Effect of hydrogen on the fracture behavior of high strength steel during slow strain rate test

The effect of hydrogen on the fracture behavior of the quenched and tempered AISI 4135 steel at 1450 MPa has been investigated by means of slow strain rate tests on smooth and circumferentially-notched round-bar specimens. Hydrogen was introduced into specimens by electrochemical charging and its content was measured by thermal desorption spectrometry (TDS) analysis. Results showed that the steel had high hydrogen embrittlement susceptibility. For both smooth and notched specimens, the fracture mode was changed from microvoid coalescence (MVC) to brittle intergranular (IG) fracture after the introduction of a small amount of diffusible hydrogen. Fracture initiated in the vicinity of the notch root for notched specimens, while it started from around the center in smooth specimens. The fracture stress decreased with increasing diffusible hydrogen content, and the decreasing trend was more prominent for specimens with a higher stress concentration factor. Taking into account the stress-driven hydrogen diffusion and accumulation in the vicinity of the notch root, the local diffusible hydrogen concentration and local fracture stress in notched specimens have been calculated. According to numerical results, the relationship between the local fracture stress and local diffusible hydrogen concentration was independent of stress concentration factor, which could account for the effect of hydrogen on the fracture stress of the steel.     

The influence of aging treatments on sulfide stress corrosion cracking of PH 13-8 Mo steel welds

Slow displacement rate tensile tests were carried out to study sulfide stress corrosion cracking (SSCC) of PH 13-8 Mo stainless steel welds in a saturated H₂S solution. The welds aged in the temperature range of 482-593 °C were susceptible to SSCC; the fracture surfaces revealed mainly quasi-cleavage fractures after notched tensile tests. However, the SSCC susceptibility in terms of the percentage loss of the notched tensile strength (NTS) of the welds was dependent on the aging treatment. The SSCC resistance and the austenite content of the welds increased with the aging temperatures. The presence of greater amounts of austenite, mainly reverted austenite, in the W1100 specimen (the weld aged at 1100 °F or 593 °C) than that in other aged specimens could account for its lower hardness and better SSCC resistance. On the other hand, the AW (as-welded) specimen containing a small amount of retained austenite films in a soft matrix exhibited a slightly improved SSCC resistance than that in the W1100 specimen. The lower hardness of the AW specimen was owing to the absence of fine coherent precipitates, leading to a reduced local stress and an enlarged plastic zone located in front of the notch in the test. With lower hardness, the local stress would also be lower and less likely to exceed some critical stress for failure in the saturated H₂S solution. For the aged specimens, the hardness/strength level and the amount of reverted austenite were the important factors that affect SSCC susceptibility.     

The effect of residual stresses induced by prestraining on fatigue life of notched specimens

The effect of tensile prestraining-induced residual stress on the fatigue life of notched steel parts was investigated. The study was performed on AISI 4140 steel. Rotating bending fatigue tests were carried out on semicircular notched specimens with different notch radii in the as-quenched and tempered conditions. Metallography of the specimens was performed by means of light optical microscopy. The finite-element method was used to evaluate the residual stress distribution near the notch region. Fatigue tests revealed fatigue life improvement for notched specimens, which changes depending on the notch radii and applied stress. Scanning electron microscopy was used to examine the fracture surfaces of the specimens.     

Determination of the critical hydrogen concentration for delayed fracture of high strength steel by constant load test and numerical calculation

The critical hydrogen concentration for hydrogen induced delayed fracture of the AISI 4135 steel at 1320 and 1450 MPa has been determined by constant load tests in combination with numerical calculations, and thus the concept of a critical hydrogen concentration has been verified. The time to fracture was obtained for circumferentially notched round bar specimens under a constant load after electrochemically pre-charged with various hydrogen contents. A numerical model was then developed for calculating the accumulated hydrogen concentration in the vicinity of the notch root, taking into account the driving effect of the hydrostatic stress on hydrogen transport. The results showed that the delayed fracture of the steel occurred when a critical hydrogen concentration at the location of the stress peak was reached by accumulation, and that the time to fracture was related to the stress-driven hydrogen accumulation process. The critical hydrogen concentration was dependent not only on the strength level, but also on the stress concentration factor of the specimens.     

