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Vacuum heat treatment process of tool steel
S7 steel can be used to make cold work and vacuum hot work molds. The hardness of the steel after forging is about HRC52. It must be softened and vacuum annealed to reduce the hardness. It is convenient for cutting, and then vacuum quenched and vacuum tempered to meet the performance requirements of the mold.
S7 steel is softened by ordinary vacuum annealing process in metallurgical plants, so it is necessary to measure the continuous cooling transformation curve (CCT curve) of S7 steel under vacuum annealing and supercooled austenite. According to the measured zero boundary point Ac 78n ℃ of S7 steel, the austenitization temperature for measuring the CCT curve of S7 steel is set as 820 ℃.
Since the austenitization temperature is slightly higher than the Ac1 temperature of S7 steel, there are undissolved alloy carbides in the steel. Therefore, when the cooling rate is greater than 20 ℃/s, martensite and undissolved carbide structures are obtained. The lower the cooling rate, the easier it is to obtain granular pearlite structure, and the coarser the granular pearlite, the lower the hardness of the steel.
S7 steel vacuum annealing process and carbides
According to the determined critical point of S7 steel and the supercooled austenite transformation curve for annealing, formulate the annealing process. Carry out the vacuum annealing process.
The austenitizing temperature was set at 820 ℃, and the samples after cooling at different cooling rates were used to determine the types of carbides of S7 steel after annealing by X-ray diffractometer. The average diameters of carbides measured by scanning electron microscope are: 15 ℃/h cooling, the average carbide diameter is 369 nm; 50 ℃/h cooling. The average carbide diameter is 262 nm.
It can be seen from the results of the annealing test that if the annealing temperature of S7 steel is slightly higher than that of Ac1. That is, annealing at 820 ~ 840 °C, and the cooling rate is less than 50 °C/h. The softening effect of steel is the best, because when it is slightly higher than Ac1, more undissolved carbides remain in the austenite. At the same time, the amount of carbon and alloy elements in austenite is small, and the stability of supercooled austenite is poor. When cooled at a slower cooling rate, the supercooled austenite transforms into granular pearlite with undissolved carbide as the core. And when the heating temperature is low and the cooling rate is slow, the carbide particles have the opportunity to aggregate and grow, and the hardness of the steel decreases.
The annealing temperature of S7 steel is 810 ~ 830℃. It is cooled below 50℃/h and the hardness is below HB200. Granular carbides are distributed on the ferrite matrix, and when the particle size of the carbides is larger, the softening effect of the steel is the best. it is good.
Quenching at 850~910℃, tempering at 550~630℃, the hardness reaches about HRC48. It can meet the performance requirements of hot work die made of S7 steel.
Equipment selection: The RVA vacuum annealing furnace provided by SIMUWU is an excellent product for processing this kind of process. It has the characteristics of good temperature uniformity and high temperature control accuracy. SIMUWU provides a professional team of engineers who can solve various problems encountered in the production process and are committed to giving customers the most convenient and efficient experience.