Cracking Of Steel Research Articles

Evaluation of the “nickel effect” in sulfide stress cracking of low alloy steels

The “nickel effect” refers to the cause of the 1 wt %Ni maximum limit in low alloy steels for the oil and gas production industry to avoid sulfide stress cracking (SSC). The Ni restriction dates to the first edition of the NACE MR 0175/ISO 15156 standard in 1975 and the specific reasons for that limitation remain disputed. This work evaluates the nickel effect with electrochemical tests in the presence and absence of tensile loads at anodic and cathodic potentials and at the open circuit potential (OCP). Thiosulfate was used as a satisfactory alternative to H2S bubbling. The effect of nickel on hydrogen transport and trapping was analyzed with gaseous hydrogen permeation tests. A strong influence of Ni, even at Ni contents below 1 wt%, on SSC initiation at OCP due to trench formation was observed. Compared to other common alloying elements, Ni had a modest role in hydrogen trapping.

On the suitability of single-edge notch tension (SENT) testing for assessing hydrogen-assisted cracking susceptibility

Combined experiments and computational modelling are used to increase understanding of the suitability of the Single-Edge Notch Tension (SENT) test for assessing hydrogen embrittlement susceptibility. The SENT tests were designed to provide the mode I threshold stress intensity factor (Kth) for hydrogen-assisted cracking of a C110 steel in two corrosive environments. These were accompanied by hydrogen permeation experiments to relate the environments to the absorbed hydrogen concentrations. A coupled phase-field-based deformation–diffusion-fracture model is then employed to simulate the SENT tests, predicting Kth in good agreement with the experimental results and providing insights into the hydrogen absorption–diffusion–cracking interactions. The suitability of SENT testing and its optimal characteristics (e.g., test duration) are discussed in terms of the various simultaneous active time-dependent phenomena, triaxiality dependencies, and regimes of hydrogen embrittlement susceptibility.

Influence of cathodic polarization on stress corrosion cracking susceptibility of 35CrMo steel for high strength bolt in simulated deep-sea environment

The effect of cathodic polarization on stress corrosion cracking (SCC) of 35CrMo steel for high strength bolt in the simulated deep-sea environment has been studied. The results show that 35CrMo steel has a higher SCC sensitivity at open circuit potential in the simulated deep-sea environment than that in the shallow seawater. With potential negatively shifted to −750 mV, 35CrMo steel presents the lowest SCC sensitivity. And the SCC susceptibility is increased significantly with the further negative shift of potential because of the intensive hydrogen evolution. When the potential is below −950 mV, hydrogen embrittlement with hydrogen-enhanced-plasticity mediated decohesion should be the dominant SCC micro-mechanism.

Experimental studies of the influence of hydrogen and sulfur on sulfide-corrosion destruction under tension of pipelines

For the first time, comprehensive experimental studies of the influence of hydrogen and sulfur on sulphide stress corrosion cracking (SCR) of pipe steels in a corrosive-aggressive environment of NACE were carried out. In the research, experimental steels of the 06G2BA and 08KHMCHA brands, which are widely used in the construction of pipelines for various purposes, were used an analytical system (analyzer of non-stationary processes – ANP) was developed and tested in laboratory conditions, into which the installation for tests on corrosion-mechanical cracking under stress was integrated. For the first time, comprehensive experimental studies of pipe steel samples of grades 06G2BA and 08KHMCHA under the influence of sulfur and hydrogen were carried out, which made it possible to develop an understanding of the mechanisms of SCDS (sulphide-corrosion destruction under stress) and HID (hydrogen-initiated destruction). For the first time, experimental studies of the corrosion kinetics of 06G2BA and 08KHMCHA steels were carried out depending on the duration of the tests in the NACE model environment, while three periods of the formation of corrosion products were determined. It was found that sulfur and hydrogen, which are dissolved in test pipe steels, have a strong influence on the growth rate of corrosion cracks of SCDS, which made it possible to determine the optimal content of sulfur and hydrogen in steel 06G2BA. It is shown on the basis of the results of experimental studies that to ensure high crack resistance of pipe steels under conditions of static and cyclic loads both in air and in conditions of corrosive-aggressive environments, the sulfur content should not exceed 0.015-0.020%, and the dissolved hydrogen content should not exceed 2-3 ppm . The obtained results make it possible to improve pipe steels in the process of their smelting at metallurgical enterprises thanks to the use of economical modification with niobium, chromium, cerium and other impurities, which contribute to the fragmentation of the structure and reduce the content of non-metallic inclusions and harmful impurities.

