What is color case hardening on firearms?

Answer 1

First we must discuss basic metal terms - iron, steel and "high carbon" and "low carbon" steel. From http://en.wikipedia.org/wiki/Carbon_steel we see that Steel is usually defined as

* Mild (low carbon) steel: approximately 0.05 - 0.29% carbon content

Mild steel has a relatively low tensile strength, but it is cheap and

malleable; surface hardness can be increased through carburizing.

* Medium carbon steel: approximately 0.30 - 0.59% carbon content.

Balances ductility and strength and has good wear resistance; used for

large parts, forging and automotive components. * High carbon steel: approximately 0.6 - 0.99% carbon content. Very strong,

used for springs and high-strength wires. "Case Hardening" is a form of "carburizing" - a process that applies a thin carbon skin to the surface of low-carbon steel objects in order to make the surface more wear resistant.

This carbon skin is achieved by

* applying carbon bearing materials (usually in the form of powdered charcoal, bone, leather, or a commercial preparation such as "Casenite" or other similar materials) to the surface of the steel object * wrapping the whole thing to keep the powder in direct contact with the steel and to keep air (oxygen) away from the surface

* Heating the entire mass to "red heat" which causes the carbon to be absorbed at the surface of the steel forming a thin skin of "high carbon" steel. * "Quenching" the part by plunging the red-hot mass into cold water to harden the "carburized skin".

This results is a slightly mottled gray colored finish which is "glass hard" and brittle for a few thousandths of an inch in thickness, but leaves the body of the steel object tough, strong, and ductile.

"Color Case Hardening" adds steps to the above process, such as bubbling the water, or packing the part in loose carbon material in an airtight box prior to heating, then dumping the contents of red hot box into the cooling water. This causes almost random contact of air, gases from the heated carbon material, the steel object, and water producing a pleasing mottled blue/purple/ gray, brown/ red colored surface in addition to the hard, wear resistance surface.