Annealing method
Annealing method
One of the most important process parameters for 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 the iron-carbon balance diagram. 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 at a temperature above or below the solidus temperature.
1. 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, known as complete annealing, annealing temperature between Ac1 and Ac3 (hypoeutectoid 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 isomeric transformation when heated and cooled. 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 grains, recrystallization annealing is also performed, that is, slowly heating to a temperature not higher than the phase transition temperature range, and the alloy is transformed into fine 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.
2. Incomplete annealing
Incomplete annealing is an annealing process in which an iron-carbon alloy is heated to a temperature between Ac1-Ac3 to achieve incomplete austenitization followed by slow cooling.Incomplete annealing is mainly applied to medium and high carbon steel and low alloy steel forgings, etc. The purpose is to refine the structure and reduce the hardness. The heating temperature is Ac1+(40-60) °C, and the temperature is slowly cooled after the insulation.
3. 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 (hypoeutectoid steel and pro-eutectoid ferrite; hypereutectoid steel and pro-eutectoid cementite) So far, finally cool down at any speed (usually it is cooled in the air). The approximate temperature range for isothermal maintenance 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 recrystallization annealing, but the process operation and equipment required are relatively complicated, so it is usually mainly applied to alloy steel in which the 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 more uniform microstructure and performance of the entire workpiece. Isothermal annealing can also be used at different stages of the thermal processing 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 too cold 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.
4. 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 high heating temperature due to a high heating temperature and a long heat retention time.
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