Comparative Analysis of Carburizing Media on Surface Hardness, Mechanical, and Thermal Properties of ST 40 Steel

Selecting an appropriate carburizing medium requires balancing surface hardness, mechanical strength, ductility, and thermal behavior. This study compared the mechanical and thermal properties of ST 40 steel carburized using coconut shells, durian peels, and bovine bones with those of untreated steel. The specimens were evaluated through tensile testing, Rockwell hardness testing, and thermal analysis. Coconut shells produced the highest tensile strength and surface hardness, increasing tensile strength from 312.47 to 378.62 MPa and hardness from 35.0 to 42.8 HRC, while reducing elongation from 11.83% to 8.64%. Bovine bones provided intermediate improvements in mechanical strength, whereas durian peels preserved the highest elongation among the carburized specimens. All specimens retained approximately 99.75–99.83% of their initial mass at 1000 °C. Thermal analysis revealed that the principal calorimetric peak shifted from approximately 805 °C in untreated steel to 794–798 °C following carburizing. These findings demonstrated that different carburizing media produced distinct mechanical and thermal responses, highlighting the importance of selecting a medium according to specific engineering requirements. Coconut shells were the most effective medium for improving tensile strength and surface hardness, although these improvements were accompanied by reduced ductility. Furthermore, high mass retention during thermal analysis did not independently establish the thermal stability or durability of the carburized surface during subsequent heating.