Vacuum air quenching heat treatment process for ring die of feed pelletizer

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The overall quenching of the ring die of the feed pelletizer generally refers to the heat treatment in an electric furnace or a salt bath furnace in an atmospheric environment. However, in the past 20 years, more and more foreign feed machinery manufacturers have begun to use vacuum furnaces to quench ring dies as a whole. This heat treatment method is called vacuum quenching. That is, under a certain degree of vacuum, the ring die placed in the closed vacuum furnace is heated to above the austenitizing temperature, and then the high-pressure, high-purity cooling gas is used for rapid gas quenching. Compared with traditional overall quenching, this vacuum heat treatment technology has the following advantages: the deformation of the workpiece is small; the metal surface after heat treatment is very smooth, and there is no oxidation, corrosion, decarburization or carburization; especially the smoothness of the ring die hole The vacuum is basically not reduced after quenching, which is very important to ensure the output of pellet feed.

In addition, the vacuum heat treatment furnace has a wide range of uses, and is energy-saving, safe and reliable, pollution-free, and easy to realize automatic operation. At present, there are still relatively few feed machinery manufacturers adopting vacuum heat treatment technology on a large scale in China. Based on previous practical experience, the author analyzes and studies the characteristics, main influencing factors and issues that need to be considered in the ring die vacuum quenching process, and gives The relevant process parameter values ​​and solutions are for reference. The author uses martensitic high-chromium stainless steel AISI420 (equivalent to domestic steel grade 3Cr13) as the material of the ring die for vacuum quenching. This material has good corrosion resistance, wear resistance and strength, and is the most commonly used by American feed machinery manufacturers. One of the ring die materials.

Vacuum heat treatment furnace

The vacuum heat treatment furnace is a large heat treatment equipment used for vacuum quenching of ring dies. When selecting a vacuum heat treatment furnace, feed machinery manufacturers should consider the following main factors: vacuum furnace form (vertical or horizontal), vacuum chamber space size (diameter×height), maximum loading capacity of a heat-treated workpiece (kg) , The highest heating temperature in the vacuum chamber, the gas pressure used as the gas quenching cooling medium (also known as the gas quenching pressure), etc. Generally speaking, a vertical vacuum furnace is used to heat-treat the ring die. This is mainly because the ring die needs to be placed flat in the cylindrical vacuum chamber to reduce the deformation caused by vacuum quenching. The size of the space in the vacuum chamber and the maximum loading capacity of the workpiece are mainly determined by the maximum outer diameter and total weight of the ring die that the feed machinery manufacturer needs to vacuum. The maximum heating temperature of the ring die in the vacuum chamber will not exceed 1100°C, which can be met by general vacuum furnaces. The choice of gas quenching pressure has a great influence on the heat treatment quality and cycle length of the ring die. This will be discussed in detail in the next section of the process parameter setting.

There are many manufacturers of vacuum heat treatment furnaces at home and abroad, such as the vertical vacuum heat treatment furnace manufactured by China SIMUWU Company. It is mainly composed of a vacuum chamber with built-in heating elements, a control cabinet with a human-machine interface, a high-pressure gas tank for storing the gas quenching cooling medium, a gas quenching system equipped with a blower and a heat exchanger, and maintenance and operation It is composed of workbench and vacuum pump. The RVGQ-B series of vertical vacuum furnaces have vacuum chambers of various sizes. The lower end covers of these vacuum chambers can be opened and moved down to the ground automatically along the column, and then automatically slide out along the guide rails on the ground, thus loading and unloading the ring die Very convenient. The maximum heating temperature calibrated in the vacuum chamber can reach about 1400C, and the gas quenching pressure can be selected in three specifications: 0.2, 0.6 and 1MPa. Therefore, the RVGQ-B vertical vacuum furnace is very suitable for the vacuum quenching process of ring molds.

A touch screen with a good man-machine dialogue interface is built in the control cabinet of the SIMUWU vacuum furnace. All the actions of the vacuum furnace can be operated and controlled through this touch screen. It is worth noting that for each ring die, a so-called heat treatment recipe program can be established through the touch screen. In this program, the temperature change curve in the vacuum chamber can be set in advance during the entire vacuum quenching period (see the next section for details), so that precise automatic control of the vacuum quenching process of the ring die can be realized. In addition, the heat treatment formula program and the temperature and air pressure in the vacuum chamber can be downloaded, printed and archived later. This is necessary and convenient for the ISO-certified feed machinery manufacturer.

Heat treatment quality inspection of ring die

The surface of the ring die after vacuum quenching should be very smooth and light yellow. If black marks appear in some places on the surface, you need to check whether the vacuum meets the requirements, whether the vacuum chamber and all pipelines are leaking, and whether the purity of the argon or nitrogen used is high enough, and so on. For the martensitic stainless steel material AISI420, using the above heat treatment process, the surface hardness of the ring die after vacuum quenching is about 56~58 HRC, and the surface hardness after tempering is about 54~56 HRC, which conforms to the ring die heat treatment specification.