Carbon content variations have a pronounced effect on steel; apart from the obvious chemical changes that occur as carbon content changes, there are also important changes to the mechanical properties of a steel as well. Carbon content affects the hardness, strength, crack resistance, and formability of a steel, to name a few characteristics. Decreases in carbon content are commonly caused by a phenomenon known as decarburisation. It is critical to understand the amount of decarburisation a steel undergoes as it is exposed to different types of processing and fabrication.
What is Decarburisation?
Decarburisation is the reduction of carbon content in steel. It typically occurs when a steel is heated above the lower limit of its recrystallisation temperature, usually around 700 degrees Celsius. When steel is elevated to this temperature and exposed to gases such as hydrogen and oxygen, the steel’s carbon bonds with those gases and leaves the steel with a reduced amount of carbon. Since the gases are only in contact with the surface of a steel, decarburisation generally occurs most heavily on the outermost layers of a steel. The carbon reduction results in a loss of tensile strength, fatigue strength, and hardness.
How to Prevent Decarburisation
Decarburisation can be detrimental, but it can readily be controlled or removed. One way to prevent decarburisation from happening is to prevent it from reaching its recrystallisation temperature. This prevents the ability of the carbon to migrate from the surface of the steel to the ambient gases. Because steel must sometimes be heated to this temperature, another method for decarburisation prevention is to flood the surroundings of the steel with gases that do not react with the carbon. Popular gases for this method of prevention include argon and nitrogen, although further research should be done to determine which gas is best to use for a given application.
How to Remove Decarburisation
If decarburisation is unable to be prevented, it can still be removed through several methods. One way to remove decarburisation is to remove the decarburised material entirely. This can be done through milling, grinding, or a variety of other machining processes. Another popular process for getting rid of decarburisation is to restore the carbon that was lost back to the steel substrate. This can be done after a steel has been decarburised by heating it back up to its recrystallisation temperature and flooding the area around it with hydrocarbons. Care needs to be taken when doing this to ensure that the right amount of carbon is returned to the surface of the steel.
How to Measure Decarburisation
Decarburisation cannot be adequately inspected simply by visual means alone. To accurately determine the amount of decarburisation that a steel has undergone, proper tools must be used. One quick method of measuring decarburisation is to take a hardness measurement of the steel. If possible, doing this before and after thermal processing will show how much decarburisation has occurred by comparing the differences in hardness. Cutting a specimen, polishing it, and then viewing it under a microscope is another way to get an accurate depiction of the amount of decarburisation that has occurred. This method also shows the observer the depth of decarburisation.
While unintended decarburisation can be detrimental to a steel, there are some times where purposeful decarburisation has benefits. Since carbon content is lowered during decarburisation and the surface of the steel has a reduced amount of hardness, decarburisation can improve machinability. It can also result in improved formability at the surface of the steel as well. Because of the risks associated with decarburisation, proper research and testing should be performed to make sure that intentional decarburisation is appropriate.
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