Efeito da hidrogenação catódica em ausência de tensão mecânica e durante ensaios de corrosão sob tensão em aços martensíticos

Abstract

Three low-alloy high strength steels presenting martensitic structures, were hydrogen cathodically charged, in the absence of applied mechanical tension, in a 1N H2 SO4 solution, free from poisons, to observe their tendency to crack. The "crack index" was larger for the AISI 4340 than for the AISI 8640 and at low applied current densities this difference was very marked. The values of the "index" for the AISI 8620 fell between those for the other steels, but its cracks were systematically localized at the sample's periphery and parallel to it. The nature of the craks for the three materials was transgranular, almost all being sub-surface and their number increased as the surface was removed by polishing, cracks being gradually replaced by crevicing. Under stress corrosion tests with and without applied potential, the fracture times were always greater for the AISI 8620. In the absence of applied potential AISI 4340 presented shorter fracture times than AISI 8640, but under cathodic polarisation the situation was reversed. Under the conditions of stress corrosion, the cracks were essentially intergranular. For the steels with similar mechanical properties (4340 and 8640), the fracture time due to stress corrosion without applied potential were in good agreement with the “crack index” for the lower value of applied current in the hydrogen induced test in the absence of mechanical tension.