J. Ćwiek

Plasma Nitriding as a Prevention Method Against Hydrogen Degradation of Steel

Plasma Nitriding as a Prevention Method Against Hydrogen Degradation of Steel Aim of this paper is evaluation of susceptibility of plasma nitrided structural steel to hydrogen absorption and degradation. Structural steel, nitrided at glow discharge in the gas mixture of various N2, H2, Ar content was subjected to cathodic hydrogen charging in acid solution simulating the aged engine oil. The effect of the nitrided layers on the hydrogen transport and on the irreversible trapping was evaluated by the measurements of the hydrogen permeation rate and by the vacuum extraction, respectively. Surfaces with modified layers were examined with the use of a scanning electron microscope (SEM) before and after hydrogen permeation tests. In the presence of the not defected compact nitride layer, no hydrogen permeation through the steel has been stated under the experimental conditions. Absorbed hydrogen was accumulated within this layer. Using the atmosphere of the higher nitrogen to hydrogen ratio at plasma assisted nitriding provides the formation of thin compact nitride zone, highly protective against corrosion and hydrogen degradation.

Hydrogen Enhanced-Cracking of High-Strength Steel Welded Joints

Hydrogen Enhanced-Cracking of High-Strength Steel Welded Joints Aim of this paper is evaluation of susceptibility of high-strength steel welded joints to hydrogen degradation and establishing of applicable mechanism of their hydrogen-enhanced cracking. High-strength quenched and tempered steel grade S690Q and its welded joints have been used. Susceptibility to hydrogen degradation of steel and welded joints has been evaluated using monotonically increasing load. Slow strain rate test (SSRT) was carried out on round smooth specimens in air, and seawater under cathodic polarization. Elongation and reduction in area were choosen as measures of susceptibility to hydrogen embrittlement. Fractographic examinations with the use of scanning electron microscope (SEM) were performed to establish suitable mechanism of hydrogen-enhanced cracking of the welded joints. Tested high-strength steel and its welded joints are susceptible to hydrogen embrittlement when evaluated with the use of SSRT. The loss of plasticity is higher for welded joints then for the base metal. There is no possibility to perform direct observations of exact mechanism of hydrogen-assisted cracking so far. On the base of mechanical tests and fractographic observations it is likely to deduce which of nowadays models of hydrogen degradation and cracking is the most viable. Tested steel and its welded joints could be safely utilized in marine constructions under cathodic protection provided that overprotection does not take place. Hydrogen-enhanced localized plasticity (HELP) model is more applicable mechanism of hydrogen degradation than other for high-strength welded joints in seawater environment.