Investigation of the surface reactivity on a 304L tensile notched specimen using scanning electrochemical microscopy

Local electrochemical reactivity around a notched tensile sample of 304L stainless steel under applied stress was investigated using scanning electrochemical microscopy (SECM). The plastic strain field around the notch was evaluated by finite element model (FEM). Microscopic observations of local plasticity related to grain morphology were correlated with the effective plastic strain field calculated with FEM around the notch root. Numerical results, surface observations and experimental electrochemical investigations showed the significant effect of plastic strain gradient on the surface reactivity. The effect of roughness induced by the triaxial plastic strain field around the notch was investigated with surface topography measurements.     

Susceptibility of a X80 steel to hydrogen embrittlement

The susceptibility to hydrogen of a X80 grade steel produced by a thermo‐mechanical control process (TMCP) has been investigated by keeping straining notched specimens under continuous charging conditions. Hydrogen charging was carried out either in synthetic seawater under potentiostatic control at ? 1000 mV vs. SCE or in sulphuric acid with an absorption promoter under galvanostatic control at ? 5 mA/cm². Results reported in terms of hydrogen effect on the ductility of the steel as a function of both cross head speed and root radius of the notch indicate that under the combined effect of cathodic charging, notch severity and very low strain rates the ductility of the TMCP X80 steel can be greatly affected by the presence of hydrogen. With notched specimens strained in air increasing loss of ductility in terms of reduction in area is observed as the notch severity increases. Notched specimens are fairly “more brittle” than smooth ones. As notched specimens are strained under cathodic charging at ? 1000 mV vs. SCE in the synthetic seawater, considerable decrease of reduction in area (RA) is observed. The same trend is observed for displacement and load at fracture both being connected with ductility even if a definite tendency is not always obtained. As the notched specimens are strained under cathodic charging in seawater the fracture morphology shows regions of mixed ductile and brittle fracture and zones where intergranular and/or transgranular fracture path are prevailing. Area of intergranular and transgranular fracture path, that can be more strictly associated with the presence of hydrogen, tends to increase as the strain rate decreases, which suggests a fracture behaviour influenced by hydrogen diffusion. Several mechanisms were involved in the rupture process in sulphuric acid depending on the notch geometry and, especially, on the cross head speed. Apparently, transgranular (quasi‐cleavage) rupture tends to prevail as the displacement becomes lower and lower. No evidence of intergranular fracture was observed.     

Hydrogen embrittlement susceptibility and permeability of two ultra-high strength steels

Slow displacement rate tensile tests were carried out in a saturated H₂S solution to investigate the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of two ultra-high strength steels (PH 13-8 Mo stainless steel and T-200 maraging steel). Hydrogen permeation properties were determined by an electrochemical permeation method. The results of permeation tests indicated that over-aged specimens showed a lower diffusivity/hydrogen flux and higher solubility than those solution-annealed. The great increase in reverted austenite (irreversible hydrogen traps) together with numerous precipitates at the expense of dislocations (reversible) in the over-aged specimen led to such a change in permeability. Ordinary tensile tests indicated that four tested specimens had roughly the same yield strength level. Hence, the hydrogen embrittlement susceptibility of the material could be related to their permeation properties. The uniform distribution of strong hydrogen traps in over-aged specimens instead of weak traps in the solution-annealed impeded the hydrogen transport toward the strained region, thus, the resistance to sulfide stress corrosion cracking was improved in over-aged specimens.     

