Partial-Isothermally-Treated Low Alloy Ultrahigh Strength Steel With Martensitic/Bainitic Microstructure

© 2016 by The Minerals, Metals & Materials Society. All rights reserved. We introduce a new strengthening heat treatment of a Ni-Cr-Mo-V alloyed spring steel by partial isothermal salt-bath and subsequent air-cooling and tempering. Detailed isothermal treatments were made at temperatures below or above the Ms point (2300C). The salt bath time was controlled between 10 and 80 minutes. Through the new treatment, the candidate steel developed ultrahigh tensile strength -2,100 MPa, yield strength -1,800 MPa, elongation 8-10 %, hardness 580-710 HV, and V-notch Charpy toughness 10-12 J. Optical and electron microscopic observations and X-ray diffraction revealed multi-phase microstructures of bainitic/martensitic ferrites, fine carbide precipitates and retained austenite. Carbon partitioning during the bainitic/martensitic transformation was investigated for its remarkable influence on the strengthening mechanism.