Susceptibility of Composite Floor Tube Materials to Stress Corrosion Cracking During Recovery Boiler Operation, 2000 Engineering Conference Proceedings

Martti Mäkipää, Maria Oksa, Pekka Pohjanne--During recent years the cracking of 304L type composite floor tubes in recovery boilers has been subject to active research both in North America and in Scandinavia. Most fractographic analyses support environment assisted cracking mechanism although pure thermal fatigue failures have been also observed. The objective of this study was to clarify if salt mixtures enriched in sulfur, potassium and chlorine could cause SCC of composite materials at normal operation temperatures of floor tubes. Slow strain rate tests (SSRT) were used for this purpose. Specimens of 304-type austenitic stainless steel and of Alloy 825 were tested in air and as exposed to polysulfide-chloride mixtures at 280-300 °C under an argon atmosphere. Stress-strain curves, specimen elongation and fractography were used to evaluate SCC susceptibility. Under air exposures Type 304L austenitic stainless steel specimens fractured purely by plastic deformation. 304L specimens exposed to the salt mixture fractured without substantial plastic deformation and showed fractographic features typical of TGSCC. Type Alloy 25 specimens fractured by plastic deformation irrespective of the testing conditions. The results of this study combined to practical data indicate that substituting 304L by Alloy 825 may provide in most conditions a good protection to typical cracking of floor tubes. However, considering the critical performance requirements on materials in high-pressure boilers, additional refinement of the composite tube cracking mechanisms proposed by various authors and additional testing is recommended.