Study on the Influence of Hydrogen Blended Transportation on the Safety of Existing X70 Steel in Gas Distribution Pipeline Network

Coal to gas is one of the natural gas sources utilized in Beijing, with a hydrogen blend concentration range from 1% to 3% and reaching up to 5% in specific cases. In order to evaluate the safety of transporting hydrogen blended natural gas with X70 pipeline steel in distribution pipelines, this study first conducted the slow strain rate tensile tests under a total pressure of 4 MPa in hydrogen environment. The results show that X70 steel exhibits highly sensitive to a hydrogen blending ratio of 5%. Subsequently, four types of hydrogen compatibility tests were conducted in the above sensitive hydrogen partial pressure include the impact toughness test, fracture toughness test, fatigue crack growth test, and fastening disk pressure test. The findings demonstrated that hydrogen blended natural gas has virtual no effect on the impact and fracture toughness, with only 10% reduction in fatigue crack growth threshold. Finally, based on fatigue life analysis by fracture mechanics evaluation, it can be concluded that the existing natural gas pipe of X70 steel is capable to operate safely over 40 years under a total pressure of 4 MPa with a hydrogen blend concentration of 5%.


Introduction
The National Development and Reform Commission and the National Energy Administration issued the Medium-and Long-Term Plan for the Development of Hydrogen Energy Industry (2021-2035) [1] on March 23, 2022, solidifying the position of hydrogen energy as a future pillar of the national energy strategy, and a key carrier for achieving green and low-carbon transformation.The basic rules of hydrogen energy safety are prioritized in the Plan such as hydrogen embrittlement failure, leakage, and diffusion, along with pilot demonstrations of blending hydrogen into the existing natural gas pipelines.Relevant studies and demonstration application have been carried out on hydrogen blended transportation in medium and low-pressure distribution pipelines.Shi found that the Wobbe index and combustion potential of natural gas reduced with hydrogen blend transportation, however the requirement of 12T natural gas for residential users can be achieved under a less than 20% hydrogen blend concentration [2].Based on the interchangeability calculation, Qiao concluded that the maximum upper limit of hydrogen blend concentration to meet the requirements of 12T natural gas was 23%, while a 10% hydrogen blend concentration would have no significant impact on the safety of residential users [3].Yang researched the adaptability and safety of using hydrogen blended natural gas in domestic gas appliances based on theoretical analysis and experimental tests, and concluded that the less than 20% hydrogen blended natural gas could be used in domestic gas appliances without any modification [4].In the 2004 EU NaturalHy Project, it was found that the fatigue resistance of low-grade steel pipes decreased in case of blending hydrogen, with the fatigue test results indicating that the hydrogen blend concentration should be less than 50% [5].The above research provided a theoretical basis for the feasibility of hydrogen blended natural gas transportation.
However, the compatibility of materials including hydrogen induced ductility loss, hydrogen blistering and hydrogen induced cracking is the greatest challenges of pure hydrogen and hydrogen blended natural gas transportation.
Hydrogen induced ductility loss is the decrease in material ductility caused by hydrogen dissolving in steel in the atomic or ionic form and accumulating to supersaturation at dislocations and microscopic gaps, which can be minimized by dehydrogenation to prevent pipe embrittlement.The fundamental causes of hydrogen bubbling and hydrogen induced cracks usually considered permanent and irreversible are the gradual buildup of this atomic hydrogen and molecular hydrogen which is compounded by some of this accumulation of atomic hydrogen at the structural defects of the material [6].Due to the increased likelihood of hydrogen leakage in the pipe [7], the research into pipe safety in case of hydrogen blended transportation has gained international attention.
Shang studied hydrogen induced cracking of low carbon steel that transported hydrogen blended natural gas [8].The results indicated that the coupling effect of carbon dioxide and hydrogen on materials could significantly hasten the fatigue crack growth rate (FCGR) in hydrogen blended natural gas.In Chaoyang City, Liaoning Province, China, the demonstration project of hydrogen blended natural gas confirmed that 20# steel pipe could meet the safety transmission requirements at of the medium and low pressure [9].An HSE study and analysis in Britain found that in the pressure range of 0 to 0.2 MPa, a hydrogen blended concentration of less than 20% had no effect on low grade steel pipes in natural gas distribution pipelines [10].
The delivery pressure of natural gas distribution pipelines in Europe and the United States is low, and the pipes are typically made of polyethylene (PE) or low grades steel.China's natural gas distribution pipeline network is among the world's most extensive, with a wide range of working pressures (from 3500 Pa to 6 MPa) and a wide variety of materials (from 20# to Q235B and X70).Hydrogen has been shown to have a greater impact on higher grades pipeline steel, with X80 and X70 being more susceptible to hydrogen induced cracking than X60 steel [11].Hydrogen blended transportation primarily affects tensile properties and fatigue crack growth performance of pipes.With a design pressure of 4 MPa, X70 steel is primarily used in Beijing's outer ring pipeline network.Hydrogen can cause embrittlement in pipes, which can lead to pipe failure [12][13].There are currently high-pressure rings, medium-pressure networks, and regional low-pressure connectivity in Beijing's natural gas pipeline network with the complex distribution and near to urban buildings.In case of a gas leak, there is a serious risk to the public safety.It is very important to investigate the sensitive hydrogen partial pressure, the change in mechanical properties and service life of pipes with hydrogen blended transportation.
A sensitive hydrogen partial pressure for X70 steel under a total pressure of 4 MPa was analyzed in this work.Furtherly, four types of mechanical performance tests were conducted based on sensitive hydrogen partial pressure include the impact toughness test, fracture toughness test, fatigue crack growth test, and fastening disk pressure tests.Finally, based on the fracture mechanics method, the critical crack size and fatigue life index of pipe were evaluated, and the service safety of pipes hydrogen blended natural gas transmission was determined.

