The master's thesis investigates the influence of austenitization conditions, i.e. temperature and time, on the martensitic transformation of OTV1D tool steel (used for cold rolls). The primary focus was on the martensite start temperature Ms. The impact of austenitization conditions was studied mostly through dilatometric analysis. Samples were heated in the dilatometer at austenite temperatures ranging from 960°C to 1050°C, with austenitization times ranging from 15 to 120 minutes. The samples were than quenched at a rate of 50 K/s to harden the steel. Based on the dilatometric results, the steel was also heat-treated in a chamber furnace. Samples were quenched in water from the same temperatures, with austenitization times set at 30 minutes. After the heat treatment hardness was measured, the size of the prior austenite crystal grains was determined according to the ASTM E112 standard, X-ray diffraction was conducted to determine the proportion of retained austenite, and microstructural analysis was carried out using a light microscope and a scanning electron microscope.
The results revealed that significant influence can be exerted on the martensitic transformation, especially on the martensite start temperature, by varying the temperature and time of austenitization. The increase in the austenitization temperature resulted in the dissolution of carbides, facilitating easier growth of austenite crystal grains, from 17.62 µm (at TA = 960°C/ 15 minutes) to 118.99 µm (at TA = 1050°C/ 15 minutes). With the increased content of alloying elements in austenite, the Ms temperature decreased, from 235.2°C (at TA = 960°C/ 15 minutes) to 141.6°C (at TA = 1050°C/ 15 minutes). Consequently, incomplete martensitic transformation occurred, resulting in a higher proportion of retained austenite in the microstructure of steel, with the increase from 16.1% (at TA = 960°C) to 18.5% (at TA = 1020°C). The higher content of retained austenite caused a decrease in steel hardness, from 800 HV to 737 HV. Longer retention times at the austenitization temperature caused further growth in austenite crystal grains. This is due to a larger proportion of dissolved carbide precipitates, enabling faster growth of austenite crystal grains. At an austenitization temperature of 960°C, the crystal grain size increased from 17.62 µm (at tA = 15 minutes) to 24.26 µm (at tA = 120 minutes), while the Ms temperature increased from 235.2°C to 251.1°C The increase in Ms temperature demonstrates that the growth of prior austenite grains has a greater impact on Ms temperature compared to the higher proportion of alloying elements in austenite due to carbide dissolution. With a higher Ms temperature, the proportion of martensitic transformation increased, thus raising the proportion of martensite in the steel. Consequently, this microstructure resulted in higher steel hardness, which after quenching at 960°C, increased from 800 HV to 832 HV.
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