Effect of Hydrogen Pressure and Punch Velocity on the Hydrogen Embrittlement Susceptibility of Pipeline Steels Using Small Punch Tests under Gaseous Hydrogen Environments at Room Temperature

Abstract

The in situ small punch (SP) test method is a simple screening technology developed to assess the hydrogen embrittlement (HE) characteristics of structural steels. This method can easily adjust the influencing parameters such as test temperature, gas pressure, and punch velocity depending on the hydrogen service environment. With increased hydrogen consumption, using pipelines for mass hydrogen transportation is being considered. This study evaluated the HE susceptibility of API-X52 and API-X70 steels, considering the hydrogen usage environment. The study investigated the effects of hydrogen pressure and punch velocity on the HE behaviors of each pipe steel at room temperature using the SP energy and relative reduction in thickness (RRT) to determine their effect on HE susceptibility quantitatively. The study found that hydrogen pressure produced a different HE effect; the lower the hydrogen pressure, the more HE was relieved. Particularly, when the punch velocity was high, such as 1 mm/min, the HE effect was significantly relaxed. However, when the punch velocity was below 0.01 mm/min, HE occurred even at low hydrogen pressure conditions, meaning hydrogen diffusion within the specimen during the SP testing reached a critical hydrogen concentration to create a brittle fracture. Both pipeline steels showed similar HE behaviors under a wide range of H₂ pressures and punch velocities, showing an inverse S-curve for quantitative factors of SP energy and RRT against the H₂ pressure at 1.0 mm/min punch velocity. The study classified the observed HE behaviors into four types based on quantitative and qualitative aspects. These findings confirm that the in situ SP test is a useful screening technique, and the factor RRT can be effectively applied to the HE screening of pipeline steels in low and high-pressure hydrogen environments.

1. Introduction

To realize a hydrogen economy, establishing a supply chain through mass production, transportation, and storage of green hydrogen is an important issue [1]. For mass hydrogen transport, gaseous hydrogen transport through pipelines and liquid hydrogen transport have been considered. Pure hydrogen, or a blend of natural gas and hydrogen, can be transported through pipelines [2], and the economical method depends on handling volume and distance. Hydrogen can be distributed effectively through pipelines at a pressure typically below 100 bar. Pipelines offer an affordable way of distributing hydrogen over wide distances in large quantities, and the need for pipelines increases with increased hydrogen consumption volume and transport distance. Large-diameter pipes with submerged arc welding are suitable for long-distance deliveries, while small-diameter pipes adopting electric-resistance welding are suitable for short-distance delivery [3].

Pipes typically operate at relatively low pressures compared to hydrogen storage containers made of high-strength alloy steel. When transporting and distributing a hydrogen blend gas, it is crucial to understand how pipeline steels behave under low hydrogen partial pressure conditions. For bcc-structured steels with a high hydrogen diffusion rate, evaluating their hydrogen embrittlement (HE) compatibility quantitatively under external hydrogen (in situ) conditions considering a practical hydrogen-use environment is essential. However, evaluating the HE compatibility of steels under high-pressure conditions is typically carried out using slow strain rate tensile testing (SSRT) under external hydrogen conditions. This involves placing the specimen in a large autoclave with an explosion-proof function and testing it under high-pressure hydrogen environments [4,5,6]. This method is expensive due to large-scale facility operations and strict safety protocols. Alternatively, for austenite phase stainless steels, testing can be conducted under internal hydrogen conditions (ex situ) using hydrogen pre-charged specimens [7,8,9]. However, considering the expected increase in test numbers due to chemical compositions and microstructure adjustments, it becomes challenging to perform SSRT under external hydrogen conditions using autoclaves. Therefore, simplified test methods have been proposed to screen HE susceptibility without autoclave equipment. One of these methods involves using a hollow test specimen filled with high-pressure hydrogen inside the hole and testing it in ambient conditions [10,11]. This enables SSRTs with hollow tensile specimens to evaluate the HE susceptibility of materials under high-pressure conditions, even at cryogenic and elevated temperatures. This test method is close to being established as an international standard [12].

