FFS Forum: Creep in Heater Tubes and Other Components – Part I: Understanding Creep

Editor’s Note: This is the first of a two-part column on “Creep”. It presents an engineering overview of the phenomena, answers the question: “What is Creep?”, and provides readers with foundational knowledge for evaluating and managing remaining equipment life. Part two of this series, to be published in the July/August issue of Inspectioneering Journal, will discuss how to assess creep damage in plant equipment.

What is “Creep”?

It’s probably good to start with a definition.

Creep is not yielding; creep can occur at stresses well below yield. The onset of creep can occur over a wide range of temperatures for different materials. For example:

• Lead can creep near room temperature
• Ice can creep below 30°F
• Steel can begin to creep around 650°F, or up to 1200°F for some high temperature alloys

Generally, for metals, the effects of creep begin to become noticeable at around 35% of the absolute melting temperature of the metal. For ceramics, creep begins around 45% of the melting temperature.

To get a good mental picture of creep in action, consider a carbon steel rod with a weight hanging on the bottom, inducing a stress of 5 ksi (Figure 1a).

In Figure 1(b), where load is applied at low temperature (i.e., below the creep range), there is no time-dependent aspect to the deformation – it’s all elastic. It can sit for 50 years, but when you remove the load, the bar will elastically return to its original length. Contrast that with Figure 1(c), where the same load is applied at a high temperature (i.e., above the creep range, but still below yield at temperature). Over time, the bar continues to stretch. Given enough time and/or stress, the bar will continue to stretch until it breaks. For this particular material, temperature, and stress, the creep life is predicted to be a bit over 100 years.

For a given material, increasing the stress and temperature results in increasing creep strain over time. A typical creep-time relationship is shown in Figure 2.