CO2 effect on the fatigue crack growth of X80 pipeline steel in hydrogen-enriched natural gas: Experiment vs DFT

Utilizing the existing natural gas grid presents a promising method for transporting hydrogen on a large scale. However, the effects of natural gas and its impurities on hydrogen-assisted cracking in pipeline steel have not been adequately studied. This study aims to investigate the fatigue performance of X80 pipeline steel in hydrogen-enriched natural gas (HENG) environments and in mixtures containing the impurity CO2. This is achieved through fatigue crack growth rate (FCGR) tests and density functional theory (DFT) calculations. The experimental results show that the FCGR in H2 is slightly faster than that in HENG, whereas it is slower than that in the N2/CO2/H2 mixtures. The enhanced FCGR by CO2 further increases with the increasing CO2 content. DFT computational results indicate that the adsorbed CO2 on the iron surface significantly accelerates the migration of H atoms from surface to subsurface. This promotes the entry of hydrogen into the steel.

Chloride-induced stress corrosion cracking in Austenitic steels for SNF storage canisters - Recent understanding and advances in mitigation and repair

Chloride-induced stress corrosion cracking (CISCC) is a critical threat to stainless steel (SS) spent nuclear fuel (SNF) and radioactive materials storage canisters currently in service worldwide. In the past decade, extensive research has been conducted to understand fundamental mechanisms of CISCC, as well as to develop effective repair and mitigation methods to prevent catastrophic failure of the SNF canisters from CISCC. This review article aims to: (1) summarize recent progress from fundamental studies of CISCC in SS, and (2) present state-of-the-art developments in CISCC mitigation and repair techniques that may be especially well-suited for SNF storage applications, including cold spray (CS), friction stir welding (FSW), friction surfacing (FS), laser shock peening (LSP), and ultrasonic nanocrystal surface modification (UNSM). Knowledge gaps in current understanding of CISCC and roadmaps for deployment of the repair techniques are also discussed.

Fatigue growth behavior of mode II crack in headed stud steel used in steel–concrete composite structures

The fracture mechanics method has been employed for fatigue performance evaluation of stud used in steel–concrete composite structures while the fatigue crack growth behavior in headed stud steel has not been systematically studied. The present work is proposed to investigate the fatigue growth behavior of mode II crack in headed stud steel and its welded joint. Compact tensile shear (CTS) specimens extracted from the base material (BM) and weld metal (WM) of stud are tested under mode II loading to investigate the fatigue crack growth (FCG) trajectories and rates with different stress ratio. The crack closure level and FCG threshold are determined and discussed. Results show that the crack deflection in BM meets both the MTS criterion and S-criterion, while crack deflection in WM does not meet the two criterions. The 2% compliance offset criterion in the origin ASTM method is suitable to determine the crack opening force both for BM and WM of stud. The low crack closure level in WM indicates the tensile residual stress in welded joint, resulting a more quickly fatigue crack growth rate (FCGR) in WM than that in BM. The calculation formulation of equivalent SIF range proposed by Borrego et al. is verified with the best normalization effects and is recommended to fit the FCGR model both for headed stud steel and its welded joint under mode II loading.

Analysis and control of surface cracks in a B-bearing continuous casting blooms

Abstract In this work, normal manufacture of a B-bearing steel was hampered by cracks in steel. In order to control these cracks, the formation mechanism has been examined through a comprehensive analysis of crack morphology, element segregation, high temperature mechanical properties, and precipitates. The high-temperature thermoplastic capabilities of the steel were found to be reduced by boron nitride particles precipitated at grain boundary. This led to the formation of a brittle zone in the straightening zone of the continuous casting process, which in turn caused cracks. Based on the formation mechanism of these cracks, the cracks were successfully controlled by adding an appropriate amount of Ti element to the steel and reducing the charging temperature of the heating furnace.

Assessing fracture toughness of vintage and modern X65 pipeline steels under in situ electrochemical hydrogen charging using advanced testing methodology

The susceptibility of pipeline steels to hydrogen embrittlement, which can significantly impair mechanical performance, especially fracture toughness, is a critical issue in infrastructure integrity. This study evaluates the fracture toughness of modern and vintage API X65 steels under cathodic polarization (CP) conditions, employing methods such as rising displacement testing, stepwise rising constant load testing, and constant load testing. Tearing crack growth was detected at crack tip opening displacements (CTODs) of 0.032 to 0.055 mm while subsequent crack arrest under constant force loading was observed with CTOD thresholds of 0.148 mm and 0.255 mm for modern and vintage X65, respectively. Electron channelling contrast imaging demonstrated that arrested cracks in both steels exhibited reduced plasticity and a higher propensity for crack branching compared to propagating cracks. These findings highlight the need for tailored strategies to mitigate the effects of hydrogen embrittlement in pipeline materials and underscore the differences in damage tolerance between steel generations.

