Evaluation of 850°C Heat Treatment: Effects of Used-Oil Quenching, Brine Quenching, and Pack Carburizing on the Rockwell Hardness of ST37 Shredder Blades

Waste shredder blades operate under abrasive contact and intermittent impacts from heterogeneous materials, making surface hardness a key property for maintaining cutting performance and reducing premature dulling. This study evaluates the effect of a 850°C heat-treatment route on the hardness of ST37 low-carbon steel used for shredder-blade applications. Plate specimens (100 mm × 100 mm × 10 mm) were prepared and tested under four conditions: untreated (as-received), pack carburizing using coconut-shell charcoal followed by furnace cooling, quenching in used oil, and quenching in saltwater (brine). Hardness was measured using the Rockwell C method (ASTM E18) with a diamond cone indenter (10 kgf minor load and 150 kgf major load). For each condition, ten hardness readings were collected and analyzed using one-way ANOVA (α = 0.05).The untreated specimens yielded an average hardness of 83.85 HRC. The pack-carburized and furnace-cooled condition resulted in a substantial decrease to 69.35 HRC, accompanied by higher variability, indicating a less favorable hardness response under the applied cooling route. In contrast, quenching after heating at 850°C increased hardness to 85.55 HRC for used-oil quenching and 88.35 HRC for brine quenching, with brine showing the highest mean value but also a wider scatter. ANOVA confirmed that the treatment route significantly influenced hardness (p < 0.05). Overall, the results indicate that, under the present parameters, quenching routes are more effective than the pack carburizing + furnace cooling route for improving blade hardness, while process consistency is important to control hardness dispersion, particularly for brine quenching.