A metal heat treatment process that slowly heats the metal to a temperature for a sufficient period of time and then cools it at a suitable rate (usually slow cooling, sometimes controlled cooling). Introduction to annealing definition
A metal heat treatment process that slowly heats the metal to a temperature for a sufficient period of time and then cools it at a suitable rate (usually slow cooling, sometimes controlled cooling).
purpose
It is to soften materials or workpieces that have been cast, forged, welded or machined, to improve plasticity and toughness, to homogenize chemical components, to remove residual stress, or to obtain expected physical properties. Annealing processes vary depending on the purpose, such as recrystallization annealing, isothermal annealing, homogenization annealing, spheroidizing annealing, stress relief annealing, recrystallization annealing, and stabilization annealing, magnetic field annealing, and the like.
1. When the metal tool is used, it loses its original hardness due to heat.
2. Slowly cool the metal material or workpiece after heating it to a certain temperature for a certain period of time. Annealing can reduce the hardness and brittleness of the metal and increase the plasticity. Also called bonfire.
Purpose of annealing (1) Reduce hardness and improve machinability;
(2) Eliminate residual stress, stabilize the size, and reduce the tendency of deformation and crack;
(3) Refine the grains, adjust the structure, and eliminate tissue defects.
Annealing processes are widely used in production. Depending on the purpose of the workpiece to be annealed, there are a variety of annealing specifications, such as full annealing, spheroidizing annealing, and stress relief annealing.
One of the most important process parameters for annealing annealing is the maximum heating temperature (annealing temperature). The annealing temperature of most alloys is selected based on the phase diagram of the alloy system. For example, carbon steel is based on iron-carbon balance diagram. (figure 1). The annealing temperature of various steels (including carbon steel and alloy steel) is a certain temperature of Ac3 or more and Ac1 or more of each steel grade depending on the specific annealing purpose. The annealing temperature of each of the non-ferrous alloys is below a solidus temperature of each of the alloys, and a temperature above or below the solidus temperature.
Recrystallization annealing (complete annealing)
It is used in alloys where solid phase transformation (recrystallization) occurs during equilibrium heating and cooling. The annealing temperature is a temperature above or below the phase transition temperature range of each of the alloys. Heating and cooling are slow. The alloy undergoes a phase change recrystallization in each of the heating and cooling processes, so it is called recrystallization annealing, and is often referred to as annealing.
This annealing method is quite commonly applied to steel. The recrystallization annealing process of steel is: slowly heating to Ac3 (eutectic steel) or Ac1 (eutectoid steel or hypereutectoid steel) above 30 ~ 50 ° C, for a suitable time, and then slowly cooled down. The transformation of pearlite (or pre-eutectoid ferrite or cementite) that occurs during heating into austenite (first phase change recrystallization) and the second opposite to that occurring during cooling The phase change recrystallizes to form pearlite (or proeutectoid ferrite or cementite) with finer grains, thicker layers, and uniform microstructure. Annealing temperature above Ac3 (hypoeutectoid steel) to completely recrystallize the steel, called full annealing, annealing temperature between Ac1 and Ac3 (hypegmatitic steel) or Ac1 and Acm (hyper-eutectoid steel ), the part that causes the steel to recrystallize, called incomplete annealing. The former is mainly used for castings, forgings and weldments of hypoeutectic steel to eliminate tissue defects (such as Wei's structure, banded structure, etc.), to make the structure thin and uniform, to improve the plasticity and toughness of steel. . The latter is mainly used for the forging of medium carbon and high carbon steel and low alloy structural steel. If the forging and rolling parts have a large cooling rate after forging and rolling, the pearlite formed is finer and has a higher hardness; if the forging and stopping rolling temperatures are too low, there is a large internal stress in the steel. At this time, incomplete annealing may be used instead of complete annealing to recrystallize the pearlite, and the crystal grains become fine, and at the same time, the hardness is lowered, the internal stress is eliminated, and the machinability is improved. In addition, the hypereutectic steel spheroidizing annealing at an annealing temperature between Ac1 and Acm is also incomplete annealing.
