Wear Performance Under Dry and Lubricated Conditions of Post Boriding Heat Treatment in 4140 Steel

The tribological performance of borided, post borided heat-treated and uncoated 4140 steel, under dry and lubricated conditions, was studied. The boriding process was conducted by powder-pack method at 900 °C with 1 h of exposure time. After boriding, a heat treatment (quenching and tempering) was conducted to obtain a hardened core microstructure of the 4140 borided steel. The tribological tests were performed with an $Al2O3$ ball of 6.35 mm of diameter, setting sliding distances of 25, 50, and 100 m. The loads used were 20 and 40 N in dry and lubricated conditions, respectively. Overall samples, the coefficient of friction was reduced around five times when lubricant was used, in comparison to dry tests. After 100 m of sliding distance, it was observed that the wear resistance increased around ten times with boriding. The dry results showed that borided (B) exhibited better wear-rate k_B = 7.06 × 10⁻⁷ mm³ N⁻¹ m⁻¹ than borided and heat-treated (BHT) k_BHT = 9.93 × 10⁻⁷ mm³ N⁻¹ m⁻¹, attributed to its higher hardness. Nevertheless, the lubricated results showed that the surface characteristics of BHT improved its wear resistance k_BHT = 5.25 × 10⁻⁸ mm³ N⁻¹ m⁻¹ over k_B = 1.68 × 10⁻⁷ mm³ N⁻¹ m⁻¹ of borided only, because it permitted a better adsorption of the lubricant.