Cracking of recovery boiler composite tubes:Thermal fatigue or stress corrosion cracking?

In the composite tubes of black liquor recovery boilers cracking and corrosion attack has been observed, especially in the floor tubes. Cracks typically initiate in the stainless steel cladding and penetrate through the cladding to the stainless steel/low alloy steel interface, where the cracks normally propagate along the interface and do not penetrate to the low alloy steel. Cracking occurs also on lower walls, smelt spout openings and air ports and sometimes penetration to low alloy steel base metal has also occurred. The cracking mechanism has been suggested to be either stress corrosion cracking or thermal fatigue. A special case of cracking has been the situation where cracks are present on the inner surface of the tubes, when cracking is also starting from the water side. The environmental, thermal, mechanical and metallurgical effects on these cracking phenomena are evaluated based on these two different mechanisms of cracking. Special attention is paid to the possible methods for mitigation of the recovery boiler cracking failures, e.g. by operating procedures or materials selection