UNS N06600 Research Articles

Slow Strain Rate Testing for Hydrogen Embrittlement Susceptibility of Alloy 718 in Substitute Ocean Water

The hydrogen embrittlement susceptibility of near-peak-aged UNS N07718 (Alloy 718) was evaluated by performing slow strain rate tests at room temperature in air and substitute ocean water. Tests in substitute ocean water were accomplished in an environmental cell that enabled in situ cathodic charging under an applied potential of −1.1 V versus SCE. Some specimens were cathodically precharged for 4 or 16 weeks at the same potential in a 3.5 wt.% NaCl-distilled water solution at 50 °C. Unprecharged specimens tested in substitute ocean water exhibited only moderate embrittlement with plastic strain to failure decreasing by about 20% compared to unprecharged specimens tested in air. However, precharged specimens exhibited significant embrittlement with plastic strain to failure decreasing by about 70%. Test environment (air or substitute ocean water with in situ charging) and precharge time (4 or 16 weeks) had little effect on the results of the precharged specimens. Fracture surfaces of precharged specimens were typical of hydrogen embrittlement and consisted of an outer brittle ring related to the region in which hydrogen infused during precharging, a finely dimpled transition zone probably related to the region where hydrogen was drawn in by dislocation transport, and a central highly dimpled ductile region. Fracture surfaces of unprecharged specimens tested in substitute ocean water consisted of a finely dimpled outer ring and heavily dimpled central region typical of ductile fracture.

Fracture resistance testing of dissimilar nickel–chromium girth welds for clad line pipes

This work presents an experimental investigation of the ductile tearing properties for the girth weld of a typical C–Mn pipe internally clad with ASTM UNS N06625 Alloy 625 using measured crack growth resistance curves (\(J{-}\Delta a\) and \(\mathrm {CTOD}{-}\Delta a\) curves). Here, the material of the external pipe is a typical API 5L Grade X65 pipeline steel whereas the inner clad layer is made of a nickel–chromium corrosion resistant alloy steel. Testing of the girth weld employed side-grooved, clamped SE(T) specimens with a weld centerline notch to determine the crack growth resistance curves based upon the unloading compliance method using a single specimen technique. This experimental characterization provides additional toughness data which serve to evaluate the effectiveness of current procedures in determining accurate experimentally measured R-curves for this class of material, including the effects of weld strength mismatch.

Factors Influencing Passivity Breakdown on UNS N08800 in Neutral Chloride and Thiosulfate Solutions

In this work, the passivity degradation of UNS N08800 in solutions containing Cl− and S2O32− is studied by using polarization curve, electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS). Experimental results reveal that the passivity breakdown heavily depends on anodic potentials, concentration ratios of Cl− to S2O32−, and the composition of materials. A combined effect between Cl− and S2O32− on pitting corrosion is observed at high anodic potentials at which the passive film is broken down; in this situation, S2O32− ions can enter into the pits by electromigration, and they are reduced to Sads and S2−, stabilizing the metastable pits and accelerate the pit growth rate. However, there is no such combined effect at low anodic potentials when the passive film is intact, on the contrary, S2O32− is beneficial to passivity in chloride solutions. At low anodic potentials, the interaction of S2O32− with an intact passive layer is weak, and the adsorption of S2O32− would mitigate the detrimental effect of Cl− ions.

Plane strain compression testing of Sanicro 28 by channel-die compression test: A direct microstructural observation

The split-channel plane strain die compression test was performed in Sanicro 28 (UNS N08028) high-alloy austenitic stainless steel. The same grain was examined before and after interrupted split-channel plane strain die compression tests by electron backscattered diffraction (EBSD). All deformed specimens exhibited developments in deformed microstructure. The development of near boundary gradient zone (NBGZ) was quantified for specimens deformed at different reductions. NBGZ development was gradual, it increased as a function of deformation. The image quality (IQ) map from EBSD indicated the imposed strain and formation of slip lines in the deformed microstructures. The inclined nature of slip lines indicated non-uniform distribution of strain and few grains did not develop any slip traces. The knowledge about NBGZ is important to enhance/control localized corrosion phenomena

