Effect Of Austenite Grain Size Research Articles

Acceleration of the super bainite transformation through a coarse austenite grain size

The effect of austenite grain size on the kinetics of the isothermal bainitic transformation in a high-carbon super-bainitic steel was investigated. Experimental results showed that the transformation of super bainite was accelerated by a coarse austenite grain size. This is because while coarse austenite grains provide less nucleation sites, it is beneficial for bainite sheaf growth. Meanwhile, there is a critical austenite grain size below which there is a distinct grain size effect and above which it is not evident.

Effect of Austenite Grain Size and Composition on Matrix Microstructure and Properties of Steel

The composition and the grain size of austenite together play important role in controlling the matrix phases of the steel through alteration of austenite transformation kinetics. This work presents how altering prior austenite grain size changes the product microstructure in a plain carbon steel and a Nb containing steel after several heat treatment simulations. The different phases present in the microstructure has been identified and quantified. Influence of matrix microstructures on the mechanical properties of the steels will also be discussed.

Effect of Austenite Grain Size on Transformation Behavior, Microstructure and Mechanical Properties of 0.1C–5Mn Martensitic Steel

The effect of austenite grain size on martensitic transformation, particularly with regard to martensite structure, Ms/Mf temperatures, and mechanical properties was investigated in 0.1C–5Mn martensitic steel. Utilizing a newly developed experimental technique that makes it possible to examine phase transformation behavior and conduct tensile testing with the same specimen, we examined these relationships and obtained the following results. Ms temperature decreases as much as 40 K with a decrease in austenite grain size from 254 to 30 μm. Regarding martensite structure, the packet size and the block length decrease, while the lath width does not change, with the refinement of austenite grain size by about one tenth. Grain boundary density, especially high-angle grain boundary density, increases with decreasing austenite grain size. Tensile strength slightly increases though austenite grain size decreases about one tenth. However, reduction in area significantly improves particularly at refined grain sizes of 30 μm. True stress - true strain curves obtained up to fracture elucidates that the austenite refinement substantially improves true fracture strength and greatly increases true fracture strain of martensite, potentially invalidating the conventional concept of a trade-off between strength and ductility. Low C–5Mn martensitic steel produced from fine austenite shows a great possibility having an excellent total balance of strength, ductility and toughness.

Effect of austenite grain size on kinetics of dynamic ferrite transformation in low carbon steel

The effect of austenite grain size on kinetics of dynamic ferrite transformation above Ae3 in a 6Ni–0.1C steel was studied. As the austenite grain size decreased, the onset of dynamic transformation was accelerated. The increase in the fraction of dynamically transformed ferrite was in good agreement with the change in flow stress, i.e. dynamic softening. The kinetics of dynamic transformation could be evaluated by an Avrami-type formula.

Development of dynamic recrystallization maps based on the initial grain size

A series of hot compression tests at the temperature range of 900–1100°C and strain rate of 0.01–1s−1 was carried out using a 304L stainless steel with different initial grain sizes. The hot deformation flow curves, the corresponding strain hardening curves, and the resulting microstructures were analyzed in order to reveal the occurrence of dynamic recrystallization (DRX). Results showed that despite the shape of flow curves, DRX occurs in the range of deformation conditions examined in the current work. Some DRX maps were developed by considering the effect of initial grain size and deformation conditions. These maps showed that the effect of austenite grain size on the progress of DRX in the temperature–strain rate coordinate is considerable and should be taken into account in thermomechanical processing. Since the occurrence and the extent of static and metadynamic recrystallization highly depend on the completion of DRX during deformation, the full DRX maps based on grain size can be used to find the appropriate working conditions based on the desired microstructures in the hot deformation schedules, especially in the dynamic recrystallization controlled rolling.

Effect of austenite grain size and accelerated cooling start temperature on the transformation behaviors of multi-phase steel

The transformation behaviors and microstructures of a low carbon multi-phase steel were investigated by the simulation of deformation-relaxation-accelerated cooling processing, using a Gleeble 3500 thermal-mechanical simulator. A pre-treatment of solid solution at 1200°C was implemented to minimize the influence on transformation from solid solution/precipitation qualities of 0.08%Nb in this steel. On this basis, the effect of austenite grain size and accelerated cooling start temperature were studied individually. The results indicated that the transformation of ferrite in multi-phase steel could be significantly promoted by the refinement of austenite grains and the increase of relaxation time, while the hard phase, such as lath bainite or martensite, could still be obtained with the following accelerated cooling. In contrast, more uniform lower temperature transformed microstructure could form from coarse grain austenite. The potential benefit of austenite grain size on adjusting the proportion of phases in multiphase steel was also discussed.

