EPRI led a research effort from 2013 to 2015 to identify contributing factors to the large number of hardfacing failures—a/k/a delamination or disbonding events—experienced industry-wide. The project purposely engaged stakeholders in the valve supply chain with both users and valve manufacturers sponsoring the effort. Content summaries of the three technical update documents issued in 2015 as a consequence of this effort are below. They are available at no charge to select EPRI members and for a fee to others. To purchase, contact the EPRI Order Center at 650-855-2121 or firstname.lastname@example.org.
Investigated failures occurred primarily in valve components (disc, seat, and/or stem) fabricated from CrMo Grade 91 or 400-series stainless where a cobalt-based hardfacing material (Stellite) was directly clad on the base metal. Failures were observed in components where the stated operating temperature was a nominal 975F or higher.
EPRI continues to investigate and update its guidance on the subject. The latest iteration of this ongoing research effort is being led by Dan Purdy (email@example.com), a senior technical leader in the organization’s Materials and Repair Program. Purdy now is in the early stages of integrating the first report in the three-part series into a more encompassing large-bore valve-body specification.
Cobalt-based hardfacing alloys are used to protect sealing surfaces in high-temperature valve components primarily because of their resistance to wear. Inspection of ex-service valve components has revealed early cracking and disbonding of the hardfacing from the substrate material. Analysis identified the formation of undesirable hard, brittle intermetallic phases in an intermixed zone typically between the substrate and the hardfacing layer.
Although the degree of this first weld pass dilution can affect the extent and kinetics of embrittlement, it is desirable to remove the possibility of the undesirable phase formation entirely through the application of nickel-based-alloy butter layers that do not show the tendency of phase transformation at any level of dilution with the substrate or cobalt-based hardfacing
Report’s objective is to provide scientifically based guidance in the engineering, quality control, and inspection of welded joints between ferritic valve components and cobalt-based hardfacing to avoid delamination in service.
Evaluations of service history and failed ex-service components have led to an understanding that metallurgical changes within the microstructure during welding and high-temperature service exposure contribute to disbonding. Cracking has been shown to prefer bands of unexpectedly hard layers in the weld deposit, and there is evidence of the formation of the brittle intermetallic Sigma phase in those regions. The solution appears to be not one of process—that is, dilution—control, but rather identification of the alloy combinations that remove the possibility deleterious phases will form.
The report discusses the history of hardfacing disbonding as it applies to the power-generation sector of the industry. Included in the timeline are the advances in the state-of-the-art in fabrication, the potential consequences of those changes in processing, a variety of notable failures, and a thorough look at the thermally driven stresses in valve components. Metallurgical analyses of failed components covering a range of material combinations and applications are presented. Many material combinations have been used to varying degrees of success; the report describes the causes of the issues.
This third report elaborates on an exhaustive thermodynamics methodology to predict the formation of deleterious intermetallic phases over a range of alloy combinations, and the degrees of mixing among them. The thermodynamic predictions uncovered a wide range of problematic material combinations, as well as several key parameters that lead to metallurgically stable combinations—regardless of the degree of mixing among the constituents. Stitching together these safe combinations creates a layered hardfacing welding procedure that removes the possibility of harmful phases affecting the matrix and leading to disbonding.
These alternative weld solutions were validated through laboratory trials and extended ageing to demonstrate their long-term stability. Laboratory welds that recreated the problematic combinations were observed to begin their transformation, while alloy combinations that were expected to be free of that risk did not harden.