Evaluating the Effect of Hydrogen on the Tensile Properties of Cold-Finished Mild Steel

Abstract

One of the major sources of catastrophic failures and deterioration of the mechanical properties of metals, such as ductility, toughness, and strength, in various engineering components during application is hydrogen embrittlement (HE). It occurs as a result of the adsorption, diffusion, and interaction of hydrogen with various metal defects like dislocations, voids, grain boundaries, and oxide/matrix interfaces due to its small atomic size. Over the years, extensive effort has been dedicated to understanding hydrogen embrittlement sources, effects, and mechanisms. This study aimed at assessing the tensile properties, toughness, ductility, and susceptibility to hydrogen embrittlement of cold-finished mild steel. Steel coupons were subjected to electrochemical hydrogen charging in a carefully chosen alkaline solution over a particular time and at various charging current densities. Tensile property tests were conducted immediately after the charging process, and the results were compared with those of uncharged steel. The findings revealed a clear drop in toughness and ductility with increasing hydrogen content. Fracture surfaces were examined to determine the failure mechanisms. This evaluation has enabled the prediction of steel’s ability to withstand environments with elevated hydrogen concentrations during practical applications.

1. Introduction

Hydrogen embrittlement (HE) is the deterioration of metals’ mechanical qualities, particularly their ductility, due to extended exposure to hydrogen-concentrated conditions [1] (Figure 1). HE is prevalent in metals such as low-alloy steel, precipitation-hardening steels, super alloys, and aluminum alloys. Due to the small radius of hydrogen atoms, they enter steel in the form of atoms. High-pressure hydrogen gas, electrochemical hydrogen charging, and corrosion reactions are the typical sources of hydrogen in metals. For high-pressure hydrogen gas, the dissolution of hydrogen involves three steps, namely: physical adsorption, chemical absorption, and hydrogen dissolution. Electrochemical hydrogen charging is determined by parameters such as the charging current density, potential, time, type of electrolyte, pH, and temperature.

The degree of embrittlement experienced by a metal is influenced both by the amount of hydrogen absorbed and the microstructure of the material [2]. Furthermore, stress and plastic strains have significant effects on accelerating hydrogen embrittlement by enhancing hydrogen diffusion, trapping, increasing dislocation density, and facilitating crack initiation and propagation, making materials more susceptible to brittle failure [3]. Metals with body-centered cubic and hexagonal close-packed structures are most prone to HE, whereas face-centered cubic metals exhibit little to no susceptibility to HE. Hydrogen has a very high mobility in the BCC lattice of carbon and low-alloy steels [2,4]. HE failures are low stress events that result in brittle fractures and frequently cause massive financial losses and disastrous events. Since the discovery of the adverse effects of HE on the mechanical properties of materials, it has been the subject of intense industry and academic research employing multi-scale experimental and theoretical techniques, particularly in recent times, to understand the mechanism(s) behind it and means to reduce the impact of damage.

Results and findings from the preceding research have indicated that hydrogen in metals reduces their fatigue strength [5,6], fracture toughness [7,8,9], and macroscopic and microscopic tensile strength [10]. Overall outcomes indicate a detrimental effect on the mechanical performance of metallic structures upon the introduction of hydrogen into various traps of hydrogen.

In the presence of hydrogen, the rate of crack propagation under cyclic loading is significantly increased. Hydrogen aids in the formation of microvoids and assists in their coalescence into cracks, which rapidly expand under repeated stress cycles. The presence of hydrogen reduces the threshold stress intensity factor (K), a measure of the stress intensity range below which fatigue crack growth does not occur [11,12]. This means that even lower stress amplitudes can lead to crack growth in hydrogen-embrittled steel, severely reducing the fatigue life [6,13]. The cumulative effect of enhanced dislocation mobility and bond weakening leads to a substantial reduction in the fatigue life of steel. This is characterized by an earlier onset of fatigue crack initiation and faster crack growth rates, which collectively reduce the number of cycles the material can withstand before failure [6]. Examination of fracture surfaces in hydrogen-embrittled steel typically reveals features such as intergranular cracking, quasi-cleavage facets, and secondary cracking, which are indicative of the mechanisms.

