Effect of heat on mechanical properties and microstructure of reinforcing steel bars made from scrap

Abstract

Mechanical properties of reinforcing steel bars (rebars) degrade with elevated temperatures and this deterioration has to be properly accounted for to understand the behaviour of rebars and composite structures in fire. In this study, the effect of heat on the mechanical properties and microstructures of rebars made from local scrap was studied. Rebar Samples of 10, 12, and 16 mm in diameter and 450 mm length were prepared using a power saw from samples purchased from the local steel factories and the spectrometry analysis was carried out. Seven specimens of the above types of rebars were heated in an electric furnace to temperatures ranging from 100 to 1000 oC for one hour and then cooled in air to room temperature. Thereafter, their residual mechanical properties (Yield Strength, Ultimate Tensile Strength, Percentage Elongation, and Young’s Modulus) were determined using a 1MN Universal Testing Machine. Brinell hardness testing was performed using the Universal Hardness Tester and Metallographic analysis and grain size determination was studied by an optical microscope with an inbuilt camera to correlate mechanical properties to the microstructure. Results showed that normal mechanical properties and microstructure can be assumed after exposure to temperatures up to 500oC for one hour. For higher temperatures, the retained yield stress as a proportion of normal bar properties after exposure times of one hour was as follows: - 600oC - 0.9, 900oC - 0.7, 1000oC - 0.6. Variation of the microstructure occurred above 500oC , whereby the grain size reduced from 17.3 to 12 micrometres when the temperature increased from 500 to 1000oC. The study provides precise information on mechanical properties and microstructure of rebars to steel producers, designers, building industry, and standardization bodies. The results may also be used to support other research projects aimed at studying the behaviour of rebar steels structures in fire.