Improving the Wear Resistance of 40Cr Steel by Multi-Pass Laser Hardening: Effect of the Number of Passes on the Microstructure, Microhardness and Tribological Properties

Laser hardening is normally applied in the solid-state regime, and the behaviour of chromium steels treated at a line energy high enough to melt the surface is far less documented. Here 40Cr steel (AISI 5140) was laser hardened at 3000 W and 5 mm/s (600 J/mm) with one, two and three passes and compared with the untreated state by roughness measurement, microhardness profiling, quantitative porosity analysis, X-ray diffraction, dry ball-on-disc sliding against a 100Cr6 ball (6 N, 3 cm/s, 200 m, n = 3) and abrasive testing to GOST 23.208-79. One pass produced a hard (530–590 HV) but porous remelted layer, with 6–9% porosity in the top 300 µm and a dislocation density of 1.6 × 1015 m−2. Repeated remelting reduced the porosity to below 1%, while the near-surface hardness fell to 280–310 HV, the dislocation density decreased four- to five-fold, and the hardened region extended to about 1300 µm, indicating tempering of the previously hardened material. After grinding and polishing, the specific wear rate of the steel in sliding fell significantly (p < 0.001) from 6.5 × 10−5 to 2.9, 4.3 and 1.7 × 10−5 mm3/(N·m), i.e., by 54, 34 and 74%, whereas the friction coefficient, 0.62–0.66, did not differ between the states. Under loose abrasive, the mass loss fell by 8.4, 25.5 and 14.3%. Three passes are recommended for sliding contacts and two passes for abrasive conditions; a single pass is not recommended because of the porosity of its remelted layer.