Recrystallization annealing is also used for non-ferrous alloys. For example, titanium alloys undergo homogeneous transformation during heating and cooling. The low temperature is α phase (close-packed hexagonal structure), and the high temperature is β phase (body-centered cubic structure). The α+β†two-phase region, that is, the phase transition temperature interval. In order to obtain a near-balanced room temperature stable structure and refine the crystal grains, recrystallization annealing is also performed, that is, slowly heating to a temperature not higher than the phase transition temperature interval, and the alloy is transformed into fine crystal grains of the β phase for a suitable time; Then, it is slowly cooled down to reconvert the β phase into fine crystal grains of the α phase or the α + β phase.
Isothermal annealing
A controlled cooling annealing method applied to steel and certain non-ferrous alloys such as titanium alloys. For steel, it is slowly heated to a temperature of less than Ac3 (hypoeutectoid steel) or Ac1 (eutectoid steel and hypereutectoid steel). After a period of heat preservation, the steel is austenitized and then rapidly moved into the temperature. In the other furnace below A1, the isothermal temperature is maintained until the austenite is completely transformed into lamellar pearlite (sub-eutectoid steel and pro-eutectoid ferrite; hypereutectoid steel and pro-eutectoid cementite) So far, finally cool down at any speed (usually the furnace is cooled in the air). The approximate temperature range for isothermal holding is within the range of A1 to pearlite transition tip temperature on the isothermal transition diagram of the treated steel (see the cold austenite transition diagram); the specific temperature and time are mainly based on the requirements after annealing. The hardness is determined (Figure 2). The isothermal temperature should not be too low or too high. If it is too low, the hardness will be high after annealing; if it is too high, the isothermal holding time needs to be prolonged. The purpose of isothermal annealing of steel is basically the same as that of recrystallization annealing, but the process operation and equipment required are relatively complicated, so it is usually mainly applied to alloy steel in which supercooled austenite transforms relatively slowly in the pearlite type phase transition temperature range. If the latter adopts the recrystallization annealing method, it often takes tens of hours, which is uneconomical; the use of isothermal annealing can greatly shorten the production cycle and 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 steel. For example, if the air-cooled hard alloy steel is cooled from high temperature to room temperature, when the core is transformed into martensite, cracks may occur in the outer layer where martensite transformation has occurred; if the steel is hot The steel ingot or billet is placed in an isothermal furnace at about 700 °C during the cooling process, and is kept isothermal until the pearlite phase transformation is completed, and then air-cooled to avoid cracking. A titanium alloy containing a relatively high β-phase stabilizing element has a relatively stable β phase and is easily overcooled. The supercooled β phase, its isothermal transformation kinetic curve (Fig. 3) is similar to the supercooled austenite isothermal transformation of steel. In order to shorten the production cycle of recrystallization annealing and obtain a finer, more uniform structure, isothermal annealing may also be employed.
Homogenization annealing
Also known as diffusion annealing. An annealing method for ingots or castings of steel and non-ferrous alloys (such as tin bronze, silicon bronze, white copper, magnesium alloy, etc.). The ingot or casting is heated to a higher temperature below the solidus temperature of each of the alloys, held for a long time, and then slowly cooled down. Homogenization annealing is a solid diffusion of elements in the alloy to reduce chemical composition inhomogeneity (segregation), mainly to reduce chemical composition inhomogeneities (intragranular segregation or dendrite segregation) within the grain size. The homogenization annealing temperature is so high in order to accelerate the diffusion of alloying elements and to minimize the holding time. The homogenization annealing temperature of alloy steel is much higher than Ac3, usually 1050 ~ 1200 °C. The temperature at which the non-ferrous alloy ingot is homogenized and annealed is generally "0.95 × solidus temperature (K)", and the homogenization annealing has a large heating temperature due to a high heating temperature and a long heat retention time.
