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Hydrogen embrittlement testing on austenitic stainless steel candidates for automotive applications
Given the rapid increase in hydrogen as an energy carrier and fuel , the demand for stainless steel grades compliant with the hydrogen atmosphere is growing . Alleima has several grades fulfilling the applicable standards within this area , tested at high hydrogen pressure and low temperature . The susceptibility to hydrogen embrittlement of different austenitic stainless steels was tested using various methods for use in high-pressure hydrogen . The tests included in-situ charging and precharging , cathodic hydrogen charging , and high-pressure hydrogen gaseous charging . Samples tested in an inert environment acted as references . Based on the Slow Strain Rate Testing ( SSRT ) results , the microstructure and fracture surface of some test samples were investigated to further explain the different material properties .
By Angela Philipp , Technical Marketing Specialist ; Ulf Kivisäkk , Senior Expert Corrosion Resistant Alloys ; and Charlotte Ulvin , Senior Engineer , Product Development Corrosion Resistant Alloys , Alleima
Introduction
Today , hydrogen is receiving increasing attention as an alternative to fossil-based fuels for vehicles . Stainless steel , a key material in this transition , is used in electric fuel cell cars and heavy vehicles using direct combustion . Bosses , part of composite tanks , are made from stainless steel bars . Stainless steel tubes are used for hydrogen transport , as well as in tubing and fittings in hydrogen filling stations . These systems operate at high pressures , typically 35 or 70 MPa . To avoid gas overheating during vehicle filling , cooling to temperatures as low as −40 ° C may be performed .
SAE ¹ J2579 prescribes using Slow Strain Rate Testing ( SSRT ) to qualify material . The testing should be performed at −40 to −50 ° C at 125 % of the design pressure , which results in 87.5 MPa for a 70 MPa system . Requirements for passing the SSRT in the SAE J2579 are an elongation above 12 %, yield and tensile strength above specified minimum values , and a work hardening ratio above 1.07 .¹ SSRT is a common method for testing the resistance against hydrogen embrittlement . However , the ratio of elongation or reduction of area in hydrogen gas compared to air is often used when evaluating the resistance to hydrogen embrittlement .
To increase the low mechanical strength of austenitic grades , cold working could be applied , as well as alloying utilizing dispersion hardening and grain refinement . The latter could be obtained by adding a low amount of niobium as well as nitrogen to an austenitic grade .² The influence of cold work on resistance to hydrogen embrittlement of austenitic stainless steels is negative if cold work introduces martensite and increases the dislocation density of the material . However , for UNS S21900 , good hydrogen embrittlement resistance of the cold worked plate was achieved when tested using 69 MPa hydrogen .³
Several authors have demonstrated the importance of austenite stability for hydrogen embrittlement resistance , suggesting higher nickel contents ( above 13 %) for high-pressure hydrogen gas . 1 , 3 A Ni-equivalent describing the resistance to martensite formation correlates with hydrogen embrittlement resistance in hydrogen
42 Hydrogen Tech World | Issue 17 | August 2024