Dissolution Of Cementite Research Articles

Effects of grain refinement by HPT processing in carbon steel with various cementite morphology

This article reports on the effect of the initial phase composition (martensitic, patented, spheroidized) of the middle carbon steel C45 (0.45 wt.% C) on grain refinement during high-pressure torsion (HPT) at elevated temperature. It is established that the cementite dissolution and the final grain size after HPT strongly depend on the processing regime, as well as on the cementite morphology in the initial state of the steel. As result of this study, new approaches to obtain a very high-strength state in carbon steels have been proposed.

Microstructure and crystallographic orientation evolutions below the superficial white layer of a used pearlitic rail

Although several studies have been conducted on the mechanism of the formation of white layer by cyclic large shear deformation, and its effect on rolling contact fatigue, limited research has been carried out to find a correlation between sub-superficial layer of running contact surface and the crystallographic orientation, microstructural changes, and distribution of shear deformation and dislocation density. In order to understand the microstructural evolution (i.e., microstructure and crystallographic orientation) a used pearlitic rail sample removed from the heavy-haul railroad was investigated in the current work. A very thin superficial white layer was observed, approximately 15 μm below the running contact surface. X-ray diffraction analysis confirmed the formation of a supersaturated ferrite phase with carbon content of approximately 3.78 C wt%. It could be attributed to the cementite dissolution due to severe shear and compressive stresses, during intense shear plastic deformation, from the rail-wheel interaction. The dominance of {110} ferrite grains parallel to the rail direction was characterised, in the transition layer between white layer and non-deformed pearlite structure, by X-ray diffraction and electron backscattered diffraction techniques. Formation of these grains, corresponding to the closed-pack plane of the ferrite matrix at the transition region, leads to great ductility and retarded crack formation.

The Effect of Ultra-Fast Heating on the Microstructure, Grain Size and Texture Evolution of a Commercial Low-C, Medium-Mn DP Steel

The effect of ultra-fast heating on the microstructures of steel has been thoroughly studied over the last year as it imposes a suitable alternative for the production of ultra high strength steel grades. Rapid reheating followed by quenching leads to fine-grained mixed microstructures. This way the desirable strength/ductility ratio can be achieved while the use of costly alloying elements is significantly reduced. The current work focuses on the effect of ultra-fast heating on commercial dual phase grades for use in the automotive industry. Here, a cold-rolled, low-carbon, medium-manganese steel was treated with a rapid heating rate of 780 °C/s to an intercritical peak temperature (760 °C), followed by subsequent quenching. For comparison, a conventionally heated sample was studied with a heating rate of 10 °C/s. The initial microstructure of both sets of samples consisted of ferrite, pearlite and martensite. It is found that the very short heating time impedes the dissolution of cementite and leads to an interface-controlled α → γ transformation. The undissolved cementite affects the grain size of the parent austenite grains and of the microstructural constituents after quenching. The final microstructure consists of ferrite and martensite in a 4/1 ratio, undissolved cementite and traces of austenite while the presence of bainite is possible. Finally, it is shown that the texture is not strongly affected during ultra-fast heating, and the recovery and recrystallization of ferrite are taking place simultaneously with the α → γ transformation.

Enhanced Corrosion Resistance of SA106B Low-Carbon Steel Fabricated by Rotationally Accelerated Shot Peening

The corrosion resistance of a SA106B carbon steel with a gradient nanostructure fabricated by rotationally accelerated shot peening (RASP) for 5, 10, 15 and 20 min was investigated. Electrochemical tests were carried out in the 0.05 M H2SO4 + 0.05 M Na2SO4 and 0.2 M NaCl + 0.05 M Na2SO4 solutions. The experimental results showed that the sample RASP-processed for 5 min exhibited the best corrosion resistance among them. TEM analysis confirmed that the cementite dissolution and formation of nanograins, which improved the corrosion resistance of the steel. Prominent micro-cracks and holes were produced in the samples when the RASP was processed for more than 5 min, resulting in the decrease of corrosion resistance.

