UNS N06600 Research Articles

Additive Manufacturing: Corrosion Proofing by Infusion of Interstitial Solute—Exemplified for Alloy 22

The corrosion resistance of Cr-containing alloy parts made by additive manufacturing can be significantly improved by a post-treatment of gas-phase-based infusion of concentrated interstitial solute (carbon and nitrogen). We demonstrate this universal approach for the example of low-temperature nitrocarburization by solid-reagent pyrolysis applied to Alloy 22 (UNS N06022) parts made by laser powderbed fusion. We show that the post-treatment improves the crevice-corrosion resistance of these parts, as well as the corrosion resistance of corresponding parts made from wrought Alloy 22 to surpass the maximum crevice corrosion test temperature specified in ASTM G48-D, whereas non-treated samples typically fail well below. Similarly, cyclic potentiodynamic polarization testing (ASTM G61-86) demonstrates that the post-treatment makes the additively manufactured alloy and the wrought alloy more corrosion-resistant than the non-treated wrought alloy.

Passivation of precipitation-hardened UNS N07718 in a shallow sour aqueous solution.

Passivation of precipitation-hardened UNS N07718 in 5 wt% NaCl + 0.5 wt% CH3COOH was investigated. Cyclic potentiodynamic polarisation revealed that the alloy surface was passivated without active-passive transition behaviour. The alloy surface was in a stable passive state during potentiostatic polarisation at 0.5 VSSE for 12 h. Bode and Mott-Schottky plots showed that the passive film became electrically resistive and less defective with n-type semiconductive properties during the polarisation. X-ray photoelectron spectra revealed that Cr- and Fe-enriched hydro/oxide layers were formed on the outer and inner layers of the passive film, respectively. The thickness of the film was almost constant with the increase of the polarisation time. The outer Cr-hydroxide layer changed into a Cr-oxide layer during the polarisation, resulting in a decreased donor density in the passive film. The film's composition change during the polarisation should be related to the corrosion resistance of the alloy in the shallow sour conditions.

Failure Analysis of Once-Through Steam Generator (OTSG) Tube

Abstract The once-through steam generator (OTSG) produces superheated steam using purified feed water. The plant-specific water quality, steam quality, high temperature, and pressure operations lead to the leakage of the OTSG tubes with economic, safety, and environmental consequences. Tube leakage is one of the most frequent causes of OTSG tube failure. A leaking tube was discovered within the OTSG unit of the 110 MW cogeneration plant. The failed section of the tube was removed from the steam generator. Several metallurgical examinations of this tube segment were performed to identify the failure mode and cause. A portion of the tube was analyzed using optical emission spectroscopy (OES) to determine the alloy composition. The results confirmed that the tubing was fabricated from a material consistent with chemical specifications for ASME Specification SB 407 Inconel Alloy 800 (UNS N08800). Glass bead blasting was used to determine the deposit-weight-density (DWD). The DWD value was a maximum of 5.1 g/ft2. The maximum internal deposit thickness was 0.002 in. No evidence of overheating was observed. Scanning electron microscope-energy-dispersive x-ray analysis (SEM-EDXA) was used to determine the elemental composition of the internal deposits. The results indicated that the internal gray deposits primarily comprised iron, chromium, and nickel compounds. There were also fewer amounts of sodium, silicon, aluminum, potassium, and calcium species. The subject tube failure involved a through-wall crack that occurred as stress corrosion cracking (SCC). Additions of caustic solution used in OTSG water treatment practices potentially induced corrosive substances into the tube.

Failure analysis of a heavy gauge fastener of the ITER toroidal field gravity support system

