide precipitation, retained austenite decomposition and cementite precipitation. The main difference is the partial or total absence of transition carbides. In the present work, the effect of the carbon segregation on the mitigation of the precipitation of the latter is analyzed by combining multi-scale experimental investigations with a new precipitation model accounting for the carbon heterogeneities induced by the segregation. The full precipitation sequence is considered in order to study the impact of the segregation not only on the transition carbides, but also on the cementite which forms afterwards. The experimental work includes APT characterization of carbon segregation, in-situ HEXRD experiments to reveal precipitation kinetics, and TEM observations for carbide size measurements.The here-in developed precipitation mean-field and physics-based model combines two previous ones dedicated to the nucleation and growth of transition carbides and cementite and to the segregation of carbon at dislocations. It is shown that, even after water-quench, carbon atoms are already segregated on dislocations. The mitigation of transition carbide precipitation is caused by the presence of such segregations, which decrease the driving force for the transition carbide nucleation and enhance cementite precipitation. In agreement with previous experiments, the model also demonstrates that inside a martensitic microstructure, the precipitation sequence is different between the first (formed close to Ms temperature) and the last martensite (formed at room temperature) formed upon cooling, because of the difference in dislocation density, which influences the intensity of the segregation phenomenon.

Dilatometric study of high silicon bainitic steels: Solid-state transformations

High silicon bainitic steels have gained significant recognition in various applications due to their exceptional properties, such as high strength, favourable corrosion resistance, and excellent high-temperature stability. This study investigates an alloy capable of generating a finer bainitic structure through a transformation at 260 °C, leading to the desired microstructure. Interestingly, other phenomena were discovered while pursuing optimal heat treatment conditions. At temperatures exceeding 850 °C, the alloy exhibits a tendency for graphite formation, which has intriguing implications for its mechanical properties. The high silicon concentration in the alloy significantly retards cementite growth, resulting in a microstructure composed solely of bainitic ferrite and residual austenite through the transformation of austenite below the bainite start temperature. Furthermore, it is observed that pearlite formed during rapid transformation at 650 °C does not exhibit the predicted equilibrium chemical composition. This discrepancy challenges the existing models of pearlite growth, which assume local equilibrium at the shared interface with austenite. This research aims to investigate the influence of silicon content on solid-state transformations in high-silicon steels using dilatometry, optical microscopy, scanning electron microscopy, and X–ray diffraction techniques. These analytical methods will provide insights into the intricate processes occurring during isothermal transformation temperatures, contributing to a deeper understanding of the material's behaviour and its potential applications.

The effect mechanism of Si on the cementite growth behavior in Fe–Cr–C steel: first-principles calculations and experiments

The effect mechanism of Si on the cementite growth behavior in Fe–Cr–C steel: first-principles calculations and experiments

Kinetic Modeling of Grain Boundary Cementite Evolution

Prediction of grain boundary cementite growth kinetics in hypereutectoid steels is necessary to control its thickness. It is a question of major industrial importance but has remained unresolved to date. This paper presents and compares two different and new modeling approaches. The first one relies on diffusion-based nucleation and growth of cementite precipitates using a modified SFFK model with short-circuit grain boundary diffusion and accounting for heterogeneous nucleation site energy during isothermal treatments and continuous cooling. It is compared to previously published simulations of diffusion-controlled reaction with moving phase boundaries and a similar simulation using the software Dictra. The second approach implies that cementite thickening is driven by the nucleation of ledges at the stepped austenite/cementite interface, controlled by a structure barrier to ledge formation previously assumed in the literature.[1] We suggest a semi-empirical formulation of this barrier to ledge nucleation during isothermal transformation. Both approaches lead to an excellent match to experimental data for an almost pure Fe–C system. This implies that modeling the stepped structure of the austenite/cementite interface is not imperative for simulation of thickening kinetics, but also that understanding the governing physics of ledge formation allows for a comprehensive description of secondary cementite formation.

