Hydrogen-assisted Fatigue Crack Research Articles

A mechanism behind hydrogen-assisted fatigue crack growth in ferrite-pearlite steel focusing on its behavior in gaseous environment at elevated temperature

Hydrogen-assisted fatigue crack growth in gaseous environment was comparatively examined at room temperature (RT) and 423 K, based on analysis of the deformation structure evolution around crack-wakes using scanning electron microscopy techniques. In hydrogen-gas at RT, the propagating crack displayed weakly-evolved dislocation arrangement, accompanied by a significant acceleration of fatigue crack growth. However, in hydrogen-gas at 423 K, the crack-wake plasticity was well-evolved and analogous to that observed in an inert environment. This apparent recovery of deformation micro structure coincided with suppressed crack growth acceleration, the rationale for which can be interpreted by the trapping/de-trapping equilibrium between hydrogen and dislocations.

Fatigue crack growth rates in high pressure hydrogen gas for multiple X100 pipeline welds accounting for crack location and residual stress

Fatigue crack growth rates (FCGR) of multiple X100 pipeline steel welds and heat affected zones were measured in high-pressure hydrogen gas to investigate their behavior compared to lower strength pipeline welds. A total of five high strength welds and two heat affected zones (HAZ) were examined all of which were fabricated using the same X100 base material. Different welding wires and techniques were used to fabricate the welds to provide a variety of end products to evaluate susceptibility to fatigue in high pressure hydrogen gas. Residual stresses were measured for each weld and HAZ using the slitting method and the effect of residual stress on the stress intensity factor, Kres, was determined. Using Kres, the fatigue crack growth rate curves were corrected to remove the effects of residual stress by examining the influence of Kres on stress ratio, R. Comparisons were then made between the high strength welds, which were corrected for residual stress, and lower strength welds from the literature. It was found that the higher strength welds and heat affected zones exhibited comparable fatigue crack growth rates to lower strength welds, as the FCGR data of the high strength welds overlaid the lower strength welds. This suggests that despite distinct differences in strength and microstructure between the different welds, hydrogen-assisted fatigue crack growth susceptibility is similar. A comparison was made between the Kres measured in extracted coupons and residual stress estimates provided in relevant welded pipe assessment standards such as API 579-1/ASME FFS-1. It was found the residual stress values in the test coupons extracted from welded pipe were significantly lower than those expected in the intact welded pipes and highlights the importance in quantifying and removing coupon residual stresses when fatigue crack growth rates are measured and including expected weld joint residual stress when making structural assessments.

Role of retained austenite in hydrogen trapping and hydrogen-assisted fatigue fracture of high-strength steels

The Interaction of hydrogen with retained austenite under fatigue loading of dual-phase and complex-phase high-strength steels with strength of about 1200 MPa was studied. A load-controlled fatigue test was performed in the air with the maximum applied tensile strength of 900 MPa, which is just below the offset yield point of the studied steels. The trapping of hydrogen accumulated into the studied steels under the fatigue loading was studied by thermal desorption spectroscopy. Measurements of hydrogen trapping evolution and microstructure changes during fatigue testing reveal a complicated hydrogen trapping behavior driven by hydrogen interaction with deformation defects and retained austenite. Hydrogen concentration increases in the studied steels during the fatigue testing in the air without preceding hydrogen charging. The fracture surfaces were studied by scanning electron microscopy evidencing the relationship between the hydrogen concentration increase related to retained austenite and initiation of the intergranular fatigue fracture. The role of retained austenite in hydrogen-assisted fatigue cracking is discussed and a possible mechanism of the hydrogen-assisted fatigue crack initiation in the high-strength steels is proposed.

Hydrogen isotope effect on the embrittlement and fatigue crack growth of steel

The deleterious effect of hydrogen on steel is a long-standing problem. Embrittlement in the presence of hydrogen is, in part, a consequence of the fast diffusion of hydrogen in ferritic steels. Because of the identical chemical properties but large differences in mass between hydrogen isotopes, interaction of hydrogen isotopes with steel structures are of interest as a tool to study the role of diffusion in hydrogen embrittlement under all-else-equal conditions. In this paper, we present tensile and fatigue data for steel samples measured in air, in hydrogen, and in deuterium. Tensile measurements show deuterium causes a loss of ductility comparable to the loss due to hydrogen. In fatigue, a large pressure effect on deuterium-assisted fatigue crack growth was observed, an effect which is not exhibited in uniaxial tensile measurements. With sufficient pressure to show an enhanced fatigue crack growth effect, the fatigue crack growth rate is well-predicted assuming a scaling of the diffusion coefficient by 1/2. These results suggest surface adsorption kinetics play a large role in hydrogen-assisted fatigue crack growth at low pressure. At higher pressures, hydrogen-assisted fatigue crack growth is primarily affected by the kinetics of diffusion of hydrogen within the steel lattice.

