Visualisation of diffusion sites and measurement of hydrogen traps in hot-rolled pipes

Abstract

Inclusions are very often founded in the centre of thickness in hot-rolled pipeline steel. In this research, the role of these inclusions on hydrogen diffusion, accumulation and distribution in pipeline steel is studied using the electrochemical permeation and visualisation experiments. While the permeation experiment estimates the parameters of the diffusion process, the visualisation experiment reveals the diffusion pathway in the steel microstructure. Results from the permeation test allowed to show that the steel samples selected from the segregation zone at the mid-layer of thickness have higher permeability and effective diffusion coefficient than samples from the top-layer. Also, the hydrogen visualisation results permit to reveal an increased discharge of hydrogen around inclusions as represented by a pattern of superimposed white particles. Moreover, the calculated diffusion parameters also allowed us to establish that the density of hydrogen traps in mid-layer was lower than top-layer of the steel.

Analysis of Mechanical Properties of Welded Joint Metal from TPP Steam Piping after Its Operational Degradation and Hydrogenation

“…In particular, in the structure of heat-resistant steel for reactor vessels alloyed with vanadium, nano-sized carbides (Mo, V) C are released, which are traps for hydrogen absorbed by the metal and thereby reduce its activity in initiating cracks inside the metal [27]. Visualization of hydrogen diffusion paths in the steel structure made it possible to show an increased discharge of hydrogen around inclusions [28]. Easy capture of hydrogen (absorbed by steel during long-term operation) should promote the decohesion of both non-metallic inclusions and carbides from the surrounding matrix.…”