The Gravity Supports (GS) of the ITER Toroidal Field (TF) coils are positioned at the bottom of the machine and are situated under the 18 toroidal field coils. Each support includes 26 high strength bolts with M60 and M85 gauge. The TFGS will sustain a total load of about 11,000 tonnes of dead weight of the magnet system. Moreover, they will be exposed in operation to large electromagnetic forces and possible seismic dynamic loads. The bolts are manufactured from forged rods of double aged UNS N07718 (also known as Inconel® 718), a high strength nickel base superalloy. Stringent material specification and quality controls requirements apply to the threaded bolts and the other components of the GS. A complete break of one M85 bolt was discovered, that occurred about 40 days after its installation and preloading, prior to the application of any operating stress. Half of the part blew out from the bolthole and was found lying horizontally on the GS top area. Following the incident, a comprehensive failure analysis was carried out, based on a combination of non-destructive and destructive examinations including advanced techniques such as computed microtomography, immersion ultrasonic testing (UT) and fracture mechanics. The delayed rupture was understood as due to a combination of cavities not closed by the forging operations associated to a continuous network of brittle secondary phases at the grain boundaries, resulting in a local lack of ductility, poor impact toughness and mechanical properties locally lower than specified. As a consequence of the incident, UT procedures were developed to confirm that the remaining bolts, including the installed ones, are free of cavities using sufficiently conservative criteria and fit for purpose prior to operation. In particular, in-situ axial inspections allowed installed bolts that could contain relevant imperfections to be identified and replaced.

Pitting corrosion resistance of additive manufactured alloy 718 in alkaline brines at elevated temperatures

AbstractThe feasibility of additive manufacturing (AM) to produce industrial components, also for the oil and gas industry, has been demonstrated in the past. Therefore, current research efforts focus on demonstrating the reliability of AM materials subjected to demanding service conditions. In this regard, special attention has been paid to its corrosion resistance in harsh environments as one of the most critical properties in these applications. The beneficial combination of high strength, excellent thermal stability, and outstanding corrosion resistance makes alloy 718 (UNS N07718) the most used wrought and AM nickel alloy in the oil and gas industry. Wrought 718 has an extensive record of field performance in demanding oil and gas applications including directional drilling tools that undergo extreme mechanical loads in corrosive drilling fluids. On the other hand, limited data regarding the corrosion behavior of AM718 in typical drilling environments has been collected so far. In this research work, the pitting susceptibility of selective laser melted alloy 718 in simulated drilling environments has been studied. Open circuit potential measurements as well as cyclic potentiodynamic and potentiostatic polarization tests were used to characterize the pitting corrosion resistance of AM718 in alkaline brines at elevated temperatures. In addition, microstructural particularities in alloy 718 were investigated to explain the observed differences in the electrochemical behaviors.

Numerical investigation of creep crack growth behavior of UNS N10003 alloy based on the creep damage model

Creep crack growth (CCG) analysis is extremely important for integrity evaluation of the defective structures at elevated temperatures in a thorium molten salt reactor (TMSR). However, a numerical method of CCG analysis that can be applied to the TMSR has not yet been developed. In this study, the CCG behavior of a UNS N10003 alloy was investigated based on the Liu and Murakami (L-M) creep damage model. First, the material constants were obtained according to the uniaxial creep test data. Second, finite element analysis (FEA) of CCG for compact tensile (CT) specimens was conducted, and the influence of factors such as element meshing, crack tip radius, side groove, and fillet was analyzed. The results indicated that the crack tip radius and fillet at the side groove demonstrated a significant effect on the accuracy and rationality of the FEA model. Moreover, the multi-axial parameter α in the L-M model was obtained by comparing the calculated results with the CCG test data. The calculated results were found to be more conservative than the experimental results. Finally, the proposed numerical analysis method was applied to the CCG life-span prediction of the pressure vessel in the TMSR. Thus, this study suggests an effective, reliable numerical method for CCG behavior prediction of the UNS N10003 alloy, which has important engineering significance for structural integrity evaluation at elevated temperatures in the TMSR.

Mechanism of tellurium induced nickel alloy corrosion in molten LiCl-KCl salt

Fission product tellurium causes severe embrittlement of nickel alloys in nuclear molten fuel salt systems. This paper investigates the tellurium chemistry in molten LiCl-KCl-Li2Te salts and the tellurium reactions at the alloy interface. The reactions for the two-step reduction of Te0 and the electrochemical formation of nickel tellurides have been identified from cyclic voltammograms. The corrosion of UNS N10003 alloy with different tellurium salt chemistries and applied potentials found that tellurium corrosion attack is only present at potentials more positive than the Ni1.43Te/Te2− potential. The critical potential is thus suggested for mitigating tellurium embrittlement using the redox control technique.

