Predicting the Austenite Fraction After Intercritical Annealing in Lean Steels as a Function of the Initial Microstructu
- PDF / 704,079 Bytes
- 5 Pages / 593.972 x 792 pts Page_size
- 109 Downloads / 210 Views
austenite decomposition and formation kinetics[1–11] during the intercritical annealing have been of great technological and scientific interest as the mechanical properties of advanced steels, like transformation-induced plasticity (TRIP) and dual-phase (DP) steels, depend critically on the balance between the austenite and ferrite fractions realized during the intercritical holding stage of its production process. Therefore, it is very crucial to develop a physical model to accurately predict the fraction of ferrite or austenite upon intercritical annealing. The model should be able to handle different initial microstructures (fully austenite, martensite, and ferrite + pealite) before the intercrtitical annealing stage and capture the effects of chemical composition; in particular, the effects of Mn and C as these elements play crucial roles in controlling solid-state transformation in low-alloyed steels. In recent decades, three main concepts have been applied to determine the fraction of ferrite or austenite at temperatures in the two-phase region: (i) Full
HAO CHEN, Postdoctoral Researcher, XIAOJUN XU, PhD Candidate, WEI XU, Assistant Professor, and SYBRAND VAN DER ZWAAG, Professor, are with the Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands. Contact e-mail: [email protected], [email protected] Manuscript submitted October 24, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
equilibrium (FE):[12] it means the equilibrium of the whole system. In the case of FE, the fraction of ferrite or austenite in the intercritical region is fixed, once the temperature is fixed. (ii) Paraequilibrium (PE):[13,14] in the case of PE, it is assumed that the phase transformation can proceed without any redistribution of the substitutional alloying element M (M = Mn, Ni, Cr, etc.) and the chemical potential of carbon across the interface is constant. At a certain temperature, there is only one PE tie-line, which means that the PE fraction of ferrite or austenite is also fixed at that specific temperature. (iii) Local equilibrium (LE):[15,16] In the LE model, the interface is assumed to migrate under full LE with partitionings of both C and M. Due to the large difference in the diffusivities of C and M, there are two different transformation modes: (a) local equilibrium with negligible partitioning (LENP) mode. In this mode, the concentration of Mn in ferrite is the same as that in austenite, but due to LE condition, there is a Mn spike in front of the migrating interface. The transformation rate is effectively controlled by carbon diffusion and is relatively fast; and (b) local equilibrium with partitioning (LEP) mode, in which the carbon gradient in austenite is negligible, while that of M is large. In this mode, the transformation rate is controlled by the rate of M partitioning, and is extremely sluggish. In practice, the point at which the transformation mode switches from LENP to LEP is regarded as the termination of the transformation. The transition from LENP to LEP d
Data Loading...
