Cracking in water tubes in black liquor recovery boilers, especially in boiler floors and lower waterwalls, has been the source of unscheduled downtime in the pulp and paper industry for many years. For this reason, Sandvik Materials Technology has launched an installation reference for stainless/carbon steel composite tubes, which show no signs of cracking after eight years in service. Composite or co-extruded tubes, comprising a 304L stainless steel outer layer and an SA210 Grade A1 carbon steel inner layer, have been around for over 30 years, but it was only in the early 1990's that it became apparent that cracking in the 304L layer - particularly in floor tubes - was a widespread problem.
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