Stress Corrosion Cracking of Additively Manufactured Alloy 625

Laser bed powder fusion (LPBF) is an additive manufacturing technology for the fabrication of semi-finished components directly from computer-aided design modelling, through melting and consolidation, layer upon layer, of a metallic powder, with a laser source. This manufacturing technique is particularly indicated for poor machinable alloys, such as Alloy 625. However, the unique microstructure generated could modify the resistance of the alloy to environment assisted cracking. The aim of this work was to analyze the stress corrosion cracking (SCC) and hydrogen embrittlement resistance behavior of Alloy 625 obtained by LPBF, both in as-built condition and after a standard heat treatment (grade 1). U-bend testing performed in boiling magnesium chloride at 155 and 170 °C confirmed the immunity of the alloy to SCC. However, slow strain rate tests in simulated ocean water on cathodically polarized specimens highlighted the possibility of the occurrence of hydrogen embrittlement in a specific range of strain rate and cathodic polarization. The very fine grain size and dislocation density of the thermally untreated specimens appeared to increase the hydrogen diffusion and embrittlement effect on pre-charged specimens that were deformed at the high strain rate. Conversely, heat treatment appeared to mitigate hydrogen embrittlement at high strain rates, however at the slow strain rate all the specimens showed a similar behavior.

Fabrication and Mechanical Characterization of Aluminium 7075 Reinforced with Nano Graphene / B4C / Inconel alloy 625 using Ultrasonic Stir Casting Method

Aluminium 7075 widely used in aerospace and defence applications. At the present time, hybrid composite material utility is refining in all engineering areas as a result of high strength /weight ratio, good corrosive and wear resistance. In this research work deals with the fabricating of hybrid metal matrix composite of aluminium 7075 as base metal reinforced by varying weight percentage of Nano graphene (0.5,1,1.5,2) , B4C (2,4,6,8,) and Inconel alloy 625 (2,4,6,8). This boron carbide, Inconel alloy 625 particles having particle size of 50μm. The hybrid aluminium composite material fabricated utilizing liquid metallurgy ultrasonic stir casting method. Tension test, compression test, hardness test and flexural test were conducted to determine mechanical properties of the hybrid composite material. Morphology of the hybrid metal matrix composite were analysed by using SEM.

Increasing the Cost Effectiveness of Mechanically Lined Pipe for Risers Installed by Reel-Lay

The use of mechanically lined pipe (MLP) using a thin liner, i.e. 2.5mm, can provide a more cost effective linepipe material solution relative to a standard 3.0mm liner. This is especially the case for the more expensive liner materials with higher corrosion resistance, including Alloy 625. Thin liners, i.e. 2.5mm, can be used without compromising pipeline integrity and performance, whilst still fulfilling design requirements defined in most pipeline design standards, including DNVGL-ST-F101. The suitability of 2.5mm liner MLP has previously been demonstrated in service over a range of pipeline bundle projects installed with the controlled depth tow method, but not to date for risers installed by reel-lay. This paper presents the details and test results of the qualification programme to support its use for both flowlines and risers installed by reel-lay. The qualification MLP test pipes, which comprised an outer diameter (OD) 219.1mm × wall thickness (WT) 15.9mm X65 + 2.5mm Alloy 625, were manufactured using established manufacturing procedures and facilities. Reeling and fatigue test strings were fabricated using qualified welding solutions. The fabricated test strings were subject to internal visual inspection and dimensional measurement using laser metrology in order to provide a benchmark for comparison post reeling. The test strings were given a simulated reeling procedure using bending and straightening formers representative of a reel-lay vessel with the smallest reel hub diameter, this being a conservative material straining condition. An internal pressurisation technique, as per standard installation practice for the present pipe lay contractor for MLP, was applied during the simulated reeling procedure. Post reeling the internal laser metrology inspection procedure was repeated in order to confirm the integrity of the liner and to check for the presence of any evidence of liner wrinkling or damage. Subsequently, full scale fatigue testing was performed using the high frequency resonance bending procedure. Testing was performed to ultimate failure to determine the fatigue endurance limit of the thin liner MLP. Additionally Finite Element Analysis (FEA) was performed to further validate the satisfactory reeling performance of the thin liner MLP. The FE numerical analysis embraced manufacture of the MLP pipe and test samples coupled with the reeling procedure. Sensitivity analysis on pipe strength and geometrical mismatch was performed to demonstrate the robustness of the linepipe material solution and reeling procedure. All of the critical qualification activities were performed and verified by DNVGL and in accordance with the guidance of DNVGL-RP-A203 Technology Qualification process. The paper highlights the qualification programme performed to enable the cost effective use of thin liner MLP, specifically Alloy 625, for risers installed by reel-lay.

Fatigue Variability of Alloy 625 Thin-Tube Brazed Specimens

As an advanced heat exchanger for aero-turbine applications, a tubular-type heat exchanger was developed. To ensure the optimum performance of the heat exchanger, it is necessary to assess the structural integrity of the tubes, considering the assembly processes such as brazing. In this study, fatigue tests at room temperature and 1000 K were performed for 0.135 mm-thick alloy 625 tubes (outer diameter of 1.5 mm), which were brazed to the grip of the fatigue specimen. The variability in fatigue life was investigated by analyzing the locations of the fatigue failure, fracture surfaces, and microstructures of the brazed joint and tube. At room temperature, the specimens failed near the brazed joint for high σmax values, while both brazed joint failure and tube side failure were observed for low σmax values. The largest variability in fatigue life under the same test conditions was found when one specimen failed in the brazed joint, while the other specimen failed in the middle of the tube. The specimen with brazed joint failure showed multiple crack initiations circumferentially near the surface of the filler metal layer and growth of cracks in the tube, resulting in a short fatigue life. At 1000 K, all the specimens exhibited failure in the middle of the tube. In this case, the short-life specimen showed crack initiation and growth along the grains with large through thickness in addition to multiple crack initiations at the carbides inside the tube. The results suggest that the variability in the fatigue life of the alloy 625 thin-tube brazed specimen is affected by the presence of the brazed joint, as well as the spatial distribution of the grain size and carbides.

Friction Stir Welding: Mitigating the Susceptibility to Intergranular Corrosion of Alloy 625

In this work, friction stir welding (FSW) was employed to alloy 625 grade I (soft annealed) sheets. Therefore, solid-state based welding was undertaken with a tool rotational speed of 200 rpm and welding speed of 1 mm/s. Microstructural features were analyzed by light optical and scanning electron microscopy (SEM). Moreover, microhardness measurements were performed. The susceptibility to intergranular corrosion was verified by the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test. Complementary, intergranular corrosion was evaluated by ASTM G28 Method A. FSW promoted grain refinement, increased microhardness, and reduction in the degree of sensitization. Finally, the mean corrosion rate observed in the ASTM G28A test was 0.4406 mm/year, which suggests a good weld quality.