Annealing process

      Annealing process

      The heat treatment process of heating the metal or alloy to an appropriate temperature, holding it for a certain period of time, and then slowly cooling (usually cooling with the furnace), is called annealing. The essence of annealing is to heat the steel to austenitize and transform into pearlite, and the structure after annealing is close to the equilibrium structure. The purpose of annealing: (1) Reduce the hardness of the steel, improve the plasticity, and facilitate machining and cold deformation processing; (2) Uniform the chemical composition and structure of the steel, refine the grain, improve the performance of the steel or prepare the structure for quenching; (3) Eliminate internal stress and work hardening to prevent deformation and cracking. Annealing and normalizing are mainly used for preliminary heat treatment. For parts with low stress and low performance requirements, annealing and normalizing can also be used as final heat treatment.

      Classification of annealing methods

      Commonly used annealing methods are divided into heating temperature: phase change recrystallization annealing above the critical temperature (Ac1 or Ac3): complete annealing, diffusion annealing, incomplete annealing, spheroidizing annealing. Annealing below the critical temperature (Ac1 or Ac3): recrystallization annealing, stress relief annealing.

      Seven types of annealing methods

       1. Complete annealing process: heat the steel to 20~30℃ above Ac3, keep it for a period of time and then slowly cool down (with the furnace) to obtain a heat treatment process close to the equilibrium structure (complete austenitization). Complete annealing is mainly used for hypoeutectoid steel (wc=0.3~0.6%), generally medium carbon steel and low and medium carbon alloy steel castings, forgings and hot rolled sections, and sometimes also used for their welded parts. The hardness of low carbon steel is low after being fully annealed, which is not conducive to cutting. When hypereutectoid steel is heated to austenite state above Accm and slowly cooled and annealed, Fe3CⅡ will precipitate along the grain boundary in a network shape, which will increase the strength, hardness and plasticity of the steel. And the toughness is significantly reduced, leaving hidden dangers to the final heat treatment. Purpose: Refine the grain, uniform structure, eliminate internal stress, reduce hardness and improve the machinability of steel. The structure of hypoeutectoid steel after complete annealing is F+P. In actual production, in order to improve productivity, annealing and cooling to about 500 ℃, namely out of furnace air cooling. 2. Isothermal annealing. It takes a long time for complete annealing, especially for alloy steels with stable undercooled austenitization. For example, if the austenitized steel is quickly cooled to a temperature slightly lower than the Ar1 temperature isothermal, A is transformed into P, and then air-cooled to room temperature, which can greatly shorten the annealing time. This annealing method is called isothermal annealing. Process: Heat the steel to a temperature higher than Ac3 (or Ac1). After holding for a suitable period of time, it will be quickly cooled to a certain temperature in the pearlite zone and maintained isothermally to transform austenite into pearlite, and then air-cooled to room temperature The heat treatment process. Purpose: Same as complete annealing, the transformation is easier to control. Suitable for A relatively stable steel: high carbon steel (wc>0.6%), alloy tool steel, high alloy steel (total amount of alloying elements>10%). Isothermal annealing is also conducive to obtaining uniform structure and performance. But it is not suitable for large cross-section steel parts and large batches of charge, because isothermal annealing is not easy to make the inside of the workpiece or the batch of workpieces reach the isothermal temperature. 3. Incomplete annealing process: The steel is heated to Ac1~Ac3 (hypoeutectoid steel) or Ac1~Accm (hypereutectic steel), and then slowly cooled to obtain a heat treatment process close to equilibrium structure. Mainly used for hypereutectoid steel to obtain spherical pearlite structure to eliminate internal stress, reduce hardness, and improve machinability. Spheroidizing annealing is a kind of incomplete annealing. 4. Spheroidizing annealing A heat treatment process to spheroidize carbides in steel to obtain granular pearlite. Process: Heat to a temperature of 20~30℃ above Ac1, and the holding time should not be too long, generally 2~4h. The cooling method is usually furnace cooling, or isothermal for a long time at about 20℃ below Ar1. Mainly used for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. The air-cooled structure of hypereutectoid steel after rolling and forging is lamellar pearlite and network cementite. This structure is hard and brittle, not only difficult to cut, but also easy to deform and crack in the subsequent quenching process. . Spheroidizing annealing obtains spherical pearlite. In the spherical pearlite, the cementite is spherical fine particles, which are dispersed on the ferrite matrix. Compared with flaky pearlite, spherical pearlite not only has lower hardness and is convenient for cutting, but also when quenching and heating, the austenite grains are not easy to be coarse, and the tendency of deformation and cracking during cooling is small. If the hypereutectoid steel has network cementite, it must be eliminated by the normalizing process before the spheroidizing annealing to ensure the normal spheroidizing annealing. Purpose: Reduce hardness, uniform structure, improve machinability and prepare structure for quenching. There are many spheroidizing annealing process methods, mainly including: a) One-time spheroidizing annealing process: heating the steel to 20~30℃ above Ac1, holding it for an appropriate time, and then slowly cooling with the furnace. The original structure before annealing is required to be fine flake pearlite, and no cementite network is allowed. b) Isothermal spheroidizing annealing process: After heating and holding the steel, it is cooled to a temperature slightly lower than Ar1 with the furnace for isothermal (generally 10~30℃ below Ar1). After the isothermal is completed, the furnace is slowly cooled to about 500°C and then air-cooled. It has the advantages of short cycle, uniform spheroidization organization, and easy quality control. c) Reciprocating spheroidizing annealing process. 5. Diffusion annealing (homogenization annealing) process: a heat treatment process in which steel ingots, castings or forgings are heated to a temperature slightly lower than the solidus for a long time, and then slowly cooled to eliminate uneven chemical composition. Purpose: Eliminate dendrite segregation and regional segregation during the solidification of the ingot, and homogenize the composition and structure. The heating temperature of diffusion annealing is very high, usually 100~200℃ above Ac3 or Accm. The specific temperature depends on the degree of segregation and the steel grade. The holding time is generally 10~15 hours. After diffusion annealing, complete annealing and normalizing treatment are required to refine the structure. It is applied to some high-quality alloy steel and alloy steel castings and steel ingots with serious segregation. 6. Stress-relieving annealing process: Heat the steel to a temperature lower than Ac1 (generally 500~650℃), keep it warm, and then cool it with the furnace. The stress relief annealing temperature is lower than A1, so the stress relief annealing does not cause structural changes. Purpose: Eliminate residual internal stress. 7. Recrystallization annealing Recrystallization annealing, also known as intermediate annealing, is to heat the cold-deformed metal to a temperature above the recrystallization temperature and maintain it for an appropriate time, so that the deformed grains can be transformed into uniform equiaxed grains to eliminate work hardening and residual stress. Heat treatment process. The occurrence of recrystallization must firstly have a certain amount of cold plastic deformation, and secondly, it must be heated above a certain temperature.

       Selection of annealing method

       The selection of annealing method generally has the following principles: (1) All kinds of steels with hypoeutectoid structure are generally fully annealed. In order to shorten the annealing time, isothermal annealing can be used; (2) Spheroidizing annealing is generally used for hypereutectoid steels. When the requirements are not high, incomplete annealing can be used. Spheroidizing annealing is often used for tool steel and bearing steel. Cold extruded parts and cold headed parts of low carbon steel or medium carbon steel sometimes use spheroidizing annealing; (3) In order to eliminate work hardening, recrystallization annealing can be used; (4) In order to eliminate various processing processes For internal stress, stress-relieving annealing can be used; For some large-scale steel castings of high-grade high-quality alloy steel, in order to improve the unevenness of the structure and chemical composition, diffusion annealing is often used.