Hydrogen trapping in work-hardened alloys

The ingress of hydrogen in two work-hardened nickel-base alloys (Inconel 625 and Hastelloy C-276) exposed to an acetate electrolyte (1 mol L⁻¹HAc/1 mol L⁻¹ NaAc where Ac = acetate) was studied using a potentiostatic pulse technique. The data were shown to fit a diffusion/trapping model under interface control, and values were determined for the irreversible trapping constants (k) and the flux of hydrogen into the alloys. The density of irreversible trap defects in Inconel 625 was calculated from k and found to be in excellent agreement with concentration of NbTi(C) particles. Hastelloy C-276 was characterized by two trapping constants; one is associated with quasi-irreversible traps that saturate, leaving only irreversible traps. The density of irreversible traps was shown to agree with the concentration of phosphorus segregated at grain boundaries. The irreversible trapping constants for these alloys are consistent with their relative susceptibilities to hydrogen embrittlement.     

The effect of aging on hydrogen trapping in ß-titanium alloys

The ingress of hydrogen in three ß-titanium alloys (Beta-C, Ti10V2Fe3Al, and Ti13V11Cr3Al) and an α-ß titanium alloy (Ti6Al4V) was investigated with a view to characterizing their interaction with hydrogen. A technique referred to as hydrogen ingress analysis by potentiostatic pulsing (HIAPP) was used to obtain anodic current transients for the unaged and aged ß-Ti alloys and as-received Ti-6-4 in an acetate buffer (1 mol L⁻¹ HAc/l mol L⁻¹ NaAc, where Ac = acetate). The transients were analyzed using a diffusion/trapping model under interface control conditions to evaluate the trapping constants and hydrogen entry flux in each case. A marked increase in irreversible trapping was observed for the ß-titanium alloys with aging and was attributed to precipitation of secondary α phase. Aging also induced changes in the passive film and hence the hydrogen entry flux. Ti-13-11-3 and Ti-10-2-3 are predicted to become less resistant to hydrogen embrittlement with aging as a result of increases in both the trapping constant (at least for Ti-13-11-3) and the flux. In contrast, the change in resistance of Beta-C Ti with aging is subject to the opposing effects of a reduced flux and an enhanced trapping capability, though the latter appears to have the primary effect, rendering aged Beta-C Ti less resistant to hydrogen embrittlement than the unaged alloy.     

A quantitative analysis of hydrogen trapping

The complex hydrogen trapping characteristics of iron-titanium-carbon alloys, contain-ing both reversible and irreversible traps have been fully analyzed. The key to this quantitative analysis is a complete identification of the type and number of each operating trap. The trapping parameters were obtained from an analysis of the relevant hydrogen permeation transients. Titanium substitutional atoms have been shown to be reversible, low occupancy traps with an interaction energy with hydrogen,E (Ti-H), of 0.27 eV. Typi-cal rate constants for these alloys are; a hydrogen capture rate constant of approximately 10^-24 cm³/atom .s a release rate constant of approximately 10^-3 s^-1 and a trapping rate of the order of 10¹⁵ atoms, H/cm³ .s. TiC particles are irreversible traps with a large oc-cupancy and an interaction energy, .E(TiC-H), of 0.98 eV. The irreversible trapping parameters are calculated from the first permeation transient, where mixed trapping oc-curs. The trapping kinetics are about an order of magnitude faster than when only rever-sible trapping exists. The role of trapping on the effective diffusivity of hydrogen is dis-cussed as is, briefly, its role in affecting hydrogen-induced damage. Finally, guidelines are given to permit the trapping behavior of more general alloys to be analyzed. Formerly at Carnegie-Mellon University, Pittsburgh.     

