Abstract
The bubbles formed by hydrogen attack in carbon and low alloy steels were observed by scanning and transmission electron microscopies. The results summarized are:
(1) The bubbles nucleate primarily at heterogeneous nucleation sites on grain boundaries and the bubble density is essentially independent of temperature over the range of our experimental conditions. Carbides are found to be the main sites and MnS also provides the site.
(2) The density and growth rate of bubbles depend on the heat-reatment and local microstructure and are high on the boundary of the grain containing much amount of carbide. In general, the bubble density in 2.25 Gr-1 Mo steel (except HAZ) is very low (-5×105cm-2) compared to that of other steels (-107cm-2).
(3) Tensile stress accelerates bubble growth but does not influence the order of magnitude of the bubble density.
(4) Coalescence of bubbles occurs, leaving low and rounded traces of the walls between bubbles.
(5) The above observations support Shewmon's model based on Raj and Ashby's equation and his predictions for carbon steel.
(6) The internal pressures in the bubbles were estimated by applying our data of the bubble density and size to Raj and Ashby's equation and it was found that the pressure was very sensitive to the contents of Cr and Mo in steel. This suggests that the resistivity of a given steel to hydrogen attack depends on the attainable internal pressure as well as the bubble density.
