Dissolution Of Cementite Research Articles

MODIFICATION OF THE STRUCTURE AND PHASE STATE OF A FERRITE-CEMENTITE COMPOSITION BY AN ELECTRON BEAM

− The phase composition and defect substructure formed in carbon steel subjected to a microsecond high-current e-beam have been studied by thin-foil electron diffraction microscopy. It has been shown that the mechanism and the degree of globular cementite dissolution, the phase composition and the morphology of the appearing structure depend on the distance from a melt spot and are determined by the temperature field gradient. As the electron beam approach the “spot” (an increase in steel temperature), the evolution of the material volume containing cementite particles is accompanied by the formation of structures which alternate in a regular manner.

A Criterion for the Change from Fast to Slow Regime of Cementite Dissolution in Fe–C–Mn Steels

The present study clarifies the role of Mn in cementite on the driving force of cementite dissolution and the growth of austenite. From an experimental study, the effects of manganese composition and temperature on the cementite dissolution were shown. From a theoretical analysis based on thermodynamic and kinetics considerations, a criterion for the change from fast to slow regime of cementite dissolution was proposed. This criterion is in good agreement with the experimental results. It can be easily calculated and can define the composition and temperature ranges where the cementite dissolution is slow or fast.

The influence of electrolyte composition on electrochemical ferrate(VI) synthesis. Part III: anodic dissolution kinetics of a white cast iron anode rich in iron carbide

The anolyte composition and process temperature could improve the kinetics of iron anode dissolution and subsequent ferrate(VI) production significantly. This also holds for the anode composition. Following pure iron and silicon-rich steel (SRS), white cast iron (WCI) was the last representative of anode material tested that is typically used to produce ferrate(VI). Using anolytes 14 M NaOH, 14 M KOH, and mixtures thereof, the systems were studied by potentiodynamic methods, electrochemical impedance spectroscopy, and batch electrolysis experiments. Additionally, metallographic analysis of the material was performed. The dissolution kinetics increases with increasing temperature and also, at 60 °C, with increasing K+ content in the anolyte, but less progressively than in the case of SRS. Similar to SRS, WCI also easily dissolves into ferrate(VI) even at 20 °C in pure NaOH, thus indicating the inferior protective properties of oxo-hydroxide surface layers. In general, a maximum current efficiency of approx. 60 % was obtained at 60 °C in pure KOH solution. The authors conclude that, at 60 °C, the high efficiency of the synthesis is caused by the low protective properties of the oxo-hydroxide surface layer caused by the preferential dissolution of cementite and at the same time by the precipitation of the potassium salt of the product in the electrolyte immediately after its formation. This minimizes the effect of its decomposition.

Texture and Microstructural Evolution in Pearlitic Steel During Triaxial Compression

This article presents the deformation behavior of high-strength pearlitic steel deformed by triaxial compression to achieve ultra-fine ferrite grain size with fragmented cementite. The consequent evolution of microstructure and texture has been studied using scanning electron microscopy, electron back-scatter diffraction, and X-ray diffraction. The synergistic effect of diffusion and deformation leads to the uniform dissolution of cementite at higher temperature. At lower temperature, significant grain refinement of ferrite phase occurs by deformation and exhibits a characteristic deformation texture. In contrast, the high-temperature deformed sample shows a weaker texture with cube component for the ferrite phase, indicating the occurrence of recrystallization. The different mechanisms responsible for the refinement of ferrite as well as the fragmentation of cementite and their interaction with each other have been analyzed. Viscoplastic self-consistent simulation was employed to understand deformation texture in the ferrite phase during triaxial compression.

Modeling of Growth and Dissolution of Grain Boundary Cementite in Vacuum Carburizing Process

In order to predict microstructures during vacuum carburizing, the model which simulates not only the carbon(C) diffusion but also growth/dissolution of cementite(θ) is required. For development of a new model we applied vacuum carburizing to low alloy steels and analyzed the distribution of C and θ by GD-OES and image analysis of microstructures. The C in retained austenite(γ) phase after carburizing was also measured by lattice constants obtained from XRD. We also simulated multi-component diffusion with γ matrix and θ layer to analyze a velocity of the moving interface. The new carburizing model was proposed based on the findings, which suggest that C in γ phase at the carburizing surface is supersaturated and corresponds to C concentration for metastable equilibrium condition to graphite. The growth and dissolution of the θ follow a square root of time with the coefficients controlled by diffusion of Si in γ and Cr in θ respectively. The required parameters such as diffusivity coefficients are obtained by the CALPHAD method. The calculated C distributions and volume fractions of θ represent the experimental results.

