The ability to withstand abrasion and resist indentation are two very important attributes of certain types of metals. Hardness, the measurement of those attributes, is a key consideration when deciding which type of metal to choose. Arguably the most important area of a metal for which the optimal hardness must be found is its outermost surface. This is because it is the surface that undergoes the most abrasion and direct impact. In order to ensure that the hardness of a metal substrate is appropriate, some metals have their surface hardness altered through a method known as case hardening.
What is Case Hardening?
Case hardening is a material processing method that is used to increase the hardness of the outer surface of a metal. Case hardening results in a very thin layer of metal that is notably harder than the larger volume of metal underneath of the hardened layer. Case hardening almost always requires elevated temperatures to perform. Through heating, the hardening can be caused by altering the crystal structure of a metal or adding new elements to the composition of the exterior surface of a metal. Since hardening processes reduce formability and machinability, case hardening is typically done once most other fabrication processes have been completed.
Why Case Harden?
There are several reasons to case harden a material rather than attempt to harden an entire metal object. One reason is efficiency. Less energy and less time are required to heat the outermost surface of a metal as opposed to its entire cross section. These efficiencies can result in huge cost savings in large-scale manufacturing operations. Another reason why case hardening is widely used is because of performance. It can be advantageous to have a metal with a hard outside shell and a more ductile interior. An example of this would be when a metal is needed to resist abrasion, but still needs to be able to absorb an impact without resulting in a complete brittle fracture.
Types of Case Hardening Methods
Case hardening can be performed through several different means. One of the most popular methods for higher carbon steels or other heat-treatable metals is through heating and quenching. Heating and quenching involves using some sort of heat source, such as induction coils or an oxyfuel flame to get the outer surface of a steel up past the temperature where its microstructure begins to change, known as its critical temperature (generally somewhere around 700 degrees Celsius). Once this has been accomplished, the steel surface needs to be rapidly cooled by being placed into contact with a quenching medium. This can be brine, water, oil, or air. Different media will be used for different applications depending on the cooling rate required. This rapid cooling causes the steel to form martensite, which is a very hard, abrasion-resistant microstructure.
Another case hardening method is nitriding. Nitriding gets its name from the formation of nitrides that the process forms on the surface of a metal. To perform the nitriding process, metals are heated to an elevated temperature and exposed to ammonia or other nitrogen carrying substances. The elevated temperature and exposure to nitrogen promote the formation of nitrides, which by nature are very hard and resistant to abrasion. This process only works when there are elements on the metal being hardened that can form nitrides such as chromium and molybdenum. Nitriding generally requires lower temperatures than heating and quenching and also does not require a quench process, resulting in less distortion.
Carburising is another form of case hardening that is widely used to improve the mechanical properties of a steel substrate. During carburising, a steel alloy is heated to an elevated temperature and then is exposed to high amounts of carbon on its surface. The external carbon source can be a gas, liquid, or solid depending on the application requirements. The high amounts of external carbon will then form carbides with other elements on the surface of the steel. These carbides provide increased hardness and wear resistance. Similar to nitriding, the heating requirements are generally less, potentially resulting in less distortion.
What Types of Metals Can Be Case Hardened?
Metals that can be case hardened are generally limited to ferrous materials, although there are special cases such as the nitriding of some titanium or aluminium alloys. The ferrous metals commonly case hardened are:
- Low carbon steel
- High carbon steel
- Cast iron
- High strength low alloy steel
- Tool steel
- Stainless steels
Some common components that are case hardened include:
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