TRATAMIENTOS TERMICOS: 03/09/08

domingo, 9 de marzo de 2008

video de prueba sobre metalografia....

metalografia

Estructuras cristalinas

Estructuras cristalinas

Los materiales sólidos se pueden clasificar de acuerdo a la regularidad con que los átomos o iones están ordenados uno con respecto al otro.Un material cristalino es aquel en que los átomos se encuentran situados en un arreglo repetitivo o periódico dentro de grandes distancias atómicas; tal como las estructuras solidificadas, los átomos se posicionarán de una manera repetitiva tridimensional en el cual cada átomo está enlazado al átomo vecino más cercano.Todos los metales, muchos cerámicos y algunos polímeros forman estructuras cristalinas bajo condiciones normales de solidificación.

Celda Unitaria.- es el agrupamiento más pequeño de átomos que conserva la geometría de la estructura cristalina, y que al apilarse en unidades repetitivas forma un cristal con dicha estructura. La estructura cristalina de un sólido depende del tipo de enlace atómico, del tamaño de los átomos (o iones), y la carga eléctrica de los iones en su caso).

Existen siete sistemas cristalinos los cuales se distinguen entre sí por la longitud de sus aristas de la celda (llamados constantes o parámetros de la celda) y los ángulos entre losbordes de ésta.Estos sistemas son: cúbico, tetragonal, ortorhómbico, rombohédrica (o trigonal), hexagonal, monoclínico y triclínico.

PRINCIPLES OF HEAT TREATING

Principles of heat treating
A steel is usually undefined as an alloy of iron and carbon with the carbon content between a few hundreds of a percents up to about 2 wt%. Other alloying elements can amount in total to about 5wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels and stainless steels. Steel 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
The basis for the understanding of the heat treatment of steels is the Fe-C phase 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 for different combinations of carbon concentration and temperature. We distinguish at the low carbon end ferrite, which can at most dissolve .028 wt% C at 727 oc and austenite which can dissolve 2.11wt% C at 11480c. at the carbon rich side we find cementite Fe3C. of less interest, except for highly alloyed steels, is the ferrite existing at the highest temperatures. Between the single phases fields are found regions whit mixtures of two phases, such as ferrite+austenite, austenite+cementite, and ferrite+austenite. At the highest temperatures, the liquid phase field can be found and below thus are two phase fields liquid+austenite, liquid+cementite, and liquid+ferrite. In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single phase fields have been given special names that facilitate the discussion.

Transformation diagrams
The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. The metastable phase martensite and morphologically metastable micro constituent bainite, which are of extreme importance to the properties of steel, can generally form with comparatively rapid cooling to ambient temperature, that is, when the diffusion of carbon and alloying elements is suppressed or limited to a very short range. Bainite is a eutectoid decomposition that is a mixture of ferrite and cementite. Martensite, the hardest constituent, forms during severe quenches from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to about 0.7 wt% if these unstable metastable products are subsequently heated to a moderately elevated distributions of ferrite and carbide. The reheating process is sometimes known as tempering or annealing.
The transformation of an ambient temperature structure like ferrite pearlite or tempered martensite to the elevated temperature structure of austenite or austenite+carbide is also of importance in the heat treatment of steel.
ISOTHERMAL TRANSFORMATION DIAGRAMS

This type of diagram show what happens when steel is held at a constant temperature for a prolonged period. The development of the microstructure with time can be followed by holding small specimens in a lead or salt bath and quenching them one at a time after increasing holding times and measuring the amount of phases formed in the microstructure with the aid of a microscope.
Ith diagrams (formation of austenite).
During the formation of austenite from an original microstructure of ferrite and pearlite or tempered martensite, the volume (and hence the length) decrease with the formation of the dense austenite phase.

ASM Metal handbook
Torsten Ericsson, Linkoping Institute of technology, Sweden Principles of heat treating
A steel is usually undefined as an alloy of iron and carbon with the carbon content between a few hundreds of a percents up to about 2 wt%. Other alloying elements can amount in total to about 5wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels and stainless steels. Steel 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
The basis for the understanding of the heat treatment of steels is the Fe-C phase 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 for different combinations of carbon concentration and temperature. We distinguish at the low carbon end ferrite, which can at most dissolve .028 wt% C at 727 oc and austenite which can dissolve 2.11wt% C at 11480c. at the carbon rich side we find cementite Fe3C. of less interest, except for highly alloyed steels, is the ferrite existing at the highest temperatures. Between the single phases fields are found regions whit mixtures of two phases, such as ferrite+austenite, austenite+cementite, and ferrite+austenite. At the highest temperatures, the liquid phase field can be found and below thus are two phase fields liquid+austenite, liquid+cementite, and liquid+ferrite. In heat treating of steels, the liquid phase is always avoided. Some important boundaries at single phase fields have been given special names that facilitate the discussion.

Transformation diagrams
The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. The metastable phase martensite and morphologically metastable micro constituent bainite, which are of extreme importance to the properties of steel, can generally form with comparatively rapid cooling to ambient temperature, that is, when the diffusion of carbon and alloying elements is suppressed or limited to a very short range. Bainite is a eutectoid decomposition that is a mixture of ferrite and cementite. Martensite, the hardest constituent, forms during severe quenches from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to about 0.7 wt% if these unstable metastable products are subsequently heated to a moderately elevated distributions of ferrite and carbide. The reheating process is sometimes known as tempering or annealing.
The transformation of an ambient temperature structure like ferrite pearlite or tempered martensite to the elevated temperature structure of austenite or austenite+carbide is also of importance in the heat treatment of steel.
ISOTHERMAL TRANSFORMATION DIAGRAMS

This type of diagram show what happens when steel is held at a constant temperature for a prolonged period. The development of the microstructure with time can be followed by holding small specimens in a lead or salt bath and quenching them one at a time after increasing holding times and measuring the amount of phases formed in the microstructure with the aid of a microscope.
Ith diagrams (formation of austenite).
During the formation of austenite from an original microstructure of ferrite and pearlite or tempered martensite, the volume (and hence the length) decrease with the formation of the dense austenite phase.

ASM Metal handbook
Torsten Ericsson, Linkoping Institute of technology, Sweden

TRATAMIENTOS TERMICOS

domingo, 9 de marzo de 2008

video de prueba sobre metalografia....

Estructuras cristalinas

CLASE 2008

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