In some steels, particularly those containing titanium or vanadium, hydrogen forms metal hydrides which are brittle and prone to cracking [14]. These hydrides precipitate at grain boundaries or within grains, creating internal stress concentrations that act as crack nucleation sites. Hydrogen atoms occupy interstitial sites in the steel’s crystal lattice, creating zones of high volumetric strain due to lattice expansion [15,16]. This local lattice distortion weakens bonds and facilitates crack initiation under tensile stress. The hydrogen-enhanced decohesion (HEDE) mechanism fundamentally affects the metallic bonds within the steel. Hydrogen reduces the cohesive force between iron atoms, particularly at grain boundaries [17], leading to easier separation of the grains under stress and promoting intergranular fracture [18]. The presence of hydrogen restricts the size of the plastic zone at a crack tip. Normally, the plastic zone helps to blunt the crack tip and spread the stress over a larger area, thus impeding crack growth. Hydrogen’s effect on dislocation mobility can paradoxically increase local plasticity but decrease the overall ductility of the material [7,8], leading to a sharper crack tip and enhanced crack propagation. Hydrogen can alter the typical fracture path in steel, promoting brittle fracture mechanisms such as cleavage or quasi-cleavage instead of ductile tearing. This change in fracture mode is associated with a reduced energy absorption capacity of the material, thereby lowering its fracture toughness [9].

Reduced cohesion between metal atoms, particularly at critical stress concentrators such as inclusions, voids, grain boundaries, and residual stresses from processing, promotes crack initiation and propagation at lower stress levels than in non-embrittled steel, which can lead to both intergranular and transgranular [19] fractures, which are more brittle in nature and contribute to a lower macroscopic and microscopic tensile strength [10].

Figure 1 shows various hydrogen traps which capture and localize hydrogen atoms, intensifying their negative effects on material properties. Dislocations are linear defects in the crystal structure of metals where the regular ordering of atoms is disrupted [20]. These defects provide sites for hydrogen atoms to collect, particularly at the dislocation cores. The strain fields around dislocations increase the chemical potential for hydrogen, making these areas preferred sites for hydrogen accumulation. The accumulation of hydrogen at dislocations can significantly enhance localized plasticity, leading to the hydrogen-enhanced localized plasticity (HELP) mechanism, which contributes to embrittlement. Grain boundaries, which are the interfaces between different crystalline grains, are regions of high atomic mismatch and thus have excess free volume and energy, making them favorable sites for hydrogen segregation [21,22]. Hydrogen accumulation at grain boundaries can weaken the metallic bonds at these locations, promoting intergranular cracking through the HEDE mechanism.

Vacancies act as effective traps for hydrogen atoms because the missing atom in the lattice creates a space into which a hydrogen atom can fit, forming a stable hydrogen-vacancy complex [23]. Hydrogen atoms trapped in vacancies can diffuse through the material, leading to the formation of hydrogen clusters that can initiate cracks or facilitate crack propagation. Microvoids and cracks can act as physical traps for hydrogen. These features can develop during manufacturing processes or due to corrosion. When hydrogen collects in these voids, it can lead to increased local pressure and stress concentration, which exacerbate crack initiation and propagation. Interfaces between different materials, such as those in multi-layered steels or coatings, can also serve as hydrogen traps. The difference in material properties across the interface creates a barrier to hydrogen diffusion, causing hydrogen to accumulate at these interfaces. The accumulation of hydrogen at material interfaces can lead to delamination or decohesion, particularly under tensile stress.