The Effects of Ultrasonic Nanocrystal Surface Modification Technique Temperature on Microstructure and Wear of Alloy 600

Alloy 600 (UNS N06600) is an austenitic nickel-based alloy with superior corrosion resistance and high-temperature endurance, which determines its widespread applications in aeronautical, aerospace, marine and nuclear industries. Particularly, a number of nuclear components used Alloy 600 as their structure materials due to their high corrosion resistance, high-temperature endurance and excellent fabricant characteristics. Many failures have occurred in Alloy 600 with various forms of environmental degradations during long-term operation. In this study, an ultrasonic nanocrystal surface modification (UNSM) technique was applied to Alloy 600 at a room and a high temperature of 500 OC. The effects of UNSM treatment temperature on the microstructure and wear behavior including a compressive residual stress were investigated. The hardness, compressive residual stress with respect to depth from the top surface were measured. Also, the wear behavior of UNSM-treated at a room and a high temperature Alloy 600 specimen was compared to that of the untreated specimen. The increase in wear resistance by UNSM technique was discussed in terms of increased hardness, refined grain size and induced compressive residual stress.

Microstructure and mechanical properties of UNS N10003 alloy welded joints

Microstructure and mechanical performance of the welded joints of UNS N10003 alloy have been investigated in this work. Primary precipitates in base metal and eutectic precipitates in Heat Affected Zone (HAZ) and weld metal have been characterized and identified as M6C type. The hardness value of HAZ (Eutectic zone) is significantly higher than the rest of joint, including weld and base metal. Tensile tests suggested the welded joints possess a very stable mechanical performance at elevated temperature from 650 °C to 725 °C. Moreover all the tensile samples were fractured in base metal, indicating that the eutectic carbides have no adverse effects on the short-time mechanical performance of joint. The fine carbides, acting as dispersion strengthening in weld metal, are main contribution to enhance the hardness and strength of weld. The good mechanical performance of HAZ is ascribed to the presence of eutectic carbides and twins.

Microstructure Evolution and Properties of Hastelloy UNS N06200 and UNS N06022 as a Function of the Applied Thermal Treatment

The aim of this study is focused on the mechanical and the microstructural characterization of two Ni-based cast alloys: Hastelloy UNS N06200 and Hastelloy UNS N06022. The analysis has been performed through the observation of different microstructures obtained as a function of different cooling rates performed by the use of test blocks. After solidification, the specimens produced by different cooling rates have been extracted, and UNS N06200 samples have undergone a solution treatment process, while UNS N06022 samples have undergone a solution treatment followed by an ageing process. The mechanical properties have been measured, and some significant and consistent relationships have been pointed out with the heat treatment and the obtained microstructures.

Materials Selection for Use in Concentrated Hydrochloric Acid

Hydrochloric acid (HCl) is an important mineral acid with many uses, including the pickling of steel, acid treatment of oil wells, and chemical cleaning and processing. This acid is extremely corrosive and its aggressiveness can change drastically depending on its concentration, the temperature, and contamination by oxidizing impurities. One of the most commonly encountered oxidizing impurities is the ferric ion. In general, stainless steels cannot tolerate aggressive HCl solutions, hence the need to use corrosion resistant nickel-based alloys. A part of this study focused on the role of alloying elements on the corrosion performance of commercial nickel-based alloys UNS N10276, UNS N06022, UNS N06200, UNS N07022, UNS N10362, UNS N10675, UNS N06059, and UNS N06625, in HCl solutions, with and without the presence of oxidizing impurities (ferric ions). Aggressive HCl solutions can also be used to simulate the critical crevice solution. Therefore, another aspect of this research is to investigate the role of...

