Coarsening Of Austenite Grains Research Articles

Suppress Austenite Grain Coarsening by Nb Alloying in High-Temperature-Pseudo-Carburized Bearing Steel.

The effect of Nb alloying on the suppression of austenite grain coarsening behavior during pseudo-carburizing is investigated in high-temperature-carburized SAE4320 bearing steel. To explore the role of the Nb element in the pseudo-carburizing process, the morphology, composition, size, and distribution of NbC precipitates were analyzed. The results show that the fine austenite grain observed in Nb micro-alloyed steel is caused by the pinning effect of NbC precipitates, which hinders the coarsening of austenite grains and changes the growth dynamics of austenite grains. After the SAE4320 carburized bearing steel with the addition of 0.45 wt.% Nb element is kept at 1150 °C for 4 h, the PAG size is still below 20 μm, which indicates the Nb element has obvious advantages in limiting PAG growth at high temperatures and shows great potential for the development of high-temperature carburized bearing steel.

Microstructure evolution induced by solidification and ferrite–austenite massive-like transformation in Fe–C alloys

The coarsening of austenite grains produced through a massive-like transformation from the ferrite to the austenite phase during/after solidification in Fe–0.45mass%C steel was examined through time-resolved X-ray diffraction measurements using time-resolved computed tomography. The δ phase solidification resulted in a single δ grain in the observation region (0.6 mm in diameter, 0.1 mm in height). The massive-like transformation produced multiple γ grains, and coarsening immediately followed the transformation. The coarsening in the observation region was completed even within 200 s. A correlation of crystallographic orientation between the γ grains was found in the γ grain structure. The massive-like transformation frequently produced fcc (111) twinned grains, and the twinned austenite grains tended to remain during coarsening because the twin interface energy is lower than the grain boundary energy. The massive-like transformation following the solidification contributed to the texture formation in the γ grain structure.

Influence of hot isostatic pressing post-treatment on the microstructure and mechanical behavior of standard and super duplex stainless steel produced by laser powder bed fusion

This study investigates the properties of standard duplex steel 2205 and super duplex steel 2507 processed by laser powder bed fusion. The focus of the analysis is on the changes in microstructures and the mechanical properties caused by hot isostatic pressing (HIP) as post treatment. For that purpose, samples produced by laser powder bed fusion (LPBF) were exposed to hot isostatic pressing post-treatment followed by solution annealing. The properties of this condition were compared to properties of the duplex steels without post-heat treatment and of those of samples with only solution annealing as post-treatment.Both duplex steels had a predominantly ferritic structure after the LPBF process. Subsequent heat treatments were able to establish a ferritic-austenitic duplex structure. HIP applied before solution annealing leads to a coarsening of austenite grains and a reduction in porosity.All post-heat treated samples meet the minimum requirements of the standard DIN EN 10088-3 regarding tensile properties. An effect of hot isostatic pressing was seen in the fatigue strength. An increase in the fatigue strength was measured in both steels, which was mainly attributed to the decrease in inner defects size.

Microstructure evolution and mechanical properties of reduced activation steel manufactured through laser directed energy deposition

A new laser metal desposition process based on an inside-laser coaxial powder feeding system was successfully applied to manufacture reduced activation steel, which is featured with fine microstructure and excellent mechanical properties. In addition, infrared thermal imaging experiments and Abaqus numerical simulation were conducted to characterize the complex thermal history during the laser metal deposition process. The microstructure and mechanical properties of reduced activation steel were systematically investigated in the as-fabricated and heat-treated samples. The results indicat that the peak temperature increased and the cooling rate decreased in the melt pool when the additional layers were deposited as a result of a cumulative effect of heat in the fabricated thin wall samples. The reduced cooling rate directly contributed to the decreased heterogeneous nucleation rate and the coarsening of austenite grains in the top domain. The differences in terms of microstructure and hardness of the as-fabricated samples along the building direction were also in a good agreement with the evolution of temperature field. The thermal cycling experimental and cyclic heat treatment results confirmed that in-situ thermal cycles were unable to trigger recrystallization because the stored strain energy was insufficient to induce nucleation of new austenite grains during laser directed energy deposition.

