Elemental Effects of Fe, Mo, C, and Hf (or Nb) on Solidification Behavior, Microstructure, and Weldability of High-Cr, Ni-base Filler Metals

<p>The mitigation of primary water stress corrosion and ductility-dip cracking in Ni-Cr-Fe base materials and filler metals has led to increased Cr, Nb, and Mo contents in these alloys, respectively. Nb additions however, have resulted in a significant increase in the solidification cracking susceptibility of Ni-Cr-Fe-Nb filler metals, especially when dissimilar welding pressure vessel and stainless steels. These dissimilar weldments are commonplace practice in the nuclear power generation industry, so the Welding Repair Technology Center at the Electric Power Research Institute has been tasked with finding a solution to this dilemma. The solution that is being investigated in this thesis is the replacement of Nb with an alternative carbide former, namely, Hf. The goal of this work is to develop a Hf-bearing 30 wt % Cr Ni-base alloy that is resistant to both solidification cracking and ductility-dip cracking.</p><p>Thermocouple plunging experiments and quantitative microstructural analysis, used in conjunction with a design of experiment methodology, were employed to statistically determine the elemental effects of Fe, Mo, Hf, and C on the solidification behavior of Ni-Cr-Fe-Hf alloys. These effects were compared to their effects in the Ni-Cr-Fe-Nb system. As a result of this investigation, optimized Ni-Cr-Fe-Hf weld metal compositions have been developed that demonstrate potential for increased solidification cracking resistance, while maintaining excellent ductility-dip cracking resistance. This is in contrast to current commercially available Ni-Cr-Fe-Nb filler metals. </p><p>Further microstructural characterization via scanning electron microscopy, x-ray energy dispersive spectroscopy and electron backscatter diffraction was employed to determine the effects of Mo on the microstructure and weldability of Hf and Nb-bearing high-Cr, Ni-base alloys. Commercially available Nb-bearing Ni-Cr-Fe alloys like filler metal 52MSS have depended on the addition of 4 wt % Mo to improve the ductility-dip cracking resistance of the filler metal. Hf-bearing Ni-Cr-Fe alloys with 0.75 and 1.0 wt % Hf, resulting from statistical optimization of computational and experimental data in this investigation, have presented microstructures that would indicate excellent resistance to ductility-dip cracking, regardless of Mo content. Mo was observed to have a significant influence on the distribution of NbC and ¿¿/Ni7Hf2 along grain boundaries. A theory is presented that explains how the eutectic distribution is directly associated with the increases in ductility-dip cracking resistance of filler metal 52MSS, which was reported in previous investigations, but not fully explained.</p>

Name thesis:degree_name

Master of Science

Level thesis:degree_level

masters

Discipline thesis:degree_discipline

Welding Engineering

Grantor dc:publisher

The Ohio State University

Year dc:date

2013

Author dc:creator

• Fusner, Eric

Contributors dc:contributor

• Lippold, John

• Engineering

Statement dc:rights

• unrestricted
• This thesis or dissertation is protected by copyright: all rights reserved. It may not be copied or redistributed beyond the terms of applicable copyright laws.

Language dc:language

English

Repository record dc:identifier

http://rave.ohiolink.edu/etdc/view?acc_num=osu1356985236

OAI identifier oai:identifier

oai:etd.ohiolink.edu:osu1356985236