Master of Science, The Ohio State University, 2016, Welding Engineering

Pipes of solid solution strengthened Ni-based alloys, as Alloy 826 and Alloy 800H, have been used for high temperature service in once through steam generators (OSTGs) on off-shore platforms. The oil and gas industry is seeking to increase service temperature, improve service reliability, and extend service life to 40 years of such installations. Alloy 230 has better-high temperature stability and mechanical properties, and higher service temperature than Alloys 825 and 800H, and is therefore considered as a potential replacement of these alloys in newly built OTSGs. However, the weldability and the high temperature service behavior in welds of Alloy 230 have not been thoroughly investigated yet. This study is a comprehensive comparative research focused on susceptibility to solidification cracking and stress relief cracking in Alloys 800H, 825, and 230. To evaluate the solidification behavior and solidification cracking susceptibility in these alloys, the Cast Pin Tear Test (CPTT), thermodynamic simulations with Thermo-Calc, and the technique of Single-Sensor Differential Analysis (SS-DTA) were used. The results revealed that Alloy 230 and Alloy 825 were more resistant to solidification cracking than Alloy 800H, due to narrower solidification temperature range and crack back filling with eutectic constituents. The OSU Stress Relief Cracking (SRC) Test was applied to evaluate the susceptibility to stress relief cracking in autogenous gas tungsten arc welds of the investigated alloys. None of the three alloys failed by stress relief cracking mechanism while loaded at stress equal to 90% of the high temperature yield strength at 650 oC for 8 hours.. Alloys 825 and 800H showed significant amount of stress relief, while Alloy 230 sustained the applied load at 650C with almost no stress relief. Tensile testing at 650 oC after the 8 hours SRC test showed that the autogenous weld in Alloy 230 had significantly higher yield and tensile strength and slightly lower el (open full item for complete abstract)

Committee: Boian Alexandrov (Advisor); Avraham Benatar (Advisor) Subjects: Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2022, Welding Engineering

Ductility-dip cracking (DDC) in face-centered cubic (FCC) alloys, such as nickel-based alloys and 300-series stainless steels, is a challenge faced by nuclear power generation. Aging reactors need to be repaired via multipass weld overlays to extend their lifetime. DDC often occurs in the first few layers of these overlays, and the nuclear industry has low flaw tolerance, making DDC subject to costly repair and rework. The prevailing theory describing DDC is based on observations of grain boundary (GB) sliding, microvoid formation, and the effect of GB tortuosity. This work aims to quantify the effect welding process parameters and welding generated stresses have on the formation of DDC and to provide clear avenues for productive future research. The main project objectives include the development of methodology, based on combining physical experiments and computational modeling, for prediction of DDC in multipass welds of austenitic alloys that is applicable for materials selection and process optimization. An additional study on the DDC fracture surface was conducted due to findings from the experimental component. Research began with the development of a Gleeble-based experimental procedure that evaluates a material's susceptibility to elevated temperature embrittlement. The procedure is called simulated strain ratcheting (SSR), and preliminary testing led to the use of the imposed mechanical energy (IME), defined as the integral of experienced stress vs. strain, as a parameter for quantification of thermo-mechanical loading in Gleeble tests and FEA models of multipass welds. This experimental procedure was used to successfully generate DDC in various nickel-based alloys and 310 stainless steel. Fracture surfaces generated from this testing were found to exhibit thermal faceting (TF), which warranted further study. Samples which contained high amounts of DDC, or those which experienced fracture, also generally experience higher IME than those which showed no s (open full item for complete abstract)

Committee: Boian Alexandrov (Advisor); Avraham Benatar (Committee Member); Carolin Fink (Committee Member); John Lippold (Committee Member); Michael Mills (Committee Member) Subjects: Engineering; Materials Science; Metallurgy