Although many studies have been carried out on the hydrogen embrittlement of steels, there is still insufficient data in the open literature to determine the level of hydrogen that would constitute a threshold for the safe operation of mild steel subjected to hydrogen environments. Regulatory bodies and industry standards organizations rely on the latest research to develop and update guidelines for the safe use of materials. Ongoing research ensures these standards are based on the most current understanding of HE, promoting safer engineering practices. However, despite the extensive existing research, the precise mechanisms of HE are not fully understood. There are multiple theories and models, but no single comprehensive explanation. Further study is needed to develop a more complete understanding, which can lead to more effective prevention and mitigation strategies.

Due to increased interest in lowering carbon emissions by blending natural gas with hydrogen [24,25], there is a renewed interest in HE in low-carbon steel, as such steels are traditionally used to transport natural gas. To achieve such a goal, more studies need to be carried out and more data generated. This study seeks to establish a trend on the behavior of the tensile properties of cold-finished mild steel under varying hydrogen concentrations.

2. Experimental Method

2.1. Electrochemical Charging

Cold-finished mild steel (AISI 1018) coupons were taken through a cathodic hydrogen process such that the samples acted as cathodes. The charging cell setup used (Figure 2) had two electrodes, working and counter electrodes, whose functions were to induce a reduction reaction in the steel sample and to complete the circuit to allow the flow of the current, respectively. A potentiostat with a maximum current capacity of 1 A was connected to supply a constant current to the samples through the working electrode. Charging was performed for three hours for all samples to enable comparison between steels subjected to different charging current densities.

The temperature and pH of the solution (electrolyte) in the cell were determined by immersing a thermometer and pH meter in the solution. The setup was connected to a computer system with software (Aftermath) in which charging parameters such as the charging time, charging current, and data collection rate were entered. The software produced a plot of the current and potential against the charging time. In order to ensure accuracy in the outcome, all the samples had the same orientation during the charging process. The threads of the samples were masked with chemical-resistant tapes to ensure that only the gauge surface area of the samples experienced charging. Prior to the charging process, the samples were cleaned with acetone to remove oil and debris from the surfaces.

Alkaline solution (NaOH) with an average pH and temperature of 11.4 and 23.1 °C, respectively, was used as the electrolyte in the charging cell. A recombination poison, ammonium thiocyanate (NH₄SCN), was added to the solution to prevent hydrogen atoms from recombining into hydrogen molecules (H₂). Uniformity of the solution was achieved using a magnetic stirrer, after which it underwent deaeration through argon purging for approximately 5 min.

The hydrogen concentration was determined by the indirect method based on the charging current and time [26].

2.2. Tensile Testing

In this study, samples of cold-finished mild steel were used. Samples had respective surface and cross-sectional areas of 373.3 mm² and 8.6 mm². In order to have a good overview and comprehensible trend of the behavior of the samples under different hydrogen concentration conditions [0.00, 0.03, 0.05, 0.07, 0.10, 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, 1.20, 1.40, 1.60, 1.80 and 2.00 wppm], 19 samples of cold-finished mild steel were prepared.

Each of the samples was subjected to uniaxial tensile testing. This was performed to enable comparison of the variation in the mechanical (tensile) properties of the samples before and after charging under different conditions resulting from the diffusion of hydrogen into samples. The samples were fixed in the upper and lower grips of the tensile tester. An extensometer with a 1-inch gauge length was connected to the gauge section of the samples to accurately measure the strain. The setup included a computer system (Figure 3) and a software package, PASCO Capstone, for data collection.

A crosshead speed of 5 mm/min and tensile strain rate of 0.2 min⁻¹ were applied to each sample until failure.

The samples were prepared in dog-bone shapes, as shown in Figure 4a, with the specified dimensions.

2.3. Characterization Techniques

2.3.1. Chemical Composition Analysis

To achieve the highest accuracy level possible, the chemical composition of cold-finished mild steel samples was analyzed. This was performed to investigate the probable presence of dissolved hydrogen in the samples possibly stemming from processing methods, preparation, storage, and transportation conditions, as this could affect the outcome of the experiment.