Hydrogen permeation and diffusion in inconel 718 and incoloy 903

Hydrogen permeation and diffusion have been measured in two high strength superalloys, Inconel 718 and Incoloy 903. Measurements were made over the temperature range of 150 to 500°C, with applied hydrogen pressures of 1 to 3 atm. The permeabilityp and dif-fusivityD values were used to derive the hydrogen solubility S in these materials. For Inconel 718 the results depend to some extent on the heat treatment of the alloy. For this alloy in the solution treated condition, the results for φ andD are: D.1.07xl0^-2exp(-11,900/RT) cm²/s For Incoloy 903, the results are: φ = 9.50x10^-3 exp (-13,710/RT) cm³(NTP)/cm-s-atm¹/ RT cm-s-atm1/2D = 2.46 x 10^-2(-12,590/RT) cm²/s where the activation energies are in cal/mole. The measured diffusivities and permeabilities are very similar for the two alloys.     

Hydrogen-enhanced cracking of superalloys

Internal hydrogen embrittlement was studied in two nickel-base superalloys (IN718 and IN625) and one iron-base superalloy (A286). Subcritical crack growth (SCG) measurements were made on uniformly precharged specimens containing up to 50 weight parts per million (wt ppm) hydrogen, and the behavior was correlated with metallographic observations. For intermediate hydrogen concentrations, three-stage SCG ratevs stress intensity behavior was observed in IN718 and IN625 but not in A286. For all alloys, the threshold stress intensity decreased with increasing hydrogen concentration. Cracking in the nickel alloys was transgranular, and there was a transition from dimpled to faceted failure as the hydrogen concentration increased. Failure in A286 was mainly by intergranular microvoid coalescence at high hydrogen concentrations. Enhanced localization of plasticity and void pressurization due to hydrogen are proposed to explain the observed hydrogen embrittlement of these alloys. The effects of hydrogen on the stacking fault energy, trapping sites, microstructure, and cracking ahead of the main crack front are discussed with reference to the above alloys and their hydrogen embrittlement. Formerly Research Assistant, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign     

A study of the hydrogen-induced degradation of two steels differing in sulfur content

Two steels with different sulfur contents: 0.003 and 0.024 wt pct, were cathodically charged under three different conditions and brought to fracture in tension immediately after charging or after aging at room temperature. All hydrogen charged specimens showed embrittlement, with a little higher loss of ductility in the high sulfur steel. The hydrogen embrittlement was reversible in both steels when specimens were charged in arsenic-free sulfuric acid solution at room temperature but was irreversible when charged in arsenic-containing acid at the same temperature. After charging in molten salts at 200 °C, some of the low sulfur steel specimens exhibited irreversible hydrogen damage with the appearance of quasicleavage fractures, while all high sulfur steel specimens were restored to the uncharged ductility by aging at room temperature. These results are interpreted by assuming that an increased sulfur content in steel increases the density of trapping sites for hydrogen at the sulfide/matrix interfaces. These traps are inactive above 150 °C and become operative after cooling. Therefore, at the same hydrogen content in steel after cooling, the greater content of sulfur results in a decreased activity of the lattice dissolved hydrogen, hence in reduced embrittlement.     

Quantitative analysis on hydrogen trapping of TiC particles in steel

The change in the hydrogen-trapping behavior of a TiC particle accompanying its coherent to incoherent interfacial-character transition in a 0.05C-0.20Ti-2.0Ni steel that was quenched and tempered in a partially protective argon atmosphere and in ultrahigh vacuum (UHV) has been studied by thermal desorption spectrometry (TDS). The results indicated that (semi)coherent TiC precipitates demonstrate distinctly different hydrogen-trapping features from that of incoherent TiC particles with respect to hydrogen capacity, interaction energy with hydrogen, locations available for hydrogen occupation, and the capability of hydrogen absorption from the environment. The broad (semi)coherent interface of the disc-shaped (semi)coherent TiC precipitate does not trap hydrogen during tempering in a partially protected argon atmosphere, but traps hydrogen during cathodic charging at room temperature. The semicoherent interface traps 1.3 atoms/nm² of hydrogen at the core of the misfit dislocation with short-time charging (1 hour), which is characterized by a desorption activation energy of 55.8 kJ/mol. The side interface of the (semi)coherent TiC precipitate acts like the broad interface when the precipitate is small. As the precipitate grows, the side interface gradually loses its coherency and results in a simultaneous increase in the trapping activation energy and the binding energy. An increase in the trapping activation energy, i.e., the energy barrier for trapping, makes hydrogen trapping more difficult in cathodic charging at room temperature, while an increase in the binding energy enhances the capability of hydrogen absorption from the atmosphere during heat treatment. An incoherent TiC particle is not able to trap hydrogen during cathodic charging at room temperature due to its high energy barrier for trapping, but absorbs hydrogen during heat treatment at high temperatures. The amount of hydrogen that is trapped by incoherent TiC particles depends on their volume, which strongly indicates that incoherent TiC particles trap hydrogen within them rather than at the particle/matrix interface. Octahedral carbon vacancies are supposedly the hydrogen trap sites in incoherent TiC particles.     

