The overall quenching of the ring die of feed pelletizing machine generally refers to the heat treatment carried out by electric furnace or salt bath furnace in atmospheric environment. But in the past 20 years, more and more foreign feed machinery manufacturers began to use vacuum furnace to quench the ring die as a whole, this heat treatment method is called true air quenching process. That is, under a certain vacuum degree, the ring mold placed in the closed vacuum furnace is heated to above the austenitizing temperature, and then it is rapidly gas quenched with high pressure and high purity cooling gas. Compared with the traditional monolithic quenching, this vacuum heat treatment technology has the following advantages: the workpiece deformation is small; The metal surface after heat treatment is very smooth and clean, and no oxidation, corrosion, decarburization or carburization occur; In particular, the finish of the ring die hole is basically not reduced after real air quenching, which is crucial to ensure the yield of pellet feed. In addition, vacuum heat treatment furnace is widely used, and energy saving, safe and reliable, pollution-free, easy to realize automatic operation. At present, there are few feed machinery manufacturers that really use vacuum heat treatment technology on a large scale in China. According to the previous practical experience, the characteristics, main influencing factors and problems that need to be considered in the ring mold true air quenching process are analyzed and studied, and the relevant process parameter values and solutions are given for reference.
Vacuum heat treatment furnace
Vacuum heat treatment furnace is a large heat treatment equipment used for true air quenching of ring die. When choosing a vacuum heat treatment furnace, feed machinery manufacturers should consider the following main factors: vacuum furnace form (vertical or horizontal), vacuum room space size (diameter × height), the maximum load of a heat treatment workpiece (kg), the highest heating temperature in the vacuum room, the gas pressure used as a gas quenching cooling medium (also known as gas quenching pressure) and so on. Generally speaking, vertical vacuum furnaces are used for heat treatment of ring molds. This is mainly because the ring mold needs to be placed flat in a cylindrical vacuum chamber to reduce the deformation caused by true air quenching. The space size of the vacuum chamber and the maximum load of the workpiece mainly depend on the maximum outer diameter and total weight of the ring die that the feed machinery manufacturer needs to quench the real air. The maximum temperature of the ring mold heating in the vacuum chamber will not exceed 1100℃, and the general vacuum furnace can meet this requirement. The choice of gas quenching pressure has a great influence on the heat treatment quality and cycle length of the ring die, which will be discussed in detail in the setting of process parameters in the next section.
There are many manufacturers of vacuum heat treatment furnaces at home and abroad, and vertical true air quenching heat treatment furnaces are mainly composed of vacuum chambers with built-in heating elements, control cabinets with human-machine interfaces, high-pressure gas storage tanks for storing gas quenching cooling media, gas quenching systems equipped with blowers and heat exchangers, working platforms for maintenance and operation, and vacuum pumps. RVGQ-B series vertical vacuum furnace has a variety of sizes of vacuum chamber, the lower end cover of these vacuum chambers can be fully opened and automatically moved down to the ground along the column, and then automatically slide out along the guide rail on the ground, so the loading and unloading ring die is very convenient. The maximum heating temperature of vacuum chamber calibration can reach about 1400C, and there are three specifications of gas quenching pressure 0.2, 0.6 and 1MPa for choice. Therefore, RVGQ-B vertical vacuum furnace is very suitable for true air quenching process of ring mold.
The control cabinet of the SIMUWU vacuum furnace has a built-in touch screen with a good human-machine interface, and all the actions of the vacuum furnace can be operated and controlled through this touch screen. It is worth noting that for each type of ring mold, a so-called heat treatment formulation program can be established for it through the touch screen. In this program, the temperature change curve of the vacuum chamber with time during the whole true air quenching period can be set in advance (see the next section for details), so that the precise automatic control of the ring mold true air quenching process can be achieved. In addition, this heat treatment formulation program and the data of temperature and pressure changes over time in the vacuum chamber can be subsequently downloaded, printed and archival, which is necessary and convenient for ISO-certified feed machinery manufacturers.
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