Austenite Grain Growth Kinetics Research Articles

Grain Growth during Mechanical Processing of Austenitic Stainless Steel AISI 321

The kinetics of austenite grain growth during thermomechanical treatment of AISI 321 steel with a relatively high content of carbon (0.07 wt. %) and titanium (0.50 wt. %) were studied. Hot deformation was carried out by the uniaxial compression of cylindrical specimens on a Gleeble 3800 thermomechanical simulator. A dependence is obtained for calculating the kinetics of austenite grain growth for a temperature range of 1150–1250 °C. The proposed dependence makes it possible to evaluate grain growth under non-isothermal conditions. The verification of the adequacy of the proposed dependence and the method for calculating the grain size at cooling rates 0.2, 1 and 5 °C/s showed its high convergence. The difference between the calculated and experimental grain size did not exceed 8%. The suppression of grain growth is due to the precipitation of titanium carbides and carbonitrides. Using the developed grain growth model, an analysis was made of the reasons for the formation of large grains in the shell after the elongating in the production process.

Formation of austenite in ferrite-beinite, beinite-martensite and martensitic shipbuilding steels and its influence on the transformed structure

This article investigates the kinetics of austenite grain growth during heating and the features of phase transformations depending on the austenite grain size in ferrite-bainitic, bainite-martensitic and martensitic shipbuilding steels. The kinetics of dynamic and static recrystallization is studied depending on the holding time at a given temperature. The studies carried out made it possible to determine the effect of the austenite grain size in shipbuilding steels of manganese, manganese-nickel and nickel alloying composition on the transformed structure.

Effect of boron accompanied by chromium, vanadium and titanium on kinetics of austenite grain growth

ABSTRACT The work was aimed at analysing the effect of boron on austenite grain size of low-alloy cast steels during austenitization in the range between 850°C and 1200°C. The effect of boron was investigated in presence of other alloying additives: chromium, vanadium and titanium. Eight melts with various chemical compositions were subjected to heat treatment started from soaking at eight different temperatures. After quenching, former austenite grain size was determined by a method of quantitative metallography. Influence of boron on austenite grain size, especially in presence of other alloying elements, was evaluated by one-way variance analysis ANOVA and the Kruskal–Wallis tests. The analyses showed a significant influence of boron on austenite grain size, which can be compensated by alloying elements creating stable carbides and nitrides.

Study of the kinetics of austenite grain growth by dynamic Ti-rich and Nb-rich carbonitride dissolution in HSLA steel: In-situ observation and modeling

The effect of carbonitrides on the growth of austenite grains in high-strength low-alloy (HSLA) steel was investigated through microstructure analysis and high-resolution dilatometry. The austenite grain growth at various temperatures was measured in situ using high temperature laser scanning confocal microscopy. Two kinds of precipitates with different morphologies containing large amounts of Ti and Nb were observed in the austenitized samples at different temperatures. The smaller Nb-rich spherical carbonitrides dissolved at the higher temperature, while the lager Ti-rich rectangular carbonitrides have a higher thermal stability and can pin at the austenite grain boundary. This study reveals that the dissolution of Nb in austenite causes the rapid growth of austenite grains and slows down the ferrite phase transition, which is beneficial to the formation of bainite. A kinetic model of austenite growth considering the dynamic dissolution of different types of carbonitrides is proposed. The results of the model show the same overall trend as the experimental results, and the individual differences can be explained. The proposed model can be used to predict the austenite grain size during the austenitizing process of HSLA steel and can reflect the austenite grain change throughout the process more continuously.

Study of the Effect of Austenite Grain Inclination Towards Growth on Impact Strength of Low-Carbon Low-Alloy Steel Welded Joint Heat-Affected Zone

Kinetics of austenite grain growth in the heat affected zone (HAZ) in low-carbon low-alloy steels welded joints is studied. The effect of metal structure and phase composition of the metal section of the near weld region of the heat affected zone (NWR HAZ) on the change in hardness and to brittle failure resistance is evaluated. The maximum nominal austenite grain diameter that provides a standard impact strength value is determined.