The small punch (SP) test is another simple method for evaluating steels’ HE behavior. However, most SP tests are performed using hydrogen (H₂) pre-charged specimens (ex situ) equivalent to internal hydrogen conditions [13,14]. However, it has been pointed out that they provide different results from HE evaluation under external hydrogen conditions in a high-pressure environment [7,13]. The SP test is helpful in investigating non-uniform features such as welds due to miniature-sized samples [15,16,17]. Recently, to overcome the difficulty of applying hydrogen-precharged specimens to bcc-structural steels, a method of penetrating hydrogen by electrochemical-hydrogen charging while loading the test specimen has been proposed, called in situ testing [18]. However, even in this case, a high hydrogen concentration exists at the region near the specimen surface, so adjusting the compatibility with the actual environment is still a key issue.

We recently developed a simple in situ SP test method that evaluates the HE susceptibility of structural steels under high-pressure external hydrogen environments without relying on large-scale high-pressure vessels [19]. It can directly assess the embrittlement effect of hydrogen penetrating through the surface of the specimen and diffusing into the material during in situ SP testing. We tested the mechanical performances without and with the hydrogen effect of several metallic materials by in situ SP tests [19,20,21]. They employed reduction in thickness (ROT) and relative reduction in thickness (RRT) to quantitatively access the HE susceptibility, similar to the used ductility loss of the area reduction (RA) and relative reduction in area (RRA) in tensile testing results. The results showed the fitness for applying SP tests to evaluate the HE effects. However, unlike conventional uniaxial tensile tests, the SP test does not provide straightforward information such as the stress and induced strain. Therefore, we also conducted a numerical analysis to obtain information on crack formation due to the HE effect by SP testing, such as the stress and strain distribution [22].

Recently, Garcia et al. investigated the effects of strain rate and hydrogen contents on HE for pipeline steel of API X65 steel through SP tests with the online hydrogen pre-charging technique [23,24]. Tao et al. also reviewed HE characterization using micro-sample testing [25]. In this review, the HE sensitivity achieved through small sample tensile testing, SP testing, and nanoindentation testing is quantitatively analyzed and discussed. However, due to the limitations of these online hydrogen charging test methods, they could not provide a correlation with the hydrogen pressure used. Similarly, the effect of strain rate on the HE behaviors of austenitic stainless steels has been studied extensively by performing in situ tests under external hydrogen conditions. A significant strain-rate effect on HE susceptibility has been observed, and researchers are trying to explain the relationship between the strain-rate effect during SSRT and the diffusivity of hydrogen atoms on metallic microstructures [26,27,28]. It is well-established that HE susceptibility increases with a decrease in the strain rate or an increase in the hydrogen content [29,30].

It is essential to study how hydrogen pressure and punch velocity affect the quantitative screening factors for the HE susceptibility of pipeline steels, depending on the types of hydrogen transported through pipelines, which may be pure H₂ or blend H₂. In the case of a blend of natural gas and hydrogen, HE susceptibility under low hydrogen partial pressure environments should be examined under external hydrogen conditions. However, there are little data available on this topic [31]. In particular, the effect of strain rate (punch velocity for SP testing) on the HE behavior may differ under low hydrogen partial pressure compared to high-pressure hydrogen conditions. Although it is not easy to evaluate them using the conventional hydrogen-precharged specimen or even using online in situ methods, in situ SP testing in a gaseous hydrogen environment can easily be used to perform these evaluations. Therefore, it is worthwhile to investigate the effect of strain rate, i.e., punch velocity, on the behavior of reaching the critical hydrogen concentration for hydrogen-induced cracking due to HE effects during SP testing at low hydrogen partial pressure in external hydrogen environments [32].

Therefore, in this study, we aimed to examine the influences of hydrogen pressure and punch velocity on the HE behaviors of API X52 and X70 steels, which are bcc-structured pipeline steels, using the in situ SP test method under gaseous hydrogen environments. The in situ SP test was conducted under a wide range of hydrogen and nitrogen gas pressures and different punch velocities at room temperature (RT). The influence of hydrogen pressure and punch velocity on HE behaviors was examined qualitatively and quantitatively. This process provides a fundamental understating for screening HE susceptibility of bcc-structured pipeline steels for the hydrogen service.