Environmentally-assisted cracking of electropolished 316L stainless steel in molten FLiNaK salt

Environmentally-assisted cracking is a well-known and extensively studied phenomenon in light water reactor environments but remains relatively unexplored in proposed molten salt reactor systems where the mechanisms at play are expected to be quite different. Here, slow strain rate testing has been performed on 316L stainless steel tensile specimens during simultaneous exposure to a molten LiF-KF-NaF eutectic salt environment at 600°C with a strain rate between 1E-5 to 1E-7 s-1. Sample ductility and ultimate tensile strength were both observed to decrease for slower strain rates/longer salt exposures. Post-mortem characterization of fractured specimens revealed Fe deposits and Cr- and O-rich precipitates decorating surface grain boundaries and crack surfaces, suggesting a cracking mechanism driven by embrittlement of the crack tip due to oxide formation. Unlike previous studies, intergranular Cr corrosion into the bulk was not observed, and distinct differences in surface morphology between electropolished and non-electropolished sample surfaces are reported. Trace metal salt chemistry measurements showed accumulation of 316L alloying elements over the course of the tests. It appears likely that the surface oxide layer induced by electropolishing, in addition to oxygen and moisture impurities in the salt, led to the formation of LiCrO2 which had a net passivating effect that prevented significant bulk corrosion, though some interplay between mechanical and environmental stressors is still observed.

Towards understanding stress corrosion cracking of austenitic stainless steels exposed to realistic sea salt brines

Stress corrosion cracking behavior of stainless steel 304 L was investigated in full immersion, evaporated artificial sea salt brines (ASW) at 55 °C. It was observed that brines representative of thermodynamically stable brines at lower relative humidity (40% RH, MgCl2-dominant) had a faster crack growth rate than high relative humidity brines (76% RH, NaCl-dominant). Observed crack growth rates (da/dt) under constant stress intensity (K) conditions were determined to be independent of transitioning procedure (rising K or decreasing frequency) regardless of solutions investigated for the orientation presented. Further, positive strain rates had little to no impact on the observed da/dt. The observed behavior suggests an anodic dissolution enhanced hydrogen embrittlement mechanism for SS304L in concentrated ASW environments at 55 °C. Additional explorations further examined environmental influences on da/dt. Nitrate additions to 40% ASW at 55 °C solutions were shown to decrease measured da/dt and further additions stopped measurable crack growth. After sufficient nitrate had been added to fully stifle crack growth, a temperature increase to 75 °C induced cracking again, and a subsequent decrease to 55 °C once again stopped da/dt. These tests demonstrate the importance of ascertaining both brine-specific chemical and dynamic environmental influences on da/dt.

Specifics and Methods of Inhibiting the Underfilm Corrosion of Carbon Steel.

The process of metal dissolution under a delaminated insulating polymer coating (underfilm dissolution) has been studied. For this purpose, we used an experimental setup that simulates the process of corrosion of underground metal structures in the presence of through defects in the polymer coating and/or extended areas of peeling of the polymer coating from the metal (loss of adhesion)-subfilm cavities partially or completely filled with electrolyte. In particular, the distribution of the protective current under a peeled polymer coating was studied, and a sharp decrease in the value of the protective current was shown at a distance of 1-3 cm from the edge of the defect with a gap between the metal and the coating of 1-6 mm. The localized nature of metal corrosion under the exfoliated polymeric coating has been demonstrated. The ratio of the areas with accelerated corrosion to the total area of the metal can be 1 to 100. It has been established that there are areas of anodic dissolution of the metal during cathodic polarization of the entire sample with a peeled coating. The activating effect of carbon dioxide and hydrogen sulfide on the corrosion and anodic dissolution of steel under the coating was shown. So, it has been established that the dissolution current flowing from the anodic sections on a surface can increase approximately 10 times in the presence of carbon dioxide and hydrogen sulfide. A synergistic effect of these compounds on the process of localized underfilm corrosion of steel was detected. It has been developed a mechanism for the formation of localized corrosion damage to steel under a delaminated polymeric coating, which can be the nuclei of corrosion cracks upon reaching a certain level of mechanical loads, i.e., stress corrosion cracking (SCC) of carbon steel. Possible manners of inhibiting underfilm dissolution of metals are considered, and a method for pre-treatment of the surface with solutions of organosilanes, which ensures the formation of surface self-assembled polymeric siloxane nanolayers responsible for inhibiting underfilm corrosion of steel, is proposed.

