Austenite Grain Size Research Articles

Effect of Partitioning Treatment on the Mechanical Property of Fe-0.19C-1.47Mn-1.50Si Steel with Refined Martensitic Microstructure

In order to understand the effect of microstructural features on the mechanical property, quenching and partitioning (Q&P) and quenching and tempering (Q&T) treatments were carried out on a cold-rolled low-carbon Fe-0.19C-1.47Mn-1.50Si steel sheet. It has been shown that because of the rolling in advance, the grain size of prior austenite was dramatically reduced, which resulted in a great decrease in martensite packet/block size and an increase in dislocation density in martensite in the as-quenched state. However, there was no obvious change in average lath size. Different from Q&T treatment, Q&P not only stabilized a large amount of retained austenite, but also led to a serious carbon depletion in martensite as revealed by X-ray diffraction and three-dimensional-atom-probe analyses. In Q&T and Q&P samples, refining martensitic microstructure improves both the strength and impact toughness markedly but does not affect the elongation very much. Compared with Q&T sample, Q&P one is softer due to the existence of considerable amount of retained austenite and less carbon content in martensite, i.e., it has higher elongation and impact toughness but lower strength. Analyses indicated that the strength loss caused by carbon depletion in martensite is critical which has even completely covered up the strengthening effect of microstructural refinement. On the other hand, the carbon depletion in martensite is more essential in improving impact toughness, comparing the role of microstructural refinement and the existence of more retained austenite. Through a combination of rolling and Q&P processes, the refined Q&P microstructure was achieved for a greatly improved product of strength and elongation and a much lower ductile-to-brittle transition temperature.

Prediction of the austenite-grain size of microalloyed steels based on the simulation of the evolution of carbonitride precipitates

Kinetic calculations of the evolution of carbonitride precipitates in low-alloy steels with Nb and Ti have been performed for different temperatures of austenitizing. Based on the data of the kinetic simulation of the ensembles of carbonitride precipitates, the expected size of the austenite grain has been calculated using different models. The results obtained have been compared with experimental data. It has been shown that the best agreement with the experiment is achieved for the high-temperature region (1150–1250°C) when using the Gladman model (with the parameter Z = 2) with allowance for the polydispersity of the ensemble of precipitates.

Effect of Intercritical Annealing Temperature on Martensite and Bainite Start Temperatures After Partial Austenitization

The microstructure evolution of dual-phase steel during partial austenitization was investigated for different intercritical annealing temperatures between Ac1 and Ac3 temperatures. Partial austenitization may result in different austenite volume fraction, chemical composition, and grain size of austenite depending on the intercritical annealing temperature. This study examines the effect of intercritical annealing temperature on M s and B s temperatures for dual-phase steels. M s and B s were measured experimentally for different intercritical annealing temperatures by using dilatometry and were compared with calculated values from empirical formulas. The grain sizes of the final microstructures were also quantitatively analyzed. It was shown that M s depends on the intercritical annealing temperature and austenite grain size. It was concluded that this double effect is attributed to the intercritical annealing temperature, which is responsible for both austenite chemical composition and grain size.

Three-Dimensional Numerical Model Considering Phase Transformation in Friction Stir Welding of Steel

A three-dimensional (3D) thermo-mechanical model is developed considering the phase transformation occurring during the friction stir welding (FSW) of steel, and the simulated result is compared with both the measured temperature distribution during FSW and the microstructural changes after FSW. The austenite grain size (AGS) decreases significantly because of the frictional heat and severe plastic deformation generated during FSW, and the decreased AGS accelerates the diffusional phase transformation during FSW. The ferrite phase, one of the diffusional phases, is developed mainly in mild steel, whereas the bainite phase transformation occurs significantly in high-strength steel with large hardenability. Additionally, transformation-induced heat is observed mainly in the stir zone during FSW. The measured temperature distribution and phase fraction agree fairly well with the predicted data.

