Vacuum Heat Treatment of Hot Extrusion Dies

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In order to prevent the surface oxidation, decarburization and carbon increase of the mold during heat treatment, salt bath or box furnace protection heating was mainly used in the past, and controllable atmosphere heating has also been used in recent years. When heated at high temperature, it is difficult to keep the gas medium, protective agent or salt bath neutral during the whole heating process to prevent them from reacting with the metal. However, the use of inert gases such as argon and nitrogen can achieve the purpose, but it is difficult to popularize due to the high cost. Vacuum acts as an ideal heating medium to treat molds. After the mold is heated in vacuum, neutral gas is passed into the cooling chamber in the furnace (air-cooled vacuum heat treatment furnace), so that quenching can be carried out in the furnace. With the development of vacuum heat treatment equipment, mold vacuum heat treatment technology has been widely developed internationally, and it has also been used more and more domestically.

Vacuum heat treatment has the functions of preventing the surface oxidation of the workpiece, purifying the surface, degreasing and degassing. Long life, can eliminate or reduce grinding. In addition, vacuum heat treatment can also realize rapid heating and rapid cooling, which is convenient for automatic operation, and has the advantages of no pollution, no pollution, energy saving, economical price and so on.

According to the characteristics of vacuum heat treatment, when selecting mold materials, materials with high hardenability should be used as much as possible. If the end quenching test is used to judge the hardenability of low-hardenability materials, the hardenability of vacuum-quenchable high-hardenability materials can usually be judged by the semi-cooling curve method. The half cooling time is the time required to cool from the quenching temperature to half (intermediate temperature) from the quenching temperature to the air temperature (20°C). The relationship between the half-cooling time and the quenching hardness is the half-cooling curve. Usually, in the vacuum air quenching operation, the semi-cooling time is mostly 5 to 60 minutes. At least within 10 minutes of the semi-cooling time, the quench hardness should change little.

Hot extrusion die steels have high hardenability, especially 4Cr5MoSiV and other steel types. On the premise of ensuring sufficient quenching hardness, gas quenching can be used, which effectively reduces quenching deformation and can give full play to the advantages of vacuum heat treatment. Effect.

When the high-speed steel mold is heated, the first preheating temperature is 800 °C, and the second preheating temperature is 1000 °C, in order to reduce the temperature difference. When heating above 1000℃, the vacuum degree is controlled at 1～2×10-1 Torr to prevent the evaporation of chemical elements. The cooling method can choose oil cooling or air cooling, or pressurized air cooling according to the purpose of the mold. It is recommended to temper twice, each time should be cooled to 50 ~ 60 ℃. Cr12MoV steel can be heated at 1000-1020℃ when toughness is required, and can be heated at 1030-1050℃ when wear resistance is required. When the size of the mold is large, the preheating should be sufficient. Cooling should use pressure quenching as much as possible, and tempering should be twice tempered, which is more effective than one long time tempering. 3 Cr2W8V steel has low hardness and high tempering temperature, so secondary tempering is generally used. For more complex extrusion dies of aluminum alloy profiles, especially large-sized plane splitting combined dies, reasonable pretreatment and high temperature quenching are generally recommended. , 2 to 3 times of high temperature tempering process.

Analysis of vacuum treatment process:

(1) Quenching heating time

The heating in the vacuum heat treatment furnace is carried out by radiation, so the heating speed is relatively slow. Because the radiative heat transfer is proportional to the fourth power of the temperature, it is slower for medium and low temperature heating. When heating the mold in a vacuum furnace, the hysteresis of heating is very serious. For example, when a steel rod with a diameter of 50×100 mm is heated in a vacuum, the time for the core to warm up is 6 times that of heating in a salt bath and 1.6 times that of heating in an air furnace. The diathermy time at 850℃ can be 1.5min/mm, and the diathermy time from 850℃ to 1280℃ can be 0.45～0.52min/mm. When quenching the tool and die, it should be extended for 10 to 15 minutes, so that the carbides can be baked and the austenite can be alloyed. Excessive heating time wastes energy, increases costs, and may degrade mechanical properties. When the holding time in the vacuum furnace reaches 10 times that of the salt bath furnace, the bending strength of the heated mold will decrease by 5-8%. Therefore, accurate simulation temperature measurement should be carried out for large and important molds. In order to carefully decide the heating time.

The mutual shielding of the workpieces during furnace loading, the shape and size of the heated workpieces in the same basket, and the inconsistency of wall thicknesses are also factors that affect the vacuum heating holding time. The placement of the workpieces cooperates with each other, and the reasonable furnace loading with the same diameter and thickness as possible is a problem that must be paid attention to during vacuum heat treatment.

(2) Cooling method during quenching

The cooling of the extrusion die after vacuum quenching and heating can be carried out by air cooling or oil cooling. In air cooling, when the diameter and thickness are too large, the ideal hardness value cannot be obtained after quenching.

(3) Heating during tempering

If the mold after vacuum quenching needs to be tempered at low temperature, it can be carried out in an oil furnace. If high temperature (400 ~ 650 ℃) tempering is required, it must be carried out in a vacuum state. However, severe heating hysteresis may occur at this time. It takes 3.5 hours for the steel rod of φ164×180mm to heat up to 600℃ in the vacuum quenching furnace. When tempering in a vacuum quenching furnace, it is necessary to maintain a high degree of vacuum (10-3 Torr), otherwise the brightness after vacuum quenching will be lost. During vacuum tempering, it is best to recharge 200~300 Torr high-purity nitrogen, and use a fan for forced stirring or conduct it in a specially designed vacuum tempering furnace. so. The heating time of tempering can be effectively shortened. The tempering heating time of the steel bar of φ164×180mm can be shortened to about 2 hours. Cooling after high temperature tempering is preferably air cooling, which can reduce residual stress and avoid unnecessary cleaning.

Equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum heat treatment process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.