Commonly used annealing processes are:
① Complete annealing. It is used to refine the coarse overheated structure with poor mechanical properties that appears after casting, forging and welding of medium and low carbon steel. The workpiece is heated to 30 to 50°C above the temperature at which all ferrite is transformed into austenite, kept warm for a period of time, and then slowly cooled in the furnace. During the cooling process, the austenite transforms again, which can make the steel structure finer.
② Spheroidizing annealing. Used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to 20 to 40°C above the temperature at which the steel begins to form austenite, and is slowly cooled after heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.
③ Isothermal annealing. It is used to reduce the high hardness of some alloy structural steels with high nickel and chromium content for cutting processing. Generally, it is first cooled to the most unstable temperature of austenite at a faster speed, and then kept for an appropriate time. The austenite transforms into troostite or sorbite, and the hardness can be reduced.
④ Recrystallization annealing. It is used to eliminate the hardening phenomenon (increased hardness and decreased plasticity) of metal wires and sheets during the cold drawing and cold rolling processes. The heating temperature is generally 50 to 150°C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened.
⑤ Graphitization annealing. It is used to turn cast iron containing a large amount of cementite into malleable cast iron with good plasticity. The process operation is to heat the casting to about 950°C, keep it warm for a certain period of time and then cool it appropriately to decompose the cementite to form flocculated graphite.
⑥Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve their performance. The method is to heat the casting to the highest possible temperature without melting, keep it warm for a long time, and slowly cool it after the diffusion of various elements in the alloy tends to be evenly distributed.
⑦ Stress relief annealing. Used to eliminate internal stress in steel castings and welded parts. For steel products that are heated to a temperature 100 to 200°C below the temperature at which austenite begins to form, and then cooled in the air after heat preservation, the internal stress can be eliminated.
Quenching, a heat treatment process for metals and glass. Heating alloy products or glass to a certain temperature and then rapidly cooling it in water, oil or air is generally used to improve the hardness and strength of the alloy. Commonly known as “dipping into fire”. Metal heat treatment in which the quenched workpiece is reheated to an appropriate temperature below the lower critical temperature, kept warm for a period of time and then cooled in air, water, oil and other media.
Steel workpieces have the following characteristics after quenching:
① Unbalanced (that is, unstable) structures such as martensite, bainite, and retained austenite are obtained.
② There is large internal stress.
③ The mechanical properties cannot meet the requirements. Therefore, steel workpieces generally undergo tempering after quenching.
Purpose of annealing process:
① Improve or eliminate various structural defects and residual stress caused by steel casting, forging, rolling and welding processes to prevent workpiece deformation and cracking.
② Soften the workpiece for cutting.
③ Refine the grains and improve the structure to improve the mechanical properties of the workpiece.
④ Prepare the structure for final heat treatment (quenching and tempering).