Creep cracking has been experienced in recent years in the welds of thick wall high temperature components of aged fossil power boilers in Europe, the United States and also in Japan. In response, MHI has developed a reliable life assessment system that integrates three categories of advanced technologies: nondestructive flaw inspection (NDI), non-destructive damage evaluation (NDE), and theoretical analysis (TA) including crack growth analysis. For NDI, an advanced ultrasonic testing method, TOFD (Time Of Elight Diffraction Method) is applied to detect and quantitatively measure sub-surface cracks, termed Type III and Type IV cracks. Cracks can be distinguished from other defects by comparing the characteristics of ultrasonic signals from the tips of defects. For NDE, a metallurgical life assessment method has been developed to evaluate the consumed creep life of the component. In this method, replicas and carbon extraction replicas are taken from the component and the degree of mechanical and metallurgical damage is classified and quantified through microstructural investigation. The consumed life is evaluated from the life evaluation diagram. Creep crack growth behavior and residual life are analyzed using a life evaluation database which takes into account the effects of trace element content and creep damage on crack growth rates.
展开▼
