Hydrogen Embrittlement of Nanostructred Bainitic High Strength Steel

Abstract

Characterization of nanostructured bainitic high strength steel revealed austenitic and bainitic-ferritic constituents. Hydrogen diffuses through austenite slower than bainitic ferrite. Discovering the effective hydrogen diffusion coefficient, subsurface hydrogen concertation and number of traps of such a microstructure leads to a deeper understanding of the role of retained austenite, as the dominant trap in such microstructures. Devanathan– Stachurski hydrogen permeation experiments determined the permeation parameters and subsequently numbers of reversible and irreversible traps. Volume of the retained austenite correlated well with the total number of traps and the mean free path. Lower mean free path, higher austenite content and trap density and more importantly finer dispersed distribution of films of retained austenite alongside with thin plates of bainitic ferrite satisfied percolation through the austenite. Therefore, permeation experiments demonstrated the lowest diffusivity in 2000 MPa microstructure between all the bainitic high strength membranes. On the contrary, combination of granular morphology and smaller volume of retained austenite triggered the loss of percolation and yielded to the lowest diffusivity for 1000 MPa microstructure. Higher volume of the retained austenite in isolation in the nanostructured bainitic steel does not produce lower diffusivity. With a semi analytical nonlinear fracture mechanics model and NTSSRT, we evaluated the hydrogen embrittlement susceptibility of the 1600 MPa exposed to H2S and the 2000 MPa steels exposed to hydrogen charging. At a certain hydrogen concertation, pre-charged samples showed greater decrease in hydrogen embrittlement index and J integral drop.