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Steel and Embrittlement Tests for Hydrogen Transport

Introduction

It is now clear that the UK is on the path towards Net Zero for the major Industrial Clusters by 2040 with the aim to achieve Net Zero nationally by 2050. The ‘pathways’ towards these goals will impact transport, domestic heat and of course our many large industrial and energy plants and processes. The vectors which will deliver our new low carbon future will include the capture of CO2 and the replacement of hydrocarbon fuels with alternatives, such as renewably generated electricity and hydrogen.

Hydrogen provides one ready means of concentrating, storing and distributing energy, whether for use as a large scale source of heat, for example furnaces and boilers, as an alternative to natural gas or when stored in smaller quantities at very high pressure as the fuel for hydrogen powered transport where, in combination with fuel cells, hydrogen offers significant advantages in weight reduction and range when compared to current battery technologies. Key to assuring this weight reduction is storage of hydrogen at high pressure, with typically 7 kg of hydrogen stored at 700 bar for a hydrogen powered car or light vehicle and in greater quantity, stored at 350 bar, for heavier road vehicles, such as buses and future rail applications.

Hydrogen is also known for its potential to cause cracking and failure in many metallic materials, often as a direct result of corrosion mechanisms or indeed when trapped at source during manufacture. Equally, there is little evidence that hydrogen, when used at high purity and low pressures as an alternative to natural gas, has significant adverse effects on materials. However, when higher pressures are involved or where contaminants and impurities are present, hydrogen presents a greater challenge to materials performance.

Figure 1. CT specimen for testing to ISO 11114-4 Method B
Figure 1. CT specimen for testing to ISO 11114-4 Method B
Figure 2. Lowering the pressure vessel down over the CT specimen, loaded and instrumented for testing
Figure 2. Lowering the pressure vessel down over the CT specimen, loaded and instrumented for testing

Work Programme

TWI is supporting one of our Industrial Members, Carten Controls, to better qualify expected materials performance in this relatively new market by performing fracture mechanics tests in high pressure pure hydrogen in accordance with ISO 11114 Part 4.

This standard method is to ensure the compatibility of cylinder and valve materials used for gas transportation from hydrogen embrittlement from the gas contents. The standard was developed in response to catastrophic failure of cylinders containing hydrogen gas or hydrogen enriched gases, due to the embrittlement of the containment steels. It also provides a method to approve newer steels, with higher tensile strength but reduced impurities, such as sulphur and phosphorous, to be demonstrated for their suitability for hydrogen storage service, alongside more conventional lower grade steels.

Test Method B in the standard is a fracture mechanics test (Method A is a disc test), and the test method involves incrementally loading a 26mm wide pre-cracked compact tension specimen in steps, with a twenty minute hold duration at each step to determine whether any crack propagation can be detected from the direct current potential drop (DCPD) monitoring on the specimen. The tests are done at room temperature within a high pressure gas environment containing the relevant embrittling gas, in this case high purity hydrogen. The largest applied load (in terms of stress intensity factor, K, for the specimen) before crack extension occurs is defined at the threshold value, KIH, for the specimen. If two repeat specimens confirm values of KIH above the target value (equal to 60/950 multiplied by the UTS) then the material is qualified to ISO 11114-4 Method B.

 

The Solution

TWI has adapted its existing high-pressure hydrogen testing facility to enable these tests to be performed routinely, by developing automated control software to carry out the incremental load increase and hold durations while monitoring the DCPD signal throughout the test. The test method that was developed and then proven through work carried out allowed material to be successfully qualified to the ISO 11114-4 standard.

Dr Ruth Donnelly and Dr Philippa Moore, from TWI’s Materials and Structural Integrity Department, combined their knowledge and expertise to lead this work to a successful conclusion for Carten Controls.

For more information or to enquire how you might use the equipment and test methods described above to support your own business please contact us, below.

Figure 3. The incremental load test being carried out inside high pressure hydrogen inside the test vessel
Figure 3. The incremental load test being carried out inside high pressure hydrogen inside the test vessel
Avatar Philippa Moore Metallurgical Integrity Engineer - Materials & Structural Integrity Group

Philippa has over 15 years’ experience in project management and research related to fracture toughness testing, welding engineering, steel metallurgy, failure investigation, engineering critical assessment, and structural integrity.

For nearly a decade, she has represented the UK on standards work related to fracture toughness testing for BSI and ISO, and was leading the development of Standard BS 8571 on SENT testing. She has published more than 45 papers in various international journals and conferences.

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