Composite Tube Cracking in Kraft Recovery Boilers - A State-of-the Art Review, 1997 Engineering Conference Proceedings

Cracks in co-extruded tubes installed in kraft recovery boilers have been found at spout and air port openings and in the floors and portions of the walls covered by the smelt bed. In some cases, the cause of cracking is readily apparent, but in many others, there is no apparent reason for cracks to form. A review of public and private domain documents pertaining to co-extruded tube cracking in kraft recovery boilers was conducted to help guide a subsequent research program on composite tube cracking.

Based on the information collected, it appears likely that all 304L/SA-210 co-extruded tubes exposed to the smelt bed in a kraft recovery boiler are susceptible to cracking in service. Neither boiler design nor operating pressure could be correlated with the occurrence or severity of cracking in floor tubes. In some cases, particularly at spout and air port openings, cracking can be attributed to thermal fatigue. Many other cracks appear to be due to a form of environmentally assisted cracking, but the actual mechanism could not be determined from the reviewed information. Most cracks have been confined to the outer stainless steel layer. Penetration of cracks across the interface into the carbon steel is rare, and occurs predominantly near highly restrained welds.