Solid-State Phase Transformation And Kinetics Simulation In Continuously-Cooled High Cr Fe-based Alloys

As we all known, the microstructure of austenite after cooling largely determine the physical properties (mechanical) and chemical properties of many steels.Thus, the investigation of the microstructural evolution and an accurate description of the phase transiton mechanism in austenite under different cooling conditions have important practical significance to formulate correct heat treatment process and achieve the desired performance. Based on theory and model of phase transformation, simulating the evolution of austenite during phase transitons provides the possibility for systematic and in-depth study of phase-transformation mechanism, and a lot of achievements have been made.In resent years, The austenite→ferrite transformation kinetics in iron-based, such as Fe-Ni, Fe-Mn and Fe-C alloy, have been investigated successfully on the phase transformation model, involving site saturation, interface-controlled growth and incorporation of an appropriate impingement.Though the site saturation nucleation model has been proved to be valid in these cases, the number of nuclei may be varying accompanying the formation of the product. In view of that, in chapter 3,on the dilatometric data, the austenite→ferrite phase transformation kinetic feature in Fe-Cr alloy has been studied on the phase transformation model, involving site saturation and continuous nucleation model respectively, interface-controlled growth, randomly dispersed nuclei and isotropic growth impingement model, the conclusions as follows:(1) The values of activation energy for growth, QG, which are obtained by adopting continuous nucleation and site saturation nucleation respectively, are comparable and are all decreasing with the increase of cooling rate.(2) The value of the activation energy,QG, for growth is larger than activation energy,QN, for nucleation. This indicates that the formation of nucleation is mainly determined by a single atomic thermal activation jump in the matrix, while the growth of nucleation is mainly controlled by groups of atomic thermal activation jumps.(3) By changing the density of high angle boundaries, cooling rate influences the active energy QG and pre-exponential factor v0.Thus, the kinetics of austenite→ferrite transformation was affected.(4) As compared with site saturation nucleation, besides activation energy for growth QG, activation energy for nucleation QN can also be obtained by adopting continuous nucleation model. Therefore, it seems reasonable to deem that continuous nucleation model could be used to investigate the overall kinetic behaviors of austenite→ferrite transformation in iron-based alloys.In addition, in many cases, the service structure of many steels is martensite. Therefore, from both a fundamental and a practical point of view, it is important to investigate the mechanism of martensitic transformation to adjust the microstructure and thus improve the properties of materials. Like most solid-state phase transformation, martensitic transformation is also completed by nucleation, growth. At the same time, it has its own characteristics. A considerable amount of work has been performed in the past on the martensitic transformation kinetics on the dynamic model in iron-based alloys. While few of these models take into account the nucleation and growth process simultaneously. In view of that, in Chapter 2, a modular phase-transformation model, incorporating a classic partitioning analysis for nucleation and anisotropic growth for impingement of martensitic, has been developed, using which the mechanism of martensitic transformation and kinetic feature in binary Fe 8.98at.%Cr alloy have been studied. On this basis, in order to further test the model whether can be used to investigate the complex system, in Chapters 5 and 6, the kinetics of martensitic transformation in T91 steel and a new high-chromium ferritic heat-resistant steel, in which the content of Cris basically equal to Fe 8.98at.%Cr alloy, have been studied systematically on the dilatometric data and developed model here. The following conclusions were obtained:(i) By the analysis of martensitic transformation temperature and time of these three sample, it is found that:with the increase of cooling rate, the onset temperature Ms of binary Fe 8.98at.%Cr alloy decreases, while for T91 steel and new high chromium ferritic heat-resistant steel, the Ms exists a turning point, which shows that the internal stress generated in multi-component alloy is larger than in binary alloy. In addition, in the same cooling rate (500K/min,6000K/min), the transformation time in T91 steel as well as the new high-chromium ferritic heat-resistant steel are longer than Fe-8.98at.%Cr alloy, which fully shows that increasing alloying elements will hinder the martensite-formation rate.(ii) By the comparison of martensite-formation rate in three samples, we conclude that there is a competition between the number of martensite plate and the interface speed. At a certain cooling rate, which is dominant, namely it will control the rate of martensite-formation.(iii) The experimental curve, the rate of martensite-formation varys with temperature, has been fitted by isochronal phase transformation kinetic model developed here. The results indicat that:martensitic transformation is governed by a thermally activated process, the growth activation energy QG is relatively low,and about 40-60 kJ mol-1 for lath martensite; the increase of cooling rate has little effect on QG, while has significant effect on the pre-exponential factor v0, thus martensite interface speed is mainly determined by v0.In addition, at a certain cooling rate, the aspect factor of martensite plate, the growth activation energy, and the pre-exponential factor have the same trends for certain components alloy.For binary Fe-8.98at.%Cr alloy, the motion of martensitic interface is mainly retarded by the dislocation through the parent austenite, while for T91 steel and the new high Cr ferritic heat-resistant steels, it is mainly controlled by the interaction between the interface dislocation and the stress field generated by interstitial atoms, which is affected not only by cooling rate, transformation temperature and the content of carbon.