A computational approach to thermomechanical fatigue life predictions of dissimilarly welded superheater tubes

Inhaltsverzeichnis (Table of Contents)

• Introduction
• Background
• Thesis outline
• Thermal effects on power plant steels
• Typical cracking phenomena in weldments
• Weld metallurgy
• High temperature behavior of power plant steels
• Fatigue
• Creep
• Creep-fatigue interaction
• Viscoplastic deformation and TMF life prediction model
• Deformation models
• Simple material law
• Viscoplastic material law
• Mechanism based model for fatigue lifetime prediction
• Welding and heat treatment of boiler tubes
• Materials
• Multipass welding of dissimilar boiler tubes
• Post weld heat treatment of the welded tubes
• Hardness distribution of welded and post weld heat treated tubes
• Component test
• Damage assessment of welded superheater tube
• Detection of cracks
• Metallographic analyses
• Summary of the chapter
• Experimental program for material characterization
• Introduction
• Relaxation experiment
• Low cycle fatigue experiment (LCF)
• Thermomechanical fatigue experiments on VM12 base material
• Thermomechanical fatigue experiments on VM12/Alloy617 cross weld specimens
• Parameter identification for deformation model
• Parameter identification for viscoplasticity model
• Validation through thermomechanical experiments
• Parameter identification for stress relaxation model
• Parameter calibration for heat affected zone
• Summary of the chapter
• Parameter identification for lifetime model
• Parameter identification for mechanism-based lifetime model
• Lifetime prediction of T91 base material and HAZ
• DTMF parameter identification for VM12
• Manson-Coffin, Ostergen and SWT models
• Summary of the chapter
• Numerical simulation of welding and post weld heat treatment
• Overview
• Modelling of multipass welding process
• Deposition of filler material
• Model geometry and material properties
• Thermal analysis
• Mechanical Analysis
• Simulation of post weld heat treatment
• Results of welding and PWHT simulations
• Results of thermal simulation
• Effects of PWHT on residual stresses
• Summary of the chapter
• Fatigue life prediction of welded superheater tubes
• Influence of heat affected zone
• Influence of heat affected zone on crossweld specimen model
• Influence of heat affected zone on welded component
• Influence of weld angle
• Influence of weld angle on crossweld specimen model
• Influence of weld angle on welded component
• Influence of residual stress and distortion
• Influence of residual stresses on TMF lifetime prediction
• Influence of distortion on the thermomechanical fatigue lifetime
• Summary
• Discussion
• Mechanism based TMF lifetime prediction
• Applicability of mechanism based lifetime models
• Residual stresses after welding and PWHT
• Failure assessment of welded component
• Conclusion

Zielsetzung und Themenschwerpunkte (Objectives and Key Themes)

The main objective of this dissertation is to predict the lifetime of dissimilarly welded superheater tubes operating under thermomechanical fatigue (TMF) conditions. This involves understanding the complex interaction of thermal and mechanical loads, the influence of residual stresses, and the degradation of the material due to creep and fatigue. The key themes investigated in the dissertation are:

• Thermomechanical Fatigue (TMF): The dissertation examines the effects of combined thermal and mechanical cyclic loads on welded superheater tubes and analyzes their influence on the fatigue behavior and life prediction.
• Residual Stresses: The study explores the generation of residual stresses during multipass welding and their redistribution during post-weld heat treatment (PWHT). The influence of these stresses on the TMF lifetime of the welded tubes is thoroughly investigated.
• Heat Affected Zone (HAZ): The dissertation investigates the properties of the HAZ, which is a critical region influenced by the welding process. The dissertation introduces a novel method for modeling the HAZ properties based on hardness distribution, contributing to a more accurate lifetime assessment.
• Material Characterization: The dissertation details the experimental characterization of the materials (VM12 and Alloy617) under various conditions, including low cycle fatigue, thermomechanical fatigue, and stress relaxation. These experiments provide the data necessary for model calibration and validation.
• Computational Modeling: The dissertation extensively uses computational methods, including finite element analysis (FEA), to simulate the welding process, PWHT, and TMF behavior of the welded tubes. This modeling allows for a detailed investigation of stress evolution, damage accumulation, and lifetime prediction.

Zusammenfassung der Kapitel (Chapter Summaries)

The introduction provides a comprehensive overview of the research topic, outlining the background of dissimilarly welded superheater tubes in power plants and highlighting the significance of TMF lifetime prediction. The thesis outline provides a structured roadmap for the reader.

