Fully annealed. Used to refine the coarse overheated structure with poor mechanical properties of medium and low carbon steels after casting, forging, and welding. Heat the workpiece to a temperature 30 to 50 ℃ above the temperature at which all ferrite is transformed into austenite, hold it for a period of time, and then slowly cool it with the furnace. During the cooling process, the austenite undergoes another transformation, which can refine the microstructure of the steel.
Spheroidizing annealing. Used to reduce the relatively high hardness of tool steel and bearing steel after forging. “The workpiece is heated to a temperature 20 to 40 ℃ above the temperature at which the steel begins to form austenite, and then slowly cooled after heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby reducing the hardness.”.
Isothermal annealing. Used to reduce the high hardness of certain alloy structural steels with high nickel and chromium content for machining purposes. Generally, the hardness can be reduced by first cooling to the most unstable temperature of austenite at a faster speed and holding it for an appropriate time. After the austenite is transformed into troostite or sorbite, the hardness can be reduced.
Recrystallization annealing. It is used to eliminate the hardening phenomenon (increase in hardness and decrease in plasticity) of metal wires and sheets during cold drawing and cold rolling. The heating temperature is generally 50 to 150 ℃ below the temperature at which steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal soften.
Graphitization annealing. Used to convert 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 ℃, hold it for a certain time, and then properly cool it to decompose the cementite into flocculent graphite.
Diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve their service performance. The method is to heat the casting to the highest possible temperature without melting, hold it for a long time, and slowly cool after the diffusion of various elements in the alloy tends to uniform distribution.
Stress relieving annealing. Used to eliminate internal stress in steel castings and weldments. For steel products that begin to form austenite after heating at a temperature below 100 to 200 ℃, internal stress can be eliminated by cooling in air after insulation.
Quenching, a heat treatment process for metals and glass. Heating alloy products or glass to a certain temperature and then rapidly cooling in water, oil, or air is generally used to improve the hardness and strength of the alloy. Commonly known as “dipping in fire”. Reheating a quenched workpiece to an appropriate temperature below the lower critical temperature, holding it for a period of time, and then cooling it in air, water, oil, and other media for metal heat treatment. After quenching, steel workpieces have the following characteristics:
Unbalanced (i.e. unstable) structures such as martensite, bainite, and residual austenite were obtained.
There is a large internal stress.
The mechanical properties cannot meet the requirements. Therefore, steel workpieces generally undergo tempering after quenching.