Heat Treating of Steel
Heat treating of steel is a process of heating and cooling steel to change its structure and properties. It is done to metallurgically alter the structure and properties of the steel, primarily to make it harder, stronger, and more durable. Depending on the desired results, several different heat treatment processes can be used.
Annealing is the process used to soften steel and increase its ductility. The steel is heated to above the critical point and held at that temperature for a period of time to allow the heat to penetrate deep enough into the steel. Then, the steel is cooled at a rate that prevents it from becoming harder due to thermal shock. It is usually cooled in a furnace.
Normalization is a similar process but with different results. Steel is heated above its upper critical temperature and allowed to cool in the air, instead of a furnace, until it reaches an acceptable hardness.
Quenching is another heat treating process. Steel is heated above its critical temperature and then quickly submerged in cold water or oil. This process is used to produce incredibly hard and strong steel.
Tempering is the process used to reduce the hardness of steel that is too hard due to the quenching process. It is done by reheating the steel to a lower temperature then allowing it to cool slowly. This relieves some of the stress that has built up.
The heat treating process can vary depending on the type of steel and desired results. For example, certain high carbon steels are often subjected to a process of heat treating known as carburizing in which the steel is heated to high temperatures and exposed to carbon-rich gases. This allows the carbon atoms to diffuse into the steel and harden it while still keeping it ductile.
Despite its complexity, the heat treating of steel is a process that has been used to alter the properties of steel for centuries. It requires knowing the exact temperature and holding time required, as well as the correct cooling rate. If done correctly, it can create an incredible durable and strong steel.