The role of intergranular fracture on hydrogen-assisted fatigue crack propagation in pure iron at a low stress intensity range

Hydrogen-assisted fatigue crack growth (HAFCG) in pure iron at a relatively low stress intensity range exhibits brittle-like intergranular (IG) fracture, while the macroscopic crack acceleration is not significant. The present study focuses on the mechanism of IG fracture in terms of the microscopic deformation structures near the crack propagation paths. We found that the IG fracture is attributed to hydrogen-enhanced dislocation structure evolution and subsequent microvoid formation along the grain boundaries. The impact of such IG cracking on the macroscopic fatigue crack growth (FCG) acceleration is evaluated according to the dependency of IG fracture tendency on the hydrogen gas pressure during testing. It is demonstrated for the first time that increased hydrogen pressure results in a larger fraction of IG fracture and correspondingly faster FCG. On the other hand, the gaseous hydrogen environment also has a positive role in decelerating the FCG rate relative to air due to the absence of oxygen and water vapor. The macroscopic crack propagation rate in hydrogen gas is eventually determined by the competition between the said positive and negative influences.

Interpretation of hydrogen-assisted fatigue crack propagation in BCC iron based on dislocation structure evolution around the crack wake

A new model for hydrogen-assisted fatigue crack growth (HAFCG) in BCC iron under a gaseous hydrogen environment has been established based on various methods of observation, i.e., electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM), to elucidate the precise mechanism of HAFCG. The FCG in gaseous hydrogen showed two distinguishing regimes corresponding to the unaccelerated regime at a relatively low stress intensity factor range, ΔK, and the accelerated regime at a relatively high ΔK. The fracture surface in the unaccelerated regime was covered by ductile transgranular and intergranular features, while mainly quasi-cleavage features were observed in the accelerated regime. The EBSD and ECCI results demonstrated considerably lower amounts of plastic deformation, i.e., less plasticity, around the crack path in the accelerated regime. The TEM results confirmed that the dislocation structure immediately beneath the crack in the accelerated regime showed significantly lower development and that the fracture surface in the quasi-cleavage regions was parallel to the {100} plane. These observations suggest that the HAFCG in pure iron may be attributed to “less plasticity” rather than “localized plasticity” around the crack tip.

Correlation between Fatigue Crack Growth Behavior and Fracture Surface Roughness on Cold-Rolled Austenitic Stainless Steels in Gaseous Hydrogen

Austenitic stainless steels are often considered candidate materials for use in hydrogen-containing environments because of their low hydrogen embrittlement susceptibility. In this study, the fatigue crack growth behavior of the solution-annealed and cold-rolled 301, 304L, and 310S austenitic stainless steels was characterized in 0.2 MPa gaseous hydrogen to evaluate the hydrogen-assisted fatigue crack growth and correlate the fatigue crack growth rates with the fracture feature or fracture surface roughness. Regardless of the testing conditions, higher fracture surface roughness could be obtained in a higher stress intensity factor (∆K) range and for the counterpart cold-rolled specimen in hydrogen. The accelerated fatigue crack growth of 301 and 304L in hydrogen was accompanied by high fracture surface roughness and was associated with strain-induced martensitic transformation in the plastic zone ahead of the fatigue crack tip.

Development of a Model for Hydrogen-Assisted Fatigue Crack Growth of Pipeline Steel1

Hydrogen has been proposed as a potential partial solution to the need for a clean-energy economy. In order to make this a reality, large-scale hydrogen transportation networks need to be engineered and installed. Steel pipelines are the most likely candidate for the required hydrogen transportation network. One historical barrier to the use of steel pipelines to transport hydrogen was a lack of experimental data and models pertaining to the fatigue response of steels in gaseous hydrogen. Extensive research at NIST has been performed in conjunction with the ASME B31.12 Hydrogen Piping and Pipeline committee to fill this need. After a large number of fatigue crack growth (FCG) tests were performed in gaseous hydrogen, a phenomenological model was created to correlate the applied loading conditions, geometry, and hydrogen pressure to the resultant hydrogen-assisted fatigue crack growth (HA-FCG) response of the steels. As a result of this extensive data set, and a simplification of the above-mentioned phenomenological model, the ASME B31.12 code was modified to enable the use of higher strength steels without penalty, thereby resulting in the potential for considerable installation cost savings. This paper details the modeling effort that led to the code change.

