Principles of Heat Treating of Steels :: essays papers

Principles of Heat Treating of SteelsPrinciples of Heat Treating of SteelsA steel is usually defined as an alloy of iron and carbon with the content betwixt a few hundreds of a percent up to about 2 wt%. Other alloying elements bottom amount in total to about 5 wt% in low-alloy steels and higher in to a greater extent highly debase steels such(prenominal) as tool steels and stainless steels. Steels can exhibit a gigantic variety of properties depending on composition as well as the sorts and microconstituents present, which in turn depend on the heat treatment. The Fe-C Phase DiagramThe hindquarters for the understanding of the heat treatment of steels is the Fe-C phase diagram. Because it is well explained in forward volumes of Metals Handbook and in many elementary textbooks, the stable iron-graphite diagram and the metastable Fe-Fe3 C diagram. The stable condition usually takes a very languish time to develop, especially in the low-temperature and low-carbon range, a nd therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium for variant combinations of carbon concentration and temperature. We distinguish at the low-carbon and ferrite, which can at about dissolve 0.028 wt% C at 727 oC and austenite which can dissolve 2.11 wt% C at 1148 oC. At the carbon-rich side we find cementite. Of less interest, except for highly alloyed steels, is the d-ferrite existing at the highest temperatures. Between the single-phase palm are found regions with mixtures of 2 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 + d-ferrite. In heat treating of steels the liquid phase is always avoided. Some important boundaries at single-phase fields have been assumption special names. These include the carbon con tent at which the minimum austenite temperature is reach is called the eutectoid carbon content. The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite particles dispersed with a ferrite ground substance after extended holding close to A1. The Fe-C diagram is of experimental origin. The companionship of the thermodynamic principles and modern thermodynamic data now permits very finished calculations of this diagram.