The microstructure of high strength steel issusceptible to delayed failure caused by the absorption ofhydrogen produced either during cathodic charging orelectroplating. When 0.8%C and AISI 4340 steel aresubjected to constant load testing, a wide range of failuretimes is observed. By applying Weibull statistics smallchanges in experimental parameters such as heat-treatmentare detected readily and are explained in the terms ofhydrogen trapping at microstructural defects.During the electroplating of steel in a double-cell,quantitative measurements are made of the amount ofhydrogen permeated. Current densities are measured in therange 2- 40 mAcm2 and it is shown that, although thelowest current density produces the most mechanically soundplate, it also causes the largest amount of hydrogenabsorption. The nickel deposit is found to act as areservoir for reversibly trapped hydrogen allowingdiffusion to continue into the steel after the cessation ofplating.Permeation measurements were taken on AISI 4340 steelusing an electrochemical probe developed from the BarnacleElectrode. The effects of cathodically charging andelectroplating with nickel are compared. Exposure of thesteel to the atmosphere is shown to have an importantinfluence on the hydrogen content after a period of timedue to a limited occurrence of corrosion. Various post-plating treatments are commonly used to remove a damagingconcentration of hydrogen and the quantitative effects ofsuch treatments are described.Finally, a mathematical model is proposed whichexplains the reason for the wide spread of delayed failuretimes. It is found that if the stress intensity necessaryto initiate a crack is known and, provided either thenominal stress or the crack size is known, it is possibleto calculate either the allowable defect size or theallowable stress below which cracking is not expected tooccur.
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