Towards a better understanding of hydrogen-assisted cracking in multiphase stainless steel

Given the high strength of multiphase stainless steel, it is imperative to study its hydrogen embrittlement behavior. However, the understanding of hydrogen-assisted crack initiation and propagation is limited. In this study, we investigate the hydrogen-assisted cracking behavior of multiphase stainless steel through crack analysis, hydrogen permeation test, and hydrogen desorption experiments. Our findings reveal that hydrogen-assisted cracks initiate at the martensite packet boundaries and propagate along the packet boundaries, block boundaries, prior austenite grain boundaries and through ferrite. Additionally, we have elucidated the hydrogen trap sources and proposed a hydrogen-assisted cracking mechanism.

Fitness-for-Service assessment of a Hydrogen-Induced crack in an inlet gas separator pressure vessel using computational modelling

Hydrogen-induced cracking is the cracking of carbon steel or low-alloy steel in a wet H2S environment, driven by the diffusion of hydrogen atoms into the steel due to corrosion reaction. The formation of blisters and hydrogen-induced cracks in pressurized components in the oil and gas or petrochemical industries imposes a serious risk to personnel safety and production continuity. In this study, a fitness-for-service assessment of a gas separator pressure vessel containing 3.76 % H2S and 3.47 % CO2 was analyzed using finite element analysis as required by level III assessment in API 579–1/ASME FFS-1 2021. Different load scenarios, including internal pressure, static head from liquid, dead weight, and wind loads, were considered using elastic-perfectly plastic stress analysis. The hydrogen-induced affected area, with dimensions of 2650 mm × 900 mm, was shown to sustain an internal pressure of 14.8 MPa, which is above 1.35 times the design pressure (i.e., 12.1 MPa). The assessment of API 579–1/ASME FFS-1 in level I and II failed due to the presence of a hydrogen-induced crack near the weldment. However, the evaluation in level III assessment considered the influence of residual welding stresses on both longitudinal and circumferential cracks in their as-welded state. The findings indicated that the affected area fell within the acceptable region of the failure assessment diagram, thereby confirming the continued safe operation of the HIC damaged pressure vessel while accounting for welding residual stresses.

Evaluation of corrosion behaviour and susceptibility to hydrogen-induced cracking in high-strength low-alloy X70 steels under sour gas conditions

High-strength low-alloy (HSLA) steels find extensive usage in the oil industry for manufacturing pipes due to their requirement for materials with high mechanical and corrosion resistance. Among these steels, the HSLA X70 steels are particularly of great interest to the petroleum industry because of its high mechanical resistance, ductility and weldability. In this study, it was aimed to assess the corrosion behaviour of HSLA API X70 pipelines using electrochemical impedance spectroscopy (EIS) technique and evaluate its resistance to hydrogen-induced cracking (HIC). The analyses were conducted in non-sour and sour media, as per NACE TM0284 standard. Microstructural characterisations of polished and corroded samples were also conducted using scanning electron microscopy (SEM) and optical microscopy (OM). The results indicate that the resistance to HIC in API 5L X70 steel is compromised when exposed to surface corrosion in sour gas media. This susceptibility may be attributed to the presence of microconstituents MnS and M/A along the grain boundaries, which were found in the crack trajectory. The identified microstructural features are deemed to exert a significant influence on the propagation of cracks and the consequent susceptibility to HIC. Moreover, the EIS tests revealed the lowest corrosion resistance when exposed to H2S media, as MnS inclusions accelerate the active regions in the matrix, consequently leading to the formation of a plethora of corrosion products and the dissolution of iron.

Type IV cracking of P92 steel welded joint used in China power plants

ABSTRACT P92 (9Cr-1.8W–0.5Mo-NbV) martensite steel is widely used in high-temperature pipes of ultra-supercritical boilers in China. During high-temperature operation, microstructural evolution of the heat affected zone (HAZ) across welded joint leads to type IV cracking failure. In this article, microstructural evolution and type IV cracking of P92 steel welded joint in an actual 605℃ supercritical boiler were analysed. The results show the cracking appeared in the fine-grained heat affected zone (FGHAZ). Laves phase and M23C6 phase clustered around the cavity. Their precipitation and coarsening promote the nucleation and growth of cavities. The Z phase was also observed in the heated affected zone. Compared with other zones, FGHAZ has lower dislocation density and more degeneration of tempered martensite lath, the coarsening of Laves phase and M23C6 phase is more severer in FGHAZ. These factors led to the deterioration of the creep properties and type IV cracking.

