The Influence of Hydrogen on Plain Carbon and Low Alloy Steels Containing Strain Induced Cavities

Abstract

Cavities were created near carbide particles in heat treated steels by various plastic pre-strains on notched tensile bars. The cavities formed preferentially in the central region of the bar. In the quenched and tempered low alloy steels "void sheet" formation occurred, whereas only isolated cavities were seen in the plain carbon steels. Small tensile specimens were then machined from the strained notched bars so that the maximum cavity density corresponded to the central region. After vacuum annealing, this region was electrolytically charged with hydrogen. It was found that the presence of cavities showed an increase in hydrogen absorption. Hydrogen charging caused a reduction in the Yield, Ultimate and Fracture strength and the ductility of the steels; and subsequent modification of the stress-strain curves. These effects were more pronounced in the steels with higher cavity densities. The strength and ductility have been expressed as a function of the heat treatment, the degree of prior strain and the amount of hydrogen absorbed. Scanning electron micrographs of the fracture surfaces of the hydrogen charged plain carbon steels revealed a reduction in dimple size in the central region when compared with those of the uncharged specimens. The severely hydrogen charged low alloy steels showed quasi-cleavage fracture. This indicates a fracture transition from ductile to quasi-cleavage micro mechanism, which is a function of the amount of hydrogen absorbed into the steel. Hydrogen absorption is related to the size and distribution of the pre-strain cavities. Based upon the observed effects of hydrogen on the fracture mechanisms and tensile properties of the steels, a proposed explanation for the enhancement of hydrogen damage and for the structure sensitivity of this phenomenon is presented. These implications are discussed in terms of the existing theories of hydrogen damage.