Thermo-mechanical analysis of strength degradation of 30SiMn2MoVA gun barrel material during continuous shooting

The internal bore of a machine gun barrel is subjected to the complex effects of cyclic high-temperature and mechanical loadings during the continuous shooting process, resulting in the accumulation of bore damages and eventually the end of the lifespan. The correlation between the ballistic performances and the material properties is a critical issue for the development of gun barrels with a long lifespan. However, it is still not fully understood yet. In this study, the thermo-mechanical analysis of the strength degradation of 30SiMn2MoVA gun barrel steel during continuous shooting is investigated. Irregular rifling deformation of the failed machine gun barrel with a 12.7 mm caliber was observed. Mechanical tensile tests revealed that severe softening occurs in the 30SiMn2MoVA steel when the temperature exceeded 600 °C, where the yield strength decreased significantly from 862 MPa at room temperature to 114 MPa at 700 °C. Transmission electron microscope observations further revealed that the recrystallization of martensitic lath and the growth of cementite softened the gun barrel steel. Moreover, the finite element models are established using the Abaqus software for the evolution of the temperature and stress of the internal wall within 180 cycles. This shows that the peak temperature increases to 730–740 °C and the peak total stress exceeds 246 MPa during 170–180 cycles, which is much higher than the yield strength of 30SiMn2MoVA gun barrel steel. Therefore, the strength of gun barrel steel at 700 °C can be suggested as the key parameter for the development of long-life gun barrels for continuous shooting.

Tailoring cementite precipitation and mechanical properties of quenched and tempered steel by nickel partitioning between cementite and ferrite

Abstract The effect of Ni on the evolutions of cementite and mechanical properties in quenched and tempered Cr–Ni–Mo steel was investigated. The results of tensile and Charpy impact tests showed that an increase in the Ni content was more effective in improving the low-temperature impact toughness than in increasing yield and tensile strengths. Experimental observations revealed that Ni was beneficial to cementite refinement but had almost no impact on refining the sizes of both the quenched martensite structure and tempered effective grain size. Further, energy dispersive spectrometry (EDS) and thermodynamic studies on cementite and adjacent ferrite indicated that Ni partitioning resulted in the formation of a Ni-rich layer at the ferrite/cementite interface and thereby retarded cementite growth. In order to clarify the different effects of Ni on the impact toughness and yield and tensile strengths, the role of cementite refinement in strengthening and toughening mechanisms was analyzed in detail.

The X80 Pipeline Steel produced by a Novel Ultra Fast Cooling Process

The relation between microstructure characteristics and mechanical properties of X80 pipeline steels produced by ultra fast cooling process was investigated using energy dispersive spectrometer, scanning and transmission electron microscopy. Results showed that ultra fast cooling kept hot rolled austenitic work hardened, induced a large number of carbonitride precipitation, limited the growth of cementite. Therefore ultra fast cooling was beneficial to increase the steel strength and improve the steel ductility and toughness.

Redistribution of Mn between α-Fe matrix and θ cementite during long-term thermal aging in a low alloy steel

