Advanced High Strength Steels IV: Session VI
Sponsored by: TMS Structural Materials Division, TMS: Steels Committee
Program Organizers: Ana Araujo, Vesuvius USA; Mary O'Brien, Los Alamos National Laboratory; Tilmann Hickel, Bam Federal Institute For Materials Research And Testing; Amy Clarke, Los Alamos National Laboratory; Kester Clarke, Los Alamos National Laboratory; C. Tasan, Massachusetts Institute of Technology; MingXin Huang, University of Hong Kong
Thursday 2:00 PM
February 27, 2020
Room: Balboa
Location: Marriott Marquis Hotel
Session Chair: Mary O'Brien, Colorado School of Mines; Kester Clarke, Colorado School of Mines
2:00 PM Cancelled
Cryogenic Tensile and Microstructural Behaviors of High Manganese Steel Welds: Myeonghwan Choi1; Junghoon Lee1; Hyunbin Nam1; Namhyun Kang1; Myung-Hyun Kim1; Dae-Won Cho2; Dae-Geun Nam3; Seunghwan Lee4; 1Pusan National University; 2Busan Machinery Research Center; 3Korea Institute of Industrial Technology; 4Korea Aerospace University
We investigated the weldability and relationship between microstructure and tensile properties in 24 wt% Mn steel welds and, specifically, submerged arc welds (SAWs) were produced using these welds for cryogenic applications. The base metal (BM) and weld metal (WM) exhibited a stacking fault energy (SFE) that maintained a stable austenite phase for 27.1 and 17.0 mJ/m2, respectively. Deformation twins were observed after tensile testing of the BM and WM at 298 K. Weld metals using undermatched fillers showed a lower SFE and coarser grain size compared to that of the BM. Therefore, the tensile testing at 110 K produced deformation twins and ε-martensite. The formation of ε-martensite with deformation twins antedated necking during tensile testing and elongation decreased at 110 K. However, the SAWs of high Mn steels maintained excellent low-temperature mechanical properties such as elongation, tensile strength, and yield strength with values of 20%, 1150, and 617 MPa, respectively.
2:20 PM
Impact of Cr and Mn on the Hydrogen-carbide Interaction in High-strength Steels: Lekshmi Sreekala1; Poulumi Dey2; Tilmann Hickel1; Jörg Neugebauer1; 1Max-Planck-Institut für Eisenforschung GmbH; 2Technische Universiteit Delft
Understanding hydrogen-assisted embrittlement of advanced high- strength steels is decisive for their application in automotive industry. Since the addition of Cr to the composition of high-Mn steels improves their corrosion resistance and simultaneously changes the microstructure, the influence of Cr and Mn on hydrogen solubility in the context of carbide formation is scientifically challenging. Density functional theory based calculations have been employed to study the hydrogen interaction of carbides and interfaces containing Cr and Mn. Cr has an indirect impact on hydrogen, since it strongly influences the thermal stability of the carbides like cementite and Fe23C6, but the individual Cr atom interacts only weakly with H. On the other hand, Mn has a weaker impact on carbide formation, but its incorporation into the carbides yields a remarkable non-linearity in the interaction with hydrogen, which is not consistent with the H-Mn interaction known in the austenitic steel matrix.
2:40 PM
Interstitial-free Bake Hardening Realized by Epsilon-martensite Reverse Transformation: Shaolou Wei1; Menglei Jiang1; Cemal Tasan1; 1Massachusetts Institute of Technology
Benefitting from thermally- or mechanically-induced martensitic transformation, ferrous-based alloys exhibit an exceptionally broad range of strength-ductility combinations. Due to technological importance, the forward austenite-to-martensite transformation is intensively studied, whereas its reverse counterpart remains comparatively less explored and unutilized. Here in this presentation, by exploiting an interstitial-free Fe-Mn-rich alloy as a model material, we report a latent strengthening mechanism that is associated with the thermally-activated epsilon-martensite-to-austenite reverse transformation. We will show that this sort reversion-induced hardening effect can be achieved in the same time-scale and temperature range similar to the conventional bake-hardening treatment, but leads to both improved strength and cumulative ductility. We will discuss the key mechanisms regarding transformation kinetics, kinematics, strengthening and ductilization modules.
3:00 PM
Strength and Toughness of Nano-structured Pearlite: Kushal Mishra1; Vaibhav Khiratkar1; Aparna Singh1; 1Metallurgical Engineering and Materials Science, IIT Bombay
Nano-structured pearlitic steels are extremely strong with significant amount of ductility. This is because of the extremely fine ferrite-cementite lamellar structure that hinders dislocation motion effectively due to insufficient pile-up in the refined ferrite structure. Prior studies have shown a decrease in toughness with lamellae refinement in pearlite and a subsequent increase as the the spacing reaches around 200 nm. Fracture toughness and fatigue life of pearlite with lamellar spacing below 100 nm have been investigated and the crack growth characteristics as a function of the microstructure have been examined using post-martem scanning electron microscopy studies. A combination of high strength, fracture toughness and higher fatigue crack growth life has been achieved by decreasing the lamellar spacing in pearlite.
3:20 PM Cancelled
Martensite Transforamtion Induced Unprecedented Strength in Pure Iron: Hongwang Zhang1; 1Yanshan University
Martensitic transformation can easily induce a maximum hardness value of 800-900 HV (Vickers hardness) for steels with carbon contents of 0.6 wt.% and above. However, the occurrence of martensitic transformation in pure iron requires exceptionally high cooling rates (105 to 106 °C/s), and the maximum achievable hardness is only about 150 HV. Here we report an extreme hardness of 830 HV in pure iron obtained through high pressure induced martensitic transformation at a rather slow cooling rate of just 10 °C/s. This unprecedented strength originates from the formation of twin-related martensitic laths with an average thickness of 3.8 nm.