In Situ Synchrotron Diffraction Studies on Hot Deformation of Austenite in a High Strength Quenched and Tempered Structural Steel

Abstract

The effect of plastic deformation of austenite at elevated temperatures on the kinetics ofphase transformations during continuous cooling was studied in a high strength quenched and tem-pered structural steel S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt.%)) by means of in-situsynchrotron diffraction. The steel was heated to 900 C (above Ac3) in the austenite region and elon-gated by 6% followed by quenching to room temperature. Time-temperature-load resolved 2D syn-chrotron diffraction patterns were recorded and used to calculate the local d-spacings between latticeplanes. The plane specific diffraction elastic constants of austenite at 900 C in the steel were deter-mined from the local d-spacings. The effect of the deformation of austenite on the phase transforma-tion kinetics was studied. The evolution of lattice parameters and the phase fraction of the bcc phasesduring the quenching process were calculated.The calculated plane specific elastic constants of austenite at 900 C varied between 32 GPa to140 GPa for the different fhklg reflections of austenite. The deformation of austenite at 900 C re-sulted in the formation of a mixture of 38 % bainite, 59 % martensite and 3 % retained austenite afterquenching to room temperature. Without hot deformation, austenite transformed to 9 % bainite and88 % martensite with 3 % retained austenite. The presence of the bainitic and the martensitic phaseswas observed fromthe change in the slopes of the lattice parameters of the bcc phase during quenchingand confirmed by microscopy.