Five case studies involving waterwall and roof tubes from conventional high-pressure boilers are presented. Each tube was affected by cracking initiated on the waterside surface. The cracking was associated with cyclic thermal stresses. In each case, results of visual examination, waterside deposit density and elemental composition, and optical metallography are provided. The effects of the environment and cyclic thermal stresses on the crack morphology are discussed.
Cyclic thermal stresses can contribute to waterwall tube cracking. Thermal stresses have been defined as "stresses in metal resulting from nonuniform temperature distribution" 1. The stresses can be grouped into the following categories for waterwall tubes:
Expansion of two pieces of metal at different rates. For example, weld attachments where thermal expansion or contraction of one or both pieces of metal results in tensile stresses (strain) on the tube interior or exterior. Pad welding and dissimilar metal weld overlay also can add to surface stresses. During operation, the resultant stress can be fixed or cyclic. Stressed surfaces are more susceptible to cracking and corrosion (stress-assisted corrosion).
Sudden longitudinal expansion or contraction of the waterwall tube causing brittle fracture of the protective oxide coating. Where the oxide is ruptured, oxide rapidly reforms on the exposed metal surface. If this cycle is repeated many times or if the environment is particularly corrosive, significant corrosion can result. Corrosion results in thinner walls, and thinner walls result in higher hoop stresses which increases the potential for cracking. Excessive cyclic stresses can result in predominantly cracking mechanisms (e.g., thermal fatigue cracking or circumferential waterwall cracking). In waterwall tubes, corrosion often is a contributing factor due to the corrosivity of the fireside environment of a boiler burning coal or other mixed fuels (e.g., black liquor, refuse-derived fuels, etc.) or the rapidity of the iron/water reaction on the waterside surfaces of all boilers and heat recovery steam generators via the Schikorr reaction 2. Cracking usually is circumferential.
Varying hoop stress caused by temperature fluctuations. Cyclic hoop stress can cause tube expansion resulting in creep-like longitudinal waterwall tube cracking. These cracked areas may be subject to corrosion as well. Once-through and other high pressure boilers will tend to be more susceptible to this type of failure due to the higher operating temperatures and higher internal pressures.
Thermal stress-related failure mechanisms for steam generators and other power plant equipment are not new and have been well described in past references 1, 3. However, there has been a plethora of these types of failures in recent years in heat recovery steam generators (HRSGs) and a number of papers have been written discussing factors to reduce thermal stresses 4, 5. Also, thermal fatigue cracking of waterwalls is reported to be the leading boiler tube failure mechanism in supercritical boilers 6. The focus of this discussion is on conventional boilers.
Thermal stresses in boilers have been attributed to the following:
Rapid boiler startups or shutdowns. Boiler manufacturers generally have guidelines on the rate of heating and cooling for proper startup and shutdown of boilers. Reference 7 provides an example for once-through supercritical boilers 7.
