Hydrogen-assisted Cracking Research Articles

Effect of ball-burnishing on hydrogen-assisted cracking of a martensitic stainless steel

Slow strain rate tensile tests under hydrogen cathodic charging are conducted on 17-4 PH steel with two surface conditions: mirror polished and ball-burnished. In both cases, significant subcritical cracking initiating at the surface is observed leading to considerable reduction in elongation to fracture. However, ball-burnished specimens show better elongation and much less secondary cracking than the polished ones. Ball-burnishing introduces high compressive residual stresses in the specimen sub-surface. However, EBSD showed a very limited impact of ball-burnishing on the microstructure, so little effect on hydrogen trapping is expected. The beneficial effect of ball-burnishing on the resistance of the hydrogen assisted cracking is mainly explained by the high compressive longitudinal stress at the specimen surface, which makes crack initiation more difficult and hence delays specimen failure. In addition, it is estimated that the amount of hydrogen introduced at the specimen surface is decreased by approximately 30% due to the high compressive hydrostatic stress.

Strength-Toughness Balance and Hydrogen Embrittlement Susceptibility of a Precipitation-Strengthened Steel Adopted Tempering Process

Two steels with different nickel (Ni) content were investigated to reveal the role of Ni on strength-toughness balance and hydrogen embrittlement susceptibility. Although they were similar in microstructure, i.e., nano-particles were precipitated on martensitic laths, different mechanical behaviors were exhibited. After tempering, the yield strength of 3.5 Ni steel reached a peak at 500 °C, while that of 2.5 Ni steel kept a downward trend, indicating that precipitation strengthening was significant in 3.5 Ni steel. Combined with thermodynamic and kinetic analyses, it was shown that when the Ni content increased, the rich-copper (Cu) precipitation transformation driving force would be enhanced and the reverse of austenite transformation accelerated to improve its stability. Moreover, the increase of Ni content also induced the increase in high-angle grain boundaries (HAGBs), which could inhibit crack propagation. Under the comprehensive effects of strengthening and ductility mechanism, 3.5 Ni steel exhibited excellent cryogenic toughness. Although it was not possible to obtain the ideal balancing of strength-toughness for the steel with lower Ni content, its hydrogen embrittlement susceptibility is satisfying. The results showed that the increase of grain boundary density caused by the grain refinement of 2.5 Ni steel is the key factor for its lower hydrogen embrittlement sensitivity index. Moreover, with the reduction of Ni content, the decrease of HAGBs and the increase in Σ11 boundary were conducive to reducing hydrogen-assisted cracking, while the residual Fe3C in 3.5 Ni steel would deteriorate the hydrogen embrittlement resistance.

On the role of the stacking fault energy in the beneficial effect of aluminium on the hydrogen embrittlement sensitivity of twinning-induced plasticity (TWIP) steel

The hydrogen embrittlement sensitivity of three twinning-induced plasticity steels is investigated. A reference TWIP steel (18Mn-0.6C) is compared to an aluminium added TWIP steel (18Mn-0.6C-1.5Al) and a TWIP steel with an increased manganese content (24.5Mn-0.6C), exhibiting an identical stacking fault energy as the aluminium added material. As such, the importance of the stacking fault energy in the reduction of the hydrogen embrittlement sensitivity with aluminium addition is evaluated, which is considered as one of the possible underlying mechanisms. Through thermal desorption spectroscopy analysis, it is shown that both aluminium and manganese addition increase the hydrogen solubility while the hydrogen diffusivity is increased by manganese addition and decreased by aluminium addition. These effects can be considered as the bulk effects of both alloying additions on the hydrogen properties. With respect to the mechanical properties, aluminium addition leads to the expected increased resistance to hydrogen while manganese addition reduces the hydrogen resistance, despite the equal stacking fault energy. The aluminium added TWIP steel shows increased resistance to hydrogen-assisted crack initiation and propagation suppressing intergranular fracture in the presence of hydrogen. On the contrary, the increased manganese content increases the tendency for intergranular cracking in the presence of hydrogen. The critical hydrogen concentration might therefore be higher with the addition of aluminium.

