The susceptibility to hydrogen embrittlement of a bainite/martensite dual-phase high strength steel with different morphologies obtained by the conventional or thermomechanical heat-treatment has been investigated by means of electrolytic hydrogen charging in this paper. The results show that the finer the microstructure, the lower the sensitivity of steel to hydrogen embrittlement is. The fractographic analysis suggests that the fracture mode of the hydrogen-charged specimens is a mixture of quasicleavage and dimple for both treating processes. The quasicleavage facet of the thermomechanical treated specimens is smaller than that of the conventional heated ones. The observation of the fracture profile specimen shows that the crack propagates preferentially along bainite/martensite laths boundary, suggesting that the fracture mode is predominantly lath boundary separation. In addition, the fractographic analysis indicates that the rhombic Ti(N, C) inclusion is the predominant type of inclusions on the fracture surface of the hydrogen-charged specimens of thermomechanical heat-treatment. The separation between Ti(N, C) inclusions and matrixes is a brittle fracture with small facets, which indicates that inclusions such as Ti(N, C) are harmful to hydrogen embrittlement.
In the present work, scanning tunneling microscopy has been used to study Bainite steel surfaces prepared by polishing and etching, and then covered with a layer of glycerol. The success of sample preparation method of this work offers a general way of studying easy-oxidized metal surfaces in an ambient environment. It is found that Bainitic ferrite is composed of structural units much smaller than subunits that are generally accepted as the smallest building block of Bainitic ferrite and that the structural units at different levels have certain self-similarity, an important characteristic of fractals. Two kinds of fractal dimension calculations have been carried out. The straight line in the ln–ln plot of scan size versus the number of ‘‘boxes’’, as well as the obtained fractal dimension, DB = 2.74, indicate that Bainitic ferrite is a sort of self-embedded fractal, at least in the scale range of the images.
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