Creep is a topic in metallurgy that can be quite confusing; while not common knowledge to many people, the subject of creep is one that those in charge of selecting metals must be aware of. Creep is a type of deformation, and metals that undergo excessive creep deformation can oftentimes fail completely. Unfortunately, creep-related failures happen frequently and are hazardous to both property and human life. Being informed during the metal selection process can help avoid metal failure due to creep and the serious consequences that come with it.
What is Creep?
Creep is a type of metal deformation that occurs at stresses below the yield strength of a metal, generally at elevated temperatures. One of the most important attributes of any metal is its yield strength because it defines the stress at which metal begins to plastically deform. Creep is unique in the fact that it is a phenomena that causes materials to plastically strain even though yield stresses have not been reached. The mechanics behind creep are complicated, but they can be broken down into three stages.
Stage 1: Primary Creep
Primary creep occurs first during the deformation process. At this stage, elastic deformation is initialised. Elastic deformation occurs from atomic bond stretching and is not permanent. Following the elastic deformation, permanent plastic deformation starts to take place. During the primary creep stage, this deformation occurs more rapidly at first and then slows with time. The reduction in the creep rate that occurs near the end of the primary creep stage is due to work hardening.
Stage 2: Secondary Creep
Secondary creep begins once the strain rate begins to stabilise and becomes constant. The strain during secondary creep occurs relatively slow when compared to the first stage and the third stage of creep. The creep rate remains constant and relatively slow because no microstructural damage has taken place yet.
Stage 3: Tertiary Creep
Tertiary creep is the final phase of the creep deformation process. This stage of the creep process begins once damage to the microstructure of the metal takes place. The strain rate accelerates as more and more deterioration of the microstructure continues to happen. After enough microstructural voids have been created, the metal eventually fractures and fails completely.
Common Instances of Creep
Creep is commonly found in some applications more than others. For instance, automobile frames are designed more with impact strength in mind since their static loads are small and normal operating temperatures are low. On the other hand, certain automobile engine components subjected to high loads and temperatures from engine combustion may experience creep if the right material is not selected.
Typically, applications that have high heat and high stress can be susceptible to creep. Examples include nuclear power generation, industrial engine components, heated metal filaments, jet engine components, and pressurised high-temperature piping.
How to Avoid Creep
The effects of creep can be circumvented or reduced through several different methods. One way to reduce creep is to lower the working temperature of the metal being used, although this is not always possible. Another method is reducing the constant load the metal has to withstand, but again, this may not be achievable depending on the application. Using a metal with large grains can reduce creep because less grain boundary sliding occurs. Certain metals with specific alloying element additions can avoid creep by eliminating microstructural vacancies.
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