what is decarburization of steel?

What is Decarburization?

Decarburization is a technique of reducing the amount of carbon constituents in a metal (generally steel). The process is the process opposite of carburization, namely the reduction of carbon content. As we know, carbon is the primary alloying element in steel & hence the term Decarburization is more concerned with steels such as low-carbon, medium-carbon, and high-carbon steels. As a rule of thumb, As the strength and hardness increase, the amount of carbon increases. So both properties rise along with rising in carbon percentage.

Decarburization occurs when the metal is heated to temperatures of 700 °C or above when carbon in the steel surface reacts with gases containing oxygen or hydrogen and is removed from the steel as a gaseous phase.

At 700 °C or above temperature, carbon disperses out of the steel and the metal develops further vulnerable to absorbing hydrogen and oxygen gases. The rate of carbon diffusion rises with growing temperature.  The elimination of carbon removes hard carbide phases resulting in a softening of the metal, mainly at the surfaces of the material that are in touch with the decarburizing gas.

Effects of Decarburization in Steel?

Decarburization may have desirable or adverse effects on the steel. Some of the influences include reduced ductility, reduced strength, and rise in susceptibility to crack initiation. These effects are mostly concentrated at the surface of the metal. In general, equipment failure is rarely caused directly due to decarburization. Although, most people in the industry see the process as a serious issue that has adverse effects on the fatigue life of steel parts and may be indicative of a more severe underlying problem.

Decarburization is a serious problem because surface properties are inferior to core properties, resulting in poor
wear resistance and low fatigue life.

Chemical reactions during Decarburization

The most common reactions during Decarburization are:

The general test for decarburization is by a hardness test on the material. This will provide a general idea of how much carbon diffused from the surface of the metal because the metal will be softer than its original state. Again, this is because there is less carbon, and the lower the carbon content the less hard the material will be.

Other forms of decarburization testing exist that estimate the depth of decarburization along the surface of steel (e.g. ASTM E1077). The results of the test shows if the surface has experienced complete decarburization (100% loss of carbon), if more than 50% has experienced decarburization, or if less than 50% has experienced decarburization. These types of decarburization are classified as Type 1, Type 2, and Type 3, respectively.

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