Hydrogen-assisted Cracking Behavior of a Fine-grained Equiatomic CoCrFeNi High-entropy Alloy

Hydrogen-assisted Cracking Behavior of a Fine-grained Equiatomic CoCrFeNi High-entropy Alloy

Damage associated with interactions between microstructural characteristics and hydrogen/methane gas mixtures of pipeline steels

There is no common standard for blended hydrogen use in the natural gas grid; hydrogen content is generally based on delivery systems and end-use applications. The need for a quantitative evaluation of hydrogen-natural gas mixtures related to the mechanical performance of materials is becoming increasingly evident to obtain long lifetime, safe, and reliable pipeline structures. This study attempts to provide experimental data on the effect of H2 concentration in a methane/hydrogen (CH4/H2) gas mixture used in hydrogen transportation. The mechanical performance under various blended hydrogen concentrations was compared for three pipeline steels, API X42, X65, and X70. X65 exhibited the highest risk of hydrogen-assisted crack initiation in the CH4/H2 gas mixture in which brittle fractures were observed even at 1% H2. The X42 and X70 samples exhibited a significant change in their fracture mechanism in a 30% H2 gas mixture condition; however, their ductility remained unchanged. There was an insignificant difference in the hydrogen embrittlement indices of the three steels under 10 MPa of hydrogen gas. The coexistence of delamination along with the ferrite/pearlite interface, heterogeneous deformation in the radial direction, and abundance of nonmetallic MnS inclusions in the X65 sample may induce a high stress triaxiality at the gauge length at the beginning of the slow strain rate tensile process, thereby facilitating efficient hydrogen diffusion.

Evaluation of Fatigue Life of Ultra-High-Strength Steel under Stress Corrosion Environment

Ultra-high-strength steel (UHSS) structures are exposed to corrosive environments during service, and hydrogen-assisted cracking (HAC) may occur owing to stress corrosion cracking and hydrogen embrittlement. In this study, the HAC threshold stress intensity factor and fatigue life of UHSS steel were evaluated by applying stress in a corrosive environment to prevent structural fracture. For specimen with semicircular slits by electric discharge machining, fatigue limit was obtained by static fatigue test under corrosive environment. The fatigue limit of the crack specimen was evaluated by the fatigue limit of the experiment and HAC threshold stress intensity factor, and comparative evaluation was performed. On the surface of cracks, grain boundaries were embrittled by corrosion, and grains were clearly observed. Meanwhile, cracks in the surface direction propagated slightly, unlike cracks in the depth direction. The static fatigue limit of UHSS (SKD11:HV670) was determined to be 400 MPa, and the fatigue limit of the crack specimen could be evaluated. The experimental results agreed well with the evaluation results.

Combined thermal desorption spectroscopy, hydrogen visualization, HRTEM and EBSD investigation of a Ni–Fe–Cr alloy: The role of hydrogen trapping behavior in hydrogen-assisted fracture

The role of hydrogen trapping behavior in hydrogen-assisted cracking in a Ni–Fe–Cr alloy has been investigated using thermal desorption spectroscopy (TDS), hydrogen visualization, and high-resolution microstructural characterization methods. The results indicate that hydrogen resides at interstitial lattice sites, dislocations, and vacancies with the ascending order of the desorption activation energies. This study firstly shows that hydrogen is trapped reversibly at dislocations including misfit dislocations at the austenite/δ phase interface, and deformation-induced dislocations. The hydrogen trapping behavior is proved by TDS and hydrogen visualization technique. The weakly trapped hydrogen at dislocations provides sufficient hydrogen to build up a required critical hydrogen concentration at potential flaws for crack nucleation and to activate hydrogen enhanced decohesion mechanism. Further, it was found that δ phase precipitation exacerbates the hydrogen embrittlement sensitivity. For the first time, evidence for hydrogen transport by dislocations in a Ni-based alloy is observed by the hydrogen visualization technique.

