When I think back to when I first started in the industry over 18 years ago, I remember hearing the word “creep” discussed on numerous occasions. I would chuckle to myself, and think “Creep? What type of industry have I gotten involved in? Are there scary people looming around that I need to be careful of; and if so, where are they?”. I learned very quickly that “creep” is a form of damage that effects steel materials that are under high temperature and stress over a period of time, reducing the yield strength of the material. As I have come to realize, however, these two types of creep have some similarities. They can sneak up on you, often going unnoticed until it is too late. They can cause people who are aware of their existence to experience uneasiness and even fear. If they are not avoided or monitored, they can cause severe harm or injury.
I recently presented at the ASME conference in Baltimore. In his speech, something that the keynote speaker, James Chiles, author of “Inviting Disaster” stated was entirely fitting when considering creep. Mr. Chiles investigated disasters in many different industries and found several commonalities that occurred in all instances. He said that “failure never happens out of the blue, it propagates from flaws that eventually link up.” This is a very interesting statement when we examine the development of creep, as illustrated in the diagram below.
Creep at elevated temperatures frequently is identified by a curve, which shows four portions. The first part involves the initial extension (or expansion) at high temperatures. This extension is partially elastic. In the next stage, the extension (or expansion) rate decreases with time. This phase is also called a transient or primary creep stage. The primary creep stage is not considered to represent damage.
Subsequently, a stage occurs on the creep curve located where the rate of creep (expansion) is nearly constant. This stage is also called secondary creep. The period of secondary creep after some development (i.e., in the latter period of the stage) is evidenced by initial void formation along the grain boundaries. In time, these tend to join or “link up”. Void formation tends to develop after approximately 50% of the life of the pipe material has been consumed. Consequently, as recognized by Mr. Chiles, the linking up of these “flaws” will eventually lead to failure if not monitored for progression.
The fourth stage of the creep curve, located beyond the constant creep rate portion, involves a rapidly increasing creep rate. This stage is called tertiary creep. It includes grain boundary fissuring (or microcracking). After this stage is reached, the material would tend to develop failure. This may represent a remaining life period of 5% to 10% of the prior operating period. Thus, when the material reaches the tertiary creep stage, the end of the useful life of the steel material, at the particular area of high-stress levels, is being approached.
In many components, the tertiary creep stage may not be reached until after 200,000 to 500,000 hours of operation, or longer. However, in other instances, particularly involving overheating of Superheater tubes, the tertiary creep stage has been reached after only 10,000 to 50,000 hours of operation. Such overheating, however, would require temperatures of 1100°F to 1200°F. These are higher than most applicable design temperatures in modern power plants.
It is imperative to conduct routine non-destructive examinations on the steel components in your facility that are exposed to high temperatures and pressures to determine if creep damage has begun, and if so what stage it is in. Determining the stage of creep provides insight into the remaining useful life of the equipment, giving you the ability to be proactive in preventing failures caused by creep damage.
Avoid allowing “creep” to loom around the corner and sneak up on you! For more information on non-destructive examinations for creep determination, please contact Peter Kennefick at Pkennefick@thielsch.com.