Material and Test Environment
The base material and welds of X70 steel consists of polygonal ferrite (PF), granular bainite (GB) and a small amount of acicular ferrite (AF), as test materials with the microstructure shown in Figure 1.The mass fractions of the main chemical components are C0.09,Si0.35, Mn1.7, P0.015, S0.005, V0.06, Nb0.03, and Ti0.025.The test parameters were selected based on the actual operating conditions of the distribution pipeline network in Beijing Sixth Ring Road, with a maximum operating pressure of 4 MPa and hydrogen blending ratios of 1%, 3%, and 5%.The samples were placed in an autoclave pre-filled with natural gas blended with hydrogen to investigate the impact of different hydrogen blending ratios on the mechanical properties of X70 steel.

Test of Sensitive Hydrogen Partial Pressure
The strength, elongation, reduction of area, and fracture morphology in the samples under room temperature and pressure and different hydrogen partial pressure were compared based on the slow strain rate tensile test method considering the greatest impact on hydrogen embrittlement susceptibility of the material, in order to determine the hydrogen concentration that has the impact on the material properties.The samples were smooth bar-shaped tensile samples according to the ISO 7539-7-2005 [14] and ASTM G142-98 (2016) [15], which were sampled along the axial and circumferential direction of the pipeline, with a cross-section diameter of 5 mm.The samples were placed in a normal temperature and pressure environment, and a hydrogen environment that was simulated in an autoclave, with hydrogen blending ratio of 1%, 3%, and 5%.Uniaxial tensile stress was applied at the tensile rate of 1 × 10-6 s-1 until the samples cracked, to obtain the stress-strain curves under different environments.

Relevant Mechanical Test under Sensitive Hydrogen Partial Pressure
Hydrogen-blended natural gas was prepared based on the selected sensitive hydrogen partial pressure.The components are shown in Table 1.Impact toughness, fracture toughness, fatigue crack growth, and fastening disk pressure tests were conducted to evaluate the impact of hydrogen on the mechanical properties, hydrogen embrittlement susceptibility, and crack growth of materials.[16].Consequently, the work conducted the Charpy impact test on V-notched samples exposed to 5% hydrogen partial pressure for 30 days at 0 ℃, and compared the test results with V-notched samples not exposed to hydrogen at 0 ℃ to evaluate the impact of hydrogen components on material toughness.The samples had a length of 55 mm, height of 10 mm, and width of 5 mm, with a notch angle of 45° and a notch depth of 2 mm.

Fracture Toughness Test.
The crack tip opening displacement (CTOD)test method of materials as specified in ISO 12135-2016 [17] was used for the fracture toughness test under normal temperature and pressure, with samples taken along the axial and circumferential direction of the pipeline.For the prefabricated crack process and test method of the samples referring to ISO 7539-9-2017 [18].The samples had a length of 100 mm, width of 20 mm, and thickness of 10 mm, with a notch width of 1.2 mm and an effective crack notch length of 7 mm.Double cantilever beam (DCB) samples as specified in ISO 7539-6-2011 [19] was used for the fracture toughness test in the hydrogen environment.The crack was prefabricated on the samples before the test.The samples had a total width of 120 mm, net width of 110 mm, thickness of 20 mm, with a notch width of 0.7 mm and a notch length of 20 mm.Bolt loading was adopted to enable the crack tip to reach the stress intensity factor, and the sample was then placed in a hydrogen autoclave.The hydrogen embrittlement susceptibility of the material was evaluated based on the crack growth.

