Effect of hydrogen on the properties and fracture characteristics of TRIP 800 steels

The paper describes effect of hydrogen on the properties and fracture characteristics of two variants of TRIP 800 C–Mn–Si steels. The effect of hydrogen was studied by means of tensile tests on specimens previously charged by hydrogen. Hydrogen provoked embrittlement in both variants but only for very high hydrogen content. Hydrogen embrittlement manifested itself mainly by a loss of plasticity. Both steel variants were able to absorb a large amount of hydrogen, up to 50 ppm. Concerning fractographic characteristics, steels containing higher hydrogen content displayed transgranular cleavage fracture. In exceptional cases, an irreversible embrittlement was revealed initiating on non-metallic inclusions.     

TWIP钢不同温度变形的力学性能变化规律及机理研究

通过控温拉伸实验分析了在298,373,473和673 K温度下变形时,TWIP钢(Fe-25Mn-3Si-3Al)力学性能和显微组织的变化规律.结果表明,TWIP钢的强度和延伸率均随温度的升高而降低.通过热力学公式对不同温度下TWIP钢层错能Γ的估算可以推断,温度T≥673 K时,Γ≥76 mJ/m2,滑移为TWIP钢主要的变形机制;298 K≤T≤373 K时,21 mJ/m2≤Γ≤34 mJ/m2,孪生为TWIP钢主要的变形方式,此时产生"TWIP"效应,可获得较高的加工硬化速率,从而获得高强度及高塑性.     

Wasserstoff-induzierte Spannungsrißkorrosion von Stählen durch dynamisch-plastische Beanspruchung in Promotor-freien Elektrolytlösungen

Hydrogen induced stress corrosion cracking of steels subjected to dynamic loading involving plastic deformation in promotor free electrolytic solutions Plain carbon steels and low alloy steels suffer internal cracking and a relatively high embrittlement when they are subjected to dynamic loading involving plastic deformation in any type of electrolytic solution where there is a simultaneous cathodic hydrogen evolution. These conditions can be encountered in service in the case of cathodic polarisation and free corrosion in acids if the mechanical stresses lead to plastic deformation, e.g. at notches. There is an upper limit to the potential range in which internal cracking occurs. This limit is independent of the yield strength (300 to 500 N mm^?2) of the materials tested and lies at U_H = ?0.5 V in oxygen free salt waters. It is more negative in oxygen bearing electrolytic solutions or in alkalaine media. Materials containing hard transformation products such as martensite and bainite are more susceptible to cracking in the regions of these hard transformation products. There is only a small decrease in susceptibility with increasing temperature. Ultra high strength, quenched and tempered steels with yield strengths > 1000 N mm^?2 undergo hydrogen induced stress corrosion cracking even when they are subjected to static loading in the elastic region. The critical potential is very negative and shifts to less negative values with increasing strength of the material. Stainless steels with stable austenitic microstructure are resistant to this type of corrosion. However, if the mechanical deformation can lead to the formation of martensite surface cracking and brittle fractures occur. The hydrogen induced damage decreases with increasing temperature.     

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.     

Comparative assessment of electrochemical hydrogen absorption by pipeline steels with different strength

The assessment of ability to absorb hydrogen of three API grade pipeline steels: X52, X70 and X100 have been evaluated. The factors of cathodic hydrogen charging, time of exposure, and applied stress were taken into account. It has been shown that all steels demonstrate the sensitivity to hydrogenating in deoxygenated, near-neutral pH NS4 solution under relatively “soft” cathodic polarisation, although the efficiency of hydrogen permeation in metal is quite low and depends on time of exposure. Applied tensile stress, which equivalent to gross hoop stress in pipe wall under operating conditions, can accelerate the hydrogen absorption in several times. For studied steels the resistance to hydrogen absorption decreases with decreasing of steel strength.     

Hydrogen environment embrittlement testing at low temperatures and high pressures

Hydrogen environment embrittlement (HEE) of several heats of austenitic stainless steels was measured by slow strain rate tensile testing in hydrogen atmosphere at different temperature–pressure combinations.The relative reduction of area decreased (means increasing hydrogen embrittlement) in the following order: 20 °C/10 bar > 20 °C/700 bar > −50 °C/10 bar > −50 °C/400 bar ≈ −80 °C/200 bar. An assessment of the most severe error sources revealed that tensile tests in hydrogen can be performed with a very high repeatability and reproducibility with a total deviation of about 10% with respect to the relative reduction of area (RRA = RA_H2/RA_He). Higher deviations in RRA are due to local deviations in metallurgy and chemical composition, which are very sensitive to HEE.Screening tests of several austenitic SS at 20 °C/700 bar show decreasing HEE with increasing Ni content. For RRA > 90%, a minimum Ni content of >11.5 wt% is required.     

