Determination of Critical Hydrogen Concentration and Its Effect on Mechanical Performance of 2200 MPa and 600 HBW Martensitic Ultra-High-Strength Steel

: The inﬂuence of hydrogen on the mechanical performance of a hot-rolled martensitic steel was studied by means of constant extension rate test (CERT) and constant load test (CLT) followed with thermal desorption spectroscopy measurements. The steel shows a reduction in tensile strength up to 25% of ultimate tensile strength (UTS) at critical hydrogen concentrations determined to be about 1.1 wt.ppm and 50% of UTS at hydrogen concentrations of 2 wt.ppm. No further strength degradation was observed up to hydrogen concentrations of 4.8 wt.ppm. It was observed that the interplay between local hydrogen concentrations and local stress states, accompanied with the presence of total average hydrogen reducing the general plasticity of the specimen are responsible for the observed strength degradation of the steel at the critical concentrations of hydrogen. Under CLT, the steel does not show sensitivity to hydrogen at applied loads below 50% of UTS under continuous electrochemical hydrogen charging up to 85 h. Hydrogen enhanced creep rates during constant load increased linearly with increasing hydrogen concentration in the steel.


Introduction
There is an ever-increasing need for hard and tough steels for demanding wear and impact resistance industrial applications.These include mining equipment in severe corrosion environments [1], ballistic resistance in armored and patrol vehicles, and protected buildings in civil construction [2].As the need for safe operation of higher strength steels for challenging applications is increasing, so are concerns about their susceptibility to hydrogen.Over the years, several industrial failures related to hydrogen have been reported ranging from small components such as fasteners to large ones like boilers, hydrogen storage tanks, oil, and gas structures [3,4].Hydrogen interacts with metallic materials in a way that reduces their ductility, toughness and even their strength [5].It has been reported that local stresses and local hydrogen concentrations are controlling factors of the loss of fracture strength in steels [6].The primary conditions responsible for the undesired failure depend often on dislocation process, and are controlled by hydrogen diffusion and trapping, coupled with the state of stresses in the material [7][8][9].
Generally, the susceptibility of steels to hydrogen increases with their strength [10][11][12][13].As steels are produced with increased strength, they become harder, less ductile, less tough, and more susceptible to hydrogen embrittlement (HE).The susceptibility of quenched and tempered ferritic-martensitic steels increases significantly above 1200 MPa with hardness of about 360 HBW [14].To a large extent, this can be attributed to two phenomena.One is the high diffusivity of hydrogen in ferritic-martensitic steels [15].The other is the segregation of alloying elements resulting from the high alloying and high carbon content used for hardening, leading to the formation of carbides and precipitates that act as stress concentrators, affecting the diffusion and trapping of hydrogen in these materials.Over the years, HE mechanisms such as hydrogen enhanced decohesion (HEDE), and hydrogen enhanced localized plasticity (HELP) have been proposed as damage mechanisms in steels in the presence of hydrogen.The HEDE mechanism suggests that embrittlement is due to localized reduction in cohesive strength of the iron lattice in hence assists the separation of cleavage planes or grain boundaries under lower stresses [16,17].While the HELP mechanism focuses on the fact that atomic hydrogen accelerate the dislocation mobility through an elastic shielding effect that causes a local reduction in shear stress and hydrogen transport by dislocation motion, which could lead to localized high concentrations at distances further ahead of crack tip [7,18,19].
The evaluation of the hydrogen embrittlement property of steels, particularly new ultra-high strength steels, is an important task allowing their safe and reliable use in conditions under which their susceptibility to hydrogen is found to be minimal.This is a difficult task to perform because many variables considering that factors like chemistry, microstructure, metallurgical defects, operating temperatures, and stress states simultaneously affect the sensitivity of steel to hydrogen.It is widely reported in literature [20][21][22][23][24] that the degradation of mechanical properties of steels in the presence of hydrogen occurs only when hydrogen reaches a certain critical concentration in the steel.Hence the critical hydrogen concentration (H crit ) was proposed as a parameter to evaluate the hydrogen embrittlement property of high-strength steels [23,24].
It has been determined that tensile strength of steels decreases with increasing diffusible hydrogen content leading to the formulation of a power law relationship between fracture strength and diffusible hydrogen content [25].However, it was also observed the power law was not always applicable, especially for notched specimens.The strong dependence of the notch tensile strength on the stress intensity factor makes it unlikely to be used as fracture criterion for HE, except for specified geometries [6,25].In addition, it was found that the power law relationship between fracture stress and hydrogen content is mostly applicable only when the fracture mode is intergranular [26] limiting its application to various steels with complex microstructures.In recent years, many studies have explored promising H-resistant additively manufactured steels [27,28].However, until their full development, conventionally manufactured ultra-high-strength, hard and impact resistant steels are still the primary options.
Slow strain rate tests has been used in several studies to evaluate the effect of hydrogen on mechanical properties of steels, the technique is believed to allow enough time for hydrogen activity within the material [22].Although SSRT and hydrogen concentration measurement techniques have been employed in several studies to determine H crit for high-strength steels up to 1500 MPa [29][30][31][32][33][34], much more work is still required.Particularly in determining H crit for higher strength steels (>1500 MPa) and its effect on the mechanical performance of these steels under constant loads, focusing specifically on the hydrogen enhanced creep rates.
In this study, we determine the critical hydrogen concentration and evaluate its effect on the mechanical performance of a modern steel for demanding applications.The material is a martensitic ultra-high-strength steel (2200 MPa), with hardness of 600 HBW.The research methods include constant extension rate testing (CERT), constant load testing (CLT), hydrogen thermal desorption spectroscopy (TDS), and fractography.

