Influence Of Austenite Grain Size Research Articles

Correlation of austenite grain size with ε-/α′-martensitic transformation and mechanical properties in a high-manganese damping steel

One of the core studies on low stacking fault energy (SFE) high-manganese steel is to maintain excellent damping capacity while achieving high strength and toughness. The austenite grain size (AGS) in high-Mn steel affects its SFE and even the strength-plasticity and damping capacity of the steel, but the detailed mechanism related to martensitic transformation has not been clarified. In this study, Fe-17Mn high-Mn damping steel is used as the material to obtain austenite grains of different sizes by controlling the cold rolling and annealing processes. By using in-situ tensile testing, damping capacity testing, and various microscopic analytical techniques, the influence of AGS on the thermal/deformation induced martensitic transformation, work hardening behavior, mechanical properties and damping capacity of the steel is mainly studied. The results indicate that the refinement of austenite grain increases the SFE of high-Mn damping steel and reduces the amount of thermal induced ε-martensite (εth) and stacking faults (SFs). The deformation induced martensitic transformations (DIMTs) in high-Mn damping steel during tensile deformation include γ→εD, εth→α′ and εD→α′ transformation. The deformation induced ε-martensitic transformation is more helpful to improve the work hardening rate than deformation induced α′-martensitic transformation. Due to the concentration of strain partitioning at grain boundaries, twin boundaries and phase boundaries, refinement of austenite grain can promote deformation induced ε-martensitic transformation and improve the steel strength. However, excessively small austenite grains can suppress deformation induced α′-martensitic transformation leading to a significant decrease in elongation. Under the synergetic effects of grain refinement strengthening and multiple martensitic transformations, when the average AGS is about 4.0 μm, the steel has high strength and high plasticity, with a tensile strength of 919 MPa, a yield strength of 518 MPa and an elongation of 46.4 %. The product of tensile strength and elongation is up to 42.6 GPa·%. At low or high strain amplitudes, the damping capacity of this steel shows an opposite trend with the refinement of AGS. The damping capacity of the steel gradually increases with grain refinement at low strain amplitudes of 0.01 %–0.065 %, mainly due to the reduction of the content of weak pinning points. At high strain amplitudes of 0.065 %–0.1 %, the damping capacity of the steel gradually decreases for the reduction of damping sources with the refinement of grain size.

Influence of Austenite Grain Size on the Variant Configurations of Martensite in a Fe-30.5Ni-0.155C Alloy

A Fe-30.5wt%Ni-0.155wt%C alloy was annealed at two different temperatures to produce two different austenite grain sizes. In the coarse-grained specimen, hierarchical configurations of variants are formed and carefully analyzed using EBSD. These typical patterns result from the alternate formation of two perpendicular plate groups of variants over several length scales, and two distinct types of mechanical couplings are shown to occur sequentially in the process of the transformation of an austenitic grain. In the fine-grained specimen, the martensite start temperature is depressed below liquid nitrogen temperature, and the martensitic transformation can only occur under stress assistance. Grain size reduction brings about a dramatic change in the morphology of martensite and its configurations. Martensite is fully twinned, and martensite variants arrange themselves into self-accommodating configurations involving all four variants of the same plate group. Those specific configurations share striking similarities with those usually encountered in conventional shape memory alloys. The reversion of such microstructures upon heating is believed to be at the origin of the observed shape memory effect.

Simulation of B Segregation at Austenite Grain Boundary in Low Carbon Steel

The diffusion and segregation of boron (B) at the austenite grain boundary have been simulated in continuously cooled low carbon steels to elucidate the role of vacancy, the influence of austenite grain size and thermodynamic interaction between solutes. First‐principles calculations were carried out to obtain the stable configuration of B‐vacancy complexes and the binding energies therein. The thermodynamic equations of a hybrid interstitial‐substitutional solid solution were utilized to evaluate the contribution of interstitial B atoms and B‐vacancy complexes to boundary enrichment. The latter contribution did not appear to be significant probably due to their small concentration and/or low mobility. The grain growth of austenite is likely to play a significant role in B enrichment at elevated temperatures. Due to the strong C‐Mo interaction, the carbon flux in the grain interior did decrease, but the interaction within the grain boundary had a much greater influence on the segregation amount. The B enrichment in an Fe‐C‐B‐Mo quaternary alloy was simulated for comparison with experiment. A sophisticated approach, e.g., segregation energies spectrum, may be necessary to reproduce adequately the B segregation behavior, which also depends sensitively on process parameters and microstructure.