Diffusion model for austenite formation kinetics during inter-critical annealing of dual-phase steels

A numerical solution for the ferrite-to-austenite phase transformation in a ternary iron alloy is proposed that predicts austenite formation during isothermal treatment. As the solution ensures perfect mass conservation of the solutes, it can be applied for long holding times until the system reaches final thermodynamic equilibrium. The solution was validated using previous experimental results on Fe–Ni–Cr alloys. Comparison with experimental data for a low-C, low-alloyed dual-phase steel enabled investigation of the effect of the initial concentration of the substitutional solute in cementite and the dissolution of cementite on the austenite formation kinetics.

In situ analysis of the effect of high heating rates and initial microstructure on the formation and homogeneity of austenite

Decreasing processing time of a quench and temper heat treatment is of high interest for industry due to the possibility of cost reduction. One option to reduce processing time is to shorten the austenitizing cycle by applying high heating rates and minimum holding times. However, due to the high heating rates, the analysis of their influences on the formation kinetics of austenite and its crystallographic parameters is challenging. Thus, this work concentrates on the in situ analysis of the austenitization process by means of high-energy X-ray diffraction to study a range of heating rates applied to ferritic–pearlitic and soft annealed initial microstructures. The transformation kinetics from ferrite/pearlite and soft annealed state to austenite, the cementite dissolution behavior and the homogeneity of the freshly formed austenite were analyzed. The results indicate three distinct steps of austenite formation independent of initial microstructure and heating rate: (1) nucleation of carbon-rich austenite at cementite–ferrite interfaces, (2) growth of austenite phase fraction accompanied by a reduction of the carbon content, until reaching the mean carbon content of the steel, followed by growth of the austenite grain size, (3) regarding austenite homogeneity, the combination of austenitization temperature and initial microstructure are the main influencing factors.

Effect of Ultra-Fast Heat Treatment on the Subsequent Formation of Mixed Martensitic/Bainitic Microstructure with Carbides in a CrMo Medium Carbon Steel

The current work focuses on complex multiphase microstructures gained in CrMo medium carbon steel after ultra-fast heat treatment, consisting of heating with heating rate of 300 °C/s, 2 s soaking at peak temperature and subsequent quenching. In order to better understand the microstructure evolution and the phenomena that take place during rapid heating, an ultra-fast heated sample was analyzed and compared with a conventionally treated sample with a heating rate of 10 °C/s and 360 s soaking. The initial microstructure of both samples consisted of ferrite and spheroidized cementite. The conventional heat treatment results in a fully martensitic microstructure as expected. On the other hand, the ultra-fast heated sample shows significant heterogeneity in the final microstructure. This is a result of insufficient time for cementite dissolution, carbon diffusion and chemical composition homogenization at the austenitization temperature. Its final microstructure consists of undissolved spheroidized cementite, nano-carbides and martensite laths in a ferritic matrix. Based on EBSD and TEM analysis, traces of bainitic ferrite are indicated. The grains and laths sizes observed offer proof that a diffusionless, massive transformation takes place for the austenite formation and growth instead of a diffusion-controlled transformation that occurs on a conventional heat treatment.

Enhancing austenite stability in a new medium-Mn steel by combining deep cryogenic treatment and intercritical annealing: An experimental and theoretical study

We have systematically investigated the effect of deep cryogenic treatment (DCT) and intercritical annealing (IA) on the microstructural evolution and mechanical properties of a hot rolled medium-manganese transformation-induced plasticity (TRIP) steel. By intercritical annealing of DCT-treated steel in the fully martensitic state, ultra-fine-grained (UFG) austenite with different morphologies, namely lath-like and granular, was obtained. It is interesting that carbon segregation caused by DCT promoted the formation of carbide which can be additional nucleation sites for austenite besides the boundary of lath-martensite during austenite revision treatment (ART). Furthermore, the process of austenite growth was simulated by DICTRA and it revealed that austenite nucleated at the boundary of lath-martensite and carbide, and then grew into different morphologies, i.e. lath-like and globular, respectively. Moreover, the dissolution of cementite and finite diffusion distance of Mn limited austenite growth and partitioning of Mn, which resulted in refined grain size and high degree of Mn-enrichment. Consequently, enhanced mechanical-stability of austenite was obtained by DCT+IA, which ensured outstanding combination of mechanical properties, i.e. yield strength of 807 MPa, tensile strength of 1650 MPa and total elongation of 25.3%. The improved strength and ductility was attributed to stabilized austenite that enabled the moderate evolution of TRIP-related work hardening.

