Commonly used internal combustion engine exhaust valve material, the solution quenching temperature as high as 1150 ~ 1190 °C. To reduce the heating oxidation, the traditional method is to use high-temperature salt bath heating. High-temperature salt bath furnaces have problems of environmental pollution, high energy consumption, and poor working conditions. Although the high vacuum heat treatment furnace is competent for such work, the equipment is expensive, the use and maintenance costs are high, the production cost of the valve is increased, and the staff is required to have a high technical level, and therefore is rarely used in the valve industry. The commonly used protective (or controllable) atmosphere furnaces, such as the box type multi-purpose furnace, have a maximum working temperature of generally lower than 1000° C. The valve is heat-treated using a simple protective atmosphere high-temperature resistance furnace (1200*C), but the furnace door Other parts of the poor sealing, workpiece oxidation is more serious. And the protective atmosphere of high temperature resistance furnace due to the high operating temperature, the heating element is easily blown, requiring frequent shutdown maintenance and repair. Therefore, salt bath furnaces are often forced to be reopened due to urgent production needs. In recent years, domestically produced low vacuum heat treatment furnaces (hereinafter abbreviated as SLV furnaces) have been used to satisfactorily solve the problem of heat treatment of 2-4N valves. This article introduces the structure and process characteristics, application experience, and precautions of the furnace in order to obtain in-line corrections.
1 Structural features of the low vacuum heat treatment furnace The structure of the SLV furnace is shown schematically.
1.1 Furnace Structure The guarantee of the gas tightness of the furnace is the key to achieving less non-oxidative heating, and the sealing of the furnace door is critical. SLV furnace uses double doors, high temperature resistant rubber rings and cooling water jacket seals to achieve the door seal. According to the double door separating the heating chamber and the cooling chamber of the high-vacuum furnace, the SLV furnace adds a double door near the furnace mouth in the furnace, and the electric push rod lifts and turns the double door to open and close. The area of ​​the double door is slightly larger than the teaching and research work of the furnace mouth, metal materials and heat treatment, published nearly 20 papers, won the second prize of teaching achievement in Guangdong Province (1997), and the third prize of Guangdong Province scientific and technological achievements (2004 ). Contact Tel Fund Project: Foshan City, Guangdong Province, special fund project for production and research (2003S58) State 4 4 ​​gas cooler 5. Inner furnace door 6 liquid drip chamber 7. Outer furnace door 8 Inner door lift mechanism It blocks the furnace radiation to Most of the heat of the outer furnace door and the corresponding front plate of the furnace greatly reduces the temperature rise (refers to the low temperature of the outer furnace door and the corresponding front plate of the furnace). A sheet-shaped steel boss is machined at the outer furnace mouth of the furnace, and the size is equal to that of the outer furnace door cover. The interior of the outer door cover is a light brick surrounded by about 5mm steel plate. Outside the furnace port boss plate and the contact with the outer furnace cover after the plane plate has a very high degree of smoothness and flatness with the accuracy. A rectangular slot with a semi-circular cross section is machined at the near edge of the steel plate of the outer furnace door cover, and a high-temperature rubber sealing ring is mounted thereon. The outer furnace cover and the port bosses have inner circulating cooling water channels to ensure that the actual working temperature of the seal ring is far below its maximum allowable operating temperature. Touching the furnace cover by hand is only a feeling of slight heat. In addition, the use of bevel wedge-type clamping device, so that the furnace cover and the outer port boss has a great pre-pressure, in order to ensure high sealing ring seal. As the furnace operates in a low vacuum state, the outer furnace door is further pressed by the negative pressure to press the outer furnace port bosses. During ramping and vacuuming of the furnace, ramp wedges need to be constantly adjusted to ensure preload pressure until furnace temperature and furnace pressure stabilize. From the above analysis, it can be seen that the sealing performance of the furnace mouth and furnace cover is much better than that of a general protective or controlled atmosphere furnace.
1.2 Structure of heating element terminal The terminal of SLV furnace is a special structure with multiple sealing rubber rings and cooling water jacket. It can be easily removed from the back plate of the furnace, and a special structure is used to connect the heating wire outside the furnace. The contact is good without "returning" due to temperature fluctuations or vibrations, so as to prevent the resistance wire temperature from rising sharply and melting due to the sharp rise of the contact resistance. Traditional resistance furnaces, protective atmospheres, or controlled atmosphere resistance furnaces often cause the carbon black or solid particles to accumulate in the vicinity due to the overflow of furnace gas from the terminals, resulting in a short circuit between the resistance wires.
The SLV furnace adopts the sealant ring and the cooling water jacket structure to eliminate high-temperature furnace gas spillage and intrusion of foreign air.
1.3 Structure features of other parts After high-quality welding of the outer shell of the furnace, the water pressure test is not leaking to ensure that the furnace gas cannot be invaded by the furnace shell and the atmosphere.
For the double door lifting device, thermocouple hole, controllable or protective atmosphere medium drip hole, there are corresponding sealing ring and cooling water jacket device to prevent the furnace gas from overflowing and reduce the invasion of the external air.
In addition to the two sides of the furnace, a resistance wire is also arranged on the bottom and the back of the furnace and uses a three-phase “A* connection†to increase the distance between the resistance wire coils at the same power, which facilitates local heat dissipation between the rings and reduces carbon black or solid impurity particles. The possibility of short circuit due to accumulation.