A generalised, multi-phase-field theory for dissolution-driven stress corrosion cracking and hydrogen embrittlement

We present a phase field-based electro-chemo-mechanical formulation for modelling mechanics-enhanced corrosion and hydrogen-assisted cracking in elastic–plastic solids. A multi-phase-field approach is used to present, for the first time, a general framework for stress corrosion cracking, incorporating both anodic dissolution and hydrogen embrittlement mechanisms. We numerically implement our theory using the finite element method and defining as primary fields the displacement components, the phase field corrosion order parameter, the metal ion concentration, the phase field fracture order parameter and the hydrogen concentration. Representative case studies are addressed to showcase the predictive capabilities of the model in various materials and environments, attaining a promising agreement with benchmark tests and experimental observations. We show that the generalised formulation presented can capture, as a function of the environment, the interplay between anodic dissolution- and hydrogen-driven failure mechanisms; including the transition from one to the other, their synergistic action and their individual occurrence. Such a generalised framework can bring new insight into environment–material interactions and the understanding of stress corrosion cracking, as demonstrated here by providing the first simulation results for Gruhl’s seminal experiments.

Ranking the susceptibility to hydrogen-assisted cracking in dissimilar metal welds

Ranking the susceptibility to hydrogen-assisted cracking in dissimilar metal welds

Local fracture criterion for quasi-cleavage hydrogen-assisted cracking of tempered martensitic steels

Electrochemical permeation cell built on a Instron tensile testing machine allows fracturing notched specimens under hydrogen flux while monitoring simultaneously the flow stress and the permeation anodic current. Analysis of the fracture surfaces reveals that cracking initiates at the hydrogen-entry surfaces as quasi-cleavage regions followed by ductile propagation. Quasi-cleavage zones were fully developed at the maximum engineering stress and often initiate around inclusions for unnotched specimens. The observation of multiple cracks at the hydrogen-entry surfaces and correlations between quasi-cleavage features and martensitic boundaries suggest that cracking is related to decohesion of cropping out and inner boundaries. The local mechanical states and diffusible hydrogen concentration and flux at quasi-cleavage failure were calculated by finite element method (FEM). The results predict local failure criteria in terms of decrease of local maximum principal stress and equivalent plastic strain with local diffusible hydrogen concentration, revealing that the inclusion of hydrogen-mechanical-structural interactions at the charging surfaces is necessary to complete fracture analysis.

Comparing Stress Corrosion Cracking Behavior of Additively Manufactured and Wrought 17-4PH Stainless Steel

As a high-strength corrosion-resistant alloy, stress corrosion cracking (SCC) behavior is a key consideration for the conventional, wrought form of 17-4PH stainless steel. With the increasing popularity of the additively manufactured (AM) form of 17-4PH, understanding the SCC behavior of AM 17-4PH will be similarly critical for its presumed, future applications. The current study quantifies and compares the SCC behavior of both the wrought form, as a baseline, and AM form of 17-4PH at peak-aged (∼1,200 MPa) and overaged (∼1,050 MPa) strength levels. The laser powder bed fusion technique followed by post-process hot isostatic press (HIP), solution annealing, and aging heat treatments is used to produce AM 17-4PH with similar microstructures and strength levels to wrought 17-4PH and facilitate the comparison. SCC behavior is quantified using fracture mechanics-based rising (dK/dt = 2 MPa√m/h) and constant (dK/dt = 0 MPa√m/h) stress intensity tests in neutral 0.6 M NaCl at various applied potentials. Limited SCC susceptibility was observed at open-circuit and anodic potentials for both forms of 17-4PH. At cathodic applied potentials, AM consistently underperforms wrought with up to 5-fold faster crack growth rates and 200 mV to 400 mV wider SCC susceptibility ranges. These results are interrogated through microstructural and fractographic analysis and interpreted through a decohesion-based hydrogen-assisted crack model. Initial analyses show that (1) increased oxygen content, (2) porosity induced by argon processing, and (3) slow cooling (310°C/h) during conventional HIP processing might contribute to degraded SCC performance in AM 17-4PH.

Characterization of Hydrogen Diffusion in Offshore Steel S420G2+M Multi-layer Submerged Arc Welded Joint

