LEE- Malzeme Mühendisliği Lisansüstü Programı

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http://hdl.handle.net/11527/19378

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Konu "austempering" ile LEE- Malzeme Mühendisliği Lisansüstü Programı'a göz atma
  • Öge
    The effect of successive quenching and austempering heat treatments on the microstructure of a high silicon steel
    (Graduate School, 2024-11-08) Uğur, Şeyma ; Baydoğan, Murat ; 506201407 ; Materials Engineering
    Quenching and tempering are very common heat treatment methods for low- and high alloy steels that have sufficient carbon to achieve high strength. Austempering is an isothermal heat treatment utilized to produce bainite with various morphologies in the microstructure in order to obtain good toughness, high wear resistance, and less distortion. However, to obtain maximum benefit from this heat treatment, two important issues should be considered. The first one is the formation of carbides in microstructures, which can be suppressed using high-Si steels. The other is that the bainite transformation may take a very long time to complete, which needs to be accelerated by forming martensite in the microstructure before the bainite transformation. In this point of view, martensitic-bainitic duplex microstructures have the potential to offer a superior strength-toughness combination, as opposed to fully martensitic or fully lower bainitic microstructures. By increasing the nucleation sites at the martensite-austenite interface, the ensuing bainite transition may be sped up and improved mechanical characteristics can be achieved. This study was therefore undertaken to investigate the effect of different quenching temperatures below Ms, which form different volume fractions of martensite, on the bainite morphology developed during the subsequent austempering heat treatment and on hardness as a measure of the mechanical properties of the successively quenched and austempered steel. Quenching and austempering heat treatments were successively applied to high-Si steel in this study. For the quenching heat treatments, the Ms temperature was first determined to be 250 °C by using JMatPro software. Then the specimens were austenitized at 900 °C for 30 min and quenched in a salt bath at 180, 200, 220, and 240 °C (70, 50, 30, and 10 °C below the Ms temperature, respectively) to form different volume fractions of martensite in the microstructure. The quenched samples were then austempered at 280 °C for 2 hours. After the quenching and austempering heat treatments, it was seen that the microstructure was mainly composed of martensite and bainite. The volume fraction of martensite decreases and the volume fraction of bainite increases, which is accompanied by a decrease in hardness, with the increase in the quenching temperature. X-ray diffraction (XRD) analysis indicated that a small amount of retained austenite exists in the microstructure. Variation of the bainite morphology was also evaluated depending on the quenching temperature. In addition to hardness, tensile, impact and fatigue properties were also examined; The yield and tensile strength of martensitic and bainitic duplex microstructured steels are higher than the microstructure consisting fully of bainite. These values are 1920 MPa, 2050 MPa, 1825 MPa, 1910 MPa, respectively. In terms of impact properties, the microstructure consisting fully of bainite has an impact energy of 33 J, while the impact energy of the martensite and bainitic duplex structure is 10 J. Therefore, it is determined that as the bainite ratio increases and the martensite ratio decreases in the structure, the impact resistance increases. No significant difference could be detected in fatigue properties, as in other mechanical properties.