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Vacuum Heat Treatment of Cr12MoV Steel Ceramic Die
Because of its good wear resistance and long service life, Cr12MoV steel has been widely used to make ceramic moulds in Guangdong, Fujian and other regions in China. There is a large amount of retained austenite in the structure of Cr12MoV steel after quenching, which can theoretically ensure small volume deformation. However, in the actual quenching process, the traditional process cannot obtain satisfactory heat treatment effect due to improper quenching control.
In order to prevent surface oxidation decarburization, deformation and cracking during die quenching, the traditional process is to use barium chloride high temperature salt bath with good deoxidization, or use protective atmosphere electric furnace for heating, or use box type electric furnace for heating after packing protection, so there are many problems. For example, the labor intensity of heat treatment is high, the working environment is bad, the gas released during heating, and the waste slag, salt, and old electric furnace pollute the environment. At the same time, the surface of the workpiece after salt bath quenching is easy to be corroded by the salt bath, which leads to the reduction of the service life of the workpiece. However, due to the fast heating speed and large temperature difference of workpiece section, the deformation caused by thermal stress will also increase. During salt bath quenching, the cooling is uneven, and soft spots are easy to appear, resulting in uneven hardness and reduced service life.
Advantages of vacuum heat treatment:
The vacuum heat treatment technology was used to treat the steel die. Vacuum heat treatment has the following advantages:
1.High efficiency, low energy consumption, no pollution, low labor intensity and high product quality.
2.During vacuum heating, the workpiece is free of oxidation, decarburization, degreasing, degassing (especially the harmful gas hydrogen inside the mold), and its surface is clean and bright. The surface performance is also significantly improved, which can improve the wear resistance and fatigue strength.
3.During vacuum heating, the heating speed is controllable, the temperature difference of die section is small, and the deformation caused by thermal stress is small, about 70% less than that of salt bath quenching. The cooling is uniform, the workpiece will not have soft spots, and the quenching hardness is uniform.
Difficulties in mold processing:
The shape of the ceramic mold is shown in the figure. The shape of Cr12MoV steel ceramic mold is relatively simple, but there is a right angle groove on its centerline, and several assembly holes are distributed in it. Due to phase transformation stress and thermal stress during heat treatment, especially when the mold length L ≥ (400 ～ 600) mm, it is easy to deform along the Y axis and Z axis. The deformation of ceramic mould is required to be less than 0.25mm. After quenching by traditional process, the deformation is beyond the limit, and the hot straightening method is required to meet the deformation requirements.
Vacuum heat treatment process:
The process is carried out in VOGQ-669-2 double chamber vacuum oil and gas quenching furnace produced by SIMUWU. The vacuum furnace with double chamber structure separates the cold chamber from the hot chamber. At the same time, nitrogen can be filled into the cold chamber to achieve better heat treatment effect.
The main form of heating in vacuum is thermal radiation. The austenitizing temperature is stronger than the thermal radiation at the preheating temperature. In the process, the preheating time is extended accordingly to make the molds in the furnace reach the preheating temperature, which is conducive to reducing the deformation caused by the thermal stress of the mold.
In Cr12MoV steel, Cr, Mo and V are all elements that form stable carbides. Considering the dissolution of alloy elements and the homogeneity of composition, the thermal insulation time is longer than that of ordinary carbon steel and low alloy structural steel. However, the holding time is too long, because the carbide is dissolved too much, the amount of retained austenite increases, and the hardness decreases after quenching. In practice, it was found that the hardness reached the maximum value after holding at 1030 ℃ for 50 min. Cr12MoV steel has low critical quenching cooling rate and high hardenability. Before quenching, the die is filled with N2 and forced to circulate and pre cool to 900 ℃, which can ensure the quenching hardness, reduce the deformation of the die and increase the surface brightness of the die. The process shortens the cooling time in the quenching oil, increases the die oil outlet temperature, further reduces the stress caused by the structure transformation during die quenching, and greatly reduces the tendency of die quenching cracking and deformation. During quenching operation, do not open the oil stirring, and only move the mold up and down, which is also helpful to reduce deformation.
After quenching, the structure of Cr12MoV steel is quenched martensite and retained austenite. The cold treatment process is added after the quenching of the die. Since the retained austenite of the high carbon and high chromium die steel is rapidly stabilized after it stays at room temperature for 30~50 min, the cold treatment should be carried out immediately after the quenching.
The specific process curve is as follows:
At present, the ceramic mould treated by vacuum quenching process has been put into production on a large scale, and there are few phenomena such as excessive deformation and low hardness of the mould, which has achieved good economic and social benefits.
Heat treatment equipment selection: The VOGQ series vacuum heat treatment furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment process of tools and dies. Good temperature control accuracy and uniformity ensure the effective implementation of the vacuum heat treatment process. SIMUWU focuses on the manufacturing 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 gas quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.
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