Annealing process types
Annealing process types
Annealing is one of the metal heat treatment processes, in addition to normalizing, tempering, and quenching. Annealing refers to slowly heating the metal to a certain temperature for a sufficient period of time and then cooling it at a suitable rate.
The purpose of annealing is to reduce the hardness, improve the machinability; eliminate the residual stress, stabilize the size, reduce the tendency of deformation and cracking; refine the grains, adjust the structure, eliminate the defects of the structure. The annealing process varies depending on the purpose, such as isothermal annealing, diffusion annealing, complete annealing, incomplete annealing, spheroidizing annealing, recrystallization annealing, intermediate annealing, stress relief annealing, magnetic field annealing, etc.
Annealing process types
Diffusion annealing: An annealing of ingots or castings applied to steels and non-ferrous alloys (such as tin bronze, silicon bronze, copper bronze, magnesium alloys, etc.)
Method: The ingot or casting is heated to a relatively high temperature below the solidus temperature of each of the alloys, kept warm for a long period of time, and then slowly cooled down. Homogenizing annealing is the solid diffusion of elements in the alloy to reduce the chemical composition non-uniformity (segregation), mainly to reduce chemical composition heterogeneity (grain segregation or dendritic segregation) within the grain scale. Homogenizing the annealing temperature is so high that the alloy element diffusion is accelerated and the holding time is shortened as much as possible.
The homogenization annealing temperature of alloy steel is much higher than that of Ac3, usually 1050-1200°C. The temperature for the homogenization annealing of non-ferrous alloy ingots is generally “0.95×solidus temperature (K)”. Since the homogenization annealing has a high heating temperature and a long holding time, the heat energy consumption is large.
Completely Annealed (Recrystallization Annealing)
Applied to balance heating and cooling there is an alloy that undergoes a solid phase transformation (recrystallization). The annealing temperature is above the phase transition temperature range of each alloy or
A certain temperature within. Both heating and cooling are slow. The alloy undergoes a phase change recrystallization during heating and cooling, so it is called recrystallization annealing and is often simply referred to as annealing.
This annealing method is quite commonly applied to steel. The recrystallization annealing process of the steel is: slowly heating to Ac3 (eutectoid steel) or Ac1 (eutectoid or hypereutectoid steel) above 30-50°C for a suitable period of time and then slowly cooling down.
Transformation to austenite (first phase change recrystallization) occurs through pearlite (or ferritic or cementite that has previously eutectoided) that occurs during the heating process and vice versa. The phase transition recrystallizes to form pearlite with finer grains, thicker slices, and even microstructure (or pre-eutectoid ferrite or cementite). Annealing temperature above Ac3 (subeutectoid steel) to completely recrystallize the steel, known as complete annealing, annealing temperature between Ac1 and Ac3 (eutectoid steel) or between Ac1 and Acm (hypereutectoid steel The partial recrystallization of the steel is called incomplete annealing.
The former is mainly used for sub-eutectoid steel castings, forgings, and weldments to eliminate structural defects (such as Wei's and banded structures) and to make the structure finer and more uniform to improve the plasticity and toughness of steel parts. .
The latter is mainly used for forgings of medium and high carbon steels and low alloy structural steels. If the cooling rate after forging and rolling of such forgings and rolling stocks is relatively high, the formed pearlite is finer and has higher hardness; if the forging and stopping temperatures are too low, there is still a large internal stress in the steel parts. Incomplete annealing may be used instead of full annealing to recrystallize the pearlite, and the crystal grains become finer. At the same time, the hardness is reduced, the internal stress is eliminated, and the machinability is improved. In addition, the hypereutectoid steel spheroidizing annealing in which the annealing temperature is between Ac1 and Acm is also incomplete annealing.
Recrystallization annealing is also used for non-ferrous alloys, such as titanium alloys undergoing allotropy conversion during heating and cooling, with low temperature alpha phase (close-packed hexagonal structure), high temperature beta phase (body-centered cubic structure), and between them " α + β "two-phase region, which is the phase transition temperature range. In order to obtain a stable equilibrium at room temperature and refine the crystal grains, recrystallization annealing is also performed, that is, slowly heating to a temperature higher than the phase transition temperature, and keeping the temperature for a suitable time, so that the alloy is transformed into fine grains of β phase; Then slowly cooled down, the β phase is reconverted into fine grains of α phase or α+β phases.