切欠きを有する高周波焼入れ丸棒の捩り強度に及ぼす旧オーステナイト粒径の影響

An increase in the torsional strength of automotive shafts has been a subject of particular interest. The purpose of this study is to establish a method of increasing strength of shaft steels to mode fracture. The effect of austenite grain size on torsional strength in mode fracture is investigated. The torsional strength in mode fracture improves according to the grain refining of the austenite grain. A ductility fracture is observed partially in the fine grained steel though the initiation site of fracture presents the intergranular fracture in all aspects in the coarse grained steel. The amount of P of the grain boundary segregation decreases by the grain refining as a result of investigating the grain boundary properties by AES. The improvement of strength by the grain refining is possibly guessed that the following effect superimposed it above. The amount of the grain boundary segregation must decrease along with an increase of the area of the grain boundary. The ratio of the low energy grain boundary must increase. It must accompany an increase of the ratio of the low energy grain boundary and the amount of the grain boundary segregation must decrease.

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (∼20 μm) and a coarse average grain size (∼260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ∼534°C.

Effect of Austenite Grain Size on the Fatigue Crack Growth and Arrest Behavior of Carburized Steel

The effect of austenite grain size on the fatigue crack growth and arrest behavior of carburized steel was studied. On the fatigue test of same stress amplitude, the fatigue crack initiation of fine austenite grain steels is delayed in comparison with coarse austenite grain steels, but the difference of crack initiation life between fine grain steels and coarse grain steels is very slight against 107 cycles. The improvement of crack arrest property is dominant for increasing fatigue limit by refinement of austenite grain size. The critical stress intensity factor for crack arrest under fatigue tests at the stress amplitude of the each fatigue limit of carburized steels is calculated by considering the effect of compressive residual stress. As a result, it becomes clear that the finer the austenite grain size, the larger becomes the critical stress intensity factor for crack arrest, and crack arrest property is improved remarkably by the refinement of austenite grain size.

Simulation of the effect of austenite grain size scattering on the kinetics of the γ → α transformation

The effect of scattering of the grain size of initial austenite on the kinetics of the γ → α transformation is analyzed for plain carbon and chromium-bearing two-phase steels. Components of a computer model describing the kinetics of the transformation, which are directly related to the size of the austenite grains, are considered. The structural parameters computed by the model and determined experimentally are compared.

Grain Size Effect on the Martensite Formation in a High-Manganese TWIP Steel by the Rietveld Method

The aim of the present work was to study the effect of austenite grain size (AGS) on the martensite formation in a high-manganese twinning-induced plasticity (TWIP) steel. The results of a quantitative microstructural characterization of the steel by the whole X-ray pattern fitting Rietveld software, materials analysis using diffraction (MAUD), indicated that the volume fraction of a bcc -martensite increases with increasing AGS. However, the value of the stacking fault probability ( P sf ) does not show a large variation for samples with different values of AGS under water-quenching conditions.

The Effect of Austenite Grain Size on the Growth of Different Ferrite Morphologies in a Nb-Microalloyed Steel

The effect of austenite grain size on the austenite to ferrite transformation temperature and different ferrite morphologies and growth behaviour in one Nb-microalloyed (HSLA) steel has been investigated. Three different austenite grain sizes were selected and cooled for obtaining austenite to ferrite and growth behaviour of ferrite. Moreover, samples with specific austenite grain size have been quenched, partially, for investigation of the microstructural evolution. The optical microscopy observation suggested that the nucleation site of ferrite is on edge and inside of austenite grains in Nb- microalloyed steels. Micrographs of different ferrite morphologies show that at high temperatures, where diffusion rates are higher, grain boundary ferrite nucleates both at the edge and corner of austenite grains and grows into both austenite grains. As the temperature is lowered and the driving force for ferrite formation increases, intragranular sites inside the austenite grains become operative as nucleation sites and suppress the grain boundary ferrite growth. With more undercooling,intragranular ferrites are seen to nucleate and grow more extensively , indicating the beginning of displacive transformation. Furthermore, growth rate of intragranular ferrite shows that by increasing of austenite grain size, the growth rate of intragranular ferrite increases extensively and growth rate of grain boundary ferrite decreases. The growth kinetics of grain boundary ferrite shows that this transformation is controlled by the diffusion of carbon in the austenite ahead of the interface.

Effect of austenite grain size on the transformation kinetics of upper and lower bainite in a low-alloy steel

The effect of austenite grain size on the continuous cooling transformation kinetics of upper and lower bainite in a low-alloy steel has been investigated using a dilatometer. The transition temperature between upper and lower bainite is almost constant (430 °C) irrespective of the grain size. The transformation rate of upper bainite increased with a decrease in grain size. The instantaneous transformation rate of lower bainite at each volume fraction is not related to the austenite grain size.

Microstructure Evolution during Tempering of Martensite in a Medium-C Steel

To identify the characteristic microstructural length determining the mechanical properties of a quenched and tempered medium-C steel and its dependence on the prior austenite grain size, different tempering treatments have been carried out after a fully martensitic quenching. The resulting microstructures have been analyzed by Orientation Imaging Microscopy (OIM) and two kind of features have been taken into consideration: packets (i.e. domains delimited by high-angle boundaries) and cells (domains bounded by low-angle grain boundaries). The main results can be summarized as follows: 1. A very weak effect of austenite grain size on packet size was found. 2. A finer packet size was measured at mid-thickness with respect to surface after external and internal water quenching process. This phenomenon was attributed to the effect of the strain path on the phase transformation during quenching. 3. The through-thickness microstructural gradient remains substantially unchanged after tempering. 4. Grains with high-angle boundaries do not significantly grow after tempering; on the contrary, low-angle grain boundaries move, fully justifying the hardness evolution with the tempering temperature.