Hydrogen assisted crack initiation and propagation in a nickel-based superalloy

To understand the mechanism for hydrogen-induced embrittlement in a nickel-based superalloy, detailed electronmicroscopy characterisation has been employed on the UNS N07718 (Alloy 718) after hydrogen charging and slow strain rate testing to investigate the strain localisation and damage accumulation caused by hydrogen. Transmission Electron Microscopy analysis demonstrates that the microstructure of the material after tension is characterised by planar dislocation slip bands (DSBs) along {111}γ planes. Consistent results from Electron Channeling Contrast Imaging (ECCI) reveal that cracks always propagate along planar DSBs in the presence of hydrogen. This phenomenon is rationalized by the evident nucleation of nanoscale voids along the DSBs, especially at the intersections between nonparallel DSBs. The proposed mechanism, confirmed by both the ECCI analysis and fractographic study by Scanning Electron Microscopy, indicates that the interaction between the hydrogen and dislocations along the DSBs leads to void nucleation. Furthermore, the results suggest that coalescence and widening of voids via the dislocation process promote the crack propagation along the DSBs in hydrogen charged Alloy 718.

Corrosion Performance of Inconel 625 in High Sulphate Content

Inconel 625 (UNS N06625) is a type of nickel-chromium-molybdenum alloy with excellent corrosion resistance in a wide range of corrosive media, being especially resistant to pitting and crevice corrosion. However, in aggressive environment, Inconel 625 will suffer corrosion attack like other metals. This research compared the corrosion performance of Inconel 625 when exposed to higher sulphate content compared to real seawater. The results reveal that Inconel 625 is excellent in resist the corrosion attack in seawater. However, at increasing temperature, the corrosion resistance of this metal decrease. The performance is same in seawater with high sulphate content at increasing temperature. It can be concluded that sulphate promote perforation on Inconel 625 and become aggressive agents that accelerate the corrosion attack.

Localized Corrosion of Corrosion Resistant Alloys in H2S-Containing Environments

Corrosion resistant alloys (CRAs) are widely used in harsh oil and gas production environments where low-alloy steels cannot be used. It is recognized that stress corrosion cracking (SCC) of stainless steels and Ni-based alloys can occur at potentials above the repassivation potential (Erp) for localized corrosion. This paper focuses on experimental work that has been conducted to determine Erp of six CRAs, i.e., super-martensitic stainless steels S13Cr (UNS S41425) and S15Cr (UNS S42625), super-duplex stainless steel 2507 (UNS S32750), and solid solution Ni-based Alloys 2535 (UNS N08535), 29 (UNS N08029), and 28 (UNS N08028) in chloride-H2S environments. A generalized approach is applied to determine Erp values from electrochemical polarization results at temperatures ranging from 85°C to 232°C. The use of completely deaerated solutions and ensuring the stable growth of localized corrosion are both critical to obtain conservative Erp values for CRAs in environments that are similar to downhole conditions...

The high-temperature corrosion of Hastelloy N alloy (UNS N10003) in molten fluoride salts analysed by STXM, XAS, XRD, SEM, EPMA, TEM/EDS

The effect of Fe ion impurity on the corrosion behavior of Hastelloy N (UNS N10003) alloy in molten FLiNaK salts at 850°C has been investigated by combined synchrotron radiation and other characterization techniques. Results showed that Mo and Cr were depleted from the alloy surface, where Fe-rich layer formed. The corrosion process was mainly controlled by the redox reaction between Fe ion and Cr, and no new compound with high valence state formed in the alloy surface. The loss of Mo and Cr in M12C carbide occurred due to the presence of a concentration-gradient between M12C carbide and matrix.

Sensitivity analysis of the Expansion Process for Alloy UNS N08028

Due to the good mechanical properties of forged parts, the forging process plays a decisive role in the manufacturing of seamless stainless steel pipes for oil country tubular goods (OCTG) lines. Tough competition between manufacturers gives them plenty of incentive to make their processes in raw material and energy usage more and more efficient. In this context the expansion process is one of the critical production steps in the manufacturing of seamless stainless steel pipes. This work presents a sensitivity analysis of a finite element method (FEM) for the simulation of the expansion of the alloy UNS N08028. The input parameters ram speed, tool angle, initial ID and final ID of the billet as well as temperature were used to describe responses like tool wear and material loss. With the aim to minimize the tool wear and to reduce the material waste, a study of influence of the input parameters on the mentioned responses were performed. This development is supported by experimental work in order to validate the simulation model. The sector demand for new materials with specific properties and the cost-intensive experimental trials justifies the use of such simulation tools and opens great opportunities for the industry.

