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

Table of Contents

1 Introduction

1.1 Background

1.2 Thesis outline

2 Thermal effects on power plant steels

2.1 Typical cracking phenomena in weldments

2.2 Weld metallurgy

2.3 High temperature behavior of power plant steels

2.3.1 Fatigue

2.3.2 Creep

2.3.3 Creep-fatigue interaction

3 Viscoplastic deformation and TMF life prediction model

3.1 Deformation models

3.1.1 Simple material law

3.1.2 Viscoplastic material law

3.2 Mechanism based model for fatigue lifetime prediction

4 Welding and heat treatment of boiler tubes

4.1 Materials

4.2 Multipass welding of dissimilar boiler tubes

4.3 Post weld heat treatment of the welded tubes

4.4 Hardness distribution of welded and post weld heat treated tubes

4.5 Component test

4.6 Damage assessment of welded superheater tube

4.6.1 Detection of cracks

4.6.2 Metallographic analyses

4.7 Summary of the chapter

5 Experimental program for material characterization

5.1 Introduction

5.2 Relaxation experiment

5.3 Low cycle fatigue experiment (LCF)

5.4 Thermomechanical fatigue experiments on VM12 base material

5.5 Thermomechanical fatigue experiments on VM12/Alloy617 cross weld specimens

6 Parameter identification for deformation model

6.1 Parameter identification for viscoplasticity model

6.2 Validation through thermomechanical experiments

6.3 Parameter identification for stress relaxation model

6.4 Parameter calibration for heat affected zone

6.5 Summary of the chapter

7 Parameter identification for lifetime model

7.1 Parameter identification for mechanism-based lifetime model

7.1.1 Lifetime prediction of T91 base material and HAZ

7.1.2 DTMF parameter identification for VM12

7.2 Manson-Coffin, Ostergen and SWT models

7.3 Summary of the chapter

8 Numerical simulation of welding and post weld heat treatment

8.1 Overview

8.2 Modelling of multipass welding process

8.2.1 Thermal analysis

8.2.2 Mechanical Analysis

8.3 Simulation of post weld heat treatment

9 Results of welding and PWHT simulations

9.1 Results of thermal simulation

9.2 Effects of PWHT on residual stresses

9.3 Summary of the chapter

10 Fatigue life prediction of welded superheater tubes

10.1 Influence of heat affected zone

10.1.1 Influence of heat affected zone on crossweld specimen model

10.1.2 Influence of heat affected zone on welded component

10.2 Influence of weld angle

10.2.1 Influence of weld angle on crossweld specimen model

10.2.2 Influence of weld angle on welded component

10.3 Influence of residual stress and distortion

10.3.1 Influence of residual stresses on TMF lifetime prediction

10.3.2 Influence of distortion on the thermomechanical fatigue lifetime

10.4 Summary

11 Discussion

11.1 Mechanism based TMF lifetime prediction

11.2 Applicability of mechanism based lifetime models

11.3 Residual stresses after welding and PWHT

11.4 Failure assessment of welded component

12 Conclusion

Objectives & Topics

The primary objective of this thesis is the thermomechanical fatigue (TMF) life prediction of dissimilarly welded superheater tubes for power plant applications. The work aims to develop and validate a computational simulation chain that accounts for complex thermal and mechanical loading, welding residual stresses, and microstructure evolution in the heat-affected zone (HAZ).

• Thermomechanical fatigue (TMF) behavior of dissimilarly welded joints (VM12 and Alloy617).
• Development of a cyclic viscoplastic material model for life prediction under high-temperature conditions.
• Numerical simulation of multipass welding processes and post-weld heat treatment (PWHT).
• Assessment of the influence of residual stresses, welding distortions, and HAZ properties on component integrity.

Excerpt from the book

Summary of the Chapters

1 Introduction: Provides background on the use of 9-12% Cr martensitic steels in power plants and outlines the thesis structure and scope.

