During the heat treatment process, there are two mechanisms that may alter the properties of the alloy: the formation of martensite leads to internal deformation of the crystal, and the diffusion mechanism leads to changes in the uniformity of the alloy.
The crystal structure is composed of atoms arranged together in a very special way, called a lattice. In most elements, this order will rearrange itself based on conditions such as temperature and pressure. This rearrangement, known as allotrope or polymorphism, may occur multiple times at many different temperatures of a specific metal. In alloys, this rearrangement may cause elements that typically do not dissolve into the base metal to suddenly become soluble, while the reversal of allotropes will cause the elements to be partially or completely insoluble.
When in a soluble state, the diffusion process causes the atoms of the dissolved element to disperse, attempting to form a uniform distribution within the crystal of the base metal. If the alloy cools to an insoluble state, atoms of dissolved components (solutes) may migrate out of the solution. This type of diffusion is called precipitation, which leads to nucleation, where migrating atoms gather together at grain boundaries. This forms a microstructure typically composed of two or more different phases. For example, a steel heated above the austenitizing temperature (red to orange hot, or approximately 1500 ° F (820 ° C) to 1600 ° F (870 ° C), depending on the carbon content), then slowly cooled to form a layered structure composed of alternating layers of ferrite and cementite, becoming soft pearlite. After heating the steel to the austenite phase and then water quenching, its microstructure will be in the martensitic phase. This is because the steel undergoes a transformation from austenite phase to martensite phase after quenching. If quenching does not quickly cool all the steel, there may be some pearlite or ferrite present.
