No, this article is not about creepy people. In boiler-lingo, creep is the term for the distortion or warping of components due to high temperatures and constant stress.
The end of useful service life of high-temperature boiler components, like the superheater and reheater tubes and headers, is usually a failure caused by creep or a stress-rupture. An understanding of how high-temperature materials behave is beneficial in evaluating failure in these types of systems.
Usually the first signs of creep damage in a superheater of reheater tube are longitudinal cracks in the steam-side scale. As creep deformation expands the tube diameter, the brittle ID scale cannot follow the expansion. Cracks develop in an axial or longitudinal direction which is perpendicular to the principle hoop stress. With time, the tube continues to expand, and these cracks widen. This wide crack shortens the path from steam to steel, iron oxide forms preferentially at the tip of the crack, as there is less oxide thickness to protect the steel, and a cusp forms within the steel tube. The cusp acts as a notch or stress raiser, reducing the local wall thickness. Creep voids form here, often before any other obvious grain-boundary damage appears elsewhere within the microstructure. With continued high-temperature operation, creep cracks grow from the cusp and ultimately weaken the cross section to the point where it fails.
Failures involving creep are easy to identify because of the type of deformation that occurs. It may appear ductile or brittle. Cracking can be either transgranular or intergranular. While creep testing is done at constant temperature and load, actual components may experience damage at various temperatures and loading conditions.
Depending on the type of metal, creep can occur at various temperatures:
- Carbon steel = 800°F
- Carbon + ½ Molybdenum = 850°F
- 1¼ Chromium – ½ Molybdenum = 950°F
- 2¼ Chromium – 1 Molybdenum = 1000°F
- Stainless steel = 1050°F
The first two stages of creep will not leave any microstructural evidence damage. The first evidence of damage will appear as individual voids or pores, often in the junction of three or more grains at nonmetallic inclusions. These voids or pores grow in size until they connect and form longer cracks until failure occurs. The final rupture occurs by a tensile overload, when effective wall thickness is too thin to contain the steam pressure.
There are several things to look for to predict creep failures:
- Building of blisters in the tube
- Thick-edged fractures, often with very little obvious ductility
- Longitudinal “stress cracks” in either or both ID and OD oxide scales
- External or internal oxide-scale thicknesses that suggest higher-than-expected temperatures
- Intergranular voids and cracks in the microstructure
Creep failures are expected for superheaters and reheaters when operating at design conditions, but even a slight deviation from the appropriate parameters can cause premature failures. In individual tubes, the temperature of the steam will always vary slightly, and the design should allow for some variability. However, if the range of temperatures is larger than accounted for, the highest temperature tubes will fail sooner than expected.
Information for this article was taken from The National Board of Boiler and Pressure Vessel Inspectors. More information can be found at www.nationalboard.org.