Hydrogen permeation in a submerged arc weldment of TMCP steel

The effects of the heterogeneous microstructure at the base metal, the heat affected zone (HAZ) and weld metal on hydrogen permeation in thermo-mechanical controlled rolling (TMCP) steel weldments have been investigated. The base metal with equiaxed refined ferrite and scattered fine grain pearlite has the highest permeation rate and effective diffusivity. The HAZ with bainite shows the lowest values of the permeation rate and effective diffusivity. Weld metal yields a higher permeation rate coupled with an intermediate diffusivity value. The hydrogen apparent solubility is low for the base metal, intermediate for the HAZ and high for the weld metal. The mechanisms of hydrogen diffusion path and hydrogen traps are discussed and experimentally confirmed using the hydrogen microprint technique. The high diffusivity paths and the hydrogen trapping site are the grain boundary and the ferrite/carbide interfaces for both the base metal with refined ferrite and the HAZ with bainitic microstructure. The spaces among the basket-weave acicular ferrite, where the martensite and the retained austenite (M/A) constituents present, are the main hydrogen trapping sites for the weld metal.