Simulation of hydrogen-induced failure in high strength steel

Analysis of hydrogen-induced subcritical intergranular crack growth of iron and nickel

This paper presents the results of experimental and in-service observations of the nucleation and growth of hydrogen-induced cracking (HIC) in hydrocarbon transport pipelines made of type API 5L steel. The experimental work was done by inducing HIC on steel plates by electrochemical cathodic hydrogen charging and using a straight beam ultrasonic inspection technique to observe the crack growth behavior. Scanning electron microscopy was also used to observe the crack nucleation and propagation mechanisms. The study was complemented by the fractographic analysis of a pipe segment removed from a sour gas pipeline after an in-service rupture caused by HIC, so the pipe segment contained a significant group of blisters and laminations caused by HIC. The results of the cathodic charging indicated that HIC cracks nucleated in less than one hour of hydrogen charging at specific non-metallic inclusions and not necessarily the largest ones as commonly thought. It is observed that the HIC cracks propagated by a quasi-cleavage mechanism in transgranular paths, linking to other cracks by ductile tearing. However, after a few hours of hydrogen charging, the crack growth rate dropped to almost zero, and the overall HIC growth was due almost solely to the interconnection of previously formed individual cracks. The examination of the in-service failed pipe showed similar fractographic and growth characteristics as compared to the laboratory-induced ones. It showed that HIC was little affected by the primary stresses and the proximity of other defects and structural discontinuities.

Though intergranular (IG) and quasi-cleavage (QC) fractures have been widely recognized as typical fracture modes of the hydrogen-induced cracking in high-strength steels, the main factor has been unclarified yet. In the present study, the hydrogen content dependence on the main factor causing hydrogen-induced cracking has been examined through the fracture mode transition from QC to IG at the crack initiation site in the tempered martensitic steels. Two kinds of tempered martensitic steels were prepared to change the cohesive force due to the different precipitation states of Fe3C on the prior γ grain boundaries. A high amount of Si (H-Si) steel has a small amount of Fe3C on the prior austenite grain boundaries. Whereas, a low amount of Si (L-Si) steel has a large amount of Fe3C sheets on the grain boundaries. The fracture modes and initiations were observed using FE-SEM (Field Emission-Scanning Electron Microscope). The crack initiation sites of the H-Si steel were QC fracture at the notch tip under various hydrogen contents. While the crack initiation of the L-Si steel change from QC fracture at the notch tip to QC and IG fractures from approximately 10 µm ahead of the notch tip as increasing in hydrogen content. For L-Si steels, two possibilities are considered that the QC or IG fracture occurred firstly, or the QC and IG fractures occurred simultaneously. Furthermore, the principal stress and equivalent plastic strain distributions near the notch tip were calculated with FEM (Finite Element Method) analysis. The plastic strain was the maximum at the notch tip and the principle stress was the maximum at approximately 10 µm from the notch tip. The position of the initiation of QC and IG fracture observed using FE-SEM corresponds to the position of maximum strain and stress obtained with FEM, respectively. These findings indicate that the main factors causing hydrogen-induced cracking are different between QC and IG fractures.

Cohesive zone modeling of hydrogen-induced delayed intergranular fracture in high strength steels

Hydrogen absorption property of nanocrystalline-magnesium films

The effects of small amounts of dissolved hydrogen on crack propagation were determined for two austenitic stainless steel alloys, AISI 301 and 310S. In order to have a uniform distribution of hydrogen in the alloys, they were cathodically charged at high temperature in a molten salt electrolyte. Sustained load tests were performed on fatigue precracked specimens in air at 0 ‡C, 25 ‡C, and 50 ‡C with hydrogen contents up to 41 wt ppm. The electrical potential drop method with optical calibration was used to continuously monitor the crack position. Log crack velocityvs stress intensity curves had definite thresholds for subcritical crack growth (SCG), but stage II was not always clearly delineated. In the unstable austenitic steel, AISI 301, the threshold stress intensity decreased with increasing hydrogen content or increasing temperature, but beyond about 10 wt ppm, it became insensitive to hydrogen concentration. At higher concentrations, stage II became less distinct. In the stable stainless steel, subcritical crack growth was observed only for a specimen containing 41 wt ppm hydrogen. Fractographic features were correlated with stress intensity, hydrogen content, and temperature. The fracture mode changed with temperature and hydrogen content. For unstable austenitic steel, low temperature and high hydrogen content favored intergranular fracture while microvoid coalescence dominated at a low hydrogen content. The interpretation of these phenomena is based on the tendency for stress-induced phase transformation, the different hydrogen diffusivity and solubility in ferrite and austenite, and outgassing from the crack tip. After comparing the embrittlement due to internal hydrogen with that in external hydrogen, it is concluded that the critical hydrogen distribution for the onset of subcritical crack growth is reached at a location that is very near the crack tip.

