What is heat treatment?
The idea of heat treatment is to change both metal's physical and mechanical properties with a series of heating and cooling process, without altering its fundamental shape.
The main purpose of heat treatment is to strengthen materials, to improve formability, or to make materials ideal for machining. In order to come to the desired results, the basic process of this treatment invovles the heating and cooling process to an extreme high/ low temperature.
The importance of heat treatment process is not only helping the manufacturing process, but optimizing the performance, numerous characteristics and improvements it can provide to a finished product.
Common Process for Heat Treatment:
Hardening:
The basic process of hardening involves two steps: heating and quenching. By heating a steel and maintaining it at an ideal temperature to a period of time, the pearlite can be transformed into austenite.
To prevent the steel to be too brittle to machine properly, quenching process is required to be taken over rapidly by using water or oil onto the steel. The microstructure of a hardened steel part is ferrite, martensite, or cementite
Tempering:
Achieving an equilibrated state is the main purpose of tempering. A steel, after being hardened and quenched for a suitable period of time, is oftentimes being heated in a strictly controlled temperature in order to obtain the ideal hardness and strength - that is what we call "tempering".
Higher temperatures will result into high ductility, but low strength and hardness. Low tempering temperatures will produce low ductility, but high strength and hardness. Almost all carbon steels that have been hardened, need to go through tempering process to reduce their brittleness, so that they can work properly in application.
Annealing:
Annealing is by slowly cooling down steel that is in high temperature to room temperature. It aims at achieving high ductility and toughness, but low hardness. The common practice is by heating a component to the appropriate temperature, soaking it at that temperature, and then shutting off the furnace while the component is in it.
Normalizing:
The process of normalizing involves heating steel, and then keeping it at that temperature for a period of time, and then cooling it in air.
The microstructure in the steel will then result in a mixture of ferrite and cementite, which has a higher strength and hardness, but lower ductility. It's a common practice for structures and structural components that will be used for machining, as it improves the machinability of carbon steels.
Other Heat Treatments and Their Purposes:
Nitirding
Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. Low-carbon and low-alloy steels are two types of materials that are commonly applied by this process. Common applications include gears, crankshafts, camshafts, cam followers, valve parts, extruder screws and other tools and dies. There are three main methods/ medium used for this process - gas nitriding, salt bath nitriding, and plasma nitriding.
Carburization
This process aims at heating steel or iron to a desired temperature that's below melting point, in the presence of a liquid, solid, or gaseous material which decomposes in order to release carbon when heated to the temperature used. The surface will have higher carbon content than the main material. When the steel or iron is quickly cooled by quenching, the higher carbon content on the outer surface becomes hard, while the core remains soft.
Sub-Zero Treatment
Sub-zero treatment is by cooling down components to a very low temperature. The main purpose of this treatment is to increase hardness and resistance, and reduce metal fatigue. Sub-zero treatment is usually done immediately following quenching and before any tempering is applied. Common practice includes gears, bearings, tools and other items that need a higher wear resistance and durability.
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