Microstructure And Properties Study Of Laser Cladded Nanostructured Bainite Coatings

In this paper, laser cladding was carried out by using CO₂ laser and the method of the reverse synchronous powder feeding to obtain the nanostructured bainite coatings with good hardness, strength and toughness and the chemical composition of the cladding powder prepared is (wt.%) Fe-0.78C-1.49Si- 1.95Mn-0.97Cr-0.24Mo-1.6Co-0.99Al.The systematic study of the influences of the change of the laser scanning speed Vs, preheating temperature and the speed of powder feeding Vp on the surface formability of the coatings and dilution were carried out and the combination of optimum parameters were: laser power P = 7KW, the speed of powder feeding Vp=30g/min, the flow of argon gas for anti-oxidation protection Vflow=20L/min, and the values of the laser scanning speed Vs range from 0.5-1.0m/min. Meanwhile, the cladding powders were prepared in two different processing ways, namely the self-made mixed powder and the melting powder, both with the same chemical composition. And we studied the influences of two different cladding powders on the qualities of laser cladding coatings. The results showed that the quality of the cladding coatings using the melting powder, which with good forming, cracks–free, low dilution, and uniformly distributed microstructure and without obvious segregation and inclusions, is much better than the self-made mixed powder.We systematically studied the influences of three different heat treatment processing ways, which were no heat treatment, 200℃and 300℃heat treatment (hereinafter referred to as room temperature, 200℃and 300℃),on the microstructure transformation of nanobain. The results showed that at room temperature, the microstructure of cladding coatings are very fine lath martensite + retained austenite with the water-cooling quenching. At 200℃, we got the fine nanostructured bainite coatings successfully after isothermal transformation for 24h.The plates of bainitic ferrite are 50～80 nm thick, and each plate is separated by an even finer film of retained austenite with a width of 10～30 nm; dislocation debris can be founded in both the bainitic ferrite and the surrounding austenite. At 300℃, we got the fine nanostructured bainite coatings successfully as well after isothermal transformation for 4h and 24h respectively, however the volume fraction of retained austenite increased slightly. In addition, because of the increase of the laser scanning speed, the grain will be refined, and the transformation speed of bainite is faster with the increase of the volume fraction of retained austenite. Therefore, whatever increasing the laser scanning speed or heat treatment temperature, both have the same effect on the bainite transformation.The systematic study of the change rules of mechanical properties of prepared nanostructured bainite coatings, such as micro-hardness and tensile, with different heat treatment processing ways were carried out. The results showed that the micro-hardness of the cladding coatings increased with the increase of the laser scanning speed and decreased slightly with the increase of the volume fraction of retained austenite. At 200℃,when the laser scanning speed of 0.5 m/min, the average micro-hardness of this coatings are 422.4HV1.0 for isothermal transformation for 24h and 334.3HV1.0 for isothermal transformation for 48h respectively. The values of the micro-hardness are low, probably due to the incomplete bainite transformation and higher volume fraction of the retained austenite. The tensile strength and elongation of the coatings isothermal transformed for 24h are 1280 MPa and 6.41 % respectively, which indicated high strength and good plasticity. At 300℃,when the laser scanning speed of 0.5 m/min and 1.0 m/min respectively, each isothermal transformed for 4h and 24h, and the average micro-hardness of these four groups coatings are 438.7HV1.0,which consistent with the micro-hardness characteristic of nanobain at the same temperature well. But the tensile test results showed that all the laser cladding specimens are brittle fracture, the tensile strength are not more than 1000MPa, elongation is less than 1%, both strength and plastic are poor, which probably due to the existence of a large number of retained austenite islands.