Stress corrosion cracking (SCC) of high-strength steel in aqueous environment and hydrogen induced cracking (HIC) during dynamic charging under Mode III loading were investigated. The threshold stress intensities for SCC and HIC under Modes III and I were measured and compared. It was found that both SCC and HIC under Mode III loading initiated and propagated on the planes inclined at 45 deg to the notch plane, differing from that under Mode I loading. The fracture surfaces, however, revealed intergranular facets, similar to that under Mode I loading. The addition of thiourea decreased the threshold value for SCC under Mode III and Mode I loading, which was still higher than that for dynamic charging. The threshold values of both SCC and HIC under Mode III were larger than that under Mode I,i.e., KIIIH> KIH, KIIISCC > KISCC. Based upon the fracture mechanics analysis, this difference is attributed to the different equilibrium hydrogen concentration between Modes III and I loading. These results give strong evidence that the SCC mechanism in high strength steel under Mode III loading is also related to hydrogen induced cracking.

Combined effects of temperature and of high hydrogen and oxygen contents on the mechanical behavior of a zirconium alloy upon cooling from the βZr phase temperature range

In this work the influence of the stresses induced by hydrogen loading on the position of phase boundaries in niobium and yttrium thin films is studied. The films were loaded with hydrogen electrochemically. The hydrogen concentration was calculated by use of Faraday"s law. The stresses were measured using an optical beam deflection setup. The strains and the position of the phase boundaries were determined in situ during hydrogen (un-)loading at the synchrotron. The results were compared to a one-dimensional linear elastic model using bulk elastic constants and bulk hydrogen induced expansion data. The measurements on epitactic niobium thin films of different thicknesses and on nanocrystalline niobium thin films on thin polymer film substrates in comparison to nanocrystalline films on silicon substrates showed a strong correlation between the hydrogen induced stresses and the shift of phase boundaries in these thin films. Measurements on yttrium thin films however revealed only small stresses built up by hydrogen loading and unloading. Additionally performed synchrotron measurements could show that there is in contrast to the literature no significant shift of phase boundaries compared to the bulk yttrium-hydrogen system.

Hydrogen Induced Cracking (HIC) is one of failure types of in-service pressure vessels. Acoustic emission (AE) is a good method to monitor HIC. In order to investigate the characteristic of AE signals produced by HIC of carbon steel used in pressure vessel, the hydrogen charging progress of 20R steel was monitored using AE technique. There were three useful AE sources during the test, corrosion and charging, FeS film forming and breaking, and HIC growth, which all can be detected through AE instrument. The amount of big amplitude AE signals were increased obviously with cracking growth and microscopic examinations provided good confirmation. The peak frequency ranges of these three kinds of AE waveform signals were similar, but their distribution of the spectrum magnitude to the corresponding frequency were different and can be used as the discriminating acoustic parameters.Therefore, a pattern recognition method based on frequency feature extraction, fisher ratio and trained back-propagation (BP) network was developed for the sources' signals class decision. Finally some testing data study was given to show the high efficiency the proposed method. These results provide a useful way of monitoring HIC development in pressure vessels for practical inspection.

Mechanisms of hydrogen-induced cracking in ultrahigh-strength steels

A finite difference hydrogen induced crack (HIC) growth model has been developed, enabling for the first time an accurate simulation of the growth of HIC cracks in the wall of pipelines and/or pressure vessels. In the model, parameters such as defect size, location, fracture toughness and hydrogen concentration, can be varied. The predictions of the HIC model are compared with experimental HIC crack growth curves, which were generated in a unique hydrogen permeation-ultrasonic scanning experiment. Both the observed time until initiation for HIC cracks and the crack growth rate show excellent agreement with the model calculations. The experiment in combination with the HIC model shows that defects initiate and propagate through segregation zones with a low fracture toughness. As soon as the defects reach the end of the segregation zone the HIC growth stops. The fracture toughness of the segregation zone as well as the bulk fracture toughness were determined by fitting the HIC model to the experimental data.

