Metal Structure & Heat Treatment
Metals are an important material in industrial production, and the variation in their physical and chemical properties is caused by the different arrangements and bonds of atoms that compose them. The most significant variations can be created by thermomechanical processing, which, by changing the arrangement and arrangement of the atoms of the material, changes its properties. Heat treatment is one of the most important thermomechanical processes used to produce metals with desired properties.
Heat treatment is a process in which the metal is heated up to an optimum temperature and then either cooled or cooled quickly. This stimulus induces phase transformations that are part of the manufacturing of modern metals and alloys. When heated or cooled, differential atomic motions occur which result in the formation of new structures and rearrangements of the metallic lattice. This causes a change in the properties of the material which can be categorised as hardness, toughness, ductility, and corrosion resistance; among others.
The first step before heat treating is to choose the alloy. This selection is important since some alloys have favourable response to heat treatments while others have less. For instance, steels have a very good response to heat treatments, while aluminium alloys do not. Aluminium alloys require a different process known as ageing, which consists in a complex sequence of heating and cooling cycles to obtain the desired properties. The selection of the alloy determines the growth of crystal structures, microstructure formation and grain boundaries, which influence the microstructure of the material.
Once the alloy has been selected, the heat treatment process begins. The fundamental heat treatment processes used on metals can be divided into two categories:
•Annealing - this is a process of applying heat to a metal, which increases its plasticity. Annealing is commonly used to soften metals and improve the ductility of their structures.
•Hardening - this is a process of applying heat to a metal, which increases its hardness. Hardening is typically used to improve the strength of the metal and increase its wear resistance.
To apply the heat treatment correctly, it is important to consider the temperature at which the metal reaches different phases, known as the phase diagram of the alloy. The temperature used in the heat treatment determines the reaction of the atoms in the metal and the final structure obtained. Depending on the alloy, temperature and cooling rate chosen, different microstructures can be obtained, from single-phases to complex mixtures of different compositions, grain boundaries and distributed phase diagrams.
Once the microstructure is attained, other treatments or heterogeneous thermal treatments may be used to give the metal surface a final finish. Different procedures like carburising, treating with nitrides, chromates or surface finishing can be used. Depending on the treatment chosen, different properties can be obtained in the material, such as non-slip feel, machinability, wear resistance, corrosion protection and luster.
It is important to note that heat treatments can only be used to modify properties within the boundaries of the given alloy. Heat treatments, therefore, can be an effective tool to adjust the mechanical, physical and chemical properties, but they have their limitation due to the nature of the material and alloys used.
Heat treatments are essential to produce modern metals with desired properties. The process requires knowledge of metal, alloy selection and the phase diagram of the alloy, as well as careful consideration of temperature, cooling rate and treatment chosen for the application desired. This is why metal structure and heat treatment is a complex and important branch in metal industry and engineering.
