Role of Microstructure on Stress Assisted Corrosion Cracking of Carbon Steel Boiler Tubes

摘要

Waterside tube cracking is one of the major reasons for carbon steel boiler tube failure. Previous research has shown that the waterside cracking depends on environmental factors such as temperature, and dissolved oxygen. Failure analysis of failed industrial boiler tubes have shown that grain size and decarburized layer on the inner tube surface may play important roles on SAC cracking. Carbons steel samples were heat-treated to produce different microstructure features. A recirculation autoclave facility was set up, in which slow strain rate tests (SSRT) were carried out on heat-treated tensile samples to investigate the role of environment and microstructure on SAC crack initiation and propagation under simulated boiler water environment. Preliminary results have shown that larger grain size and less carbon in the surface layer of carbon steel tend to accelerate SAC cracking under industrial boiler conditions, and pearlitic structure shown more resistance of SAC cracking than ferrite. Initial fatigue tests (Freq=20 Hz, R=0.3) using pre-cracked CT samples of different grain sizes, were conducted in lab air to investigate the role of microstructure on SAC crack initiation & propagation under inert conditions. Results have indicated that the AK threshold value for CT sample with larger grain is lower and microstructure properties had more effects on crack initiation than crack growth rate.