Excess driving force to initiate martensite transformation in fine-grained austenite

The effect of austenite grain size as the driving force to initiate martensite transformation was analyzed. It is proposed that the constrained propagation of the seminal martensite plate results in a lower transformation temperature in fine-grained austenite, and favors the formation of clusters of partially transformed grains.

Progress in Modeling of Phase Transformation Kinetics

Abstract Several methods representing the evolution of microstructure were introduced, which include the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, Internal State Variable (ISV) framework, Koistinen-Marburger (K-M) equation, modified Magee's rule and phase field model, etc. By combining calculation of martensite transformation kinetics, considering the selection of parameters with the effect of austenite grain size (AGS), some suitable ways of obtaining better results have been proposed.

Effect of Austenite Grain Size on Martensitic Transformation of a Low Alloy Steel

There are many empirical equations for predicting martensite start temperature (Ms) and the kinetics models of martensitic transformation of plain carbon and low alloy steels. The Ms temperature equations are only dependent upon the chemistry, while the martensite transformation kinetics models are based on the degree of undercooling below Ms temperature. However, the prior austenite grain size (AGS) is also expected to influence both Ms temperature and martensite transformation kinetics as it does in diffusive transformations. In this study, herefore, both Ms temperature and martensite transformation kinetics of a low alloy steel with different austenite grain sizes were investigated using a dilatometer. The new Ms equation and martensite transformation kinetics model including the AGS effect are proposed.

Effect of Austenite Grain Size on the Morphology and Crystallography of Lath Martensite in Low Carbon Steels

The crystallography, microstructure and mechanical property of as-quenched martensite of Fe-0.2C-Mn(-V) alloys of which the prior austenite grain sizes are 370-2 μm were studied. The prior austenite grain, whose size is larger than 28 μm, is divided by several packets. Those packets are subdivided by blocks containing sub-blocks, each of which corresponds to the Kurdjumov-Sachs variant. When the prior austenite grain size is about 2 μm, one packet tends to grow predominantly. Each packet is divided by blocks containing sub-blocks.

Effect of Grain Refinement on Thermal Stability of Metastable Austenitic Steel

In martensitic steels, it is well known that a certain chemical driving force (about 180 MJ/m3) is required to start martensitic transformation (Ms), and additional driving force has to be charged further to complete the transformation (Mf). In the case of metastable austenitic steels with Ms temperature at around room temperature, however, only the chemical driving force needed to start martensitic transformation has been stored at room temperature. Hence, the state of austenite is very unstable thermally. It has already been known that such a metastable austenite undergoes a partial martensitic transformation during isothermal holding at room temperature or cooling to a low temperature. It is very convenient to investigate the behavior of martensitic transformation of austenite. In this study, the effect of austenite grain size on martensitic transformation is introduced from the viewpoint of microstructural analysis and thermo-dynamics. The steel used in this investigation is an Fe-16 mass%Cr-10 mass%Ni ternary alloy, which has Ms temperature at around room temperature. The grain size of this steel can be controlled from 0.8 μm to 80 μm using the technique of reversion of deformation induced martensite. In the material with coarse grain size (80 μm), about 18% of martensite was detected at room temperature and the amount of martensite was increased to 50% by the following subzero treatment to 77 K. However, martensite was hardly detected in the material with ultra fine grains (0.8 μm) even after the subzero treatment. It was found that such a stabilization occurs in the materials with the grain size below 10 μm and the stabilization was reasonably explained by considering the relation between austenite grain size and elastic strain energy which is required on the single variant martensitic transformation.

The Dynamic, Static and Metadynamic Recrystallization of a Nb-microalloyed Steel.

Using hot torsion tests, the dynamic (DRX), static (SRX) and metadynamic (MDRX) recrystallization characteristics of a Nb-microalloyed steel were studied. The torsion tests were carried out at temperatures in the range 850 to 1050°C with strain rates ranging from 0.5 to 5/sec. At the higher temperatures, the Nb remained in solution, while precipitation was underway in the lower temperature range. The results indicate that Nb precipitation has little influence on the value of the critical strain (εc ) for dynamic recrystallization. The peak strain/Zener–Hollomon parameter equation is derived and the effect of austenite grain size on the peak strain is considered. The times for 50 % recrystallization for static and metadynamic recrystallization were established by means of interrupted torsion tests and are compared. The rate of metadynamic recrystallization increases with strain rate and temperature and is observed to be independent of the pass strain; this contrasts sharply with the observations for static recrystallization. Finally, an example is given of an industrial rolling process in which DRX/MDRX can play important roles. Here, the occurrence of dynamic/metadynamic recrystallization causes the load to drop or else to increase less rapidly than in the case of pure strain accumulation in the absence of SRX.