Creep failure of fasteners, creep failure phenomenon of […]
Creep failure of fasteners, creep failure phenomenon of fasteners is how is it?
I. Overview Creep is the failure phenomenon of permanent deformation of metal parts under the long-term effect of stress and high temperature. Grain deformation along the grain boundary is the main mechanism of creep. When the deformation temperature rises to 0.35 to 0.7 Tm (Tm is the absolute temperature of the melting point), the thin layer region near the grain boundary recovers and softens, and the shape becomes stable. After deformation, distortion occurs again, and then it is necessary to recover and re-soften so as to keep the deformation continuing in these areas. This is called grain boundary sliding. Because the recovery requires a certain temperature and time, the grain boundary sliding can only be performed under a certain temperature.
Metal tensile creep curve, which is divided into three phases:
In the first stage, the creep rate gradually slows down, which is related to the redistribution of crystal defects.
The second stage shows that the two mechanisms of hardening and recovery are in equilibrium and the creep rate is constant. This stage takes up a large proportion of the entire creep process.
In the third stage, the creep rate is accelerated. At this time, the metal's deformation hardening 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.
The above three stages do not appear in any creep curve of any material, and the phenomenon of failure due to the creep process changing the dimension of the pre-tensioned part is called thermal relaxation. For example, the bolts used for the flanges of fastener pressure vessels are prolonged due to creep under the long-term effects of temperature and stress, leading to a decrease in the pre-tightening force, which may cause leakage of the pressure vessel.
Second, characteristics and judgments The most important feature of creep is the slow speed of permanent deformation. The condition of the creep (temperature, stress, and time) can be analyzed based on the specific conditions of the part. Without proper temperature and sufficient time, creep or creep rupture will not occur. In the final rupture zone of the creep fracture, the torn ridge is not as clear as the normal temperature tensile fracture. Observed under the scanning electron microscope, the grain shape near the creep fracture often does not appear elongated, but at high times, sometimes Creep holes can be seen.
Third, the identification method of creep failure Thermal relaxation and plastic deformation, from the macroscopic residual deformation are easily confused. Plastic fractures and long-lasting fractures (or creep fractures) are easily confused because, from a macroscopic point of view, there are permanent deformations before fracture and necking near the fracture. The difference can be considered from the following aspects.
1, the difference in working conditions It is well known that plastic deformation and plastic fracture occur under the action of tensile stress, the process proceeds faster and the temperature is lower. Thermal relaxation and long-lasting fracture are the failure processes where temperature and time play an important role. Higher operating temperatures and longer service times are necessary conditions for this failure mode. For the understanding of working conditions, in addition to checking the textual information, directly check whether there are any hot spots on the remains, such as oxidation color. Be cautious when analyzing operating conditions. For example, a high-temperature pressure vessel is working under a relatively low pressure for a long time. Suddenly, the pressure rises, causing the connecting bolt to break. Only specifically understand the relevant pressures, temperatures and differences. The service time under working conditions can determine specifically whether the serve creep fails.
2. The difference in fracture morphology The dimples on the plastic fracture are very clear, and the sites of micropore polymerization are sharp. These areas are observed with scanning electron microscope to show white and bright lines. On creep fractures, the microporous polymer is relatively blunt. Observed under scanning electron microscopy, these sites have no obvious bright white lines. On creep fractures, it is possible to see oxidized colors and sometimes creep holes.
3, the microstructure of the fracture near the metallographic structure is mostly intergranular fractures, and plastic fractures are mostly transgranular fractures. Creep samples may see creep holes. In addition, carbon steel stays at high temperatures for a long time, and carbides are stoned to a certain degree.
Fourth, to improve the creep resistance measures The design aspect is based on the characteristics of the product, and the correct choice of materials and the size of the parts are crucial. In recent years, many new materials have been developed to meet the requirements for increasing the use temperature and load of products, but the creep performance data that can be provided to designers is not sufficient. In this case, it may happen on the one hand that early failure due to high stress levels in the design. On the other hand, the design may be too conservative and cause unnecessary waste. For example, the thermal power plant design life is generally 100,000 hours. In China, there are a lot of 540 degrees, 10MPa power station high pressure boiler main steam pipeline appearance has successively reached the design life, but according to recent life estimates that can be sure to extend the service life of these boilers to 200,000 h.
In general, this type of failure takes a long time, so the response speed is slow, and effective measures are based on the testing and accumulation of the creep properties of materials, and further research and decisions are made.
2. Strict quality management in manufacturing and avoidance of parts assembly products that do not conform to technical specifications are particularly important for products with long periods of failure. Of course, specific measures should be formed on the basis of failure analysis in product service.
3. The use of overloaded measures is a common cause of product creep failure. Therefore, the strict control of use conditions during use is the most important measure to improve product life and reliability. Strengthening the monitoring of the quality of products in service and critical parts is an effective measure to ensure product reliability.