STUDY OF THE HYDROGEN EMBRITTLEMENT SENSITIVITY OF AN X4CRNIMO 16.5.1 STAINLESS STEEL AND THE ASSOCIATED ELECTRON BEAM WELD

X4CrNiMo 16.5.1 steel (commercial name APX4) is a low carbon martensitic stainless steel known for its remarkable mechanical characteristics and its good resistance to corrosion. The use of APX4 in the manufacture of high pressure gas vessels requires a thorough understanding of its resistance to Hydrogen Embrittlement (HE) as the gas can contain traces of hydrogen, and martensitic steels, and their welds, are generally very sensitive to HE. This paper deals with the first part of this study, involving the characterization of the microstructure and the mechanical properties of each zone of the electron beam weld (melted zone and four different heat affected zones), and the investigation of their embrittlement in a hydrogen environment. For this purpose, tensile tests were performed in hydrogen gas at various pressures on both the base metal and the welded samples. In situ tensile tests with picture correlation analysis have been planned for the subsequent part of our study. These experiments will enable us to track the strain in the different zones and examine the effect of hydrogen on their mechanical behavior. These results, combined with permeation tests, should lead to a complete model for predicting hydrogen location inside the vessel as a function of time and local properties, as well as the associated embrittlement.