The inductively-coupled plasma (ICP) spectroscopy technique was used to achieve this outcome. This technique has a multi-element capability, which allows multiple elements to be measured simultaneously in a single analysis and measures and identifies elements within a sample matrix based on the ionization of the elements within the sample.

The results obtained were compared with the reported ASM standard composition for mild (low-carbon) steels [27], as shown in Table 1 and Table 2.

2.3.2. Metallography

The structure; spatial distribution of grains, voids, and inclusions; and phases in the cold-finished mild steel samples were examined. The samples were sectioned, mounted, ground, polished, and etched as outlined in the standards [28,29]. After preparation, the aforementioned characteristics were observed under both a scanning electron microscope and a confocal microscope to improve the analysis at low and high magnifications. Rockwell hardness testing was conducted on the sample using B scale according to refs. [30,31].

2.3.3. X-ray Diffraction Analysis

The samples of cold-finished mild steel were analyzed using the X-ray diffraction technique. A portion of the sample was sectioned to a height of 2.5 mm using an abrasive cutting machine. The surface of the sectioned sample was polished and cleaned. This was performed using the advanced Bruker D8 Advance X-ray diffraction (XRD) system (Bruker, USA) featuring a high-speed LynxEye™ detector and a copper tube, operating at 40 kV and 40 mA, and utilizing Cu Kα radiation. Samples were scanned over a 2θ range from 20° to 100° with a step size of 0.049°. The diffraction patterns obtained were analyzed with Bruker’s EVA software and compared against the International Centre for Diffraction Data (ICDD) Powder Diffraction File (PDF) database for pattern matching. The obtained diffraction pattern of this analysis is discussed below.

3. Results and Discussion

3.1. Microstructure (XRD, Rockwell Hardness, Confocal, SEM)

The X-ray diffraction (XRD) pattern of the cold-finished mild steel sample exhibited a clearly defined iron (Fe) peak with a body-centered cubic structure, suggesting that the material is crystalline, as given by the Powder Diffraction File named in Figure 5 below.

The sample under study had an average hardness of 97.38 HRB. A Rockwell hardness value of 97.38 HRB suggests that the mild steel samples have a significant degree of resistance to indentation or penetration by a hard object. This level of hardness is suitable for many engineering and structural applications where moderate strength and hardness are required, such as gas pipelines.

The cross-sectional surface of the polished and etched sample was analyzed under a confocal microscope and scanning electron microscope for comparison at low and high magnification, respectively. In both cases, the grains and grain boundaries were clearly defined by primarily ferrite and pearlite phases, as shown in Figure 6 and Figure 7 below.

A uniform grain size was observed across the samples. The grains were equiaxed, with roughly equal dimensions in all directions. The microstructure consisted of a mixture of ferrite (α-iron), a body-centered cubic (BCC) phase with relatively large, soft, and ductile grains, and pearlite, with a lamellar structure of alternating layers of ferrite and cementite (Fe3C) providing strength and hardness to the steel. Imaging from the scanning electron microscopy showed the presence of microvoids which could serve as trapping sites for induced hydrogen. The indications ‘g’ and ‘gb’ denote grains and grain boundaries, respectively.

3.2. Tensile Properties

As shown in Figure 8 below, the percent elongation (ductility) and toughness of the samples decreased gradually with increasing hydrogen content from 7.1% for the uncharged sample through to 4.4% for the 2.0 wppm-charged sample. This occurred as a result of embrittlement from hydrogen trapping, HEDE, internal stress formation, and hydrogen-induced strain localization. Hydrogen atoms have a high affinity for certain defects in the metal lattice, such as dislocations, grain boundaries, and vacancies. These defects act as traps for hydrogen atoms, effectively concentrating them in localized regions within the metal. This accumulation of hydrogen atoms can lead to embrittlement by weakening the material at these specific sites, reducing its ability to deform plastically and elongate before failure.