A comparison of hydrogen embrittlement susceptibility of two austenitic stainless steel welds

Slow displacement rate tensile tests were carried out to assess the effect of hydrogen embrittlement on notched tensile strength (NTS) and fracture characteristics of AISI 316L and 254 SMO stainless steel (SS) plates and welds. 254 SMO generally exhibited a better resistance to hydrogen embrittlement than 316L. The strain-induced transformation of austenite to martensite in the 316L SS was responsible for the high hydrogen embrittlement susceptibility of the alloy and weld. Sensitized 254 SMO (i.e., heat-treated at 1000 °C/40 min) base plate and weld comprised of dense precipitates along grain boundaries. Interfacial separation along solidified boundaries was observed with the tensile fracture of 254 SMO weld, especially the sensitized one. Dense grain boundary precipitates not only reduced the ductility but also raised the susceptibility to sulfide stress corrosion cracking of the sensitized 254 SMO plate and weld.     

Korrosionswechselfestigkeit von Kupfer und Kupferlegierungen,insbesondere von Mehrstoff-Aluminiumbronzen

Corrosion fatigue strength of copper and copper alloys, especially of multi-component aluminium bronzes The bending fatigue strength of multicomponent copper-aluminum alloys subjected to different beat treatments has been investigated by means of smooth and notched specimens exposed to air, distilled water or synthetic sea water. The corrosion fatigue strength is lowest in synthetic sea water. The influence of the structure is but small. Distilled water has less effect on the corrosion fatigue strenght. In the case of the tests in air, a correlation between fatigue strenght and strucure was found. With bending fatigue strenght tests in air, notched specimens mit certain from quotients yield, in some cases, higher values than smooth specimens. This abnormal influence of the notch disappear with additional corrosion.     

Role of Bayer solution concentration and temperature in stress corrosion cracking susceptibility of steel

To improve the efficiency of the Bayer process for the extraction of alumina from Bauxite ore, there is a push for increasing processing temperature and caustic concentrations, which has also led to an increased concern for caustic embrittlement. In this study, the caustic cracking behaviour of steel in Bayer solutions of 2.5, 5, 7.5 and 10 mol dm⁻³ “free caustic” concentrations have been studied at different temperatures using pre-cracked circumferential notch tensile specimens. It has been observed that at 100 °C, steel is susceptible to caustic cracking in each of the four Bayer solutions. Caustic cracking has also been observed at temperatures as low as 55 °C. Tests were also conducted using only the notched specimens (i.e., without pre-cracking) in a 7.5 mol dm⁻³ “free caustic” Bayer solution at 120 °C to study the stress corrosion crack formation and propagation behaviour in blunt notches.     

Determination of threshold stress intensity factor for stress corrosion cracking (KISCC) of steel heat affected zone

Threshold stress intensity factor for stress corrosion cracking of heat affected zone (HAZ) of mild steel in caustic solution has been determined using circumferential notch tensile (CNT) technique. HAZ microstructure produced during manual metal arc welding of grade 250 steel was simulated over a length of 35 mm of CNT specimens, using a thermo-mechanical simulator. Inter-granular stress corrosion cracking has been confirmed with scanning electron microscope. The results presented here validate the ability of CNT technique for determination of K_ISCC of HAZ and base metal.     

Effect of hydrogen environment on the notched tensile properties of T-250 maraging steel annealed by laser treatment

In the present work, slow displacement rate tensile tests were performed to find out the influence of ageing condition and hydrogen-charging on the notched tensile strength and fracture characteristics of T-250 maraging steel aged at various conditions. The influence of embrittling species in the environment on the notched tensile strength was accessed by comparing the measured properties in air, gaseous hydrogen and H₂S-saturated solution. The hydrogen diffusivity, permeation flux and apparent solubility of various specimens determined by electrochemical permeation method, were correlated well with the microstructures and mechanical property. The results indicated that the peak-aged (H900) specimen was highly sensitive to hydrogen embrittlement even in gaseous hydrogen. In contrast, the microstructures of over-aged (H1100) specimen comprising of reverted austenite and incoherent precipitates could trap large amount of hydrogen atoms, resulting in decreased hydrogen permeability and hydrogen embrittlement susceptibility. The solution-annealed specimen had the highest diffusion coefficient and the lowest quantity of trapped hydrogen among the specimens, showing high susceptibility to sulfide stress corrosion cracking. In the presence of notches, hydrogen atoms were prone to segregate and trap at grain boundaries, resulting in the formation of intergranular fracture.     