2. Experimental Procedures

2.1. Specimens

Specimens used in this study were machined from two commercial-grade pipeline steels: API X52 steel and API X7 steel pipes. The API-X52 steel pipe had an outer diameter of 215 mm (8 in) and a wall thickness of 15.0 mm, while the API-X70 steel pipe had an outer diameter of 762 mm (30 in) and a wall thickness of 15.9 mm. The pipes were supplied by POSCO and KOGAS, respectively.

Table 1. Chemical composition of supplied steels (in wt. %).

	Elem’	Fe	C	Mn	P	S	Cr	Ni	Cu	Others
Mat’
API X52		Bal.	0.03	0.8	<0.01	<0.01	Ni + Cr + Mo + Cu < 0.5			Ti + Nb + V ≤ 0.1
API X70		Bal.	0.07	1.68	0.012	0.01	0.07	0.14	0.10	Ti + Nb + V ≤ 0.15

displays the chemical compositions of both types of steel used, and

Table 2. Mechanical properties of supplied steels at room temperature.

	Property	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)
Materials
API X52 steel		449	515	26.7
API X70 steel		600	631	26.3

lists their mechanical properties at room temperature. The microstructure of API X52 steel mainly consisted of a coarse polygonal ferrite and fine pearlite structure. In contrast, the API X70 steel had acicular ferrite and granular ferrite with a small amount of bainitic ferrite. Based on the microstructure, the API X70 steel is expected to be superior in resistance to HE due to its acicular ferrite microstructure [33,34].

The specimens used for SP tests were wire cut directly from the pipes perpendicular to the pipe’s longitudinal direction without applying thermal treatment, as shown in Figure 1. The surfaces of the wirecut SP specimens were polished using 800-grit sandpaper. The dimensions of the SP specimen are 10 mm × 10 mm and 0.5 mm in thickness, a miniature size.

2.2. Experimental Procedure

Figure 2a shows a schematic diagram of the in situ SP test apparatus used to apply a pressurized gas environment to the specimen at RT and Figure 2b a cross-sectional view [19]. The test fixture consists of an upper and lower die, with the specimen placed between them. An O-ring was inserted into the groove formed in the lower die to prevent gas leakage during SP testing. The upper and lower dies were tightened to a uniform torque (1.5 N_m) using a torque wrench to maintain even pressure on the O-ring. The space between the specimen and the lower die was vacuumed and purged three times with high-purity nitrogen gas (N₂) to remove impurities [19,20]. Then, hydrogen was charged into the space to a specified pressure so that one side of the specimen was exposed to the external hydrogen condition. The SP test fixture was mounted to the material testing machine (Model: AG-IS, 5 kN load cell, Shimadzu, Osaka, Japan). The SP test was conducted by placing a 3 mm diameter steel ball (rated HRC60 or greater) on the specimen and applying a compressive load via a punch at specified punch velocities. The lower die had a 4 mm diameter hole.

Table 3. Test conditions of in situ small punch tests used to examine hydrogen embrittlement behaviors of pipeline steels.

Steel type	API X52 and X70 steels
Gas purity	N2 gas (99.999%), H2 gas (99.999%)
Gas pressure (MPa)	0.1, 0.2, 0.5, 1.0, 5, 8, 10 and 20
Punch velocities (mm/min)	1.0, 0.1, 0.01 and 0.004
Test temperatures (°C)	Room temperature (RT)

presents the test conditions adopted; SP tests were performed under eight different levels of N₂ and H₂ pressure ranging from 0.1 MPa to 20 MPa. Punch velocity was 0.1 mm/min under the N₂ environment, and four different punch velocities (1.0, 0.1, 0.01, and 0.004 mm/min) were used under the H₂ environment. The applied load and displacement were measured at each test condition.

