Ausformed high-strength low-alloy steel exhibits exceptional resistance to fatigue crack-growth in high-pressure hydrogen environments

• FCG tests on ausformed and non-ausformed JIS-SCM440 were conducted in 90 MPa H 2 . • Ausforming improved FCG resistance in H 2 , even at a higher strength level. • Suppression of intergranular cracking enhanced hydrogen embrittlement resistance. • Modified grain morphology and cohesive strength changes influenced FCG resistance. Ausformed specimens of the chromium-molybdenum steel JIS-SCM440 were subjected to fatigue tests in both air and 90 MPa hydrogen gas. The results were compared with those of non-ausformed specimens of the same material with similar tensile strengths (≈ 950 MPa and ≈ 1050 MPa). The ausformed materials demonstrated excellent resistance to hydrogen-induced acceleration of fatigue crack-growth (FCG), effectively reducing the crack propagation rate under cyclic loading in hydrogen environments compared to their non-ausformed counterparts. They maintained an acceleration ratio ( i.e ., relative FCG rate in hydrogen with respect to that in air) within 10 to 40 times, an order of magnitude lower than that of the non-ausformed counterparts. Despite their high strength levels (i.e., tensile strengths greater than 900 MPa), the FCG rate in the ausformed materials was almost independent of loading frequency at a stress intensity factor range of 20 and 30 MPa·m 1/2 . Fractographic observations revealed that no intergranular fracture occurred in the ausformed materials, unlike in the non-ausformed ones. These findings suggest that two factors possibly caused the mitigation of FCG rate in hydrogen: (i) modification of the microstructure morphology, i.e., refinement and elongation, and (ii) an increase in the cohesive strength of interfaces under the influence of hydrogen.