Hardening (metallurgy) - Processes

Processes

The five hardening processes are:

• The Hall–Petch method, or grain boundary strengthening, is to obtain small grains. Smaller grains increases the likelihood of dislocations running into grain boundaries after shorter distances, which are very strong dislocation barriers. In general, smaller grain size will make the material harder. When the grain size approach sub-micron sizes, some materials may however become softer. This is simply an effect of another deformation mechanism that becomes easier, e.g. grain boundary sliding. At this point, all dislocation related hardening mechanisms become irrelevant.
• In work hardening (also referred to as strain hardening or cold working) the material is strained past its yield point. The plastic straining generate new dislocations. As the dislocation density increases, further dislocation movement becomes more difficult since they hinder each other, which means the material hardness increases.
• In solid solution strengthening, a soluble alloying element is added to the material desired to be strengthened, and together they form a “solid solution”. A solid solution can be thought of just as a "normal" liquid solution, e.g. salt in water, except it is solid. Depending on the size of the dissolved alloying element's ion compared to that of the matrix-metal, it dissolves either a substitutionally (large alloying element substituting an atom in the crystal) or interstitially (small alloying element taking a place between atoms in the crystal lattice). In both cases, the size difference of the foreign elements make them act as sand grains in sandpaper, resisting dislocations that try to slip by, resulting in higher material strength. In solution hardening, the alloying element does not precipitate out solution.
• Precipitation hardening is a process where particles are distributed throughout the metal and cause resistance to slip dislocations. This is achieved by first heating the metal to a temperature where the elements forming the particles are soluble then quenching it, trapping them in a solid solution. Had it been a liquid solution, the elements would form precipitates, just as supersaturated saltwater would precipitate small salt crystals, but atom diffusion in a solid is very slow at room temperature. A second heat treatment at a suitable temperature is then required to age the material. The elevated temperature allows the dissolved elements to diffuse much faster, and form the desired precipitated particles. The quenching is required since the material otherwise would start the precipitation already during the slow cooling. This type of precipitation results in few large particles rather than the, generally desired, profusion of small precipitates. Precipitation hardening is one of the most commonly used techniques for the hardening of metal alloys.
• Martensitic transformation, more commonly known as quenching and tempering, is a hardening mechanism specific for steel. The steel must be heated to a temperature where the iron phase changes from ferrite into austenite, i.e. changes crystal structure from BCC (body centered cubic) to FCC (face centered cubic). In austenitic form, steel can dissolve a lot more carbon. Once the carbon has been dissolved, the material is then quenched. It is important to quench with a high cooling rate so that the carbon does not have time to form precipitates of carbides. When the temperature is low enough, the steel tries to return to the low temperature crystal structure BCC. This change is very quick since it does not rely on diffusion and is called a martensitic transformation. Because of the extreme supersaturation of solid solution carbon, the crystal lattice becomes BCT (body centered tetragonal) instead. This phase is called martensite, and is extremely hard due to a combined effect of the distorted crystal structure and the extreme solid solution strengthening, both mechanisms of which resist slip dislocation.

All hardening mechanisms introduce crystal lattice defects that act as barriers to dislocation slip.