HYDROGEN TRAPPING AT PHOSPHORUS-SEGREGATED GRAIN-BOUNDARIES IN NICKEL-CHROMIUM STEEL

Abstract

The effect of phosphorus segregated at grain boundaries on hydrogen trapping in nickel-chromium steel as compared to pure iron has been studied by using gas-phase charging and electrochemical-detection techniques. The segregation of phosphorus at grain boundaries was produced by tempering phosphorus-doped nickel-chromium steel specimens at 500-degrees-C for 72 h and then revealed from SEM metallography of the steel following a special chemical etching procedure in a picric acid solution. The hydrogen trap densities and trap binding energies were determined from the plot of [(t(T)/t(L)) - 1] vs the inverse square root of input hydrogen pressure. The trap density in nickel-chromium steel was clearly increased with a reduced effective diffusion coefficient by the addition of the alloying elements, Ni, Cr and Mn. The segregation of phosphorus to grain boundaries slightly increased the hydrogen trap binding energy and hence reduced the effective diffusion coefficient. From the results, it is suggested that phosphorus-segregated grain boundaries act as saturable trap sites for hydrogen in nickel-chromium steels.