Vacuum Quenching of Micro-gap Blanking Dies

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Vacuum quenching technology is a new process developed in recent years. This process is used to process the micro-gap blanking die. It can meet the strict requirements of high hardness, high wear resistance, micro-deformation and long life of the mold. The so-called micro-gap punching die generally refers to a cold punching die with a double-sided gap between 0.003 and 0.01 mm when the convex and concave dies are matched.

Alkaline batteries produced by our factory are made of thin steel strips with a thickness of 0.05mm or 0.10mm. First, the thin steel strip is punched out with a diameter of φ2mm on a high-speed punching machine with a diameter of 300-600 times/min to form a plate skeleton, and then a series of processes are used to make the plate. Due to the huge demand for polar plates, the skeleton punching die has become the key die that determines the output and quality of the product.

When the die is working, firstly, because the punched strip is too thin and burrs (flashes) are not allowed on the edge of the hole, the matching clearance of the punch and die must be controlled below 0.01mm; The center distance between the φ2mm head hole and the tail hole cannot be changed, so the bending deformation and length deformation of the die after heat treatment must be controlled below 0.08mm; three because the mold must withstand the service conditions of a high-speed punch, so the mold is required to have high hardness and high resistance. Grinding, micro-deformation and high life.

At this time, if the conventional salt bath furnace is used for quenching, the deformation requirements cannot be met. Therefore, our factory adopts a vacuum furnace for quenching treatment.

The vacuum quenching process is a new technology developed rapidly in recent years. Compared with ordinary quenching, it has the advantages of bright surface after quenching, no oxidation, no decarburization, no carbon increase, small deformation, uniform hardness, good comprehensive mechanical properties, No pollution, good working conditions and so on. All this has also been fully confirmed in our production practice. Therefore, it can be said that vacuum heat treatment is the development direction of workpiece and mold heat treatment.

Cr12MoV steel is selected as the material of the skeleton punching die and the die material, and the heat treatment process scheme is shown in the figure:

In the vacuum chamber, two concave dies are installed in each furnace, and the working surfaces of the two concave dies (the surface of 2mm in the inner diameter of the small hole) are opposite to each other, so that the working surface of each concave die faces the heating body. Turn on the empty pump, evacuate to below 4pa, power on and heat up to the process temperature, after a period of heat preservation, turn off the pump and stop
Pump air and open the inflation valve at the same time to fill with argon. When the argon pressure in the vacuum chamber is slightly higher than one atmosphere, the valve at the lower part of the vacuum chamber is automatically opened, and the die is surrounded by argon and falls into the oil tank due to its own weight for hardening, and stops at the same time. Fill with argon. When the die is cooled to below 100°C, it is transferred to a nitrate bath furnace for tempering, and the hardness of the die after treatment is HRC58-62.

Selection of Quenching and Tempering Process Temperature
Cr12MoV series high carbon and high chromium steel has the advantages of high hardenability, small volume variation and high wear resistance. Generally speaking, Cr12MoV steel adopts low temperature quenching (950~1000℃) and low temperature tempering (~200℃) to obtain high hardness and high toughness, while the compressive strength is slightly lower, using medium temperature quenching (1030℃) and medium temperature tempering Fire (460°C) can obtain better strength and toughness and improved fracture resistance. High temperature quenching (1100°C) and high temperature tempering (500-520°C) can obtain higher hardness and compressive strength, but the toughness is too poor; The amount of deformation increases with the high quenching temperature.

For the skeleton punching die, not only considering its comprehensive mechanical performance indicators, but also controlling the deformation to a minimum, a heat treatment process plan of preheating → low temperature quenching → low temperature tempering is formulated. If this process scheme is still heated and quenched in a salt bath furnace, most of the workpieces will not meet the deformation requirements. After switching to vacuum heating and quenching, the qualified rate of finished products will reach 100%.

