Microstructure evolution in the context of fracture in austenitic steels under complex loads at cryogenic temperatures

• Published in: Materials characterization 2023-03, Vol.197, p.112654, Article 112654
• Main Authors: Nalepka, Kinga, Skoczeń, Błażej, Schmidt, Rafał, Ciepielowska, Marlena, Schmidt, Elwira, Chulist, Robert
• Format: Article
• Language: English
• Subjects: Cryogenic temperatures; Electron backscatter diffraction; fcc-bcc transformation; Fracture; XFEM simulation
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2023.112654

[Display omitted] •Microstructure changes and fracture of 304 grade steel in liquid helium are studied.•The torque and tensile force controls austenite – martensite transformation.•The secondary phase is found to initiate in sites of the highest shear stresses.•A new structure of martensite α′ is found along a direction invariant during torsion.•Martensite with a compact microstructure efficiently blocks fracture. The microstructure evolution and its coupling with fracture under complex load conditions at the temperature of liquid helium are investigated. Tension combined with torsion was applied to thin-walled cylindrical samples made of AISI 304 steel. The second component load is the key. The torque causes the sample to buckle, and the resulting folds and wells differentiate the conditions in which the transformation occurs. The feritoscopic examinations combined with the profilometer surface characterization revealed that martensite α′ initiates in the slopes of elevations, where the highest shear stresses arise. The secondary phase has a special cross-lamellar microstructure, uncovered in detailed studies using the electron backscatter diffraction (EBSD) method and synchrotron X-ray diffraction. The new phase prefers the transverse direction of the cylinder surface, which remains invariant during torsion. Along it, the martensite α′ grain develops, which then twins so as to continue growing along the second plane of the maximum shear stresses. In the advanced stage of the phase transformation, the resulting martensite α′ has compact microstructure able to block fracture propagation and to deflect its path. The macrocrack trajectory and the secondary phase distribution coupled with it were reconstructed with high accuracy in the extended finite element method (XFEM) simulation.