Modeling and Verification of Creep Strain and Exhaustion in a Welded Steam Mixer

Abstract

Structures operating in the creep regime will consume their creep life at a greater rate in locations where the stress state is aggravated by triaxiality constraints. Many structures, such as the welded steam mixer studied here, also have multiple material zones differing in microstructure and material properties. The 3-dimensional structure as such in addition to interacting material zones is a great challenge for finite element analysis (FEA), even to accurately pinpoint the critical locations where damage will be found. The studied steam mixer, made of 10CrMo 9-10 steel (P22), has after 100 000 hours of service developed severe creep damage in the several saddle point positions adjacent to nozzle welds. FE-simulation of long term behaviour of this structure has been performed taking developing triaxiality constraints, material zones and primary to tertiary creep regimes into account. The creep strain rate formulation is based on the logistic creep strain prediction (LCSP) model implemented to ABAQUS, including primary, secondary and tertiary creep. The results are presented using a filtering technique utilising the formulation of rigid plastic deformation for describing and quantifying the developing “creep exhaustion”.