MECHANICAL PROPERTIES AND MICROSTRUCTURAL CHARACTERISTICS OF MARTENSITIC STEEL DEVELOPED VIA THERMAL CYCLING TREATMENT

Abstract

Cyclic quenching and tempering (CQT) heat-treatment methods can improve the mechanical properties of martensitic medium carbon steel due to the development of fine grain size microstructure. To explore various avenues for enhancing steel quality, this work aims to investigate the mechanical properties and microstructural characteristics of martensitic steel developed via thermal cycling treatment.  As received hot-rolled, AISI 1040 ribbed steel bars with a diameter of 12mm containing 0.35 wt% C, 0.28wt % Si and 0.62wt%Mn were first normalized in a muffle furnace at 845℃. Samples were thereafter subjected to thermal cycling at 743℃, 761℃ and 779℃ in the furnace respectively within the dual-phase region and then tempered at 450^oC. Mechanical properties of the specimens were evaluated and characterized by optical microscopy (OM). The fracture surfaces of the samples were examined using the scanning electron microscope (SEM). The results show that with an increased in lamilarization temperature at 779℃, ultimate tensile strength, yield strength, percentage elongation, and modulus of elasticity all ncreased while the fracture strength decreased. The Optical micrographs and SEM micrographs revealed that the sample contains lath martensite and ferrite and carbide with fine grains after tempered at 450^oC. These findings indicate that thermal cycling treatment is an effective method for producing high-performance martensitic steel with improved mechanical properties suitable for all industrial applications.