What is the creep failure of the fastener and the creep failure of the fastener? I. Overview Creep is the failure phenomenon of permanent deformation of metal parts under the long-term effects of stress and high temperature. The deformation of the grain along the grain boundary is the main mechanism of creep. When the deformation temperature is raised to 0.35 to 0.7 Tm (Tm is the absolute temperature of the melting point), recovery occurs in the thin layer region near the grain boundary and softens, and the shape proceeds. Distortion occurs after deformation, so that it needs to be restored and softened again to keep the deformation going in these areas. This is called grain boundary sliding. Since the recovery requires a certain temperature and time, the grain boundary sliding can be performed under conditions higher than a certain temperature. Metal tensile creep curve, which is divided into three stages: In the first stage, the creep rate gradually slows down, which is related to the redistribution of crystal defects. In the second stage, it is shown that the two mechanisms of hardening and recovery are in equilibrium and the creep rate is constant. This stage occupies a large proportion in the whole process of creep. In the third stage, the creep rate is accelerated. At this time, the deformation hardening of the metal is insufficient to prevent the deformation of the metal, and the reduction of the effective cross section accelerates the creep rate and finally leads to fracture. Not all creep curves of any material appear in the above three stages. The phenomenon that the creep process changes the size of the pre-tightened part and causes failure is called thermal relaxation. For example, bolts used in fastener pressure vessel flanges, due to long-term effects of temperature and stress, elongate due to creep, resulting in a reduction in pre-tightening force, which may cause leakage of the pressure vessel. Second, characteristics and judgment The most important feature of creep is that the rate of permanent deformation is very slow. It can be analyzed according to the specific working conditions of the part, whether there are conditions for generating creep (temperature, stress and time). Without proper temperature and sufficient time, creep or creep rupture does not occur. In the final fracture zone of the creep fracture, the tearing ridge is not as clear as the normal temperature tensile fracture. Under the scanning electron microscope, the grain shape near the creep fracture often does not appear elongated, but at high magnification, sometimes Can see creep holes. Third, the identification method of creep failure Thermal relaxation and plastic deformation, from the macroscopic residual deformation is easy to be confused. Plastic fracture is easily confused with permanent fracture (or creep fracture) because, from a macroscopic point of view, there is permanent deformation before fracture and necking near the fracture. The difference can be considered from the following aspects. 1. The difference in working conditions is well known. Plastic deformation and plastic fracture occur under tensile stress, and the process proceeds faster and the temperature is lower. Thermal relaxation and permanent fracture are the failure processes that play an important role in temperature and time. Higher operating temperatures and longer service times are necessary conditions for this failure mode. In addition to reading the text, the understanding of the working conditions directly checks whether there are high temperature traces on the wreckage, such as oxidized color. Be very careful when analyzing the working conditions. For example, a high temperature pressure vessel has a long time to work at a lower pressure, and suddenly the pressure rises, causing the connecting Bolt to break. This is only to know the pressure, temperature and difference. The service time under working conditions can specifically determine whether the creep of the ball is invalid. 2, the difference of the fracture morphology of the plastic fracture on the upper dimple is very clear, the micropore polymerization is relatively sharp, under the scanning electron microscope to observe these places appear white bright lines. On the creep fracture, the place where the micropores are polymerized is relatively blunt, and under the scanning electron microscope, there is no obvious white bright line in these places. On the creep fracture, it is possible to see oxidized color and sometimes see creep holes. 3. The metallographic creep near the fracture is mostly along the crystal fracture, and the plastic fracture is mostly transgranular fracture. In the creeping sample, it is possible to see the creep hole. In addition, the carbon steel stays at a high temperature for a long time, and the carbide undergoes a certain degree of stone grinding. Fourth, measures to improve creep resistance 1. Design aspects According to the characteristics of the product, it is important to select the right material and determine the part size. In recent years, in order to meet the requirements of increasing product temperature and load, a number of new materials have been developed, but the creep performance data that can be provided to designers is not sufficient. In this case, on the one hand, it is possible to cause early failure due to the high stress level of the design. On the other hand, it is also possible to design too conservative and cause unnecessary waste. For example, thermal power plants typically have a design life of 100,000 h. In China, there are many 540-degree, 10MPa high-pressure boilers, the main steam pipeline has been designed to achieve the design life, but according to recent life estimates, it is safe to extend the service life of these boilers to 200,000 h. In general, this form of failure takes a long time, so the reaction speed is slow, and effective measures are further determined based on the testing and accumulation of creep properties of the material. 2. Strict quality management in manufacturing, avoiding parts assembly products that do not meet technical specifications, which is especially important for products with long failure periods. Of course, specific measures should be based on failure analysis in product service. 3. The use of overload measures is a common cause of product creep failure. Therefore, strict control of the use conditions during use is the most important measure to improve product life and reliability. Strengthening the monitoring of the products being serviced and the quality of key parts is an effective measure to ensure product reliability. The eye nut refers to the part that the nut and bolt or Screw are screwed together to play a fastening role. It is an essential part for all production and manufacturing machinery. The lifting eye nut is a hanger commonly used in engineering. There is a screw thread under the nut, which can be drilled according to its different specifications and fixed by the screw mouth. 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