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Charles Foster
Charles Foster

GM SI Service Information 1980 Through 2009 Crack _VERIFIED_



Low-alloy steels (LAS) are extensively used in oil and gas (O&G) production due to their good mechanical properties and low cost. Even though nickel improves mechanical properties and hardenability with low penalty on weldability, which is critical for large subsea components, nickel content cannot exceed 1-wt% when used in sour service applications. The ISO 15156-2 standard limits the nickel content in LAS on the assumption that nickel concentrations above 1-wt% negatively impact sulfide stress cracking (SSC) resistance. This restriction excludes a significant number of high-strength and high-toughness alloys, such as Ni-Cr-Mo (e.g., UNS G43200 and G43400), Ni-Mo (e.g., UNS G46200), and Ni-Cr-Mo-V grades, from sour service applications and can be used only if successfully qualified. However, the standard is based on controversial research conducted more than 40 years ago. Since then, researchers have suggested that it is the microstructure that determines SSC resistance, regardless of Ni content. This review summarizes the advantages and disadvantages of nickel-containing LAS in terms of strength, weldability, hardenability, potential weight savings, and cost reduction. Likewise, the state of knowledge on the effect of nickel on hydrogen absorption as well as SSC initiation and propagation kinetics is critically reviewed.




GM SI Service Information 1980 Through 2009 Crack


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A synergy between temper embrittlement and hydrogen-assisted cracking has been reported by Craig and Krauss (1980). In the presence of hydrogen, P segregation can contribute to intergranular HE of Ni-Cr-Mo steels that are a priori immune to temper embrittlement. This form of embrittlement was observed when steels such as UNS G43400 and UNS G41400 were quenched and tempered to a strength level in excess of 1000 MPa (145 ksi) (Craig & Krauss, 1980; Oriani, 1987), i.e., at temperatures below 500C, and it was associated with a phosphorus-hydrogen interaction (Craig & Krauss, 1980). This type of failure can be prevented by rapid quenching and tempering at temperatures above 500C.


In recent years, O&G companies have abandoned the NACE TM0177 Method B test in favor of the four-point bent-beam approach. The four-point bent-beam method is a constant displacement test performed by supporting a smooth beam specimen with no stress concentrators on two loading rollers and applying a load through two other loading rollers. This configuration leaves the outer face of the specimen in tension and the inner one in compression (EFC, 2009; Total, 2005b). Although NACE TG085 is currently developing a four-point bent-beam procedure that will be ultimately incorporated in a revised version of the TM0177 specification (NACE, 2014), no systematic studies on the effect of Ni on SSC resistance have been conducted using this test method.


Although no atomistic HE mechanism has gained the consensus of the community, it is generally accepted that a hydrogen-assisted crack advances only when a critical hydrogen concentration (Hcrit) is reached ahead of a potential crack site. A potential crack site is a defect in the lattice that acts as a hydrogen trap (Pressouyre, 1980). In this regard, Hcrit is a function of the applied stress level as well as the type and shape of the trap sites. For example, elongated MnS inclusions have a lower Hcrit than rounded inclusions and thus have a higher tendency to act as potential crack sites. Other hydrogen traps, like fine carbides, have a higher Hcrit, retaining hydrogen and slowing down hydrogen transport to sites with lower Hcrit, where cracks are expected to be nucleated first.


The beneficial effect of decreased grain size on hydrogen-assisted cracking resistance of LAS is well established (Bernstein & Thompson, 1976). Given that nickel decreases the Ac1 temperature of the steel and refines microstructure, nickel additions to LAS are expected to increase HE resistance (Chavane, Habashi, Pressouyre, & Galland, 1986). The grain boundary area per unit volume and the number of carbides per unit volume both increase when the microstructure, i.e., both grain and carbide size, are refined. In this regard, high-angle grain boundaries and carbide interphases are considered irreversible traps (Pressouyre, 1980). Although carbides can act as potential flaws (Yoshino, 1983), this effect is minimized when they are finely distributed in the microstructure. Finely distributed carbides, being irreversible traps, act as sinks of hydrogen and therefore slow down hydrogen transport to the crack tip. As a result, nickel is expected to increase SSC resistance (Pressouyre, 1979). In contrast, other irreversible traps, such as MnS and oxide interphases, are undesirable because they can act as potential flaws themselves (Chavane et al., 1986). Stringent maximum sulfur content limits and strict control of shape and distribution of inclusions is paramount for maximum SSC resistance (EFC, 2009; Kimura et al., 1989; NACE-ISO, 2001), e.g., API 6A Table 10 and API 5CT Tables C.5 (API, 2004, 2010).


