Effect of hydrogen charging on Charpy impact toughness of an X70 pipeline steel

Abstract

Hydrogen uptake in steel structures can cause a degradation in mechanical properties such as toughness, and can induce cracks. This phenomenon is widely known as hydrogen embrittlement. For structural steels subjected to cathodic protection or pipelines transporting high-pressure hydrogen gas, hydrogen embrittlement represents an important challenge. Charpy V-notch testing provides a fast and inexpensive method for quantifying the impact toughness of a steel. However, its validity for assessing hydrogen embrittlement is uncertain. In this work, the influence of hydrogen uptake on the impact toughness of an API 5L X70 pipeline steel is investigated. Charpy V-notch impact tests are performed in air, both uncharged and after electrochemical hydrogen pre-charging. Different charging times are used, and the influence of hydrogen-induced cracking is studied. The temperature range of the Charpy impact tests is between -80°C and +20°C. A rising upper shelf phenomenon is observed in the uncharged specimens and the ductile-to-brittle transition temperature (DBTT) is not reached for the tested temperatures. For this material, hydrogen uptake causes a reduction in Charpy impact energy at the higher test temperatures, with the highest reduction measured at room temperature. A post-mortem analysis of the fracture surfaces suggests that the presence of hydrogen in the lattice aids the formation of separations during fracture, lowering the absorbed energy.