Punch displacement obtained from the testing machine was used to infer specimen displacement at the midpoint of its lower surface. The in situ SP test was performed twice or three times at each test condition. The test was terminated if either a fracture of the specimen or a gas leak occurred during SP testing. After the in situ SP test, a load–displacement curve was obtained for each test condition. The area under the curve until specimen fracture was calculated as the energy absorbed, designated as the SP energy. If the final fracture did not occur abruptly, the SP energy was calculated until fracture displacement corresponding to the maximum load. The fractographic morphologies, such as the crack patterns produced on the specimen surface due to the HE effect and the crack surface, were observed under a scanning electron microscope (TESCAN, Model: Vega 2 LMS SEM/EDS, Prague, Czech Republic).

When the in situ SP test assessed the HE susceptibility of pipeline steels quantitatively, as a characterizing factor, the relative reduction in specimen thickness (RRT) was used together with SP energy [19]. This is a ductility-based factor similar to the relative reduction in area (RRA) obtained by SSRT, and the derivation process is as follows. First, the reduction in specimen thickness (ROT) is calculated using Equation (1) once the final thickness (t_f) and the initial thickness (t₀) of the specimen are measured after and before the in situ SP test. t_f is the average thickness measured at four locations at a 90-degree angle at the fractured part of the specimen using a point micrometer after the in situ SP test. The ROT values are determined in both gas environments of H₂ and N₂, and its ratio provides RRT as shown in Equation (2).

$$\mathrm{ROT}=\left(1-\frac{{\mathrm{t}}_{\mathrm{f}}}{{\mathrm{t}}_{\mathrm{o}}}\right)\times 100\%,$$

(1)

$$\mathrm{RRT}={\mathrm{ROT}}_{{\mathrm{H}}_2} /{ \mathrm{ROT}}_{{\mathrm{N}}_2.}$$

(2)

3. Experimental Results

3.1. Hydrogen Embrittlement Behaviors for API-X52 Steels

To investigate the behavior of HE in API-X52 steels, in situ SP tests were conducted under different N₂ and H₂ pressures at RT. The load–displacement curves obtained are shown in Figure 3. SP tests in the N₂ environment were performed at a punch velocity of 0.1 mm/min because there was no effect on punch velocity during the SP test. However, for the H₂ environment, SP tests were conducted with different punch velocities at each H₂ pressure to determine the correlation between H₂ pressure and punch velocity on HE behaviors.

The load–displacement curve obtained under the N₂ environment can be divided into five regions according to the deformation mechanism [35]. Increasing N₂ pressure slightly increased the maximum load (P_max) while the fracture displacement (δ_f) remained similar. However, under the H₂ environment, there was a significant HE effect leading to early fracture deviating from the curves under corresponding N₂ environments. This decreased maximum load and fracture displacement, as shown in Figure 3. The P_max and δ_f obtained at a punch velocity of 1 mm/min for two different H₂ pressure cases of 0.1 MPa and 20 MPa were marked in red arrow and compared.

The extent of the HE varied depending on the H₂ pressure used. At 1 MPa or lower H₂ pressures, there was a distinct difference in maximum load/fracture displacement depending on the punch velocity tested. However, the HE effect was greatly alleviated at higher punch velocity, and fracture occurred at Region V in the late part of the load–displacement curve. On the other hand, under conditions where the H₂ pressure was higher than 5 MPa, similar load–displacement curves were obtained regardless of the punch velocity tested, and fracture occurred in the late part of Region III or the early part of Region IV on the curve. Consequently, the effect of punch velocity on the HE behavior was significant during the hydrogen compatibility screening of API X52 steel in a low-pressure H₂ environment. However, it appeared insignificant with the noticeable HE under high-pressure H₂ conditions for bcc-structured steels with a high H₂ diffusion rate.