Selection of holding time
In the vacuum state, heating is mainly completed by thermal radiation. Because of the directionality of the radiation, the surface of the workpiece toward the radiation surface is heated faster. The back radiation part and the core of the workpiece are mainly heated by the heat conduction of the workpiece itself. Therefore, the workpiece is heated. The heating rate in the vacuum furnace is slower than that of the general box-type resistance furnace or salt bath furnace, and the actual temperature of the workpiece is lower than the furnace temperature. Therefore, in order to ensure the uniform heating of the workpiece; the holding time should be longer than that of the salt bath furnace and the box furnace heating. According to a large number of literatures, the heating and holding time of vacuum quenching should be 4 to 6 times that of salt bath quenching heating and holding time. In addition, since there is a certain temperature difference between the surface facing the heating body and the surface facing away from the heating body, some literatures introduce that the temperature difference is between 4 and 10 °C, so in order to ensure the quenching hardness of the working surface of the die, we treat each heat Two female molds, and the working surfaces of the two are facing away from each other so that they face the heating body.

vacuum requirements
When heating in a vacuum state, in order to prevent the Cr element from evaporating from the workpiece and cause the surface layer to be depleted of Cr and reduce the quenching hardness, we use the front-stage mechanical pump of the vacuum unit to maintain the vacuum at 4-0.1Pa. Practice has proved that this has been Can fully meet the use requirements.

The higher the original brightness of the workpiece, the higher the brightness after vacuum quenching. For the working surface of the mold, the brightness can meet the requirements of use. The main reasons that affect the brightness of the surface of the vacuum oil quenched workpiece are: the pollution of impurities (oxygen, moisture, etc.) contained in the argon gas charged into the vacuum chamber before quenching and the pollution of the quenching oil.

Under the premise of the same processing technology and the same service conditions, the service life of the vacuum quenched die is 4 to 6 times higher than that of the salt bath quenched die, which is consistent with the examples introduced in many foreign literatures. The reasons for this are as follows:

1. No oxidation and decarburization on the mold surface
Oxidation on the surface of the workpiece will cause oxides to form on the surface layer and intergranular, and cracks may easily occur in the surface layer when subjected to stress, and the expansion of cracks will lead to failure of the workpiece; similarly, after decarburization of the workpiece surface, the composition of the core material is different, and the surface may appear during quenching. Troostite, pearlite or ferrite, while the core generates martensite, resulting in great structural stress. Under the action of external force, the surface layer is prone to cracks, which leads to the failure of the workpiece. In addition, the surface layer has low hardness and poor wear resistance, which makes the workpiece fail prematurely. Vacuum quenching can ensure no oxidation and decarburization on the surface of the workpiece, which is the key to improving the service life of the workpiece.

2. Purification of vacuum degassing and degreasing
Very trace amounts of gas elements in metals: nitrogen, hydrogen, oxygen and their compounds, deposited on grain boundaries will drastically reduce their mechanical properties, especially toughness and fatigue strength. When the workpiece is heated in a vacuum state, the trace gas elements in the metal will diffuse to the surface layer, escape from the surface, and be discharged by the vacuum pump. At the same time, the vacuum heating also dissipates the trace oxides on the metal surface, so that the surface of the workpiece after treatment is clean and bright, and the surface purification effect is beneficial to the improvement of the comprehensive mechanical performance index of the workpiece.

3. Vacuum heat treatment reduces deformation
Vacuum heating mainly conducts heat by radiation, the heating speed of the workpiece is slow, the temperature difference of the watch core is small, the expansion and contraction of the workpiece are consistent in all directions, and the tissue transformation is uniform, thereby reducing the deformation caused by thermal stress and tissue stress. The deformation of vacuum quenching is about 1/3~1/10 of that of salt bath quenching, which meets the strict deformation requirements of the original design.

4. Good microstructure
Observed under the metallographic microscope, the workpiece after vacuum heat treatment, compared with the salt bath treatment, the surface microstructure is dense and uniform, the carbide distribution is uniform, the grain size is finer, the amount of retained austenite is reduced, and the hardness of the workpiece is uniform. Soft spots have never been found, which improves the mechanical properties of the workpiece and prolongs its service life.

Equipment selection: The VOGQ series vacuum oil quenching furnace produced by SIMUWU is a high-quality product for this type of vacuum quenching process. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum quenching process. SIMUWU has more than ten years of sales and manufacturing experience, and is exported to developed regions in Europe, America and Asia. It is a well-received vacuum furnace product.