Papers by Keyword: Austenite Grain Growth

Abstract: To predict austenite grain growth behavior in the heat-affected zone (HAZ) in low alloy steels, a new calculation model is proposed herein. This model mainly considers the solute-drag effect and pinning effect, which restrain the austenite grain growth. To calculate the solute-drag effect, the grain boundary concentration of each element is obtained by Hillert’s Law. Calculations are performed by simulating the HAZ with a temperature gradient using the phase field method for two dimensions. This calculation demonstrates the possibility of quantitatively predicting the pinning force for welding heat inputs.

Abstract: Spatial distribution of microalloy precipitates have been characterized in a low carbon microalloyed steel containing Nb, Ti and V. Micro-segregation during casting resulted in an inhomogeneous distribution of Nb (and also Ti) precipitates in the as-cast slab. Austenite grain growth has been investigated in the above mentioned steel, using different reheating temperatures between 1000°C and 1250°C for 1 h. Inhomogeneous distribution of Nb-rich precipitates created austenite grain size bimodality after reheating to an intermediate temperature range (1150-1200°C). Uniformly fine and uniformly coarse grain structures were found after reheating at lower- (≤ 1075°C) and higher-reheating temperatures (≥ 1250°C). A model has been proposed for the prediction of austenite grain size variation in the reheated steel.

Abstract: The effect of B and microalloying additions of V, V+Ti, V+Nb on austenite grain growth of low alloy steel containing 0.3% C, 1 % Cr and 0.2 % Mo was investigated. As a measure of austenite grain size the mean chord length of austenite grains was assumed. The boron content in investigated steel was in the range of 0 to 0.008 %. The investigations were carried out in austenitising temperature range of 850 to 1100oC. Using the thermodynamic model the contents of undissolved compounds of carbonitride V(C,N) and boron nitride BN were calculated and the effect of undissolved compounds content on austenite grain size was investigated. Obtained results showed, that vanadium content below 0.1 % was ineffective as austenite grain growth inhibitor of boron containing steel and austenite grain size of steel was higher compare to non-alloyed steel. The most effective for decreasing of austenite grains size of boron containing steel was addition of 0.18 % V + 0.03 % Nb

Abstract: Non-isothermal austenite grain growth kinetics has been studied in a microalloyed linepipe steel with complex precipitates containing Ti, Nb and/or Mo. The goal of these experimental studies is to provide the basis for the development of a grain growth model to predict the austenite grain size evolution in the weld heat affected zone (HAZ). Detailed electron microscopic investigations of the as received steel proved the presence of Ti-rich, Nb-rich and Mo-rich precipitates. The steel was subjected to austenitizing heat treatments to selected peak temperatures of 950, 1150 and 1350 °C at heating rates of 10, 100 and 1000 °C/s, respectively. Thermal cycles have been found to have a strong effect on the austenite grain size. Austenite grain sizes increase with peak temperature and decreasing heating rate. However, the increase in heating rate from 100 to 1000 °C/s has a negligible effect on the austenite grain size. The observed austenite grain growth kinetics can be explained taking into account the potential dissolution of Nb-rich precipitates.

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.

Abstract: The solid solution of the second phase particle and austenite grain growth behavior of the high niobium-containing RE steel was studied by mathematical calculation and extraction replica technique. The purpose of the study was to investigate the effects of Rare Earth La on austenite grain growth and propose an empirical equation for predicting the austenite grain size of RE steel. Austenite grain grows in an exponential law with the increase of heating temperature, while approximately in a parabolic law with the increase of holding time. Results show that the RE steel has good anti-coarsening ability at elevated temperatures. When soaking temperature is lower than 1250°C , AGS and growth rate are small for high niobium steel, but soaking temperature is lower than 1220°C , AGS and growth rate are small for RE steel. RE La can promote solid solution of second-phase particles Nb(C, N), the solution temperature decrease 30°C than high niobium steel.

Abstract: Austenite grain growth in microalloyed steels is governed by the coarsening of fine precipitates present at grain boundaries below the grain coarsening temperature. Zener model is widely used in metals to describe the pinning effect of second phase particles precipitated in the matrix. In this work it has been discussed whether grain boundary or volume diffusion is the rate controlling process for the coarsening of the niobium carbonitrides. Calculations on austenite grain growth kinetics, obtained coupling Zener theory and both rate controlling processes of precipitate coarsening, have been compared against experimental austenite grain size results under nonisothermal heating conditions. In this sense, it has been concluded that the coarsening of niobium carbonitrides is mainly controlled by volume diffusion of Nb in austenite.