Variant Selection in Phase Transformation and its Influence on Texture and Martensite Starting Temperature in Steel

A mechanism is proposed that can predict the textures as well as the influence of austenite grain size on martensite starting temperature in steels. The anisotropic elastic energy by shape change of martensite changes depending upon variants in neighbour austenite grains, whereas it does not in immediate parent grains in which martensite grows. A mechanism based on this behaviour of elastic energy can explain not only the texture formation of martensite but also the well-known austenite grain size dependence of Ms, particularly the abrupt drop of about 30K in the grain size range below 20 μm.

THE CALCULATION OF STRESS INTENSITY FACTOR STEEL OF RAILWAY WHEELS

From an analysis of the dependence complex of carbon steel properties on structural parameters, it was found that for an isostructural state, the influence of austenite grain size on impact strength exceeds the dependence on carbon content. As a result of explaining correlation relationships between individual mechanical characteristics, to evaluate critical stress intensity factor, a relationship is proposed based on the use of impact strength. The proportionality coefficient in proposed dependence is determined by ratio of elongation to narrowing at tensile test.

Effect of prior austenite grain size on bainitic transformation above and below Ms in medium Mn steel

The effect of prior austenite grain (PAG) size on bainitic transformation kinetics below Ms in 0.2C-3.92Mn-1.6Si steel has been investigated using a dilatometer. The samples were austenitized at two different temperature to achieve the average PAG size of ∼10.9 μm and ∼17.3 μm. The results show that above Ms the austenite grain size plays a significant role in bainitic transformation kinetics. However, the influence of austenite grain size on bainitic transformation kinetics below Ms becomes weaker which is ascribed to the acceleration of prior martensite on bainitic transformation below Ms.

Austenite stability and deformation-induced transformation mechanism in cold-rolled medium-Mn steel

The austenite stability and tensile properties of intercritically annealed samples of cold-rolled medium-Mn steel (0.15C–5Mn-1.1Al) were investigated. The influence of austenite grain size and element concentration on the mechanical stability of austenite was studied, which demonstrated that the C content in austenite was a critical factor governing the mechanical stability of austenite. The thermal stability of austenite in cold-rolled medium-Mn steel samples annealed for different durations at different temperatures was quantitatively analyzed. The results showed that the effect of annealing temperature on the thermal stability of austenite was stronger than that of annealing time. In addition, the samples subjected to heat treatment under different conditions resulted in different work hardening (WH) behavior and mechanical properties due to variations in the mechanical stability of the retained austenite. It was concluded that these differences were related to the changes in stress- and strain-induced transformation modes of the retained austenite.

Influence of Austenite Grain Size on Mechanical Properties after Quench and Partitioning Treatment of a 42SiCr Steel

This paper examines how the initial austenite grain size in quench and partitioning (Q-P) processes influences the final mechanical properties of Q-P steels. Differences in austenite grain size distribution may result, for example, from uneven heating rates of semi-finished products prior to a forging process. In order to quantify this influence, a carefully defined heat treatment of a cylindrical specimen made of the Q-P-capable 42SiCr steel was performed in a dilatometer. Different austenite grain sizes were adjusted by a pre-treatment before the actual Q-P process. The resulting mechanical properties were determined using the upsetting test and the corresponding microstructures were analyzed by scanning electron microscopy (SEM). These investigations show that a larger austenite grain size prior to Q-P processing leads to a slightly lower strength as well as to a coarser martensitic microstructure in the Q-P-treated material.

The influence of the austenite grain size on the microstructural development during quenching and partitioning processing of a low-carbon steel

The influence of the prior austenite grain size (PAGS), varying between 6 and 185 μm, on the microstructural development of a low carbon steel during quenching and partitioning (Q&P) processing is investigated. The effect on the size and morphological aspects of the microconstituents is discussed based on the kinetics of carbon redistribution between martensite and austenite upon partitioning conditions of 400 °C and 50 s. Under fixed quenching and partitioning conditions, decreasing the PAGS leads to a more efficient carbon partitioning process through the smaller and more homogeneously distributed phases developed during the first quench. In contrast, the microstructural heterogeneity obtained with larger PAGSs makes it more difficult to control the degree of carbon enrichment in austenite during partitioning and thus the austenite stability. Additionally, large volumes of fresh martensite are more likely to form in the interior of large-scale austenite grains due to the incomplete carbon homogenisation process. To consider the PAGS in the design of Q&P microstructures the selection of an optimum fraction of primary martensite is proposed, which ensures the minimisation of fresh martensite in the final microstructure and the sufficient stabilisation of the austenite phase. This new methodology facilitates the applicability of the Q&P process providing a controlled and reproducible development of optimised Q&P microstructures.

