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DISSIMILAR metal welds (DMWs) between low-alloy steels and austenitic alloys are used extensively in power plants, refineries, and petrochemical industries. The more expensive austenitic alloys are used in regions requiring superior corrosion resistance and/or greater creep strength, while the less-expensive low-alloy steels are used primarily for structural strength. Failures of DMWs occur often and can have a serious impact on facilities efficiency and availability.[1–4] Premature failure of DMWs can cause plant outages that result in a significant loss in revenue.[3] The study of DMWs has become an area of concern as there is an increased demand for life extension of existing power plants. Of the failures report, the majority of cracking in DMWs occurred either along the fusion boundary or at the locations adjacent to the fusion boundary depending on service conditions, welding parameters, and the

F.C. LIU and T.W. NELSON are with the Department of Mechanical Engineering, Brigham Young University, Provo, UT 84602. Contact e-mail: [email protected] S.L. MCCRACKEN is with the Electric Power Research Institute, Charlotte, NC, 28262. Manuscript submitted October 12, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

alloys involved in the DMWs.[2] The difference in chemical composition and crystal structure across the fusion boundary create a sharp change in mechanical properties and stress corrosion cracking susceptibility. In addition, type II boundaries which run parallel to the fusion boundary often form in the weld metal near the fusion boundary.[5,6] It is necessary to develop techniques to modify the microstructure across the fusion boundary and to improve the performance of the DMWs. Changing base metal dilution (BMD) has been an effective method to alter the evolution of boundaries and microstructures near the fusion boundary.[5] In addition, high BMD (50 to 70 pct) can produce a transition zone whose chemical composition lies between the low-alloy steel and austenitic alloy. This transition zone is desirable to reduce the mismatch in chemistry and the coefficient of thermal expansion across the fusion boundary. Thus, high BMD is commonly used to produce transition joints in pipelines and pressure vessels while the BMD is typically minimized in cladding applications to maintain the high-alloy level of the cladding. However, investigations about the microstructure and mechanical properties of high-BMD DMWs are limited. DMWs in power plants are often subjected to PWHT to relieve residual stress. When a weld repair during manufacturing or later repair service is made on a

DMW, an extended PWHT is often required following such repairs. According to previous investigations,[7,8] a PWHT at 1000 C and 4 hours or at 720 C and 72 hours produced a wider soft zone in the low-alloy steel adjacent to the welding interface due to carbon migration across the welding interface. Thus, the effect of PWHT on the development of carbon-depleted zone needs to be evaluated to avoid the premature failure of the repaired DMWs. I