The use of a special shape of light weight wire rod as little as possible with firebrick, ceramic fiber accounted for more than two-thirds of the entire lining material, the furnace heat and heat less, heating up quickly, good insulation, thermal efficiency.
The above furnace structure features greatly reduce the heat dissipation of the furnace gas, the furnace warms up quickly, the heat preservation performance is good, and the temperature of the heating wire is slightly higher than the set temperature 1160*C, which is much lower than the maximum allowable temperature of the iron-chromium aluminum resistance wire 1400*Cl1 With the addition of vacuum to reduce the carbon black, the accumulation of foreign particles in the heating wire ring, so that the chance of a short circuit between the ring and the fuse is reduced, significantly extending the furnace maintenance cycle.
2 Characteristics of vacuum equipment and related processes Vacuum furnaces generally use a rotary vane mechanical pump, a Luoji pump or even an oil diffusion pump or an ion pump to form a vacuum system with a vacuum degree of 10-3 Pa. The SLV furnace uses only a water ring vacuum pump. Approximate ordinary water pump, vacuum degree is about 104Pa, far lower than high vacuum furnace. However, the ingenious process of evacuation and the associated simple control device can greatly reduce the oxygen concentration in the furnace.
When the 2-4N valve is quenched, the furnace temperature is still 900*C. At this time, the preheated valve workpiece can be removed from the preheating furnace and loaded into the SLV furnace. After lowering the double door and closing the outer furnace door, heat the electricity while heating, and pump the air inside the furnace with a water ring vacuum pump. The vacuum in the furnace is reduced to mVa in less than 1 minute. At this time, the vacuum contact valve switches off the vacuum valve and connects the industrial pure nitrogen gas to the valve through the vacuum gauge. This greatly dilutes the residual oxygen concentration in the furnace. When the pressure in the furnace is close to 1105 Pa (1 atm), it is switched to the water ring vacuum pump to pump off the nitrogen. This cycle vacuums a pure nitrogen process. According to the gas equation PnRT, if oxygen in pure nitrogen is ignored and oxygen released from linings, workpieces, etc., is neglected, the oxygen concentration is reduced by about 90% per cycle. Thus, the cycle operation is performed several times for several minutes, although the degree of vacuum in the furnace is not high. Only 104Pa, but the oxygen concentration can be close to the oxygen concentration of the high vacuum furnace. The residual oxygen in the furnace reacts with the workpiece, creating a negligible oxide film on the surface of the workpiece. After the pumping is completed, pure nitrogen is poured into the industry. The pressure is slightly lower than the atmospheric pressure, so that the furnace is in a state of negative pressure. The door of the furnace is subjected to atmospheric pressure and its sealing ring presses the plane of the boss plate in the mouth of the furnace mouth. Due to the excellent sealing of the furnace body, it can even be considered that the valve is heated under no oxidizing conditions within about 45 minutes after the 2-4N valve is installed into the furnace. After the valve works, whether it is water-cooled, oil-cooled, or air-cooled, it requires only a simple cleaning of its surface to be quite smooth. In 45min heating time, the furnace works under negative pressure. The lid of the oven door is tightly pressed against the port boss under the action of atmospheric pressure. The sealing rubber ring strictly prevents the infiltration of foreign air. Therefore, a more accurate and more appropriate definition of such a low vacuum resistance furnace is a low (no) oxygen concentration protection atmosphere furnace. At the same power and process time, only one basket of valves can be treated with a salt bath, while six baskets of valves can be treated with a low-vacuum furnace, which has high productivity, energy saving and environmental protection.
3 Precautions for using high-temperature and low-vacuum furnaces In order to ensure the normal operation of the furnace and extend the maintenance period as much as possible, special attention should be paid to the following points: At all times, it is necessary to ensure the supply of water for cooling water and water ring vacuum pumps, preferably with a dedicated circulating pool. This saves water.
Always check the contact condition between the lid and the contact surface of the port boss to ensure a high degree of smoothness and smoothness. The quality of the contact surface is degraded due to a change in the temperature of the furnace or the impact of the workpiece loading and unloading, and even when it is deflected, repairs must be made in time.
Every time before installing the furnace, it is necessary to carefully observe the quality condition of the lid seal rubber ring, and rigidly provide regular (about 3 months) to replace the new apron with good quality.
When the furnace is repaired, replace the sealing rubber ring of the double door lifting mechanism, terminal, thermocouple insertion hole, etc., and apply a vacuum. Check whether the cooling water jacket is working properly. The water ring vacuum pump and its electrical contacts. Pressure gauges, changeover switches, etc. are also subject to routine inspections and maintenance.
4 Discussion and Conclusion General high-temperature resistance furnace due to poor sealing, thermal insulation performance, resistance wire under high-load operation in an oxidizing atmosphere for a long time, or adding a simple protective atmosphere caused by carbon black and other impurity particles to accumulate in the resistance wire ring caused by a short circuit, the resistance Easy to break, short maintenance cycle, this problem has been plaguing many domestic users. Low vacuum heat treatment furnace has good sealing performance and uses vacuum circulation and other measures to solve this problem.
The low vacuum heat treatment furnace adopts a water ring vacuum pump to pump down the vacuum. The vacuum system has a long life, low energy consumption, low price, easy operation and maintenance. Unique circulating vacuum technology is easy to get a protective atmosphere with low oxygen concentration, and the workpiece achieves less oxidation-free heating.
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