Disposition of fracture zones under hydrogen embrittlement

Disposition of the fracture zones during hydrogen embrittlement is discussed by using experimental data on crack growth kinetics by stress corrosion cracking and internal hydrogen embrittlement of the titanium alloy Ti-7 AL-2 Zr-1,5 Mo-1 V. The situation of these zones ahead of the crack tip depends on the kind of embrittlement and on K_I and can change within wide limits.     

Elucidating the variables affecting accelerated fatigue crack growth of steels in hydrogen gas with low oxygen concentrations

The objective of this study was to quantify the effects of mechanical and environmental variables on oxygen-modified accelerated fatigue crack growth of steels in hydrogen gas. Experimental results show that in hydrogen gas containing up to 1000 v.p.p.m. oxygen fatigue crack growth rates for X52 line pipe steel are initially coincident with those measured in air or inert gas, but these rates abruptly accelerate above a critical ΔK level that depends on the oxygen concentration. In addition to the bulk gas oxygen concentration, the onset of hydrogen-accelerated crack growth is affected by the load cycle frequency and load ratio R. Hydrogen-accelerated fatigue crack growth is actuated when threshold levels of both the inert environment crack growth rate and K_max are exceeded. The inert environment crack growth rate dictates the creation of new crack tip surface area, which in turn determines the extent of crack tip oxygen coverage and associated hydrogen uptake, while K_max governs the activation of hydrogen-assisted fracture modes through its relationship to the crack tip stress field. The relationship between the inert environment crack growth rate and crack tip hydrogen uptake is established through the development of an analytical model, which is formulated based on the assumption that oxygen coverage can be quantified from the balance between the rates of new crack tip surface creation and diffusion-limited oxygen transport through the crack channel to this surface. Provided K_max exceeds the threshold value for stress-driven hydrogen embrittlement activation, this model shows that stimulation of hydrogen-accelerated crack growth depends on the interplay between the inert environment crack growth increment per cycle, load cycle frequency, R ratio and bulk gas oxygen concentration.     

The effect of grain size and aging on hydrogen embrittlement of a maraging steel

Notched tensile tests were conducted under a slow displacement rate to evaluate the influences of grain size and aging on hydrogen embrittlement (HE) of T-200 maraging steel. In addition, an electrochemical permeation method was employed to measure the effective diffusivity (D_eff) and apparent solubility (C_app) for hydrogen of various heat-treated specimens. The results indicated that the aged (482 °C/4 h) specimens comprised of numerous precipitates led to a raised C_app and a decreased hydrogen diffusivity as compared to those of the solution-treated ones. The solution-treated specimens were resistant to gaseous HE, whereas aged specimens were susceptible to it, implying the strength level was the controlling factor to affect the HE susceptibility of the specimens. Nevertheless, all specimens suffered from sulfide stress corrosion cracking (SSCC) severely but to different degrees. The aged specimens were more likely to form intergranular (IG) fractures in H₂S but quasi-cleavage (QC) in H₂. For the solution-treated specimens, a fine-grained structure was susceptible to HE in H₂S and revealed mainly QC that differed from the IG fracture of the coarse-grained one. The fracture mode of the specimens could also be related to the transport path and / or the supply of hydrogen to the plastic zone of notched specimens in hydrogen-containing environments.     

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 study of competitive anion adsorption at the tip of a crack in stress corrosion cracking of AI 27-1 alloy

A method to determine chloride ion concentration adsorbed at the tip of crack in stress corrosion cracking of AI27-1 aluminium alloy in chromic acid based solutions is discussed. The dependance of hydrogen permeability of passivating film at the tip of crack on the concentration of adsorbed chloride ions is the backbone of the method. Critical stress intensity factor, K_HE, and critical crack growth rate, V_cr data form an experimental base for the method and are a necessary prerequisite of hydrogen embrittlement as the main mechanism of cracking in stress corrosion conditions.     