Figure 4a,b illustrates the macroscopic fracture appearances and fracture surface morphologies of API-X52 steel under different H₂ pressures and punch velocities compared to those under the corresponding N₂ pressure at RT. Fracture morphologies observed in N₂ environments typically represent large circular cracks on the specimen surface, as shown in Figure 4a, and dimple patterns (DP) on the crack surface, shown in Figure 4b, developed regardless of the N₂ pressure levels. On the other hand, in an H₂ environment, different crack shapes appeared depending on the H₂ pressure and punch velocity tested. At low H₂ pressure and high punch velocity, multiple circular-shaped cracks formed on the specimen surface exposed to H₂ gas. At relatively high H₂ pressure and low punch velocity, small ring-shaped cracks formed and became clearer with punch displacement. Further increase in punch displacement led to additional multiple short radial cracks around the ring crack when the H₂ pressure was higher than 8 MPa, as shown in Figure 4a. Additionally, the blue line shown in Figure 4a represents the change in fracture appearances depending on the hydrogen pressure and punch velocity. These crack patterns exhibit a unique form of brittle fracture due to the HE effect induced by in situ SP tests, with the crack surface characterized by a generally flat quasi-cleavage fracture (QC) surface, as shown in Figure 4b. When the H₂ pressure was less than 0.5 MPa, and the punch velocity was as high as 1.0 mm/min, a slight HE effect was observed, forming multiple larger circular cracks, and it showed a combination of some dimples and quasi-cleavage fractures on the fracture surface. These observed fracture morphologies support the deformation behaviors of load–displacement curves.

This study investigated the effects of H₂ pressure and punch velocity on the quantitative HE susceptibility factors for pipeline steels, specifically SP energy, ROT, and RRT, under an H₂ environment. Figure 5 shows the SP energy, ROT, and RRT variation for API-X52 steel against different H₂ pressures at RT. Although the punch velocity had an influence in low H₂ pressure ranges of less than 5 MPa, other punch velocities except 1.0 mm/min produced similar values. A significant HE occurred in the high-pressure H₂ pressure region over 5 MPa, but the punch velocity effect was insignificant.

Under the N₂ environment, SP energy and ROT had nearly a constant value regardless of the N₂ pressure exposed, of ~2.6 J and ~55%, respectively. However, under the H₂ environments, SP energy, ROT, and RRT varied significantly, depending on the H₂ pressure and punch velocity tested. Generally, higher H₂ pressure and lower punch velocity resulted in greater HE susceptibility. An inverse S-curve for HE behavior can be observed against the H₂ pressure [31]. Due to the HE phenomenon, a ductile–brittle transition pressure (DBTP) can be defined against the H₂ pressure at 1.0 mm/min punch velocity for API-X52 steel at RT. It was determined to be approximately 1.0 MPa H₂, based on the median of each influencing factor SP energy and RRT. In the low H₂ pressure ranges, the HE effect occurred except for a punch velocity of 1.0 mm/min, which has minimal effect on the HE effect. SP energy decreased to 1.5 J, and RRT was 0.7 at 0.1 MPa H₂ condition, declining further as H₂ pressure increased. Beyond 5 MPa H₂, significant HE occurred, but punch velocity had minimal effect on SP energy and RRT, with a value of approximately 1.0 J and 0.4 at 20 MPa H₂ condition. Therefore, when the punch velocity was low, applying the inverse S curve was not easy since the transition in HE behaviors was not significant enough.

It was observed that when the H₂ pressure was reduced to 0.1, 0.2, and 0.5 MPa with a punch velocity of 1.0 mm/min, the HE effect was significantly alleviated. The values approached those observed under the N₂. However, a similar extent of the HE effect was observed when the punch velocity was below 0.1 mm/min, regardless of the tested punch velocity. This means that hydrogen diffusion occurred sufficiently to reach the critical hydrogen concentration that causes brittle fracture due to the HE effects during in situ SP tests on API-X52 steel, even at relatively low H₂ pressures like 0.1 MPa (1 bar) and with a punch velocity of 0.1 mm/min.

