What is Creep?
Creep is a process of degradation observed in steels and other materials used for pressure equipment. Creep is one of the most complicated processes of degradation observed in pressure equipment.
The pressure vessel will expand when a pressure vessel is subjected to internal pressure and is made of a material that displays creep. If during expansion the internal pressure is kept constant, the load on the wall will increase. The thickness of the wall reduces simultaneously.
The result of these two simultaneous effects is that the vessel’s expansion is constantly accelerated until the thickness of the wall has decreased and the load has increased to such an extent that the material’s strength is no longer sufficient and the vessel fractures.
Creep is dependent on two parameters: temperature and time.
What damage does creep cause?
The primary impact is a steady and gradually deteriorating decrease in tensile strength. The creep mechanism leads the composition of the metal to flow leaving holes or voids in the matrix of the material. Therefore, structures under pressure can deform and then fail.
How is creep detected?
Deformation is recorded at specified time intervals and a creep versus time diagram is plotted. The slope of the curve at any point is the creep rate.
A sample is deformed to determine the stress relaxation of
Using metallographic replication (often simply termed a ‘replica’).
This is a non-destructive method that allows a visual examination of the grain structure of the material to search for the vacuum that is proof of creep harm. The method includes:
- Preparation of the metal surface by mechanical
cleaning / milling and chemical etching - bond a special plastic tape to the surface and allow the adhesive to cure
- Remove the tape and transfer the metal surface replica to the tape
- Examine the replica under low magnification and compare with reference microstructure pictures.
