The primary objective of this master's thesis was to determine the thermal properties and chemical composition, as well as to evaluate the microstructure of PROTAC 500, PROTAC 550, and PROTAC 600 high-strength ballistic protection steels. These steels are produced by Slovenia's largest steel manufacturer, SIJ Acroni d.o.o. The evaluation was conducted across a temperature range from room temperature to 400 °C.
Thermal conductivity, thermal diffusivity, and specific heat were determined using the transient planar source (TPS) method with a Hot Disk TPS 2200 instrument. Density was determined using the Archimedes method in water, and the automated Densitec method in silicone oil. The microstructure of the steel was analyzed using scanning electron microscopy (SEM), and the chemical composition was determined using energy-dispersive X-ray spectroscopy (EDS). For the density measurements, the classic Archimedes method in water proved to be more stable and reproducible than the automated Densitec device in silicone oil. Due to the viscosity of the medium, the Densitec device systematically overestimated the density values by up to 2.26%.
At room temperature, PROTAC 500 steel exhibited the lowest thermal conductivity at approximately 19 W/(m·K) and the lowest thermal diffusivity at approximately 5.2 mm²/s. The highest values were recorded for PROTAC 550 steel, which reached approximately 29 W/(m·K) for thermal conductivity and 7.7 mm²/s for thermal diffusivity. Up to a temperature of 400 °C, no uniform trend in thermal conductivity variation was observed among the three steels. Furthermore, a pronounced influence of specific heat on the calculation of thermal diffusivity was demonstrated. This highlights the necessity of considering temperature dependence during measurements at elevated temperatures. SEM analysis revealed a tempered martensitic microstructure consisting of fine martensitic laths across all three steels. EDS analysis confirmed differences in the chemical composition of the matrix and secondary phases and identified the presence of titanium- and niobium-enriched particles in the PROTAC 550 steel. The obtained results complement the existing data on the thermal properties of high-strength ballistic protection steels and enable a comprehensive comparison of their thermal behavior at elevated temperatures.
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