Views:559 Author:Site Editor Publish Time: 2021-09-14 Origin:Site
I believe everyone knows very well that it is very important in construction and industrial projects, whether it is a small socket or a large refrigerator, fasteners must be used. There are many types of fasteners because the market needs are diverse, and the commonly used fasteners are bolts, nuts, washers, screws, electrical contact rivets, bimetal contact rivets, silver bimetal contact rivets and tungsten contact rivets.
When the fastener is in use, sometimes the temperature is very high. Under the operation of the high-temperature fastener, it is easy to damage the performance of the fastener and even some failures. For example, bolts are important connecting parts widely used on boilers, steam turbines, and steam pipeline valves of high-tech power plants. This is because most of the fasteners are made of different types of metals. When encountering high temperatures, the fasteners will be oxidized, which will cause the melting point of the metal to rise, which will cause the fasteners to corrode and grow. rust. And metal fasteners will expand when they encounter high temperatures. Such as copper brass silver contact parts, brass stamping terminal parts silver contact and silver bimetal contact rivets. Under the action of expansion, once they are cooled, the metal fasteners will shrink and become loose, and even cause positional movement, which will cause a lot of trouble.
Regarding the high-temperature fasteners in these aspects, what should we pay attention to during the use of high-temperature fasteners?
1. Sealing stress is a basic requirement for bolt connection to ensure the tightness of connecting parts. When the flange geometry, the processing quality of the joint surface, the pressure and temperature of the steam, and other conditions remain unchanged, the sealing stress is determined by the tightening force of the bolt.
2. When the group starts and runs, due to the temperature difference between the flange and the bolt (the temperature of the flange is higher than that of the bolt), and the linear expansion coefficients of the two materials are different, the free thermal expansion of the flange is higher than that of the bolt. If the bolt is large, the bolt receives an additional tensile force, and the stress caused by the additional tensile force in the bolt is called temperature stress.
3. The thermal tightening of the bolt is a bolt that is pre-tightened during installation. When it is put into operation, under the action of steam pressure, the force of the bolt is increased than the initial pre-tightening force. For general horizontal flanges, The bolt tension is increased by 5%-15%, so that the initial stress of the bolt during cold tightening can be lower than that. To meet the requirements of this initial stress, it is sometimes impossible to tighten the bolts to the required degree by manpower in the cold state. Therefore, it is usually necessary to use a hot tightening method. At this time, you need to manually tighten the bolt to a certain degree (cold tightening), and then use a special electric heater to heat the bolt through the bolt center hole. After the bolt is heated and stretched, tighten the nut. This method is called As hot tight. Controlling the thermal tightening makes it easier to control the pre-tightening force.
4. In addition to metal materials, we can use fasteners made of ceramics such as alumina and zirconia to withstand high temperatures. After all, ceramic fasteners can withstand temperatures above 3000 degrees Fahrenheit, but the disadvantage is that ceramic fasteners are not as durable as metal fasteners.
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