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Heat Treatment and Metal Properties

written by: kkaarthic • edited by: Lamar Stonecypher • updated: 9/29/2011

This guide will help you learn about the various methods of heat treatment and measurement aspects of the mechanical properties of metals. Topics involved include the hardening, softening (annealing), and tempering processes carried out on critical components.

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    Heat Treatment - The basics

    Ever wondered how the gears in your automobile work for years without any breakdown or how a drill is able to make a hole in steel even after numerous holes are made? These are basically due to the composition of the material and to the metallurgical alteration of its properties. The art of controlling the metallurgical properties of steel or any ferrous material is termed "heat treatment." Through this process the material structure of the metal is altered in order to meet the requirements of a specific application. Included in this guide are several articles that will provide more information on the basics of heat treatment and its need.

    Hardness and hardenability are important factors for a material. The measurement and the modes of measurement of hardness of the material is also important for understanding heat treatment terminology.

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    Types of Heat Treatment and Their Effect on Metal Properties

    The properties of steel or other ferrous metals are mainly altered by heating the metals. The effect of heating the metals to a particular temperature level like the lower critical limit temperature or the higher critical temperature tends to alter various qualities of the material. Parameters like toughness, tensile strength, wear resistance, etc. are greatly improved due to the heat treatment process. The only criterion that plays an important role in heat treatment is the presence of carbon in steels and other ferrous metals. The amount of carbon present in the steel determines the type of heat treatment that can be carried out on it.

    There are various processes of heat treatment which are mainly used depending on the applications that the metal is subjected to. Processes like annealing are used to improve the machinability of the steel.

    1. Processes like stress relieving and annealing are used reduce the internal stress developed in the material during its course of manufacture like forging, welding, etc. Further, the annealing process tends to improve the machinability of the material to a very great extent.

    2. Case hardening and through-hardening tend to improve the hardness of the material to a great extent, thereby increasing the material's wear-resistance property. The two methods are dependent on the amount of carbon present in the steel. Low carbon steel can only be case hardened. Medium and high carbon steels can be through-hardened and surface hardened by using surface hardening methods.

    3. Tempering is an important process in heat treatment that eliminates the brittleness in the material after other heat treatment processes and provides a uniform grain structure of the material. The process tends to improve the toughness of the material to a very great extent.

    4. Carburizing is a method of increasing the carbon content in a material by inducing carbon into it. This process in particular enhances the hardenability of the case of the material.

    5. Nitriding is a process of improving the wear resistance of the material, which can be done even after the completed machining process of the component. Cementation is another process for improving the surface hardness of the material thermo-chemically, though it is not used much these days. Here are some articles that will explain in detail on each of the above processes.

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    Post Weld Heat Treatment

    Heat treatment of steel can also be carried out on completely finished products. Pressure vessels and piping carrying hot oils, steam, etc. are examples of critical items that require post-weld heat treatment. These structures are quite big and they need to have separate arrangements for heat treating them. These structures tend to develop a good amount of internal stress due to the intensive welding carried out on them. Heat treatment is required to subdue the internal stresses in the structure and provide a safe working environment. The post weld heat treatment in some case is mandatory due to meet safety norms.

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    Wrapping Up

    Heat treatment plays a vital role for any steel material to work properly. Day to day developments are making heat treatment processes more dependable. In general, it's always best to use a heat treated metal rather than the metal in its orginal form.

References


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