Effects of α′ martensite and deformation twin on hydrogen-assisted fatigue crack growth in cold/warm-rolled type 304 stainless steel

The effects of α′ martensite and deformation twin on hydrogen-assisted fatigue crack growth (FCG) were investigated in cold/warm-rolled type 304 stainless steel in 5 MPa hydrogen and argon gas atmosphere. The rate of FCG is reduced in argon gas, while greatly enhanced in hydrogen gas after cold-rolling. The FCG rates of warm-rolled specimens, no matter tested in hydrogen gas or argon gas, are reduced comparing with as-received specimens. After cold-rolling, α′ martensite formed around the grain boundary promotes hydrogen-assisted crack initiation and propagation. The deformation twin plays an important role during FCG besides α′ martensite after warm-rolling, and hydrogen-assisted cracking along the twin boundary and slip band can enhance the FCG rate during cycle loading.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part II: Accelerated regime manifested by quasi-cleavage fracture at relatively high stress intensity range values

Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part I: Intergranular crack propagation at relatively low stress intensities

The role of hydrogen on intergranular (IG) fracture in hydrogen-assisted fatigue crack growth (HAFCG) of a pure iron at low stress intensity was discussed in terms of the microscopic deformation structures near crack propagation paths. The main cause of IG fracture was assumed to be the hydrogen-enhanced dislocation structure evolution and subsequent microvoids formation along the grain boundaries. Additionally, the impact of such IG cracking on the macroscopic FCG rate was evaluated according to the dependency of IG fracture propensity on the hydrogen gas pressure. It was first demonstrated that the increased hydrogen pressure results in the larger area fraction of IG and corresponding faster FCG rate. Moreover, gaseous hydrogen environment also had a positive influence on the FCG rate due to the absence of oxygen and water vapor. The macroscopic crack propagation rate was controlled by the competition process of said positive and negative effects.

Hydrogen-assisted fatigue crack growth in ferritic steels – a fractographic study

Fatigue crack growth (FCG) behavior of a Fe-3wt.%Si ferritic alloy under different environmental conditions using in-situ electrochemical (cathodic) hydrogen (H) charging has been investigated. Three frequencies have been applied. Results clearly show that the FCG rate increased by a factor spanning from 20 to 1000 times, depending on the loading frequencies, when compared to the reference test in air. Lower frequency leads to higher FCG rate. A comprehensive fractographic analysis was carried out: the area fraction of different fracture surface features was measured and taken into statistical analysis. Based on these investigations, the possible mechanisms of H-enhanced FCG are discussed. Similar tests in high-pressure H gas from other studies were also compared and discussed. These results give a preliminary understanding of H effect in fatigue crack propagation procedure in ferritic alloys.

Fatigue Properties of Ultra-Fine Grain Austenitic Stainless Steel and the Effect of Hydrogen

The fatigue properties of ultra-fine grain austenitic steel (UFG16-10), which has a 1 μm average grain size, were studied as part of the project aimed at the development of high-strength low-cost stainless steels for hydrogen service. The fatigue properties of the UFG16-10 were compared with that of a coarse grain material with the same chemical composition (CG16-10) and two kinds of commercial steels, JIS SUS316 and JIS SUH660. The fatigue strength of the UFG16-10 was 2.8 times higher than that of the CG16-10. The effect of hydrogen on the fatigue limit of the UFG16-10 was not significant. However, the fatigue life of the UFG16-10 was reduced by hydrogen in the short life regime. In the fatigue crack growth test, the UFG16-10 showed a good crack growth resistance that was equivalent to that of the SUH660 and significantly higher than that of the SUS316. However, the crack growth rate was significantly accelerated by hydrogen. The cause of the hydrogen-assisted fatigue crack growth of the UFG16-10 was transformation of the microstructure at the crack tip from austenite to strain-induced martensite. This was also the cause of the reduced fatigue life of the hydrogen-charged UFG16-10.

Interpretation of hydrogen-assisted fatigue crack acceleration in a pure iron based on crack tip microstructure evolution

Interpretation of hydrogen-assisted fatigue crack acceleration in a pure iron based on crack tip microstructure evolution

Assessing Gaseous Hydrogen Assisted Fatigue Crack Growth Susceptibility of Pipeline Steel Weld Fusion Zones and Heat Affected Zones