Localized Corrosion of Chromium-Containing Conventional and Multiprincipal Element Alloys in Methanol and Aqueous Chloride Environments

The localized corrosion behavior of Type 304L stainless steel (UNS S30400), Alloy 625 (UNS N06625), and three experimental NiCoCrMo multiprincipal element alloys (MPEA), exposed to methanol/lithium chloride solutions with different water contents, is examined in this paper. Cyclic potentiodynamic polarization tests indicated that water increased the pitting and repassivation potentials and the critical temperature at which localized corrosion occurred in chloride solutions. The effect of water on repassivation potential in methanol-chloride solution depended on the chloride concentration and alloy content. At lower chloride concentration and higher alloy content, water had a greater inhibiting effect on repassivation potential, suggesting a competitive reaction between chloride and water on the metal surface in the pit. The localized corrosion resistance of the MPEAs followed the same trend as that of commercial stainless steels and Ni-base alloys in terms of the effect of alloying elements. The implications of these results on the mechanisms of localized corrosion are discussed.

Effect of Post-Weld Heat Treatment on the Corrosion Resistance of Super Austenitic Stainless Steel

Seaborne transportation by merchant vessels amounts to more than 90% of the world’s trade volume and it represents about 20% to 30% of the materials generated by the combustion of the total fuel. The International Maritime Organization (IMO) has enforced the use of 0.5% or less low-sulfur fuel oil when operation in all since 2020, and the installation of a scrubber is considered the most realistic alternative in compliance with IMO’s new regulation. However, the desulfurization process unavoidably creates a corrosive environment, and corrosion from it is mainly concentrated on welded materials. To solve the corrosion problem in that area, post-weld heat treatment was applied on Super Austenitic Stainless Steel (UNS N08031) which is commonly used for scrubbers. Upon heat treatment at 1,160 °C or at a higher temperature after welding, microsegregation of welds and base metals was dissolved. As a result, elements were distributed equally and the corrosion resistance increased due to the formation of a uniform oxide film.

Properties of Hastelloy joint welded via laser wire‐filling

AbstractIn this study, UNS N10003 Hastelloy is welded using laser wire‐filling welding. The welded joint exhibits favorable tensile strength at room and high temperatures. A molten salt corrosion experiment is performed, and it is discovered that the corrosion depth increases gradually as corrosion progresses, although the growth rate decreases. When the corrosion time reaches 96 h, the corrosion depths of the weld bead and base metal are 23.39 and 24.71 μm, respectively, indicating their similar corrosion resistance. The main corrosion mechanism of N10003 Hastelloy in molten salt is the diffusion of Cr and Mo. Ni3Fe and pure Ni phases appear near the outer surface area after corrosion, which can reduce the corrosion rate.

Application of graphite rods in producing Inconel 625 (UNS N06625) joints through the use of microwave radiation energy

Microwave energy is very efficiently being harnessed to join metallic materials these days. Although, use of microwaveenergy to join metallic materials is in its initial stage but, it has led to an outstanding development in the field of manufacturing. In this research work, joining of Inconel 625 (UNS N06625) without any filler material has been performed with the help of a novel process by harnessing microwave energy from a 900 W microwave applicator. Major novelty of thiswork is the application of graphite rods in accelerating the joining process based on the use of microwave radiation energydue to which it could become possible to join the Inconel specimens without using any filler powder. Selective microwave hybrid heating has been performed using six graphite rods and the process time taken for successful joining of Inconel625 specimens has been 360s. Three repetitions have been done using the mentioned process parameters. Mechanical characterization of the developed joints has been done with the help of Vickers micro-hardness tester and tensile testingmachine. The mean microhardness of the joints has been observed to be 325.1 HV at the joint region which came out to be 10.32 % more than that of the base alloy. The mean ultimate tensile strength has been observed to be 319.9 MPa with mean elongation of 5.3% which has been observed to be less than that of the base alloy.