Dilatometric analysis of cementite dissolution in hypereutectoid steels containing Cr

Cementite dissolution in hypereutectoid steels containing Cr is analyzed using dilatometry combined with consideration of the carbon content in the austenite. The results suggest that the austenite transformed from the mixture of ferrite and cementite has a carbon content that corresponds roughly to equilibrium with ferrite. The overall dissolution behavior of cementite is described well with the suggested analytical procedure.

Experimental and numerical analysis on formation of stable austenite during the intercritical annealing of 5Mn steel

Microstructure evolution during the intercritical annealing at 650 °C for various durations up to 144 h in 5 wt.% Mn-containing steel, from the initial martensitic microstructure to mainly ultrafine lamellar microstructures consisting of austenite laths and ferrite laths, was examined experimentally and analyzed numerically in this paper. Annealing for longer duration results in a larger volume faction of austenite and thicker γ laths with an enrichment of Mn, which significantly improves elongation and lowers the yield stress. The γ fraction increases almost linearly with the logarithm of annealing time until it is saturated after 12 h annealing. The thickening of the austenite lath was numerically simulated by DICTRA software and the MOB2 database under the local equilibrium. The simulation result is in fair agreement with the measurements, and also shows a proportional dependence on the logarithm of the annealing time up to 12 h. Furthermore, numerical simulations on growth of the austenite lath nucleated at the ferrite–cementite interface were also performed, and indicated that such growth should be very sluggish due to the slow dissolution of cementite. As a result, it is concluded that the growth of austenite nucleated at the ferrite lath boundaries, instead of the growth of austenite nucleated at the ferrite–cementite interface, plays a major role in the increase in austenite volume fraction during the annealing, which is controlled by the diffusion of Mn in both austenite and ferrite phases.

Grain Boundary Segregation and Amount of Bulk Carbides in Severely Deformed Fe–C Alloys

The microstructure, phase composition, Mössbauer spectra, grain boundary segregation and magnetic properties of binary Fe–C alloys with carbon concentration of 0.05, 0.10, 0.20, 0.25, 0.45, 0.60, 1.3, 1.5 and 1.7 wt. % were studied in the as-cast state, after a long annealing at 725°C and after high-pressure torsion (HPT) at the ambient temperature and 5 GPa with 5 anvil rotations (shear strain about 6). The grain size after HPT was in the nanometer range. Only Fe3C (cementite) and -Fe remain in the alloys after HPT. It was also shown that the less stable Hägg carbide (Fe5C2) and retained austenite disappear, and phase composition closely approaches the equilibrium corresponding to the HPT temperature and pressure. Measurements of saturation magnetization and Mössbauer effect reveal that the amount of cementite decreases after HPT. The reason for partial cementite dissolution is the formation of the carbon-rich segregation layers in the ferrite grain boundaries.

Nanoscale co-precipitation and mechanical properties of a high-strength low-carbon steel

Nanoscale co-precipitation in a novel high-strength low-carbon steel is studied in detail after isothermal aging. Atom-probe tomography is utilized to quantify the co-precipitation of co-located Cu precipitates and M 2C (M is any combination of Cr, Mo, Fe, or Ti) carbide strengthening precipitates. Coarsening of Cu precipitates is offset by the nucleation and growth of M 2C carbide precipitate, resulting in the maintenance of a yield strength of 1047 ± 7 MPa (152 ± 1 ksi) for as long as 320 h of aging time at 450 °C. Impact energies of 153 J (113 ± 6 ft-lb) and 144 J (106 ± 2 ft-lb) are measured at −30 °C and −60 °C, respectively. The co-location of Cu and M 2C carbide precipitates results in non-stationary-state coarsening of the Cu precipitates. Synchrotron-source X-ray diffraction studies reveal that the measured 33% increase in impact toughness after aging for 80 h at 450 °C is due to dissolution of cementite, Fe 3C, which is the source of carbon for the nucleation and growth of M 2C carbide precipitates. Less than 1 vol.% austenite is observed for aging treatments at temperatures less than 600 °C, suggesting that transformation-induced plasticity does not play a significant role in the toughness of specimens aged at temperatures less than 600 °C. Aging treatments at temperatures greater than 600 °C produce more austenite, in the range 2–7%, but at the expense of yield strength.

Microstructure Characteristics of Eutectoid Pearlitic Steel after Equal Channel Angular Pressing at Room Temperature

An ultrafine microduplex structure was successfully produced through equal channel angular pressing (ECAP) at ambient temperature. According to the morphological characteristics of cementite lamellas, the deformed pearlite consists of shear break lamella, locally thinned lamella and irregularly bent lamella. The proportion of locally thinned lamella and irregularly bent lamella has increased with the pass number of ECAP. In addition, the deformed pearlitic ferrite became a supersaturated solid solution of carbon due to the partial dissolution of cementite during ECAP.

