Introduction: Dissimilar metal welds (DMWs) are extensively used in power generation, heavy fabrication industries and the oil, gas, and petrochemical industries. It is well-known that hydrogen deteriorates mechanical properties of metals and alloys during their service by the phenomenon of hydrogen embrittlement. Hydrogen cracking is the most significant mechanism of failure in many welded components often resulting in a failure below the design limits. Examples of catastrophic failures of DMWs in subsea service under cathodic protection and in petrochemical applications highlight the importance of further investigations on the role of hydrogen in the failure mechanism of welded components. Hydrogen embrittlement is related to the interaction of hydrogen with microstructural features and defects such as dislocations, precipitates, segregations, grain boundaries and cracks. Microstructural features in the vicinity of the fusion boundary in DMWs significantly influence the susceptibility to local hydrogen-assisted cracking. Despite extensive reports on the microstructures-hydrogen interactions, there is a lack of understanding of hydrogen-assisted cracking mechanisms in DMWs. In this study, we attempt to investigate the susceptibility to hydrogen embrittlement of two types of subsea DMWs of nickel-based filler metals on low alloy steel substrate.
Experimental procedure: DMW#1 was made of 25.4 mm thick F22 steel pipes internally clad with 6 mm layer of Alloy 825 and welded in a V-groove configuration. The first 4 passes were performed with GTAW process using FM82 2.4 mm diameter filler wire. The remaining 30 passes were performed with SMAW using Alloy 182 3.2 mm diameter shielded electrodes. The DMW was subjected to a 4-hour PWHT at 720oC. DMW#2 was made of 21.95 mm thick carbon steel pipes internally clad with 3 mm layer of Alloy 825 and welded in a V-groove configuration. The first 3 passes were performed with GTAW process using FM82 1.6 mm diameter filler wire. The remaining 25 passes were performed with SMAW using Alloy 182 3.2 mm diameter shielded electrodes. The DMW was subjected to a 1-hour PWHT at 610oC.
Smooth specimens are cut from welded pipes containing fusion boundary in the center of the gauge section using electrical discharge machining (EDM). The DMW microstructures are characterized using optical microscope (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The Dissimilar fusion boundary is subjected to the delayed hydrogen cracking test (DHCT) using a constant tensile load testing (CTLT) method. In this method, the sample is held at the constant tensile load corresponding to 0.9 yield strength of the base metal. The sample is placed at the center of a platinum wire coil used as the anode. Hydrogen is introduced into the sample by electrochemical charging in a 0.1N H2SO4 and 0.1g/l Na2S2O3 aqueous solution with a pH of 1.2±0.1 at a current density of 10 mA/cm2 at room temperature. The solution is continuously circulated through the test cell and pH is monitored throughout testing to ensure consistent charging. Then, fracture surfaces of failed specimens are observed using SEM.
Results / Discussion: To evaluate the susceptibility of DMWs to hydrogen cracking, the test results, in terms of time to failure, sustained mechanical energy and sustained displacement are evaluated. The sustained mechanical energy and the sustained displacement are determined by multiplying the applied load with the total displacement at the time of failure, and by integrating the displacement vs. time curve, respectively. The post-mortem microstructural characterization and fracture surface illustrate the influence of microstructural features in the vicinity of the fusion boundary on hydrogen-assisted crack initiation and propagation and develop the mechanisms of hydrogen-related failure.
This research is still ongoing and additional investigations are planned for the susceptibility of DMWs to hydrogen-assisted failure and microstructural characterization. Further results and discussions will be provided as soon as experiments and observations have been finished.
Conclusion: It is expected that the microstructure in the fusion boundary region plays a crucial role on the hydrogen-assisted cracking susceptibility in the tested DMWs. The final conclusions will be added after evaluating the results of the ongoing experiments and metallurgical characterization.