Research On The Stabilities Of Microstructure And Properties In Heavy-section Ductile Iron

Due to the slow solidification process of heavy section ductile iron, the graphite degeneration, especially the chunky graphite often appeared in the center part of the casting, which lowers the mechanical properties of the casting greatly. In the past 40 years, scientists and producers around the world has given this problem much attention, and made a great progress in realizing and resolving the problem. But until now, the reason and mechanism for the formation of chunky graphite are still unavailable. The related researches on this issue are still in process.This paper firstly produced a heavy section ductile iron castingΦ590×800mm in factory, then made a research on the solidification, microstructure and mechanical properties of the casting, as well as the reason for the formation of chunky graphite. Then A 1500℃Knockdown Multifunctional Resistance Furnace was developed and used to control the solidification process of a small casting in its hearth which was named simulation casting in the paper, in order to make the simulation casting solidify in the manner of the centeral part of the castingΦ590×800mm. The effects of minor elements Sb and Bi, addition of more inoculant through in-stream inoculation on the simulation casting was studied. The effects of nanoparticle SiC on simulation casting and common casting was studied. The results are as follows:(1) The solidification process of the castingΦ590×800mm is extremely slow. The solidification time in the center is 372 minute, which represents the longest time. The solidification time in position 255mm away from the center is 232 minute, which represents the shortest time.The change of microstructure and mechnical properties in different positions on the casting shows special trends. The worst graphite morphology and mechnical properties appeared in the position 170mm away from the center, where lots of chunky graphite appeared, the tensile strength, brinell hardness and elongation are 362MPa,195HBW,1.67% respectively. In position 255mm away from the center, the graphite morphology and the mechanical properties are the best, where the tensile strength, brinell hardness are 494MPa and 225HBW respectively. In the circle area with radius of 85mm around the center, there are many noduar graphites and much chunky graphite. The machenical properties in this area are comparatively good.(2) Along the radius direction of the casting, macrosegregation of Mg and RE were discovered. In the position 170mm away from the center, the contents of Mg and RE are extremely high, where the content of Mg and RE are 26% and 20% higher than in the position 255mm away from the center respectively.(3) It was easy and safe in using the 1500℃Knockdown Multifunctional Resistance Furnace. The cooling curve of simulation casting controled by the furnace is similar to the real cooling curve of the castingΦ590×800mm. The maximal comparative error between the two curves is 1.7%. The results show that it's effective in simulating the solidification process of heavy section casting by this furnace.(4) The minor elements Sb and Bi was effective in improving the morphology of the graphite in the simulation castings, and the effect of 0.01wt.% Sb addtion was better than that of 0.01wt.% Bi addition. When 0.03wt.% Sb alloy is added into the iron, the morphology of graphite and mechnical properties of the simulation casting was the best; the tensile strength, yield stress and elongation are 707MPa, 610MPa and 2.96% respectively, which are 74.4%, 81.9% and 22.9% higher than that of simulation casting without minor elements.(5) The addition of 0.1wt.% and 0.2wt.% inoculant by in-stream inoculation has good effects in increasing the mounts of graphite, but bad effects on the morphology of graphite, which also promotes the ferrite in the matrix, lower the machenical properties at last.(6) The addition of 0.2wt.% nanoparticle SiC in common casting can increase the mounts of nodular graphite, improve the graphite morphology and enhance its mechnical properties. The addition of 0.2wt.% nanoparticle SiC in simulation casting promotes the growth of the graphite but lower its tensile strength and brinell hardness.