Surface Modification of Oilfield Alloys by Ultrasonic Impact Peening: UNS N07718, N07716, G41400, and S17400

Ultrasonic impact peening (UIP) is a severe plastic deformation process to induce localized surface hardening combined with compressive residual stresses which therefore extends the useful life of mechanical parts. In this investigation, UIP has been applied to four widespread alloys in use in the oilfields. These include two premium NiCrMo alloys, UNS N07718 (718) and UNS N07716 (625 Plus®), both characterized by satisfactory oilfield performance but lacking hardness and abrasive wear resistance, and two relatively low-cost alloys, UNS G41400 (4140) and UNS S17400 (17-4PH), both limited by their corrosion fatigue. To promote comparisons and determine important alloy parameters for successful UIP, all four alloys were carefully selected so that their respective yield strengths were within relative proximity (~780 to ~910 MPa), and then ultrasonically impact peened under identical conditions. Among major findings from microstructural examinations, micro-hardness indentations, and residual stress measurements, surface topological changes (roughness), alloy microstructural evolution (depth and extent of strain hardening, including mechanical twinning in the NiCrMo alloys), and compressive residual stresses were found to be well correlated. Among all four alloys, the NiCrMo alloys, in particular UNS N07716 was found to be best suited for UIP. This is explained by its FCC austenitic microstructure, relatively low stacking-fault energy (prone to mechanical twinning), and in practical terms high yield strength and high tensile-to-yield strength ratio, both related to its excellent plastic flow behavior under ultrasonic rates of plastic deformation.

Electrochemical Corrosion Performance of Mechanically Polished Alloy 690TT at High-Temperature Water (200°C)

The electrochemical corrosion performance of thermally treated (TT) Alloy 690 (UNS N06690) was investigated in 200°C pressurized water before and after mechanical polishing/grinding. The surface analysis of the specimens was performed using atomic force microscopy, x-ray photoelectron spectroscopy, transmission electron microscopy coupled with energy dispersive spectroscopy, and focused ion beam microscopy. A Cr-rich outer layer formed during the TT process was detected at the surface of the as-received Alloy 690TT specimens. As a result of the greater formation of the Cr-based oxide film, the as-received specimen showed a more desirable corrosion behavior in both oxygenated and deoxygenated water environments. The near-surface microstructure of the as-received specimen was changed by mechanical polishing/grinding of the surface. The ground specimen showed a higher degree of deformation/dislocation than the polished specimen.

The Role of Alloying Elements on the Crevice Corrosion Behavior of Ni-Cr-Mo Alloys

The roles of the alloying elements Mo, Cr, and W in resisting crevice corrosion of UNS N06022, UNS N06625, and UNS N10362 have been studied under galvanostatic conditions in 5 mol/L NaCl at 150°C. Corrosion damage patterns were investigated using surface analytical techniques such as scanning electron microscopy and optical imaging, and the corrosion products characterized by energy dispersive x-ray spectroscopy. While the Cr content of the alloy is critical in controlling initiation of crevice corrosion, the rate of activation (passive-to-active transition) is influenced by both the Cr and the Mo (and W) contents. The alloy’s Mo content also determines the distribution of corrosion damage within the crevice. In alloys with high Mo content, corrosion propagates laterally across the surface, while in alloys with low Mo content, it penetrates into the alloy. This can be attributed to the accumulation of molybdates (and tungstates), which stifle alloy dissolution at active sites. Thus, as the Mo content of the alloy increases in the order N06625 (9 wt% Mo) < N06022 (13 wt% Mo [3 wt% W]) < N10362 (22 wt% Mo), the depth of corrosion penetration decreases. In addition, once crevice corrosion initiates and the crevice acidifies, metal oxidation can also couple to proton reduction inside the crevice. The role of internal proton reduction in driving the crevice corrosion of these Ni alloys was found to be quite significant; greater than 50% of the corrosion damage is caused by proton reduction inside the crevice.

