As austenite cools, the carbon diffuses out of the austenite and forms carbon-rich iron-carbide (cementite) and leaves behind carbon-poor ferrite. Depending on alloy composition, a layering of ferrite and cementite, called pearlite, may form. If the rate of cooling is very swift, the carbon does not have sufficient time to diffuse, and the alloy may experience a large lattice distortion known as martensitic transformation in which it transforms into martensite, a body centered tetragonal structure (BCT). The rate of cooling determines the relative proportions of martensite, ferrite, and cementite, and therefore determines the mechanical properties of the resulting steel, such as hardness and tensile strength. 
A high cooling rate of thick sections will cause a steep thermal gradient in the material. The outer layers of the heat treated part will cool faster and shrink more, causing it to be under tension and thermal straining.  At high cooling rates, the material will transform from austenite to martensite which is much harder and will generate cracks at much lower strains.  The volume change (martensite is less dense than austenite) can generate stresses as well. The difference in strain rates of the inner and outer portion of the part may cause cracks to develop in the outer portion, compelling the use of slower quenching rates to avoid this. By alloying the steel with tungsten, the carbon diffusion is slowed and the transformation to BCT allotrope occurs at lower temperatures, thereby avoiding the cracking. Such a material is said to have its hardenability increased. Tempering following quenching will transform some of the brittle martensite into tempered martensite. If a low-hardenability steel is quenched, a significant amount of austenite will be retained in the microstructure, leaving the steel with internal stresses that leave the product prone to sudden fracture.