As the steel is heated above the critical temperature, about 1335°F (724°C), it undergoes a phase change, recrystallizing as austenite (i.e. it changes from body centered cubic to face centered cubic) Continued heating to the hardening temperature, 1450°F - 1500°F (788°C - 816°C) ensures complete conversion to austenite, the springs are then cooled suddenly by quenching in a bath of water, a new crystal structure, martensite, is formed as seen in the micro structural analysis above. Martensite is characterized by an angular needle like structure and very high hardness, as seen from the hardness test carried out. While martensitic steel is extremely hard, it is also extremely brittle and will break, chip, and crumble with the slightest shock. Furthermore, internal stresses remain in the spring from the sudden quenching; these will also facilitate breakage of the spring. Tempering relieves these stresses and causes partial decomposition of the martensite into ferrite and cementite. The amount of this partial phase change is controlled by the tempering temperature. The tempered steel is not as hard as pure martensite, but is much tougher. This can be observed from the result tabulated above where by the water quenched sprigs have the highest hardness and are the least ductile from all the springs, but after the water quenched steel were tempered it was observed that the hardness of the spring reduced, the toughness and ductility increased when compared to the water quenched springs.