Abstract
3rd generation advanced (ultra)high-strength steels (AHSS), such as quenched & partitioned martensitic-austenitic steels, are one prominent solution for producing a combination of high strength and deformability. This study is motivated by both the relatively unknown susceptibility of such high-strength microstructures to hydrogen and the current strong drive towards developing hydrogen-resistant steels. Here, the material under consideration is a medium-carbon direct-quenched and partitioned (DQ&P) 0.3C - 1.0Si - 1.9Mn - 1.0Cr steel, whose increased residual austenite content (RA > 9%) was stabilised with interrupted quenching and holding at ~175 °C. A direct-quenched DQ (RA < 3%) is used as a reference for comparison of different RA contents. Varying hydrogen concentrations (CH) were introduced with pre-charging before doing the slow strain rate tensile tests under the same environment, and measured with TDS. Both DQ&P and DQ have uncharged CH ≈ 0.2 wt.ppm, and tested with thin square tensile bars in air, DQ&P has tensile strength (σTS) of ~1920 MPa, and σTS of DQ is 2080 MPa. The as-quenched DQ is more vulnerable to elevated hydrogen contents: its σTS drops to ~1570 MPa (-24.5 %) with CH ≈ 1.4 wt.ppm, when DQ&P tolerates CH ≈ 1.7 wt.ppm at σTS of ~1620 MPa (-16.0 %). Furthermore, DQ&P can tolerate even CH ≈ 3.0 ppm at σTS ≈ 1470 MPa before its properties drop proportionally as much as DQ with 1.4 wt.ppm H. Thus, elevated RA content via partitioning mitigates hydrogen embrittlement in given conditions. Density functional theory calculations verify that hydrogen prefers energetically to stay rather in pure fcc-Fe than in bcc-Fe, encouraging development of hydrogen resistant bcc+fcc steels. With partitioning-enriched carbon content of RA (fcc-Fe + Cint, “int” = interstitial), Hint is energetically most favourable at the second nearest-neighbour position to Cint, indicating their important interplay within austenite containing carbon steels.