Comparison of cracking in recovery boiler composite floor and primary air port tubes

Cracking of the stainless steel outer layer of co-extruded 304L stainless steel/SA210 Gd A1 carbon steel tubes in kraft recovery boilers has been the subject of intensive worldwide investigation for more than a decade. One such multi-task project, funded by the U.S. Department of Energy (DOE) and launched in 1994, focused initially on cracking in co-extruded floor tubes. Results of this study led to recommendations on selection of alternate tube materials and operating practices to minimize cracking. More recently, new reports of extensive cracking in co-extruded tubes that form primary air ports caused this research program to be broadened to include cracking around primary air ports. In this paper, we compare the cracking,in the two locations and describe the similarities and differences in the mechanisms, based on the findings of the DOE-funded project.