New High Strength Steels with Reduced Hydrogen Embrittlement Sensitivity

Experience indicates a high sensitivity of high strength materials against hydrogen embrittlement (HE). Steels with a yield strength exceeding 1000 MPa are usually not employed for applications where a critical hydrogen (H) uptake during processing or service might take place. But especially in transportation, steels with higher strength would allow increased weight reductions. Our approach is to understand the mechanisms of HE which not only depend on H concentration and stress level, but also on the microstructure of the steel itself. Exposing weak microstructural features enables to design modified thermomechanical processing routes to avoid them. With taylored modifications of the microstructure, we were able to increase the yield strength plus reduce the HE sensitivity of the steel.

Fig. 1: Stress-strain curves for the steel from slow strain rate tensile tests. Conventionally processed sample without H (curve 1): ductile fracture and high total elongation. Charging with H (curve 2): intercrystalline fracture and high loss of total elongation. Modified processing (curve 3 and 4) increases yield strength and lower loss of total elongation by H.

We investigated a medium Mn steel with the chemical composition 0.2C-10.2Mn-2.8Al-1Si (in wt.%). Medium Mn steels combine high strength and ductility and are candidates for future automotive applications. But not much is known about HE sensitivity and embrittlement mechanisms in these multiphase steels. A method to quantify the sensitivity against HE is the slow strain rate tensile test. Curve 1 in Fig. 1 shows the stress-strain curve for this steel after conventional processing and no H charging. The yield strength is 890 MPa and the total elongation 51 %. When the steel is charged with H (curve 2), the total elongation is significantly reduced to 13 %. The difference between the total elongations of uncharged and H charged samples allows to quantify the HE sensitivity. Here, this loss in total elongation is high (38 %) and indicates a high HE sensitivity. The appearance of the fracture surfaces after tensile testing (Fig. 1) provides information about the failure mechanisms: Dimples on the fracture surface of the sample without H (curve 1) reveal a ductile fracture. This changes for the sample with H (curve 2), which shows a more brittle failure along interfaces.

Fig. 2: Initial microstructure: austenite (g) with fcc structure and ferrite (a) with bcc structure. During plastic deformation austenite transforms to martensite (a').

A detailed characterization reveals the mechanisms leading to HE: The microstructure of the medium Mn steels shows two phases: face centered cubic (fcc) austenite (g) with a volume fraction of 60 % and body centered cubic (bcc) ferrite (a). In the bcc lattice H diffusion is much faster, but H solubility is much lower than in austenite. When charging with H, finally austenite contains much more H than ferrite. During deformation metastable austenite transforms partially into martensite (a`) (Fig. 2). This martensitic transformation is very fast. Therefore, martensite inherits all the H from austenite. This leads to drastic H oversaturation of martensite as martensite has a bcc lattice and hence a very low H solubility (Fig. 3).

Due to the high diffusivity of H in martensite, the H can escape the oversaturated lattice by moving to interfaces like a`/ a` grain boundaries. This leads to the observed embrittlement of these interfaces.

Fig. 3: Schematically: H in oversaturated martensite (a') segregates to grain boundaries. Embrittlement of these boundaries leads to observed intercrystalline failure.

To avoid these mechanisms, we lowered the heat treating temperature. This provides a finer grain size (higher yield strength of modified processed steel curve 3) and more Mn and C partitioning to austenite stabilizes the austenite. During deformation, less martensite is formed and the sensitivity against HE is decreased: The H charged sample (curve 4) in comparison to the uncharged sample (curve 3) shows a low loss in total elongation of 8 % and a ductile fracture mode. This indicates a much lower HE sensitivity, even with the higher yield strength of the modified processed steel of 1160 MPa.

This description is highly simplified but published in more detail elsewhere [1].

References:

[1] B. Sun, W. Krieger, M. Rohwerder, D. Ponge, D. Raabe: Acta Mat 183 (2020) 313.

Author: D. Ponge (MA)

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