Doctoral Dissertations

Date of Award

5-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Engineering Science

Major Professor

Sudarsanam Suresh Babu

Committee Members

Zhili Feng, Hahn Choo, Chad Duty

Abstract

Dissimilar Metal Welds (DMWs) made between ferritic low alloy steel (BCC) and austenitic alloys (FCC) are widely used in the high temperature components of power plants. Ex-service data from power plants suggests these bimetallic welds fail prematurely by creep mechanism, with lifetimes much lesser than the creep lives of either of the base materials. Earlier creep studies have demonstrated that failures are associated with creep cavities along the ferritic steel HAZ close to BCC/FCC boundary, due to the local detrimental microstructure. Structure-property relationships have not been established for these heterogeneous materials due to the limitation in the spatial measurement of creep strain rates. Hence, the objective of this research study is to develop a methodology to extract the local creep constitutive properties from heterogenous weld configurations and correlate these properties with the underlying microstructure. The following heterogeneous weld configurations were considered:I. Conventional DMWs made between 2.25Cr-1Mo steel and Alloy 800H base materials using Inconel weld consumable,II. Graded Transition Joints (GTJs) made between 2.25Cr-1Mo steel and Alloy 800H base materials using each of the three candidate filler metals viz., (i) Inconel 82, (ii) P87, and (iii) 347HLocal creep studies discretized the heterogeneous creep behavior in both these welded configurations. Global creep strain from both these welded configurations was a result of creep strain evolution from the 2.25Cr-1Mo base material and regions inside 2.25Cr-1Mo HAZ, while the other austenitic regions showed negligible creep formation. In both DMWs and GTJs, creep strain was accumulating inside 2.25Cr-1Mo HAZ and was driving the premature failure in these welded joints.Research findings from these local creep studies were summarized as follows:1. In DMWs, creep strain accumulation and the creep damage occurred close to BCC/FCC boundary due to the localized decarburization (depletion of carbides) in those regions,2. In all the GTJs, creep strain accumulation and the creep damage occurred in the FGHAZ at 3.5mm away from the weld interface, as a result of carbide coarsening during weld processing.Microstructure based creep model framework was developed to model the discrete creep strain rates with the local microstructures of 2.25Cr-1Mo steel.

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