The addition of certain amounts of Mn in steel has long been known to retard the growth and coarsening of cementite during tempering, which can increase the tempering resistance of carbon steels. It is now well-established that the retarding effect is inherently correlated with the partitioning of Mn between ferrite(α) matrix and cementite(θ). According to the equilibrium thermodynamics, Mn would diffuse from α-Fe matrix to θ cementite after the initial stage of tempering until equilibrium is reached.However, the manner in which Mn diffuses from α-Fe matrix to θ cementite is unclear, which is key in understanding the mechanism in which the partitioning of Mn can retard the growth and coarsening of cementite. Therefore, the measurement of Mn content across the α-Fe/θ interface is of importance to achieve this goal. In this study, the redistribution characteristics of Mn between α-Fe matrix and θcementite after long-term aging at 370 or 400 °C with quenched–tempered or quenched samples of reactor pressure vessel model steel was investigated by atom probe tomography. Results show that Mn diffuses from the α-Fe matrix and enriches in the θ cementite under all heat treatment conditions. The concentration of Mn in cementite is the highest when the specimen is thermally aged directly after quenching. Moreover, Mn is not distributed uniformly within cementite after long-term aging at 400 °C for 35000 h. Instead, a Mnsegregated zone exists within cementite adjacent to theα-Fe/θinterface,with concentration increasing by aging temperature,which acts as a barrier to the coarsening of cementite by hindering the dissolution of small-sized cementite.The redistribution characteristics of Mn between the two phases is correlated with the difference of diffusivities in theα-Fe matrix andθcementite during thermal aging,and the diffusivity of Mn inθcementite is slower than that inα-Fe matrix.

The Effect of Hot-Mounting on the Microstructure of an As-Quenched Auto-Tempered Low-Carbon Martensitic Steel

The effect of hot-mounting for metallographic studies of as-quenched low-carbon martensitic steels has been studied. Hot-mounting is typically carried out at 150–200 °C, i.e., a low-temperature tempering regime. Cold- and hot-mounted specimens from an as-quenched low-carbon auto-tempered steel were examined using a scanning electron microscope and their hardness levels were also compared. It was found that hot-mounting causes additional tempering that manifests as the appearance of new precipitates in those regions that are free of auto-tempered cementite. The observations were rationalized using DICTRA simulations to calculate the potential growth of cementite. Hot-mounting was also shown to cause a small but statistically significant increase in the hardness of the martensite.

Application of the Mixed-Mode Model for Numerical Simulation of Pearlitic Transformation

The multiscale model of pearlitic transformation, including the finite element solution of the second Fick’s equation with moving boundary in the microscale and Fourier equation in the macroscale, is presented in this paper. According to the mixed-mode approach, both the volume diffusion and the interface mobility were considered. Model describes sidewise and frontal growth of cementite and ferrite plates in a single grain of the austenite. Assessment of the possibility of the developed model application to determine the key parameters in terms of strength properties of pearlitic steel, i.e., pearlite grain size and colony size as well as interlamellar spacing for various cooling conditions, was the main aim of this work. The model was validated and verified on the basis of experimental tests performed for two eutectoid steels. In practice, developed model can support design process for a technology of high-strength rods and long products manufacturing. Rails were selected as a case study in this work, and therefore, numerical simulations of accelerated cooling of the rail head were performed and the relationship between the parameters of heat treatment, parameters of the structure and properties of the finished product was determined on the basis of obtained results.

One-step annealing optimizes strength-ductility tradeoff in pearlitic steel wires

In this paper, the mechanical properties of a cold-drawn wire (ε=2.43) are modulated by simple annealing and the variation of its microstructure is characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and molecular dynamics (MD) simulation. The tensile ductility of the wire can be improved for about three times without compromising its strength when being annealed at 325 °C for 10–30 min. It is convinced that solid solution of carbon atoms from decomposed cementite lamellae improve the wire strength at low temperature annealing (up to 250 °C) and make the wire strength basically equal the as-drawn state even though cementite lamellae are weakened by cementite recrystallization at 325 °C. And reversely the weakening cementite layers lead to the great improvement of wire ductility at this time since it relaxes the restriction to the moving of dislocations. At higher annealing temperature, the wire strength decreases with the growth of cementite and ferrite grains. The appearance of nano-recrystallized cementite grains at a medium annealing temperature may be a critical factor governing the enhanced wire mechanical properties.

