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

摘要

U-tube heat exchangers are often used in the condition of high temperature and pressure. Mostrnof failures occur in the tube-to-tubesheet weld joint, the key part of the dual-shell U-tube heat-exchanger,rnsuch as a feedwater preheater. Leakage took place after working a period of time. Some joints rupturedrnalong welding line, and some tubes break away from the tubesheet.rnWith analyzing the Macroscopical shape and fractograph of the debris, and employing the scanningrnelectron microscope to research the crack surface, there are clear fatigue crack lines, the typical corrosionrnfatigue pattern. Therefore, the failure of the tube-to-tubesheet weld joint is diagnosed as corrosion fatiguerncracking.rnBy measuring the width of fatigue cracks approximately, glancing life was calculated, andrnmiddling-high-cycle corrosion fatigue cracking is confirmed.rnBasing on the analysis of corrosion fatigue mechanism, find out the dynamic load, which is resulted fromrnwork pressure and temperature difference. Considering the plastic deformation caused by stressrnconcentration at the root of welding line, elasto-plastic finite element analysis was employed to acquirernthe relationship between strain and stress of the critical point, and Von Mises yield rule and multilinearrnkinematic hardening rule were used in the process.rnBy assuming that the corrosion fatigue process is consisted of two phases, initiating a crack andrnpropagating it to failure, life estimation is divided to two periods accordingly. The crack initiation life ofrncorrosion fatigue is estimated by application of the local strain approach, using the famousrnManson-Coffin formula considering the corrosion effect. The crack propagation life is estimatedrnaccording to the criteria of fracture mechanics. Aiming at the specific style of the joint, ?K is calculated.rnIn addition, the experimental corrosion fatigue crack growth rate equation of 304 stainless steel in waterrnwas discovered.rnMaking great efforts on a relatively accurate estimation of corrosion fatigue life of the tube-to-tubesheetrnweld joint of dual-shell U-tube heat-exchanger, the paper seeks to provide helpful data for makingrnservicing cycle and security estimation. By analyzing the total life, the conclusion and some suggestionrnwere educed.