A quantified concept of the hydrogen permeability of passivating films at a crack tip during stress corrosion cracking of structural materials

A general theoretical and methodological approach to local dissolution and hydrogen embrittlement contributions to stress corrosion cracking in various structural materials is discussed. According to this approach a quantified determination of the hydrogen embrittlement contribution to stress corrosion cracking in high-strength steels, titanium, aluminium, and zirconium, and zirconium alloys appears to be possible. On the basis of a few postulates, a quantitative concept of hydrogen permeability of the passivating film at the tip of a crack in the metal is developed; this concept allows for the first time a quantified determination of the relationship between critical hydrogen concentration and stress intensity factor in steels, titanium, and aluminium, and aluminium alloys under hydrogen embrittlement. Two new methods for studying adsorption processes at the tip of a growing crack during stress corrosion cracking are an additional outflow of this concept.     

Effect of microstructure on the hydrogen trapping efficiency and hydrogen induced cracking of linepipe steel

The hydrogen trapping efficiency in different microstructures is compared, and the critical hydrogen flux for hydrogen induced cracking (HIC) is determined for API X65 grade linepipe steel. By controlling the start cooling temperature (SCT) and the finish cooling temperature (FCT) in thermomechanically controlled process (TMCP), three different kinds of microstructure such as ferrite/degenerated pearlite (F/DP), ferrite/acicular ferrite (F/AF), and ferrite/bainite (F/B) are obtained. A modified ISO17081(2004) standard method is used to evaluate the hydrogen trapping by measuring the permeability (J_ssL) and the apparent diffusivity (D_app). Microstructures affecting both hydrogen trapping and hydrogen diffusion are found to be DP, AF, BF and martensite/austenite (M/A) constituents. The hydrogen trapping efficiency is increased in the order of DP, BF and AF, with AF being the most efficient. HIC is initiated at the local M/A concentrated region when the steel has such microstructures as F/AF or F/B. Although the trapping efficiency of bainite is lower than that of AF, bainite is more sensitive microstructure to HIC than to AF.     

The stress corrosion cracking behavior of austenitic stainless steels in boiling magnesium chloride solutions

The change in the mechanism of stress corrosion cracking with test temperature for Type 304, 310 and 316 austenitic stainless steels was investigated in boiling saturated magnesium chloride solutions using a constant load method. Three parameters (time to failure; t_f, steady-state elongation rate; l_ss and transition time at which a linear increase in elongation starts to deviate; t_ss) obtained from the corrosion elongation curve showed clearly three regions; stress-dominated, stress corrosion cracking-dominated and corrosion-dominated regions. In the stress corrosion cracking-dominated region the fracture mode of type 304 and 316 steels was transgranular at higher temperatures of 416 and 428 K, respectively, but was intergranular at a lower temperature of 408 K. Type 310 steel showed no intergranular fracture but only transgranular fracture. The relationship between log l_ss and log t_f for three steels became good straight lines irrespective of applied stress. The slope depended upon fracture mode; −2 for transgranular mode and −1 for intergranular mode. On the basis of the results obtained, it was estimated that intergranular cracking was resulted from hydrogen embrittlement due to strain-induced formation of martensite along the grain boundaries, while transgranular cracking took place by propagating cracks nucleated at slip steps by dissolution.     

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.     

Coarsening of γ′ in Ni–Al alloys aged under uniaxial compression: II. Diffusion under stress and retardation of coarsening kinetics

An elasticity-based model is developed for predicting how the coefficient of diffusion, D, changes under an applied compressive stress. Stress affects the activation energy for motion, Q_M, of an atom, as well as its jump distance, ℓ. Since a cubic crystal is slightly distorted tetragonally under uniaxial stress, Q_M and ℓ, hence D, both become different in directions normal and parallel to the stress axis. Q_M is calculated using a dynamical model of diffusion due to Flynn [Point defects and diffusion. London: Oxford University Press; 1972]. Using the elastic constants for Ni–Al alloys, and taking into account the effect of the hydrostatic component of the applied stress on D, the model predicts that D decreases by about 6% under an applied stress of 150 MPa. This reduction does not account entirely for the retardation of coarsening kinetics observed experimentally, but given the simplicity of the model the result is satisfactory.     

