WELDING RESEARCH Multiphysics Modeling of a Welded Furnace Roll for Improving CreepFatigue Life This study proposes methods to improve the creepfatigue lifetime using numerical analyses, including heat transfer analysis, creepfatigue analysis, and CFD analysis to simulate the multiphysics involved in the roll service NOVEMBER 2016 / WELDING JOURNAL 431-s Introduction Creep and fatigue in combination is the main reason for the failure of many engineering components operating under high temperature and cyclic loading. Many studies have been conducted to investigate creep-fatigue crack initiation and growth (Refs. 1–3), creep and fatigue interactions (Refs. 4, 5), damage mechanism (Refs. 6, 7), lifetime predictions (Refs. 8–12), and hold time effect on creep and fatigue life (Refs. 13–15). Karl (Ref. 7) tested and modeled smooth and notched specimens of Type 304 stainless steel by applying several types of idealized fatigue loading to obtain a clear picture of the types of damage occurring in a steam turbine and similarly loaded mechanical systems. Zhu (Ref. 11) proposed a low-cycle, fatigue creep life prediction model for general use in isothermal and thermomechanical loading based on the theory of ductility exhaustion. With better understanding of the creep-fatigue damage process, creep-fatigue assessment standards and codes (Refs. 16–19) were developed such as the ASME Boiler and Pressure Vessel Code (BPVC) Section III, Subsection NH, and the R5 assessment procedure. ASME BPVC Section III, Subsection NH (Refs. 16–17), considers cyclic failure modes at elevated temperatures and provides creep-fatigue interaction rules and damage limits. The R5 assessment procedure (Refs. 18, 19) was developed in the UK to address both creep-fatigue crack initiation in initially defect -free components and the growth of flaws by creep and creepfatigue mechanisms. Although significant progress has been made to understand the creepfatigue damage process and design the engineering components according to the ASME code or the R5 assessment procedures for elevated-temperature service, creep-fatigue failures are still observed in industries because realworld problems are far more complex than a well-controlled research environment. A large number of real-world problems have to be solved using multiphysics simulation tools to model the physics involved in a process or service. However, it is challenging to conduct a fully coupled numerical analysis among a separate continuum physics phenomena. Many successful multiphysics modeling efforts are based upon loose or one-way coupling because of computational cost and results mapping issues between multiphysics simulation tools (Ref. 20). A catastrophic roll failure was observed in a continuous hot-dip coating BY Y. P. YANG AND W. C. MOHR ABSTRACT Heattransfer analysis, creepfatigue analysis, and computational fluid dynamics (CFD) analysis were conducted to understand the failure of a welded roll in a continuous coating line and to propose solutions to improve the creepfatigue lifetime. Analysis results showed three factors that contributed to the roll failure: difference of material properties between filler metal and base metal, furnace temperature variation during service, and high operating temperature. Based on these findings, two fatiguelife improvement methods, using the electron beam welding (EBW) process without filler metal to weld the roll and cooling the roll from inside to lower the shell temperature, were proposed. Creepfatigue analyses showed both methods were effective in reducing the creep strain and stress to improve the roll fatigue life. Using EBW, the creep strain and stress in the interface between the filler metal and the base metal were completely eliminated. By lowering the shell temperature using the inside roll cooling, the creep strain and stress near the weld were significantly reduced. A cooling system was designed and evaluated using a fully coupled CFD and heat transfer analysis. A preliminary welding test showed that EBW could be implemented to weld the roll, and the CFD analysis results confirmed that effective inside roll cooling was difficult to achieve with the current design. In addition, the predicted highly localized maximum principal stress at the weld root can be used to explain the observed crack at the weld root, which indirectly validates the models. KEYWORDS • Creep • Fatigue • Multiphysics Modeling • Crack • Welding Y. P. YANG (yyang@ewi.org) and W. C. MOHR are with the Edison Welding Institute, Columbus, Ohio.
Welding Journal | November 2016
To see the actual publication please follow the link above