What is metal creep? What are its influencing factors?

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The phenomenon in which a metal, under the combined action of high temperature and stress for an extended period of time, maintains a constant stress while its plastic deformation increases gradually over time, is known as metal creep. High temperature, stress, and time are the three elements required for creep to occur. The greater the stress and the higher the temperature, and the longer the exposure time at high temperatures, the more severe the creep becomes. Under what circumstances does one need to consider the issue of creep in pressure vessels? How is creep design carried out? The necessary condition for creep to occur is high temperature. Therefore, whether to consider the creep issue in the design depends first on the temperature of the metal. Generally, the issue of creep should be considered at 400°C for carbon steel, at 450°C for low-alloy steels, and above 600°C for heat-resistant alloys. In the creep design of pressure vessels, codes in various countries adopt a simple approach: the equivalent stress within the wall is determined using the design formulas for pressure vessels at normal temperatures, while the allowable stress is taken as the smaller of the creep limit and endurance limit of the material at high temperatures, each divided by the corresponding safety factor.