Effects of tempering process on microstructure and mechanical properties of ZG30Mn

Through microstructure analysis, hardness testing, tensile and impact tests, the effects of tempering process on the microstructure and mechanical properties of ZG30Mn were studied. The results show that the microstructure of ZG30Mn treated by different tempering processes is tempered sorbite. When the holding time is 60 min, with the increasing of the tempering temperature, the hardness and strength gradually decrease, and the elongation and impact energy gradually increase. When the tempering temperature is 600 °C, the hardness and strength of ZG30Mn gradually decrease with the prolongation of holding time, while the elongation and impact energy increase first and then decrease. According to the analysis, as the tempering temperature increases, the recovery and recrystallization of α-phase occurs and dispersed fine cementite gradually grows and spheroidizes, resulting in a decrease in hardness and strength, and an increase in elongation and impact energy. With the prolongation of holding time, the aggregation growth of alloy cementite results in the decrease of the elongation and the impact energy.

Interface dominated cooperative nanoprecipitation in interstitial alloys

Steels belong to one of the best established materials, however, the mechanisms of various phase transformations down to the nano length scale are still not fully clear. In this work, high-resolution transmission electron microscopy is combined with atomistic simulations to study the nanoscale carbide precipitation in a Fe–Cr–C alloy. We identify a cooperative growth mechanism that connects host lattice reconstruction and interstitial segregation at the growing interface front, which leads to a preferential growth of cementite (Fe3C) nanoprecipitates along a particular direction. This insight significantly improves our understanding of the mechanisms of nanoscale precipitation in interstitial alloys, and paves the way for engineering nanostructures to enhance the mechanical performance of alloys.

Cementite coarsening during the tempering of Fe-C-Mn martensite

Cementite precipitation during martensite tempering has become an important processing step in some advanced high strength steels (AHSS). This tempering can lead to the formation of Mn-partitioned cementite which has a strong influence on subsequent austenite reversion during intercritical annealing. In this contribution, both the cementite Mn content and particle size evolution have been quantitatively monitored during the tempering of Fe-C-Mn martensite at temperatures between 400 °C and 600 °C. Mn progressively partitions to cementite during the coarsening reaction from the earliest tempering times. This coarsening of the cementite occurs in a matrix that contains strong Mn concentration gradients that drive partitioning of the Mn to the cementite. A model is developed to simultaneously describe the time, temperature and bulk alloy dependence of the mean cementite size and Mn composition evolution. In the model, both the Mn flux, driven by the composition gradient, and C flux, driven by capillarity, have been taken into account to determine the operative tie-lines at the cementite and martensite/ferrite interface. The effect of the cementite Mn content on the dissolution of smaller cementite particles that supply C to the larger particle growth is considered by coupling the growth and dissolution of cementite under conditions of multi-component diffusion.

Diffusion processes during cementite precipitation and their impact on electrical and thermal conductivity of a heat-treatable steel

The thermal conductivity of heat-treatable steels is highly dependent on their thermo-mechanical history and the alloying degree. Besides phase transformations like the martensitic upgamma rightarrow upalpha ^{prime } or the degree of deformation, the precipitation of carbides exerts a strong influence on the thermal conductivity of these steels. In the current work, thermal and electrical conductivity of a 0.45 mass% C steel is investigated during an isothermal heat treatment at 700 ^{circ }C and correlated with the precipitation kinetics of cementite. To include processes in the short-term as well as in the long-term range, annealing times from 1 s to 200 h are applied. This investigation includes microstructural characterization, diffusion simulations, and electrical and thermal conductivity measurements. The precipitation of carbides is connected with various microstructural processes which separately influence the thermophysical properties of the steel from the solution state to the short-term and long-term annealing states. In the early stages of cementite growth, an interstitial-dominated diffusion reaction takes place (carbon diffusion in the metastable condition of local equilibrium non-partitioning). Afterwards, substitutional-dominated diffusion controls the kinetics of the reaction. The electrical and thermal conductivity increase differently during the two stages of the carbide precipitation. The increment is associated to the binding of alloying elements into the carbides and to the reduction of the distortion of the martensitic matrix. Both factors increase the electron density and reduce the electron and phonon scattering. The correlation of the precipitation kinetics and the thermophysical properties are of general interest for the design of heat-treatable steels.

