Stress Corrosion Cracking Susceptibility of Alternative Composite Tube Alloys in Simulated Kraft Recovery Boiler Environment

Abstract Cracking of co-extruded 304L stainless steel/carbon steel (composite) tubes in the lower-furnace section of kraft recovery boilers continues to be a serious problem. One remedial approach being used by pulp mills involves replacing the conventional 304L-based composite tube system with alternative composite tube systems based on Alloy 825 and Alloy 625. U-bend stress corrosion cracking tests were conducted on these alloys in a simulated wash-water environment. Attention was given to studying relative SCC resistance in relevant metallurgical conditions, which include mill-annealed, sensitized, cold-worked and solution-annealed conditions. The experimental approach was validated by comparing the lab test results with field observations. It was concluded that 304L stainless is susceptible to SCC even with an ideal microstructure. Alloys 825 and 625 were clearly susceptible after heavy cold work, yet significantly more resistant than 304L stainless steel in other conditions tested.