Quenching and Partitioning of Martensite and Other Advancements in Steels: Session 3
Program Organizers: Emmanuel De Moor, Colorado School of Mines
Wednesday 8:00 AM
July 12, 2017
Location: Hyatt Regency Chicago
Session Chair: Antti Kaijalainen, University of Oulu
Tough Ductile Ultra High Strength Steels Through Direct Quenching and Partitioning – An Update: Mahesh Somani1; David Porter1; Jukka Kömi1; Devesh Misra2; 1University of Oulu; 2University of Texas at El Paso
The TMR-DQP* processing route comprising thermomechanical rolling followed by direct quenching and partitioning, has shown huge potential for the development of not only tough ductile ultra-high strength structural steels, but also hard abrasion-resistant steels. The approach comprised designing suitable chemical compositions, establishing appropriate DQP processing conditions with the aid of physical simulation and finally testing laboratory rolled DQP material with the emphasis on cost-effective process development, amenable for industrial hot strip production. Evaluation of DQP processed samples cooled slowly following DQP processing thus simulating coiling confirmed achieving the desired martensite-austenite microstructures and targeted mechanical properties. Ausforming in Tnr regime resulted in extensive refining and randomization of the martensite packets/laths besides fine division of interlath austenite, thus resulting in an all-round improvement of mechanical properties, including low temperature toughness and uniform elongation. Preliminary investigations on alloys designed with 0.2C and 0.3C have shown promising properties for structural and wear-resistance applications, respectively.
Microstructural Evolution of 4340 Steel Martensite After Quenching and Tempering: Amy Clarke1; Virginia Judge1; John Speer1; Daniel Coughlin2; Dean Pierce3; Michael Miller3; Jonathan Poplawsky3; Bjorn Clausen2; Kester Clarke1; Jonathan Almer4; Robert Field1; Don Williamson1; George Krauss1; 1Colorado School of Mines; 2Los Alamos National Laboratory; 3Oak Ridge National Laboratory; 4Argonne National Laboratory
Steel is one of the most versatile structural materials, because it can be heat-treated to exhibit a wide range of microstructures and properties. In medium-carbon low-alloyed steels, the high temperature austenite phase undergoes diffusionless phase transformation to form carbon supersaturated martensite during quenching, which may be relieved by tempering to yield disastrous or desirable mechanical properties, due to local chemical and structural evolution. Here we reveal the atomic and nanoscale chemical and structural variations that occur in a medium carbon, low-alloyed 4340 steel after quenching and tempering with atom probe tomography and complementary techniques, such as transmission electron microscopy, Mössbauer effect spectroscopy, and synchrotron x-ray diffraction. The effect of quench rate and short and long time tempering at temperatures ranging from 100 to 575 C is discussed, along with the characterization of mechanical properties. These results suggest that short-time tempering can yield martensite by design through advanced manufacturing.
Electron Diffraction Analysis of Martensite in Quenched High Carbon Steels: Tianwei Liu1; Dehai Ping1; Ikumu Watanabe1; Takahito Ohmura1; Masato Ohnuma2; 1National Institute for Materials Sceince; 2Hokkaido University
Martensitic substructure in a quenched Fe-1.4wt.%C sample has been investigated in detail using selected area electron diffraction (SAED) technique in a conventional transmission electron microscopy (TEM). The experimental SAED patterns from various zone axes were used for building a reciprocal lattice space of the martensite. The reciprocal lattice space suggests that two sets of diffraction spots exist in the martensite. The two sets of diffraction spots (face-centered cubic (fcc) spots and hexagonal spots) suggest that the two crystalline phases with body-centered cubic (bcc or α-Fe) and hexagonal (ω-Fe, which exists at the α-Fe twinning boundaries) structures actually exist in the quenched martensite. The diffraction pattern from the reciprocal lattice space matches perfectly with the experimental one.
M7C3 - M23C6 In Situ Carbide Transformation in the Cast Heat Resistant Fe-Cr-Ni Alloy and Non-Crystallographic Symmetry of this Transformation: Valentin Kraposhin1; Alexander Talis2; Sergej Kondrat'ev3; Gregory Anastasiadi3; Ekaterina Svjatysheva3; 1Bauman Moscow State Technical University; 22A.N.Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Sciences,; 3Peter the Great St. Petersburg Polytechnic University
The microstructure and phase composition of the heat resistant 0.45C-25Cr-35Ni alloy have been investigated in the as cast condition and after holding at 1150C with the durations of 2 – 100 hours. After 2 h holding the primary M7C3 carbide particle has been fragmented onto domains of 500 nm size with the partial transformation of some domains into the М23С6 carbide. The total transformation of the hexagonal М7С3 carbide into the М23С6 carbide with the FCC lattice was observed after 100 hours duration. The fine structure of the М7С3 particle consists of periodical set of flat parallel defects. The mechanism of the in situ М7С3 - М23С6 transformation including the transition of a carbon atom coordination number from 6 to 8 has been explained in the frameworks of the previously proposed combinatory model for polymorph transformation in metals  Financial support by RFBR-17-02-00225. .Talis A., Kraposhin V. Acta Cryst. A70 (2014) 616-625.
Case Study of Deformation-induced Martensitic Transformation in Steel for Presshardening: Hana Jirková1; Katerina Opatova1; 1University of West Bohemia in Pilsen
Deformation-induced martensitic transformation is used for improving mechanical properties of AHS steels which contain metastable retained austenite (RA). TRIP steels are one of the categories that fall into this group. Their microstructures consist of ferrite, bainite, and metastable RA. Cold working causes RA in these steels to transform to deformation-induced martensite. A technical complication to their treatment routes is the isothermal holding stage. At this stage, bainite forms and RA becomes stabilized which is the key aspect of the process. A CMnSi-type low-alloy steel with 0.2 % carbon was subjected to various experimental cooling sequences which represented presshardening operations at tool temperatures ranging from 500 °C to the room temperature, followed by isothermal holding in the bainitic transformation region. By varying the cooling parameters, one can obtain a broad range of mixed martensitic-bainitic structures containing RA, with strengths of about 1300 MPa, and A5mm elongation levels of 10 %.
Effect of Carbon Content on Bainite Transformation Start Temperature on Fe–9Ni–C Alloys: Hiroyuki Kawata1; Kazuki Fujiwara1; Manabu Takahashi1; Toshiyuki Manabe1; 1Nippon Steel & Sumitomo Metal Corporation
Upper bainite in steels has many common features with lath martensite in steels. But there are some studies which indicate that bainite transformation start temperature (Bs) is over T0 on steels containing high carbon content. We measured Bs on Fe–9 mass% Ni alloys containing 0.003–0.89 mass% C. In low carbon alloys, Bs is below T0, and the increasing of carbon content decreases Bs standing in a line which is parallel to T0. On the other hand, in high carbon alloys, carbon content does not affect Bs which stand around 753K. The border between these two tendencies and Bs in high carbon alloys seem to correspond to the intersection point between the line of Bs on low carbon alloys and the calculated γ/(γ+θ) phase boundary.
10:00 AM Break