Material
A hot-rolled and quenched medium carbon steel with ultimate tensile strength of 2200 MPa and hardness of 600 HBW (58 HRC) was studied.The steel was obtained from steel manufacturer SSAB, in Finland.An optical microscopic observation of the studied steel (after etching with Nital 2% for 20 s) shows a dominant martensitic microstructure with islands of bainite as shown in Figure 1.The relevant major chemical composition and the mechanical properties of the studied steel are summarized in Table 1.
Metals 2021, 11, x FOR PEER REVIEW 3 of 16 steel (after etching with Nital 2% for 20 s) shows a dominant martensitic microstructure with islands of bainite as shown in Figure 1.The relevant major chemical composition and the mechanical properties of the studied steel are summarized in Table 1.

Specimen Preparation
Two types of specimens were used in the study.TDS specimens with characteristic size of 1 mm × 4 mm × 10 mm were used for investigation of hydrogen charging parameters and hydrogen uptake of the steel.Secondly, sub-sized specimens were used for mechanical tensile testing (MT) with size of 5 mm × 10 mm × 300 mm and gauge part size of 1.0 mm × 5.0 mm × 20 mm, shown in Figure 2a.All the specimens were cut by EDM and polished mechanically finishing with emery paper No. 1200.Additionally, the MT specimens were Teflon-taped to expose only the gauge part to hydrogen charging, as shown in Figure 2b.

Specimen Preparation
Two types of specimens were used in the study.TDS specimens with characteristic size of 1 mm × 4 mm × 10 mm were used for investigation of hydrogen charging parameters and hydrogen uptake of the steel.Secondly, sub-sized specimens were used for mechanical tensile testing (MT) with size of 5 mm × 10 mm × 300 mm and gauge part size of 1.0 mm × 5.0 mm × 20 mm, shown in Figure 2a.All the specimens were cut by EDM and polished mechanically finishing with emery paper No. 1200.Additionally, the MT specimens were Teflon-taped to expose only the gauge part to hydrogen charging, as shown in Figure 2b.

Hydrogen Charging
Electrochemical hydrogen charging was performed in a glass, three-electrode electrochemical cell combined with a Gamry potentiostat framework.Calomel reference electrode and platinum wire counter electrode were used in the cell together with the steel specimen as the working electrode.
To obtain a suitable hydrogen charging parameters for the studied steel, the TDS specimens were charged from 3% of NaCl and 0.1% NH 4 SCN as hydrogen atom recombination poison [35].The charging time was varied from 10 min to 12 h at an applied electrochemical potential of −1 V SCE , followed with hydrogen concentration measurement using TDS method.Hydrogen uptake of the studied steel as a function of the applied electrochemical potentials was also measured by varied potential from −0.8 to −1.3 V SCE for a charging time of 2 h.