In-situ TEM Observation of Cementite Coarsening Behavior of 5Mn Steel during Tempering

The cementite formation and coarsening behaviors of 0. 2 mass% C-5 mass% Mn steel during tempering at 500 °C were investigated by in-situ transmission electron microscope (TEM). In-situ TEM observation showed uniform distribution of cementite particles at the early stage of tempering in the rapidly heated (500 °C/s) sample. Elemental analysis confirmed that the cementite growth was dominated by Mn diffusion. During the cementite growth, the coarsening behavior of intragranular cementite was significantly controlled by the matrix diffusion, while that of the intergranular cementite was mainly governed by the boundary diffusion. The in-situ TEM observation revealed that the dislocation pipe diffusion of Mn took place during tempering, which accelerated the Mn diffusion between cementite particles. The coarsening rates of individual cementite particles were calculated based on the in-situ TEM observation.

Effect of Al on Growth Kinetics of Cementite in Fe–C–Al Alloys

The effect of substitutional element aluminum (Al) on the cementite growth from austenite phase in Fe–C–Al alloy was simulated using ThermoCalc–Dictra software. The simulation is based on diffusional reactions of multi components in the system. The distribution of elements across the interface of cementite and austenite phases and cementite fraction as a function of time for different Al concentrations in Fe–C–Al system were evaluated. It has been found that the presence of Al retards the growth of cementite which attributes to the negligible solubility of Al in the cementite phase.

Influence of a 2-D defect on the partitioning during the formation of a cementite particle in steels

The spatial distribution of alloying elements in a cementite particle is analysed by Atom Probe Tomography (APT) during the tempering of a ferrite+martensite cold rolled microstructure. The local equilibrium conditions of formation of the cementite particle are at the origin of a strong enrichment in Manganese, Chromium and Boron. The presence of a 2-D defect such as a grain boundary modifies these local conditions and increases the enrichment. The diffusion controlled reaction of cementite growth is simulated with DICTRA software.

Thermodynamic mapping of austenite decomposition’s approach toward equilibrium in Fe–C–Mn at 700 °C

Transformation of Fe–0.85C–11.56Mn (wt.%) at 700°C begins by formation of grain boundary cementite in conjunction with carbon and manganese partitioning. Grain boundary cementite formation initiates regional partitioning of manganese from austenite to cementite that continues over months of cementite growth. The large disparity between manganese volume and grain boundary diffusion in austenite highlights why grain boundaries are associated with all stages of the 700°C phase transformation in this system. Therefore, unlike carbon, manganese partitioning associated with cementite growth is localized to the immediate region near the grain boundary/cementite reaction front. The reaction path involves austenite ‘neighborhoods’, defined to have manganese chemistries and, consequently, thermodynamic properties different from the majority of the remaining matrix. The remaining matrix does partition carbon but not manganese. Along with differences between grain boundary and matrix manganese diffusivity in austenite, diffusion of carbon is up to 106 times greater than that of manganese resulting in carbon activity in austenite equilibrating at each step of cementite growth. The concept of neighborhood thermodynamics is developed for these altered austenite regions and directed toward producing a transformation pathway analysis. The concept of local equilibrium is also demonstrated not to be viable as a method for phase transformation tracking and thermodynamic mapping as the reaction moves toward equilibrium.

On the direct nucleation and growth of ferrite and cementite without austenite

The direct nucleation and growth of ferrite and cementite has been investigated in an Fe–C metallic glass using X-ray diffraction, transmission electron microscopy and atom probe tomography. The nucleation and growth proceeds via primary crystallization of ferrite, without the formation of austenite. There is no carbon gradient at ferrite/amorphous interfaces, indicating that the transformation rate is controlled by the low mobility of these interfaces compared to the mobility of C atoms. A thermodynamic description is proposed to explain the delayed nucleation of cementite.