讲座4Fe-C相图与相转变图总结与热处理基础.ppt

Summary and Principles of Heat Treating of Steels Fe-C相图与非平衡相转变 总结（热处理依据原理） Steel is usually defined as an alloy of iron and carbon with the carbon content between a few ppm up to about 2.11 wt%. Other alloying elements can amount in total to about 5 wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels, stainless steels (10.5%) and heat resisting CrNi steels (18%). Steels can exhibit a wide variety of properties depending on composition as well as the phases and micro-constituents present, which in turn depend on the heat treatment. The Fe-C Phase Diagram/相图 The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram (Fig 1). Figure 1 actually shows two diagrams; the stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium (or metastable equilibrium) for different combinations of carbon concentration and temperature. Stable iron-graphite diagram (dashed lines) and the metastable Fe-Fe3C diagram(solid line). We distinguish at the low-carbon end ferrite (α-iron), which can at most dissolve 0.028% C, at 727°C (1341°F) and austenite -iron, which can dissolve 2.11 wt% C at 1148°C (2098°F). At the carbon-rich side we find cementite (Fe3C). Of less interest, except for highly alloyed steels, is the δ-ferrite existing at the highest temperatures. Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + δ-ferrite. In heat treating of steels, the liquid phase is always avoided. Some im