Case hardening

Case hardening is a simple method of hardening steel. It is less complex than hardening and tempering. This techniques is used for steels with a low carbon content. Carbon is added to the outer surface of the steel, to a depth of approximately 0.03mm. One advantage of this method of hardening steel is that the inner core is left untouched and so still processes properties such as flexibility and is still relatively soft.

Case hardening is a technique in which the metal surface is reinforced by the adding of a fine layer at the top of another metal alloy that is generally more durable. Case hardening steel is normally used to increase the object life. This is particularly significant for the manufacture of machine parts, carbon steel forgings, and carbon steel pinions. Case hardening is also utilized for other applications. Case hardening is also called surface hardening. Case hardening has been in use for many centuries, and was frequently used for producing horseshoes and different kinds of cooking utensils that were subjected to substantial wear and tear. Case hardening is essentially a group of processes that are used to increase the surface hardness to an extent that is higher than that of the bulk material. Case hardening is performed normally locally on the top surface, and for a limited depth. Greater hardness is usually related with better wear and fatigue resistance.

The addition of carbon to the iron surfaces is common. Case hardening involves the use of metal that has low carbon contents, and combining it with a metal that has more carbon content. The grouping of metals is likely to produce the product that is much harder. The adding of the low carbon metal creates a material that can be molded easily into the desired shapes. The surface improvement not only increases the product strength, but also assists to avoid the iron weakening. Consequently, items like fireplace equipment, cast iron wash pan, and frying utensils would continue to be serviceable for long periods of time. Case hardening is frequently utilized in the constructing industry for reinforcing girders, metal doors, and metal panels. Case hardening is generally performed after the formation of the component into its ultimate form.

Case hardening involves the use of metal that has low carbon contents, and combining it with a metal that has more carbon content.

The traditional method of applying the carbon to the surface of the iron involved packing the iron in a mixture of ground bone and charcoal, or a combination of leather, hooves, salt and urine, all inside a well-sealed box. This carburizing package is then heated to a high temperature, but still under the melting point of the iron, and left at that temperature for a length of time. The longer the package is held at the high temperature, the deeper the carbon will diffuse into the surface. Different depths of hardening is desirable for different purposes: sharp tools need deep hardening to allow grinding and resharpening without exposing the soft core, while machine parts like gears might need only shallow hardening for increased wear resistance.

The resulting case hardened part may show distinct surface discoloration. The steel darkens significantly, and shows a mottled pattern of black, blue and purple, caused by the various compounds formed from impurities in the bone and charcoal. This oxide surface works similarly to bluing, providing a degree of corrosion resistance, as well as an attractive finish. Case coloring refers to this pattern and is commonly encountered as a decorative finish on replica historic firearms.

Case Hardening is a process of hardening ferrous alloys so that the surface layer or case is made substantially harder than the interior or core. The chemical composition of the surface layer is altered during the treatment by the addition of carbon, nitrogen, or both. City Steel Heat Treating provides the most common processes of Carburizing, Carbonitriding, and Gas Nitriding.

Components that are subjected to severe impacts and high pressures are generally case hardened. The surfaces that need special hardness may be selectively hardened, without performing case hardening of the remaining object. Firearms are a usual item that is case hardened, as they need accuracy in machining and higher hardness for performing the desired functions. Another general application of the case hardening is on camshafts and special purpose screws, mainly the self drilling screws. Case hardening is less complex for fasteners and screws since it is performed simply by heating and quenching. Case hardening of smaller items is performed by repetitive heat application.

Another common application of case hardening is on screws, particularly self-drilling screws. In order for the screws to be able to drill, cut and tap into other materials like steel, the drill point and the forming threads must be harder than the material(s) that it is drilling into. However, if the whole screw is uniformly hard, it will become very brittle and it will break easily. This is overcome by ensuring that only the case is hardened and the core remains relatively soft. For screws and fasteners, case hardening is achieved by a simple heat treatment consisting of heating and then quenching.