A fatigue-life prediction methodology for notched aluminum-magnesium alloy in gulf seawater environment

Local-strain and linear-elastic fracture-mechanics (LEFM) methodologies have been investigated for prediction of the corrosion-fatigue life of notched components of specially developed Al-2.5Mg alloys exposed to Arabian Gulf seawater environment. Corrosion-fatigue crack initiation life estimates were obtained using strain-life relationships; corrosion-fatigue crack propagation life estimates were obtained using LEFM relationships. The total corrosion-fatigue life was considered to be the sum of the crack initiation and crack propagation lives. Estimated corrosion-fatigue lives were compared with experimentally obtained corrosion-fatigue life data using center-notched specimens of three types of Al-2.5Mg alloys (containing different amounts of chromium) exposed to Arabian Gulf seawater environment. Two notch geometries, a circular notch (K _t= 2.42) and an elliptical notch (K _t= 4.2), were investigated. Good corrosion-fatigue life predictions can be obtained using local-strain and LEFM methodologies by determining the relevant material constants via a few simple fatigue tests on smooth specimens and a few crack-growth-rate tests in the environment at the frequency of interest.     

Notched tensile tests of cold-rolled 304L stainless steel in 40 wt.% 80 °C MgCl2 solution

The effects of cold-rolling (20% thickness reduction) and sensitization treatment (600 °C/10 h) on the microstructure, tensile properties and susceptibility to stress corrosion cracking of 304 stainless steel in 80 °C MgCl₂ (40 wt.%) solution were investigated. The increase in hydrogen traps, which retarded hydrogen diffusion to the strained region, accounted for the low loss in notched tensile strength (NTS) of such a cold-rolled specimen, as compared to the solution-treated specimen in the corrosive environment. By contrast, the high NTS loss of sensitized specimens in MgCl₂ solution was attributed mainly to the formation of stress-induced martensite near grain boundary regions.     

Strengthening of NiAl nanofilms by introducing internal stresses

Molecular dynamics simulations of notched NiAl nanofilms tension are carried out. The stress–strain curves are calculated for a dislocation-free nanofilm and for nanofilms with periodic arrays of prismatic dislocations introduced in order to create tensile internal stress in the inner part of the nanofilms and compressive stress near their surfaces. It is demonstrated that under uniaxial tensile load the nanofilms with the dislocation loops can show higher strength, strain to failure, and energy to failure compared to the dislocation-free nanofilm. Larger strength of the nanofilms with dislocations is naturally explained by the compressive internal stress at the surfaces which detains crack initiation at the notch under tensile loading. Increase in the strain and energy to failure is due to the particular mechanism of elastic strain observed for the nanofilm. There exists a domain of strain where homogeneous deformation of the nanofilm is thermodynamically unstable. As a result, domains with larger elastic strain appear and elastic deformation of the nanofilm occurs at practically constant stress by growth of the domains with larger elastic strain in expense of the domains with smaller elastic strain. We believe that this mechanism of non-homogeneous elastic deformation is due to competing interaction of atoms of different sorts and thus, it cannot be realized in pure metals but can happen in ordered alloys and intermetallic compounds. Our results demonstrate that strengthening by introducing internal stresses, widely used for macroscopic structures, can also be applied for nanomaterials such as nanofilms and nanowires.     