Effect of microstructure on the sulphide stress cracking susceptibility of a high strength pipeline steel

The sulphide stress cracking (SSC) susceptibility of a newly developed high strength microalloyed steel with three different microstructures has been evaluated using the slow strain rate testing (SSRT) technique. Studies were complemented with potentiodynamic polarization curves and hydrogen permeation measurements. Material included a C–Mn steel having Ni, Cu, and Mo as main microalloying elements with three microstructures: martensitic, ferritic and ferritic + bainitic. Testing temperatures included 25, 50, 70 and 90 °C. Detailed SEM observations of the microstructure and fracture surfaces were done to identify possible degradation mechanisms. The results showed that in all cases, the corrosion rate, number of hydrogen atoms at the surface and the percentage reduction in area increased with temperature. The steel with a martensitic microstructure had the highest SSC susceptibility at all temperatures, whereas the ferritic steels were susceptible only at 25 °C, and the most likely mechanism is hydrogen embrittlement assisted by anodic dissolution.     

Evaluation of delayed fracture property of outdoor-exposed high strength AISI 4135 steels

Delayed fracture properties of AISI 4135 high strength steels with 1490 and 1310 MPa of tensile strength, represented as B15 and B13, respectively, have been studied by means of slow strain rate test (SSRT) of notched bar specimens after outdoor exposure at rural and coastal areas. The exposed specimens were kept at humid medium before SSRT to reproduce active hydrogen entry influenced by the rust layer and to homogenize hydrogen distribution. The influences of exposure site and exposure time on fracture stress have been investigated. The susceptibility of B15 to delayed fracture was obviously higher than that of B13.     

Hydrogen transport and embrittlement in 300 M and AerMet100 ultra high strength steels

This paper describes how hydrogen transport affects the severity of hydrogen embrittlement in 300 M and AerMet100 ultra high strength steels. Slow strain rate tests were carried out on specimens coated with electrodeposited cadmium and aluminium-based SermeTel 1140/962. Hydrogen diffusivities were measured using two-cell permeation and galvanostatic charging methods and values of 8.0 × 10⁻⁸ and 1.0 × 10⁻⁹ cm² s⁻¹ were obtained for 300 M and AerMet100, respectively. A two-dimensional diffusion model was used to predict the hydrogen distributions in the SSR specimens at the time of failure. The superior embrittlement resistance of AerMet100 was attributed to reverted austenite forming around martensite laths during tempering.     

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.     

In situ electrochemical nanoindentation: A technique for local examination of hydrogen embrittlement

Nanoindentation combined with AFM (NI-AFM) has been used to study the effect of electrochemically in situ charged hydrogen on the deformation of small volumes of nickel and copper single crystals. Hydrogen reduces the unstable elastic plastic transition load (pop-in) in nickel, but does not have any effect on copper. It has been shown that the activation energy for the onset of plasticity (dislocation nucleation) is reduced by dissolved hydrogen. This is because hydrogen reduces shear modulus and stacking fault energy in nickel, whereby the former results in hydrogen-enhanced decohesion (HEDE) and the latter in the hydrogen-enhanced plasticity (HELP) mechanism.     

Hydrogen uptake in 316L stainless steel: Consequences on the tensile properties

Different charging conditions aimed at introducing significant hydrogen concentrations without microstructural damages in a 316L austenitic stainless steel were investigated. The equivalent hydrogen pressure developed at the surface of the samples during cathodic charging was estimated from hydrogen concentration measurements. A clear hydrogen absorption, controlled by diffusion, was evidenced during the immersion of 316L steel samples in 30% MgCl₂ at the open circuit potential at 117 °C. Deuterium profiling by SIMS was performed to check the validity of the few literature data on hydrogen diffusivity in the near room temperature range in this material. On the other hand, the macroscopic effects of hydrogen on the tensile characteristics of the steel were investigated and compared at 20 °C and at −196 °C with samples cathodically pre-charged, charged during tensile straining or pre-charged at high temperature-high pressure in gas phase. Hydrogen is shown to affect both the short range and the long range forces exerted on the strain-induced mobile dislocations. The hydrogen-induced softening effect observed at 20 °C and the systematic decrease of the ductility support a mechanism involving the enhanced transport of hydrogen atoms by mobile dislocations. This mechanism is confirmed by the absence of softening and of ductility loss at −196 °C and by the strain-enhanced tritium desorption from samples cathodically pre-charged with tritium, measured by β counting during tensile deformation.     

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 environmental and metallurgical factors on hydrogen induced cracking of HSLA steels

Hydrogen induced cracking (HIC) resistance of two high strength low alloy (HSLA) steel plates equivalent to API X70 grade was evaluated in various test solutions with different H₂S partial pressures and pH values. Results showed that H₂S partial pressure is the key parameter affecting HIC resistance. Hydrogen permeation rate was affected by both H₂S partial pressure and pH of test solutions, whereas the apparent hydrogen diffusivity was determined mainly by pH value in case of H₂S partial pressure less than 0.1 atm. HIC in the steels primarily nucleated at inclusions and/or clusters containing the Al and Ca oxides. HIC resistance was determined by diffusible hydrogen amount with different microstructures.     

