Hardening (metallurgy)

Hardening is a metallurgical and metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.

Processes

The five hardening processes are:

• The Hall–Petch method is used to change the grain size in a material, which can affect the dislocation density. Smaller grain size will make the material harder, but if the grains get too small the hardness can actually decrease.
• In work hardening (also referred to as strain or cold hardening) the material is strained past its yield point. The work done on the material adds energy and has the ability to move and generate dislocations. This process usually takes place at a temperature below the materials recrystallization temperature.
• In solid solution strengthening, an alloying element is added to the material desired to be strengthened, and together they form a “solid solution”. Different alloying elements can be used to cause either a substitutional or an interstitial solid solution.
• Precipitation hardening is a process where impure particles are distributed throughout the metal. This is achieved by first heating the metal above its phase transition temperature and then rapidly cooling the metal. Particles of the second phase become trapped and form anchor points to impede the movement of dislocations. Precipitation hardening is one of the most commonly used techniques for the hardening of metal alloys. In steels, a similar method is referred to as a martensitic transformation. In this transformation, austenite is rapidly cooled off before the dissolved carbon atoms have a chance to escape, forming martensite, a higher hardness phase of steel.
• Martensitic transformation, more commonly known as quenching and tempering.

All hardening mechanisms, except of the martensitic transformation, introduce dislocations or defects in a crystal lattice that act as barriers to slip.

Applications

Material hardening is required for many applications:

• Machine cutting tools (drill bits, taps, lathe tools) need be much harder than the material they are operating on in order to be effective.
• Knife blades – a high hardness blade keeps a sharp edge.
• Bearings – necessary to have a very hard surface that will withstand continued stresses