A number of black liquor recovery boilers in North America have experienced severe, localized cracking in the co-extruded 304L stainless steel/SA210 carbon steel tubes at or near the bottom of primary air ports. Some of these cracks have been found to advance into the carbon steel. Studies have shown that excessive temperature fluctuations on the surface of the tubes are common to all tubes that experience this cracking. This project has addressed this problem by identifying 1) changes in the alloy used for the cladding on the composite tubes to reduce the likelihood of cracking, 2) changes in the design of the primary air port to one that appears to reduce crack frequency, and 3) process changes that can reduce the magnitude and frequency of the temperature fluctuations. This paper utilizes the information obtained from this study to address questions regarding the nature of the cracking, the cracking mechanism, prevention of cracks advancing into the carbon steel and complete avoidance of any cracking.
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