Thermal Aging of PM-HIP and Forged Inconel 690 and 625

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Nuclear structural components are conventionally fabricated by casting or forging, but these processing methods are subject to fabrication defects and cavities, and require considerable post-fabrication machining and welding.  Powder metallurgy with hot isostatic pressing (PM-HIP) is an alternative processing method that creates dense, homogeneous microstructures containing fewer defects.  PM-HIP heavy components can also be produced near-net shape, reducing the need for machining and welding.  Hence, there is interest in qualifying PM-HIP alloys for nuclear structural applications.  Amongst the extreme conditions these alloys must withstand is long-term operation at elevated temperatures >200°C.

Here, we study the Ni-base superalloys Inconel 690 and 625, and their microstructural and mechanical evolution under thermal aging.  We compare the behaviors of the PM-HIP alloys to their forged/cast counterparts.  Thermal annealing is conducted at 400°C, 600°C, and 800°C, to 100 hr and 1000 hr.  Subsequently, we utilize light optical microscopy to characterize grain size evolution.  We also use dynamic nanoindentation to evaluate hardness, yield strength, and elastic modulus.

In general, we find that 100 hr aging has little effect on the grain size or the mechanical behavior of all alloys.  However, the alloys exhibit an increase in hardness and yield strength after 1000 hr aging. Because this strengthening is not accompanied by grain size changes, it may be attributed to precipitation.  Overall, the PM-HIP versions of the alloys perform comparably to the cast or forged version of the alloys, lending confidence to the implementation of PM-HIP alloys in power plant components.

Collaborators:

  • D.W. Gandy, Electric Power Research Institute
  • D.P. Guillen, Idaho National Laboratory

Support:

  • DOE Nuclear Science User Facilities project 15-8242
  • Network for Computational Nanotechnology SURF Program
  • Purdue SURF Program

Products:

  1. A.L. Bullens, E. Bautista, E.H. Jaye, N.L. Vas, N.B. Cain, K. Mao, D.W. Gandy, and J.P. Wharry. Comparative thermal aging effects on PM-HIP and forged Inconel 690.  Journal of Materials (JOM), in press.
  2. E. Bautista, A. Bullens, and J.P. Wharry. Understanding the role of fabrication method on metal alloys used in nuclear power plants. Purdue University Network for Computational Nanotechnology (NCN) Summer Undergraduate Research Fellowship (SURF) Symposium, West Lafayette IN, August 2017.
  3. A. Bullens, E. Bautista, K. Mao, and J.P. Wharry. Comparison of PM-HIP to cast Alloy 625 for nuclear applications. Purdue University College of Engineering Summer Undergraduate Research Fellowship (SURF) Symposium, West Lafayette IN, August 2017.
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