Austenite Grain Growth Behavior Research Articles

A new study on the growth behavior of austenite grains during heating processes

In this paper, the effects of the heating temperature and holding time on the austenite grain growth of SCM435 steel were studied and analyzed in terms of the average, macro-axis and minor-axis size of the grains. The results indicated that the classical Sellars model was highly consistent for describing the growth stage of the new austenite but poorly described the initial nucleation-growth stage and stability stage of the austenite. A new model that expresses the average austenite grain growth of SCM435 steel was developed, and the values calculated based on this model were highly consistent with the actual measured values. The standard deviation and expected grain-size expressions increased as the heating temperature and holding time increased. The macro-axis and minor-axis size of the grains were linearly correlated with the average size, and the ratio of the macro-axis to the minor-axis size remained generally constant during grain growth. Furthermore, this paper provides a new way of thinking about heredity in materials science from the perspective of mathematical characteristics.

Austenite Grain Refinement and Isothermal Growth Behavior in a Low Carbon Vanadium Microalloyed Steel

The austenite grain refinement through control of the grain growth during reheating process after thermo-mechanical controlled process (TMCP) in a vanadium microalloyed steel was achieved. The formation of ultra-fine grained austenite was attributed to the high density of austenite nucleation at the ferrite/martensite structure and to the inhibition of austenite growth by (Ti,V)C particles at the relatively low reheating temperature. Corresponding with the precipitation behavior of (Ti,V)C with temperature, the growth behavior of austenite in the vanadium microalloyed steel could be divided into two regions. At lower reheating temperature, austenite grains grew slowly, and ultra-fine grained austenite smaller than 5 μm was successfully obtained. By contrast, the austenite grains grew rapidly at high temperature due to the dissolution of (Ti,V)C particles. According to the measured and predicted results of austenite growth kinetics, two models were developed to describe the growth behavior of austenite grains in two different temperature regions, and the apparent activation energy Qapp for grain growth was estimated to be about 115 and 195 kJ/mol, respectively.

The deformation behavior in isothermal compression of 300M ultrahigh-strength steel

The flow stress, the apparent activation energy for deformation and the growth behavior of austenite grains in isothermal compression of 300M steel are investigated at the deformation temperatures ranging from 850 °C to 1250 °C, the strain rates ranging from 0.1 s−1 to 25.0 s−1, and the height reductions ranging from 30% to 70%. Through the analysis of flow curves and microstructure, it is confirmed that dynamic recovery is the dominant softening mechanism at a deformation temperature of 900 °C. At a deformation temperature of 1100 °C, the process of dynamic recrystallization has completed and the austenite grain growth is in progress in isothermal compression of 300M steel. Coarse macro-grains in isothermal compression of 300M steel are observed at the deformation temperatures above 1140 °C. The apparent activation energy for deformation of 300M steel is in the range of 332 ± 61 to 397 ± 31 kJ mol−1. The processing maps of 300M steel at a series of strains are constructed based on the dynamic materials model (DMM). The results show that the unstable region in instability maps decreases with an increase in strain. At a strain of 0.7, the unstable region occurs at the deformation temperatures ranging from 900 °C to 1140 °C and the strain rates ranging from 0.1 s−1 to 2.1 s−1. And the processing maps exhibit the peak efficiency of power dissipation of 0.86 occurring at the deformation temperatures ranging from 1100 °C to 1140 °C and the strain rates ranging from 0.1 s−1 to 0.16 s−1, which is correspondent to the optimal deformation condition of 300M steel.

Kinetic Analysis of the Austenite Grain Growth in GCr15 Steel

The growth behavior of austenite grains in GCr15 steel was investigated through the isothermal annealing tests of the steel under different heating temperatures and holding times. The tests were performed on a Gleeble-3800 thermo-mechanical simulation machine. Austenitizing temperatures 1223, 1323, 1373, and 1423 K were chosen, and holding time varied from 0 to 480 s. Experimental results suggest that austenite grains grow gradually with the increase of heating temperature, and holding time has an important effect on the growth of austenite grains. The time exponent for the growth is bigger at higher temperature, and the growth rate decreases with increasing time. On the basis of previous models and experimental results, a mathematical model that can describe the growth behavior of austenite grains in the tested steel under different heating temperatures and holding times was obtained using regression analysis. The predicted grain sizes by the model are in good agreement with measured ones.

Growth behavior of ultrafine austenite grains in microalloyed steel

Ultrafine austenite grains (UFAGs) with size of 1–5 μm were prepared through repetitive treatment, four times, of rapid heating and quenching, and the growth behaviors of these UFAGs during both the reheating and cooling stages were investigated. The results indicated that UFAGs without pinning particles appeared with significant coarsening when the reheating temperature reached 1000 °C. Although coarsening still occurred in the cooling stage, the growth was obscured during the isothermal holding process at temperatures between 900 °C and Ar 3 .

Growth of Individual Austenite Grains Measured with 3DXRD Microscopy

Studying austenitisation in steel, so far, was either limited to observations at the surface of the material or to the determination of the average grain growth behavior in the bulk. The development of the three-dimensional X-ray diffraction (3DXRD) microscope at beam line ID11 of the European Synchrotron Radiation Facility in Grenoble, France, made it possible to study the transformation kinetics in-situ and at the level of individual grains in the bulk of the material. Unique in-situ observations of austenite growth kinetics during continuous heating experiments were made for two commercial low-alloy steels (C22 and C35). The observed growth behavior of individual austenite grains gives a valuable contribution to understanding the phase transformations on heating, i.e. austenite formation from ferrite and pearlite.

Pinning of Austenite Grain Boundaries by AIN Precipitates and Abnormal Grain Growth

The growth behavior of austenite grains in the presence of A1N precipitates varies with the temperature and time of anneal. To study this behavior, two iron alloys, (in weight percent) a 0.1 carbon base chemistry with 0.03A1/0.01N and 0.09A1/0.04N, respectively, were annealed between 1000 °C and 1200 °C for times of up to 180 minutes. Using optical microscopy, as many as 1000 austenite grains per heat-treatment condition were measured. Conditions of sup- pressed, abnormal, and uniform grain growth were observed. Using an extraction replica tech- nique, the size, shape, and distribution of the A1N particles were determined using transmission electron microscopy (TEM). The largest grain boundary curvatures calculated, using the Hellman- Hillert pinning model, were in close agreement with independent calculations of curvatures using the grain size data. The largest grains in the lognormal size distribution of austenite grains were found to be the ones with the potential to grow to abnormally large sizes.