Microstructure and Properties of 1.0C–1.5Cr Bearing Steel in Processes of Hot Rolling, Spheroidization, Quenching, and Tempering

A novel online subcritical spheroidization annealing technology is proposed. To verify the validity and advantage, microstructure, and properties of 1.0C–1.5Cr bearing steel in three process of hot rolling, spheroidization, quenching, and tempering are investigated. It is demonstrated that the refining of hot rolled microstructure is beneficial to obtaining finer spheroidized microstructure, which accelerates cementite dissolution during the austenitization process of subsequent quenching treatment. When the ferrite grain size of spheroidized microstructure decreases from 8.3 to 1.7 µm, and mean diameter of spheroidized cementite decreases from 0.38 to 0.22 µm, the prior austenite grain size of the microstructure quenched from 850 °C decreases by about 27%, and mean diameter of undissolved cementite decreases by at least 20%. The fatigue limit of the tested steel after quenching and tempering is measured. It is increased from 789 to 986 MPa due to the comprehensive effect of the refining of both undissolved cementite and prior austenite grain, and the increase of retained austenite content in the martensite matrix.

Effect of Sliding Wear on Surface Microstructure and Wear Property of D2 Wheel Steel

In this paper, the surface microstructure and wear property of D2 wheel steel under sliding wear condition were studied by MRH-30 sliding wear tester. After testing, a transmission electron microscope (TEM), scanning electron microscope (SEM) with electron backscatter diffraction (EBSD), and micro-hardness testers were used to characterize the surface microstructure of samples with different cycles. The results show that the wear losss samples are increased as the increase of cycles, and the wear loss of wheel samples is higher than that of rail samples. The surface hardness and thickness of deformation layer of wheel samples are increased as the cycles increase. After sliding wear, the samples surfaces form the white etching layer with the thickness of several microns. Through the analysis of surface microstructure of sample with 12,000 cycles, the lamellar cementite in pearlite is fragment into cementite particles with the decrease of depth from surface, and the cementite is dissolved at surface to lead to the form of white etching layer. The ferrite grains are refined gradually and the fraction of high angle grain boundary is increased with the decrease of depth from surface. The nanosgrains layer of ferrite grains with 5 μm thickness is formed. According to the result of finite element simulation of contact surface temperature, the formation of surface nanograins and the dissolution of cementite are caused by the severe plastic deformation. The fiber structure of samples is formed after sliding wear, with direction of .

Homogenisation of austenite during non-equilibrium heating in hypereutectoid steels

ABSTRACTThe homogenisation (Stage III) of austenite during non-equilibrium heating of hypereutectoid steels has been characterised for the first time through dilatometric strain measurements and electron microscopic techniques. It has been found that the homogenisation temperature (Ach) is significantly higher than the cementite dissolution temperature (Acm). Though there is a complete conversion of low-temperature constituents to austenite above the Acm, a significant segregation of carbon in the austenite is observed below the Ach. Performing heat treatment with an incomplete austenitisation between the Acm and the Ach temperature can affect the subsequent on-cooling phase transformations. It is proposed that the Ach temperature should be used as the full austenitisation temperature in hypereutectoid steels.