Hydrogen atoms also weaken the atomic bonds in the metal lattice, making it easier for cracks to propagate. In most cases, hydrogen does not substitute metal atoms in the lattice but occupies interstitial sites within the lattice. This introduces lattice strain due to the size difference between hydrogen and the metal atoms, leading to local distortions in the lattice structure. These distortions weaken the overall bonding within the lattice. This process, known as hydrogen-enhanced decohesion, reduces the material’s ductility and elongation by promoting premature crack initiation and propagation, even under relatively low stress levels.

The presence of hydrogen in the metal lattice can induce stresses due to the size mismatch between hydrogen atoms and the metal lattice. These internal stresses can lead to the formation of microcracks and voids, which act as nucleation sites for subsequent crack propagation. As cracks propagate through the material, they reduce its ability to elongate before failure, resulting in decreased elongation.

Induced hydrogen atoms also promote strain localization in the metal lattice, a phenomenon termed hydrogen-induced strain localization [32], leading to the formation of localized regions of high stress concentration. These stress concentrations can accelerate crack initiation and propagation, further reducing the material’s elongation and ductility.

The obtained results align closely with those found by researchers [33,34], emphasizing the consistency and reliability of our findings in this area of study. This agreement in outcomes enhances the credibility of our individual research efforts and contributes significantly to the body of evidence supporting our shared hypotheses. This corroboration between our studies is particularly encouraging, as it paves the way for further exploration and validation of these findings within the scientific community.

The results further demonstrated a distinct correlation between elongation and toughness, as illustrated in Figure 9. Both properties displayed synchronized fluctuations, increasing and decreasing together, which indicated a consistent pattern of behavior. This simultaneous change in elongation and toughness is indicative of a continuous degradation process, where the material exhibits an altered ductility and resistance to fracture. This trend provides evidence that the material undergoes progressive embrittlement due to the continuous introduction of hydrogen into its structure. This embrittlement effect, reflected in both the elongation and toughness measurements, highlights the detrimental impact of hydrogen on the mechanical properties of the material. As aforementioned, HE occurs when hydrogen atoms diffuse into the material’s lattice, weakening its structure and making it more prone to fracture under stress. Embrittling occurs as hydrogen builds up within the material’s internal voids and microstructural defects, creating high internal pressures that can initiate and propagate cracks. A key mechanism involved is HEDE [35], where hydrogen weakens metallic bonds at grain boundaries or interfaces, facilitating crack nucleation. Additionally, hydrogen can prompt the formation of brittle hydrides at localized sites, further contributing to embrittlement [36]. At the tips of cracks, hydrogen adsorption is known to increase dislocation emission [37]—a displacement of atoms within the crystal structure—exacerbating crack expansion. Collectively, these mechanisms lead to the degradation of the steel’s structural integrity, which contributes to the observed changes in both elongation and toughness behaviors.

The drop and subsequent slight increase in elongation and toughness with increasing hydrogen content may be explained as follows. Initially, as hydrogen content increases, hydrogen atoms diffuse into the steel and accumulate at microstructural defects such as grain boundaries, dislocations, and voids. This process weakens the steel by promoting crack initiation and propagation, leading to a significant reduction in ductility and elongation. As the hydrogen content increases further, its influence on dislocation dynamics at higher concentrations can sometimes lead to conditions where dislocation movement is facilitated, resulting in a temporary increase in ductility. This phenomenon is known as hydrogen-enhanced localized plasticity (HELP) [38,39,40]. The HELP mechanism suggests that hydrogen atoms reduce the energy barriers for dislocation motion. Hydrogen atoms can accumulate at dislocations and create local stress fields that lower the resistance to dislocation movement. This makes it easier for dislocations to glide through the crystal lattice, thereby enhancing plasticity and potentially increasing ductility.