Fatigue Crack Growth
Test.The fatigue crack growth test was carried out according to ISO 7539-6-2011 and ISO 12108-2012 [20].For the compact tensile C (T) samples that were taken along the axial and circumferential direction of the pipeline.The samples had a width of 70 mm and thickness of 14 mm.The fatigue load that is sine wave loading and frequency 1 Hz was applied to the samples by a testing machine, with maximum load 5.2kN, minimum load 0.52 kN.The tests were conducted at normal temperature and pressure and 5% hydrogen blending ratio.

Fastening Disk Pressure Test.
The fastening disk pressure test (disk blasting) was carried out according to the ASTM F1459-06 [21].Different gases were used to pressurize the ambient chamber at a certain pressure rise rate until the sample with a diameter of 58 mm and a thickness of 0.7 mm was blasted after reaching the blasting pressure recorded under different gas environment.Pure hydrogen (99.995%), and 5% hydrogen-blended components were used for the tests.

Sensitive Hydrogen Partial Pressure of X70 Steel
To analyze the hydrogen sensitivity of X70 steel base metal and weld joint under a pressure of 4 MPa, a series of slow strain rate tensile tests were conducted by varying the hydrogen partial pressure of 1%, 3%, and 5%.The hydrogen embrittlement susceptibility was evaluated.The results in Figure 2 indicate that different levels of hydrogen partial pressure ratio have virtually no effect on the yield strength and ultimate tensile strength of both the base metal and weld joint.However, there is a slight decrease in the weld elongation of under 5% hydrogen partial pressure.Hydrogen induced degradation in pipeline steels is typically manifested as a ductility loss of materials.The elongation (δ) and reduction in area (Ψ) of the X70 steel were calculated based on cross-sectional area  2 and gauge section length  at initial and final fracture.The calculation formulas for elongation (δ) and reduction of area (Ψ) are shown in Formula (1) and Formula (2), respectively.The embrittlement index (EI) is defined as a measure of hydrogen embrittlement susceptibility, as shown in Formula (3).The higher the reduction of area and elongation, the better the ductility of the material.The fracture morphology in the sample weld is shown in Figure 3.The same samples tested in the hydrogen environment were compared with those in the non-hydrogen environment to evaluate hydrogen embrittlement susceptibility.The greater the ratio deviation, the higher the cracking sensitivity.It is shown that there is a decrease in the reduction of area and elongation of weld material under 5% hydrogen partial pressure in Figure 4, which indicates that 5% hydrogen partial pressure has an impact on the mechanical properties of the material and increases brittleness of the material.As a result, 5% hydrogen partial pressure is determined as the sensitive hydrogen partial pressure for X70 steel.

Impact of Hydrogen on Relevant Mechanical Properties of X70 Steel under Sensitive Partial Pressure
3.2.1.Impact Toughness Test.The impact toughness test results of X70 steel base material and welds under normal temperature and pressure and 5% hydrogen partial pressure are shown in Table 2.As shown in Figure 5, the impact toughness of samples in a hydrogen environment is not significantly different from that in a non-hydrogen environment, indicating that 5% hydrogen partial pressure has little impact on the impact toughness of materials.

3.2.2.
Fracture Toughness Test .The tests were conducted on 4 groups of samples, carried out from the transverse and circumferential direction of the pipeline, using DCB samples.The DCB samples were loaded up to the yield strength, with the stress intensity factor at crack tip reaching 25% of the K IC (fracture toughness).The results, shown in Figure 6 and Figure 7, indicated that the crack tip of the weld and base material samples did not grow forward under 5% hydrogen partial pressure within 30 days and there were no secondary cracks.Consequently, when service stress below the yield strength (e.g., Class 1 natural gas pipelines with a design factor of 0.72), it appears that the blending natural gas of 5% hydrogen has no detrimental effect on the damage tolerance of both X70 parent material and weld joint.This finding holds significant importance in terms of practical applications.

