Chapter 12 – Change of ConditionThe Iron-Carbon Phase Diagram THERMAL PROCESSES Methods of Softening Steels Annealing is the softening of a metal to its softest possible condition. For steels, the metal must be heated into the austenitic range and cooled very slowly. Normalizing is a heat treatment used to give steel an even GRAIN size. It is used prior to machining or other heat treatments. Methods of Hardening Steels Quenching is the rapid cooling of a metal to harden it. Surface Hardening - If steel is hardened all the way through the part, it will be brittle. In parts that have wearing surfaces such as gear teeth, shafts, lathe beds, and cams, only the surface of the part should be hardened so as to leave the inside soft and ductile. CHEMICAL PROCESSES Case Hardening  If a low-carbon steel is needed to give toughness to the workpiece, its surface cannot be significantly hardened.  Case Hardening is any of several methods of hardening the surface of steel.  A low-carbon steel is immersed in a compound containing carbon, nitrogen, or hydrogen, which diffuses into the outer layers of the steel.  The steel surface can then be hardened by QUENCHING. METHODS OF MODIFYING THE PROPERTIES OF STEELS Tempering – is the removal of internal stresses in a metal by heating. No steel should be left in its “as-hardened” condition. Quenched steels have internal stresses that make the steel brittle. They must be tempered to relieve these stresses.Martensite is the hardest form of steel. Formed when austenite is cooled at a high rate, such as by quenching it in water, its fcc structure is transformed to a body-centered tetragonal (bct) structure. Martempering is a heat treatment process in which steel is quenched from the austenitic temperature to just above the MARTENSITE start temperature, held there for a few seconds to a few minutes, and then quenched. It is used to provide an even-sized martensite throughout the part. Austenite is the face-centered-cubic structure of steel. Between 1394 °C (2541 °F) and 912 °C (1674 °F) iron undergoes a polymorphic transformation from the bcc to an fcc structure. This structure has a solid solubility of up to 2.11% carbon at 1148 °C (2098 °F). It is nonmagnetic. Large amounts of nickel and manganese can also be dissolved in free iron to impart various properties. 1Austempering is a heat treatment of steel in which the steel is quenched to just above the MARTENSITE start temperature and held there for several hours before lowering the temperature to room conditions.