Quenching and Partitioning of Martensite and Other Advancements in Steels: Session 4
Program Organizers: Emmanuel De Moor, Colorado School of Mines

Thursday 2:00 PM
July 13, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: Amy Clarke, Colorado School of Mines

2:00 PM  
The Thermal and Mechanical Stabilities of Blocky and Lamellar Austenite in Advanced High Strength Steels: jinliang WANG1; peipei SUN1; wei xu1; 1Northeastern University
    The stability of retained austenite and its characteristics of transformation to martensite play the vital role in the design of the 3rd generation advanced high strengthen steels. The stability of austenite does not only depend on their chemistry and size, but also correlate to their morphologies, e.g. blocky island in TRIP steels while more often a lamellar structure in the 3G AHSS. Moreover, the martensitic transformation can be motivated by either a thermal process or a mechanical driving force. In the present study, prototype alloys with different stabilities of austenite are investigated systematically. The thermal stability is studied by dilatometry and Vibrating Sample Magnetometer. The evolution of the transformation is analysed by in-situ confocal and electron microscope, upon both thermal and mechanical driving forces. The transformation mechanisms and correlations of thermal and mechanical stabilities are attempted. Finally, effects of their size, morphology and magnetic field on the transformation are discussed.

2:20 PM  
Self-stabilization of Untransformed Austenite by Hydrostatic Pressure via Martensitic Transformation: Nobuo Nakada1; Yuji Ishibashi2; Toshihiro Tsuchiyama3; Setsuo Takaki3; 1Tokyo Institute of Technology; 2Sanyo Special Steel; 3Kyusyu University
    Hydrostatic pressure in untransformed austenite which is generated via martensitic transformation was evaluated from macro- and micro-viewpoints and then its effect on austenite stability was investigated in Fe-27%Ni austenitic alloy. X-ray diffractometry macroscopically reveals that lattice parameter of untransformed austenite is continuously decreased via martensitic transformation only when martensite becomes a matrix of duplex microstructure. This suggests that untransformed austenite is isotropically-compressed by surrounding martensite grains. On the other hand, microscopic strain mapping using electron backscatter diffraction technique indicates that finer untransformed austenite grain has higher hydrostatic pressure, while a high density of dislocations is also introduced in untransformed austenite near austenite/martensite interface due to lattice invariant shear characterized by non-thermoelastic martensitic transformation. Furthermore, it was experimentally demonstrated that the hydrostatic pressure stabilizes untransformed austenite definitely, but the austenite stabilization effect is not large enough to fully explain a large gap between Ms and Mf in steel by itself.

2:40 PM  
Influence of Manganese Content and Finish Rolling Temperature on the Martensitic Transformation of Ultrahigh-strength Strip Steels: Antti Kaijalainen1; Mahesh Somani1; David Porter1; Jukka Kömi1; 1University of Oulu
    The effects of manganese content and finish rolling temperature (FRT) on the transformed microstructures and properties of two low-alloyed thermomechanically rolled and direct-quenched steels were investigated. The materials were characterized in respect of microstructures and tensile properties. Besides, microhardness measurements were made both at the surface and mid-thickness of the hot rolled strips to characterize phase constituents. Detailed microstructural features were further revealed by LCSM and FESEM-EBSD. It was apparent that decreasing the temperature of controlled rolling, i.e. lower FRT, resulted in reduced amounts of martensite fraction, as a result of strain induced fine ferrite formation at the surface. The mid-thickness of the strip, however, consisted essentially of martensite and upper bainite. In contrast, high FRT and increased manganese content resulted in essentially martensitic microstructure due to enhanced hardenability. The paper presents a detailed account of the hot rolling and hardenability aspects, microstructures and properties of ultra-high strength strip steels.

3:00 PM  
In situ SEM-EBSD Observations of the Austenite to Martensite Phase Transformation in a Low Carbon Steel: Michael Pfund1; Moritz Wenk1; Reiner Moenig1; 1Karlsruhe Institute of Technology
    For many modern steels, martensite is an important microstructural component. The distribution and morphology of the martensitic phase strongly affects their mechanical properties. In order to investigate martensite formation on the microscopic scale, in situ experiments have been performed inside a scanning electron microscope. Small polished samples of low carbon steel were heated and quenched and the microstructural evolution was monitored by electron backscatter diffraction. This experiment enables the direct observation of the sample at the same location before and after martensite formation and during ferrite-austenite transformations. The recorded data shows morphological changes at the sample surface and reveals the characteristic orientation relationship and variant selection for these transformations. The investigations focus on analyzing local deviations from these established orientation relationships and highlight how the microstructural evolution during quenching is affected by the selection of martensite variants.

3:15 PM  
Carbon Segregation and Partitioning between Martensite and Retained Austenite in Steels: Jiayi Yan1; 1KTH Royal Institute of Technology
    During the aging of a microstructure consisting of fresh martensite and retained austenite, carbon can segregate to martensite/austenite interface before partitioning into austenite. Such a process, detailed decades ago to explain the thermal stabilization of retained austenite, recently has had an impact on quenching-and-partitioning. We have simulated the interfacial segregation and partitioning of carbon in a one-dimensional cell. The distribution of carbon is also affected by other concurrent processes such as carbon clustering and carbon segregation to dislocations. The chemical condition of the martensite/austenite interface is analyzed. Whether the interface moves or not depends on the driving force and the resistance caused by, for example, forest dislocations. The present simulator can be a general tool for predicting the kinetics during quenching-and-partitioning.

3:30 PM Break