| The thin-walled ductile castings with step ladder-like thickness were prepared. The effects of different holding time of alloy melts in vacuum on the microstructure and graphite morphology of thin-walled ductile cast iron were studied. The Y-block specimens with different graphite nodule counts were prepared and the relationship of the ballistic work between the standard charpy impact specimen with U-notcth and miniature one at subzero (-20℃) was studied. The effects of different graphite counts and matrix on the Rockwell Hardness and the ballistic work at room temperature and subzero were studied.The results indicated that in order to obtain more graphite nodule counts, different holding time in vacuum was needed for the ductile castings with different thickness. The microstructure of the plate specimen with 2mm thickness was better when the melt was held in vacuum for three minutes. For the annealing castings, the standard ballistic work increased with the graphite nodule count increasing. The relationship of the ballistic work between the miniature (Emin) and standard (E) specimens at subzero (-20℃) was linearly fitted as E min =0.005E.For the annealing thin-wall ductile castings, the miniature ballistic work increased with the increase of the graphite nodule counts. And for the specimen with 2mm thickness, whose graphite nodule count is 1719 nod/mm2, the ballistic work was 0.092J and 0.088J at room temperature and subzero respectively, which transformed to standard ballistic work were 18.4J and 17.6J. For the as-cast thin-walled ductile iron, the pearlite rates of which was varying from 34.4% to 38.9%, when the graphite nodule count was 972 nod/mm2, the brittleness caused by the pearlite for the material was restrained and the subzero ballistic work was improved as most as possible. And when the nodule counts was less or more than 972 nod/mm2, the impact fracture patten was changed from dimple pattern to cleavage one and the ballistic work for miniature specimen descreased.
|
PDF Full Text Request