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 kB = 7.06 × 10-7 mm3 N-1 m-1 than borided and heat-Treated (BHT) kBHT = 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 kBHT = 5.25 × 10-8 mm3 N-1 m-1 over kB = 1.68 × 10-7 mm3 N-1 m-1 of borided only, because it permitted a better adsorption of the lubricant.