Design against Fatigue of Super Duplex Stainless Steel Structures Fabricated by Wire Arc Additive Manufacturing Process

Additive manufacturing (AM) is increasingly used to make complex components for a wide spectrum of applications in engineering, medicine and dentistry. Wire arc additive manufacturing (WAAM), as one of AM processes, utilises electric arc and metal wire to fabricate fully dense and heavy metal parts at relatively low costs and high-energy efficiencies. WAAM was successfully applied in the production of several welding-based metal structures. Recently, there was a growing interest in WAAM processing of super duplex stainless steels (SDSS) due to their high strength and excellent corrosion resistance, which make them the prime choice for load-bearing structures in marine applications. Although a number of studies investigated the microstructural and mechanical properties of WAAM-processed SDSS components, little is known regarding their fatigue performance, which is critical in engineering design. This study reports on the outcomes of fatigue tests and fracture surface fractography of WAAM-processed SDSS. The results obtained indicate a significant anisotropy of fatigue properties and fatigue crack initiations resulting from internal defects rather than surface flaws. Based on these experimental results, we suggest an effective design methodology to improve the fatigue life of the WAAM-fabricated SDSS components. We also indicate that post-manufacturing surface treatments should not be underlined for the enhanced fatigue resistance of WAAM-processed SDSS structures.

Designing and Optimising Duplex/Superduplex Compact Flanges to Negate HISC in Subsea Applications

Hydrogen Induced Stress Cracking (HISC) is a phenomena related with metals, with potentially severe consequences when utilizing Duplex / Super Duplex stainless steels in subsea equipment exposed to Cathodic Protection (CP). In general, the three conditions need to be present: hydrogen source (CP), susceptible material (Duplex/Super Duplex stainless steel) and high stresses/strains. The third one can be related with loads, calculation promises and parts design in general (for example risers). The designing rules to minimize the risk of HISC failure are still in development and the latest state of art is presented in DNVGL-RP-F112 June 2018 edition. The Compact Flange (CF) is much lighter and more robust connection than standard ASME or API types as the CF design methodology is based on plastic capacity which is presented in the ISO-27509 code. However, ISO standardization covers only topside applications (with IX seal ring type), pressure class range up to CL2500 and simplified face angles methodology. The ISO-27509 flanges are not HISC proven. Therefore, to deal with HISC requirements and subsea conditions the SPO-S (Subsea) flange series with HX seal ring type was defined and type approved by DNVGL based on extensive FEA documentation, testing and field experience. The SPO-S range has, currently, only HISC compliant designs for the 5K, 7.5K, 10K and 15K flange ranges. For the projects where the Duplex / Super Duplex flanges are used subsea with CP, consideration for HISC still must be addressed. For such cases, the process is not straight forward and often many FEA rounds are needed depending on the engineer’s experience and project related restrictions. The process is time consuming and every decision reflects many aspects to finalize the design. Genetic Algorithms (GA) are a good optimization tools to use in such situations, because at the end, the engineer will only get few design propositions to choose from, and the process itself, is highly automated. In the following paper the results for HISC optimized CF flange connection will be presented and compared with standard ISO-27509 flange design. The GA optimization steps and designing process will also be presented in the same example. Commercial FEA code ANSYS will be used for calculations based on parametrical model and GA implemented by APDL scripting. The following optimization method is already in use for HISC related projects.

Nanoscale phase separations in as-fabricated thick super duplex stainless steels

Nanoscale phase separations, and effects of these, were studied for thick super duplex stainless steel products by atom probe tomography and mechanical testing. Although nanoscale phase separations typically occur during long-time service at intermediate temperatures (300–500° C, our results show that slowly cooled products start to develop Fe and Cr separation and/or precipitation of Cu-rich particles already during fabrication. Copper significantly slowed down the kinetics at the expense of Cu-rich particle precipitation, where the high-copper material subjected to hot isostatic pressing (HIP), with Δt500–400 of 160 s and the low-copper hot-rolled plate with Δt500–400 of 2 s had the same level of Fe and Cr separation. The phase separations resulted in lower toughness and higher hardness of the HIP material than for hot-rolled plate. Therefore, both local cooling rate dependent and alloy composition governed variations of phase separations can be expected in as-fabricated condition.

Corrosion Resistance of Dissimilar GTA Welds of Pipeline Steel and Super Duplex Stainless Steels in Synthetic Brine

The effect of weld passes and single V grove designs, on the corrosion resistance of dissimilar welds of a low alloy steel and a super-duplex stainless steel, was studied in synthetic brine. Welds were manufactured in argon by gas tungsten arc (GTA) technique and joined by a high nickel wire of super-duplex stainless steel. Samples of weld regions were characterized by composition scans, electrochemical measurements, micro-hardness and scanning electron microscopy. In X52/ER2594, a transition region (TR) of grain boundaries type II and a band of martensite were formed. The base metal of X52 underwent the highest corrosion rate and localized corrosion occurred in the heat affected zone. Interface ER2594/25Cr7Ni and 25Cr7Ni showed the presence of pitting near intermetallics.

Effect of Ultrasonic Shot Peening on the Microstructure of Austenitic Stainless Steel 316 and Super Duplex Stainless Steel UNS S32750

Shot peening for nanocrystallization of the surface region is a good way of improving corrosion-, fatigue-, and wear-resistance, etc. of metallic parts (Lu, K., 2014. Making strong nanomaterials ductile with gradients. Science, 345(6203), pp.1455–1456). The technique has been widely used for various materials as a method of surface modification. SUS316 has excellent corrosion and oxidation resistance with good formability However, its application is limited by the low mechanical strength and hardness. S32750 (duplex stainless steel) is one of most used tubing materials for oil/gas delivery systems in a corrosive environment under high pressure (Nilsson, J.O., 1992. Super duplex stainless steels. Materials Science and Technology, 8(8), pp.685–700). Thus, improving corrosion resistance is a key for the wider application and better maintenance of S32750. In our study, the alloy S32750 was heat-treated at 1070°C to obtain a precipitation-free microstructure (γ and δ dual-phase structure). It was then ultrasonic shot peened and microstructures were analyzed for: (1) surface nanocrystallization, (2) effect of the treatment processing parameters, and (3) the determination of microstructural evolution and the effect of the shot peening process.

Use of the Critical Acidification Model to Estimate the Influence of W in the Localized Corrosion Resistance of 25Cr Super Duplex Stainless Steels

Two super duplex stainless steels (SDSS) with different W content, namely UNS S32750 (W-free) and UNS S39274 (2.1 wt.% W), were tested in simulated crevice corrosion environments to determine the influence of W on their corrosion resistance. Anodic potentiodynamic polarization experiments were performed in two different crevice-like-solutions: 1 M HCl (as reference for a pH = 0 environment), and 7 M LiCl, adjusted to the same pH value. Galvele’s critical acidification model was used to estimate the theoretical critical potential (Ecrit) and comparatively evaluate the corrosion resistance of the two SDSS. The anodic potentiodynamic polarization results showed a statistically significant difference between the two materials in only one test condition, i.e., 7 M Cl− at 60 °C. Additionally, the quantification of chemical dissolution of the metal cations after the tests suggested a surface enrichment in W only in the 7 M chloride solution. Scanning electron microscope (SEM) analysis indicated a uniform dissolution experienced by UNS S32750 in this environment, whereas UNS S39274 suffered selective corrosion of the ferrite-phase. These observations were reflected in a slight increase in the Ecrit values of UNS S32974 estimated with Galvele’s model.