Annealing is a heat treatment process in which metals and alloys are heated to an appropriate temperature, kept for a certain period of time, and then slowly cooled. After annealing, the structure of hypoeutectoid steel is ferrite plus lamellar pearlite; eutectoid steel or hypereutectoid steel is granular pearlite. In short, the annealed structure is a structure close to equilibrium.
Purpose of annealing
①Reduce the hardness of steel and increase plasticity to facilitate cutting and cold deformation processing.
② Refine the grains, eliminate the structural defects caused by casting, forging, and welding, uniform the structure and composition of the steel, improve the performance of the steel or prepare for the subsequent heat treatment.
③ Eliminate internal stress in steel to prevent deformation and cracking.
Types of annealing process
① Homogenization annealing (diffusion annealing)
Homogenization annealing is to reduce the segregation of the chemical composition of metal ingots, castings or forging billets and the unevenness of the structure. It is heated to a high temperature, maintained for a long time, and then slowly cooled, in order to homogenize the chemical composition and structure. The annealing process.
The heating temperature of homogenization annealing is generally Ac3 (150～200℃), that is, 1050～1150℃, and the holding time is generally 10～15h to ensure the full progress of diffusion and eliminate or reduce the unevenness of composition or organization. Because the heating temperature of diffusion annealing is high, the time is long, and the grains are coarse, for this reason, complete annealing or normalizing is performed after diffusion annealing to re-fine the structure.
Complete annealing, also known as recrystallization annealing, is an annealing process in which the iron-carbon alloy is completely austenitized and then slowly cooled to obtain a structure close to the equilibrium state.
Complete annealing is mainly used for hypoeutectoid steels, generally medium carbon steel and low and medium carbon alloy structural steel forgings, castings and hot-rolled sections, and sometimes also used for their welding components. Complete annealing is not suitable for hypereutectoid steels, because the complete annealing of hypereutectoid steels needs to be heated to above Acm. During slow cooling, cementite will precipitate along the austenite grain boundaries and distribute in a network shape, resulting in increased brittleness of the material. Leave hidden dangers to the final heat treatment.
The heating temperature for complete annealing is generally Ac3 (30～50℃) for carbon steel; Ac3 (500～70℃) for alloy steel; the holding time depends on the type of steel, the size of the workpiece, the amount of furnace installed, and the selected equipment model And other factors to determine. In order to ensure that the supercooled austenite undergoes the pearlite transformation completely, the cooling of the complete annealing must be slow, and the furnace is cooled to about 500°C and then air-cooled.
Incomplete annealing is an annealing process in which the iron-carbon alloy is heated to a temperature between Ac1 and Ac3 to achieve incomplete austenitization, followed by slow cooling.
Incomplete annealing is mainly suitable for 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) ℃, and the temperature is slowly cooled after heat preservation.
④ Isothermal annealing
Isothermal annealing is to heat the steel or blank to a temperature higher than Ac3 (or Ac1), and after keeping it for a proper time, it will quickly cool to a certain temperature in the pearlite temperature range and maintain it isothermally to transform austenite into pearlite. The organization is then cooled in the air by an annealing process.
The isothermal annealing process is applied to medium carbon alloy steel and low alloy steel, and its purpose is to refine the structure and reduce the hardness. The heating temperature of hypoeutectoid steel is Ac3 (30～50)℃, and the heating temperature of hypereutectoid steel is Ac3 (20～40)℃. Keep it for a certain period of time. After the furnace is cooled to slightly lower than the Ar3 temperature, it will undergo isothermal transformation, and then air-cooled out . The structure and hardness of isothermal annealing are more uniform than that of complete annealing.
⑤ Spheroidizing annealing
Spheroidizing annealing is an annealing process to spheroidize carbides in steel. The steel is heated to 20-30°C above Ac1, kept for a period of time, and then slowly cooled to obtain a structure of spherical or granular carbides uniformly distributed on the ferrite matrix.
Spheroidizing annealing is mainly suitable for eutectoid steel and hypereutectoid steel, such as carbon tool steel, alloy tool steel, bearing steel, etc. These steels are air-cooled after rolling and forging, and the resulting structure is lamellar pearlite and network cementite. This structure is hard and brittle, not only difficult to cut, but also easily deformed and cracked in the subsequent quenching process. The spherical pearlite structure obtained by spheroidizing annealing, in which cementite is spherical particles, is dispersed on the ferrite matrix.
Compared with flaky pearlite, it has low hardness and is convenient for cutting and processing. When heated, the austenite grains are not easy to grow, and the deformation and cracking tendency of the workpiece is small when cooling. In addition, spheroidizing annealing can sometimes be used for some hypoeutectoid steels that need to improve cold plastic deformation (such as stamping, cold heading, etc.).
The heating temperature of spheroidizing annealing is Ac1 (20-40) ℃ or Acm-(20-30) ℃, and isothermal cooling or direct and slow cooling after heat preservation. During the spheroidizing annealing, the austenization is “incomplete”, but the flaky pearlite is transformed into austenite, and a small amount of excess carbide is dissolved. Therefore, it is impossible to eliminate network carbides. If the hypereutectoid steel has network carbides, normalizing must be carried out before spheroidizing annealing to eliminate them to ensure normal spheroidizing annealing.
There are many spheroidizing annealing processes, and the two most commonly used processes are ordinary spheroidizing annealing and isothermal spheroidizing annealing. Ordinary spheroidizing annealing is to heat the steel to 20-30°C above Ac1, hold it for a proper time, then slowly cool it with the furnace, and cool it down to about 500°C. Isothermal spheroidizing annealing is the same heating and holding process as the ordinary spheroidizing annealing process,
followed by the furnace cooling to a temperature slightly lower than Ar1 for isothermal, and the isothermal time is 1.5 times the heating and holding time. After isothermal, the furnace is cooled to about 500℃ and then air-cooled. Compared with ordinary spheroidizing annealing, spheroidizing annealing can not only shorten the cycle, but also make the spheroidized structure uniform, and can strictly control the hardness after annealing.
⑥ Recrystallization annealing (intermediate annealing)
Recrystallization annealing is a heat treatment process in which the cold-deformed metal is heated to above the recrystallization temperature and maintained for an appropriate time to recrystallize the deformed grains into uniform equiaxed grains to eliminate deformation strengthening and residual stress.
⑦ Stress relief annealing
Stress-relief annealing is an annealing process to eliminate residual stress caused by plastic deformation processing, welding, etc. and the residual stress in the casting.
There are internal stresses in the workpiece after forging, casting, welding and cutting. If it is not eliminated in time, the workpiece will be deformed during processing and use, which will affect the accuracy of the workpiece. It is very important to use stress relief annealing to eliminate internal stress generated during processing.
The heating temperature of stress relief annealing is lower than the phase transition temperature A1, therefore, no structural transformation occurs during the entire heat treatment. The internal stress is mainly eliminated by the workpiece in the process of heat preservation and slow cooling. In order to eliminate the internal stress of the workpiece more thoroughly, the heating temperature should be controlled during heating.
Generally, it enters the furnace at low temperature, and then heats to the specified temperature at a heating rate of about 100°C/h. The heating temperature of welding parts should be slightly higher than 600℃. The holding time depends on the situation, usually 2 to 4 hours. The holding time of stress relief annealing for castings is taken as the upper limit, the cooling rate is controlled at (20-50) ℃/h, and it can be air-cooled after cooling to below 300 ℃.