2 Thermal effects on power plant steels: Reviews the failure modes of welded joints, weld metallurgy, and high-temperature material behavior including fatigue and creep.

3 Viscoplastic deformation and TMF life prediction model: Details the constitutive equations for cyclic viscoplastic deformation and the mechanism-based model used for lifetime prediction.

4 Welding and heat treatment of boiler tubes: Describes the materials used (VM12 and Alloy617), the welding procedure, heat treatment, and the assessment of damage through destructive evaluation.

5 Experimental program for material characterization: Documents the experimental setup for low cycle fatigue, thermomechanical fatigue, and relaxation tests used to calibrate material models.

6 Parameter identification for deformation model: Focuses on the parameter fitting of the Chaboche viscoplastic model and the method for characterizing HAZ properties based on hardness.

7 Parameter identification for lifetime model: Describes the procedure for identifying parameters for the mechanism-based lifetime model using experimental data.

8 Numerical simulation of welding and post weld heat treatment: Outlines the finite element simulation framework for the multipass welding process and subsequent post-weld heat treatment.

9 Results of welding and PWHT simulations: Presents the findings regarding thermal and mechanical simulations, specifically focusing on the redistribution and relaxation of residual stresses.

10 Fatigue life prediction of welded superheater tubes: Analyzes the influence of HAZ properties, weld angles, residual stresses, and geometric distortions on the TMF life of the welded tubes.

11 Discussion: Evaluates and interprets the computational results, the reliability of the methods used, and the applicability to power plant components.

12 Conclusion: Summarizes the key achievements and provides recommendations for future research in fatigue life assessment.

Keywords

Thermomechanical fatigue, TMF, superheater tubes, residual stresses, VM12, Alloy617, multipass welding, finite element simulation, viscoplasticity, post-weld heat treatment, PWHT, heat-affected zone, HAZ, life prediction, creep-fatigue interaction.

Frequently Asked Questions

What is the core subject of this dissertation?

This work focuses on the numerical and experimental prediction of the thermomechanical fatigue (TMF) life of dissimilarly welded superheater tubes used in power plants.

What are the primary thematic areas?

The core themes include welding process simulations, cyclic viscoplastic material modeling, thermal-mechanical fatigue behavior of 9-12% Cr steel (VM12), and the effect of residual stresses on lifetime predictions.

What is the primary goal or research question?

The main goal is to create a reliable computational chain of simulations to predict the fatigue lifetime of dissimilarly welded components, minimizing the need for costly and time-consuming physical experiments.

Which scientific methods are applied?

The work employs finite element analysis (FEA) for welding and heat treatment simulations, combined with experimental material characterization (LCF, TMF, and relaxation tests) to calibrate viscoplastic constitutive laws.

What topics are discussed in the main body?

The main body covers the experimental characterization of VM12 base material, the parameter identification for deformation and lifetime models, the simulation of welding and PWHT, and the analysis of how distortions and residual stresses impact lifetime.

Which keywords characterize this research?

Key terms include thermomechanical fatigue (TMF), residual stresses, VM12, Alloy617, viscoplasticity, heat-affected zone (HAZ), and lifetime prediction.

How is the Heat Affected Zone (HAZ) modeled in this study?

A novel method is presented that scales the cyclic viscoplastic model parameters of the base material based on hardness distributions measured in the individual HAZ zones.

What effect do residual stresses have on the fatigue life of the welded components?

The study finds that while residual stresses exist after welding and PWHT, they significantly relax during the initial TMF loading cycles, making their impact on long-term fatigue life less severe than initial conservative assumptions might suggest.

How do geometric distortions influence the results?

The analysis demonstrates that accounting for welding-induced geometric distortions is critical, as calculations using undistorted models significantly overestimate the cycles to failure compared to models with distortions.

What is the practical value of this approach for power plant operators?

The proposed computational framework allows for reliable lifetime assessment of welded power plant components, leading to a substantial reduction in expensive and lengthy physical component tests.