Chapter 2 delves into the complex thermal effects on power plant steels, including typical cracking phenomena in weldments, the metallurgical aspects of welding, and the high-temperature behavior of these steels under operating conditions. This chapter lays the foundation for understanding the challenges in accurately predicting the lifetime of such components.

Chapter 3 introduces the constitutive equations for cyclic viscoplastic deformation models, including both simple material laws and more advanced models with internal state variables. This chapter focuses on the Chaboche model for viscoplasticity, which is used to describe the material behavior under TMF loading. Additionally, the chapter presents a mechanism-based lifetime model based on fracture mechanics, known as the DTMF parameter, which serves as the foundation for lifetime predictions in this research.

Chapter 4 details the materials used in the study (VM12 and Alloy617) and the procedures for multipass welding and post-weld heat treatment of the dissimilar tubes. The chapter also describes the component test setup and provides a summary of the experimental results, including the failure mechanisms observed in the welded component after TMF loading.

Chapter 5 documents the experimental program for material characterization, outlining the methods used for relaxation, low cycle fatigue (LCF), and thermomechanical fatigue (TMF) testing of the VM12 base material. The chapter concludes by describing the experimental challenges encountered while performing TMF tests on VM12/Alloy617 crossweld specimens.

Chapter 6 focuses on parameter identification for the deformation model, particularly the Chaboche viscoplasticity model, using the results of the complex low cycle fatigue (CLCF) tests. The chapter also describes the validation of the model parameters using TMF experiments on VM12 base material. Additionally, this chapter introduces a novel method for characterizing the heat affected zone (HAZ) properties based on hardness distribution and validates the method using data from T91 steel.

Chapter 7 addresses the parameter identification procedure for the mechanism-based lifetime model (DTMF) used in this work. The chapter explains the process of adjusting the model parameters using experimental LCF and TMF data from VM12 base material. The chapter also briefly compares the DTMF model with other classical models like Manson-Coffin, Ostergen, and Smith-Watson-Topper.

Chapter 8 provides an overview of the numerical simulation of welding and post-weld heat treatment using the finite element method. The chapter details the modeling of the multipass welding process, including the application of element activation techniques, the heat source calibration, and the mechanical analysis considering temperature-dependent material properties and solid-state phase transformations. The chapter concludes with a description of the PWHT simulation process, highlighting the importance of incorporating residual stresses and welding distortions for accurate lifetime assessment.

Chapter 9 presents the results of the welding and PWHT simulations. This chapter analyzes the thermal cycles simulated for the welded VM12/Alloy617 tubes and compares them with the experimental data. The chapter also investigates the redistribution of residual stresses due to PWHT, emphasizing the importance of the PWHT process in reducing the risk of premature cracking caused by these stresses.

Chapter 10 focuses on fatigue life prediction of welded superheater tubes under TMF loading. This chapter investigates the influence of HAZ properties, weld angle, residual stresses, and geometrical distortions on the predicted lifetimes. The chapter explores the effects of these factors on the DTMF parameter and provides valuable insights into the complexities of TMF lifetime assessment.

Chapter 11 discusses key findings and interpretations from the dissertation, analyzing the role of residual stresses in TMF lifetime prediction, the applicability and limitations of the mechanism-based lifetime model, and the significant effects of welding residual stresses and distortions. The chapter also analyzes the results of the component test and highlights the importance of future experimental validation.

The conclusion summarizes the significant contributions of the dissertation to the field of TMF lifetime prediction for dissimilarly welded superheater tubes. The dissertation highlights the effectiveness of the developed computational approach and emphasizes the importance of accounting for residual stresses, distortions, and HAZ properties. The chapter also provides recommendations for future research, including the need for experimental validation of the proposed models and the potential for further refining the HAZ modeling method.

Schlüsselwörter (Keywords)

The dissertation explores the topic of thermomechanical fatigue (TMF) life prediction of dissimilarly welded superheater tubes, emphasizing the crucial role of residual stresses, heat affected zones (HAZ), and welding distortions. Key concepts include:

• Dissimilar Welding
• Superheater Tubes
• Thermomechanical Fatigue (TMF)
• Residual Stresses
• Heat Affected Zone (HAZ)
• Welding Distortions
• Cyclic Viscoplasticity
• Mechanism-Based Lifetime Models
• Finite Element Analysis (FEA)
• Material Characterization
• Computational Modeling