This process converts remaining austenite in the steel to martensite. The strength of the martensite is reduced as the amount of retained austenite grows. There is no thermal activation energy for the transformation from austenite to martensite. Further excessive heat-treatment brings about the decomposition of the martensite and reversion to austenite. As previously discussed, this detwinning is reversible when transforming back from martensite to austenite. The volume change (martensite is less dense than austenite) can generate stresses as well. At this temperature pearlite changes to austenite because of nucleation process. Heating white cast iron above causes the formation of austenite in crystals of primary cementite. The transformation of pearlite to austenite takes place at lower critical temperature of 723C. In lower bainite, cementite nucleates on the interface between ferrite and austenite. An increase in alloying agents or carbon content causes an increase in retained austenite. This allows the formation of bainite, an austenite decomposition product. Manganese and nickel lower the eutectoid temperature and are known as "austenite stabilizing elements". For instance, when steel is heated above the upper critical-temperature, small grains of austenite form. While at this temperature, more C is allowed to enrich the retained austenite.