The influence of gradient difference in microstructure on hydrogen embrittlement susceptibility of extra-thick Q690DR steel plate

Abstract

To investigate the hydrogen embrittlement susceptibility and hydrogen permeation behavior of 90 mm extra-thick Q690DR steel plate in the thickness direction, samples were taken from three regions: Subsurface layer, 1/4 Thickness layer (1/4T), and 1/2 Thickness layer (1/2T). The results showed that the matrix structure in each region was mainly tempered martensite, and the grain size gradually increased from the Subsurface to 1/2T; a discontinuous banded structure existed in the Subsurface region. In-situ hydrogen permeation and mechanical testing under 6 MPa hydrogen revealed that hydrogen embrittlement susceptibility: Subsurface > 1/2T > 1/4T. The Subsurface region was significantly affected by rolling deformation, with grains elongated along the rolling direction (RD) and forming a banded structure, providing a rapid channel for hydrogen diffusion and making hydrogen more likely to permeate the material interior, resulting in the formation of fine elongated cracks during fracture. Therefore, it exhibited the highest hydrogen embrittlement susceptibility. The 1/2T region formed coarse martensite structure due to a slower cooling rate, and a large number of secondary cracks were generated during fracture. In contrast, the 1/4T region had a homogeneous distribution of tempered martensite and fine grains, with a stronger hindrance effect on hydrogen diffusion, and thus had the best hydrogen embrittlement resistance. In summary, the degree of deformation during the rolling process and the cooling rate during the cooling process are the key factors that cause the microstructure to show a gradient distribution and thereby affect hydrogen embrittlement susceptibility.