Quenching and Partitioning of Martensite and Other Advancements in Steels: Session 5
Program Organizers: Emmanuel De Moor, Colorado School of Mines

Thursday 4:00 PM
July 13, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: Xianwen Lu, Shanghai Jiaotong University

4:00 PM  
Transformation Strains and Orientation Relationship Models in Steels: Konstantinos Koumatos1; Anton Muehlemann2; 1University of Sussex; 2University of California, Berkeley
     By employing a criterion of least atom movement, it is shown that the Bain strain is the optimal stretch component of the γ-to-α phase transition in steels. Exploiting the transformation mechanism implied by the Bain strain, a method is then introduced that identifies transformation strains with their corresponding orientation relationships and used to develop a strain-based approach for the γ-to-α phase transition, with common orientation relationship models analyzed. One of the advantages of this approach is that it naturally takes into account the tetragonality of the product phase and leads to a generalization of typical orientation relationship models for arbitrary values of the ratio of tetragonality. The generalized orientation relationships predict a sharpening of texture with increasing tetragonality - as experiments suggest - and provide some insight into the Greninger-Troiano model.The methods presented are general enough to be extended to any martensitic transformation between Bravais lattices.

4:20 PM  
Revisiting the Microstructures of Martensite in Carbon Steels: Xinqing Zhao1; 1Beihang University
    It has been well accepted that the martensite formed by quenching in carbon steels is a supersaturated solid solution of carbon in ferritic iron, and that the carbon atoms in the interstices of martensite lead to the body-centred tetragonal (bct) lattice. In this work, the microstructures of martensite in carbon steels were characterized. The results of the study indicated that the martensites formed in high carbon steel and low carbon steel by quenching are twin type martensites. Large numbers of nanometer-scaled ultrafine particles, i.e. ω phase with a primitive hexagonal crystal structure, were observed to embed in the interfaces of twins. We believe that the ω phase embedded in the twin interface and coherent with the ferrite matrix should be responsible for the outstanding hardness of martensite in carbon steel. In addition, it was foundthat the strain induced martensite in high carbon steel does not exhibit tetragonal feature.

4:40 PM  
Initial Investigation on the Corrosion Mechanism of a Low Carbon Quenching and Partitioning Steel in Natural Seawater: Jilan Yang1; Yisi Song1; Yuankai Jiang1; Zhenghong Guo1; Jianfeng Gu1; 1Shanghai Jiao Tong University
    To understand corrosion resistance in the coast environment, different treatments including traditional quenching and tempering (Q&T), typical quenching and partitioning (Q&P) and critical partial quenching and partitioning (P&QP) were carried out to obtain different microstructures in Fe-0.2C-1.5Mn-1.5Si steel. The relationship between microstructure and the corrosion resistance in the natural seawater was investigated initially by weight loss test and related electrochemical characterizations. The result shows that oxygen absorption corrosion dominates corrosion mechanism for present three kinds of specimens. Q&P one exhibits a slight better resistance than Q&T one because retained austenite reduces corrosion rate. However, the resistance of P&QP specimen is the worst among three kinds of specimens. Further analysis indicated that Cl- ion in the solution is easy to penetrate into the film formed by corrosion product and then is absorbed by ferrite, leading to accelerate pitting behavior and then form galvanic corrosion with either austenite or martensite.

5:00 PM  
Microstructure and Variant Selection of Martensite in Ferrite-martensite Dual Phase Steels Undergone Thermo-mechanical Control Process: Anh Pham1; Takuya Ohba1; Shigekazu Morito1; Taisuke Hayashi1; 1Shimane University
    A dual-phase microstructure of ferrite and martensite is often produced from low-carbon, low-alloy steel by a thermo-mechanical control process. The effect of deformation in hot rolled prior-austenite on formation of martensite microstructure has not been clarified, because information on the former is not readily available. In this study, the prior-austenite microstructure of two dual phase steels was reconstructed from martensite orientations measured by electron backscatter diffraction. Owing to development of deformation structure in prior-austenite, variant selection of martensite was observed with dominance of the variants, whose (011)α’ plane is aligned with primary (111)γ or secondary (1 ̅11)γ slip planes of austenite. Two kinds of martensite morphology were often found within one prior-austenite grain. Coarse-grained martensite developed inside the austenite grain, whereas fine-grained martensite formed at ferrite-austenite boundaries due to partitioning of carbon. During a tensile test, micro-voids tended to nucleate at the boundaries between ferrite and fine-grained martensite.

5:20 PM Break