Case hardening
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Case hardening or surface hardening is the process of hardening the surface of steel whilst leaving the interior unchanged. The idea behind case hardening is to have two different types of steel in the same item. This allows a relatively soft, tough core of a component to be combined with a hard (but potentially brittle) surface. Case hardening improves the wear resistance of machine parts without affecting the tough interior of the parts. Many processes are available for surface hardening.
Both carbon and alloy steels are suitable for case-hardening providing their carbon content is low, usually less than 0.2%. Case hardened steel is usually formed by diffusing carbon (carburization) and/or nitrogen (nitridization) into the outer layer of the steel at high temperature.
The term case hardening is derived from the practicalities of the process itself. The steel work piece (e.g. a firing pin, the head of a rifle bolt, or an engine camshaft) is placed inside a case packed tight with a carbon-based case hardening compound. This is collectively known as a carburizing pack. The pack is put inside a hot furnace for a variable length of time. Time and temperature determines how deep into the surface the hardening extends. However, the depth of hardening is ultimately limited by the inability of carbon to diffuse deeply into solid steel and a typical depth of surface hardening with this method is up to 1.5mm.
Case hardened is also used to describe lumber or timber that has been improperly kiln-dried. If dried too quickly, wood shrinks heavily on the surface, compressing its still damp interior. This results in unrelieved stress. Case hardened wood may warp considerably and potentially dangerously when the stress is released by sawing.
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 less complicated as it is achieved by heating and quenching in the form of heat treatment.
See also
External links
- [Case Hardening]
- [Surface Hardening of Steels]
- [Workshop practice -- Hardening and Tempering]
- [Case Hardening Steel and Metal]
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