Hydrogen Charging
Electrochemical hydrogen charging was performed in a glass, three-electrode electrochemical cell combined with a Gamry potentiostat framework.Calomel reference electrode and platinum wire counter electrode were used in the cell together with the steel specimen as the working electrode.
To obtain a suitable hydrogen charging parameters for the studied steel, the TDS specimens were charged from 3% of NaCl and 0.1% NH4SCN as hydrogen atom recombination poison [35].The charging time was varied from 10 min to 12 h at an applied electrochemical potential of −1 VSCE, followed with hydrogen concentration measurement using TDS method.Hydrogen uptake of the studied steel as a function of the applied electrochemical potentials was also measured by varied potential from −0.8 to −1.3 VSCE for a charging time of 2 h.
The MT specimens were H-charged electrochemically from 3% NaCl + 0.1% and 0.3% NH4SCN for 2 h determined from the charging time dependency of the hydrogen concentration when the concentration plot approaches saturation, which corresponds to almost homogeneous distribution of hydrogen across the thickness of the specimen's gauge part.The applied electrochemical potential for hydrogen charging was varied between −0.8 to −1.4 VSCE.A combination of the applied potential and H-charging poison content was used to provide a rather wide range of hydrogen concentration in the MT specimens.
The pH of the electrolyte was measured to be 5.5 and 4.5 for the electrolyte solution containing 0.1% and 0.3% NH4SCN, respectively.The electrolyte is replaced with a fresh one if the pH changes due to evaporation of the solution over the testing time.During H-charging, the electrolyte was kept under constant stirring and deaeration by nitrogen gas flow.Hydrogen charging was performed at room temperature, about 20 °C.

Mechanical Testing
After 2 h of hydrogen pre-charging, mechanical testing comprising of CERT and CLT is initiated under continuous hydrogen charging.CERTs were performed with a 30 kN MTS benchtop tensile test machine at the strain rate 10 −4 s −1 and CLTs at the same strain rate until the applied load was attained.In the case of CLT, mechanical testing was The MT specimens were H-charged electrochemically from 3% NaCl + 0.1% and 0.3% NH 4 SCN for 2 h determined from the charging time dependency of the hydrogen concentration when the concentration plot approaches saturation, which corresponds to almost homogeneous distribution of hydrogen across the thickness of the specimen's gauge part.The applied electrochemical potential for hydrogen charging was varied between −0.8 to −1.4 V SCE .A combination of the applied potential and H-charging poison content was used to provide a rather wide range of hydrogen concentration in the MT specimens.
The pH of the electrolyte was measured to be 5.5 and 4.5 for the electrolyte solution containing 0.1% and 0.3% NH 4 SCN, respectively.The electrolyte is replaced with a fresh one if the pH changes due to evaporation of the solution over the testing time.During H-charging, the electrolyte was kept under constant stirring and deaeration by nitrogen gas flow.Hydrogen charging was performed at room temperature, about 20 • C.

Mechanical Testing
After 2 h of hydrogen pre-charging, mechanical testing comprising of CERT and CLT is initiated under continuous hydrogen charging.CERTs were performed with a 30 kN MTS benchtop tensile test machine at the strain rate 10 −4 s −1 and CLTs at the same strain rate until the applied load was attained.In the case of CLT, mechanical testing was stopped, if fracture does not occur after 85 h under applied load and continuous hydrogen charging.Figure 2c shows a general view of mechanical testing setup with continues hydrogen charging.
After fracture or abortion of testing, the gauge part of the MT specimen was cut into two parts.One is cut to the characteristic size of a TDS specimen and taken for hydrogen concentration measurement with TDS.The other is cleaned with distilled water and stored for fractography in a vacuum chamber to prevent the formation of any oxide layers.

Hydrogen Concentration Measurement
The TDS apparatus used for hydrogen measurement was designed, manufactured, and assembled at Aalto University, Finland.The general schematic view of the TDS apparatus is shown in Figure 3.
stopped, if fracture does not occur after 85 h under applied load and continuous hydrogen charging.Figure 2c shows a general view of mechanical testing setup with continues hydrogen charging.
After fracture or abortion of testing, the gauge part of the MT specimen was cut into two parts.One is cut to the characteristic size of a TDS specimen and taken for hydrogen concentration measurement with TDS.The other is cleaned with distilled water and stored for fractography in a vacuum chamber to prevent the formation of any oxide layers.

