Heat Treatment Annealing of Die Steel
Annealing is to heat the die steel to the critical temperature of the steel above 20-30 C. After holding for a certain period of time, it slowly cooled to obtain a nearly balanced structure. The annealing structure of the die steel is generally carbide distributed on the ferrite matrix. The purpose of annealing is to eliminate stress, reduce hardness of steel, make steel easy to process and obtain good structure, and prepare for the final heat treatment structure. According to the requirements and objectives of die steel in different production processes, there are several common annealing processes for die steel.
1) Diffusion annealing and diffusion annealing are generally used for ingots in die steel production. After ingot pouring, different degrees of segregation (dendritic segregation) occur during solidification. When the segregation is serious, there is uneven microstructure and chemical composition in ingot or billet. Diffusion annealing is to heat ingots or billets at high temperatures and keep them warm for a long time, so that segregation elements in steel can diffuse at high temperatures to reduce or eliminate the influence of segregation as much as possible, so as to improve the quality of steel. Diffusion annealing requires a higher temperature, generally between 1100 and 1200 C. The holding time is related to ingot size, steel grade and segregation degree.
2) Complete annealing is to heat hypoeutectoid steel to more than Ac3 for enough time to completely transform the steel into austenite and homogenize the austenite, and then slow cooling. The purpose of complete annealing is to soften the steel so as to facilitate subsequent mechanical or plastic deformation processing, refine the grain size of the steel and eliminate the internal stress so as to prepare the appropriate structure for quenching. The heating temperature of complete annealing is usually higher than Ac320-30 C. However, the die steel contains strong carbide forming elements (such as Cr, W, Mo, V, etc.), and the austenitizing temperature should be properly increased so that the formed carbides can dissolve into austenite quickly. The holding time of annealing heating should be enough to homogenize the austenite structure. The cooling rate after heat preservation should be determined according to the purpose. Generally, the complete annealing process takes a long time. In order to shorten the process time, the steel can be reduced from annealing temperature to heating temperature (slightly lower than the lower critical temperature) as soon as possible after austenitizing. Thereafter, appropriate slow cooling speed is adopted to obtain the required structure and properties within the pearlite transformation temperature range.
3) Incomplete annealing is to heat the steel to a temperature between the upper and lower critical temperatures, usually slightly higher than the lower critical temperatures. For hypoeutectoid steels, the heating temperature of incomplete annealing is between Ac1 and Ac3, while that of hypereutectoid steels is between Ac1 and Acm. The difference between incomplete annealing and complete annealing is that the former is only partially recrystallized to form austenite, while the latter is completely recrystallized to form austenite, so the former is inferior to the latter in grain refinement. But the heating temperature of incomplete annealing is low, which is very beneficial to the treatment of die steel billet.
4) Isothermal annealing is a kind of complete annealing process commonly used in die steel. The isothermal annealing process is to keep the steel above the critical temperature (hypoeutectoid steel above Ac3, eutectoid steel and hypereutectoid steel above Ac1) for a certain period of time, to austenite and homogenize the steel, and then to keep the steel at a temperature slightly lower than Ar1, so that the austenite is isothermal transformed at this temperature to form pearlite and carbide. The isothermal annealing process consists of three stages: austenitizing heating temperature and holding time, fast cooling to isothermal temperature and holding time for a period of time and out-of-furnace air cooling. The selection of austenitizing temperature depends not only on steel grades, but also on technical requirements and original structure. For example, higher austenitizing temperature can promote the formation of flaky pearlite structure; lower austenitizing temperature can easily obtain spheroidized structure. The isothermal temperature of austenitized steel should be determined according to the final properties obtained and the isothermal transformation curve (TTT curve) of the steel. Generally speaking, the closer the isothermal temperature is to A1, the thicker the pearlite layer is, the lower the hardness of steel; the farther away from A1, the finer the pearlite layer is, the higher the hardness of steel. Therefore, in order to obtain the softest structure, the lowest austenitizing temperature and higher isothermal temperature can be used. However, it should be noted that the time of pearlite transformation of undercooled austenite should also be considered when choosing isothermal temperature, that is, the isothermal temperature at which the required hardness can be obtained with a shorter time should be chosen as far as possible.
5) Spheroidizing annealing is the most common annealing process used in die steel. Its technological characteristics are that by controlling heating temperature, holding time and cooling speed, the carbide is sphericized, the spherical pearlite structure is obtained, the hardness is reduced, the plasticity is improved and the mechanical properties are improved. The microstructures after spheroidizing annealing are generally composed of ferrite matrix and granular carbide, and their properties are the superposition of two kinds of mechanical mixtures. The hardness of ferrite itself depends on solution strengthening, grain boundary strengthening and dislocation strengthening of alloy elements. The distribution, quantity and shape of carbides on the matrix play an important role in the properties. With the increase of carbon content in steel, the number of carbides increases, and the carbide dispersion distribution correspondingly increases the annealing hardness of steel. Isothermal annealing or continuous annealing is generally used in metallurgical plants, which is essentially the same. Reasonable austenitizing temperature, isothermal temperature and cooling rate are important factors for the success of spheroidizing annealing process. That is, when isothermal, we must pay attention to the isothermal temperature. When continuous annealing, the cooling rate should not exceed 30 C/h.
6) In the cold working process of softening annealing and recrystallization annealing, with the increase of deformation, the hardness of steel increases and ductility decreases gradually, so that the processing can not continue. In order to eliminate the hardening caused by cold working, softening annealing is necessary to reduce the hardness of steel, and then continue cold working to achieve the desired size. Softening annealing usually takes place in the middle of two cold working successively, so it is called intermediate annealing. Softening annealing is to heat the steel below A1 (about 650 C) and keep it for a certain period of time before cooling. Recrystallization annealing is to heat a cold-plastic deformed metal material to a temperature higher than its recrystallization temperature, so that it can be recrystallized and grain growth, in order to obtain a new stable structure with the same crystal structure (without phase transformation) and no internal stress.