The basics of case-hardening
In surface heat-treating, or case hardening, the surface of a piece of steel is heated to a suitable temperature at which it begins to change its internal structure, and then it is cooled very rapidly. The heating and rapid cooling process causes a permanent transformation in the steel, and it becomes harder. This process is known as transformation hardening, because the molecular structure of the metal surface is transformed to another state.
The transformation involved when steel is hardened is actually a change in the arrangement of the atoms or a change in the crystallographic lattice network of the metal. This change occurs at critical transformation temperatures both when the metal is being heated and as it is cooling.
The transformation temperature is the temperature above or below which solid-state transformation takes place. The exact temperature depends on the alloy composition of the steel being treated. The changes and the structures they produce in the metal all take place while the metal is in the solid state. In other words, the transformation occurs before the surface material reaches its melting temperature.
If a piece of steel is heated to its transformation temperature and allowed to cool slowly in ambient air, it passes through several states or arrangements in its microstructure. One of these arrangements is called martensite, a microstructure required to form high hardness in ferrous metals. The greater the degree of martensite formation, the higher the hardness.
To obtain the desired martensite formation, the heated steel must cool at the critical cooling rate, which is the rate at which the heated material is cooled from the transformation temperature to the desired lower temperature. With most heat-treating methods, critical cooling rates are achieved after the heating process by quenching.
In quenching, the surface is sprayed or immersed in water or oil to make it cool very rapidly. Quenching causes the heat-treated area of the steel to bypass other possible states and go directly to martensite.
The most commonly used scale for determining the hardness of steel is the Rockwell C (RHC) scale. The requirements for typical industrial hardening applications are 56 to 62 points Rockwell C.
Although many types and compositions of steel are used, machinable steels usually need a minimum of 0.4% carbon content before they can be successfully hardened to this range by any heat treating method. The carbon content of the metal being treated is the most important factor in determining the maximum attainable surface hardness of a steel, and the higher the carbon content, the greater the potential hardness.
The depth of the hardened case depends on how far below the surface the martensite can form in sufficient quantities. The chemical composition or alloy content of the steel determines the depth to which the maximum hardness can be achieved on a given piece of steel.
The most widely used case hardening technologies are flame hardening, induction hardening, laser hardening.
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