Papers by Keyword: Austenite Grain Size

Abstract: The effects of austenite grain size on hardenability and impact toughness were investigated. The results show that: Since the beginning of the two-phase region with quenching temperature, the austenite grain size from the initial 4+6 mixed crystal at 740°C, and gradually increased to 10 at 860°C; Austenite grain size and hardenability was directly proportional to the austenite grain size increased from 8μm to 36μm, the biggest change is the hardness 10HRC; Austenite grain size and impact toughness is linear, with the decrease of grain size, the impact energy increases linearly, and the austenite grain size and impact toughness curve fitting. Comprehensive analysis for ensuring the hardenability of cold heading steels should be considered optimal matching of material strength and plasticity.

Abstract: In order to investigate the effect of austenite grain size on martensite start temperature of Nb-V-Ti micro-alloyed ultra-high strength steel, the phase transformation features of Nb-V-Ti micro-alloyed steel was investigated. It has been found that martensite start temperature increased with the increase of austenite grain size as a consequence of the increase of austenitizing temperature. Based on microstructure observation, two types of MX carbonitrides with different compositions and morphologies have been identified. With the increase of the austenite grain size, both the volume fraction of precipitates and the dislocation density decreased, which may be induced by the strengthening of the austenite matrix directly and increasing the resistance of austenite to plastic deformation. Hence, the increase of martensite start temperature could be attributed to a decrease in volume fraction of precipitates and dislocation density.

Abstract: The effect of hot deformation behavior on austenite grain size refinement of low carbon multi-microalloyed steel was investigated. The morphology of austenite grains was revealed by thermal etching and observed using optical microscope. The results showed that single pass compression can only marginally refine austenite grain size by dynamic recrystallization, even under severe plastic deformation. However, when the specimens were held for a while after hot deformation, the fine austenite grain size can be obtained due to static recrystallization behavior.

Abstract: Laser ultrasonics for metallurgy (LUMet) is an innovative sensor technology for in-situ measurement of microstructure evolution during thermomechanical processing. This unique sensor has been attached to a Gleeble 3500 thermomechanical simulator for dedicated laboratory studies during processing of steel, aluminum, magnesium and titanium samples. Advanced processing software has been developed for the measurement of grain size and texture evolution from laser ultrasonic signals. Results of austenite grain growth measurements in low carbon steels will be described to demonstrate the capabilities of the LUMet technique. Further, applications of the system to measure recrystallization of ferrite and austenite formation during intercritical annealing simulations of dual phase steels will be presented. The ability to rapidly acquire data both during a single test and for multiple conditions over a range of conditions from different samples has important implications on expediting process modelling and alloy design. Although certain limitations exist, the LUMet technique offers a very reliable characterization platform with a number of potential applications in metallurgical process engineering.

Abstract: The original austenite grain size, the inclusions and the isothermal annealing microstructure of 20MnCrNi2Mo wear-resistant cast steel without and with Rare Earth were observed by metallographic microscope and QUANTA-400 environmental scanning electronic microscope. The effect mechanism of Rare Earth in 20MnCrNi2Mo wear-resistant cast steel was investigated. The results show that adding Rare Earth in 20MnCrNi2Mo wear-resistant cast steel can refine the original austenite grains. Meanwhile, it can reduce the inclusions size and change the inclusions shape from irregular to nearly spherical. In addition, the Rare Earth in 20MnCrNi2Mo wear-resistant cast steel can increase the amount of ferrite, reduce that of pearlite in the isothermal annealing microstructure and refine the microstructure of pearlite.

Abstract: In hot metal forming processes, the material is subjected to the thermo-mechanical processing. A fully three dimensional thermo-mechanically coupled FEM-simulation of an eleven pass hot rough rolling process of H-beam has been performed. Microstructure evolution equations available in literatures were incorporated into the commercial FE solver ABAQUS/Explicit, through user defined subroutine VUMAT, to simulate the microstructure evolution. Since it’s impractical to obtain the austenite grain size distribution in the beam blank during industrial hot rolling, the calculated rolling loads are compared with the mills loads instead of grain size comparison between the predicted average value and the real ones.

Abstract: 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.

Abstract: The microstructure and carbide precipitate of simulated coarse grain heat affected zone(CGHAZ) in modified high Cr ferritic heat-resistant steel at different cooling rates have been investigated by means of thermal simulator, optical microscope, SEM and TEM . It was found that the microstructure of CGHAZ of testing steel was mainly lath martensite and δ-ferrite under the different welding thermal cycles. However, the prior austenite grain size reduced with increasing the cooling rate. Furthermore, with increasing the cooling rate, the amount of carbide precipitate inside laths of martensite increased, and the size and morphology of precipitates have changed from elongated and coarse to needlelike and fine.

Abstract: The effect of austenization treatment temperature 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 the coarsening temperature of austenite grains in the 30CrNi3MoV steel is raised to about 1000°C due to the inhibition to the migration of austenite grain boundaries, not only by the fine and disperse vanadium carbides, but also by the solute atoms adsorbed near the boundaries. The martensite obtained in 30CrNi3MoV samples with different austenization temperatures varied in the structural constituent, as well as in the size. The martensite microstructures obtained in the samples austenized at relatively low temperatures were composed of both lath martensite and acicular martensite and they are small in size. Yet the microstructures in the 30CrNi3MoV samples with relatively high austenization temperatures were occupied mostly by coarse lath martensite. For the 30CrNi3MoV steel, the austenization heating temperature should be kept below 1000°C in order to achieve the optimum mechanical property.

Abstract: Non-isothermal austenite grain growth kinetics under the influence of several combinations of Nb, Ti and Mo containing complex precipitates has been studied in a microalloyed linepipe steel. The goal of these studies is the development of a grain growth model to predict the austenite grain size in the weld heat affected zone (HAZ). A detailed electron microscopic investigations of the as-received steel proved the presence of Ti-rich, Nb-rich and Mo-rich precipitates. Inter and intragranular precipitates of ~5-150 nm have been observed. The steel has been subjected to austenitizing heat treatments to selected peak temperatures of 950, 1150 and 1350°C at various heating rates of 10, 100 and 1000°C/s. Thermal cycles have been found to have a strong effect on the final austenite grain size. The increase in heating rate from 100 to 1000°C/s has a negligible difference in the austenite grain size irrespective of the austenitizing temperature. However, the increase in grain size has been noticed at 10°C/s heating rate for all the austenitizing temperatures. The austenite grain growth kinetics have been explained taking into account the austenite growth in the presence of precipitates.