Detection of hydrogen trap distribution in steel using a microprint technique

Abstract

The distribution of hydrogen trapped at defects has been visualized by means of a hydrogen microprint technique (HMT). A low carbon steel having a ferrite/pearlite structure with second phases along grain boundaries was subjected to notch-tensile and three-point bending tests with/without hydrogen pre-charging. HMT was applied to tested specimens re-charged with hydrogen. Accumulation of hydrogen was confirmed in strain-concentrated areas as well as along grain boundaries and in pearlite. A noteworthy finding was a substantial increase in defects acting as traps of diffusive hydrogen when specimens were deformed in the presence of hydrogen. The increased defects were annealed out at temperatures as low as 150°C, indicating their point defect nature. Another finding was the appearance of a zone denuded of silver precipitation in highly strained areas such as near grain boundaries and the fracture surface, where tritium autoradiography revealed the evolution of defects having high binding energy with hydrogen.