Effect of austenite grain size on acicular ferrite transformation in a HSLA steel

Austenite grain size is well known to have a significant influence on various phase transformations in steels. Although the effects of many thermomechanical processing parameters on acicular ferrite (AF) transformation in HSLA steels have been investigated, little attention has been paid to the influence of austenite grain size. Therefore, in this research, different parameters of solid-solution heat treatment and rough deformation were adopted to generate austenite with different grain sizes and the effects of austenite grain size before deformation on the AF transformation and grain refinement were investigated. It was found that the reduction of prior-austenite grain size (PAGS) from 62.8 μm to 37.0 μm before austenite deformation promotes the AF transformation and simultaneously refines and homogenises the transformed microstructures. A reciprocally increased density of deformation induced dislocations with the reduction of PAGS was proposed to account for these results, which not only increase the nucleation sites of AF but also suppress the lengthening of BF laths. Further reducing PAGS from 37.0 μm to 22.3 μm, the fraction of AF is not increased and the transformed microstructure is not refined. Possible differences in the type, distribution, and density of austenite deformation substructures between austenite with PAGSs of 37.0 μm and 22.3 μm were argued to be responsible for the cease of grain refinement. The relationship between effective grain sizes and the Sv parameter was investigated and an equation relating effective grain sizes with processing parameters was also proposed.

Effect of Nb micro-alloying on microstructure and properties of thermo-mechanically processed high carbon pearlitic steel

Two C-Mn-Si steels with and without Nb micro-alloying are selected in the present study. Both these high carbon steels have been thermo-mechanically processed using Gleeble 3800 simulator. Optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM) were utilised for microstructural characterisation. The austenitising temperature was varied from 1150 °C to 1200 °C followed by hot compression and subjected to free cooling to evaluate the influence of austenite grain size on transformed microstructure and mechanical properties. It is evident that higher austenite grain size is obtained for both types of steels subjected to higher austenitising temperature. The refinement of pearlite interlamellar spacing (<0.20 μm) with degenerated morphology as well as finer prior austenite grain size (<40 μm) is found to be more prominent for Nb micro-alloyed sample. The average hardness values of both the steels are higher for the specimens, treated at a higher austenitising temperature which is attributed to finer interlamellar spacing. Finally, the correlation between the evolving microstructure and resulting mechanical properties (hardness and yield strength) has been made.

Simulation Study on Heating Process of Hot Rolled Steel Strip for X80M Pipeline Steel

The simulation experiments of heating process for X80 pipeline steel were carried out to investigate the influence of austenite grain size and micro alloy element solution. The results showed that a reasonable industrial production process of slab heating control system was developed, and the guidance of good industrial test could be provided for the acquisition of reasonable microstructure of 22mm thickness X80 pipeline steel hot rolled steel strip.

The Influence of Austenite Grain Size on the Mechanical Properties of Low-Alloy Steel with Boron

This study forms part of the current research on modern steel groups with higher resistance to abrasive wear. In order to reduce the intensity of wear processes, and also to minimize their impact, the immediate priority seems to be a search for a correlation between the chemical composition and structure of these materials and their properties. In this paper, the correlation between prior austenite grain size, martensite packets and the mechanical properties were researched. The growth of austenite grains is an important factor in the analysis of the microstructure, as the grain size has an effect on the kinetics of phase transformation. The microstructure, however, is closely related to the mechanical properties of the material such as yield strength, tensile strength, elongation and impact strength, as well as morphology of occurred fracture. During the study, the mechanical properties were tested and a tendency to brittle fracture was analysed. The studies show big differences of the analysed parameters depending on the applied heat treatment, which should provide guidance to users to specific applications of this type of steel.

CRESCIMENTO ANORMAL DO GRÃO AUSTENÍTICO E SUA INFLUÊNCIA NA FRAÇÃO VOLUMÉTRICA DE AUSTENITA RETIDA APÓS A TÊMPERA DE UM AÇO ABNT 5135

Grain size in the steels is a relevant aspect in quenching and tempering heat treatments. It is known that high austenitizing temperature and long time provide an increase in austenitic grain sizes. Likewise, after hardening of low alloy steel, the microstructure consists of martensite and a volume fraction of retained austenite. This paper evaluates the influence of austenite grain size on the volume fraction of retained austenite measured by metallographic analyses and X-ray diffraction. The Mi and Mf temperatures were calculated using an empirical equation and experimentally determined by differential thermal analysis. The mechanical behavior of the steel was evaluated by Vickers microhardness testing. Differently from other results published in the literature that steel hardenability increases with the austenite grain size, it was observed that the increase in austenite grain promotes greater volume fraction of retained austenite after water quenching

