Page 57 - Handbook of Materials Failure Analysis

1 Introduction 51

maximum percentage of tube failures [3,7] are: Short-term overheating, high-
temperature creep, caustic corrosion, and hydrogen damage.

1.1 SHORT-TERM OVERHEATING
Stress and temperature influence the useful life of tubing steels operating in a boiler.
The strength of a boiler tube is dependent on the level of stress as well as temperature
when the tube metal temperatures are in the creep range. Since an increase in either
stress or temperature can reduce the time to rupture, attention must be given to both
factors when investigation is to be carried out regarding failure caused by stress-
rupture mechanism. Stress-rupture failure mechanisms are predominantly experi-
enced in steam cooled superheater and reheater sections where the operating temper-
ature is in the creep range. Stress rupture can also be experienced in water cooled
tubing if abnormal heat-transfer conditions exists that results in an increase in the
tubes operating temperature [8,9]. If the thickness of the tube decreases by corrosion,
or erosion, the hoop stress will increase and hence the likelihood of failure. The cir-
cumferential hoop stress in a tube is determined by the diameter and thickness of the
tube as stated in the following Equation 3.1.
σ H ¼ PD M =2W (3.1)
where: σ H ¼estimated hoop stress; P¼internal stress pressure; D M ¼mean tube
diameter; W¼tube wall thickness.
The “overheating failure” means a failure resulting from operation of a tube at
temperature higher than expected in design selection of the tube steel for a period
of time sufficient to cause a stress-rupture failure. Time at temperature is an impor-
tant factor and these types of failures are called “short-term” and “long-term” over-
heating. Figure 3.2 shows the locations of the boiler where short-term overheating
can occur and Figure 3.3 shows thick-edged short-term overheating type failure
of a boiler tube.

1.2 HIGH-TEMPERATURE CREEP
Boiler tube failure can result from high-temperature creep of superheater and rehea-
ter tube steel. Metal degradation and permanent deformation will occur with time
depending on the actual stress level and temperature. If temperatures and stresses
exceed design selection values, the tube steel will exhibit a higher creep rate and will
fail earlier than expected.
High-temperature creep failures are called “long-term” or extended overheating
failures. Such a failure results from a relatively continuous extended period of slight
overheating, a slowly increasing level of temperature or stress, or accumulation from
several periods of excessive overheating. The creep occurs along the grain bound-
aries of the steel and is aligned 90° from the direction of applied tensile stress. Creep
deformation results in little or no reduction in wall thickness but produces measur-
able creep elongation or increase in diameter in ferritic steel tubes. The creep