Hydrogen Concentration Measurement
The TDS apparatus used for hydrogen measurement was designed, manufactured, and assembled at Aalto University, Finland.The general schematic view of the TDS apparatus is shown in Figure 3. Before TDS measurement the specimens were cleaned with distilled water followed by drying in helium gas flow to prevent any effect of moisture on the TDS measurement.The measurement of partial pressure of hydrogen occurs in the UHV chamber (1 × 10 −9 mbar) coupled with a mass spectrometer (SRS residual gas analyser RGA100.To keep the UHV chamber at the required pressure and reduce pumping time before measurement, the sample was first placed in an airlock compartment and pumped to an intermediate pressure of 1 × 10 −6 mbar.After which the specimen was transported to the UHV chamber and measurement is initiated.The total time from the specimen preparation, placing it in the airlock, and transporting it to the furnace in the UHV chamber did not exceed 10 min [15].All TDS measurements were performed at a heating rate of 10 K/min.Before TDS measurement the specimens were cleaned with distilled water followed by drying in helium gas flow to prevent any effect of moisture on the TDS measurement.The measurement of partial pressure of hydrogen occurs in the UHV chamber (1 × 10 −9 mbar) coupled with a mass spectrometer (SRS residual gas analyser RGA100.To keep the UHV chamber at the required pressure and reduce pumping time before measurement, the sample was first placed in an airlock compartment and pumped to an intermediate pressure of 1 × 10 −6 mbar.After which the specimen was transported to the UHV chamber and measurement is initiated.The total time from the specimen preparation, placing it in the airlock, and transporting it to the furnace in the UHV chamber did not exceed 10 min [15].All TDS measurements were performed at a heating rate of 10 K/min.

Constant Extention Rate Test (CERT)
(Hc crit ) [21].An analytical representation of the critical hydrogen concentration can be described with an 'atan' function in form of Equation ( 1): where X is the measured hydrogen concentration and Y 0 = 1657, A = −412, X c = 1.05, and W = 0.2 are the best fitting parameters generated automatically by the OriginPro software.The fitted curve corresponds well with the experimentally obtained results with an accuracy of R 2 = 0.96 as shown in Figure 5b.Notably, X c = 1.05 corresponds to the critical hydrogen concentration.The hydrogen embrittlement index (EI) which is about 50% for the studied steel was calculated by Equation ( 2) [36]: where UTS air = 2200 MPa is the ultimate tensile strength of the steel tested in air, and UTS H(plateau) = 1100 MPa is the ultimate tensile strength of the steel tested under continuous hydrogen charging corresponding to the lower plateau on the hydrogen embrittlement curve.
where  is the measured hydrogen concentration and  0 = 1657,  = −412,   = 1.05,  = 0.2 are the best fitting parameters generated automatically by the OriginPro software.The fitted curve corresponds well with the experimentally obtained results with an accuracy of  2 = 0.96 as shown in Figure 5b.Notably,   = 1.05 corresponds to the critical hydrogen concentration.The hydrogen embrittlement index (EI) which is about 50% for the studied steel was calculated by Equation (2) [36]: where   = 2200MPa is the ultimate tensile strength of the steel tested in air, and  = 1100MPa is the ultimate tensile strength of the steel tested under contin- crostructural defects in the solid solution such as dislocatio cies, and nano-voids [38].While the higher temperature com the decomposition of the molecular hydrogen trapped in vo processes [39].

Constant Load Tests (CLTs)
CLTs were performed at varying applied constant loads under continuous electrochemical hydrogen charging at cond critical hydrogen concentration determined during CERT (i.e. about −1 VSCE).The actual time to fracture in CLTs was calc excluding the time taken to attain the actual applied load.Loa occur after 85 h under load are considered having reached aborted.The applied load as a function of time to fracture for hydrogen charging is shown in Figure 7.The susceptibility o constant load is evident as the applied load and time to fractu on a semi-logarithmic scale.For example, the hydrogen-fre zone' (no fracture after 85 h) at applied stress of 1600 MPa (72 under continuous hydrogen charging fractures in about 2 min charged specimen reaches the 'safety zone' at an applied stres

