What does "case hardened" mean?
A thin layer of harder metal is formed at the surface of the metal object after case-hardening or surface hardening.Adding carbon or nitrogen to the surface layer of iron or steel that has poor to no hardenability is a case-hardening process.After the part has been formed into its final shape, case-hardening can be done to increase the hardening element content of bars to be used in a pattern welding or similar process.When discussing modern armour, the term face hardening is used.
For metal components that are subject to sliding contact with hard or abrasive materials, hardened metal is more resistant to surface wear.Through-hardening is not always a good choice because hardened metal is more brittle than softer metal.Because of the soft core that can absorb stresses without cracking, case-hardening can produce a component that will not break, but also provide adequate wear resistance on the hardened surface.
There are two layers of metal, one with a very low carbon content which is worked into wrought iron, and the other a high carbon outer layer.It was not useful without more smelting since the high carbon iron is hot short.It was mostly unused in the west until the popularization of the finery forge.The iron was very flexible and not very hard.
Case-hardening involves packing the low-carbon iron within a substance high in carbon, then heating this pack to encourage carbon migration into the surface of the iron.A thin surface layer of higher carbon steel is formed with the carbon content gradually decreasing from the surface.The result is a product that combines the strength of a low-carbon steel core with the wear resistance of high carbon steel.
The traditional method of applying the carbon to the iron involved packing it in a mixture of ground bone and charcoal or a combination of leather, hooves, salt and urine, all inside a well-sealed box.The carburizing package is heated to a high temperature but still under the melting point of the iron and left at that temperature for a long time.The deeper the package is held at the high temperature, the more carbon will diffuse into the surface.Sharp tools need deep hardening to allow grinding and resharpening without exposing the soft core, while machine parts need only shallow hardening for increased wear resistance.
If the carbon material is mixed organic matter, the resulting case-hardened part may show surface discoloration.The dark steel shows a pattern of black, blue, and purple caused by the various compounds formed from the bone and charcoal.This oxide surface has an attractive finish and provides a degree of corrosion resistance.This pattern is commonly encountered as a decorative finish on firearms.
Hard steel alone tends to be brittle, so it's not easy to match it with case-hardened steel.
Carbon is immobile at case-hardening temperatures.The carbon monoxide was generated by the breakdown of the carburising compound and the oxygen in the sealed box.It is too slow to be workable with pure carbon.Oxygen is not required for this process and can be carried out inside a sealed box.CO leaking out or being oxidised to CO2 by excess outside air can be stopped with the seal.
Adding a carbonate that breaks down to BaO + CO2 encourages the reaction.
The activity of the carburising compound is increasing.There is a misconception that case-hardening was done with bone.Hoof and horn was the main carbon donor.Some carbonates are present in the bone, but it is mostly calcium phosphate.It doesn't have the beneficial effect of encouraging CO production and it can also introduce phosphorus into the steel alloy.
Mild steels are used with low carbon content and are suitable for case-hardening.Mild steels are not normally hardenable due to the low quantity of carbon, so the surface of the steel is altered to increase its hardenability.Case-hardened steel is formed by a mixture of carbon, nitrogen, and/or Boron in the outer layer of the steel at high temperature and then heat treating the surface layer to the desired hardness.
The ancient process of carburization is what inspired the term case-hardening.A carbon-based case-hardening compound is placed inside a steel work piece.This pack is called a carburizing pack.The pack is put in a hot furnace.The time and temperature determine how deep the hardening is.A typical depth of surface hardening with this method is up to 1.5mm due to the inability of carbon to diffuse deeply into solid steel.In modern carburizing, heating in a carbon-rich atmosphere is one of the techniques used.Small items can be case-hardened by repeated heating with a torch and quenching in a carbon rich medium, such as the commercial products Kasenit / Casenite or "Cherry Red".The newer types of compounds do not contain potentially toxic cyanide compounds.
A "case" of martensite is created when the surface of the steel is heated very rapidly to high temperatures and then cooled rapidly using water.This type of hardening requires a carbon content of less than 0.6 wt% C.
The shackle of a lock can be used where the outer layer is hardened to be file resistant and mechanical gears are needed to maintain a long service life.A defined surface area is used for flame hardening.Four factors control the result of the hardening process.
In this process steel is introduced to a carbon rich environment and quenched so that the carbon is locked in.
The carbon content of the steel will be higher if it is held in the carbon-rich environment for a long time.The carburized section will have a carbon content high enough that it can be hardened again.
It is possible to carburize only a portion of a part, either by protecting the rest with a process such as copper plated, or by applying a medium to the part.
Pack carburizing is when the carbon comes from a solid source.The outer layers of carbon are diffused by packing low carbon steel parts with a carbonaceous material.A high-carbon layer can be formed by a few hours of heating.
Liquid carburizing involves placing parts in a bath of molten carbon-based material, often a metal cyanide, which is then poured into a furnace with a methane-rich interior.
The steel part is exposed to an atmosphere of ammonia gas and dissociated ammonia during nitriding.The depth of the case is determined by the amount of time the part spends in this environment.The formation of nitrides is how the hardness is achieved.For this method to work, nitride forming elements must be present.The advantage of this process is that it causes little distortion, so the part can be case-hardened.quenching is not done after nitriding.
It is a case-hardening process that is fast and efficient and is used on low-carbon steels.The part is heated to 954 C and then washed with water or oil to remove any residual cyanide.
This process produces a thin, hard shell 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 can be used on bolts, nuts, screws and small gears.cyanide salts are poisonous.
Carbonitriding uses a gaseous atmosphere of ammonia and hydrocarbons instead of sodium cyanide.If the part is to be quenched, it is heated to a temperature of 1,450 F.