Mechanical and Tribological Characterization of Salt Bath Nitrocarburized 4150 Steel

AISI 4150 steel was nitrocarburized in a molten salt bath to investigate its applications in gun barrels. Cross-section examinations by SEM revealed layer formation in nitrocarburized 4150 steel. The oxide and compound layers were found to be 2 and 10 µm thick, respectively. The diffusion zone was measured to be 263 µm from Vickers microhardness tests performed on the cross section of nitrocarburized samples. Nanohardness (H), H/E ratio, and H³/E² ratio of nitrocarburized 4150 steel were observed to be 2.25, 3.63, and 33 times higher than those of untreated steel. Friction and wear properties of nitrocarburized and untreated steel were evaluated using a pin-on-disk tribometer against an oxygen-free copper disk, given its use in shell jackets. A total of 12 test cases, under the conditions of a sliding velocity of 2 m s^-1, normal loads of 30, 40, and 50 N, and temperatures of 25, 300, 400, and 500 °C, were investigated. A decrease of 1.03 to 1.5 times in coefficient of friction (COF) and a decrease of 1.2 to 3.3 times in wear rate was observed after nitrocarburization. A combined mechanism of adhesion, abrasion, and delamination with mild oxidation was observed at 25 °C, whereas strong oxidation was profound at 300 °C.