Constant Load Tests (CLTs)
CLTs were performed at varying applied constant loads with MT specimens in air and under continuous electrochemical hydrogen charging at conditions that correspond to the critical hydrogen concentration determined during CERT (i.e., 3% NaCl + 0.1% NH 4 SCN at about −1 V SCE ).The actual time to fracture in CLTs was calculated as overall testing time excluding the time taken to attain the actual applied load.Loads at which fracture does not occur after 85 h under load are considered having reached a 'safe zone' and the test is aborted.The applied load as a function of time to fracture for specimens under continuous hydrogen charging is shown in Figure 7.The susceptibility of the steel to hydrogen under constant load is evident as the applied load and time to fracture shows a linear correlation on a semi-logarithmic scale.For example, the hydrogen-free specimen reaches the 'safe zone' (no fracture after 85 h) at applied stress of 1600 MPa (72% of UTS), while the samples under continuous hydrogen charging fractures in about 2 min under the same load.The H-charged specimen reaches the 'safety zone' at an applied stress of 1100 MPa (50% of UTS).
In addition, the effect of hydrogen on the hydrogen-enhanced creep in CLT was evaluated.A typical creep curve retrieved from the CLT data of the studied steel (applied load 1400 MPa) is shown in Figure 8a where the derivative of the stage II of the creep curve is considered as the creep rate [40].The creep rates linearly increase with increasing total hydrogen concentration, as shown in Figure 8b.

Fractography
Fractographic observations were made after CERT in air, hydrogen charged to the critical concentration, and hydrogen charged above the critical concentrations.As shown in Figure 9a, the fracture surface manifests a ductile fracture characterized by dimpled rupture for the specimen tested in air.At increasing hydrogen concentrations around the critical range, the fracture surface shows a brittle fracture area with clearly visible long secondary cracks, as emphasized in Figure 9b.At hydrogen concentrations corresponding Metals 2021, 11, 984 9 of 16 to the plateau above the critical concentration the fracture surface shows a fine blend of transgranular and intergranular fracture mode, with high-density of secondary cracks forming most likely along the former austenite grain boundaries as well as martensitic lath, as depicted in Figure 9c.
occur after 85 h under load are considered having reached aborted.The applied load as a function of time to fracture fo hydrogen charging is shown in Figure 7.The susceptibility o constant load is evident as the applied load and time to fract on a semi-logarithmic scale.For example, the hydrogen-fre zone' (no fracture after 85 h) at applied stress of 1600 MPa (7 under continuous hydrogen charging fractures in about 2 mi charged specimen reaches the 'safety zone' at an applied stre  In addition, the effect of hydrogen on the hydrogen-enhanced creep in CLT was evaluated.A typical creep curve retrieved from the CLT data of the studied steel (applied load 1400 MPa) is shown in Figure 8a where the derivative of the stage II of the creep curve is considered as the creep rate [40].The creep rates linearly increase with increasing total hydrogen concentration, as shown in Figure 8b.

Fractography
Fractographic observations were made after CERT in air, hydrogen charged to the critical concentration, and hydrogen charged above the critical concentrations.As shown in Figure 9a, the fracture surface manifests a ductile fracture characterized by dimpled rupture for the specimen tested in air.At increasing hydrogen concentrations around the critical range, the fracture surface shows a brittle fracture area with clearly visible long secondary cracks, as emphasized in Figure 9b.At hydrogen concentrations corresponding to the plateau above the critical concentration the fracture surface shows a fine blend of transgranular and intergranular fracture mode, with high-density of secondary cracks rupture for the specimen tested in air.At increasing hydrogen concentrations around the critical range, the fracture surface shows a brittle fracture area with clearly visible long secondary cracks, as emphasized in Figure 9b.At hydrogen concentrations corresponding to the plateau above the critical concentration the fracture surface shows a fine blend of transgranular and intergranular fracture mode, with high-density of secondary cracks forming most likely along the former austenite grain boundaries as well as martensitic lath, as depicted in Figure 9c.

