INTRODUCTION
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 ?K threshold value for CT sample with larger grain is lower and microstructure properties had more effects on crack initiation than crack growth rate.
Waterside cracking of carbon steel tubes is one of the major reasons for waterwall failure in industrial and utility boilers and has been recognized as a major cause for the reduced boiler availability [1-4]. As early as in the 1960s [5], some utility boilers were experiencing casing side tube leaks at attachment welds. It was assumed that those cracks originated at the outside surface of the tube, because of the proximity of the leak to the attachment weld, which was generally considered as ?stress raiser?. Efforts were made to modify the attachment to reduce thermally or mechanically introduced stress concentration effects which might promote outside surface crack initiation [5]. It took a while before it was recognized that actually the large majority of those leaks at attachments were originating from the inside surface of the boiler tube because leaking tubes were rarely removed and examined. In the utility industry, waterside tube cracking is generally referred to as corrosion fatigue (CF) whereas in industrial boilers cracks found in areas with heavy attachment welds on the outer surface are typically referred to as stress assisted corrosion (SAC). The typical CF cracks found in utility boiler are generally un-branched, long and sharp, as shown in Figure 1.(a). However, SAC cracks in industrial boilers are typically blunt, with multiple bulbous features indicating a discontinuous growth, as shown in Figure 1.(b). There are differences in the water chemistry and frequency of upsets in the operating conditions between the utility boilers and industrial boilers. In general utility boilers operate at higher temperatures and pressures, and it has been speculated that the water chemistry control in utility boilers, during operating and shutdown, may be tightly controlled as compared to most low pressure industrial boilers.