The effect of the cementite phase on the surface hardening of carbon steels by shot peening

This study investigates the effect of the cementite phase on the surface hardening of carbon steels when they are shot-peened. Three carbon steels were used in this study: 0.1% C, 0.45% C, and 0.8% C steels. The results show that the surface hardness after shot peening is proportional to its carbon content. Grains at the surface were transformed into grains with lamellar structures. The cementite was spheroidized by energy generated from the severe plastic deformation at the surface, the ferrite grain size was refined noticeably, and the carbon dissolved in the ferrite. The higher the carbon content in the ferrite, the higher the degree of grain refinement was observed. The primary reason for the noteworthy enhancement of surface hardening likely originates from both the grain refinement and supersaturation of the carbon in the ferrite following cementite dissolution.

An Alternate Approach to Accelerated Spheroidization in Steel by Cyclic Annealing

In this work an annealed 0.6 wt.% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 810 °C (above Ac3 temperature) followed by cooling in a flowing air medium (flow rate: 6 m3/h). After 8 cycles (about 1 h and 20 min), the microstructure mostly contains spheroidized cementite and ferrite along with trace amount (3%) of pearlite. In addition to the diffusion within lamella, the disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac3 temperature, and the generation of defects (lamellar faults) during non-equilibrium cooling in a flowing air medium are the main reasons of accelerated spheroidization.

Effect of cyclic heat treatment on microstructure and mechanical properties of 0.6 wt% carbon steel

In this work an annealed 0.6 wt% carbon steel was subjected to cyclic heat treatment process that consisted of repeated short-duration (6 min) holding at 810 °C (above Ac 3 temperature) followed by forced air cooling. After 8 cycles (about a total 1 h and 20 min duration of heating and cooling cycles), the microstructure mostly contained fine ferrite grains (grain size of 7 μm) and spheroidized cementite. This microstructure possessed an excellent combination of strength and ductility. The disintegration of lamellae through dissolution of cementite at preferred sites of lamellar faults during short-duration holding above Ac 3 temperature, and the generation of defects (lamellar faults) during non-equilibrium forced air cooling were the main reasons of accelerated spheroidization. The strength property initially increased mainly due to the presence of finer microconstituents (ferrite and pearlite) and thereafter marginally decreased with the elimination of lamellar pearlite and appearance of cementite spheroids in the microstructure. Accordingly, the fractured surface initially exhibited the regions of wavy lamellar fracture (pearlite regions) along with dimples (ferrite regions). With increasing number of heat treatment cycles the regions of dimples gradually consumed the entire fractured surface.

Atom Probe and Mössbauer Spectroscopy Investigations of Cementite Dissolution in a Cold Drawn Eutectoid Steel

Mossbauer spectrum and three dimensional atom probes (3DAP) were combined to investigate the mechanism of cementite dissolution in a cold-drawn eutectoid steel at a true strain of 2.89. The experimental results suggest that the dislocations play an important role in the dissolution of the cementite by sweeping across the nano-scaled cementite, and transferring carbon from cementite to ferrite inducing cementite decomposition. The mechanism of cementite dissolution in the steel is discussed in association with the investigation of nonstoichiometric cementite structure after heavy deformation.

Abnormal α to γ Transformation Behavior of Steels with a Martensite and Bainite Microstructure at a Slow Reheating Rate

The same coarse austenite (γ) grains as those before austenitizing emerge when a martensite or bainite steel with coarse grains is reheated to an austenite region at a slow reheating rate. This is called abnormal ferrite (α) to austenite (γ) transformation or γ grain memory. In this paper, α to γ transformation behavior is investigated in order to clarify the mechanism of abnormal α to γ transformation from the viewpoint of the roles of cementite and retained γ. (1) Coarse γ grains and fine globular γ grains that nucleate along the coarse γ grain boundaries are formed when bainite or martensite steel is reheated above the AC3 temperature. The size distribution of γ grain is the same as that before reheating. (2) Coarse γ grains are formed by the growth, impingement, and coalescence of acicular γ grains that corresponds to retained γ between laths. (3) Abnormal α to γ transformation is suppressed by decreasing the amount of retained γ and by increasing the amount of cementite before reheating. These results suggest that α to γ transformation behavior is governed by competition between the nucleation and growth of newly formed γ from the dissolution of cementite and the growth and coalescence of retained γ. Abnormal α to γ transformation occurs when the growth and coalescence of retained γ dominates rather than the nucleation and growth of globular γ grains.

Mechanisms of the strain-induced dissolution of phases in nanostructured metals

The possible mechanisms of the strain-induced dissolution of phases in metals subjected to severe plastic deformation have been investigated. The mechanisms of impurity absorption by new interphase and grain boundaries formed under severe plastic deformation and the high-temperature phase at its strain-induced nucleation are shown to be the most effective. The possible dislocation mechanisms of phase dissolution are low-efficient. In carbon steels, the strain-induced dissolution of cementite continues until the limiting nanostructure absorbs about 10–12 at % carbon from the mixture volume.

