Role of hydrostatic stress in hydrogen diffusion in pearlitic steel

Abstract

The relevant role of hydrostatic stress in hydrogen diffusion in pearlitic steel is outlined from both theoretical and experimental points of view. The theoretical development offers the formulation of hydrogen diffusion equations where hydrogen flux density depends not only on the concentration gradient, but also on the hydrostatic stress distribution in the sample. The experimental programme consisted of slow strain-rate tests on axisymmetric notched samples at different strain rates under simultaneous hydrogen charging by cathodic polarization. The use of different notch geometries allows a study of the influence on hydrogen diffusion of the hydrostatic stress state in the vicinity of the notch tip. A specific microscopic mode of fracture different from classical cleavage was found, associated with hydrogen effects: the tearing topography surface. In the quasi-instantaneous tests, the value of hydrostatic stress at the sample boundary (just the notch tip) at the failure instant is relevant from the fracture point of view. In the quasi-static tests, the tearing topography surface depth equals that of the maximum hydrostatic stress point, and the maximum value of the stress triaxiality in each geometry (ratio of the hydrostatic to the equivalent stress, almost constant during the tests) seems to govern the diffusion process. These facts emphasize the relevant role of hydrostatic stress in the vicinity of the notch in hydrogen diffusion.