Synchrotron diffraction studies on the transformation strain in a high strength quenched and tempered structural steel

摘要

In-situ phase transformation behaviour of a high strength (830 MPa yield stress) quenched and tempered S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt.%)) structural steel, during continuous cooling under different mechanical loading conditions to promote the bainitic transformation, was studied. Time-temperature-load resolved 2D synchrotron diffraction patterns were recorded and used to calculate the transformation strains. The temperature dependent elastic constants of ferrite in the steel were also determined using in-situ tensile tests at different temperatures in a synchrotron X-ray diffractometer. The transformation strains were calculated under different loading conditions. The elastic constants were calculated from the lattice parameters at 25 ℃, 200 ℃, 300 ℃, 400 ℃, 500 ℃ and 600 ℃. The elastic constants varied from 202 GPa at 25 ℃ to 143 GPa at 600 ℃. The variation in lattice plane strains during phase transformation under small external loads were calculated. Bulk measurement techniques such as dilatation experiments give the averaged transformation strains. However, in-situ synchrotron measurements performed in this work describe the transformation strains of the individual transforming phases and the strains arising due to possible variant selection.