Virtual commissioning can find problems in the PLC program earlier and improve the efficiency of on-site commissioning. When constructing the FMS digital twin system with Beckhoff TwinCAT as the master control system for virtual commissioning of the production line, in order to select the most suitable modeling environment, this paper conducts research of predecessors based on commercial industrial simulation software. The four common commercial industrial simulation software with virtual commissioning function (Visual Components, Tecnomatix Process Simulate, Emulate3D, Flexsim) are introduced, analysed and compared from the two perspectives of software functions and features. Finally, it explains which commercial simulation software should be used for simulation and virtual commissioning in different scenarios.

It has been well documented that slab internal quality is one of the key factors for reduced susceptibility of hydrogen induced cracking (HIC) in line pipe steels designed for sour gas service. In addition, the creation of a homogeneous microstructure which is heavily influenced by the slab internal quality is also a critical key parameter to reduce the HIC susceptibility in higher strength line pipe steel grade X60 and above. For the application of deep sea linepipe exposed to higher external pressure environments, heavy gauge in combination with higher strength steel is essential. Homogeneity of the steel microstructure is a key to success for thicker plates used in sour service HIC applications in combination with a deep sea environment. In this paper, various microstructures were compared along with an evaluation of the effects of the various microstructures on HIC susceptibility in grades X52, X65 and X70 designed for sour service. The various microstructures compared consisted of polygonal ferrite and pearlite in the X52 and polygonal ferrite, pearlite, acicular ferrite and bainite in the X65 and X70. The effect of microstructural inhomogeneity on HIC susceptibility was comparatively lower for the X52 than that of the X65 and X70. The microstructure of grade X65 and X70 were different due to the different conditions of rolling and cooling that were applied. Grades X65/X70 had a microstructure of polygonal ferrite/pearlite with bainite islands that resulted in a high crack length ratio (CLR) value caused by different hardness regions across the microstructural matrix. A homogeneous fine acicular ferrite microstructure produced by optimizing temperature control during rolling and cooling showed no hydrogen induced cracking. In addition, this alloy/process/microstructure design resulted in improved toughness results in low temperature drop weight tear test (DWTT). This paper will describe the successful production results of plate and pipe for high strength heavier gauge line pipe steels with highly homogeneous microstructures designed for sour service by controlling chemical design and process conditions in rolling and cooling. In addition, HIC evaluation methods utilizing both a traditional NACE TM0284 method versus that of a Scan-UT method were conducted and compared. A proposal to make the NACE TM0284 testing method more reliable by using Scan-UT method will be presented.

Hydrogen induced cracking (HIC) occurs by the poisoning effect of hydrogen sulfide (H2S) which promotes hydrogen absorption and entry at steel surface. Therefore, it is important for linepipe steels to have sufficient HIC resistance in sour environments. The HIC resistance is usually evaluated by measuring cracks after the standardized immersion test such as NACE TM0284. However, the general evaluation method cannot investigate HIC initiation and propagation behavior separately. It is necessary to understand the effect of metallurgical factors on the cracking behavior of sour service linepipe. In this study, in-situ ultrasonic inspection equipment was applied to the HIC test for the several linepipe steels with bainitic microstructure in order to clarify crack initiation and propagation behavior quantitatively. The three dimensional (3-D) distribution of cracks in the specimen was successfully captured as time sequence, and the temporal change of the crack area ratio (CAR) was investigated. It was revealed that the CAR-time curves are consist of four stages with different CAR increment rate. The first stage is the incubation of crack initiation. In the second stage, cracks occur and grow, and adjacent cracks coalesced rapidly. Regarding the first and second stages, sensitivity for the HIC initiation was well correlated with the hydrogen diffusion coefficient and the density of crack initiation site, such as MnS and Nb inclusions. In the third stage, the coalesced cracks propagate along the center segregation region. From the investigation of individual crack behavior, the crack along harder region showed higher propagation rate. In the fourth stage, the crack propagation rate was decreased to be in stasis. It can be stated that crack growth in the final stage is strongly affected by the hardness of base material and the crack easily propagate when HIC occurs in high strength steels.