Hydrogen Embrittlement in Medium Mn Steels 

Why are medium Mn steels interesting materials?

Medium Mn steels containing 3 to 12 wt.% Mn have receive high attention, due to the increasing demand for strong and compositionally lean materials with good formability from the automotive industry.

The most widely investigated microstructure of this type of steels consists of ultrafine grained ferrite and austenite, realized by intercritical annealing. Different alloy compositions, starting microstructures before heat treatment as well as the intercritical annealing conditions can result in a wide range of microstructural characteristics, such as the austenite fraction (typically ranging from 20 to 70 vol.%, grain size (from below 200 nm up to a few micrometers), phase morphology (globular or laminated), microstructure percolation and dislocation density. Among these parameters, the fraction of austenite has a crucial influence on the mechanical properties of such steels. For a given alloy composition, a higher austenite fraction typically corresponds to a lower content 
of C and Mn partitioning into austenite, an effect which leads to lower austenite stability and an enhanced transformation-induced plasticity (TRIP) effect. Most studies on such steels have focused so far on tailoring austenite conditions, in order to achieve an improved strength-ductility combination. The tensile 
strength of austenite-ferrite medium Mn steels normally ranges from ~800 to 1400 MPa. This high strength level fuels concerns about hydrogen embrittlement (HE), impeding the use of  these steels.


Why is hydrogen embrittlement important in medium Mn steels?

The risk of hydrogen embrittlement (HE) is currently one important factor impeding the use of medium Mn steels. However, knowledge about HE in these materials is sparse. Their multiphase microstructure with highly variable phase conditions (e.g. fraction, percolation and dislocation density) and the feature 
of deformation-driven phase transformation render systematic studies of HE mechanisms challenging.


Why is the investigation of hydrogen embrittlement in medium Mn steels so challenging?

The HE behavior in medium Mn steels is complex, due to 3 main reasons:


(1) High Mechanical Contrast in medium Mn steels

The high contrast between ferrite and austenite in terms of H diffusivity and solubility makes the H migration and trapping behavior in such steels more complicated than in materials consisting of a single phase or multiple phases 
with similar crystal structures (e.g. ferrite-martensite dual phase steels).


(2) Austenite-ferrite interfaces in medium Mn steels

The austenite-ferrite interfaces are known as strong trapping sites for H. The density and spatial arrangement of such planar defects thus affect H trapping and migration.


(3) Austenite-to-martensite transformation  in medium Mn steels
The austenite-to-martensite transformation occurring during deformation resets the local structure and micromechanical state in terms of strain/stress partitioning and localization, as well as the local thermodynamic driving forces for H migration.


These three factors imply that the change of phase conditions (e.g. fraction, percolation, mechanical stability) in such steels might significantly affect the H trapping and migration and thus also the resulting HE susceptibility.


Which types of mechanisms may occur during the hydrogen embrittlement of medium Mn steels?

the prevalent HE micromechanisms in medium Mn steels are:


(1) Hydrogen-enhanced decohesion (HEDE)


(2) Hydrogen-enhanced local plasticity (HELP)


(3) Adsorption-induced dislocation emission (AIDE).


The former two mechanisms refer to the influence of internal H atoms in the bulk, whereas the AIDE model is based on H adsorption at the crack surface. Therefore, for obtaining a better understanding of the HE mechanisms in 
medium Mn steels, a more comprehensive investigation covering a more systematic range of phase conditions is required.


Hydrogen embrittlement mechanisms in medium Mn steels
Acta Materialia 183 (2020) 313–328
Acta 2019 Sun et al hydrogen embrittleme[...]
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