Creep crack growth behavior of several structural alloys

Creep crack growth behavior of several high temperature alloys, Inconel 600, Inconel 625, Inconel X-750, Hastelloy X, Nimonic PE-16, Incoloy 800, and Haynes 25 (HS-25) was examined at 540, 650, 760, and 870 °C. Crack growth rates were analyzed in terms of both linear elastic stress intensity factor and J*-integral parameter. Among the alloys Inconel 600 and Hastelloy X did not show any observable crack growth. Instead, they deformed at a rapid rate resulting in severe blunting of the crack tip. The other alloys, Inconel 625, Inconel X-750, Incoloy 800, HS-25, and PE-16 showed crack growth at one or two temperatures and deformed continuously at other temperatures. Crack growth rates of the above alloys in terms ofJ* parameter were compared with the growth rates of other alloys published in the literature. Alloys such as Inconel X-750, Alloy 718, and IN-100 show very high growth rates as a result of their sensitivity to an air environment. Based on detailed fracture surface analysis, it is proposed that creep crack growth occurs by the nucleation and growth of wedge-type cracks at triple point junctions due to grain boundary sliding or by the formation and growth of cavities at the boundaries. Crack growth in the above alloys occurs only in some critical range of strain rates or temperatures. Since the service conditions for these alloys usually fall within this critical range, knowledge and understanding of creep crack growth behavior of the structural alloys are important.     

Influence of the crystalline microstructure on the diffusivity of hydrogen in palladium galvanostatic permeation and X-ray diffraction measurements

The influence of crystalline microstructure upon the apparent diffusion coefficient of hydrogen in Pd samples are reported. The apparent diffusion coefficient was measured by the galvanostatic permeation method (at 20°C), while the structure was characterized by X-ray diffraction measurements. Pd membrane electrodes of different thicknesses, l = 5 × 10⁻³cm and l = 10⁻²cm, were used. The structure of Pd membrane electrodes was changed by successive annealing and sequences of absorption and desorption of hydrogen accompanied by the α⇌β phase transition. Both the irreversible and reversible traps affect the mobility of hydrogen in Pd. The irreversible traps manifest in the variation of the apparent diffusion coefficient as a function of the average stationary bulk concentration of hydrogen, while the diffusion coefficient of the free hydrogen does not depend on the actual concentration of hydrogen in Pd. This is true provided that the average stationary bulk concentration of hydrogen is higher than the irreversible trap concentration. The diffusion coefficient of free hydrogen depends on the specific internal surface area according to McNabb-Foster-like equation. The value of the diffusion coefficient of hydrogen in single crystal of palladium was estimated, D₀ = (4.2 ± 0.3) × 10⁻⁷ cm²s⁻¹ (20°C).     