Discussion
non-metallic inclusions (NMI), as shown in Figure 11c.Even though stress concentration always plays a significant role in fracture of steels, even without hydrogen, the deleterious contribution of hydrogen to the loss of steel strength is markedly enhanced [20,22,37,40].
Metals 2021, 11, x FOR PEER REVIEW 12 of 16 CLT were performed, at varying loads under continuous electrochemical hydrogen charging, after 2 h of pre-charging with parameters sufficient to provide the critical concentration of hydrogen in the studied steel.It was reported [7,20] that hydrogen in steels exposed to constant load segregates and modifies the stress fields of dislocations, enhancing their movement at lower stress levels.The enhanced movement of dislocation may be responsible for the observed increase in creep rates, with increasing hydrogen content corresponding to the proposed hydrogen enhanced local plasticity (HELP) mechanism [7,19,42].
It is worth noting that the experimental data points in Figure 8b were obtained at different applied loads.Hence the possibility that the increase in creep rates may not only be primarily due to increasing hydrogen but also increasing loads.Whereas increasing loads are a contributing factor to increased measured hydrogen for specimens charged using the same parameters.Higher applied loads result into more deformation, enhancing the ability to uptake more hydrogen.To eliminate the increasing contribution of applied loads on the creep rates, further CLT were conducted at the same applied load, corresponding to a stress level of 1400 MPa: (i) In air, (ii) in hydrogen charging conditions providing lower hydrogen content than the critical concentration, and (iii) in hydrogen charging expected to provide about the critical concentration of hydrogen.The results are summarized in Table 2.
It is difficult to assert concretely that the fracture of steels upon reaching measured hydrogen critical concentration is due to only local or total average hydrogen activity in the steel.However, the synergic interplay between local hydrogen concentrations and local stress states, accompanied with total average hydrogen presence reducing the general plasticity of the specimen, should be responsible for the observed degradation of strength of the steel around the critical concentration zone on the embrittlement curve.Despite, in literature, the term critical hydrogen concentration, in its definition, being more related to the local concentration of hydrogen responsible for crack nucleation, its measurement is difficult, as it may have diffused out of the sample at the actual time of the fracture [20,41].
CLT were performed, at varying loads under continuous electrochemical hydrogen charging, after 2 h of pre-charging with parameters sufficient to provide the critical concentration of hydrogen in the studied steel.It was reported [7,20] that hydrogen in steels exposed to constant load segregates and modifies the stress fields of dislocations, enhancing their movement at lower stress levels.The enhanced movement of dislocation may be responsible for the observed increase in creep rates, with increasing hydrogen content corresponding to the proposed hydrogen enhanced local plasticity (HELP) mechanism [7,19,42].
It is worth noting that the experimental data points in Figure 8b were obtained at different applied loads.Hence the possibility that the increase in creep rates may not only be primarily due to increasing hydrogen but also increasing loads.Whereas increasing loads are a contributing factor to increased measured hydrogen for specimens charged using the same parameters.Higher applied loads result into more deformation, enhancing the ability to uptake more hydrogen.To eliminate the increasing contribution of applied loads on the creep rates, further CLT were conducted at the same applied load, corresponding to a stress level of 1400 MPa: (i) In air, (ii) in hydrogen charging conditions providing lower hydrogen content than the critical concentration, and (iii) in hydrogen charging expected to provide about the critical concentration of hydrogen.The results are summarized in Table 2. Sample A did not fracture after 85 h, while samples B and C were fractured after 11 and 0.52 h under load, respectively.As shown in Figure 12, the increase of hydrogen content up to the critical concentration increases, markedly, the creep rates by more than two orders of magnitude in the studied steel under the same loads.This is comparable with values reported for different grades of cast iron [40] where creep rates increased for hydrogen charged samples as compared to that tested in distilled water.However, it is important to note that the effects of critical hydrogen concentration on creep rates may be dependent on experimental conditions, especially strain rates used to attain the constant load regime, but this need more investigation beyond the scope of the present paper.In addition, more work is required to ascertain effective methods that can enhance HE susceptibility of the studied steel.It has been suggested that suitable tempering after hot rolling could be applied to enhance the HE properties as well as to obtain maximum mechanical properties [43]; however, this requires further investigation to ascertain its effectiveness for the studied steel.
Metals 2021, 11, x FOR PEER REVIEW Sample A did not fracture after 85 h, while samples B and C were fra and 0.52 h under load, respectively.As shown in Figure 12, the increase of tent up to the critical concentration increases, markedly, the creep rates by orders of magnitude in the studied steel under the same loads.This is co values reported for different grades of cast iron [40] where creep rates inc drogen charged samples as compared to that tested in distilled water.How portant to note that the effects of critical hydrogen concentration on creep dependent on experimental conditions, especially strain rates used to atta load regime, but this need more investigation beyond the scope of the pr addition, more work is required to ascertain effective methods that can en ceptibility of the studied steel.It has been suggested that suitable temperin ing could be applied to enhance the HE properties as well as to obtain maxi ical properties [43]; however, this requires further investigation to ascerta ness for the studied steel.

