The mechanical properties of a material depend both on its chemical composition and crystal structure have. Heat treatments change the crystalline structure without altering the chemical composition, giving the material a specific mechanical properties, through a process of successive heating and cooling to achieve the desired crystal structure.
These features are:
Wear resistance: The resistance of a material to be left to erode when in contact friction with another material.
Toughness: The ability of a material to absorb energy without producing cracks (impact).
Machinability: The facility has a material to allow the machining by chip removal.
Hardness: The resistance of a steel to penetrate himself. Is measured in Brinell (HB) or units Rockwell C (HRC) through the test of the same name.
The mechanical properties of alloys of the same metal, and in particular of steel lies in the chemical composition of the alloy that the form and type of heat treatment to which they are subjected. Thermal treatments modify the crystal structure formed by the steel without changing the composition their chemical.
This property of having different grain structures with the same chemical composition is called polymorphism and is what makes the heat treatments. Technically, the polymorphism is the ability of some materials have different crystalline structures, with a unique chemical composition, diamond and graphite are carbon polymorphisms. The α-ferrite, austenite and δ-ferrite are polymorphisms of iron. This property in a pure chemical element called allotropy.
Steel is an alloy of iron and carbon containing other alloying elements, which confer specific mechanical properties for use in the metalworking industry.
The other main elements of composition are chromium, tungsten, manganese, nickel, vanadium, cobalt, molybdenum, copper, sulfur and phosphorus. These are chemicals that are called steel components, and different crystal structures or combination of constituents.
constituent elements according to their percentage, offer specific features for certain applications, tools, knives, stands, etc.. The difference between the various steels, as has been said depends on the chemical composition of the alloy thereof, and the type of heat treatment to which they are subjected.
thermal treatment the material is one of the key steps that can achieve the mechanical properties for which it is created. Such processes involve heating and cooling of a metal in its solid state to change its physical properties. With appropriate heat treatment can reduce internal stresses, grain size, increase toughness or produce a hard surface with a ductile interior. The key of the heat treatment consists of reactions occurring in the material, both in steel and nonferrous alloys, and occur during the heating and cooling of parts, with guidelines and timelines.
To know that the metal temperature should rise for receiving a heat treatment is recommendable to change the phase diagrams as the iron-iron-carbon. In this type of diagrams are the temperatures at which phase changes occur (changes of crystalline structure), depending on the materials diluted.
Heat treatments have become very important in industry in general and with the constant innovations will require metals with high resistance to wear and stress. The main thermal treatments are:
Temple: Its purpose is to increase the hardness and strength of steel. To this end, the steel is heated to a temperature slightly higher than the upper critical Ac (between 900-950 ° C) and then cooled more or less quickly (depending on the characteristics of the piece) in a medium such as water, oil, etc. .
Tempering: This only applies to pre-hardened steels and slightly decreased the effects of temper, still maintains the hardness and increasing toughness. Tempering is able to decrease the hardness and strength of hardened steels, has eliminated the problems created in the temple and improved toughness, leaving the steel with the desired hardness or resistance. Temple differs fundamentally in terms of maximum temperature and cooling rate.
Annealing: It's basically a warm up to austenitizing temperature (800-925 ° C) followed by slow cooling. With this treatment was possible to increase elasticity, while decreasing the hardness. It also facilitates the machining of parts to homogenize the structure, refine grain and soften the material, eliminating the bitterness produced by cold work and internal tensions.
Standard: The purpose leave a material in a normal state, ie, absence of internal stresses and a uniform distribution of carbon. It is usually used as a pretreatment to hardening and tempering.
Maria Linares EES secc1
19881179 
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