Strengthening and hydrogen embrittlement of ultrafine-grained Fe–0.01 mass% C alloy processed by high-pressure torsion

This study was conducted to elucidate the effects of annealing and hydrogenation on the tensile properties of an Fe–0.01 mass% C alloy processed by high-pressure torsion (HPT). By HPT processing, the tensile strength was increased to ∼1500 MPa through grain refinement. Low-temperature annealing further strengthened the HPT-processed specimen because of a simultaneous effect of carbide precipitation and grain refinement. Reduction in the dislocation density and the fraction of low-angle grain boundaries through warm-temperature annealing led to a decrease in hydrogen uptake when the specimens were exposed to high-pressure gaseous hydrogen, and they became less sensitive to hydrogen embrittlement (HE).     

Hydrogen entry behaviour of hot-dip Al–Mg–Si coated steel

A ternary hot-dip Al–Mg–Si coating was formed on a steel substrate and tested as an alternative to conventional zinc coatings particularly in high-strength steel application with respect to hydrogen entry behaviour. Hydrogen entry behaviour was evaluated under wet–dry conditions using a Devanathan cell. The new hot-dip Al–Mg–Si coating shows relatively low corrosion potential during the initial stage of the wet period; however, the potential shifts in a noble direction in a short time resulting in smaller amount of hydrogen entry than that in the conventional zinc coating.     

Impact of oxyfuel atmospheres H2O/CO2/O2 and H2O/CO2 on the oxidation of ferritic–martensitic and austenitic steels

In future power plant technologies, oxyfuel, steels are subjected to steam rich and carbon dioxide rich combustion gases. The effect of simulated combustion gases H₂O/CO₂/O₂ (30/69/1 mol%) and H₂O/CO₂ (30/70 mol%) on the corrosion behavior of low alloyed steels, 9–12% chromium steels and an austenitic steel were studied. It was discovered that the formation of protective chromium rich oxides is hampered due to the carburization of the base material and the formation of chromium rich carbides. The kinetics of corrosion and carburization are quantified. The effect of temperature and the effect of gas pressure are analyzed statistically.     

Characterization of inclusions of X80 pipeline steel and its correlation with hydrogen-induced cracking

In this work, the microstructures of an X80 pipeline steel were characterized, and their susceptibilities to hydrogen-induced cracking (HIC) were investigated by hydrogen-charging, electrochemical hydrogen permeation and surface characterization. It is found that the microstructure of X80 pipeline steel consists of a polygonal ferrite and bainitic ferrite matrix, with martensite/austenite (M/A) constituents distributing along grain boundaries. The inclusions existing in the steel include those enriched with Si, Al oxide, Si–ferric carbide and Al–Mg–Ca–O mixture, respectively. The majority of inclusions are Si-enriched. Upon hydrogen-charging, cracks could be initiated in the steel in the absence of external stress. The cracks are primarily associated with the Si- and Al oxide-enriched inclusions. The diffusivity of hydrogen in X80 steel at room temperature is 2.0 × 10⁻¹¹ m²/s, and the estimated hydrogen trapping density in the steel is as high as 3.33 × 10²⁷ m⁻³.     

A new aspect on intergranular hydrogen embrittlement mechanism of solution annealed types 304, 316 and 310 austenitic stainless steels

The objective of this paper is to propose a new intergranular hydrogen embrittlement mechanism of solution annealed austenitic stainless steels (types 304, 316 and 310) on the basis of the results already reported. An intergranular hydrogen embrittlement (IG-HE) took place for type 316 at potentials less noble than the open-circuit potential in a HCl solution, and for types 304 and 316 at a lower test temperature under an open-circuit condition in saturated boiling magnesium chloride solutions by using a constant load method, while type 310 suffered only a transgranular stress corrosion cracking (TG-SCC) in both solutions under the same experimental conditions, but not IG-HE. In addition, TG-SCC occurred for types 304 and 316 under an open-circuit condition in the HCl solution irrespective of test temperature and in saturated boiling magnesium chloride solutions at higher test temperatures. Thus, the occurrence of IG-HE depended upon the material and test temperature. The new IG-HE mechanism was developed that explains the results obtained in terms of martensite transformation, hydrogen-enhanced local plasticity (HELP), grain boundary sliding (GBS) and so on.     

Hydrogen permeation in Fe-40 at.% Al alloy at different temperatures

The electrochemical permeation technique for studying transport and trapping of hydrogen in Fe-40 at.% Al alloy at temperatures of 5, 25, 45 and 65 °C was used in the paper. The influence of temperature on the effective hydrogen diffusion coefficient, hydrogen permeation rate and hydrogen solubility was determined. The activation energy of hydrogen diffusion in iron aluminide in the studied temperature range was also determined.