Conclusions
The effects of hydrogen on mechanical performance of a 600 HBW m were evaluated through CERT and CLT under continuous hydrogen cha

Figure 1 .
Figure 1.Microstructure of the studied steel showing dominantly a martensitic microstructure with bainitic islands indicated by yellow arrows (after etching with Nital 2% for 20 s).

Figure 1 .
Figure 1.Microstructure of the studied steel showing dominantly a martensitic microstructure with bainitic islands indicated by yellow arrows (after etching with Nital 2% for 20 s).

Figure 2 .
Figure 2. (a) Dimensions of mechanical testing (MT) specimens; (b) MT specimens prior to H-charging; (c) assembly of MT specimen with the electrochemical cell and loading unit for tensile testing (CERTs and CLTs) under continuous H-charging.

Figure 2 .
Figure 2. (a) Dimensions of mechanical testing (MT) specimens; (b) MT specimens prior to Hcharging; (c) assembly of MT specimen with the electrochemical cell and loading unit for tensile testing (CERTs and CLTs) under continuous H-charging.

Figure 3 .
Figure 3. Schematic view of the thermal desorption spectroscopy apparatus.

Figure 3 .
Figure 3. Schematic view of the thermal desorption spectroscopy apparatus.

Figure 4 .
Figure 4. (a) Measured hydrogen concentration as a function of hydrogen charging time; (b) measured hydrogen concentration as a function of applied electrochemical potential; (c) effect of NH4SCN concentration on the measured hydrogen concentration after hydrogen charging for 2 h at the applied potential of −1.2 VSCE.

Figure 4 .
Figure 4. (a) Measured hydrogen concentration as a function of hydrogen charging time; (b) measured hydrogen concentration as a function of applied electrochemical potential; (c) effect of NH 4 SCN concentration on the measured hydrogen concentration after hydrogen charging for 2 h at the applied potential of −1.2 V SCE .

16 Figure 5 .
Figure 5. (a) Engineering stress versus strain curves of studied steel showing the effect of hydrogen on the tensile property of studied steel; (b) hydrogen embrittlement curve of tested steel showing a clear lower plateau at 1100 MPa.

Figure 5 .
Figure 5. (a) Engineering stress versus strain curves of studied steel showing the effect of hydrogen on the tensile property of studied steel; (b) hydrogen embrittlement curve of tested steel showing a clear lower plateau at 1100 MPa.

Figure 7 .
Figure 7. CLT-based results with applied load as a function of time charged specimens are performed at continuous hydrogen chargin conditions corresponding to the critical hydrogen concentrations.

Figure 7 .
Figure 7. CLT-based results with applied load as a function of time to fracture.The CLT of hydrogen charged specimens are performed at continuous hydrogen charging, after 2 h of pre-charging under conditions corresponding to the critical hydrogen concentrations.

Figure 8 .
Figure 8.(a) Engineering strain versus time curve from CLT at 1400 MPa under continuous hydrogen charging; (b) creep rates as a function of hydrogen concentration.

Figure 8 .
Figure 8.(a) Engineering strain versus time curve from CLT at 1400 MPa under continuous hydrogen charging; (b) creep rates as a function of hydrogen concentration.

Figure 12 .
Figure 12.Creep rates in CLT under applied stress of 1400 MPa as a function of mea concentration.

Figure 12 .
Figure 12.Creep rates in CLT under applied stress of 1400 MPa as a function of measured hydrogen concentration.

Table 1 .
Chemical composition and mechanical properties of the studied steel.

Table 1 .
Chemical composition and mechanical properties of the studied steel.

Table 2 .
Effect of increasing critical hydrogen content on creep rates during constant load tests.

Table 2 .
Effect of increasing critical hydrogen content on creep rates during consta