Creep behavior and in-depth microstructural characterization of dissimilar joints

Area of Science:

• Materials Science
• Metallurgy
• High-Temperature Engineering

Background:

• Advanced 700 °C power plants require Ni-base alloys (e.g., alloy 617) for high-temperature components (>650 °C).
• Economic considerations necessitate using 2% Cr or 9-12% Cr steels for components below 650 °C.
• The dissimilar joining of Ni-base alloys and Cr steels is essential for constructing these power plants.

Purpose of the Study:

• To investigate the microstructural reasons for premature failure in welds between alloy 617 and 2% Cr steel under creep loading.
• To identify the specific location and mechanisms of damage initiation and propagation in these dissimilar joints.
• To understand the role of microstructural features at the fusion line in the creep behavior of the welds.

Main Methods:

• Welding of alloy 617 and 2% Cr steel.
• Creep loading experiments.
• Microstructural analysis of the fusion zone using Transmission Electron Microscopy (TEM) and Focused Ion Beam (FIB) techniques, both in the as-welded and post-creep conditions.

Main Results:

• Fracture under creep load consistently occurred near the fusion line between the 2% Cr steel base metal and the alloy 617 weld metal.
• TEM analysis revealed a distinct zone within the weld metal adjacent to the fusion line.
• This zone exhibited significant chromium depletion and a reduction in chromium carbide precipitates, indicating localized weakening.

Conclusions:

• The observed chromium depletion and reduced carbides in the weld metal near the fusion line are the primary cause of the joint's weakness under creep conditions.
• This microstructural alteration creates a preferential site for damage initiation and fracture.
• Understanding and mitigating this localized weakening is critical for ensuring the structural integrity and operational safety of high-temperature power plants utilizing these dissimilar materials.