The obtained results from this part of the study contrast with the results reported by the researchers in refs. [7,8], where a reduction in toughness was observed throughout the experiment. The disparity in experimental outcomes emphasizes the intricate factors influencing material behavior under hydrogen exposure. Minor variances in chemical composition or microstructure can dramatically affect mechanical performance and hydrogen interaction. Experimental variables, such as the specifics of equipment, procedures, and environmental settings like temperature and pressure, also play critical roles in divergent results. The degree of hydrogen exposure, defined by concentration and exposure time, could also add layers of variation to the data obtained. Pre-experimental history, involving any mechanical modifications or thermal processing, might have influenced various samples’ responses to hydrogen permeation. Measurement discrepancies arising from different techniques and calibration standards only further emphasize the nuanced and delicate nature of assessing hydrogen’s impact on materials.

The ultimate tensile strength and yield strength of the samples exhibited minimal variation (Figure 10), demonstrating a uniform response to the applied stresses. This consistency suggests that the effects of hydrogen induction were either negligible or uniformly distributed across the samples, indicating that under the specific experimental conditions used, the hydrogen atoms did not significantly compromise the mechanical integrity of the material with respect to these properties.

Contrasting this result to those reported by refs. [13,41], which observed a notable reduction in both macroscopic and microscopic tensile strengths, highlights a potential divergence in the material’s response to hydrogen. This contrast suggests that factors such as an inherent resistance to HE, a potentially lower than critical hydrogen concentration, or a microstructure less susceptible to hydrogen’s adverse effects might have contributed to the observed stability in this study.

In the context of industrial applications, especially those prone to hydrogen exposure, the ability of a material to reliably maintain consistent mechanical properties is of paramount importance. This consistency ensures a high degree of safety and reliability, mitigating the risks of material failure due to property fluctuation under working conditions.

Furthermore, the constancy observed in critical mechanical properties highlights their value in substantiating theoretical predictions and enhancing the understanding of material performance in hydrogen-rich environments. These insights are vital for the design and selection of materials suited for such demanding applications, ensuring longevity and performance integrity in scenarios where hydrogen plays a pivotal role.

Furthermore, it was observed that the elastic modulus of the material remained consistent despite the incremental hydrogen concentration, as evidenced by the linear trend in Figure 11. This suggests that the intrinsic stiffness of the steel samples and their ability to undergo elastic deformation when subjected to stress are not significantly influenced by varying levels of hydrogen absorption. The minimal variation in the elastic modulus across different hydrogen concentrations supports the expectation that elasticity, as an inherent material property, should remain consistent, thus accounting for the negligible changes observed in the elastic modulus of the samples.

The obtained outcome of relationship between varying hydrogen contents and the elasticity of steel is in line with the results of ref. [42], wherein a study was conducted on a generally similar material, X65 pipeline steel.

3.3. Fractography

Samples’ fracture surfaces were analyzed using a scanning electron microscope. Observed features are discussed below.

3.3.1. Ductile Failure (Uncharged Sample)

Analysis of an uncharged sample yielded characteristics such as dimples, microvoids, and shear lips, describing severe plastic deformation prior to fracture and, thus, a ductile mode of failure. Microscopic inclusions, second-phase particles, or other heterogeneities within the material might have served as formation sites for microvoids. Inclusions could be impurities, precipitates, or other discontinuities in the metal matrix around which stress concentration increases under tensile loading and the onset of intergranular cracks. Cracks act as nucleation sites for further crack propagation. As tensile stress was applied to the material, the crack propagated gradually, driven by the stress concentration at the crack tip. This process continued until the crack grew to a critical size.

The propagation of cracks along the grains led to the coalescence of voids, leaving microscopic impressions referred to as dimples on the surfaces after failure. Additionally, two observably distinct macroscopic regions, the cup and the cone, were created on the fractured surface. The cup central depression was characterized by a concave morphology, resembling the shape of an inverted cup, and represents the area where significant plastic deformation occurred prior to fracture. This plastic deformation results from the material’s ability to accommodate tensile stress through mechanisms such as dislocation movement.