Spheroidizing annealing
An annealing method that is only applied to steel. The steel is heated to a temperature slightly lower or slightly higher than Ac1 or the temperature is periodically changed above and below A1, and then slowly cooled down. The purpose is to make the flaky cementite and the pro-eutectoid cementite in the pearlite into spherulites and evenly distributed in the ferrite matrix (this kind of structure is called spheroidized pearlite). The medium carbon steel and the high carbon steel having such a structure have low hardness, good machinability, and large cold deformation ability. For tool steel, this structure is the best original structure before quenching.
Stress relief annealing
The steel piece is heated to a temperature slightly higher than Ac1, and after heat preservation for a certain period of time, the furnace is cooled to 550 to 600 ° C. The heat treatment process of the air cooling is called stress relief annealing. The stress-free heating temperature is low and there is no tissue transformation during the annealing process. It is mainly applied to the blanks and the machined parts. The purpose is to eliminate the residual stress in the blanks and parts, stabilize the size and shape of the workpiece, and reduce the parts in the cutting process. Deformation and cracking tendency during use.
Process for spheroidizing annealing
There are: 1 ordinary (slow cooling) spheroidizing annealing (Fig. 4a), slow cooling is suitable for most steel grades, especially when the furnace capacity is large, the operation is convenient, but the production cycle is long; 2 isothermal spheroidizing annealing (Fig. 4b) Suitable for most steel grades, especially steels that are difficult to spheroidize and steels with high spheroidization quality (such as rolling bearing steel); their production cycle is shorter than ordinary spheroidizing annealing, but it needs to be able to control the cooling rate before eutectoid transformation. Furnace; 3-cycle spheroidizing annealing (Fig. 4c), suitable for steel with original lamellar pearlite structure, and its production cycle is shorter than ordinary spheroidizing annealing, but it is difficult under the condition of large furnace loading capacity. The temperature is changed according to the control requirements, so it is not widely used in production; 4 low temperature spheroidizing annealing (Fig. 4d), suitable for cold deformed steel and quench hardened steel (the latter is usually called high temperature softening tempering); 5 deformation spheroidizing annealing, deformation processing has an acceleration effect on spheroidization, and combination of deformation processing and spheroidization can shorten the spheroidization time. It is suitable for cold and hot-formed steel and steel (such as strip) (Fig. 4e is a short-time, large-form thermal deformation process between Acm or Ac3 and Ac1; Figure 4f is given at room temperature first) Deformation processor; Figure 4g is spheroidized by forging waste heat).
Recrystallization annealing process
An annealing method applied to metals and alloys subjected to cold deformation processing. The purpose is to make the internal structure of the metal into fine equiaxed grains, eliminate the deformation hardening, and restore the plasticity and deformation ability (recovery and recrystallization) of the metal or alloy. If the surface of the metal or alloy is to be kept bright, recrystallization annealing can be carried out in a controlled atmosphere furnace or in a vacuum furnace.
Stress-relieving Annealing of cast, forged and welded parts causes internal stresses due to different cooling rates of various parts during cooling. Metals and alloys also generate internal stresses during cold deformation and during workpiece cutting. If the internal stress is large and not removed in time, the workpiece often deforms or even forms cracks. Stress relief annealing is to slowly heat the workpiece to a lower temperature (for example, gray cast iron is 500 ~ 550 ° C, steel is 500 ~ 650 ° C), holding for a period of time, the metal inside relax, and then slowly cooled down. It should be noted that the stress relief annealing does not completely remove the internal stress, but only partially removes it, thereby eliminating its harmful effects.
There are also some special annealing methods, such as stainless acid-resistant steel stabilization annealing; soft magnetic alloy magnetic field annealing; silicon steel sheet hydrogen annealing; malleable iron forging annealing.