Investigation on decomposition behavior of austenite under continuous cooling in vanadium microalloyed steel (30MSV6)

In the present study, investigations are focused on microstructural evolution and the resulting hardness during continuous cooling transformation (CCT) in a commercial vanadium microalloyed steel (30MSV6). Furthermore, the effects of cooling rate and austenite grain size (AGS) on CCT behavior of the steel have been studied by employing high-resolution dilatometry. Quantitative metallography accompanied with scanning electron microscopy (SEM) has efficiently confirmed the dilatometric measurements of transformation kinetics and austenite decomposition products. A semi-empirical model has been proposed for prediction of microstructural development during austenite decomposition of the steel and the resultant hardness. The model consists of 8 sub-models including ferrite transformation start temperature, ferrite growth, pearlite start temperature, pearlite growth, bainite start temperature, bainite growth, martensite start temperature and hardness. The transformed fractions of ferrite, pearlite and bainite have been described using semi-empirical Johnson–Mehl–Avrami–Kolmogorov (JMAK) approach in combination with Scheil's equation of additivity. The JMAK rate parameter for bainite has been formulated using a diffusion-controlled model. Predictions of the proposed model were found to be in close agreement with the experimental measurements.

Microstructures and mechanical properties of dual phase steel produced by laboratory simulated strip casting

Conventional dual phase (DP) steel (0.08C–0.81Si–1.47Mn–0.03Al wt.%) was manufactured using simulated strip casting schedule in laboratory. The average grain size of prior austenite was 117±44μm. The continuous cooling transformation diagram was obtained. The microstructures having polygonal ferrite in the range of 40–90%, martensite with small amount of bainite and Widmanstätten ferrite were observed, leading to an ultimate tensile strength in the range of 461–623MPa and a corresponding total elongation in the range of 0.31–0.10. All samples exhibited three strain hardening stages. The predominant fracture mode of the studied steel was ductile, with the presence of some isolated cleavage facets, the number of which increased with an increase in martensite fraction. Compared to those of hot rolled DP steels, yield strength and ultimate tensile strength are lower due to large ferrite grain size, coarse martensite area and Widmanstätten ferrite.

Effect of Tantalum Content on Creep Properties of CLAM Steel

This paper presents the results of an experimental study about the effect of tantalum content on the creep properties of China low activation martensitic steel. Four heats of the steel with tantalum content ranging from 0.027 to 0.18 wt% have been fabricated and the creep tests have been carried out at 550 °C over a stress range of 230–240 MPa. The results indicated that the elevation of tantalum content caused an obvious increase in the rupture time of one order of magnitude and a dramatically decrease in creep rate for one order of magnitude. Both the creep rupture time and minimum creep rate revealed good linear relationships versus tantalum content on log–log scale, and empirical models were built up. The improvement of creep properties could be attribute to the change of microstructure caused by the increase of tantalum content. The microstructure observation indicated that the average grain size of the prior austenite decreased and the precipitation of Ta-riched MX was promoted with the addition of tantalum.

Effect of intercritical annealing on the Lüders strains of medium Mn transformation-induced plasticity steels

Abstract The present work systematically investigates the effects of starting microstructure and intercritical annealing temperature on the tensile properties and Luders strain of transformation-induced plasticity steels containing 5 wt.% Mn. It is found that higher intercritical annealing temperature leads to smaller Luders strain, lower yield strength and higher ultimate tensile strength. The starting cold-rolled microstructure produces much coarser microstructural constituents and larger Luders strain than the martensitic one. It is concluded that the fraction and size of austenite grains and the amount of carbide formed during intercritical annealing are the most important microstructural factors to determine the Luders strain rather than the fraction of retained austenite and the grain size of the ferritic phase.

