Cracking Susceptibility of Alternative Composite Tube Alloys in a Molten Sodium Sulfide Hydrate-Sodium Hydroxide Salt Mixture

The cracking of composite tube systems based on Type 304L stainless steel (SS) (UNS S30403) in the lower-furnace section of kraft recovery boilers continues to be a serious problem. One remedial approach used by pulp mills involves replacing the conventional system based on Type 304L SS with alternative composite tube systems based on Alloys 825 (UNS N08825) and 625 (UNS N06625). U-bend stress corrosion cracking (SCC) tests were conducted on these alloys in a sodium sulfide hydrate-sodium hydroxide ((Na2S·xH2ONaOH) molten salt mixture at 180°C (simulated wash-water environment) to help rationalize the observed service performance and to help assist with the selection of the more resistant composite tube system. Attention was given to studying relative SCC resistance of those three alloys in relevant metallurgical conditions, which include mill-annealed, sensitized, cold-worked, and solution-annealed conditions. It was concluded that Type 304L SS is susceptible to cracking even with an ideal microstructure. Alloys 825 and 625 were clearly susceptible after heavy cold work, yet significantly more resistant than Type 304L SS in other conditions tested. The results are consistent with field observations and, therefore, have meaningful implication regarding materials selection and fabrication.