DISSIMILAR GIRTH JOINTS UNDER COMBINED CYCLIC THERMAL AND MECHANICAL LOADING - EXPERIMENTAL AND NUMERICAL INVESTIGATIONS

摘要

Experimental and numerical investigations were conducted to qualify dissimilar girth welds in tubes fabricated in 9% Chromium steels T91 and E911 and the austenitic steel X3 CrNiMoN 17-13 for service temperatures up to 625℃. Girth welds were produced on tubes by different welding processes such as automatic and manual TIG welding as well as friction welding. Ni-based consumables (NiCr20Nb) were used in the arc welding processes and NiCr15Fe transition rings in case of friction welded joints. The girth welds were investigated in the annealed as well as in the quenched and tempered post weld condition. The tubes were tested under combined cyclic thermal and mechanical loading. A full material characterization (tensile tests at different strain rates, creep tests, low cycle fatigue tests at temperatures up to 650℃) for base metals and weld metals as well as weld thermal simulated heat affected zones were performed. This data was used to apply a non-isothermal viscoplastic Chaboche-type material model. This model describes primary, secondary and tertiary creep as well as the softening and hardening during cyclic loading. The viscoplastic model was implemented in the finite element code AB AQUS. Each material zone (base metals, three different heat affected zone materials, weld metal) was treated separately. Various finite element simulations were performed to analyze and predict the in-service behavior of the welded tubes. The comparison of long term laboratory tests with these predictions are promising.