Determination of threshold stress intensity for chloride stress corrosion cracking of solution-annealed and sensitized austenitic stainless steel by circumferential notch tensile technique

This paper discusses a new approach to determination of threshold stress intensity factor for stress corrosion cracking (K_ISCC) of stainless steel in 42% MgCl₂ environment at 154 °C. K_ISCC of solution-annealed and sensitized AISI 304 stainless steel in chloride environment has been determined using circumferential notch tensile (CNT) technique. K_ISCC data generated using CNT technique have been compared with those generated using traditional techniques such as compact tension and double cantilever beam (reported in the literature). The results presented here validate the ability of CNT technique for determination of K_ISCC of sensitized as well as solution-annealed austenitic stainless steel. This paper also discusses the mechanistic aspects of the difference in fractographic features of the sensitized and solution-annealed stainless steel.     

Effect of prior corrosion on short crack behavior in 2024-T3 aluminum alloy

Two thicknesses of dogbone shaped 2024-T3 aluminum alloy specimens were notched and corroded prior to constant amplitude fatigue loading. The purpose of the subject research was to examine and characterize the effects of various levels of prior corrosion on the growth rate of short fatigue cracks. The specimens were notched and exposed to a corrosive environment per one of three defined protocols prior to experimentation. The notch was manually introduced at one edge of the test section of the specimen, which was later corroded to create a more natural site for crack origination. Fatigue crack nucleation was monitored and subsequent crack growth recorded, with results presented in the form of da/dN vs. ΔK curves.     

Mechanical behaviour of SSRT specimens optimized for IGSCC concerns

Simulating testing conditions leading to evaluate the intergranular stress corrosion cracking sensitivity of structural alloys is crucial to estimate the lifetime of in-service components. Former studies have pointed out that a simple modification of the design of slow strain rate tensile specimens was particularly convenient for evaluating the susceptibility to intergranular stress corrosion cracking of nickel-base alloys. The aim of the present work is to characterize and model the mechanical behaviour of such specimens. Validation of proposed modelling relies mainly on tensile tests carried on specimens equipped with strain gages. One of the striking results is that, for a given displacement rate of the heads of the specimen, a much slower strain rate can be obtained locally in comparison with the strain rate of an equivalent smooth specimen.     

Fracture of high volume fraction ceramic particle reinforced aluminium under multiaxial stress

Circumferentially notched cylindrical bars of high volume fraction Al₂O₃ particle reinforced aluminium are tested in tension to probe the role of tensile stress triaxiality on damage and failure of such materials. The transverse strain is monitored with a specially designed video extensometer. A significant dependence of the peak average stress and failure strain on notch radius is observed. Finite-element simulations of the tests are conducted on the basis of a micromechanical model derived from earlier studies of damage and failure of these composites under uniaxial tensile deformation (Journal of the Mechanics and Physics of Solids 2009;57:1781). The simulations show that stress and strain distributions within the notched composite samples deviate significantly from predictions of Bridgman’s simplified analysis. Comparison with data shows that, whereas calculations capture satisfactorily the evolution of the average composite flow stress as a function of notch radius at small strains, the notched samples damage faster and fail at strains lower than predicted. Two phenomena may explain the discrepancy, namely (i) damage coalescence beyond a threshold level, and (ii) the incapacity of the matrix to sustain large hydrostatic stresses, which results from the presence of internal surfaces (cracked particles and possibly matrix voiding).     

近全层组织γ-TiAl基合金的室温拉伸断裂机理

通过对直缺口近全层组织的扫描电镜原位拉伸实验以及相应的断裂表面观察，结合有限元计算了TiAl基合金近全层组织拉伸的断裂机理。研究表明：许多裂纹在塑性变形前沿着层间起裂和扩展，断裂过程的驱动力是拉应力。在直缺口试样中，许多裂纹直接起裂于缺口根部，并且沿着层间扩展。随着拉应力的增加，主裂纹和新裂纹也可以通过障碍晶粒的穿层解理断裂来连接。通过有限元计算得沿层断裂强度大约为50MPa，穿层断裂强度大约为120MPa。