As onshore installation capacity is limited, the increase in the number of offshore wind turbines (OWT) is a major goal. In that connection, the OWTs continuously increase in size and weight and demand adequate foundations concepts like monopiles or tripods. These components are typically manufactured from welded mild steel plates with thickness up to 200 mm. The predominant welding technique is submerged arc welding (SAW). In accordance with the standards, the occurrence of hydrogen-assisted cracking is anticipated by either a minimum waiting time (MWT, before non-destructive testing of the welded joint is allowed) at ambient or a hydrogen removal heat treatment (HRHT) at elevated temperatures. The effectiveness of both can be estimated by calculation of the diffusion time, i.e., diffusion coefficients. In this study, these coefficients are obtained for the first time for a thick-walled S420G2+M offshore steel grade and its multi-layer SAW joint. The electrochemical permeation technique at ambient temperature is used for the determination of diffusion coefficients for both the base material and the weld metal. The coefficients are within a range of 10−5 to 10−4 mm2/s (whereas the weld metal had the lowest) and are used for an analytical and numerical calculation of the hydrogen diffusion and the related MWT. The results showed that long MWT can occur, which would be necessary to significantly decrease the hydrogen concentration. Weld metal diffusion coefficients at elevated temperatures were calculated from hydrogen desorption experiments by carrier gas hot extraction. They are within a range of 10−3 mm2/s and used for the characterization of a HRHT dwell-time. The analytical calculation shows the same tendency of long necessary times also at elevated temperatures. That means the necessary time is strongly influenced by the considered plate thickness and the estimation of any MWT/HRHT via diffusion coefficients should be critically discussed.

Criteria for hydrogen-assisted crack initiation in Ni-based superalloy 718

The tensile mechanical properties of a Ni-based superalloy 718 uniformly precharged with ≈ 90 mass ppm hydrogen were investigated under a wide range of temperatures to shed light on the long-standing uncertainties surrounding the H-related embrittlement mechanisms of the material. The detrimental effect of H on ductility was found to be substantial in the near-ambient to high-temperature range, up to 300 °C, stemming from H-assisted microcrack initiations along annealing twin boundaries (ATBs) and crystallographic slip planes (SPs). Dynamic H-dislocation interaction, which has been thought to be a prerequisite for the onset of embrittlement, was found to be unimportant, as demonstrated by employing supplemental tests that incorporated prestraining and intermediate temperature changes. By combining the insights gained from the successfully designed test program, a new model for the nucleation process of H-induced fracturing was established.

Induction Heating in Underwater Wet Welding-Thermal Input, Microstructure and Diffusible Hydrogen Content.

Hydrogen-assisted cracking is a major challenge in underwater wet welding of high-strength steels with a carbon equivalent larger than 0.4 wt%. In dry welding processes, post-weld heat treatment can reduce the hardness in the heat-affected zone while simultaneously lowering the diffusible hydrogen concentration in the weldment. However, common heat treatments known from atmospheric welding under dry conditions are non-applicable in the wet environment. Induction heating could make a difference since the heat is generated directly in the workpiece. In the present study, the thermal input by using a commercial induction heating system under water was characterized first. Then, the effect of an additional induction heating was examined with respect to the resulting microstructure of weldments on structural steels with different strength and composition. Moreover, the diffusible hydrogen content in weld metal was analyzed by the carrier gas hot extraction method. Post-weld induction heating could reduce the diffusible hydrogen content by −34% in 30 m simulated water depth.

Adaptive finite element modeling of phase-field fracture driven by hydrogen embrittlement

The effect of hydrogen on crack propagation has been studied extensively using various numerical algorithms. Recently, phase-field models have been developed that can predict crack propagation with very good resemblance of the crack paths with the experimental results. Solving the coupled equations of hydrogen transport, mechanical equilibrium and phase-field models is computationally expensive requiring a large number of time steps and high spatial resolution. Often, the spatial resolution is achieved a priori in known regions where the cracks may propagate. This leads to a large number of degrees of freedom to be solved for a large number of time steps for the coupled equations in simulating hydrogen-assisted cracking. In the present work, an adaptive refinement scheme is proposed that will remove the burden of very high uniform spatial resolution in non crack regions eventually leading to fewer degrees of freedom and hence decreases the computational cost. Several case studies are considered and the efficacy of the proposed method is demonstrated.