Metastable bcc-Cu precipitation behavior and its effect on microstructure and strengthening of as-cast Fe–B alloy containing 3 wt% copper content

The metastable bcc-Cu precipitation behavior, with particular focus on effect of metastable bcc-Cu on microstructural evolution and strengthening is investigated by experiment and calculation simulation in an as-cast Fe–B alloy containing 3 wt% Cu content. The results show that the metastable bcc-Cu precipitates with the size of 3 to 5 nm can precipitate from the α-Fe (pearlite). During the precipitation process, the metastable bcc-Cu can act as the highly effective heterogeneous nuclei of pearlitic ferrite, which results in the formation of abnormal pearlite. Additionally, the existence of bcc-Cu will lead to the refinement of abnormal pearlitie, the dissolution of abnormal pearlitic cementite, and the strengthening of pearlite. The microstructural evolution process can be essentially described as: L (liquid phase) → γ (austenite), L → γ + M2B(boride), γ → P(normal pearlite), γ → P + bcc-Cu1 (the first precipitates), γ → P + P*(abnormal pearlite) + bcc-Cu2 (the second precipitates) + bcc-Cu1, and γ → M (martensite). Moreover, the calculation simulation analyses are in good agreement with the experimental results.

Cementite evolution in medium manganese twinning-induced plasticity steels

The precipitation and dissolution of cementite in austenitic medium Mn high C steels capable of deformation-induced twinning were studied based on the associated length changes. In continuous dilatometry heating cycles, the net contraction associated with the precipitation of cementite resulted in a decrease in the apparent coefficient of thermal expansion. The expansion associated with the reverse process of cementite dissolution in austenite, on the other hand, increased the apparent coefficient of thermal expansion. The contraction associated with the cementite precipitation was found by in-situ X-ray diffraction measurements to originate from the decrease in the lattice parameter of austenite. The latter contraction more than offset the expansion expected based on the pure consideration of austenite replacement by cementite. A model was developed to determine the evolution of cementite fraction based on the dilatometry data and the corresponding X-ray diffraction results during continuous heating cycles. The introduced dilatometry method confirmed the effect of aluminum in retarding the cementite precipitation in medium Mn high C steels.

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.

Influence of Initial Pearlite Morphology on the Microstructure Evolution During Heat Treatment of 1.0C–1.5Cr Steel

Effect of initial pearlite morphology on the microstructure evolution during spheroidization annealing and subsequent hardening treatment was studied in hot-rolled 1.0C–1.5Cr bearing steel. The ferrite-to-austenite transformation in spheroidization annealing can be accelerated by refining the pearlite interlamellar spacing and pearlite colony size. Decreasing interlamellar spacing is also beneficial to increasing the number density of undissolved cementite, leading to the refining of final spheroidized cementite. During the re-austenitizing process of hardening treatment, the smaller initial spheroidized cementite leads to faster cementite dissolution and finer undissolved cementite particles. The prior austenite grain size after hardening treatment can also be decreased by refining the initial pearlite microstructure.

Mixed-mode model for ferrite-to-austenite phase transformation in dual-phase steel

A mixed-mode model is proposed to predict the austenite formation during intercritical annealing of a low-carbon steel with ferrite-cementite microstructure. The transformation kinetics is decribed by carbon diffusion in combination with an interface mobility, representative of complex phenomena slowing down the transformation rate (solute drag, slow diffusivity of other alloying elements, cementite dissolution). The model approach is formulated to be relevant for complex chemistries typical of industrial steels. Model predictions have been successfully benchmarked with experimental data including heating and holding stages at various heating rates and holding temperatures.

TRANSFORMATION OF CARBIDЕ PHASE IN RAILS AT LONG-TERM OPERATION

Evolution of carbide phase in surface layers of volume (passed gross tonnage 500 and 100 million tons) and differentially hardened rails (passed tonnage – 691.8 million tons) to a depth of 10 mm along the central axis and along the rail head fillet was studied by means of transmission electron diffraction microscopy. The grains of lamellar perlite, ferrite-carbide mixture, structurally free ferrite are analyzed. The flow of two complementary mechanisms of transformation of carbide phase of steel in the surface layers during the rails operation was identified: mechanism of cutting cementite particles and their subsequent­ transfer into the ferrite grains or plates volume (in perlite structure); mechanism of cutting and following dissolution of cementite particles, transition of carbon atoms to dislocations (into the Cottrell­ clouds and the dislocation centers), transfer of carbon atoms within dislocations to the volume of grains (or plates) of ferrite, with the following repeated formation of nanoscale cementite particles. A fragmented dislocation substructure is formed instead of former plates. Fragments boundaries decorate places where cementite-α phase interphase boundaries used to be. The main reason for dissolution of cementite is that it is energetically more preferable for carbon atoms to be on dislocation centers and on subboundaries than in cementite lattice. Binding energy of carbon atom-dislocation is 0.6 eV, for carbon atom-subboundary bond it is 0.8 eV, while in cementite it is held by 0.4 eV. Formation of elastoplastic stress fields is detected, concentrators of which are intra and interphase boundaries between grains of ferrite and perlite, cementite and ferrite plates of perlite colonies, particles of globular cementite and ferrite. The main sources of curvaturetorsion of metal lattice of rails metal are intraand interphase bounda­ries­ of grain separation of ferrite and perlite, cementite and ferrite plates of perlite colonies, particles of globular cementite and ferrite. Approaching to the rolling surface, number of stress concentrators and amplitude of internal fields of longrange stress are increasing.