All steels used by Dunlop in his investigation had a fixed nickel concentration close to 3.5-wt%. Therefore, beyond all criticism to the test methodology exposed here, no conclusions on the effect of nickel could be drawn. Schmid (1980) conducted constant load SSC tests on the same 3.5-wt% Ni ASTM A203 grade E steel. Given that the author did not specify heat treatment and microstructure, limited information can be obtained from that paper. However, Schmid concluded that this alloy had a higher σth-SSC than a 1-wt% Ni LAS of similar hardness, concluding that Ni had no adverse effect on SSC performance.


Localized dissolution, in the form of pits and trenches, is associated with the presence of surface films. Formation of a non-stoichiometric Fe(1+x) S film, where the x can take on values between 0 and 0.11 (Shoesmith, Taylor, Bailey, & Owen, 1980; Smith & Miller, 1975), can provide some degree of general corrosion protection, which is evidenced by a decrease in generalized corrosion measured with weight-loss coupons and hydrogen permeation rates with time (Azevedo, Bezerra, Esteves, Joia, & Mattos, 1999). If Fe(1+x) S films were more protective in Ni-containing steels, that would explain the reduced amount of hydrogen permeation at steady state (Asahi & Ueno, 1994; Yamane et al., 1986; Yoshino & Minozaki, 1986) and the increased amount of trenching in the alloys reported by some authors (Craig et al., 1990; Yamane et al., 1986). This is a purely speculative assumption that should be validated with long-term corrosion tests. Furthermore, potentiostatic tests held at a net cathodic potential to yield a similar rate of hydrogen permeation in H2 S-free environments, but with the addition of H recombination poisons (Berkowitz & Heubaum, 1984), could help to weigh the relative importance of hydrogen absorption vs. anodic dissolution based mechanisms on cracking of nickel-containing steels.


The enduring question remains of where the H is located in the microstructure and how such traps facilitate catastrophic failure. Several studies pointed to the critical role of GBs in the environmental degradation. GBs are locations of preferential electrochemical attack4, but also cracks propagate more easily via GB networks throughout the microstructure of the alloy24,25. An experimental validation of the H distribution in Al alloys is challenging, owing to its low solubility and to the experimental difficulty of performing spatially resolved characterization of H at relevant scales and at specific microstructural features. Recent efforts in atomic-scale H imaging in steels led to insights into the trapping behaviour of second phases and interfaces26,27,28.


Generally, avoiding the ingress of H in the first place is extremely unlikely to work, and the best approach to mitigate H embrittlement is therefore to control its trapping to maximize the in-service lifetime of the components. Our results provide indications of H-trapping sites and their respective propensity to initiate damage in environmentally assisted degradation, thus contributing towards establishing a mechanistic understanding of H embrittlement in Al alloys. On this basis of this study, we propose specific measures that may be explored to enhance resistance to H-induced damage and improve the lifetime and sustainability of high-strength lightweight engineering components. The results on the high H enrichment in second-phase particles provide a potential mitigation strategy for improving H-embrittlement resistance, namely through introduction and manipulation of the volume fraction, dispersion and chemical composition of the second phases, despite their potentially harmful effects on mechanical properties. Other strategies could aim at designing and controlling GB segregation, for instance with the goal of eliminating Mg decoration of GBs by trapping it into precipitates and keeping it in bulk solution. A third and more general avenue against environmental degradation lies in reducing the size of PFZs in these alloys, with the goal to mitigate the H-enhanced contrast in mechanical and electrochemical response between the H-decorated GBs and the less H-affected adjacent regions.


25. Climate change is a global problem with grave implications: environmental, social, economic, political and for the distribution of goods. It represents one of the principal challenges facing humanity in our day. Its worst impact will probably be felt by developing countries in coming decades. Many of the poor live in areas particularly affected by phenomena related to warming, and their means of subsistence are largely dependent on natural reserves and ecosystemic services such as agriculture, fishing and forestry. They have no other financial activities or resources which can enable them to adapt to climate change or to face natural disasters, and their access to social services and protection is very limited. For example, changes in climate, to which animals and plants cannot adapt, lead them to migrate; this in turn affects the livelihood of the poor, who are then forced to leave their homes, with great uncertainty for their future and that of their children. There has been a tragic rise in the number of migrants seeking to flee from the growing poverty caused by environmental degradation. They are not recognized by international conventions as refugees; they bear the loss of the lives they have left behind, without enjoying any legal protection whatsoever. Sadly, there is widespread indifference to such suffering, which is even now taking place throughout our world. Our lack of response to these tragedies involving our brothers and sisters points to the loss of that sense of responsibility for our fellow men and women upon which all civil society is founded.


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