A Comparative Study on the Hot Working Behavior of Inconel 718 and ALLVAC 718 Plus

Hot compression tests were performed on Inconel 718 and ALLVAC 718 PLUS (718+) at temperatures and strain rates in ranges of 1223 K to 1373 K (950 °C to 1100 °C) and 0.001–1 s⁻¹, respectively. Discontinuous yield behavior was observed in the flow curves of both alloys. For both alloys, the drop in stress at the yield point (yield drop) was maximized at 0.01 to 1 s⁻¹. The alloy 718+ showed larger yield drop than 718 over the studied deformation conditions. The different yield behaviors were attributed to the various chemical compositions. The peak strain for both alloys increased in temperature range of 1223 K to 1273 K (950 to 1000 °C) and strain rates of 0.01 to 1 s⁻¹. This uncommon behavior was ascribed to the change in the mechanism of microstructural evolution from continuous to discontinuous dynamic recrystallization (DRX). The kinetics of DRX was described by the Avrami equation and the exponent was determined at different deformation conditions. The Avrami exponent increased in the middle values of Zener–Hollomon (Z) parameters, i.e., 29.3 < lnZ < 32.9 for 718 and 31.4 < lnZ < 34.5 for 718+. The unusual variation of the Avrami exponent was attributed to the change in the mechanism of DRX.

     

Hydrogen embrittlement of amorphous alloys based on iron and nickel

Hydrogen embrittlement of amorphous alloys based on iron and nickel was examined in constant extension rate tests during cathodic polarization. Tests were carried out at a current density of 50 A·m⁻² in 0.1 M H₂SO₄ with and without addition of NaAsO₂ and NaCl. Hydrogen permeation and polarization curves were measured. Differences in the degree of hydrogen embrittlement were ascribed mainly to differences in the entry of hydrogen into the alloys. It was suggested that a low degree of embrittlement resulted largely from a hindrance of hydrogen entry. For alloys containing phosphorus, this might be associated with phosphorus-containing oxyanions and salts. For the alloy with higher silicon content it may be associated with the formation of oxides of silicon.     

Hydrogen transport in nickel base stainless alloys

Hydrogen transport parameters have been measured in two nickel base stainless alloys, HASTELLOY Alloys C-276 and G. Hydrogen diffusivity and permeability were determined by means of the electrolytic permeability technique over the temperature range of 17 to 90 °C. Although the two alloys are similar in composition and structure, they exhibited dramatically different hydrogen behavior. For Alloy C-276, the diffusivity in both the cold worked and annealed conditions decreased by a factor of two following low temperature (500 °C) aging. That behavior was related to ordering in the alloy. Unexpectedly, hydrogen trapping was not observed in Alloy C-276. An analysis of hydrogen transport in Alloy G indicated reversible and irreversible trapping of hydrogen by niobium substitutional atoms and second phase carbides, respectively. The hydrogen transport results were related to the hydrogen embrittlement tendencies of the two nickel base alloys. DAVID A. MEZZANOTTE, formerly Graduate Assistant with the Department of Metallurgical Engineering and Materials Science, University of Notre Dame. NICHOLAS F. FIORE, formerly Professor and Chairman of the Department of Metallurgical Engineering and Materials Science, University of Notre Dame.     

Effect of Microstructure and Alloy Chemistry on Hydrogen Embrittlement of Precipitation-Hardened Ni-Based Alloys

The sensitivity to hydrogen embrittlement (HE) has been studied in respect of precipitation size distributions in two nickel-based superalloys: Alloy 718 (UNS N07718) and Alloy 945X (UNS N09946). Quantitative microstructure analysis was carried out by the combination of scanning and transmission electron microscopy and energy dispersive x-ray spectroscopy (EDS). While Alloy 718 is mainly strengthened by γ″, and therefore readily forms intergranular δ phase, Alloy 945X has been designed to avoid δ formation by reducing Nb levels providing high strength through a combination of γ′ and γ″. Slow strain rate tensile tests were carried out for different microstructural conditions in air and after cathodic hydrogen (H) charging. HE sensitivity was determined based on loss of elongation due to the H uptake in comparison to elongation to failure in air. Results showed that both alloys exhibited an elevated sensitivity to HE. Fracture surfaces of the H precharged material showed quasi-cleavage and transgranular cracks in the H-affected region, while ductile failure was observed toward the center of the sample. The crack origins observed on the H precharged samples exhibited quasi-cleavage with slip traces at high magnification. The sensitivity is slightly reduced for Alloy 718, by coarsening γ″ and reducing the overall strength of the alloy. However, on further coarsening of γ″, which promotes continuous decoration of grain boundaries with δ phase, the embrittlement index rose again indicating a change of hydrogen embrittlement mechanism from hydrogen-enhanced local plasticity (HELP) to hydrogen-enhanced decohesion embrittlement (HEDE). In contrast, Alloy 945X displayed a strong correlation between strength, based on precipitation size and embrittlement index, due to the absence of any significant formation of δ phase for the investigated microstructures. For the given test parameters, Alloy 945X did not display any reduced sensitivity to HE compared with Alloy 718 when considering high-strength conditions despite the absence of intergranular δ phase.     

