Evaluation of the Mechanical Properties of Cast and Wrought CF8C-Plus Relevant to ASME Code Case Qualification

CF8C-Plus initially referred to a cast austenitic stainless steel, officially recognized under the Unified Numbering System (UNS) as J92604 and designated by ASTM as HG10MnN. This advanced alloy represented a significant improvement over its predecessor, CF8C, the composition of which closely matches the wrought type 347 stainless steel. Compared to CF8C and type 347 stainless steel, CF8C-Plus exhibits enhanced stability of the desirable austenitic phase, with suppressed formation of delta ferrite and sigma phases. Additional improvement was obtained via the refined distribution of carbides and fine carbonitrides within the alloy's microstructure, resulting in superior high-temperature properties, particularly elevated high-temperature creep strength. A later wrought version of CF8C-Plus was also developed, including production of the first ever commercial-scale heat and thick-wall seamless pipe of the alloy. The high temperature tensile and creep performance of wrought CF8C-Plus exceeds that of wrought type 347 stainless steel and is comparable to, or even outperforms, advanced austenitic stainless steels such as Super 304H and 347HFG. Mechanical properties evaluation of both the cast and wrought versions of CF8C-Plus has been and is being conducted in support of ASME code case qualification, a key step toward widespread application of this material in elevated temperature structural components. This paper comprehensively assesses and compares the tensile and creep properties of both cast and wrought versions of CF8C-Plus, including weldments. Furthermore, this study delves into the derivation of yield strength, tensile strength, and maximum allowable stress values in the context of two approved ASME code cases for cast CF8C-Plus and preliminary analysis for wrought CF8C-Plus.

This project is sponsored by the U.S. Department of Energy (DOE), Office of Fossil Energy and Carbon Management, Crosscutting Research, High Performance Materials Program, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory (ORNL) managed by UT Battelle, LLC.