Engineers from Worcester Polytechnic Institute (WPI), Worcester, Mass., will present a discussion of methods to determine the amounts of various microstructures in steel during heat treating, at Heat Treat 2019.
Austempering is a hardening process based on the isothermal transformation of austenite to bainite. The transformation percentage of bainite must be quantified to determine the bainite transformation kinetics. During the austempering process, if the austempering holding time is not long enough, the bainitic transformation will be incomplete, and the residual austenite will transform to martensite during cooling to room temperature. The result may be a microstructure containing bainite, martensite, and retained austenite.
In the present work, color tint-etching was used to distinguish phases in austempered AISI 5160 steel samples. After color tint-etching, the microstructural constituents display different colors under the optical microscope with polarized light to estimate the percentage of each microstructural constituent. X-ray Diffraction (XRD) analysis with Rietveld refinement was also conducted to determine the percentage of each microstructural feature. These experimental results are compared with those determined by tint etching.
Both results show that longer austempering holding time increases the bainite percentage, while the martensite percentage decreases. However, the martensite percentage determined by the tint-etching method is much higher than that determined by XRD analysis.
Title: “Methods for the Determination of Bainite, Martensite, and Austenite Fractions in Austempered Steel.”
Authors: Prof. Mei Yang, Mr. Hooman Sabarou, Mr. Zhida Chen, Dr. Yangyang Fan, Prof. Yu Zhong, Prof. Richard D Sisson, Worcester Polytechnic Institute, Worcester, Massachusetts.
Wednesday, October 16, 2019: 2:40 PM, 251C (TCF Center)
https://www.asminternational.org/web/heat-treat-2019
