What is creep in metals? What are its influencing factors? In what situations must creep be considered for pressure vessels? How is creep design carried out?

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What is metal creep? What are its influencing factors? Metal creep refers to the phenomenon in which, when a metal is subjected to high temperatures and stress for an extended period of time with the stress remaining constant, its plastic deformation increases gradually over time. 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 must creep issues be considered for 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, creep must be considered for carbon steel at 400°C, low-alloy steels at 450°C, and heat-resistant alloys at temperatures above 600°C. 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. The allowable stress is taken as the smaller value between the creep limit and the creep-rupture limit of the material at high temperatures, each divided by the respective safety factor.