Effects of Applied Compressive Stress on Microstructure and Mechanical Properties of Cr-Mo Steel during Continuous Cooling Bainitic Transformation

摘要

The thermal expansion curves are measured by Gleeble-1500 thermo-mechanical simulator for Cr-Mo steel during continuous cooling transformation at the cooling rate ranging from 1°C/s to 90°C/s, and CCT curve is also determined; The thermal expansion curves of continuous cooling transformation are measured under the axial compressive stresses (40MPa, 80MPa and 120MPa) at the cooling rates of 50°C/s, 70°C/s and 90°C/s respectively, then the CCT curves with applied compressive stress are determined. The transformation microstructures of steel after continuous cooling with and without stress are observed by SEM and TEM, and the hardness is measured by Vickers hardness tester. The results show that the Bs is increased and Bf is decreased owing to the effect of applied compressive stress; At the meantime the microstructure is also changed, with increasing applied compressive stress, the quantity of granular bainite, bulk ferrite and carbide increase gradually except lath bainite, the lath width of lath bainite becomes narrower gradually, and original grain boundaries become bent from straight; The hardness of transformation structure becomes lower with increasing applied compressive stress exceeding 40Mpa after continuous cooling.