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Weldability and Service Performance of Ni-base Superalloys for Advanced Ultra-Super Critical Boilers

 

In order to increase the efficiency of boilers in coal-fired power plants, the Department of Energy (DOE) is proposing steam conditions of up to 375 bar (~5300 psi) and 700°C (~1290°F) for Advanced Ultra-Supercritical Boilers.  Such operating conditions will lead to metal temperatures as high as an estimated 760°C (1400°F). Conventional boilers materials, including the P91/92 steels, do not have sufficient creep strength at these temperatures and this has led to the consideration of gamma-prime strengthened Ni-base alloys (or “superalloys”) for applications such as steam header pipes.  It is clear that without the use of superalloys, it will be impossible to meet the temperature/pressure/stress goals that the DOE has proposed. Preliminary work performed under the auspices of a DOE/NETL program led by Babcock and Wilcox, has identified INCONEL Alloy 740H as a candidate material for this application.

The overall objective of this investigation is to determine the mechanism for reduction of creep strength in Alloy 740H weld metal.  his is presumably the result of a combination of the microsegregation that occurs during weld solidification and the complex thermal cycles that the weld metal is subject to during multipass welding. It is anticipated that other Ni-base filler metals (such as Haynes Alloy 282) would be incorporated in this study for comparison.  Once the mechanism is identified, remedial measures will be identified to mitigate the problem. These could range from changes in welding procedures, alternate postweld heat treatments, or development of a more creep-resistant filler metal.  OSU will collaborate with Lehigh University on this project. OSU will be responsible for characterizing the solidification behavior and age hardening characteristics of Alloy 740H in conjunction with both matching and dissimilar filler metals. Lehigh will determine the elevated temperature creep of this alloy based on samples that have been tested at Oak Ridge National laboratory.


Sponsor:  Babcock and Wilcox (through NSF I/UCRC)

Graduate Student: David Tung (PhD)

Collaborators: Dr. John Dupont (Lehigh), Special Metals, ORNL


Publications

  1. D.C. Tung and J.C. Lippold. 2012. INCONEL™ 740H solidification behavior and postweld heat treatment, 10th Int. EPRI Conf. on Welding and Repair Technology for Power Plants, Marco Island, FL, June 2012.
  2. D. Tung and J.C. Lippold.2012, Weld Solidification Behavior of  Ni-Base Superalloys for Use in Advanced Supercritical Coal-Fired Power Plants, Superalloys 2012