What is the phenomenon of creep failure of fasteners?
1, Creep is a failure phenomenon in which metal parts undergo deformation under long-term stress and high temperature. The deformation caused by grain sliding along grain boundaries is the main mechanism of creep. When the deformation temperature rises to 0.35~0.7 Tm (Tm is the melting point temperature), the thin layer area near the grain boundary undergoes recovery and softening, forming a transition. After deformation, distortion occurs again, so it needs to be restored and softened again to maintain the deformation in these areas, which is called grain boundary sliding. Due to the temperature and time required for recovery, grain boundary sliding can only occur under conditions above a certain temperature.
The metal tensile creep curve is divided into three stages:
In the first stage, the creep rate gradually slows down from fast, which is related to the redistribution of crystal defects.
In the second stage, it indicates that the two mechanisms of hardening and recovery are in equilibrium, with a constant creep rate. This stage accounts for a significant proportion of the entire creep process.
In the third stage, it is manifested as an increase in creep rate, at which point the deformation hardening of the metal is no longer sufficient to prevent deformation, and the reduction in effective cross-section promotes an increase in creep rate, leading to fracture.
Not all materials exhibit the above three stages in their creep curve. The phenomenon of failure caused by changes in the size of pre tensioned parts during the creep process is called thermal relaxation. Bolts used for fastening pressure vessel flanges may elongate due to creep under long-term effects of temperature and stress, resulting in a decrease in preload and potentially causing leakage of the pressure vessel.
2, The main characteristic and judgment of creep is that the deformation speed is very slow. It can be analyzed based on the specific working conditions of the parts to determine whether there are conditions for creep (temperature, stress, and time). Without appropriate temperature and sufficient time, creep or creep fracture will not occur. On the final fracture zone of the creep fracture, the tearing ridge is not as clear as on the tensile fracture at room temperature. Under scanning electron microscopy, the grain shape near the creep fracture often does not show elongation, while at high magnification, creep voids can sometimes be seen.
3, The identification methods for creep failure are thermal relaxation and plastic deformation, both of which have residual deformation at the macroscopic level and are easily confused. Plastic fracture and persistent fracture (or creep fracture) are easily confused because, macroscopically, there is deformation before fracture and necking near the fracture surface. The differences can be considered from the following aspects.
1. The differences in working conditions are well known. Plastic deformation and plastic fracture occur under tensile stress, with a faster process and lower temperature. Thermal relaxation and persistent fracture are failure processes in which temperature and time play important roles. Higher operating temperatures and longer service times are necessary conditions for this failure mode. For understanding the working conditions, in addition to consulting written materials, directly check whether there are traces of high temperature, such as oxidation color, on the wreckage. When analyzing working conditions, one should be very cautious. For example, a high-temperature pressure vessel has been working at a low pressure for a long time, and suddenly the pressure rises, causing the connecting bolts to break. Only by specifically understanding the relevant pressure, temperature, and service time under different working conditions can one determine whether there is a creep failure.
2. The difference in fracture morphology is that the ductile dimples on the plastic fracture surface are very clear, and the areas where micropores aggregate are relatively sharp. Under scanning electron microscopy, these areas appear as bright white lines. On the creep fracture surface, the areas where micropores aggregate are relatively dull, and under scanning electron microscopy, there are no obvious white bright lines in these areas. On the creep fracture surface, oxidation color may be observed, and sometimes creep pores can also be seen.
3. The microstructure creep near the fracture surface is mostly intergranular fracture, while plastic fracture is mostly transgranular fracture. In samples that have undergone creep, it is possible to see creep pores. In addition, carbon steel remains at high temperatures for a long time, and carbides undergo a certain degree of stone grinding.
4, Measures to improve creep resistance
1. In terms of design, it is crucial to correctly select materials and determine part dimensions based on the characteristics of the product. In recent years, many new materials have been developed to meet the increasing requirements of product temperature and load, but the creep performance data that can be provided to designers is not sufficient. In this case, on the one hand, early failure may occur due to the high stress level of the design. On the other hand, it is also possible that the design is too conservative, resulting in unnecessary waste. For example, the design lifespan of a thermal power station is generally 100000 hours. In China, many main steam pipelines of 540 degree, 10MPa power plant high-pressure boilers have successively reached their design life. However, according to recent life estimates, it is possible to confidently extend the service life of these boilers to 200000 hours.
Generally speaking, this failure mode requires a long time, resulting in a slow response speed. An effective measure is to further study and determine based on the testing and accumulation of material creep properties.
2. Strict quality management is implemented in manufacturing to avoid assembling products with parts that do not meet technical specifications, which is particularly important for products with longer failure cycles. Of course, specific measures should be formed based on the failure analysis during product service.
3. Measures taken during use: Overloading is a common cause of creep failure in products. Therefore, strict control of usage conditions during use is an extremely important measure to improve product life and reliability. Strengthening the monitoring of the quality status of products in service and key components is an effective measure to ensure product reliability.
