Creep

 Creep in superheater tubes is the gradual and long-term deformation of the metal due to continuous exposure to high temperatures and pressures. This slow, irreversible process is a major cause of failure in high-temperature boiler components, including superheaters, reheaters, and steam pipes. 

Mechanism of superheater tube creep

The creep mechanism is accelerated by factors such as the formation of oxide scale, which insulates the tube and raises the metal temperature. This leads to microstructural degradation and eventual rupture. 

1. High operating temperature: Superheater tubes operate at very high temperatures, typically above
   

   ${482}^{\circ }C$
   (
   

   ${900}^{\circ }F$
   ). At these temperatures, materials are less able to withstand stress, and creep deformation becomes a significant concern.
2. Sustained internal pressure: The tubes are under constant internal pressure from the steam they contain. The material, weakened by high temperature, begins to deform slowly over time, a process driven by a constant internal hoop stress.
3. Oxide scale formation: A layer of magnetite (
   

   $F{e}_3{O}_4$
   ) forms on the inner (steam-side) surface of the tubes during service. This layer is an effective insulator with a thermal conductivity significantly lower than steel.
4. Increasing metal temperature: As the internal oxide layer thickens, it reduces heat transfer from the tube to the steam. To maintain a consistent steam temperature, the heat input is increased, which drives up the metal temperature. Studies show that a small increase in oxide thickness can raise the metal temperature by tens of degrees.
5. Accelerated deformation and thinning: The higher metal temperature accelerates the rate of creep deformation. The tube begins to expand, or bulge, and its wall thickness decreases.
6. Stress cracks in oxide scale: As the tube expands, the brittle internal oxide layer cracks in a longitudinal direction. This exposes fresh metal to the steam, accelerating the formation of oxide scale.
7. Microstructural changes: Prolonged exposure to high temperatures also causes the steel's microstructure to degrade. For example, carbides in the steel may spheroidize or aggregate at grain boundaries. This weakens the material and facilitates the formation of creep voids.
8. Creep void formation and coalescence: Voids, or microscopic cavities, form primarily along the grain boundaries of the steel. As creep progresses, these voids link up to form microcracks and eventually macroscopic cracks.
9. Rupture: The creep cracks ultimately weaken the tube's cross-section to the point of failure. The classic "fish-mouth" rupture, a longitudinal split with thin or thick edges, is the typical result of a creep failure.

Factors that influence creep life 

The rate of creep and the remaining service life of a superheater tube are affected by multiple factors: 

• Operating temperature and pressure: Small increases in temperature or pressure can drastically reduce the creep life of a tube. An increase of just
  

  ${33.3}^{\circ }C$
  (
  

  ${60}^{\circ }F$
  ) can shorten a tube's lifespan by 90%.
• Oxide scale thickness: The insulating effect of the steam-side oxide scale is a primary driver of increased tube temperature. Monitoring this scale provides an indication of the remaining creep-rupture life.
• Material properties: The choice of steel is critical. Alloys with superior creep resistance, such as certain chromium-molybdenum steels, are used in higher-temperature sections of superheaters.
• Flow conditions: Blocked or restricted steam flow due to scale exfoliation can cause localized overheating, which accelerates creep failure.
• Design and manufacturing: Factors like poor tube material selection, excessive temperatures, or welding defects can contribute to premature creep failure