Fe–C nanograined alloys obtained by high-pressure torsion: Structure and magnetic properties

The microstructure, phase composition, Mössbauer spectra and magnetic properties of nine binary Fe–C alloys with carbon concentrations between 0.05 and 1.7 wt.% were studied in the as-cast state, after a long annealing at 725 °C and after high-pressure torsion (HPT) at ambient temperature and 5 GPa with five anvil rotations (shear strain about 6). The grain size after HPT was in the nanometer range. Only Fe 3C (cementite) and α-Fe remain in the alloys after HPT. It was also shown that the less stable Hägg carbide (Fe 5C 2) and retained austenite disappear, and phase composition closely approaches the equilibrium corresponding to the HPT temperature and pressure. Measurements of saturation magnetization and Mössbauer effect reveal that the amount of cementite decreases after HPT. The reason for partial cementite dissolution is the formation of the carbon-rich segregation layers in the ferrite grain boundaries.

XRD analyses on dissolution behavior of cementite in eutectoid pearlitic steel during cold rolling

The changes of lattice parameter and carbon content of cold-rolled pearlitic ferrite ( α) in eutectoid pearlitic steels were studied with X-ray diffraction (XRD) analysis. The effects of rolling reduction and alloying elements on the dissolution of cementite ( θ) were discussed. In the present work, it was observed that XRD peaks of pearlitic α shift to smaller angle side after cold rolling with respect to that of the as-transformed state. The lager the cold-rolling reduction is, the more significant the shift of α peaks, and the larger its lattice parameter. The addition of Cr or Mn can accelerate the dissolution of the pearlitic θ during cold rolling with the effect of Cr more pronounced, but the influence of Si is not so evident.

Strain-induced refinement in a steel with spheroidal cementite subjected to surface mechanical attrition treatment

Steel with spheroidal cementite particles dispersed in a ferrite matrix was subjected to surface mechanical attrition treatment. Strain-induced microstructure evolution, including grain refinement of ferrite and cementite as well as dissolution of cementite, was examined using transmission electron microscopy. Upon straining, dense dislocation walls were developed in ferrite grains, which evolved gradually into sub-boundaries and highly misoriented grain boundaries (GB) at increasing strains, leading to grain refinement of the ferrite. The ferrite refinement process is greatly facilitated by the presence of dispersed cementite particles, as the cementite/ferrite interfaces are effective nucleation sites for dislocations and also barriers for dislocation motions. When ferrite grains are refined to sizes smaller than that of cementite particles, plastic deformation occurs in cementite particles, characterized by edge dislocation gliding along the {110} and {100} planes. Dislocation gliding in cementite is initiated at intersections of the cementite/ferrite interfaces and ferrite GB. Accumulated multiple gliding progressively refines cementite into nano-sized cementite particles mixed within ferrite nano-grains in the top surface layer. A decreasing volume fraction of cementite is observed with decreasing depth in the top deformed layer, indicative of dissolution of cementite induced by increased plastic straining.

Al-C-Cr-Fe-Mn-Mo-N-Ni-Si-V

The calculations were done for a steel of composition in wt.%: 1.24 Mn, 0.88 Ni, 0.47 Mo, 0.21 Cr, 0.25 Si, 0.004 V, 0.008 Al, 0.21 C, 0.005 N, and balance Fe. Comparisons were made with an experimental alloy of the same composition, but containing in addition 0.03 Cu, 0.002 S, and 0.007 P. The solid solution phases were described as a two-sublattice model with formula (Fe,Mn,Ni,Mo,Cr,Si,V,Al)a(C,N,Va)c, where Va stands for vacancy. In the case of body-centered cubic ferrite ðaÞ, a = 1 and c = 3. For facecentered cubic austenite ðcÞ, a = 1 and c = 1. Available assessments of Fe-based quaternary carbide and nitride systems (Fe-M-M-C and Fe-M-M-N, M standing for metal) and the corresponding subsystems were considered adequate for the reliability of the calculations. Complete descriptions were available for all important lower-order systems such as Fe-C, Fe-N, Fe-M, M-C, M-N, Fe-M-C, and Fe-M-N. The computed equilibrium mole fractions of austenite, ferrite and various carbides and nitrides as a function of annealing temperature are shown in Fig. 1. Figure 1(b) shows in greater detail the austenite formation as well as the cementite and n-carbide dissolution. To produce an austenite fraction of 0.4, the calculations yield an intercritical annealing temperature of 720 C. An independent experimental verification in the authors group showed that this temperature is 725 C for this steel. Despite the influence of variables such as thermal history, prior microstructure and incomplete attainment of equilibrium during the intercritical anneal, the agreement is good.