Master of Science, The Ohio State University, 2018, Welding Engineering

Throughout the oil and gas industry, dissimilar metal welds (DMW) between high strength low alloy steels and nickel base alloys are used to join pipes particularly for service in subsea environment. Using a DMW combines the high strength of carbon steel at a low price while the nickel base alloy provides corrosion resistance to the subsea environment and internal fluids being produced. Corrosive protection is provided using cathodic protection (CP) in which sacrificial anodes corrode preferentially. Failure of DMWs under these conditions have been reported in a variety of welds. Failure occurs at the dissimilar metal interface costing the operating company millions of dollars in repair and downtime. The cracking mechanism for these failures is hydrogen assisted cracking (HAC). Hydrogen generated on the pipe surface as a result of the cathodic reaction penetrates the metal lattice and makes its way to the weld interface. Failures has been experienced in a variety of welds which has led to the investigation in determining which welds are most/least susceptible to hydrogen assisted cracking. Several variables determine weld characteristics such as material combination, welding process, post weld heat treatment (PWHT), and joint geometry/interface. These factors will influence the microstructure found at the fusion boundary, which can be detrimental to HAC susceptibility. The susceptibility to HAC of four welds produced with characteristically different variables were evaluated in this study. To achieve this goal, the project was broken into two parts: 1) metallurgical characterization of the welds, which included hardness mapping, microstructural quantifications and identifying chemical compositions along the fusion boundary; 2) ranking the susceptibility to HAC based on the time to failure criterion using the delayed hydrogen cracking test (DHCT) developed at The Ohio State University. The DHCT will identify which welds/interfaces are most susceptible, while the char (open full item for complete abstract)

Committee: Boian Alexandrov (Advisor); Carolin Fink (Committee Member) Subjects: Engineering; Materials Science; Metallurgy

Doctor of Philosophy, The Ohio State University, 2018, Welding Engineering

Subsea high pressure equipment used in production of oil and gas is routinely clad with nickel base alloys for corrosion protection. In the equipment with partial clad for sealing purpose, dissimilar metal interfaces are possibly exposed to the production fluids containing H2S. After cladding, a high hardness heat affected zone (HAZ) is produced in the base metal adjacent to the fusion boundary and is possibly susceptible to hydrogen assisted cracking (HAC) and sulfide stress cracking (SSC). National Association of Corrosion Engineers (NACE) standard MR0175/International Standard Organization (ISO) 15156 requires that HAZ hardness should be less than 22 HRC or 250 VHN. Postweld heat treatment (PWHT) is applied to reduce the HAZ hardness to meet this requirement. However, PWHT causes the carbon to diffuse from the base metal to the weld metal and pile up in a narrow region adjacent to the fusion boundary, possibly causing interface embrittlement. Also, prolonged PWHT can overtemper the base metal and impair its strength. Therefore, the optimal PWHT conditions need to be determined, which reduce the HAZ hardness to meet the industry standard, do not harm base metal strength, and do not increase the HAC and SSC susceptibility near or at the fusion boundary. In this work, nickel base Alloy 625 overlays on F22 (2.25Cr-1Mo) steel and AISI 8630 steel, or F22/625 and 8630/625 dissimilar metal welds (DMWs), were studied. A wide range of PWHT conditions indicated by Hollomon-Jaffe Parameter (HJP) was investigated to determine an optimal balance between HAZ softening and interface embrittlement. Vickers hardness testing revealed that the CGHAZ hardness decreases with the HJP increase due to martensite decomposition. There is a secondary hardening effect in F22 CGHAZ. The hardness of the planar growth zone (PGZ) of the interface and the weld metal increases with HJP, and the PGZ hardness increases at a higher rate than the weld metal. Nanoindentation and optical microsc (open full item for complete abstract)

Committee: John Lippold (Advisor); Boian Alexandrov (Committee Member); David Phillips (Committee Member) Subjects: Engineering; Materials Science; Metallurgy