Editing Case-hardening (section)

== Processes ==

=== Flame or induction hardening ===
{{Main|Differential heat treatment}}
[[File:Flame hardened sprocket.jpg|thumb|A flame-hardened sprocket. The discoloration around the teeth delineates the area that was rapidly heated and then quenched.]]

Flame or [[induction hardening]] are processes in which the surface of the steel is heated very rapidly to high temperatures (by direct application of an [[oxy-gas]] flame, or by [[induction heating]]) then cooled rapidly, generally using water; this creates a "case" of [[martensite]] on the surface. A carbon content of 0.3–0.6 wt% C is needed for this type of hardening. Unlike other methods, flame or induction hardening does not change chemical composition of the material. Because it is merely a localized heat-treatment process, they are typically only useful on high-carbon steels that will respond sufficiently to quench hardening.

Typical uses are for the shackle of a lock, where the outer layer is hardened to be file resistant, and mechanical gears, where hard gear mesh surfaces are needed to maintain a long service life while toughness is required to maintain durability and resistance to catastrophic failure.
Flame hardening uses direct impingement of an oxy-gas flame onto a defined surface area. The result of the hardening process is controlled by four factors:

* Design of the flame head
* Duration of heating
* Target temperature to be reached
* Composition of the metal being treated

=== Carburizing ===
{{Main|Carburization}}

Carburizing is a process used to case-harden steel with a carbon content between 0.1 and 0.3 wt% C. In this process iron is introduced to a carbon rich environment at elevated temperatures for a certain amount of time, and then quenched so that the carbon is locked in the structure; one of the simpler procedures is repeatedly to heat a part with an acetylene torch set with a fuel-rich flame and quench it in a carbon-rich fluid such as oil.

Carburization is a diffusion-controlled process, so the longer the steel is held in the carbon-rich environment the greater the carbon penetration will be and the higher the carbon content. The carburized section will have a carbon content high enough that it can be hardened again through flame or induction hardening.

It is possible to carburize only a portion of a part, either by protecting the rest by a process such as copper plating, or by applying a carburizing medium to only a section of the part.

The carbon can come from a solid, liquid or gaseous source; if it comes from a solid source the process is called '''pack carburizing'''. Packing low carbon steel parts with a carbonaceous material and heating for some time diffuses carbon into the outer layers. A heating period of a few hours might form a high-carbon layer about one millimeter thick.

Liquid carburizing involves placing parts in a bath of a molten carbon-containing material, often a metal cyanide; gas carburizing involves placing the parts in a furnace maintained with a methane-rich interior.

=== Nitriding ===
{{Main|Nitriding}}

Nitriding heats the steel part to {{convert|482|–|621|C|F}} in an atmosphere of ammonia gas and dissociated ammonia. The time the part spends in this environment dictates the depth of the case. The hardness is achieved by the formation of nitrides. Nitride forming elements must be present for this method to work; these elements include [[chromium]], [[molybdenum]], and [[aluminum]]. The advantage of this process is that it causes little distortion, so the part can be case-hardened after being quenched, tempered and machined.
No quenching is done after nitriding.

=== Cyaniding ===

Cyaniding is a case-hardening process that is fast and efficient; it is mainly used on low-carbon steels. The part is heated to {{convert|871|–|954|C|F}} in a bath of [[sodium cyanide]] and then is quenched and rinsed, in water or oil, to remove any residual cyanide.

: 2NaCN + O<sub>2</sub> → 2NaCNO
: 2NaCNO + O<sub>2</sub> → Na<sub>2</sub>CO<sub>3</sub> + CO + N<sub>2</sub>
: 2CO → CO<sub>2</sub> + C

This process produces a thin, hard shell (between {{convert|0.25|and|0.75|mm|in|abbr=on|disp=semicolon}}) that is harder than the one produced by carburizing, and can be completed in 20 to 30 minutes compared to several hours so the parts have less opportunity to become distorted. It is typically used on small parts such as bolts, nuts, screws and small gears. The major drawback of cyaniding is that cyanide salts are poisonous.

=== Carbonitriding ===
{{Main|Carbonitriding}}

Carbonitriding is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons is used instead of sodium cyanide. If the part is to be quenched, it is heated to {{convert|775|–|885|C|F}}; if not, then the part is heated to {{convert|649|–|788|C|F}}.

=== Ferritic nitrocarburizing ===
{{Main|Ferritic nitrocarburizing}}

Ferritic nitrocarburizing diffuses mostly nitrogen and some carbon into the case of a workpiece below the critical temperature, approximately {{convert|650|C|abbr=on}}. Under the critical temperature the workpiece's microstructure does not convert to an [[austenitic]] phase, but stays in the [[ferritic]] phase, which is why it is called ''ferritic'' nitrocarburization.