Abstract The objective of this work was twofold: (1) measure reliable fatigue crack growth relationships for X65 steel and its girth weld in high-pressure hydrogen gas to enable structural integrity assessments of hydrogen pipelines, and (2) evaluate the hydrogen accelerated fatigue crack growth susceptibility of the weld fusion zone and heat-affected zone relative to the base metal. Fatigue crack growth relationships (da/dN versus ΔK) were measured for girth welded X65 pipeline steel in high pressure hydrogen gas (21 MPa) and in air. Hydrogen assisted fatigue crack growth was observed for the base metal (BM), fusion zone (FZ), and heat-affected zone (HAZ), and was manifested through crack growth rates reaching nearly an order of magnitude acceleration over rates in air. At higher ΔK values, crack growth rates of BM, FZ, and HAZ were coincident; however, at lower ΔK, the fatigue crack growth relationships exhibited some divergence with the fusion zone having the highest crack growth rates. These relative fatigue crack growth rates in the BM, FZ, and HAZ were provisional, however, since both crack closure and residual stress contributed to the crack-tip driving force in specimens extracted from the HAZ. Despite the relatively high applied R-ratio (R = 0.5), crack closure was detected in the heat affected zone tests, in contrast to the absence of crack closure in the base metal tests. Crack closure corrections were performed using the adjusted compliance ratio method and the effect of residual stress on Kmax was determined by the crack-compliance method. Crack-tip driving forces that account for closure and residual stress effects were quantified as a weighted function of ΔK and Kmax (i.e., Knorm), and the resulting da/dN versus Knorm relationships showed that the HAZ exhibited higher hydrogen accelerated fatigue crack growth rates than the BM at lower Knorm values.

The effect of phase transformation in the plastic zone on the hydrogen-assisted fatigue crack growth of 301 stainless steel

The effects of rolling and stress ratio (R) on the fatigue crack growth rates (FCGRs) of AISI 301 stainless steel (SS) in gaseous hydrogen were investigated. Accelerated fatigue crack growth occurred for the solution-annealed (SA) and cold rolled (CR) specimens tested in gaseous hydrogen, and especially for the CR specimen at high R. Moreover, a sufficiently high ΔK and/or Kmax were required to activate hydrogen-assisted fatigue crack growth of the 301 SS. Fatigue crack growth within a narrow α′ martensite layer was responsible for the rapid FCGRs of the specimens in hydrogen. The plastic zone size, identified by hardness distributions, was much larger than the region that underwent phase transformation.

Effects of microstructure banding on hydrogen assisted fatigue crack growth in X65 pipeline steels

Banded ferrite–pearlite X65 pipeline steel was tested in high pressure hydrogen gas to evaluate the effects of oriented pearlite on hydrogen assisted fatigue crack growth. Test specimens were oriented in the steel pipe such that cracks propagated either parallel or perpendicular to the banded pearlite. The ferrite–pearlite microstructure exhibited orientation dependent behavior in which fatigue crack growth rates were significantly lower for cracks oriented perpendicular to the banded pearlite compared to cracks oriented parallel to the bands. The reduction of hydrogen assisted fatigue crack growth across the banded pearlite is attributed to a combination of crack-tip branching and impeded hydrogen diffusion across the banded pearlite.

Application of Paris’ law for estimation of hydrogen-assisted fatigue crack growth

Based on the general form of Paris’ law a new method is proposed for estimating the rate of cracking of metals under Hydrogen-Assisted Fatigue. It is based on relating the fatigue crack growth rate of hydrogen embrittled metal to the fatigue crack growth rate of the metal without hydrogen embrittlement. One-dimensional hydrogen diffusion is assumed. Simulations using the proposed method on steel types X52, X70–80 and X80 are in agreement with published experimental tests results. The new method obviates numerical modelling of crack propagation and much reduces the computational costs.

Measurement of Fatigue Crack Growth Relationships in Hydrogen Gas for Pressure Swing Adsorber Vessel Steels

Hydrogen-assisted fatigue crack growth rates (da/dN) were measured for SA516 Grade 70 steel as a function of stress-intensity factor range (ΔK) and load-cycle frequency to provide life-prediction data relevant to pressure swing adsorber (PSA) vessels. For ΔK values up to 18.5 MPa m1/2, the baseline da/dN versus ΔK relationship measured at 1 Hz in 2.8 MPa hydrogen gas represents an upper bound with respect to crack growth rates measured at lower frequency. However, at higher ΔK values, baseline da/dN data must be corrected to account for modestly higher crack growth rates at the lower frequencies relevant to PSA vessel operation.

Suppression of Hydrogen-Assisted Fatigue Crack Growth in Carbon Steels

Carbon steels with high resistance to hydrogen-assisted fatigue crack growth were successfully produced by the addition of a carbideforming element and the refinement of ferrite grain size. In carbon steels containing vanadium, titanium, or niobium, fine carbide (VC, TiC, or NbC, respectively) precipitation and the refinement of grain size below 1μm were achieved by caliber rolling at 833K. Hydrogen charging increased the fatigue crack growth rate in standard JIS-S45C carbon steel to about 25 times that in uncharged steels. In contrast, in carbon steels containing carbide-forming elements, hydrogen-assisted fatigue crack growth was either absent or at most two times faster than that in uncharged steels. [doi:10.2320/matertrans.M2013237]