Fatigue Crack Growth Test.
Hydrogen has a significant impact on the fatigue performance of material, leading to an increase in the fatigue crack growth rate, and a lowering of the threshold of fatigue crack initiation.There effects can occur in combination with each other [22].If the fatigue crack growth threshold or fatigue crack growth rate of all parallel samples under hydrogen condition is degrade than that under original conditions, then it can be concluded that hydrogen has an effect on X70 pipe steel.The fatigue crack growth rate curve (da/dN v.s.△K) of X70 steel under room RT and 5% hydrogen partial pressure are shown in Figure 8.The fatigue crack growth threshold of the X70 steel base material and the weld sample is lower in the hydrogen environment than that at RT. Table 3 shows the results of determining the fatigue crack growth of X70 steel through linear regression under different environments.The coefficients n and m can be obtained by fitting the Paris' formula,expressed as  Weld #1 Weld #2 The threshold for fatigue crack growth of the two samples is slightly less than 15 MPam1/2 as shown in table 3, with a reduction of 10% when blending 5% hydrogen.It is evident that the △Kth of all parallel samples in the hydrogen environment are lower than those at RT. Therefore, there is a decrease in the fatigue properties of X70 steel under 5% hydrogen partial pressure, especially in the weld joint.Even a small amount of hydrogen can adversely affect the fatigue performance of pipeline steel, which is consistent with the findings of Meng's research.(Hydrogen effects on X80 pipeline steel in highpressure natural gas/hydrogen mixtures)

Fastening Disk Pressure Tests (Disk Blasting).
A total of 15 groups of fastening disk pressure tests were carried out on X70 steel base material including 6 groups in a helium environment and 9 groups in a hydrogen environment.The test results are shown in Table 4.The blasting pressure of X70 steel in the hydrogen environment is slightly lower than that in the helium environment, indicating an increase in material brittleness.The blasting pressure ratio PHe/PH2 in the helium and hydrogen environment is shown in Table 5. long-term exposure to hydrogen may result in hydrogen embrittlement at when PHe/PH2 is greater than 1 but less than 2. Consequently, based on the analysis of the test results, the material has certain hydrogen embrittlement susceptibility long-term exposure to hydrogen environment.The results based on slow strain tensile test, it was found that the sensitive hydrogen partial pressure ration for X70 steel pipe is 5%.b.The results from Charpy impact and DCB (Double Cantilever Beam) tests show that the presence of 5% hydrogen in natural gas does not affect the toughness of X70 base material and welds.Under the condition of applying a 25% of K IC stress intensity factor, the X70 steel demonstrates the ability to resist crack propagation in hydrogen environment.c.The results of fatigue crack growth rate indicate that the addition of 5% hydrogen reduces the fatigue crack growth threshold of X70 base material and welds by approximately 10%.Additionally, the fatigue crack growth rate is significantly increased.Fatigue life evaluation was conducted using fracture mechanics assessment methods, and the results demonstrate that X70 steel can safely operate for over 40 years under a total pressure of 4 MPa and a hydrogen blend concentration of 5%.

Figure 2 .
Figure 2. (a)-(b) Slow Strain Rate Tensile Curves of X70 Steel Base Material and Welds under Different Hydrogen Partial Pressures.

Figure 3 .
Figure 3. Slow Tensile Fracture Morphology of X70 Steel under Different Hydrogen Partial Pressures.

Figure 4 .
Figure 4. Plasticity Indexes of X70 Steel Welds under Different Hydrogen Partial Pressures.

Figure 5 .
Figure 5.Comparison of Impact Energy Test Results of X70 Steel under Normal Temperature and Pressure and 5% Hydrogen Environment.

Figure 6 .
Figure 6.Crack of DCB Samples of X70 Steel Base Material before and after Soaking under 5% Hydrogen Partial Pressure for 30 Days.

Figure 7 .
Figure 7. Crack of DCB Samples of X70 Steel Weld before and after Soaking under 5% Hydrogen Partial Pressure Ratio for 30 Days.

Figure 8 .
Figure 8. Fatigue Crack Growth Curves of X70 Steel under Different Environments.

Table 1 .
Components of hydrogen blended natural gas.

Table 2 .
Impact Toughness Test Results of X70 Steel under Normal Temperature and Pressure and 5% Hydrogen Partial Pressure.

Table 3 .
Amplitude of Fatigue Threshold Stress Intensity Factor of X70 Steel under Normal Temperature and Pressure and 5% Hydrogen Partial Pressure.

Table 4 .
Disk Blasting Test Results of X70 Steel under Different Conditions.

Table 5 .
Evaluation of Experimental Indexes in the Fastening Disk (Disk Blasting) Pressure Test.