Characterization of precipitation in VIM and PM/HIP CF8C-Plus

CF8C-Plus is a cast austenitic stainless steel developed for high temperature power plant applications including ultrasupercritical steam boilers and heat recovery steam generators.  Over the temperature range of 600-900°C, CF8C-Plus exhibits creep strength comparable to several superalloys, and is thus a cost-effective alternative to superalloys. Various manufacturing processes have been utilized to explore the impact of wrought processing on nano/microstructure, and hence on mechanical performance.  High temperature creep performance is of particular interest.  In this work, we compare CF8C-Plus specimens produced by conventional vacuum induction melting (VIM) with extrusion, to those produced by powder metallurgy and hot isostatic pressing (PM/HIP).

We utilize bright field TEM imaging, diffraction pattern analysis, energy-dispersive spectrometry (EDS), and electron energy loss microscopy (EELS) to characterize the structure and chemistry of the matrix, dislocation networks, and precipitates.  We find little to no precipitates after VIM processing, although Cr-rich precipitates form in the VIMed specimens during high temperature creep testing.  On the other hand, PM-HIP processing results in a considerable volume fraction and number density of Nb-rich and Mn-rich precipitates, neither of which are present after VIM.  The differences in precipitate volume fraction and composition explain differences in creep-rupture performance between processing pathways.

Collaborators:

  • D. Purdy, Electric Power Research Institute
  • P.J. Maziasz, Oak Ridge National Laboratory and HTA
  • D.W. Gandy, Electric Power Research Institute

Support:

  • Electric Power Research Institute agreement 10004870

Products:

  1. D. Purdy, P.J. Maziasz, J.P. Wharry, and C.K. Dolph. Microstructure impacts on mechanical properties in a high temperature austenitic stainless steel. in: Advances in Materials Technology for Fossil Power Plants, Proceedings from the 8th International Conference (EPRI 2016), eds. J. Parker, J. Shingledecker, and J. Siefert. Co-published by EPRI and ASM International, 1288 pages, 2017, ISBN 978-1-62708-131-3.
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