Research On Surface Graphite Superfine-spheroidizing Process Of Gray Cast Iron Used For Glass Molds

The working condition of gray cast iron glass molds is complex. It requires material with high thermal fatigue resistance and oxidation resistance in the mold cavity and material with good thermal conductivity in the main body of the mold. The former demand requires fine scattered graphite and spheroidal graphite is preferred while the latter requires continuous flake graphite. However, there isn’t a kind of metal that can meet both the contradictory performance demand so far. Previous studies aim to improve the mold performance by thermal spraying or casting of dissimilar metal in the mold cavity and have acquired certain achievement. But there are still some issues such as poor interface bonding, deciduous and complicated craft.This paper investigated the microstructure and performance of some domestic and imported gray cast iron used for glass molds and came up with surface graphite superfine-spheroidizing process of gray cast iron for the first time, which includes the laser surface re-melting and solid-state graphitizing annealing process. Meanwhile, this paper also optimized the laser surface re-melting and graphitizing annealing process and analyzed its microstructure and performance by instruments such as OM, SEM and XRD. The surface graphite superfine-spheroidizing process of gray cast iron has enriched the surface treatment technique of glass molds and has significant application value and potential.The results show that glass molds often fail from the mold cavity and the failure is a phenomenon of low-cycle fatigue. The mold service life is mainly determined by plasticity. The graphite morphology of domestic and imported molds is similar, and from the mold cavity to the periphery side, it ranges from D-type to A-type. The service life of glass molds is consistent with thermal fatigue resistance and it is mainly determined by the matrix of cast iron. The matrix of the imported cast iron is composed of approximately 98% of ferrite, which is purer than the domestic cast iron of 90%, therefore, it possesses better plasticity and leads to higher low-cycle fatigue life. Laser surface re-melting and solid-state graphitizing annealing process can successfully achieve surface superfine spheroidal graphite in gray cast iron. And the depth of the optimally treated layers can reach 2 mm, the diameter of the spheroidal graphite is approximately 0.66 μm, and the areal density of graphite reaches 105/mm2. Acicular martensite, lamellar cementite and retained austenite can be obtained after laser surface re-melting treatment. The depth and width of the treated layers are determined by laser power (P), scanning speed (v) and laser defocusing amount (z). The optimal laser parameters are P=2000 W, v=15 mm/s, z=18 mm. The annealing transformation process is mainly determined by temperature. Non-equilibrium structure, constituent homogenization and graphitization process co-exist during annealing process. Superfine spheroidal graphite can be prepared after annealing in the temperature of 850℃, 900℃ and 950℃. The diameter of the obtained spheroidal graphite is much smaller than the 8-degree spheroidal graphite in the national standard of GB/T 9441-2009. The spheroidizing mechanism is related to the cast iron constituent and the rapid laser re-melting process. Surface graphite superfine-spheroidizing process doesn’t change the inner microstructure of cast iron but it greatly improves the thermal fatigue resistance and oxidation resistance of gray cast iron, which is more than double the untreated D-type cast iron.