| Abstract Scope |
Introduction
Dissimilar metal welds (DMWs) are routinely used in the oil and gas industries for structural joining of high-strength steels to eliminate the need for post-weld heat treatment (PWHT) in field welding. In-service failures of subsea oil and gas extraction systems, caused by hydrogen-assisted cracking (HAC) in DMWs, have resulted in extensive damages to the environment and expensive repairs, shutdowns, and loss of production. These failures have been attributed to local hydrogen embrittlement of susceptible microstructures that form at the fusion boundary of DMWs during welding and PWHT.
Using the delayed hydrogen cracking test (DHCT), researchers at the OSU Center for Weldability Evaluation have generated an extensive database of HAC susceptibility in DMW. The database contains more than 200 DHCT results from DMWs of various base metal / filler metal combinations, weld designs, welding processes, welding and PWHT procedures.
Method
The DHCT was developed to address the need for a simple and reliable testing method for evaluating the HAC susceptibility in DMWs. This test applies a constant tensile load, below the yield strength of the tested material, on a flat gauge section specimen that is subjected to simultaneous electrolytic charging with hydrogen. The tensile load is applied normal to the DMW fusion boundary, which is located in middle of gauge section.
DHCT ranks HAC susceptibility in DMWs by time-to-failure at loads equivalent to 90% of the base metal yield strength. Conservative criterion for resistance to HAC was defined as no-failure in the delayed hydrogen cracking test (DHCT) for 1,120 hours at load equivalent to 90% of the base metal yield strength.
Additional criteria for HAC susceptibility, as sustained mechanical energy (SME) and sustained displacement (SD) are currently under evaluation. The SME and SD are calculated respectively by multiplying the test load with the time-to-failure and integrating the specimen elongation vs. time curve.
Results
A database of HAC susceptibility in more than 200 DMWs was generated using the DHCT. The database covers DMWs produced with various base metal / filler metal combinations, welding and PWHT procedures, and weld designs. The relative HAC susceptibility was ranked by time-to-failure in the DHCT. 20% of all tested DMWs did not fail the test and were ranked as resistant to HAC. 98% of the HAC susceptible DMWs failed before full saturation with hydrogen (estimated 560 hours) with 91% of the failures occurring within 200 hours.
The DMWs in the database fit into 4 general categories of HAC susceptibility: 1) resistant 20%, 2) resistant with occasional failures 7.5%, 3) susceptible with occasional non-failures 10%, 4) susceptible 53.5%, and 5) invalid results 9%. Only one base metal / filler metal combination, in varying weld designs and welding procedures, has shown consistent resistance to HAC.
Conclusions
The HAC susceptibility database validated the previously established criterion for HAC resistance in DMWs of non-failure in the DHCT for 1,120hours at load of 90% base metal yield strength.
Times to failure in the order of minutes to hundreds of hours and no failures for more than 1000 hours demonstrated the sensitivity of the DHCT to the effect of materials combinations, welding and PWHT procedures on HAC susceptibility in DMWs.
The database for HAC susceptibility in DMWs is currently being expanded with additional base metal / filler metal combinations, welding and PWHT procedures, and weld designs. It can be used in materials selection and in welding and PWHT procedures development for DMWs with improved resistance to HAC.
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