CORROSION FATIGUE OF THE TUBE-TO-TUBESHEET WELD JOINT OF DUAL-SHELL U-TUBE HEAT-EXCHANGER

摘要

U-tube heat exchangers are often used in the condition of high temperature and pressure. Most of failures occur in the tube-to-tubesheet weld joint, the key part of the dual-shell U-tube heat-exchanger, such as a feedwater preheater. Leakage took place after working a period of time. Some joints ruptured along welding line, and some tubes break away from the tubesheet. With analyzing the Macroscopical shape and fractograph of the debris, and employing the scanning electron microscope to research the crack surface, there are clear fatigue crack lines, the typical corrosion fatigue pattern. Therefore, the failure of the tube-to-tubesheet weld joint is diagnosed as corrosion fatigue cracking. By measuring the width of fatigue cracks approximately, glancing life was calculated, and middling-high-cycle corrosion fatigue cracking is confirmed. Basing on the analysis of corrosion fatigue mechanism, find out the dynamic load, which is resulted from work pressure and temperature difference. Considering the plastic deformation caused by stress concentration at the root of welding line, elasto-plastic finite element analysis was employed to acquire the relationship between strain and stress of the critical point, and Von Mises yield rule and multilinear kinematic hardening rule were used in the process. By assuming that the corrosion fatigue process is consisted of two phases, initiating a crack and propagating it to failure, life estimation is divided to two periods accordingly. The crack initiation life of corrosion fatigue is estimated by application of the local strain approach, using the famous Manson-Coffin formula considering the corrosion effect. The crack propagation life is estimated according to the criteria of fracture mechanics. Aiming at the specific style of the joint, ?K is calculated. In addition, the experimental corrosion fatigue crack growth rate equation of 304 stainless steel in water was discovered. Making great efforts on a relatively accurate estimation of corrosion fatigue life of the tube-to-tubesheet weld joint of dual-shell U-tube heat-exchanger, the paper seeks to provide helpful data for making servicing cycle and security estimation. By analyzing the total life, the conclusion and some suggestion were educed.