Steel phase transformations during

Phase relations in the C-Fe-W system are of great interest above all because of the importance of tungsten as alloying element in steels. Therefore constitution of this system has been studied quite intensively. Other important aspects of this subject, in particular the dissolution of iron carbides during austenitization of tungsten steels, phase transformations of carbides as well as diffusion interaction of carbides with iron and steels also attract attention of investigators widely. Details of experimental studies of phase relations, crystal strac-tures and thermodynamics as well as applied techniques are presented in Table 1. 

If local stresses exceed the forces of cohesion between atoms or lattice molecules, the crystal cracks. Micro- and macrocracks have a pronounced influence on the course of chemical reactions. We mention three different examples of technical importance for illustration. 1) The spallation of metal oxide layers during the high temperature corrosion of metals, 2) hydrogen embrittlement of steel, and 3) transformation hardening of ceramic materials based on energy consuming phase transformations in the dilated zone of an advancing crack tip. 

Metallurgy was one of the first fields where material scientists worked toward developing new alloys for different applications. During the first years, a large number of studies were carried out on the austenite-martensite-cementite phases achieved during the phase transformations of the iron-carbon alloy, which is the foundation for steel production, later the development of stainless steel, and other important alloys for industry, construction, and other fields was produced. 

As for pure TAG or binary mixtures (13,14), a numerical model called FEM-TTT and previously developed for phase transformations in steel (16) was adapted in the case of crystallization of cocoa butter during any cooling cycle. A finite ele-... 

Kap] Kaputkina, L.M., Kaputkin, D.E., Structure and Phase Transformations under Quenching and Tempering During Heat and Thermomechanical Treatment of Steels , Mater. Sci. Forum, 426-432, 1119-1126 (2003) (Crys. Structure, Review, Kinetics, Morphology, Phase Relations, 7)... 

Shtansky, D.V., Inden, G., Phase Transformation in Fe-Mo-C and Fe-W-C Steels-II. Eutee-toid Reaction of M23C6 Carbide Deeomposition During Austenitization , Acta Mater., 45(7), 2879-2895 (1997) (Caleulation, Crys. Structure, Experimental, Phase Diagram, Phase Relations, Thermodyn., 26)... 

Nak] Nakai, K., Shtansky, D.V., Analyses of Mierostrueture Evolutions During Reverse Phase Transformations in Alloyed Steels Using Caleulated Phase Diagrams (in Japanese), Materia Japan, 38, 975-981 (1999) (Phase Relations, Thermodyn., Calculation, 41)... 

Many phase transformations such as nucleation and crystal growth from a melt, solid-state recrystallization during annealing, austenitic to ferritic transformation in steels, etc. are stochastic events because they involve either liquid- or solid-phase nucleation and therefore must be treated in terms of probabilities as done in Chapter 11. Let Vq be the total volume of the material, Vj be the volume of the transformed material, and Vu be the volume of material that has not yet been transformed. Recall from Poisson statistics that the probability of no events occurring in a given time is exp (expected number of events in that time). In this case the expected volume to be transformed is kt" where k and n may be specified by models of the transformation mechanism or may be determined empirically. Therefore, the fraction of trarrsformed matter at time t can be written as... 

As will be described later, however, extensive research has recently been done to positively utilize these non-metallic inclusions to improve the properties of steels. That is, utilization of inclusions as nucleation sites for grain refinement during casting, welding and phase transformations, and as dispersoids for ODS alloys are anticipated. 

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