Cellular automata simulation for high temperature austenite grain growth based on thermal activation theory and curvature-driven mechanism

Based on the thermal activation theory and curvature-driven mechanism, a 2D cellular automaton model with different state transition rules was built. Validity of the model was proved by the shrinking of circular grains. Grain growth of high temperature austenite was simulated by this model; the morphology, grain size distribution, topological aspects, and local kinetics of austenite grain growth were analyzed under different simulation time. Among the grains with different sides, the 6-sided grains are the most common and 5-sided grains are the second most common. The grains with more than six sides will grow and grains with less than six sides will shrink, while the 6-sided grains will neither grow nor shrink. The kinetics of normal grain growth follows the Burke equation and the growth exponent at different temperatures and activation energies has been researched.

Kinetics of Austenite Grain Growth During Heating and Its Influence on Hot Deformation of LZ50 Steel

Grain growth behaviors of LZ50 have been systematically investigated for various temperatures and holding times. Quantitative evaluations of the grain growth kinetics over a wide range of temperature (950-1200 °C) and holding time (10-180 min) have been performed. With the holding time kept constant, the average austenite grain size has an exponential relationship with the heating temperature, while with the heating temperature kept constant, the relationship between the austenite average grain size and holding time is a parabolic curve approximately. The holding time dependence of average austenite grain size obeys the Beck’s equation. As the heating temperature increases, the time exponent for grain growth n increases from 0.21 to 0.39. On the basis of previous models and experimental results, taking the initial grain size into account, the mathematical model for austenite grain growth of LZ50 during isothermal heating and non-isothermal heating is proposed. The effects of initial austenite grain size on hot deformation behavior of LZ50 are analyzed through true stress-strain curves under different deformation conditions. Initial grain size has a slight effect on peak stress.

Influence of Carbon Equivalent Content on Phase Transformation During Inter-critical Heating of Dual Phase Steels Using Discrete Micro-scale Cellular Automata Model

In this paper, a cellular automata method based model is proposed for simulating phase transformation kinetics of inter-critical heating of dual phase (DP) steel. This developed model deals with the kinetics of pearlite dissolution, ferrite transformation and austenite grain growth based on carbon diffusion process. Diffusion equation is discretized and solved by finite difference method whereas austenite grain growth is controlled by transition rules applied in cellular automata algorithm. The model is operated in the temperature range of 730–890 °C for four different specimens of DP steel. This model predicts appropriately the microstructure and volume fraction of formed austenite during inter-critical heating of DP steel. In addition, this study shows that the presence of carbon and alloying elements enhances carbon equivalent of DP steel, helps in austenite formation.

In situ measurement and modelling of austenite grain growth in a Ti/Nb microalloyed steel

Using a novel laser ultrasonics technique in situ measurements of austenite grain growth were conducted during continuous heating (10°Cs−1) and subsequent isothermal holding at various temperatures in the range 950–1250°C in a microalloyed linepipe steel. Based on the experimental results, a grain growth model was developed, which includes the pinning effect of precipitates present in the steel. Analyzing the grain growth behaviour and using the advanced thermo-kinetic software MatCalc, an approach was developed to estimate the initial distribution of precipitates in the as-received material and their dissolution kinetics. The evolution of the volume fractions and mean particle sizes of NbC and TiN leads to a time-dependent pinning pressure that is coupled with the proposed grain growth model to successfully describe the observed kinetics of austenite grain growth. The predictive capabilities of the model are illustrated by its application to independent grain growth data for rapid heat treatment cycles that are typical of the weld heat affected zone.

Kinetics of austenite grain growth in medium-carbon niobium-bearing steel

In order to locate a reasonable heating system, the austenite grain growth behavior of Nb microalloyed medium carbon steel has been experimentally studied at various austenitizing temperatures and for different holding times. It is indicated that austenite grain growth increases with increasing austenitizing temperatures and holding times. Particularly when the austenitizing temperature was above 1100 °C, austenite grains grew rapidly, and an abnormal austenite grain growth was observed. When the austenitizing temperature was lower than 1100 °C, austenite grain size and growth rate were small. The activation energy of grain growth in the tested steel is 397 679.5 J/mol. To ensure an absence of coarse grains in microstructures, the heating technology of the tested steel should be controlled for 1 h at 1100 °C. The relationships of austenite average grain size with soaking temperature and time of tested steel were obtained by mathematical calculation, and austenite average grain size was found to be in agreement with the measured size for different holding times.

