UC San Diego

Adopting a material-level architecture gives engineers an additional tool in structural design. The exploitation of the material architecture can produce efficient and effective seismic force resisting systems. Examples of this include utilizing novel materials or functionally graded materials for constructing members with spatial variation in strength, and stiffness. This approach can be used to enforce capacity design principles; increase energy dissipation; and prevent premature component and demand critical connection failures. Recently heat-treatment has been proposed as a method to selectively reduce the strength of steel members to form weak zones with high ductility and energy absorption capabilities. So far, the method has been applied to beams in special moment frames and braces and gusset plates in special concentrically braced frames. However, no recommendations have been put forward as to the details of the heat treatment process required to achieve the desired material properties. Therefore, in this study the structure property relationships of one of the commonly used structural steel grades (ASTM A992 steel) is investigated with the goal of establishing relationships between heat treatment process and resulting steel strength.The dependence of microstructural features of these steels, including grain sizes and phase volume fractions, on heat treatments and chemical compositions is investigated. A992 steels with different chemical composition were selected and heat treated in different ways. After that, their microstructural and mechanical properties were characterized by optical microscope, electron back-scatter diffraction, energy-dispersive X-ray spectroscopy and standard mechanical tests. The austenite grain coarsening behavior upon heating, was investigated under different heating conditions, involving peak temperature and holding time. The dependence of ferrite grain size on prior austenite grain size and cooling rate has also been studied. In addition, the strengthening contributions of grain boundaries, solutes, dislocations and precipitates to the overall strength of the steel are evaluated in a quantitative manner. Finally, an empirical model for the prediction of yield strength of A992 steels was developed based on the existing experimental data.