Quenching and self-tempering (QST) for H-beams is used to enhance the mechanical properties of high-quality offshore structural steels. Quenching in QST uses high-pressure water to impose intensive cooling on surface. Self-tempering is a process to reheat surface using heat transferred from the core which is not yet cooled during the quenching process. QST is a unique process to enhance the mechanical properties of H-beam steels; however, the understanding of QST is limited due to the complexity of the process such as difficulty in estimating the quenching effect. The objective of this research is to evaluate microstructure and mechanical properties of H-beams manufactured by Hyundai Steel company. A series of quenching and tempering experiments were performed, and the resulting microstructure and mechanical properties were analyzed to better understand the adopted QST process.  

The test specimens were sampled along the through-thickness direction of the flange of H-beam. The mechanical properties of the specimens were analyzed in detail through tensile, Vickers hardness tests and nanoindentation mapping. The microstructure of the specimens was analyzed through an optical microscope (OM), scanning electron microscope (SEM) and electron backscattered diffraction (EBSD). Tempered martensitic layer and a mixture of ferrite and pearlite are formed at the surface and the core of the flange, respectively, due to the variation in cooling rate. The microstructure and mechanical properties of the surface layer were consistent with those of a specimen quenched and tempered at 600 ^oC. The core characteristics were consistent with a specimen austenitized and subsequently tempered at 600 ^oC.