Influence of short-term post welding heat treatments on corrosion resistance of UNS N06625 nickel-chromium-molybdenum alloy

UNS N06625, commonly known as Inconel 625, is a Nickel-Chromium-Molybdenum alloy with a very good corrosion resistance to a wide range of corrosive media and therefore the favorable choice for sea water applications. However, fusion welding operations may compromise its optimal corrosion resistance if a corrective post welding heat treatment (PWHT) is not carried out. In this scenario, optimum induction PWHT parameters are not yet consolidated so that there are still uncertainties that need to be investigated by proper experiments.Two short-term induction PWHTs (holding time, 20 min) at 980 and 1050 °C were investigated and correlated with both microstructural and corrosion resistance of the welded joints, according to Standard ASTM G28 A. Inter-dendritic segregations of key alloy elements have been identified as the main cause of the corrosive attack initiation. The temperature of 1050 °C was found to be the most effective to restore the corrosion resistance of the investigated welded joint.

Feasibility study for the development of Al-Si based coating on Ni-Cu alloy (UNS N04400)

Nickel-based alloys and superalloys find wide applications in marine, chemical and nuclear industries that require prolonged high-performance materials and sustenance against severe environments. Such materials are prone to corrosion due to varied kind of existing chemistries that exist within the actual operating conditions. To overcome such problems, several methods of process improvement, surface treatment and coating techniques are developed. The current research employs a different approach to develop aluminium–silicon based coatings by hot dipping technique on nickel-copper alloy (UNS N04400). An attempt has been made to deposit this coating at 750 °C, followed by diffusion heat treatment in a resistance furnace. A coating in the range of 80–100 µm thickness has been developed. Formation of intermetallic compounds are observed. The microhardness up to 140HV0.1 has been obtained for the developed coating. This paper tests the feasibility of developing aluminium–silicon based coatings on Ni-Cu alloy which can prove beneficial against various environments. The study opens a new possibility for future researches of coating.

Application of vortex air cooling in automated angle grinder: effects on surface integrity – exploratory study

Framatome has developed an automatic grinding bench to reproduce manual dry grinding applied in workshop. This surface finish operation generally leads to tensile residual stress in welding materials. The current study proposes to investigate the effect of vortex air cooling as an innovative option for the lowering of surface effects induced by dry grinding.Grinding assisted by vortex cooling was tested on AISI 304L and Alloy 690 (UNS N06690). The effect of cooling assistance on microstructural properties of these materials was evaluated using microhardness, XRD residual stress measurements, roughness, SEM & EBSD characterizations. The results suggest that vortex cooling has a lower effect on surface finish parameters of stainless steel 304L than those of alloy 690.

A study of wire tool surface topography and optimization of wire electro-spark machined UNS N06690 using the federated mode of RSM-ANN

PurposeThe purpose of this study is to examine the postprocessed wire tool surface using scanning electron microscopy and find out the streamlined conditions of input process variables using multi-objective optimization techniques to get minimum wire wear values.Design/methodology/approachA federated mode of response surface methodology (RSM) and artificial neural network (ANN) is used to optimize the process variables during the machining of a nickel-based superalloy.FindingsThe study explores that with the rise in spark-off time and spark gap voltage, the rate of wire tool consumption also escalates.Originality/valueMost of the researchers used the RSM technique for the optimization of process variables. The RSM generates a second-order regression model during the modeling and optimization of a manufacturing process whose major limitation is to fit the collected data to a second-order polynomial. The leading edge of ANN on the RSM is that it has comprehensive approximation capability, i.e. it can approximate virtually all types of nonlinear functions, including quadratic functions also.

Effect of yttrium content on the corrosion behavior of nickel-based UNS N10003 alloy in high-temperature molten LiF-NaF-KF salt

The corrosion behavior of nickel-based UNS N10003 alloy with different yttrium contents in molten fluoride salt environment was investigated. The results showed that after an 800 °C/1000 h corrosion test, the alloy contained 0.05% yttrium experienced the least weight loss than other alloys without/with higher yttrium contents. Microstructure, surface micro-chemistry analysis indicated that a YF3 and Y2O3 mixed layer formed in the near surface region for this specific alloy. The retardation of active element dissolution because of the present of YF3 was suggested to be the main reason for its superior corrosion resistance in the molten salt environment.

