Doctoral Dissertations

Cong Yue Qiao

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Carl D. Lundin

Committee Members

Charlie R. Brooks, Raymond A. Buchanan, John R. Dunlap

Abstract

The weldability of Modified 800H alloy developed at Oak Ridge National Laboratory is discussed in this dissertation in terms of HAZ liquation cracking and HAZ softening tendency. Fundamental studies of the mechanisms of HAZ liquation cracking and HAZ softening behavior in Modified 800H alloy are addressed. This research is part of a DOE project entitled "Investigation of Welding and Joining Techniques for Advanced Austenitic Alloys," therefore, an assessment on the weldability of Modified 800H and other advanced austenitic alloys, including HR3C, NF709, Modified 310 and Modified 316 is introduced. Extensive hot cracking, hot ductility and microhardness evaluations for Modified 800H were performed in order to generate a data base for fundamental understanding. The relationship between the HAZ microstructure and reactions together with weld related HAZ liquation cracking behavior is defined and a model for HAZ liquation cracking is proposed. The influence of alloy system, alloying and trace elements on HAZ liquation cracking is also presented. The influence of welding parameters on the HAZ softening tendency of Modified 800H is revealed. The precipitate dissolution and reformation, grain growth and recrystallization influence on the HAZ softening is discussed and the approach to minimization of the HAZ softening is defined. In general, hot cracking resistance of the Modified 800H materials can be improved by applying a proper thermo-mechanical treatment and controlling the base metal grain size. The minor alloying elements adjustment is suggested for further improvement of the weldability as well as high temperature mechanical properties. The evaluation results elucidate that the Modified 800H is an excellent candidate for elevated temperature application.

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