Experimental and numerical investigation on thermal fatigue behaviour of 9Cr 1Mo steel tubes

Abstract

Abstract Boiler tubing is subjected to alternate cycles of heating and cooling during operation initiating alternate thermal expansion and contraction. Alternate cycles of differential expansion and contraction causes thermal fatigue of the component. Thermal fatigue causes tube failures, significantly reducing the working life of the tubular components. Boilers have matured into super critical design using T91 grade materials thereby increasing the operating efficiency. The studies involving thermal fatigue of tubes of T91 grades for boiler components in operating conditions are thus an important area. A simple experimental set up has been developed to simulate thermal fatigue conditions in the internal diameter (ID) side of the tube. The work involves both experimental and numerical investigations of the thermal fatigue behaviour by creating a simulated environment and a Finite Element Model (FEM). FEM analyses are carried out based on the decoupled thermal and inelastic stress analyses to compute the total plastic strain range experienced by the boiler tubes. The cyclic spray cooling causes thermal fatigue cracks in the T91 tube. The number of cycles to crack initiation has been obtained from the experimentation and number cycles to failure has been calculated using modified Coffin-Manson relation. The study thus presents a reliable fatigue failure analysis of 9Cr 1Mo steel tubes used in boiler industry.