Role of chlorides on pitting and hydrogen embrittlement of Mg–Mn wrought alloy

The role of chlorides on stress corrosion cracking behavior of Mg–Mn hot rolled alloy was studied in Mg(OH)₂ saturated, 0.01 M and 0.1 M NaCl solutions. The alloy was found to fail by hydrogen embrittlement mechanism both in presence and absence of chlorides. However, the role of chloride has been found to be to damage the passive film, cause pitting and increasing hydrogen embrittlement tendency of the alloy. Crack initiation occurred through pitting and grew in a transgranular manner involving hydrogen.     

Corrosion resistance of tantalum base alloys. Elimination of hydrogen embrittlement in tantalum by substitutional alloying

The corrosion behaviour of substitutional Ta–Mo, Ta–W, Ta–Nb, Ta–Hf, Ta–Zr, Ta–Re, Ta–Ni, Ta–V, Ta–W–Mo, Ta–W–Nb, Ta–W–Hf and Ta–W–Re alloys has been investigated in various corrosive media, i.e. (1) concentrated sulfuric acid at 250°C and 200°C, (2) boiling hydrochloric acid of azeotropic composition, (3) concentrated hydrochloric acid at 150°C under pressure, (4) HF-containing solutions and (5)0.5% H₂SO₄ at room temperature (anodisation). In highly corrosive media such as concentrated H₂SO₄ at 250°C and concentrated HCl at 150°C tantalum is hydrogen embrittled, probably by stress induced precipitation of β-hydride. Both corrosion rate and hydrogen embrittlement in concentrated H₂SO₄ at 250°C are strongly influenced by alloying elements. Small alloying additions of either Mo or Re decrease the corrosion rate and the hydrogen embrittlement, while Hf has the opposite effect. Hydrogen embrittlement in concentrated H₂SO₄ at 250°C is completely eliminated by alloying Ta with 1 to 3 at % Mo (0.5 to 1.5 wt % Mo). These results can be explained in terms of the oxygen deficiency of the Ta₂O₅ film and the electronic structure of these alloys.     

Stress corrosion cracking and hydrogen embrittlement of sensitized austenitic stainless steels in boiling saturated magnesium chloride solutions

Stress corrosion cracking (SCC) and hydrogen embrittlement (HE) of the sensitized stainless steels of type 304, 310 and 316 were investigated as a function of test temperature in boiling saturated magnesium chloride (MgCl₂) solutions under a constant applied stress condition. The test temperature dependence of fracture appearance and three parameters of time to failure (t_f), steady-state elongation rate (l_ss) and transition time to time to failure ratio (t_ss/t_f) suggests that type 304 suffered SCC and HE, while type 316 suffered only HE and type 310 SCC. It was also found that the test temperature dependence of three parameters for the sensitized type 304 and 310 was almost similar to that of the solution annealed stainless steels, whereas that of type 316 showed a clear difference between sensitized and solution annealed specimens. The relationships between the logarithms of the time to failure and the steady-state elongation rate became a straight line for all stainless steels. However, its slope depended upon the fracture mode; −2.0 for SCC and −1.5 for HE. This showed that the steady-state elongation rate was the parameter for predicting the time to failure for the stainless steels in the MgCl₂ solutions. The results obtained were explained in terms of martensite transformation, hydrogen entry site, sensitization, and so on.     

Influence of the applied stress rate on the stress corrosion cracking of 4340 and 3.5NiCrMoV steels under conditions of cathodic hydrogen charging

Stress corrosion cracking (SCC) of as-quenched 4340 and 3.5NiCrMoV steels was studied under hydrogen charging conditions, with a cathodic current applied to the gauge length of specimens subjected to Linearly Increasing Stress Test (LIST) in 0.5 M H₂SO₄ solution containing 2 g/l arsenic trioxide (As₂O₃) at 30 °C. Applied stress rates were varied from 20.8 to 6 × 10⁻⁴ MPa s⁻¹. Both the fracture and threshold stress decreased with decreasing applied stress rate and were substantially lower than corresponding values measured in distilled water at 30 °C at the open circuit potential. The threshold stress values correspond to 0.03–0.08 σ_y for 4340 and 0.03–0.2 σ_y for the 3.5NiCrMoV steel. SCC velocities, at the same applied stress rate, were an order of magnitude greater than those in distilled water. However, the plots of the crack velocity versus applied stress rate had similar slopes, suggesting the same rate-limiting step. The fracture surface morphology was mostly intergranular, with quasi-cleavage features.     