Steel GX120Mn12 Modifying by Ultra Disperse Powders of Refractory Metal Oxides

The article provides a consideration of the technology of producing steel GX120Mn12 with high manganese content modified by the special modifier to improve constructional durability of details manufactured. The ultra disperse powder of refractory metal oxides was used to carry out the process of modifying. The samples were produced in the series of cast and tested mechanically. Structural research demonstrated improvement of steel microstructure and that of its mechanical characteristics. It also revealed that the size of austenite grain and nonmetallic inclusions on borders of grains were reduced, and sulphide inclusions were dissolved partially. It has positive impact on work characteristics of this steel.

DICTRA Simulation of Holding Time Dependence of NbC Size and Experimental Study of Effect of NbC on Austenite Grain Growth

The effect of austenitizing temperature and holding time on the prior austenite grain size was examined in both the transverse and the longitudinal directions of the samples that were made of the offshore structure steel EQ70. The grain size of the prior austenite was measured by using the linear intercept method. The equilibrium phase diagram was used to explain the abnormal grain growth. The equilibrium precipitation of steel EQ70 was calculated by Thermo-Calc software package, and the relationship between NbC size and the holding time was simulated based on DICTRA. The experimental results show that the prior austenite grain size is initially insensitive to increasing the austenitizing temperature from 1123 K (850 °C) with holding times from 1 to 6 hours, and presents a sudden growth at approximately 1373 K (1100 °C). The growth of the austenite grain size is also insensitive to increasing the holding time while the soaking temperature is lower than 1223 K (950 °C) or higher than 1373 K (1100 °C), and a sudden growth of grains takes place as the holding time is prolonged from 4 to 5 hours at the temperatures between 1273 K and 1323 K (1000 °C and 1050 °C). The results of DICTRA simulation and TEM observation confirm that the abnormal grain growth behavior at 1373 K (1100 °C) was influenced by coarsening of NbC with radius larger than the average and full dissolution of AlN and almost full dissolution of NbC with radius equal to or less than the average, while the same behavior between 1223 K and 1273 K (950 °C and 1000 °C) was caused by coarsening of NbC with radius larger than the average and the full dissolution of AlN but partial dissolution of NbC with radius equal to or less than the average. The present experimental and simulation results can provide a useful reference for determining the austenitizing parameters of steel EQ70.

The size effect of initial martensite constituents on the microstructure and tensile properties of intercritically annealed Fe–9Mn–0.05C steel

The size effect of α′ martensite constituents before annealing on the microstructure and tensile properties of intercritically annealed medium Mn steel was systematically investigated. The cold-rolled Fe–9Mn–0.05C (wt%) steel was austenized at three different temperatures, water-quenched to room temperature to vary the sizes of the martensite constituents, and then annealed at 640°C for 10min. When the austenizing temperature increased, the sizes of the prior austenite (γ) grains and of the packets and blocks of α′ martensite increased; however, the width of the laths changed only insignificantly. The annealed specimens had a dual-phase microstructure with lath-shaped ferrite (α) and retained γ (γR) phases. The volume fraction of γR decreased with increasing austenizing temperature because the specimen austenized at the higher temperature underwent a slower reverse transformation in the early stage of intercritical annealing. The slowed kinetics of the reverse transformation with increasing austenizing temperature was attributed to the reduction in area of block boundaries which provide nucleation sites for the reverted γ. The widths of the α and γR laths were almost independent of the austenizing temperature. The partitioning of Mn and C atoms from α into γR laths became active with increasing austenizing temperature, resulting in a more stable γR. The specimen austenized at higher temperature exhibited a lower strain hardening rate (SHR) due to the less active transformation-induced plasticity (TRIP) in γR with the higher phase stability. The ultimate tensile strength decreased with increasing austenizing temperature because the SHR was lowered by the less active TRIP with increasing austenizing temperature. The uniform elongation increased with increasing austenizing temperature due to delayed necking caused primarily by the flow stress, which dropped with increasing austenizing temperature.

