This month we continue our #epicfail series on creep failure mechanisms looking at the impact of operating at high temperature on materials properties.
Creep is the time dependent deformation of a material at elevated temperature under a constant stress. Depending on the applied stress, duration and temperature, this deformation may increase to the point where failure occurs. Creep of a turbine blade may cause the blade to contact the casing, resulting in failure of the blade. Alternatively, a boiler tube may reach a critical effective cross section where the material can no longer withstand the applied stress and therefore failure occurs, see Figure 1. In June 1985, a boiler explosion at the Mohave Power Plant in Laughlin, Nevada claimed the lives of 6 workers when a 30” diameter pipe ruptured catastrophically. The failure was thought to be a combination of issues, one of which being creep.
Figure 1 Failure of a boiler tube due to creep
There are several types of creep failure which can be characterised as follows:
Intergranular creep failure
This occurs after long-time exposure to temperature and stress. Early stages of long-term creep manifest as voids at the grain boundaries, these then subsequently link to form grain boundary fissures/cracks. As a result, there is little reduction in cross sectional area and a thick-walled fracture occurs. Non-destructive replication metallography is an effective means of determining the presence of long-term creep damage.
Furthermore, the platelets of iron carbide in the pearlite structure of carbon steels will thermally degrade to spheroidised iron carbide as a result of long-term overheating. Continued decomposition in plain carbon steels can result in total degradation to graphite plus ferrite. This degradation can also be detected using replication metallography.
Transgranular creep fracture
This type of fracture can occur in short-time creep failures. The ductility and reduction in area are usually large and much greater than at room temperature, producing a bulged, thin-walled fracture.
Point rupture fracture
At sufficiently high temperatures and low stresses, recrystallization during creep can remove microstructural creep damage. As a result, voids do not nucleate, and necking down to a point can occur.
Additions of chromium and molybdenum in steels can increase creep life. Mechanical or chemical cleaning is generally used to remove deposit build-up in boiler tubes which reduces the risk of local hot spots. An appropriate inspection programme which includes monitoring wall thickness loss, microstructural degradation and creep damage is also an effective means of reducing the likelihood of creep failure.