Abstract
Experimental results on hydrogen embrittlement by the present author were re-analyzed systematically and were discussed with the prevailing theories, reminding the fracture mode. The changes in the states of hydrogen during embrittlement were also considered.
In metal lattice, hydrogen is trapped by various kinds of defects, namely trap sites. The dislocation is the representing one. It traps hydrogen in competition with carbon and becomes mobilized. This helps to make the dislocation interact with each other so as to produce the vacancy. This vacancy can successively trap hydrogen and be stabilized by forming the vacancy-hydrogen compound (VHC). By slow deformation, this VHC is transported by dislocation motion to form vacancy cluster inside the grain. When the VHC is transported toward to grain boundary and reacts with it, more hydrogen becomes trapped there.
The, vacancy cluster is shown to be an essential factor for quasi-cleavage (QC) fracture that is a kind of ductile fracture. The oxide inclusion promotes the formation of vacancy cluster and enhances QC fracture more with hydrogen absorption. These facts support that the HESIV (Hydrogen Enhanced Strain Induced Vacancy) theory is applicable to QC fracture.
In tempered martensitic steels, hydrogen that is trapped by grain boundary causes the inter-granular (IG) fracture, which is co-existing with the QC fracture. When the prior grain boundary bears more phosphorus segregation and/or film like cementite precipitation, the grain boundary facet become prevailing in fracture surface. Along the grain boundary region where cementite is lacking, there appears “tear ridge” in between intergranular (IG) facet. In the extreme case with scares globular grain boundary cementite, the fracture surface is covered with fine wrinkling (tear striation), which is finely arranged multiple tear ridge. This is sometimes referred as IG-like or IQC (or intergranular QC) fracture.