Austenite Grain Refinement by Reverse α′ → γ Transformation in Metastable Austenitic Manganese Steel

Microstructure of metastable austenitic manganese steel after reverse transformation treatment was investigated using optical microscopy, X-ray diffraction (XRD), electrical resistivity and hardness testing. Austenite grain refinement was successfully achieved by a two-step heat treatment. First, martensite was produced by cooling the solution-treated samples to −196 °C. Then, the deep cryogenic treated samples were heated to 850 °C upon slow or rapid heating. The mean size of original austenite grain was about 400 μm. But the mean size of equiaxed reversion austenite was refined to 50 μm. Microstructure evolution and electrical resistivity change showed that martensite plates underwent tempering action upon slow heating, and the residual austenite was decomposed, resulting in the formation of pearlite nodules at the austenite grains boundaries. The refinement mechanism upon slow heating is the diffusion-controlled nucleation and growth of austenite. However, the reverse transformation upon rapid heating was predominated by displacive manner. The residual austenite was not decomposed. The plate a-phase was carbon-supersaturated until the starting of reverse transformation. The reverse transformation was accompanied by surface effect, resulting in the formation of plate austenite with high density dislocations. The refinement mechanism upon rapid heating is the recrystallization of displacive reversed austenite.

The Physical and Numerical Modelling of Heat Treatment of Experimental Steels for Pipelines

The paper presents the results of the physical and numerical modelling of heat treatment of experimental steels for pipelines. Simulation has been conducted at the Institute of Metal Forming and Safety Engineering of Częstochowa University of Technology. The numerical modelling of heat treatment has been carried using commercial program TTSteel. Based on the results of computer simulation, changes in steel microstructure during continuous cooling have been analysed, and the characteristics of temperature and the diagram of Continuous Cooling Transformation (CCT) have been constructed. Numerical research has been verified running the physical simulation of heat treatment of steel using dilatometer DIL805 A/D. The characteristic temperature of steel and the size of the former austenite grains have been determined. Also, the metallographic examination of the samples was conducted and Vickers hardness was tested. The obtained results have been used for building a real CCT diagram of steel.

Microstructural Developments and Tensile Properties of Lean Fe-Mn-Al-C Lightweight Steels

Concepts of Fe-Al-Mn-C-based lightweight steels are fairly simple, but primary metallurgical issues are complicated. In this study, recent studies on lean-composition lightweight steels were reviewed, summarized, and emphasized by their microstructural development and mechanical properties. The lightweight steels containing a low-density element of Al were designed by thermodynamic calculation and were manufactured by conventional industrial processes. Their microstructures consisted of various secondary phases as κ-carbide, martensite, and austenite in the ferrite matrix according to manufacturing and annealing procedures. The solidification microstructure containing segregations of C, Mn, and Al produced a banded structure during the hot rolling. The (ferrite + austenite) duplex microstructure was formed after the annealing, and the austenite was retained at room temperature. It was because the thermal stability of austenite nucleated from fine κ-carbide was quite high due to fine grain size of austenite. Because these lightweight steels have outstanding properties of strength and ductility as well as reduced density, they give a promise for automotive applications requiring excellent properties.

Effect of Titanium and Niobium on Modifying the Microstructure of Cast K100 Tool Steel

The effects of Ti and Nb on the microstructure of cast K100 tool steel were studied by optical and scanning electron microscopes. The amounts of Ti as 0.3, 0.7, and 1 wt pct and Nb as 0.2 and 1 wt pct were added to the studied steel. The addition of 0.3 wt pct Ti did not result in a considerable change in the size of carbides and prior austenite grain size. However, microstructure of K100 with 0.7 and 1 wt pct Ti was considerably modified (about 55 pct) and a uniform grain size was obtained at different positions (bottom, middle, and top) of the ingot. With addition of 0.2 and 1 wt pct Nb, microstructure was modified and a more uniform grain size was obtained all over the ingot. The average modification of microstructure in the bottom, middle, and top of the ingot was about 22 pct. Both Ti and Nb could effectively decrease the segregation of Cr and C from the bottom (high cooling rate positions) to the top of the ingots (low cooling rate positions). The homogeneity of chemical composition increased with increasing Nb or Ti. In alloy with 0.7 to 1 wt pct Ti, the average size of prior austenite grains was finer than alloys with 0.2 to 1 wt pct Nb. Therefore, Ti was found more capable than Nb in the modification of microstructure and decreasing the segregation of Cr and C in cast K100 tool steel.