3.2. Hydrogen Embrittlement Behaviors for API-X70 Steels

Load–displacement curves obtained through in situ SP tests for API X70 steel at RT under different N₂ and H₂ pressures are represented in Figure 6. These curves are similar to those obtained by in situ SP tests for API-X52 steel. For N₂ environments and a punch velocity of 0.1 mm/min, maximum load and fracture displacement for API-X7 steel were larger values when compared to API X52 steel. As N₂ pressure increased, the maximum load increased slightly while fracture displacement remained similar. Under an H₂ environment, API-X70 steel exhibited HE behavior similar to API-X52 steel, with early fracture and decreased maximum load and fracture displacement due to the HE effect. The HE effect varied depending on the H₂ pressure tested, with significant HE occurring regardless of the punch velocity at H₂ pressures beyond 5 MPa. To ensure the reliability of the measurement of the resistance to HE, three repeated tests were performed at 8 MPa, considering that the operating pressure of natural gas pipelines is usually below 7 MPa.

In both bcc-structured pipeline steels with a high hydrogen diffusion rate, the punch velocity significantly influenced the screening evaluation under low-pressure H₂ environments. However, the effect of punch velocity became insignificant with the occurrence of significant HE when the H₂ pressure increased. Significant HE occurred regardless of the punch velocity in H2 pressure environments of 5 MPa or higher. Failure due to the HE effect mainly occurred in the late part of Region III or the early part of Region IV on the load–displacement curves.

The fracture appearances and crack surface morphologies of API-X70 steel are observed after SP tests under different N₂ and H₂ pressure and punch velocities, as shown in Figure 7a,b. The fracture morphologies of API-X70 steel were similar to those of API-X52 steel. When tested in an N₂ environment, large circular cracks were developed on the specimen surface (as seen in Figure 7a), with dimple patterns (DP) on the crack surface (as seen in Figure 7b), regardless of the N₂ pressure levels. However, in H₂ environments, different crack shapes were observed depending on the H₂ pressure and punch velocity tested. Multiple circular-shaped cracks were formed even at a low H₂ pressure of 0.2 MPa and high punch velocity. At relatively low punch velocities, small ring-shaped cracks were formed with multiple short radial cracks developed around the ring crack, as shown in Figure 7a. At relatively high H₂ pressures, small ring-shaped cracks formed with multiple short radial cracks around the ring crack. The corresponding crack surfaces were characterized by quasi-cleavage fracture (QC), as shown in Figure 7b. The cracking morphologies in the API X70 steel under H₂ environments showed similar results at the low punch velocities of 0.01 and 0.004 mm/min, regardless of H₂ pressure. These failure modes support the behavior of quantitative influencing factors such as SP energy and RRT. When the H₂ pressure was less than 0.5 MPa, and the punch velocity was as high as 0.1 mm/min, the fracture morphologies due to the HE effect observed were similar to those of API X52 steel.

Figure 8 depicts the variation in SP Energy, ROT, and RRT for API-X70 steel against different H₂ pressures at RT. The HE behavior of API-X70 steel is generally similar to that of API-X52 steel. When tested in an N₂ environment, the SP energy and ROT of ~3.0 J and ~55% remained almost constant over the N₂ pressure tested. It can be found that the SP energy of API-X70 steel was slightly higher than that of API-X52 steel due to its higher strength. However, in H₂ environments, similar to API-X52 steel, there was a significant variation in SP energy, ROT, and RRT depending on H₂ pressure and punch velocity. Generally, low H₂ pressure and high punch velocities alleviated HE susceptibility, while high H₂ pressure and low punch velocity increased HE susceptibility. Similarly, API-X70 steel displays an inverse S-curve for HE behavior against H₂ pressure [31]. The ductile–brittle transition pressure (DBTP) due to the HE effect for API-X70 steel at RT was around 1.0 MPa H₂, based on the median of SP energy and RRT values. Therefore, it can be concluded that the DBTP behaviors of the two pipeline steels were similar.