Microstructural trapping effects on hydrogen induced cracking of a microalloyed steel

The susceptibility of a Ti microalloyed HSLA steel to internal hydrogen induced cracking has been correlated with the hydrogen trapping character of the microstructure. Both of these properties are influenced by aging reactions which determine the type and extent of carbide precipitation as well as metalloid segregation to grain boundarie. In turn, crack path susceptibility and total trapping character determine in large part the threshold and steady-state cracking propensities. Thus, metalloid segregation concurrent with potent irreversible trap (TiC) precipitation results in low inherent toughness, but no appreciables drop in threshold stress intensity. Conversely, cementite precipitates formed at lower aging temperatures provide potent crack initiation sites in the absence of deep trapping, resulting in a lower threshold for cracking.     

Study on hydrogen induced delayed fracture of 1500 and 2000MPa hot stamping steels

The application of hot-stamping steel (HS) in the automobile is an inevitable trend, but the hydrogen embrittlement sensitivity of HS steel still needs to be studied and improved. The hydrogen diffusion behavior and hydrogen embrittlement sensitivity of 1500 and 2000MPa hot stamping steels were studied by means of hydrogen penetration, slow strain rate tensile (SSRT), and fracture analysis. The results show that the apparent diffusion coefficient Dap (1.71×10-7cm2/s) of 1500HS is significantly less than the Dap (3.45×10-7cm2/s) of 2000HS; delayed fracture resistance of 1500HS is superior to 2000HS. From the fracture analysis, under the same hydrogen charging conditions, the fracture morphology of 1500HS changed from typical dimple ductile fracture to quasi cleavage brittle fracture, while 2000HS changed from dimple morphology to intergranular brittle fracture with the increase of hydrogen charging current density. While the deformation degree of 2000HS was very small, the local hydrogen content and stress value had reached the critical deal. The hydrogen reduced the bonding force between grains, resulting in the nucleation and propagation of microcracks. Therefore, with the improvement of the strength of HS steel, Ti and V micro alloyed elements should be properly added to form nano precipitates, as irreversible hydrogen traps to capture hydrogen atoms, hinder their diffusion and segregation, and effectively refine the structure and pinning dislocations, to improve the resistance to hydrogen induced delayed fracture of HS steel.     

An example of the effect of hydrogen trapping on hydrogen embrittlement

The role of internal hydrogen in reducing the tensile reduction of area of iron-titanium alloys is examined. The population of hydrogen at potential crack nucleii is shown to be controlled by its dynamic interaction with mobile dislocations and its subsequent transport to fixed traps. Expressions are developed for both the number of hydrogen atoms at a given irreversible trap, as well as the time necessary to reach such a number. Reducing the number below the critical value to nucleate a crack, or increasing the time to reach this value will improve an alloy’s performance, and this improvement is related to controllable external and metallurgical variables. These predictions of the model are shown to be consistent with companion experimental data, and with the trap theory of hydrogen embrittlement.     

