Effect of hydrogen on gigacycle fatigue properties of SNCM439 steel used for storage cylinder in 70 MPa hydrogen station

Abstract

Gigacycle fatigue properties of actual material of a storage cylinder, which had been used in a hydrogen station for verification tests, were examined in both longitudinal and circumferential directions. The storage cylinder was made of SNCM439 steel. Fatigue tests by using ultrasonic and servo-hydraulic fatigue testings were carried out on hydrogen-charged and uncharged specimens at stress ratio of R = -1. The specimen sizes were then not only φ3 mm but also φ7 mm. In the uncharged specimens, surface fracture was dominant in the longitudinal direction, while internal fracture occurred in the circumferential direction. On the other hand, the hydrogen-charged specimens showed internal fracture in both directions, and the fatigue strengths were lower than those of the uncharged specimens. The crack initiation sites of internal fracture were different in the directions: a single globular Al₂O₃ and a cluster of Al₂O₃ inclusions in the longitudinal and circumferential directions, respectively. With regard to anisotropy, the fatigue strength of the hydrogen-charged specimens was much lower in the circumferential direction than in the longitudinal direction. Concerning the size effect, the hydrogen-charged specimens showed clear size effects, i.e., the φ7 mm specimens showed much lower fatigue strength, revealing larger inclusions at the internal fracture origins.