ТЕРМОЦИКЛИЧЕСКАЯ ОБРАБОТКА (ТЦО) МЕТАЛЛОВ – ПУТЬ К ПОЛУЧЕНИЕЮ ОПТИМАЛЬНЫХ СТРУКТУРЫ И СВОЙСТВ

The aim of this work is to study the basic principles of thermal cyclic processing (TCТ) of metals to obtain structures that determine the optimal complex of mechanical properties. The basic provisions of metal heating centers using periodically repeated heating and cooling cycles are given. The TCТ method, as a heat treatment method, is based on constant accumulation from cycle to cycle of positive changes in the structure of metals. Studies have shown that with rapid heating, the growth of austenitic grain occurs slowly and, therefore, heating to high temperatures (up to 10000C) does not lead to an intensive increase in grain. It has been established that grain size increases at a variable heating temperature 3 times slower than under isothermal conditions at the corresponding temperature. Provided that the growth rate of the new phase (austenite) is small and the nucleation rate of grains is significant, it turns out that by the end of the a®g transformation, a fine-grained structure is retained. Further heating or holding at a constant temperature leads to a rapid coarsening of austenite grains. If cooling (for example, in air) of rapidly heated steel is performed 10–150C higher than the temperature of the Ас1 point, then fine perlite grain is formed due to reverse recrystallization. With one thermal cycle, ferrite in subeutectoid steels almost does not undergo changes. But if several such heating and cooling are performed, then the entire ferrite-pearlite structure undergoes a change. It has been established that the higher the heating rate during heating and heating and the less overheating above Ас1, the finer the grain in carbon structural steel. However, this increases the need to increase the number of heat treatment cycles. The mechanism of structure formation explaining these phenomena and practical recommendations on the implementation of the process of the technical and economic process are presented. This approach makes it possible to form the optimal metal structure. At the same time, opportunities can be significantly expanded in terms of obtaining materials with desired properties and improving on this basis machines, structures, individual units and parts. All this puts TCТ in the category of promising areas in metalworking.

In situ observation on microstructure evolution of 22MnB5 in hot stamping process

High temperature confocal laser scanning microscopy (CLSM) was used to investigate the microstructure evolution of high-strength boron steel 22MnB5 during hot stamping. The experimental results show that it is complete austenitized at temperatures about 810 °C during the heating process. Most of the initial austenite grain size is small and locally coarse. At 920 °C to 1000 °C, the phenomenon of B remelted and solidified was observed which played a very good pinning role at the austenite grain boundary, preventing coarsening of austenite grains. The segregation of B and the addition of Mn result in a significant reduction in both the minimum boron reverse melt content and the final solidification temperature. In the continuous cooling stage, martensitic transformation occurs at the cooling rate of 60 °C/s, and the martensite start point is 400 °C and martensite finish point is 280 °C. A large number of bursts are concentrated from 380 °C to 330 °C. There are two main forms of martensitic transformation: first, martensite begins to appear at the coarse austenite grain boundary, and grows transgranularly. Second, the new martensite laths starts from the previously formed laths and grows at a certain angle into the austenite grains. The main factor in the increase of martensite in continuous cooling is the formation of variable temperature martensite rather than the growth of martensite laths. At the cooling rate of 20 °C/s, the bainite and ferrite transformation appeared and the conversion temperature of bainite was about 600 °C. The cooling speed has a great influence on the performance of the 22MnB5 hot stamping component. The room temperature microhardness at cooling rates of 5 °C/s, 20 °C/s, and 60 °C/s was 194 HV, 243 HV, and 430 HV, respectively. Therefore, ensuring sufficient cooling rate is a key condition for obtaining ultra-high strength hot stamping components.

Austenite grain coarsening of V-microalloyed medium carbon steel during high frequency induction heating: Monte Carlo simulation study

The austenite grain coarsening behaviour of V-microalloyed (MA) medium carbon steel during induction heating at high frequency is studied, by varying induction input power, and using Gleeble simulating the ultrafast heating rates and Monte Carlo simulation. A significant coarsening of austenite grains occurs in the subsurface layers despite ultrafast heating rates. A distinctive feature is, unlike plain medium carbon steel, an abrupt grain coarsening below a critical depth from the surface. Gleeble experiments suggest that an abrupt grain coarsening is due to dissolution of VC particles. The Monte Carlo simulations, in a simplified temperature and particle fraction profiles, further show that the abrupt grain coarsening in V-MA steel during induction heating can occur not only by the particle pinning but also by the thermal pinning. The activation energy for grain growth is shown to be important in determining the relative contribution to the abrupt grain coarsening. The particle pinning plays an important role in the abrupt grain coarsening because the V-MA medium carbon steel, unlike Cr–Mo steel, shows moderate activation energy for grain growth.

Austenite grain growth in a nuclear pressure vessel steel

There is uncertainty on the physical significance of models applied to the coarsening of austenite grains in nuclear pressure vessel steels. As a result, new data are generated and analysed by adapting standard theory to account for the initial austenite grain size generated when the steel becomes fully austenitic, and any growth during heating to the annealing temperature. The experimental data reflect two regimes of isothermal grain coarsening, with grain boundary pinning dominating the kinetics at temperatures below about 940°C. The precipitates responsible for this pinning have been identified using thermodynamics, high-energy X-rays, transmission electron microscopy and microanalysis as aluminium nitrides. The model accounting for all these factors seems to generalise well on unseen data.