Trapping of hydrogen by sulfur-associated defects in steel

Reversible and irreversible trapping behaviors of sulfur-associated defects in steel were studied through electrochemical hydrogen permeation experiments and the results were analyzed to obtain the follow-ing information: an apparent diffusion constant of hydrogen influenced by reversible and irreversible trapping by sulfur-associated defects,D *, was shown to be given as a function of sulfur content, S, in wt ppm.byD * = 1.12·10⁻⁶(1 + 0.0933S^0.548)⁻¹cm² s⁻¹. Associated parameters of reversible and irreversible trapping, λ/μ and k, were also expressed as a function of sulfur content, S. Both parameters, λ/μ, and k, are shown to increase with S, suggesting an increase of both reversible and irreversible trap sites with increase in sulfur content in steel.     

Internal hydrogen effects on tensile properties of iron- and nickel-base superalloys

Two nickel-base alloys [superalloys INCONEL 718 (IN718) and INCONEL 625 (IN625)] and one iron-base superalloy (A286) were chosen to study the effects of internal H charging on their room-temperature slow strain rate mechanical behavior. Uniform internal H contents ranged from 0 to 50 wt ppm H (0 to 3000 at. ppm H), and a strain rate of 8.5 X 10^-7 m/s was used with notched strip specimens. The three alloys showed varying losses in strength and ductility, and the strongest alloy, IN718, showed a decrease of 67 pet in ductility for a dissolved H content of 40 wt ppm. Superalloy A286 showed a corresponding 50 pet decrease in ductility, and IN625 showed a 29 pet loss in ductility. Fractographic evidence and the marked decrease in strength of the alloys lead the authors to conclude that the enhanced localized plasticity mechanism for H embrittlement is possibly operative in these face-centered cubic (fcc) alloys.     

Determination of Hydrogen in Steel by Thermal Desorption Mass Spectrometry

Hydrogen embrittlement has been observed since high‐strength steels have been produced in the nineteen thirties 1 . Several different analytical methods have been developed to quantify the total and diffusible hydrogen in steel, but many aspects of hydrogen determination are still to be explored. Purely quantitative determination of hydrogen is not sufficient to fully characterize the steel regarding its resistance against embrittlement. Thermal Desorption Mass Spectrometry (TDMS) allows the investigation of hydrogen absorption and desorption mechanisms to characterize hydrogen traps in different kinds of steel microstructures. This provides valuable information for the development of new materials with a higher resistance against hydrogen embrittlement. Additionally, TDMS allows the quantitative determination of very small concentrations of hydrogen (<0.1 µg/g). Such low detection limits cannot be reached with other methods. Due to time‐consuming analysis and a rather complex construction, TDMS is usually not applied for hydrogen determination in German steel mills. The present work describes the development of a thermal desorption spectrometer at ThyssenKrupp Steel Europe AG by adapting a compact quadrupole mass spectrometer to a commercially available hot solid extraction analyzer, which has proven to be a simple and efficient solution for the determination of diffusible hydrogen in steel.     

Fatigue behavior of maraging steel 300

The cyclic stress-strain curves, the low cycle and high cycle fatigue lives and the fatigue crack growth rates of annealed (1 h 820°C) and aged (3 h 480°C) maraging steel 300 were determined. Incremental step testing and stable hysteresis loop tip measurements were used to determine the cyclic σ-ε curves. Both annealed and aged maraging steels were found to cyclically soften at room temperature over a plastic strain range from 0.1 to 20 pct. The S-N curves were determined from 10 to 10⁷ cycles to failure by plastic strain controlled low cycle fatigue tests performed in air and load controlled high cycle fatigue tests performed in dry argon. The test results compared very well with the theoretical lifetime predictions derived from Tomkins’ theory. Fatigue crack growth rates were measured in air and dry argon for the annealed and aged alloys. Crack growth rates of annealed maraging steel were found to be equal to those of aged maraging steel at rates between 10^-7 and 10^-5 in./cycle. A significant difference in crack growth rates in the two environments was found at low stress intensity factor ranges, indicating a high susceptibility to corrosion fatigue in the presence of water vapor. The mechanisms of cyclic softening in the two alloys are discussed in terms of dislocations rearrangement in the annealed alloy and dislocation-precipitate interactions in the aged alloy.