FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 5 0 In the 1920s, Edgar Collins Bain, an American metallur- gist andmember of the National Academy of Sciences, along with his colleague E.S. Davenport, discovered a microstruc- ture that forms in steels at temperatures of 250° to 550°C. While it resembledmartensite in outward appearance, it was actually quite different. In Bain’s honor, this newly identified microstructure was named bainite. Today, bainite is used in a wide variety of critical ma- terials applications—military, railway, and automotive—but there is growing pressure to find new ways to enhance per- formance of the microstructure while perfecting the cooling process. This is critical because thepreferred coolingmethod of salt bath quenching is raising environmental questions. In August 2017, the Center for Heat Treating Excellence (CHTE) at Worcester Polytechnic Institute (WPI), Mass., an- nounced it is conducting a three-year research project that will help the heat treat industry better understand process parameters related to austempering of steels. This research will help the industry to identify the potential strength, toughness, and cost benefits of bainitic steels over marten- sitic steels. Further, it will provide furnace manufacturers and commercial heat treat shops with knowledge and expe- rience in the design, validation, and control of heat treating processes designed to produce bainitic microstructures in steels. RESEARCH OBJECTIVES Under the direction of Richard Sisson, George F. Fuller Professor of Mechanical Engineering at WPI and director of CHTE, the center’s latest research project aims to answer the following questions: • How can bainite be formed without quenching in liquid salt, which is currently raising environmental concerns? • What alternative quenching media can be used? • How can cooling rates be optimized and controlled? • What temperature should the part be held at—and for how long—to provide the desired properties? According to Sisson, “We are looking into alternative quenching media, other than liquid salt, that can be used in the cooling process and still maintain strength, ductility, and toughness of the heat treated part. Andwe are exploring how precisely we need to hold the temperature. What’s the range that we can live with?” The belief is that others have attempted to do this type of research, but to date the results are proprietary and the data is not being shared. “This is an opportunity for CHTE to provide necessary answers to our members on a subject of critical importance,” says Mei Yang, assistant research pro- fessor at WPI and associate technical director of CHTE. Since 1999, CHTE, a university-industry collaborative, has helped industry to better understand the relationship of heat treated parts and the processes they undergo. It has de- mystified induction tempering, studied gas quenching, an- alyzed nondestructive testing, and explored how to extend the service life of parts and fixtures, tomention a few, always with the goal of furthering innovation in the heat treating in- dustry. NowCHTE is applying its expertise and research ethic to its latest research project—optimizing the austempering process to formbainite. STUDY PARAMETERS AND TIMELINE CHTE is currently in the early phases of research devel- opment. Following is a rough timeline for how the research will be done: Year One— Conduct literature review to obtain aus- tempering related information such as Ms, TTT, and CCCT diagrams for each alloy. Determine which materials to test. Apply computational thermodynamics to calculate T 0 and Ms temperatures. The austempering temperature should be below T 0 and above Ms. Compare the literature data and calculation results to determine the temperature range for CHTE UPDATE As shown in this time-temperature diagram, austempering involves heating to above the austenitizing temperature, cooling to just above Ms temperature, holding at that temperature until the transformation is complete, and then cooling further to room temperature. Source: Vander Voort, George F., ed., Atlas of Time-Temperature Diagrams for Irons and Steels, ASM international, 1991. 6 EXPLORING NEW WAYS TO ENHANCE THE AUSTEMPERING PROCESS FOR STEELS