Application of a Physically-Based Creep Continuum Damage Mechanics Constitutive Model to the Serviceability Assessment of a Large Bore Branch Connection

Abstract

Premature creep cracking in fabricated large bore branch connections in Grade 91 steel (9Cr-1Mo-VNbN) piping continues to be a commonly observed failure mechanism in high energy applications. Failures have been observed in components fabricated to the requirements of both ASME Section I and B31.1 codes. This paper presents the application of a physically-based creep continuum damage constitutive model developed for Grade 91 steel to the assessment of a large bore fabricated branch connection. For a specific component geometry and operating conditions, model predictions for the expected location and timing of crack initiation as well as for the crack growth behavior have been made. In addition, as validation, trends in the simulated behavior are compared to information from case studies of large bore branch cracking and failure in service. The physically-based continuum damage model is shown to accurately predict both the location and timing of local crack initiation as well as the observed crack growth behavior.

Manufacture and Performance of Welds in Creep Strength Enhanced Ferritic Steels

“…A creep continuum damage mechanics (CDM) constitutive model for Grade 91 steel developed as part of a multi-year collaborative research project has been used to assess the creep performance of structures and components. As described in previous papers [9,20], EPRI research in the area of CDM has been guided by a systematic, step wise methodology. The following list summarizes the important steps followed during the development and testing of this model, and its ability to describe high temperature behaviour:The foundational step involved identification of relevant creep data for 9% Cr tempered martensitic steels.…”

“…The broad scope of these tasks requires an integrated and sustained approach implemented and reviewed over several years. The basis for this research has been described in an EPRI published report [9]. The overall plan required that there would be key points over the course of the model development and validation where the details of the work were reviewed and further evaluated.…”

“…As an example of the application of CDM to the behaviour of a component, the physically-based creep continuum damage constitutive model was applied to the serviceability assessment of a 90° large bore, welded branch connection [9]. This connection had developed a steam leak which occurred as the result of in-service cracking.…”

“…For many operators and end users, there is a significant benefit to selecting component design and manufacturing methods which are inherently Damage Tolerant, i.e., there is a long period between crack initiation and fracture. Application of CDM has shown that the inherent advantages of reducing the risks of catastrophic fracture do not require significant greater financial investment [8,9]. These benefits can instead be realized by the application of sound engineering approaches to weld design and manufacture.A clear technical route has established the improved design and fabrication for well-engineered welds in CSEF steels.…”