This month we continue our #epicfail series on 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

Primary creep – is the initial stage of creep that occurs immediately upon the application of a stress. During primary creep, the material undergoes strain hardening, which occurs as dislocations accumulate and interact within the material’s microstructure. This results in a reduction in the rate of deformation as the material becomes stronger and less prone to further deformation. The material behaves in a manner similar to elastic deformation, but with a gradual increase in plastic strain over time, leading to a reduction in the creep rate as strain hardening counteracts further deformation.

Secondary creep – also known as steady-state creep, is the stage where the creep rate becomes constant over time. This stage typically lasts the longest and is the most significant in terms of material degradation. During secondary creep, the rate of deformation stabilises, and the material deforms at a constant rate. This occurs when the processes of dislocation motion, recovery, and recrystallisation are in balance, resulting in a steady-state creep rate. Microstructural changes, such as spheroidisation in carbon and low alloy steels, and carbide and intermetallic precipitation in austenitic materials, can lead to grain boundary sliding and dislocation motion. These processes contribute to a steady creep rate, with minimal to no increase in the rate of deformation over time.

Tertiary creep – is the final stage of creep, in which the rate of strain increases rapidly. This stage marks the accelerating phase of creep, leading to the ultimate failure of the component. In tertiary creep, the rate of strain increases significantly as microscopic internal pores, voids, and tears begin to form within the material’s structure. These voids and cracks gradually link together, causing the material to lose its structural integrity. As the internal damage accumulates, the material weakens and becomes more prone to fracture, eventually leading to rupture or complete failure of the component.

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.

The maximum allowable design temperature for a metal component is largely determined by the material composition of the tube or structural part. Materials with higher chromium and molybdenum content typically have improved high-temperature strength, making them more resistant to creep, oxidation, and other thermal damage, see the table.

Preventative measures can be undertaken to reduce the likelihood of creep failures, examples of which are as follows:

• One of the most important measures for preventing creep damage and thermal degradation is to ensure that the system is not subjected to temperatures above the design limit.
• During the design and fabrication of high-temperature components, care should be taken to avoid the introduction of stress concentrators, such as sharp corners, notches, or improper weld designs. These areas can lead to localised stress concentration, making the component more susceptible to creep failure and cracking under operating conditions.
• The use of alloys with improved creep resistance is crucial for high-temperature applications. Alloys such as high-chromium, molybdenum, and nickel-based alloys offer superior resistance to thermal degradation, creep, and oxidation.
• Regular cleaning of the process side (water/steam side) is essential to prevent fouling and deposit build-up. The build-up of materials such as ash, soot, or corrosion products can reduce the coolant flow and result in overheating.
• Fire-side scaling should also be minimised. This can be achieved through chemical or mechanical cleaning methods, which help to prevent thermal degradation and improve the efficiency of heat transfer.
• Regular inspection is crucial for detecting early signs of damage and preventing catastrophic failure:
• Replication Metallography: This technique involves taking a carbon copy of the material’s microstructure, allowing for the identification of early signs of creep damage and microstructural degradation.
• Thickness Measurements: Ultrasonic thickness measurement can be used to monitor the wall thickness of critical components.
• Magnetic Particle Inspection (MPI): MPI can be used to detect surface cracking and fissures in critical components, particularly those exposed to high stress and temperature. This technique is valuable for detecting creep-related cracking that may not be visible to the naked eye.