Fracture behavior at crack tip — a new framework for cleavage mechanism of steel

The fracture behavior at crack tip was analyzed based on: (1) observations of fracture surfaces and measurements of local critical parameters for cleavage of three point bending (3PB) precracked specimens of C-Mn steel, (2) detailed observations of configuration changes at precrack tips by metallographic cross sections in specimens unloaded at various applied loads, (3) sophisticated FEM calculations of distributions of stress, strain and triaxiality and simulations of short cracks initiated and extended at precrack tips. The results show that before a critical load (a critical COD) is reached, the crack tip is only blunted and in its vicinity three criteria for cleavage fracture (ϵ_p≥ϵ_pc for initiating a crack nucleus; σ_m/σ_e≥T_c for preventing the crack tip from blunting; and σ_yy≥σ_f for propagating the crack) are satisfied in different regions separated from each other, the nucleated cracks cannot be propagated and cleavage fracture cannot be triggered. As the applied load increases higher than this critical load, a short crack is initiated and extended at the precrack tip and then is blunted again. The plastic strain and the stress in front of the precrack are redistributed. While the plastic strain remains in front of the tip, the stress triaxiality is rebuilt. At a second critical load, the regions where the three criteria are satisfied overlap each other and a cleavage crack can be nucleated and propagated. The minimum distance for cleavage may be determined by the beginning of the overlapping of the mentioned regions. Combined with the three criteria previously suggested, the fracture behavior at crack tip and the corresponding changes of driving forces (ϵ_p, σ_m/σ_e,σ_yy) provide a complete physical model for cleavage of steels in the local scale.     

Stress corrosion cracking and hydrogen-induced cracking of amorphous Fe74.5Ni10Si3.5B9C2

Stress corrosion cracking (SCC) in NaCl solution and hydrogen-induced cracking (HIC) during dynamic charging of Fe_74.5Ni₁₀Si_3.5B₉C₂ amorphous alloy were investigated through sustained load tests. The normalized threshold stress of SCC was σ_SCC/σ_F=0.04, where σ_F is fracture strength in air. Anodic polarization and addition of As₂O₃ into the solution did not change σ_SCC/σ_F, but cathodic polarization increased σ_SCC/σ_F from 0.04 to 0.31. Cathodic polarization increased but anodic polarization decreased the time to failure during SCC at the constant load of σ=0.27σ_F.The normalized threshold stress of HIC, σ_HIC/σ_F, was linearly decreased with the increase in logarithm of hydrogen concentration (C₀) and kept a minimum constant when C₀ was larger than a critical value, i.e., σ_HIC/σ_F=1.58−0.36lnC₀ (C₀?74.4 wppm) and 0.1 (C₀?74.4 wppm). The threshold stress of HIC during dynamic charging with the maximum current was larger than that of SCC at open-circuit potential. Fracture surfaces of HIC were also different with that of SCC. Experiments indicated that SCC of the amorphous alloy in the NaCl solution is controlled by anodic dissolution process instead of hydrogen.     