The punch velocity significantly affected the HE susceptibility in a lower H₂ pressure range than 5 MPa. At higher punch velocities of 1.0 and 0.1 mm/min under 0.1 MPa H₂ pressure, the SP energy decreased to 2.2 J, and the RRT was 0.7. However, at lower punch velocities of 0.01 and 0.004 mm/min, the SP energy was further reduced to 1.2 J, and RRT was 0.4. These results indicate that when the punch velocity is higher than 0.1 mm/min under low H₂ partial pressures, there is not enough time to reach the critical hydrogen concentration at the stress-concentrated points, despite the high hydrogen diffusion rate of the bcc structured pipeline steels.

In high H₂ pressure conditions beyond 5 MPa, punch velocity had minimal effect on SP energy and RRT, although a significant HE occurred. In particular, at 20 MPa H₂ condition, SP energy and RRT were approximately 0.7 J and 0.38, respectively. For API-X70 steel, the effect of H₂ pressure on both H₂ compatibility screening factors was insignificant at lower punch velocities of 0.01 and 0.004 mm/min. It can be confirmed that in the cases of API-X70 steel, a punch velocity of 0.01 mm/min or less during the SP test is sufficient to reach the critical hydrogen concentration for the HE effect, even under low hydrogen partial pressures. For reference, a punch velocity of 0.01 mm/min can be calculated to the strain rate of 2.5 × 10⁻⁵ s⁻¹ in region III of the load–displacement curve [21].

4. Discussion

The HE susceptibility of API-X52 and API-X70 steels to be used for hydrogen transportation as types of pure hydrogen or blend gas with natural gas through pipelines was evaluated using the in situ SP test in a wide range of H₂ pressures from 0.1 MPa and 20 MPa at room temperature. Since pipeline steels have a bcc structure, the hydrogen diffusion rate is fast. Therefore, the effect of the punch velocity on the HE susceptibility of steels during screening tests should be evaluated, particularly in low-pressure H₂ environments. For this purpose, four different punch velocities, ranging from 1.0 to 0.004 mm/min, were used. It is challenging to conduct screening tests under various conditions in a gaseous hydrogen environment, particularly with conventional SSRT utilizing an autoclave facility. Additionally, when hydrogen-precharged specimens are used to evaluate HE behavior under low H₂ pressure environments, it is difficult to assess the HE behaviors properly due to the release of hydrogen charged into the atmosphere. However, the current in situ SP test makes it possible to screen the HE susceptibility of each pipeline steel qualitatively and quantitatively by its simple testing ability.

The HE susceptibility and behaviors of both pipeline steels under each test condition are detailed in the previous sections. The hydrogen compatibility can be classified into four levels depending on the extent of HE susceptibility observed in each pipeline steel under respective test conditions.

Table 4. Classification for screening HE compatibility of pipeline steels based on in situ SP test results.

Classification of HE		Quantitative Aspect		Qualitative Aspect
		SP Energy, J (For Example, Carbon Steel)	RRT	Load- Displacement Curve Pattern (Fracture Region)	Fracture Mode
Type I	Negligible	2.0 ~ 3.0	0.8~1.0	Region V	Necking, Large circular crack
Type II	Light (Mild)	1.2 ~ 2.0	0.6~0.8	Region IV	Large-size ring crack, Multiple
Type III	Moderate	0.8 ~ 1.2	0.3~0.6	Region III or IV	Small-size ring crack, Single, (radial cracks)
Type IV	Severe (Extreme)	0.2 ~ 0.8	<0.3	Region II or III	Brittle fracture without ring shape

presents a summary of HE behaviors classified in quantitative and qualitative aspects to assess the hydrogen compatibility of pipeline steels through in situ SP testing in different hydrogen environments. Quantitative screening of HE susceptibility of each pipeline steel can be carried out systematically using SP energy and RRT, and qualitative classification using load–displacement curves and fracture characteristics supports the quantitative one. The range of SP energy values shown in Table 4 was specifically tuned to the case of pipeline steels.