Hydrogen-Assisted Cracking Fracture Analysis using High-Speed Camera and Delayed Hydrogen Cracking Test

Hydrogen-Assisted Cracking Fracture Analysis using High-Speed Camera and Delayed Hydrogen Cracking Test

Hydrogen-assisted cracking (HAC) of high-strength steels as a function of the hydrogen pre-charging time

This work demonstrates that by performing hydrogen-assisted cracking (HAC) as a function of hydrogen pre-charging time, highly symmetric circumferential cracks can be generated in circumferentially notched or grooved round bars made of high-strength steels, which can be used to study HAC in terms of fracture mechanics. Using the simple-to-perform method, control over hydrogen-assisted crack initiation is achieved by applying low hydrogen pre-charging times and by determining a threshold minimum preload to failure LHAC. The uniaxial tensile testing at LHAC under constant displacement results in a characteristic load-time-curve as well as a high-symmetry circumferential crack for each material investigated in this study. Finally, the minimum preload to failure, the time to failure and the critical crack length are each exhibited as a function of hydrogen pre-charging time and an approach to the application of fracture mechanics with the aim of determining the threshold stress intensity KIHAC is discussed.

Hydrogen assisted cracking paths in cold drawn pearlitic steels

This paper analyses hydrogen assisted cracking (HAC) paths in cold drawn pearlitic steels in which progressive cold drawing produces a preferential orientation of the pearlitic microstructure in the matter of colonies and lamellae, thereby inducing strength anisotropy in the steel, and thus the resistance to HAC is a directional property that depends on the angle in relation to the drawing direction. Therefore, an initial transverse crack changes its propagation direction to approach that of the wire axis, thus producing mixed mode propagation, the deflection angle being an increasing function of the cold drawing degree. This experimental result may be explained by micro-mechanical considerations on the basis of the lamellar microstructure of the steels. A relationship is established between the microstructural angles and the deflection angles of the macroscopic HAC path, thus providing a materials science type relationship between the microstructure and the macroscopic crack.

Effect of heat treatment on the stress corrosion cracking behavior of cast Mg-3Nd-3Gd-0.2Zn-0.5Zr alloy in a 3.5 wt% NaCl salt spray environment

This study was undertaken to investigate the influence of heat treatment on the stress corrosion cracking (SCC) behavior of a low-pressure sand-cast Mg-3Nd-3Gd-0.2Zn-0.5Zr alloy in a 3.5 wt% NaCl spray environment. Results indicated that T4 heat treatment (540 °C × 10 h) could effectively improve the corrosion resistance, compared with as-cast and T6 alloys. The relatively lowest volume fraction of high-potential (HPOT) phases observed in the T4 alloy primarily accounts for its lowest corrosion rate. Slow strain rate tensile (SSRT) tests were conducted to evaluate the influence of heat treatment states on the SCC resistance. The lowest SCC susceptibility index ( I SCC I = 0.403 and I SCC II = 0.165) was obtained in T4 state, while the as-cast alloy possessed the highest SCC susceptibility. The present work revealed that hydrogen-assisted cracking (HAC) played a key role in the SCC process. Cracking was effectively suppressed in the T4 alloy owing to its retarded hydrogen diffusion. The structure and compositions of the multi-layer corrosion product film were characterized in detail. Solution treatment applied in this work was confirmed to promote the formation of protective and dense RE oxides layer, primarily contributed to the relatively highest SCC resistance achieved in the T4 alloy. • SCC susceptibility decreases in the order of as-cast > T6 > T4. • HPOT phase dissolution and hydrogen-diffusion suppression strongly improve the SCC resistance. • The structure of multi-layer corrosion product film is established. • Protective RE-oxides layer is characterized by SIMS.

Susceptibility of carbon pipeline steels operated in natural gas distribution network to hydrogen-induced cracking

The existing network of both transit and distribution gas pipelines has recently been proposed to be used for the transportation of hydrogen as an energy fuel in the multifaceted structure of the development of renewable energy sources. From this point of view, the paper considers the sensitivity of a long-term operated steel of distribution gas pipeline to hydrogen assisted cracking as a result of preliminary electrolytic hydrogenation of flat tensile specimens, followed by determination of strength and plasticity characteristics. The research methodology has two features: (i) the specimens are cut out from the pipe in the transverse direction to the pipe axis; in this case the fracture plane is parallel to the rolling direction of metal sheet from which the pipes were made; (ii) specimen thickness is 1.2 mm maximally providing through hydrogenation. The study additionally includes determination of hydrogen concentration in the steel, and fractographic analysis. It is shown that the low-carbon steel of the 52-year-old gas pipeline with high residual hydrogen content especially sensitive to hydrogen assisted cracking, indicated by signs of low-energy fracture at the micro scale.