Study of Heat-Treated Steel and Related Applications

Numerous studies conducted after the 921 earthquake in Taiwan in 1999 have reported that because heat-treated steel features relatively unstable mechanics properties, using such steel in welding, bar splicing, or as thread-cutting components in structures may result in lack of resistance to seismic activity. However, nondestructive testing methods that can be used to identify the type of steel delivered to construction sites (i.e., whether the particular steel is heat-treated steel) have not yet been developed. In this study, the materials constituting the surface and interior of heat-treated steel were determined using macroscopic or microscopic metallographic inspection; the determined materials were martensitic and ferrite-pearlite structures, respectively. The hardness levels of steel of varying thicknesses were examined using the Vickers hardness test and Leeb hardness (HL) test, after which the variance in hardness between the interior and surface of the heat-treated steel were studied. The experimental results showed that the HL test can be used to determine whether steel in concrete is heat-treated steel: Under the acceptable reliability and tolerance limits are 95 and ±5%, respectively. Steel with an HL value of 82 HL or higher between the surface and the interior of the steel (1.0 mm below the steel surface) can be considered a heat-treated steel. The HL method, which involves measuring the difference in hardness, is a nondestructive testing method and can be used to determine whether the steel used in existent buildings is heat-treated steel.