Improvement on wear resistance of citrus twig grinding hammer of 65Mn steel by laser quenching and laser cladding

In order to improve the wear resistance of the citrus twig grinding hammer, the multiple surfaces of the 65Mn steel hammer are strengthened by laser quenching and laser cladding. The metallographic microscope, microhardness tester and friction tester are used to study the effect of laser treated 65Mn steel on its microhardness and friction property. The main equipment used in this experiment is GS-HL-5000 fiber laser and SX2-5-12 type electric furnace. The power used for laser quenching is 2.5 kW and the scanning speed is 300 mm/min. For laser cladding, the power is 3.2 kW, the scanning speed is 350 mm/min, and the powder feeding speed is 15-20 g/min. Ordinary heat treatment uses water quenching and medium-temperature tempering process, and the test parameters are: With the quenching temperature of 790 °C, the time is 20 min; with the tempering temperature of 350 °C, the time is 25 min. After the laser quenching and laser cladding of the sample, a part of the metallographic sample is cut perpendicular to the scanning speed. The metallographic sample is etched by using 10% solution of nitric acid (volume fraction), and the metallographic sample is analyzed by the metallographic microscope. The loading pressure of digital microhardness tester is set to 0.981 N for 15 s pressure-keeping time. The friction experiment is implemented by MS-T-3001 friction tester with Si3N4 grinding head, time of 20 min, 500 g loading, spindle speed of 50 r/min and 0 ℃ testing temperature. The results show that 65Mn steel, processed by laser quenching, is composed of entire quenching zone, part quenching zone, heat affected zone and substrate with its microstructure sequences as acicular martensite + retained austenite, ferrite + tempered troostite, lamellar pearlite + granular pearlite, lamellar pearlite + ferrite from the surface to the center. The 65Mn steel, processed by laser cladding, is composed of cladding zone, entire quenching zone, part quenching zone, heat affected zone and substrate with its microstructure sequences as eutectic structure, acicular martensite + retained austenite, granular pearlite, lamellar pearlite + granular pearlite, lamellar pearlite + ferrite from the surface to the center. The microstructure of 65Mn steel processed by water quenching and medium-temperature tempering is acicular martensite. The maximum microhardness of 65Mn steel processed by laser quenching is 619.8 HV0.1 while the average microhardness of 65Mn steel processed by water quenching and medium-temperature tempering is 585.6 HV0.1. The surface microhardness of 65Mn steel processed by laser quenching is slightly higher than that by water quenching and medium-temperature tempering, because the surface of 65Mn steel with laser quenching is prone to form a layer of high carbon martensite. The maximum microhardness of 65Mn steel processed by laser cladding is 1 038.4 HV0.1, far more higher than the other 2 kinds of processing ways, because not only the content of WC (tungsten carbide) in composite alloy powder is high and also the higher laser energy density and the lower scanning speed can melt the WC particles to a large extent, increasing the number of carbide strengthening phase precipitated by rapid solidification. Abrasive wear mechanism appears on the surface of 65Mn steel by laser quenching, and the average friction coefficient is 0.40. Adhesive wear mechanism appears on the surface of 65Mn steel by water quenching and medium-temperature tempering, and the average friction coefficient is 0.41. The mechanism of surface abrasive wear appears on the surface of cladding layer, when the 65Mn steel is treated by laser cladding with Ni60 + 35% WC, and the average friction coefficient is 0.36, and the wear resistance is better. Based on the above results, the surface hardness of the laser-treated 65Mn steel increases substantially and the wear resistance increases significantly. This study provides a reference for extending the service life of the hammer.