High-Temperature Corrosion of UNS N10003 in Molten Li2BeF4(FLiBe) Salt

Corrosion testing of UNS N10003 in molten fluoride salt was performed in purified molten 27LiF-BeF2 (66–34 mol%) (FLiBe) salt at 700°C for 1,000 h, in pure nickel and graphite capsules. In the nickel capsule tests, the near-surface region of the alloy exhibited an approximately 200 nm porous structure, an approximately 3.5 μm chromium-depleted region, and MoSi2 precipitates. In the tests performed in graphite capsules, the alloy samples gained weight because of the formation of a variety of Cr3C2, Cr7C3, Mo2C, and Cr23C6 carbide phases on the surface and in the subsurface regions of the alloy. A Cr-depleted region was observed in the near-surface region where Mo thermally diffused toward either the surface or the grain boundary, which induced an approximately 1.4 μm Ni3Fe alloy layer in this region. The carbide-containing layer extended to approximately 7 μm underneath the Ni3Fe layer. The presence of graphite dramatically changes the mechanisms of corrosion attack in UNS N10003 in molten FLiBe salt. In t...

MATERIALS EVALUATION FOR GEOTHERMAL APPLICATIONS

In order to provide basic information on corrosion resistance to the designers and users of geothermal plants different metallic materials including duplex and austenitic stainless steels as well as a nickel alloy have been evaluated in artificial geothermal fluids simulating the conditions in some locations with geothermal potential in Germany as well as two sites in Indonesia. By electrochemical and long-term exposure tests at 100 °C and 150 °C the suitability of low alloyed steel UNS G41300, stainless steels UNS S31603 UNS S31803, UNS S32760, super austenitic steel UNS N08031 and nickel based alloy UNS N06059 was investigated in these geothermal fluids, using critical potentials and corrosion rates. In high-saline environments the crevice corrosion turned out to be the determining mechanism. The nickel based alloy shows excellent corrosion resistance against pitting corrosion. Excluding its high cost, it is very good to be used in the construction of geothermal facilities having highly saline brines. Stainless and duplex steels exhibit a limited corrosion resistance concerning pitting and crevice corrosion. Therefore they are not suitable for highly saline brines. The super austenite UNS N08031 showed a temperature depending behavior. In non-saline environments the low-alloyed steel UNS G41300 (beside of the higher alloyed materials) could be employed as a constructional material for the geothermal power plant, as long as a sufficient wall thickness of the material is considered.

Transpassive Behavior of UNS N08367 Super Austenitic Stainless Steel in LiBr Solution

This study focuses on the transpassivity of super austenitic stainless steel (UNS N08367) in a LiBr solution using electrochemical techniques, i.e., cyclic potentiodynamic polarization (CPP) and electrochemical impedance spectroscopy tests combined with the surface analysis technique, i.e., x-ray photoelectron spectroscopy (XPS). The transpassivity of UNS N08367 in 2.5 M LiBr solution corresponds to the dissolution of Cr, Mo, and Ni at high potentials. This study illustrates how the adsorption of intermediate Mo species is responsible for the appearance of an inductive loop in the Nyquist plot at low frequencies. The addition of alloying elements such as Cr and Mo successfully prevents localized corrosion; however, these alloying elements increase the susceptibility of this material to general corrosion by the formation of an anodic film. Nickel does not directly contribute to the formation of passive film in this alloy, as illustrated by the absence of passivation in the CPP curve and by the weak XPS sig...

Effect of Local Strain Distribution on Stress Corrosion Cracking of Cold-Rolled Alloy 690 With Inhomogeneous Microstructure

This article undertakes an investigation of the stress corrosion crack growth behavior of a cold-rolled Alloy 690 (UNS N06690) with microstructural inhomogeneity in the primary water of a pressurized water reactor, and ascertains the relationship between this behavior and the local distribution of the residual strain. Stress corrosion cracking (SCC) was found to propagate in a transgranular mode through a banded region composed of intragranular carbides and small-sized grains, and in an inter-granular mode through a normal matrix region of normal-sized grains, with an abnormally high growth rate when aligning the cold-rolling and crack growth directions with the intragranular carbide bands. The Vickers hardness and the degree of misorientation increased as plastic deformation as a result of cold-rolling proceeded. Moreover, these parameters were consistently higher in the banded region than in the matrix region. From an analysis of the residual strain, it was suggested that higher residual strain near int...