Microscopic Investigation for Experimental Study on Transverse Cracking of Ti-Nb Containing Micro-Alloyed Steels.

The influence of Ti on the behavior of hot ductility was examined in four different Ti-containing micro-alloyed steels with a constant content of Nb. Thermomechanical investigations using a dilatometer were carried out to simulate the conditions during casting and cooling in the strand of a continuous caster with temperatures in the range of 650-1100 °C, strain rates of 0.01 s-1 and 0.001 s-1, and reheating rates between 60 and 180 Kmin-1. To understand the fracture mechanism, optical (LOM) and scanning electron microscopy (SEM), elemental analysis via energy dispersive X-ray spectroscopy (EDX), MatCalc "Scheil-Guilliver" calculations, and precipitation kinetics calculations were carried out for the critical conditions, showing low hot ductility between Ar3 and Ae3 temperatures and a brittle to ductile transition temperature at 900 °C. The existence of TiNb(CN), thin ferrite formation, and grain boundary sliding (GBs) due to limited dynamic recrystallization (DRX) has been documented and discussed. As a result, the reheating rate has no sufficient effect on the ductility. The existence of Nb-rich TiNb(CN) of sizes below ~1 μm triggers brittle fracture by increasing the frequency of micro-voids around grain boundaries. It can be stated that if the conditions in the hot ductility trough are avoided, the addition of Ti and high strain support minimize the risk of crack formation.

Effect of strain rate on the stress corrosion cracking of TP439 stainless steel in water vapor environment at 500 ℃

The effect of water vapor on the stress corrosion cracking (SCC) behavior of TP439 stainless steel at 500 °C was investigated using slow strain rate tensile tests at three strain rates of 2 × 10–5/s, 2 × 10–6/s, and 2 × 10–7/s. Air was selected as a comparative blank test environment for the water vapor environment. The results showed that the tensile strength of the specimens increased significantly in air when the strain rate was decreased from 2 × 10–5/s to 2 × 10–7/s, while it decreased in water vapor. Dynamic strain aging (DSA) and SCC are the main factors contributing to the difference in tensile strength of the specimens in air and water vapor. The SCC of TP439 stainless steel in water vapor at 500 °C occurs at a strain rate of 2 × 10–7/s or even lower. Based on the SCC susceptibility index and SEM observation of specimen fracture, TP439 stainless steel exhibits a slight transgranular SCC in water vapor with low SCC susceptibility.

Sulfide stress cracking (SSC) of a commercial Q345 pressure vessel steel in a wet H2S environment

The steel Q345 is extensively used in pressure vessels with low and medium pressure because of good mechanical properties and low cost. Good resistance to sulfide stress cracking (SSC) is required for the pressure vessel steel, but the SSC behavior of the Q345 has received little attention. This work investigated the SSC behavior of the Q345 in a wet H2S environment. The microstructure contained the irregular various-sized polygonal-like ferrite grains plus a carbon-rich degenerate pearlite banded structure. The Q345 had high ductility but failed to pass the SSC tests. The SSC cracking involved hydrogen embrittlement. The SSC preferentially occurred at the carbon-rich degenerate pearlite banded structure. The elimination of the carbon-rich degenerate pearlite banded structure is necessary to improve the SSC resistance of the Q345 pressure vessel steel.

Study of corrosion-resistant steels in hydrogen sulfide-containing environments of oil and gas fields

Hydrogen sulfide corrosion cracking of steel occurs in deposits containing corrosive components - hydrogen sulfide and carbon dioxide (carbon dioxide). The mechanism of corrosion cracking of steel in hydrogen sulfide-containing environments of corrosive oil and gas fields is considered, and factors influencing hydrogen sulfide cracking are identified. Increasing the performance properties of carbon, low-alloy, structural steels resistant to hydrogen sulfide and sulfide cracking was carried out on the basis of improving the composition of the components of chemical elements and the structural state through the choice of heat treatment methods depending on the acidity and temperature of the environment, partial pressures of carbon dioxide and hydrogen sulfide. Laboratory and production tests have shown the durability of the proposed compositions chemical elements of steels to hydrogen sulfide cracking, sulfide cracking and hydrogen swelling in deposits containing corrosive components was 70–80% compared to existing carbon structural steels.