| Delayed failure behaviour of the different transformation structures of 0.25C-2.5Ni-2.5Cr-0.5Mo-0.1V low alloy steel has been studied. The studied microstructures are martensite, lower bainite, and mixed structure of 50% martensite and 50% lower bainite. All these structures have been tempered at 450℃ for 40 min to have the same tensile strength level of 143㎏/㎟. Delayed failure testing has been carried out with cantilever bend tester, in distilled water at 25℃. By comparing K_(ISCC) values, lower bainitic structure has shown the highest value, although it is only slightly higher than that of the martensitic structure. Mixed structure has the lowest resistance to delayed failure. The fracture modes of both martensitic and mixed structures have been observed as intergranular. In the martensitic structure, however, it is noticeable that there is a larger amount of ductile tearing between intergranular facets. The fracture mode of lower bainitic structure is the mixed topography of microplastic tearing and microvoid coalescence. The above experimental results are discussed in terms of Oriani`s decohesion theory of hydrogen embrittlement. The lowest resistance of the mixed structure to delayed failure may be due to the enhanced decohesion by hydrogen at the phase boundaries of martensite and lower bainite. |
|