Heavy Deformation of Patented Near-Eutectoid Steel

Evolution of microstructure in the patented near-eutectoid steel, forged under varying situations, is critically examined in the present investigation. Steel with 0.74 wt.% carbon is isothermally annealed at 500 °C to obtain fine pearlite microstructure. Steel samples, so patented, are subjected to mechanical deformation by forging at various temperatures with different amount of thickness reduction. Microstructural analyses have revealed that mechanical deformation by forging at lower temperatures brings about partial dissolution of cementite, which is followed by the formation of e-carbide in the microstructures. In contrast, cementite is precipitated within ferrite matrix upon warm or hot forging at higher temperatures. It is further observed that increasing deformation percent during low-temperature forging reduces interlamellar spacing of pearlite, whereas an opposite trend is noticed in case of deformation at higher temperature; moreover, deformation induced the change in interlamellar spacing and formation of fine carbide phases in microstructures has caused appreciable enhancement in hardness of the steel.

An investigation of phase transition on the microstructural characteristic and creep behavior for the SA508 Gr.3 steel used for nuclear reactor pressure vessels

The purpose of this work was to describe and discuss the effect of phase transition on the creep behavior of SA508 Gr.3 steel, used in Gen III nuclear reactor pressure vessels (RPVs). The phase transition will occur in the RPV steel, once the strategy of in–vessel retention (IVR) exerts its effect in severe accidents. The microstructural evolution of the steel during phase transition was revealed, and the creep tests were conducted at 650–850°C, to investigate the effect of phase transition on the creep behavior. The microstructural characteristics in the heat–treated and crept specimens were analyzed by scanning electron microscopy coupled with transmission electron microscopy. Results show that: the phase transition takes place at 700–800°C with the formation of austenite, dissolution of cementite, coarsening of M2C carbides and subgrains. The creep tests indicate that the phase transition significantly affects the creep rupture life. Through detailed microstructural examination, the predominant mechanism in determining the creep rupture life of the steel before and after phase transition was unveiled. The work performed will provide basic date to understand and implement the IVR strategy.

Synchrotron and neural network analysis of the influence of composition and heat treatment on the rolling contact fatigue of hypereutectoid pearlitic steels

A series of experimental hypereutectoid pearlitic steels were tested under rolling contact sliding conditions using a lubricated twin-disc setup to study the influence of different chemical compositions and heat treatments on rolling contact fatigue life. Tested samples were then characterised using microscopy and synchrotron measurements as a function of depth from the contact surface. Results, analysed through neural networks, indicate that the most influential factor in lengthening the number of cycles to crack initiation of hypereutectoid steels is hardness, attained by increasing the cooling rate from the hot rolling temperature, but adequate alloying additions can enhance it further. The harder, fast-cooled samples displayed less plastic flow at the surface than the softer slow-cooled ones. With regard to chemical composition, silicon was found to strengthen the ferrite thus reducing strain incompatibilities with the cementite, preventing in this way the fragmentation and eventual dissolution of the lamellae. This is beneficial since larger depths of cementite dissolution were found in samples with lower cycles to crack initiation for a given cooling rate (hardness). Samples containing vanadium lasted longer and displayed less plastic deformation at the surface than those without, at a similar hardness.