Other related metal heat treatment processes that heat the workpiece to a predetermined temperature and then slowly cool it after a certain period of time. The purpose of annealing is to: 1 to improve or eliminate various structural defects and residual stresses caused by steel during casting, forging, rolling and welding, and to prevent deformation and cracking of the workpiece. 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 organization for the final heat treatment (quenching, tempering). Common annealing processes are: 1 complete annealing. It is used to refine the coarse superheated structure of medium and low carbon steel which has poor mechanical properties after casting, forging and welding. The workpiece is heated to a temperature of 30 to 50 ° C above the temperature at which all of the ferrite is transformed into austenite, and is kept for a period of time, and then slowly cooled with the furnace, and the austenite is again transformed during the cooling process, so that the microstructure of the steel is thinned. . 2 spheroidizing annealing. It is used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to a temperature of 20 to 40 ° C above the temperature at which the steel begins to form austenite, and is slowly cooled after the heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby lowering the hardness. 3 isothermal annealing. It is used to reduce the high hardness of certain alloy structural steels with high nickel and chromium content for cutting. Generally, it is cooled to the most unstable temperature of austenite at a relatively rapid rate, and the austenite is transformed into torsite or sorbite at a suitable temperature for a suitable period of time, and the hardness can be lowered. 4 recrystallization annealing. It is used to eliminate the hardening phenomenon (hardness increase, plasticity drop) of metal wire and sheet during cold drawing and cold rolling. 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 make cast iron containing a large amount of cementite into a malleable cast iron with good plasticity. The process operation is to heat the casting to about 950 ° C, and after proper cooling for a certain period of time, the cementite is decomposed to form a group of flocculent graphite. 6 diffusion 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, and heat the steel for a long time, and the various elements in the alloy tend to be uniformly distributed and then slowly cooled. 7 stress relief annealing. Used to eliminate internal stresses in steel castings and weldments. After the steel product is heated, the austenite is formed at a temperature of 100 to 200 ° C below, and after cooling, it is cooled in the air to eliminate the internal stress.
In order to eliminate the internal stress of the plastic article or to control the crystallization process, the article is heated to a suitable temperature for a certain period of time and then slowly cooled.
Annealing
Heating causes the DNA double helix to unravel. Under certain conditions, the two complementary single strands rely on the base pairing of each other to re-form the double-stranded DNA, that is, the renaturation process. The heat-denatured DNA single strand can be well annealed during slow cooling. The two single strands that are annealed can be from the same double-stranded DNA molecule or from different DNA molecules. Annealing is a reversal process of denaturation, which is influenced by factors such as temperature, time, DNA concentration, and complexity of the DNA sequence. For example, in the PCR reaction, the primer is annealed to the template DNA, and the probe is annealed to the DNA to be detected in the nucleic acid hybridization.
Annealing technology semiconductor chip annealing is also often used in semiconductor technology
The semiconductor chip needs to be annealed after ion implantation. Because impurity ions are implanted into the semiconductor, high-energy incident ions collide with atoms on the semiconductor lattice, causing some lattice atoms to shift, resulting in a large number of vacancies, which will cause the atoms in the implanted region to confuse or become It is an amorphous region, so after the ion implantation, the semiconductor must be annealed at a certain temperature to restore the crystal structure and eliminate defects. At the same time, annealing also has the function of activating donor and acceptor impurities by annealing some of the impurity atoms in the interstitial position to bring them into an alternate position. The annealing temperature is generally 200 to 800 C, which is much lower than the temperature of the thermal diffusion doping.
Evaporation electrode metal annealing
Annealing of the electrode metal is required after annealing to allow the semiconductor surface to form an alloy with the metal for good contact (reduced contact resistance). The annealing temperature at this time is selected to be slightly higher than the eutectic point of the metal-semiconductor (570 degrees for the Si-Al alloy).
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