Effects of high magnetic field on austenite formation and grain size in a hypereutectoid carbon steel

Abstract

Microstructure Refinement and Property Improvement of Metastable Austenitic Manganese Steel Induced by Electropulsing

Grain refinement efficiency of electropulsing treatment (EPT) for metastable austenitic manganese steel was investigated. The mean grain size of original austenite is 300 μm. However, after EPT, the microstructure exhibits a bimodal grain size distribution, and nearly 70vol.% grains are less than 60 μm. The refined austenite results in ultrafine martensitic microstructure. The tensile strengths of refined austenitic and martensitic microstructures were improved from 495 to 670, and 794 to 900 MPa respectively. The fine grained materials possess better fracture toughness. The work-hardening capacity and wear resistance of the refined austenitic microstructure are improved. The reasonable mechanism of grain refinement is the combination of accelerating new phase nucleation and restraining the growth of neonatal austenitic grain during reverse transformation and rapid recrystallization induced by electropulsing.

Microstructures and tensile properties of 304 steel with dual nanocrystalline and microcrystalline austenite content prepared by aluminothermic reaction casting

The microstructures of 304 stainless steels with different amounts of nanocrystalline and microcrystalline austenite prepared by an aluminothermic reaction casting, without and with annealing at 1073 K for 8 h, have been investigated by X-ray diffraction, an electron probe micro-analyser, a transmission electron microscope and a scanning electron microscope. The steels, both without and with annealing, consisted of different dual nanocrystalline and microcrystalline austenite combinations and a little nanocrystalline δ ferrite, while the average grain size of the nanocrystalline austenite increased from 19 to 26 nm and volume fraction of the microcrystalline austenite increased from 17 to 30% after annealing. The tensile strength of the steel was dramatically increased from 500 to 1000 MPa and the tensile elongation ratio increased from 8 to 12% after annealing. However, the tensile strength was decreased to 600 MPa and the tensile elongation ratio increased from 12 to 22% after an annealing at 1273 K. The combination of dual nanocrystalline and microcrystalline austenite obtained after the annealing at 1073 K results in the best tensile properties.

Influence of Prior Austenite Grain Size on the Dispersion of Fatigue Crack Propagation Thresholds in Weld Metal

Fatigue crack propagation threshold is an important property for the structures with long life, such as nuclear turbine welded rotor. The fatigue crack propagation thresholds of multi-layer, multi-pass SAW weld metal of nuclear turbine rotor simulate are tested. But a big dispersion is found among test values, even at the same stress ratio . The dispersion of critical points between stable propagation zone and near-threshold zone is confirmed as an important reason. After the observation of microstructures around the critical points by backward inference method in different specimens, a good correspondence between the sizes of prior austenite grains and the maximum sizes of monotonic plastic zone at the crack tip is established. The inhomogeneous microstructures in SAW weld metal are the basic cause to dispersion of fatigue crack propagation thresholds at the same stress ratio.

From Micro to Nano Scale Structure by Plastic Deformations

Nowadays, the strategy for improving of mechanical properties in metals is not oriented to alloying followed by heat treatment. An effective way how to improve the mechanical properties of metals is focused on the research looking for some additional structural abilities of steels. Structural refinement is one of the ways. Refinement of the austenitic grain size (AGS) carried out through plastic deformation in a spontaneous recrystallization region of austenite, formation of AGS by plastic deformations in a non-recrystallized region of austenite will be considered as potential ways for AGS refinement. After classic methods of plastic deformations, next structure refinement can be obtained by an application of severe plastic deformation (SPD) methods.