As explained in the previous section, both pipeline steels exhibited similar HE behavior when subjected to a wide range of H₂ pressure and punch velocities. Generally, higher H₂ pressure environments above 5 MPa generated greater HE susceptibility, classified as Type III or Moderate HE, regardless of the punch velocity used. Under low H₂ pressure conditions of less than 1 MPa, different HE behaviors were observed depending on the punch velocity, classified as Type II or Light HE. However, for API-X52 steel, when the H₂ pressure was reduced to 0.1, 0.2, and 0.5 MPa with a punch velocity of 1.0 mm/min, the HE effect indicated Type I or Negligible HE behavior. On the other hand, when the punch velocity was lowered to less than 0.1 mm/min, a similar extent of HE susceptibility to Slight HE was assessed regardless of the tested punch velocity.

Both API-X52 and API-X70 pipeline steels exhibited an inverse S-curve regarding the factors of SP energy and RRT with respect to H₂ pressure. The ductile–brittle transition pressure (DBTP) due to the HE phenomenon for both steels was determined to be at 1 MPa H₂ pressure, which provides Moderate HE behavior. However, for API-X70 steel, the effect of H₂ pressure on both H₂ compatibility screening factors was negligible at lower punch velocities of 0.01 and 0.004 mm/min. When the H₂ pressure was low, API-X52 steel showed similar SP energy, and RRT decreased due to the HE effect when the punch velocity was lower than 0.1 mm/min. On the other hand, API-X70 steel showed similar SP energy and RRT values when the punch velocity was lower than 0.01 mm/min. Accordingly, API-X70 steel took a longer time for hydrogen diffusion to reach the critical hydrogen concentration that causes brittle fracture due to HE in a low-pressure hydrogen environment.

The in situ SP test is a valuable screening technique for determining the hydrogen compatibility of bcc-structured pipeline steels, and the factor RRT can be effectively applied to the HE screening of steels in low and high-pressure H₂ environments. However, the SP test has a limitation in determining the HE effect on strength directly, unlike the uniaxial tensile test. To overcome this limitation, it is recommended to establish a correlation and supplement comparison through in situ SSRTs using hollow specimens [34].

In summary, a punch velocity of 0.1 mm/min for API-X52 steel and a punch velocity of 0.01 mm/min for API-X70 steel is sufficient to reach the critical hydrogen concentration, even under low hydrogen partial pressures. These findings highlight the importance of using in situ SP tests and the factor RRT in the screening test of pipeline steels for hydrogen compatibility.

5. Conclusions

The hydrogen embrittlement (HE) susceptibility of two types of pipeline steels, API-X52 and API-X70, was evaluated at room temperature using the in situ SP tests under varying hydrogen (H₂) pressures and punch velocities, considering the expected operating conditions of pipeline steels for hydrogen gas transportation. The aim was to assess the hydrogen compatibility of pipeline steels through in situ SP testing in different hydrogen environments. The HE behaviors observed can be classified into four types based on quantitative and qualitative aspects.

Two kinds of pipeline steels exhibited similar HE behaviors under a wide range of H₂ pressure and punch velocities so that they can be assessed as comparable in terms of hydrogen compatibility. Higher H₂ pressure above 5 MPa resulted in greater but similar HE susceptibility classified as ‘Moderate HE’, regardless of the tested punch velocities. Under low H₂ pressure conditions below 1 MPa, different HE behaviors were observed depending on the punch velocity. When the punch velocity was high, the HE effect was classified as ‘Negligible HE’ for API-X52 steel and ‘Light HE’ for API-X70 steel.

Both pipeline steels showed an inverse S-curve for quantitative factors of SP energy and RRT against the H₂ pressure at 1.0 mm/min punch velocity. A ductile–brittle transition pressure (DBTP) due to the HE phenomenon can be defined for the H₂ pressure at RT. The corresponding DBPT for API-X52 and API X70 steels was determined as a 1 MPa H₂ pressure. It was found that a punch velocity of 0.1 mm/min for API-X52 steel and a punch velocity of 0.01 mm/min or less for API-X70 steel during the SP test was sufficient to reach the critical hydrogen concentration for the HE effect, even under low hydrogen partial pressures. These results confirm that the in situ SP test is a useful screening technique and the factor RRT can be effectively applied to the HE screening of ferritic steels in low and high-pressure hydrogen environments.