The Localized Corrosion of Ni-Cr-Mo-W Alloy UNS N06022 in Aprotic Solvents Containing Chlorides

Syntheses of pharmaceuticals often require a series of intermediate steps involving reactions in aprotic organic solvents. Some of these solutions can be aggressive towards the metallic materials that constitute the reaction vessels used, so often corrosion-resistant alloys (CRA) such as the Ni-Cr-Mo-W UNS N06022 are employed. Ni-Cr-Mo-W alloys are known for their outstanding corrosion resistance in reducing and oxidizing aqueous solutions as a result of their passive oxide film. However, the application of passive alloys in nonaqueous solvents raises additional concerns due to: i) limited availability of passivating species; ii) different behavior of aggressive halides and acids as a result of the solvent’s physicochemical properties.The aim of this work is to investigate the corrosion of Ni-Cr-Mo-W alloys in aprotic organic solvents and to understand the influence of properties of the solvent on the susceptibility to pitting and to uniform corrosion, at room temperature. A series of experiments was carried out in acetonitrile, dimethylacetamide (DMA) and dimethylformamide (DMF) solutions containing HCl or tetrabutylammonium chloride (Bu4NCl), and with the oxidant 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). A combination of electrochemical tests and subsequent microscopy has shown that N06022 corrodes uniformly in acetonitrile-HCl solutions at much lower HCl concentrations than in aqueous solutions. In DMA- or DMF-HCl solutions, however, N06022 is passive, but undergoes pitting upon anodic polarization. Furthermore, N06022 undergoes pitting in chloride concentrations as low as a few mM in neutral solutions of all aprotic solvents tested. Depending on solvent and concentration, residual water can inhibit or promote corrosion, influencing anodic kinetics and changing corrosion morphology.Ultimately, this study aims to understand why Ni-based CRAs are much more susceptible to localized corrosion in aprotic organic solvents than in water, and explain how the properties of the solvent – or mixture of solvents – can impact corrosion behavior. This work will provide fundamental understanding of the passivity of Ni-based alloys in organic solvents, assisting the selection of materials for nonaqueous applications.

Temperature dependence of He bubble evolution in UNS N10003 alloys under He ion irradiation

The effects of irradiation temperature on helium bubble evolution in UNS N10003 alloys were studied using 500 keV He ion irradiation at a dose of 2 × 1016 ions/cm2 from 650 to 850 °C. As the irradiation temperature increased, the density of the helium bubbles decreased and their size increased. Polyhedral bubbles were observed at 850 °C, and the non-spherical shapes of these bubbles reduced their surface energy. The densities and mean diameters of the bubbles at 650–850 °C show Arrhenius behaviors with activation energies of 1.06 ± 0.17 and 0.35 ± 0.02 eV, respectively. The activation energies indicate that the He bubble evolution at temperatures ranging from 650 to 850 °C is not only controlled by He diffusion via a self-interstitial/He replacement mechanism but is also affected by the migration and coalescence of He clusters or bubbles via surface diffusion. Furthermore, bubble-loop complexes were observed at 750 and 850 °C. With the temperature increasing, the helium bubbles prompted the growth of dislocation loops, suggesting that the interaction between the helium bubbles and the dislocation loops is dependent on the irradiation temperature.

Effect of heat input on microstructure and mechanical properties of laser welded dissimilar grade nickel alloys

This study reports the effect of heat input (21.3–41.3 J/mm) on the microstructure and mechanical properties of pulsed Nd: YAG laser butt welded UNS N10276 (Hastelloy C-276) and UNS N4400 (Monel 400) sheets. The different heat input was achieved by changing the experimental parameters viz., welding speed (350, 400, 450 mm/min), pulse energy (8, 10, 12 J) and pulse width (5, 6, 7 ms). Though the weld macrostructure exhibit full penetration, the morphology and top weld width differs for the attempted conditions. Weld zone microstructures at lower, medium and higher heat input conditions display unmixed islands, dendrite grains with a slender heat affected zones (HAZ) and Cr-carbide rich reaction compounds respectively. Highest micro-hardness is witnessed for a heat input of 30 J/mm. The tensile strength of the weld joints reaches a maximum and tends to decrease with the increasing heat input.

Corrosion aspects of Ni–Cu alloy (UNS N04400) and its surface improvement: a review

Corrosion aspects of Ni–Cu alloy (UNS N04400) and its surface improvement: a review