Surrounding the cup region is the cone, which exhibits a more conical or slanted morphology. The cone region represents the area where the crack has propagated further into the material, with less plastic deformation compared to the cup region. The formation of the cone is influenced by the material’s deformation behavior and fracture toughness.

Observed dimples or depressions on the surface (Figure 12) formed as adjacent grains were pulled apart, indicating regions of microvoid nucleation and growth.

A shear lip (Figure 13) was observed along the edges of the fracture surface stemming from the reduced cross-sectional area (necking). Shear lips are raised ridges or lips that develop due to localized shear deformation near the crack tip during fracture propagation. These features indicate the dominance of shear stress components in the fracture process, often associated with plastic flow and material redistribution around the crack tip.

The surface area of the material near the failure region showed wrinkles (Figure 13) which resulted from stretching and thinning, indicating that the material underwent significant plastic deformation or localized instabilities due to high tensile stresses.

3.3.2. Brittle Fracture/Failure (Charged Samples)

Examination of the fracture surface characteristics of samples charged with 1.2 wppm (Figure 14a) and 2.0 wppm (Figure 14b) of hydrogen revealed distinct brittle features including cleavage facets, quasi-cleavage facets, river marks, microcracks, and transgranular fractures.

As previously discussed, the presence of diffused hydrogen atoms, concentrated at stress points such as preexisting cracks, voids, and material discontinuity can disrupt the atomic bonds within the steel matrix due to their occupation of interstitial sites within the crystal lattice. This disruption diminishes the material’s ductility and toughness, rendering regions laden with hydrogen more prone to failure as tensile stress escalates. In mild steel, hydrogen accumulation at grain boundaries instigates the formation of brittle fracture paths that traverse through the grains, rather than along the boundaries, a phenomenon termed transgranular fracture. Once initiated, cracks propagate through the grains, guided by the crystallographic structure, resulting in the formation of cleavage planes within the grains (see Figure 14).

Cleavage facets, discernible at a macroscopic level, manifest as expansive, flat, glossy surfaces perpendicular to the applied stress direction, signifying brittle fracture along specific crystallographic planes (Figure 14). Quasi-cleavage facets, resembling cleavage facets but with less distinct boundaries and possibly slight curvature, arise from localized deformation and stress concentration, rather than along predetermined crystallographic planes. They are often encountered in materials undergoing a transition from ductile to brittle behavior.

Microscopic river marks, or striations, present a unique feature distinct from macroscopic cleavage marks. These minute features, observable at a microscopic scale, manifest as parallel lines or grooves on the fracture surface, depicting the incremental advancement of the crack front as it propagates through the material. Typically perpendicular to the crack propagation direction, river marks offer insights into the fracture process at a finer resolution.

Further examination of the sample charged with 1.2 wppm hydrogen revealed evidence of microcracks (Figure 15b) adorned with sharp ridges (Figure 15a) along the edges of the cross-section, underscoring the intricate nature of the fracture process.

This observed outcome is in agreement with those obtained by researchers in refs. [43,44,45] concerning the fracture behaviors of Mn–Ni–Mo bainitic steel, DP steel, and DP1180GI steel coils, respectively, under tension.

Mixed failure mode surface analysis of samples charged with relatively lower hydrogen content indicated a mix of microscopic ductile and brittle features, as illustrated in Figure 16, where a sample was charged with hydrogen content of 0.4 wppm. This combination of characteristics suggests that the charging process may have been insufficient, resulting in some degree of post-charging ductility. In addition, inclusions were observed. Chemical composition analysis was not performed on the inclusion phases; however, sulphides and oxides are primarily formed from processing. Typical inclusions found in plain carbon steel are silica (SiO₂), manganese oxide (MnO), iron oxide (FeO), and manganese sulphide (MnS). These inclusions act as stress concentrations due to their differing mechanical properties compared to the parent material, ultimately leading to decohesion between grains.

