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The influence of Ti content on the microstructure and corrosion behavior of nickel-based alloy 825 in acid oil and gas environments was studied. The pitting corrosion behavior and stress corrosion cracking (SCC) behavior of Alloy 825 were investigated using high-temperature and high-pressure immersion tests, electrochemical tests, and slow strain rate tensile tests. Scanning electron microscopy, electron backscatter diffraction, and 3D microscopy were applied to analyze the microstructure and corrosion performance of the samples. The results showed that the number of inclusions and residual strain increased with the increase of Ti content, and strain concentration was found around the inclusions. The rise in inclusions led to lower pitting potential and a narrower passivation interval. In addition, the enhanced SCC susceptibility of the alloy with higher Ti content may also be attributed to the increase in inclusions. [ABSTRACT FROM AUTHOR]

The effect of post-weld heat treatment (PWHT) on the microstructure, penetration and distribution of hydrogen, and sulfide stress corrosion cracking (SSCC) behavior was investigated to improve the cracking resistance of Inconel 625/X80 dissimilar weld overlay. Among nine candidate PWHT procedures, only the PWHT at 620 °C/20 h could improve the SSCC resistance of the overlay as no crack but only shallow pits were observed in the fusion boundary (FB) region. This PWHT procedure decreased the microhardness, increased the fraction of low-angle grain boundaries, eliminated the susceptible microstructure to hydrogen embrittlement, and therefore enhanced the SSCC resistance of the heat-affected zone. Further, in addition to the decrease of misorientation at the FB, the enhanced SSCC resistance of the FB after PWHT was mainly owing to precipitation of dispersed M23C6 carbides, which may reduce the amount of diffusible hydrogen within the martensite layer and prevent localized hydrogen concentration from reaching the cracking threshold. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this paper is to investigate the effect Ti on stress corrosion cracking (SCC) and flow accelerated stress corrosion cracking (FA-SCC) behavior and mechanisms of Monel K500 alloy. Design/methodology/approach: Monel K500 alloy with different Ti contents was designed. A metallurgical microscope (XJP-3C) and scanning electron microscopy (EV0 MA15 Zeiss) with an energy dispersive spectroscopy were used to analyze the microstructure of the Monel K500 alloy. In situ electrochemical tests were carried out in static and flowing seawater to study FA-SCC behavior. Findings: The number of TiCN particles in the alloy increased as the increase of Ti content. The static corrosion and SCC of Monel K500 alloy are reduced as the content of Ti increases. Generally, the SCC of alloys was caused by the synergistic effect of the anodic dissolution at exposed metal matrix and the pit corrosion of metal matrix adjacent to TiCN particles, which was further accelerated by flowing. Originality/value: The corrosion behavior and mechanism of Monel K500 alloy with different Ti contents in a complex flowing seawater environment are still unclear, which remain systematic study to insure the safe service of the alloy. [ABSTRACT FROM AUTHOR]

Monel K500 alloy is widely used in sea engineering and suffers the failure risk of flowing seawater under load. Therefore, the stress corrosion cracking (SCC) and flow accelerated stress corrosion cracking (FA-SCC) behavior and mechanism of aged Monel K500 alloy were investigated. Results showed that TiCN particles were observed in alloys, and the number and size of γ′ phase in Monel K500 alloy gradually increased as the aging temperature increased, while γ′ phase was not found in solution treated alloy. The yield strength of Monel K500 alloy after heat treatment and SCC experimentation was, from high to low, in the order of the ones aged at 630 °C, 700 °C, 560 °C, and solution treated. The corrosion resistance of Monel K500 alloy from high to low during SCC and FA-SCC experiments is in the order of alloys aged at 560 °C, 700 °C, 630 °C, and solution treated. Pit corrosion was found at the matrix adjacent to TiCN particles, and cracks were mainly observed at grain boundaries (GBs) after SCC and FA-SCC experiments and after tensile testing. Generally, the SCC of alloys was caused by the synergistic effect of the anodic dissolution at exposed metal matrix and the pit corrosion of metal matrix adjacent to TiCN particles, which was further accelerated by flowing. [ABSTRACT FROM AUTHOR]

Sulfide stress corrosion cracking (SSCC) behavior of the fusion boundary (FB) region of Inconel 625/X80 weld overlay was investigated with a focus on the role of the beach and peninsula partially mixed zones (PMZs). Compared to the martensite-ferrite boundary, the large misorientation and low deformation compatibility of the austenite-martensite boundary promote the accumulation of hydrogen and thus increase the cracking susceptibility. Further, under the effect of local anodic dissolution and hydrogen accumulation due to the large dimension PMZ, the corrosion defect formed at the junction can be easily transformed into a crack. After SSCC initiation, the crack preferentially grows along the Inconel 625/PMZ interface while the austenite matrix may oblige the crack to propagate along the FB. In addition, the beach PMZ likely shows a higher SSCC susceptibility than the peninsula PMZ mainly because severe anodic dissolution in the peninsula structure blunts the crack tip along the FB during crack propagation. [Display omitted] • Partially mixed zones (PMZs) increase sulfide stress corrosion cracking (SSCC) of the fusion boundary region. • Local anodic dissolution and hydrogen promote crack initiation at the PMZ junction. • Cracks grow preferentially along the Inconel 625/PMZ interface. • Excessive anodic dissolution makes SSCC in the peninsula PMZ weaker than the beach. [ABSTRACT FROM AUTHOR]

The stress corrosion cracking of M54 steel in marine environments at varying temperatures was investigated. The slow strain rate tensile tests show that increasing the temperature from 5 to 45°C leads to a 71.9 % and 77.4 % decrease in plastic elongation ratio and reduction in area ratio in full immersion environments, while only 7.8 % and 7.9 % in atmospheric environments. The da/dt II increases from 0.75 nm/s to 5.5 nm/s in atmospheric environments, but first decreases and then increases in full immersion environments. Compared to full immersion environments, the sufficient oxygen supply within the crack in atmospheric environments lowers da/dt II by 50–80 %. • Comparison of stress corrosion cracking (SCC) by slow strain rate tensile (SSRT) and crack growth test. • SCC susceptibility measured by SSRT increases with temperature in both environments. • SCC growth rates first decrease and then increase in full immersion environment. • Sufficient oxygen within the crack in atmospheric environment lowers crack growth rates. • The reason that SCC susceptibility does not align with the SCC growth data is discussed. [ABSTRACT FROM AUTHOR]

The use of cold metal transfer process (CMT) is an excellent alternative to depositing nickel-based alloys on oil and gas pipelines due to the relatively small dilution. However, sulfide stress corrosion cracking (SSCC) of dissimilar weld joints has proven to be an issue. Therefore, this study investigates the microstructure and SSCC behavior of an Inconel 625/X80 weld overlay fabricated CMT process focusing on the tempering effect. Four-point bending tests show a high SSCC susceptibility of the non-overlap zone due to the formation of coarse structure in the heat affected zone and narrow martensite layer along the fusion boundary. However, microstructural characterization indicates a tempering effect in the overlap zone, producing homogenized microstructure and less residual strain in the heat affected zone. Therefore, the SSCC resistance is greatly improved, especially for the fusion boundary, because reversed austenite in the martensite layer could decrease the accumulation of hydrogen at lath boundaries. [Display omitted] • Martensite layer induces sulfide stress corrosion cracking of the fusion boundary (FB). • Tempering effect produces homogenized microstructure and less residual strain. • Reversed austenite exists in the martensite layer along the FB in the overlap zone. • Sulfide stress corrosion cracking of FB is improved by the reversed austenite. [ABSTRACT FROM AUTHOR]