Influência da variação do tamanho de grão austenítico na qualidade superficial no processo de microfresamento do aço AISI H13 endurecido

O foco deste trabalho foi o estudo da influência do tamanho de grão austenítico na qualidade superficial durante o processo de micro fresamento de aços endurecidos. Corpos de prova com durezas de 46 HRC e com diferentes tamanhos de grão foram fresados com micro fresas de topo com 0,5 mm de diâmetro. Nos testes experimentais foram empregadas duas velocidades de corte e dois avanços por dente. Os resultados foram analisados empregando-se uma análise de variância e demonstraram que a velocidade de corte e o avanço não tiveram influência no comportamento da rugosidade considerando os parâmetros Ra e Rz. Dessa forma, pode-se observar que o fator mais importante foi o tamanho de grão que influenciou significativamente os valores de rugosidade Ra e Rz. Além disso, observou-se também que a interação das variáveis de entrada não demonstrou influência significativa sobre as respostas rugosidade Ra e Rz.

The Influence of Austenite Grain Size during Welding Simulations of Ferritic Steels

In recent years, considerable progress has been made in the simulation of ferritic steel welding processes. The successful validation of a single-pass autogenous TIG beam weld in SA508 Gr.3 Cl.1 steel has identified key simulation variables required for the accurate prediction of post-weld residual stress in ferritic weldments. The present work outlines a sensitivity study performed to examine the influence of austenite grain growth on predicted solid-state phase transformation kinetics and consequently, residual stress predictions.

Influence of Austenite Grain Size on the Crystallography of Allotriomorphic Ferrite in a Low Carbon Steel

The influence of prior austenite grain size on the crystallography of allotriomorphic ferrite is investigated in a low carbon steel. The results show that as the prior austenite grain size decreasing, the fraction of allotriomorphic ferrites that do not keep K-S orientation relationship with any surrounding prior austenite grains is increased. It is observed that such ferrites usually form at the grain edges or grain corners. It is known that with the grain size decreasing, the fraction of grain edges and corners increases. It is suggested that the free energy of the defects at such nucleation sites is higher than that at grain faces, and the nucleation barrier of ferrite is lower. As a result, the possibility for the ferrite to form that does not have orientation relationship with all surrounding austenite grains is increased at such sites.

Evolution of transformation plasticity during bainitic transformation

Abstract The influences of prior austenitization treatment and the state of applied stresses on the evolution of transformation plasticity strains during isothermal bainitic phase transformation and the resulting microstructures were examined. The key finding is that, upon pre-straining, the amount of transformation plasticity strain under superimposed elastic stresses is dictated by both the prior austenite grain size and the (0.01 % offset) yield strength of the supercooled austenite. Furthermore, the superimposition of internal stresses present due to pre-straining and externally applied stresses results in transformation plasticity strains similar to those obtained when a permanent stress equivalent to the 0.01 % offset yield strength of the supercooled austenite is applied. Another important result is that lower transformation plasticity strains were observed when decreasing the austenite grain size, which is accompanied by an increase in grain boundary area per volume, hindering the growth of preferred variants. Overall, the results clearly lay out the influence of austenite grain size and the particular 0.01 % offset yield strength of the supercooled austenite in limiting the transformation plasticity strains, which has to be incorporated into current models involving bainitic phase transformations.

Influence of austenite grain size on overaging treatment of continuous annealed dual phase steels

A dual phase steel with an initial coarse microstructure was refined, by means of a thermal treatment, to study the influence of the austenite grain size reached during an intercritical annealing treatment on the martensite start temperature. Thus, the effect of the austenite grain size on a subsequent overaging treatment was also investigated. It was found that a coarser austenite grain size leads to a higher martensite start temperature and a lower amount of non-tempered martensite and/or austenite (MA constituents) volume fraction after a continuous annealing and overaging treatment. The non-tempered martensite and/or austenite volume fraction always was higher for the highest overaging temperature.

Influence of austenite grain size on recrystallisation–precipitation interaction in a V-microalloyed steel

By means of torsion tests using small specimens, the influence of austenite grain size on strain induced precipitation kinetics has been determined in a vanadium microalloyed steel. Determination of recrystallisation–precipitation–time–temperature (RPTT) diagrams for two austenite grain sizes allows values of the aforementioned magnitudes to be determined. An ample discussion is made of the quantitative influence found and its relation with nucleation and growth mechanisms of precipitates. The results are compared with the quantitative influence exerted by the other variables, reaching the conclusion that the austenite grain size has a notable influence on strain induced precipitation kinetics which should not be underestimated. Finally, the influence of austenite grain size is included in a strain induced precipitation model constructed by the authors of this work and which also takes into account the other aforementioned variables.