The mixture of properties could also stem from variations in the microstructure. Metallographic analysis revealed that the samples typically consisted of a mixture of ferrite and pearlite phases, as depicted in Figure 6 above, which can significantly affect fracture behavior. In regions with a higher proportion of ferrite, ductile features may prevail due to their inherent toughness. Conversely, areas with a higher pearlite content may exhibit more brittle behavior, especially in the presence of HE.

Moreover, the distribution of applied tensile stress conditions could influence the fracture mode. In regions experiencing higher stress concentrations, such as those near defects or notch roots, brittle fracture is more likely to occur. Conversely, in areas with lower stress concentrations, the material may exhibit greater ductility before failure.

4. Conclusions

In this study, the tensile properties, particularly elongation, yield strength, ultimate tensile strength, and toughness, of cold-finished mild steel were analyzed with varying hydrogen contents in a simulated alkaline environment. The electrochemical hydrogen charging method employed in this study induced hydrogen into samples such that embrittling occurred as the hydrogen concentration was increased through a charging current. Below are some of the findings made by this study:

• Samples under study had a simple microstructure primarily containing ferrite and pearlite phases with some microvoids. The stress–strain behavior of tensile testing showed a gradual reduction in the percentage elongation and toughness with increasing hydrogen content, signifying embrittling from hydrogen diffusion into samples with extremities of 7.1% and 4.4% for uncharged and 2.0 wppm samples, respectively.

• During the fracture surface analysis, uncharged samples showed ductile features of dimpling, microvoid coalescence, and internagranular fractures as opposed to charged samples at 1.2 wppm and 2.0 wppm, which indicated embrittling with features like cleavages, microcracks, and transgranular fractures.

• It was also observed that at lower hydrogen concentration levels, i.e., 0.4 wppm, samples showed both ductile and brittle fracture features.

Author Contributions

E.S.: Writing initial draft, edit, Z.N.F.: Review, edit, supervision. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Schematic figure of the process of hydrogen diffusion into traps in a bulk alloy.

Figure 2. Setup of the cathodic charging process.

Figure 3. (a) Tensile test setup; (b) Fitted sample in grips with an extensometer.

Figure 4. (a) Schematic drawing of a tensile specimen with dimensions (mm); (b) Actual tensile specimen.

Figure 5. XRD patterns of cold-finished mild steel.

Figure 6. Microstructural features of the etched sample under a confocal microscope at 20× and 50× magnification.

Figure 7. SEM images of the etched sample: (a) 1500×, (b) 5000×.

Figure 8. Stress–strain plots for various hydrogen concentrations.

Figure 9. % Elongation/toughness vs. hydrogen content.

Figure 10. Ultimate tensile strength/yield strength vs. hydrogen content.

Figure 11. Elastic modulus vs. hydrogen content.

Figure 12. SEM images of the fracture surface of samples: (a) Uncharged sample, 1000×; (b) Uncharged sample, 1500×.

Figure 13. Cup and cone fracture (left) and wrinkling of the surface (right).

Figure 14. SEM images of (a) 1.2 wppm-charged and (b) 2.0 wppm-charged samples.

Figure 15. Observed brittle features on surface area of 1.2wppm specimen under SEM. (a) sharp-edged surface; (b) microcracks formation along edges.

Figure 16. Mixed fracture features of a 0.4 wppm-charged sample (1500×).

Table 1

Elemental composition results obtained from ICP spectroscopy analysis.

Element	Fe	C	Cr	Mg	Ni	S	Cu	Si	Mn
Composition (wt%)	98.14	0.14	0.07	0.0008	0.04	0.004	0.08	0.25	0.50

Table 2

ASM standard composition for mild (low-carbon) steels [27].

Element	C	Si	Mn
Compostion (wt%)	≤0.25	0.1–0.5	0.4–0.7

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