Calculation of Austenite Formation Kinetics of Copper-Bearing Steel During Continuous Heating

The kinetics of austenite formation in a new type of copper-bearing steel with initial microstructure composed of ferrite and bainite was investigated by using dilatometric analysis and measurement during continuous heating. The formation of austenite was observed to occur in two stages. The first stage is the dissolution of ferrite and most bainite, followed by the second stage of dissolution of bainite and formation of austenite. The first stage takes place through diffusion and the second stage through shear. The critical temperature of austenite formation during continuous heating increases with increasing heating rates, which therefore exerts a greater influence on the Ast temperature of the austenite formation. Kinetics calculation shows that the process is mainly controlled by diffusion when the heating rate is over 1℃/s.

Effect of Nb on Austenite Formation and Decomposition in an X80 Linepipe Steel

Advanced high strength steels for pipeline applications,e.g.X80 grades,have complex microstructures and are frequently microalloyed with Nb.In the hot rolled product it is sought to have Nb precipitated as Nb(CN).However,when processing these steels Nb may be in solution and critically affects the microstructure evolution,e.g.austenite decomposition on the run-out table of a hot mill.Further,microstructure changes in the heat affected zone (HAZ) during girth welding of these linepipe steels may occur with Nb precipitated or in solution.In the HAZ,depending on welding procedures,the material undergoes a number of austenite formation and decomposition cycles and the amount of Nb in solution varies along these stages.In selected positions of the HAZ,thermal cycles peak at the intercritical region and the partial formation of austenite and subsequent decomposition constitutes additional complexity.Developing reliable process models for run-out table cooling and the HAZ hinges on an accurate tracking of microstructure evolution,which is strongly influenced by the amount of Nb in solution.The present study provides more insight into the effect of Nb on austenite formation and decomposition.Firstly,a novel experimental methodology is presented to measure quantitatively the effect of Nb on transformation temperatures pertinent to austenite decomposition,notably ferrite.A model for ferrite formation that accounts for solute drag of Nb is proposed to describe the experimental observations.Secondly,an experimental study will be presented to quantify the effect of Nb in and out of solution on austenite formation in the intercritical region.It is found that the morphology of intercritical austenite,as well as the kinetics of its formation is strongly affected by the starting microstructure and the state of Nb.

Influence of initial microstructure and grain size on transformation of bainite to austenite in large size forgings

The kinetics of austenite formation in the surface and center regions of a 40 t forged ingot of a high-strength medium- carbon low-alloy steel was studied using high-resolution dilatometry. The starting microstructures from the surface or center regions had different proportions of bainite and residual austenite as well as different prior austenite grain sizes. Two heating rates representing the actual heating rates in the surface （5℃ s -1） and center regions （0.5℃ s -1） of large size forged blocks were utilized. Dilatometric curves revealed only one transformation step of austenite formation at both heating rates independent of grain size or proportion of phases. Optical microscopy, field emission gun scanning electron microscopy and X-ray diffraction were used to study microstructure evolution and confirm the results obtained by dilatometry. The kinetic parameters for austenite formation were determined from the dilatometry data by Johnson-Mehl- Avrami-Kolmogorov （JMAK） equation. The JMAK coefficients were determined for each condition of the investigated steels. The calculations indicated that the nucleation and growth of austenite in the surface region were accelerated more than 10,000 times due to a significantly smaller average prior austenite grain size, stability of initial retained austenite, and accumulation of coarse carbides at the surface. The results were discussed in the framework of classical nucleation and growth theories using the kinetic parameters for austenite formation.

Microstructure Characterization and Mechanical Properties of TRIP-aided Steel under Rapid Heating for Different Holding Time

In order to develop a comprehensive understanding about the effect of different holding time under rapid heating on the microstructural evolution and mechanical properties of transformation-induced plasticity （TRIP） steel, continuous annealing process simulations were performed using a thermal system with resistance heating method. The morphology and distribution of all phases present in the microstructure and the mechanical properties of TRIP steel were revealed. It appeared that the final tensile strength of the TRIP steel increased and retained austenite car bon content decreased with increasing holding time. An overlap between ferrite recrystallization and austenitization was observed during intercritical holding. In addition, the work hardening of the samples was evaluated by calculat ing the instantaneous ~l value as a function of the true strain. The difference in work hardening behavior corresponds to the rate of the retained austenite transformation during straining, which can be attributed to the carbon content and the morphology of the retained austenite.

Micro-scale Cellular Automaton Modeling of Interface Evolution During Reaustenitization from Pearlite Structure in Steels

A modified cellular automaton model is developed to depict the interface evolution inside the cementite plus ferrite lamellar microstructures during the reaustenitization of a pearlite steel. In this model, migrations of both the austenite- ferrite and austenite-cementite interfaces coupled with the carbon diffusion and redistribution are integrated. The capil- laxity effect derived from local interface curvatures is also carefully considered by involving the concentration given by the phase diagram modified by the Gibbs-Thomson effect. This allows the interface evolution from a transient state to a steady state under different annealing conditions and various interlamellar spacings to be simulated. The proposed cellular automaton approach could be readily used to describe the kinetics of austenite formation from the lamellar pearlites and virtually reveal the kinematics of the moving interfaces from the microstructural aspect.