Article updated on 25 January 2021.
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Image Credits Shutterstock/rommma
Air is the perfect mix of moisture, oxygen, carbon dioxide, hydrogen and other gases in small quantities, which helps keep all living things alive. However, it is not very good for the metals found in much of our surrounding infrastructure which reacts with moisture and oxygen in particular under the right conditions.
In order to manipulate metals, heat is often necessary, but the application of heat can cause metals to react with air at an accelerated rate. This heat treatment causes a skin or layer on the surface of the metal that is harder or softer and thereby changes the properties of the metal. To avoid this oxidation process – where oxygen has reacted with the metal - it is therefore helpful to remove all the air and reactive elements in a vacuum chamber.
Vacuum heat treatment is a method of hardening metals at extremely high temperatures in the absence of air. It uses a vacuum which has an absolute pressure below that of the normal atmosphere, creating an unreactive environment. During vacuum heat treatment, the metal parts go into a tightly sealed treatment chamber where a vacuum pumping system removes as much of the air as possible so that there is nothing left to react with the metal. It is then heated up to as much as 2,400°F/1316°C at a controlled rate before the temperature is lowered. The soak time - the amount of time required for a chemical purging compound to complete its reaction - depends on the needs of the part and type of metal it is made from. It can take anywhere from three to 24 hours and is controlled by a computer to ensure repeatability and uniformity.
As well as being an environmentally friendly method of hardening metals, there are three main benefits to vacuum heat treatment; the parts need no further forming, they have no scaling or discolouration, and there is no need for further cleaning. The process improves the overall condition of the metal alloy – it allows the surface of the metal to be treated evenly, the cooling process is much quicker, and it improves the lifespan and functionality of the alloy.
Applications of Vacuum Heat Treatment
Vacuum heat treatment is used on metal based super alloys (heat resistant materials based on nickel such as iron-nickel or cobalt-nickel), reactive and refractory materials like titanium, and stainless steel of various grades. Even some parts of musical instruments containing brass or bronze may also be subjected to the treatment. The process is used for parts which must withstand high heat conditions and other stresses during normal operation such as airplane engines and exhaust parts - many of which can be difficult to form in the first place.
One of the most important uses of heat treatment is in the aerospace industry in aircraft structures, where the type of vacuum heat treatment is as important as the aluminium alloy used to build the plane. The choice of alloy is critical; the lifespan of an aircraft is reliant on the grain structure of the alloy, which in turn is dependent on the vacuum heat treatment it receives. NASA has determined nine different types of vacuum heat treatment definitions – including age hardening, natural aging and artificial aging – which they utilise in the training and certification of their vacuum heat-treating personnel. The type of heat treatment is essential as it allows a designer to build a plane from higher strength materials which yield better functionality, fewer repairs and a longer lifespan.
The process of vacuum heat treatment is a means of treating difficult pieces of metal while also conferring additional benefits such as strength and cleanliness to the metal it is treating in an environmentally friendly process with no harmful side effects. At a time when industry is being encouraged to be more cautious about its environmental outputs, it is not difficult to see why vacuum heat treatment is increasing in use.
References and Further Reading
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