Design and Characterization of a Nanoscale Carbide-Free Bainite Alloy

Abstract

High carbon bainitic steel plates could surpass quench and tempered martensitic counterparts for fabrication of ammunition- and blast-resistant armours. Mechanical properties, microstructure and reaction kinetics of a commercially available carbide-free nanoscale bainite alloy were characterized. Based on the initial characterization and a comprehensive review of the literature a new alloy with lower carbon, higher silicon and cobalt additions was designed and processed into hot-rolled plates (10x10mm and 300x300mm) using CanmetMATERIALS pilot-scale facilities. The heat treated plates achieved strength above 2 GPa with elongation of 14%. Thorough analysis with electron backscattered diffraction revealed that the microstructure consisted of bainitic ferrite laths, islands of retained austenite, areas of mixed martensite-austenite (MA). Transmission electron microscopy confirmed the fine scale of bainitic ferrite and the presence of thin films of retained austenite encompassing bainite laths. Dilatometric study of the new alloy revealed that forming bainite at higher transformation temperatures, 275°C versus 250°C and 225°C, led to faster overall reaction kinetics and higher final fractions of bainite within 18 hours of isothermal holding. Although it is expected that the fraction of bainite increases at lower temperatures, substantial prolonged holding time is required for completion of the reaction. Microstructural features and particularly bainite lath thickness depended on bainite formation temperature. Ausforming, deformation of austenite at 600°C for 25-45% strain prior to decomposition to bainite, however led to a decrease in reaction rate and final fraction of bainite. Tensile testing of austempered specimens showed that higher transformation temperature yielded a stronger microstructure, which was attributed to the formation of thinner bainitic ferrite laths. Higher transformation temperatures led to an increase in ductility. Tensile testing of the ausformed specimens showed a reduction in both strength and ductility. A negative correlation was seen between the amount of MA areas in the microstructure and total elongation.