Microstructure and property of 65Mn steel preheated by laser strengthening

Abstract: At present, there exists a problem that the agricultural tools easily get worn. However, the common strengthening techniques have visible disadvantages, such as the complex process and comparatively high running cost. In order to solve the problem, the 65Mn steel was preheat-treated by the normalization or quenching + medium-temperature tempering. Subsequently, the preheat-treated 65Mn steel was strengthened by laser quenching or laser melting-hardening process with a YLR-6000 fiber laser. The microstructure was studied by optical microscope and scanning electronic microscope. The microhardness was measured using a Vickers-1000 microhardness tester with load of 1.96N and a dwelling time of 15s. Further, the controlling factors were investigated. The results showed that the preheat-treating and laser process had visible influences on the microstructure and property. In laser quenching, the microstructure was comprised of the entire quenching zone, part quenching zone and heat affected zone. The fine needle martensite was obtained in entire quenching zone by quenching + medium-temperature tempering + laser quenching. In the part quenching zone, the microstructures were fine needle martensite and troolstite. However, the fine needle martensite and a few ferrite were observed in entire quenching zone by normalization + laser quenching. In the laser melting-hardening process, the microstructure was made up of the surface melting zone, entire quenching zone, part quenching zone and heat affected zone. Compared with the microstructure of laser quenching, the surface melting zone was obtained in laser melting process, and the others were accorded with those in laser quenching. As for laser melting process, the depth of surface melting zone was within the range of 10-15 μm. In surface melting zone, the fine dendrite microstructure was got and the primary dendrite arm spacing was within the range of 4~6 μm. In addition, The microhardness of laser strengthening 65 Mn steel pretreated by normalization was relatively higher than that pretreated by quenching + medium-temperature tempering. It depended on the microstructure and its size. Moreover, the microhardness of laser melting-hardening was relatively lower than that of laser quenching. The microhardness of laser process could reach up to HV0.2830, about 1.5 times as much as that of common quenching + medium-temperature tempering. According to the working condition of agricultural tools, the surfaces needed to be hard and the interior parts needed to be with good toughness. The laser strengthening technique satisfied the request of steel property and simplified the running process. Therefore, in terms of agricultural tool strengthening approaches, the laser strengthening technique exhibits visible advantages and brings application prospects.