1. Overview Creep is a failure phenomenon in which me […]
1. Overview Creep is a failure phenomenon in which metal parts undergo deformation under the long-term effects of stress and high temperature. The main mechanism of creep is the deformation of crystal grains sliding along the grain boundary. When the deformation temperature rises to 0.35~0.7Tm (Tm is the temperature of the melting point). Distortion occurs again after deformation, so it is necessary to rehabilitate and re-soften to keep the deformation continuing in these areas. This is the so-called grain boundary sliding. Because rehabilitation requires a certain temperature and time, the grain boundary sliding must be adjusted only when the temperature is higher than a certain temperature.
The metal tensile creep curve, is divided into three stages
In the first stage, the rate slows down, which is related to the de novo dispersion of crystal defects.
The second stage indicates that the two mechanisms of hardening and rehabilitation are in equilibrium, and the creep rate is stable. This stage occupies a larger share in the whole process of creep.
In the third stage, the creep rate is accelerated. At this moment, the deformation hardening of the metal is not enough to stop the deformation of the metal, and the reduction of the useful cross-section promotes the acceleration of the creep rate and causes cracking. Not all the creep curves of any data show the above three stages. The phenomenon that the creep process changes the specifications of the preloaded parts and causes the failure is called thermal relaxation. For example, the Suzhou high-strength bolts used in the pressure vessel flange of China fasteners, under the long-term effects of temperature and stress, are elongated due to creep, resulting in a decrease in the pre-tightening force, which may constitute a leakage of the pressure vessel.
Second, the characteristics and distinguishing the main feature of creep is that the speed of deformation is very slow. It can be analyzed according to the detailed working conditions of the parts, whether there are conditions (temperature, stress, and time) for creep. Like Suzhou stainless steel bolts, there will be no creep or creep cracking when there is no suitable temperature and satisfying moment. In the final cracking zone of the creep fracture, the tearing ridge is not as clear as that on the normal temperature tensile fracture. In the scanning electron microscope, the shape of the grains adjacent to the creep fracture often does not appear elongated, and at high magnification, sometimes Can see the creep empty.
Third, the method of identifying creep failure is thermal relaxation and plastic deformation. From the microscopic point of view, there are residual deformations that are simply thick and turbid. Plastic cracking and durable cracking (or creep cracking) are simply thick and turbid, because from a microscopic point of view, there is deformation before cracking, and there is a 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 cracking occur under the effect of tensile stress, the progress is faster, and the temperature is lower. Thermal relaxation and persistent cracking are the failure processes with important effects on temperature and time. Higher operating temperature and longer service time are the necessary conditions for this failure method. For the understanding of working conditions, in addition to consulting textual data, directly check whether there are high-temperature traces on the wreckage, such as oxidation color.
2. The dimples on the different plastic fractures depicted on the fracture are very clear, and the parts where the micropores are aggregated are sharper. Check these areas with bright white lines under the scanning electron microscope. On the creep fracture surface, the areas where the micropores are aggregated are relatively blunt, and when checked under the scanning electron microscope, there are no significant white and bright lines in these areas. On the creep fracture, there may be oxidized color, and sometimes creep holes can be seen.
Suzhou high-strength bolt 3, the metallographic arrangement near the fracture, the creep is mostly intergranular cracking, and the plastic cracking is mostly transcrystalline cracking. Creep holes may be seen in the creep samples. In addition, if the carbon steel stays at a high temperature for a long time, the carbides will undergo a certain degree of stone grinding.