Incomplete annealing
Incomplete annealing is an annealing process in which the iron-carbon alloy is heated to a temperature between Ac1-Ac3 to incomplete austenitization, followed by slow cooling.
Incomplete annealing is mainly applicable to medium and high carbon steel and low alloy steel forgings, etc. Its purpose is to refine the structure and reduce the hardness, the heating temperature is Ac1 + (40-60) °C, slowly cooling after insulation.
Isothermal annealing
A controlled cooling annealing method for steel and certain non-ferrous alloys such as titanium alloys.
For steel, it is slowly heated to Ac3 (eutectoid steel) or Ac1 (eutectoid steel and hypereutectoid steel) at a temperature not more than a few degrees, holding for a period of time, austenitizing the steel, and then rapidly moving the temperature In the other furnace below A1, it is kept isothermally until all austenite is transformed into lamellar pearlite (subeutectoid steel and proeutectoid ferrite; hypereutectoid steel and proeutectoid cementite ) Finally, it is cooled down at any speed (usually it is cooled in the air). The approximate temperature range of the isothermal hold is within the interval from the A1 to the pearlite transition nose temperature of the isothermal transition of the treated steel (see the overcooling austenite transition diagram); the specific temperature and time are mainly based on the requirements after annealing. The hardness is determined.
The isothermal temperature should not be too low or too high. If the temperature is too low, the hardness after annealing is high. If the temperature is too high, the isothermal holding time needs to be extended. The purpose of isothermal annealing of steel is basically the same as that of recrystallization annealing, but the process operation and required equipment are relatively complex, so it is usually applied to the alloy steel in which the ultra-cooled austenite transforms slowly in the pearlite-type phase transition temperature interval. If the latter uses recrystallization annealing method, it often takes tens of hours, which is not economical; using isothermal annealing can greatly shorten the production cycle, and can achieve a more uniform structure and performance of the entire workpiece.
Isothermal annealing can also be used at different stages of the hot working of the steel. For example, if the air-cooled hardenable alloy steel is air-cooled from room temperature to room temperature, when the core is transformed into martensite, cracks will appear in the outer layer where martensitic transformation has occurred; if the heat of the steel is The ingot or billet is placed in an isothermal furnace at about 700°C during the cooling process, and the isothermal furnace is maintained until the pearlite transformation is completed. Then, air cracking is performed after the slab is cooled.
The titanium alloy containing the β-phase stabilizing element has a relatively stable β-phase and is easily supercooled. The subcooled β-phase, its isothermal transition kinetics curve is similar to the steel's undercooled austenite isothermal transition diagram. In order to shorten the production cycle of the recrystallization annealing and obtain a finer, more uniform structure, isothermal annealing may also be used.
Spheroidizing annealing
An annealing method applied only to steel. The steel is heated to a temperature slightly lower or slightly higher than Ac1 or the temperature is varied cyclically above and below A1 and then slowly cooled.
The purpose is to make the lamellar cementite in the pearlite and the proeutectoid cementite become spheroidal and evenly distributed in the ferrite matrix (this kind of organization is called spheroidized pearlite). Medium-carbon steels and high-carbon steels having such a structure have low hardness, good machinability, and large cold deformability. For tool steel, this structure is the best original structure before quenching.
Stress relief annealing
The stress relief annealing is to heat the workpiece to a suitable temperature below Ac1 (non-alloy steel at 500~600°C), and the heat treatment with furnace cooling after heat preservation is called stress relief annealing. The stress-free heating temperature is low, and there is no organization change during the annealing process. It is mainly applicable to blank parts and parts that have been cut and processed.
The purpose is to eliminate the residual stress in the blank and the part, stabilize the size and shape of the workpiece, and reduce the deformation and cracking tendency of the part during cutting and use.