Influence of plasma nitriding on hydrogen environment embrittlement of 1.4301 austenitic stainless steel

The aim of this work was to study if hydrogen environment embrittlement of DIN 1.4301 austenitic stainless steel can be suppressed by a nitrided surface. DIN 1.4301 was plasma nitrided in a N₂/H₂ discharge. Nitriding produced 3-layered structure consisting of a γ_N top layer, an intermediate γ/γ_C-layer and a diffusion layer. It is assumed that the γ_C phase was formed due to the decomposition of CO originating from the reactor walls and the subsequent incorporation of C into the material. The γ_C phase is characterized by distinct XRD peaks and carbon contents between 0.5 and 4 wt.% as well as nitrogen contents between 0.5 and 8 wt.%. Plastic deformation of the plasma nitrided specimen showed cracks and some delamination of the γ_N layer, whereas the γ/γ_C-layer behaved in a very ductile manner. Even at a plastic deformation of 35% no cracks or any other damage was visible. A tensile test in gaseous hydrogen showed severe embrittlement of the unnitrided steel and the nitrided steel with a γ_N layer. No cracks were observed in areas where just the γ/γ_C-layer was present.     

Hydrogen permeable Ta–Ti–Ni duplex phase alloys with high resistance to hydrogen embrittlement

Crystal structures, microstructures and hydrogen permeability Φ of as-cast Ta–TiNi alloys on the line connecting the compositions of the primary (Ta, Ti) and the ternary eutectic phases have been investigated to find out highly hydrogen permeable duplex phases alloys with high resistance to the hydrogen embrittlement. The alloys on this line show microstructures of (1) the eutectic {(Ta, Ti) + TiNi} phase, (2) the primary (Ta, Ti) phase + the eutectic {(Ta, Ti) + TiNi} phase, and (3) the (Ta, Ti) solid solution, although a little amount of unidentified (impurity) phases are included in these samples. The value of Φ increases with increasing Ta content and the volume fraction of the primary (Ta, Ti) phase, which indicates that the primary phase contributes mainly to the hydrogen permeation. The Ta₅₆Ti₂₃Ni₂₁ alloy, containing the 61 vol.% primary phase, shows the highest Φ of 2.18 × 10⁻⁸ mol H₂ m⁻¹ s⁻¹ Pa^−0.5 at 673 K, which is 1.3 times higher than that of the previous most high Φ alloy (Ta₅₃Ti₂₈Ni₁₉). The more Ta-rich alloys on this line, i.e., containing a small amount of the eutectic phase, are broken down by the hydrogen embrittlement, suggesting that the eutectic phase suppresses the hydrogen embrittlement.     

Measurement and prediction of hydrogen embrittlement in high strength carbon steel

Abstract

Hydrogen embrittlement tests were performed on 0·254 mm diameter BS 5216 M4 high strength carbon steel wire using constant loads to give initial tensile stresses in the range 48–91% ultimate tensile stress. The wires were electrolytically charged with hydrogen in 4%H₂SO₄ at current densities of 75 and 150 mA cm^–2. The failure times at each applied stress and charging rate were displayed on Weibull statistical plots and shown to correlate with a diffusion model of hydrogen transport. At high stresses, crack initiation occurred rapidly and the failure time was controlled by the rate of inward hydrogen diffusion to maintain a threshold concentration for crack propagation. At low applied stresses, crack initiation required a higher hydrogen concentration and occurred more slowly. In this case, the failure time was controlled by the size and location of the significant microstructural flaw at which crack initiation occurred. The model enabled failure times to be predicted in specimens with differing dimensions.     

Measurement of steady-state hydrogen electrode reactions on Alloys 600 and 690 tubes

The steady-state polarization measurements for HER and HOR were carried out in order to obtain the effects of hydrogen pressure, solution pH, and temperature on the current densities of Alloys 600 and 690, respectively. Optimization was performed to obtain the electrokinetic parameters of HER and HOR on Alloys 600 and 690 such as forward and reverse transfer coefficients and equilibrium corrosion densities. From the optimization process, the activation energies, E_ac, of both hydrogen reactions on the surfaces of the Alloys 600 and 690 tubes were obtained as 30.5 kJ/mole on the surface of Alloy 600 tube and 35.6 kJ/mole on Alloy 690 tube. Furthermore, the equilibrium exchange current densities of hydrogen electrode reaction, i₀(H₂), on the surface of the Alloys 600 and 690, respectively, were proposed as functions of hydrogen pressure, solution pH, and temperatures.