Effect of Heat Input on Toughness of Coarse-Grained Heat-Affected Zone of an Ultra Low Carbon Acicular Ferrite Steel

The effect of heat input on toughness of coarse-grained heat-affected zone of an ultra low carbon acicular ferrite steel were investigated when the welding was conducted with high heat input. Microstructural observations, energy dispersive X-ray spectroscopy analyses were conducted using optical microscopy, scanning electron microscopy and transmission electron microscopy, respectively. The microstructures of coarse-grained heat-affected zone consist of predominantly bainitic microstructure and a small proportion of acicular ferrite grains. The bainitic microstructures become coarsened with increasing heat input. The impact toughness of coarse-grained heat-affected zone remained at a higher level when the heat input ranged from 42 to 55 kJ/cm. It became not stable and dropped to a lower level when the heat input increased to 110150 kJ/cm. The enhancement in impact toughness was attributable to the MnS precipitation on the pre-formed Ti oxides as well as the formation of intragranular ferrite. When specimens were welded with higher heat input, the deterioration of impact toughness was caused by the coarsening of austenite grains.

Observation of changing crystal orientations during grain coarsening

Understanding the underlying mechanisms of grain coarsening is important in controlling the properties of metals, which strongly depend on the microstructure that forms during the production process or during use at high temperature. Grain coarsening of austenite at 1273 K in a binary Fe–2 wt.% Mn alloy was studied using synchrotron radiation. Evolution of the volume, average crystallographic orientation and mosaicity of more than 2000 individual austenite grains was tracked during annealing. It was found that an approximately linear relationship exists between grain size and mosaicity, which means that orientation gradients are present in the grains. The orientation gradients remain constant during coarsening and consequently the character of grain boundaries changes during coarsening, affecting the coarsening rate. Furthermore, changes in the average orientation of grains during coarsening were observed. The changes could be understood by taking the observed orientation gradients and anisotropic movement of grain boundaries into account. Five basic modes of grain coarsening were deduced from the measurements, which include: anisotropic (I) and isotropic (II) growth (or shrinkage); movement of grain boundaries resulting in no change in volume but a change in shape (III); movement of grain boundaries resulting in no change in volume and mosaicity, but a change in crystallographic orientation (IV); no movement of grain boundaries (V).

Effect of Austenization Holding Time on the Martensitic Transformation in 30CrNi3MoV Ultra-High-Strength Steel

Any heat treatment technology must begin with the process of austenization, during which the holding time at austenite region is one of the critical parameters. The effect of austenization treatment holding time on the martensitic transformation in the 30CrNi3MoV ultra-high-strength steel was investigated by means of dilatometric measurements and microstructural observations. The results showed that extending the holding time at 900 oC won’t cause obvious coarsening of austenite grains due to the dragging effect of vanadium solute atoms adsorbed at austenite grain boundaries. Martensite microstructures obtained in the 30CrNi3MoV samples that were heat treated with different holding time varied a little. The austenization holding time has obvious influence on the Ms of the 30CrNi3MoV steel, because it simultaneously affect the size of prior austenite grains and the configuration of dislocation defects.

Characteristics of precipitates and mechanical properties in Ti bearing HSLA steels.

The effect of reheating temperature on tensile strength of controlled rolled steels and on grain coarsening of austenite during solution treatment were investigated for three series of Ti bearing HSLA steels (Ti, V-Ti and Nb-Ti), by comparing with the stability and composition of precipitates in each steel. It was found that addition of 0.03wt% Nb decreases the tensile strength of Ti bearing steels reheated at a low temperature and retards austenite grain coarsening, whereas addition of 0.05wt% V increases the tensile strength and scarcely retards austenite grain growth. These differences arise from the different stability of precipitates in Nb-Ti and V-Ti bearing steels. The equilibrium thermodynamic model of quaternary compounds is proved to predict the solubility and composition of precipitates in Ti bearing steels. It is concluded that the difference in solubility of the precipitates in austenite corresponds to the different solubility between Nb and V in the precipitates.

Alキルド鋼のオーステナイト結晶粒度とその粗大化挙動に関する研究

On aluminium killed Ni-Cr-Mo case hardening steel, the austenite grain size and it’s coarsening behavior were studied in relation to the aluminium nitride precipitate observed by electron microscopy. The main results are as follows: (1) If the original structure before the austenization is same, the initial austenite grain size becomes smaller as the aluminium nitride precipitate becomes finer, in accordance with Zener’s experession. (2) When the size of the aluminium nitride precipitate is smaller than a certain degree, the discontinuous coarsening of austenite grains occurs at the temperature at which the precipitate dissolves in the matrix or aggregates to a larger size, but when larger than that, the apparently continuous or discontinuous coasening occur depending on the prior heat treatment at much lower temperatures.