Kinetics of Austenite Grain Growth during a Continuous Heating of a Niobium Microalloyed Steel

Grain growth is a thermally activated process in which the average grain size increases as temperature and time increases. The driving force for grain growth results from the decrease in the free energy associated with the reduction in total grain boundary energy. There are several known factors that influence the migration of grain boundaries such as second phase particles precipitated in the matrix and the solute elements segregated at grain boundaries. The austenite grain boundaries are revealed using the thermal etching method. Carbon extraction replicas were prepared to determine the composition and size of precipitates present in the matrix. In this work, the evolution of the average prior austenite grain size (PAGS) of a low carbon steel microalloyed with niobium is studied as a function of temperature and heating rate. Austenite grains show a two-stage growth. It has been found that as heating rate increases, the grain coarsening temperature (TGC) increases and the grain size at that temperature decreases. TGC temperature lies around 40-60°C below the temperature for complete dissolution of carbonitrides (TDISS).

Effect of Dissolved Cerium on Austenite Grain Growth in an Fe-0.20mass%C-0.02 mass%P Alloy.

The effects of dissolved cerium on the kinetics of austenite grain growth and on the grain size and grain shape distributions were studied at 1473, 1573, and 1673 K in an Fe-0.20 mass%C-0.02 mass%P alloy containing a limited amount of second-phase particles as a function of holding time. The mean planar grain diameter (Dmean) for a given holding time decreased considerably with the addition of dissolved Ce up to 300 mass ppm and then decreased gradually. The parabolic law kinetics was observed and its apparent rate constant was inversely proportional to the content of dissolved Ce in atomic pct. The spatial grain size distribution obtained by the Johnson-Saltykov transformation could be best described by the log-normal distribution function and normal grain growth was observed. The Dmax/Dmean (Dmax: the maximum grain diameter) value was independent of the content of dissolved Ce and holding time. The distribution of the number of grain sides was observed to be holding time-invariant and was not affected by the presence of dissolved Ce.

Rules of austenitic grain growth in 18KhNVA steel

1. The kinetics of austenite grain growth in 18KhNVA steel at 930°C depends upon the rate of heating to 930°C and the preliminary heat treatment as a result of the high tendency of this steel toward structural hereditary. 2. An increase in the heating rate of 18KHNVA steel leads to the formation of an unstable fine-grained structure which is prone in subsequent isothermal holding to rapid coarsening by merging of the grains as a result of their low disorientation. 3. Preliminary tempering and slow heating stabilize the grain structure of the steel and the process of merging becomes more difficult since it must be preceded by turning (accomodation) of the grains.

The effect of vanadium, nitrogen, and manganese on the kinetics of austenite grain growth in steel 35L

The effect of vanadium, nitrogen, and manganese on the kinetics of austenite grain growth in steel 35L

Hardenability of steels for precision castings

1. The standard medium-carbon steel 45L, widely used for manufacturing cast machine parts, which can be melted in high-frequency induction furnaces with an acid lining, has a high hardenability and tendency to grain growth. 2. Steel 50PPL, which can be melted in high-frequency induction furnaces with a basic lining, has a minimal hardenability. The ideal critical diameter of basic 50PPL is 8.6 mm, and does not increase at quenching temperatures of 800–1050°. Acid steel 50PPL has satisfactory hardenability, with an ideal critical diameter of 14.6 mm in quenching from 800° and 17,6 mm in quenching from 950°. 3. It was found that the fine austenite grain size (grade 12) of basic 50PPL hardly changes with quenching from 800–1000°; for acid 50PPL the austenite grain size increases from grade 12 to grade 11 at these quenching temperatures. 4. Raising the silicon content above 0,3% in acid 50PPL increases the hardenability due to the increasing resistance of austenite to decomposition. 5. Preliminary high-temperature heat treatment (quenching from 1250° and tempering at 650°) of acid 50PPL increases the dispersity of carbide and nitride inclusions and thus reduces the tendency to austenite grain growth, reduces the grain size, and lowers the hardenability. Preliminary heat treatment (quenching from 880° and tempering at 650°) has no effect on the kinetics of austenite grain growth or the hardenability of 50PPL. Hot plastic deformation (forging with cooling in air) increases the hardenability.