| Popis: |
The effect of silicon (Si) on the stability of heat-resistant ductile iron having ferrite matrix in high temperature was investigated by dilatometric test. Microstructure of heat-resistant ferritic ductile iron consists of ferrite, eutectic carbide at eutectic cell boundaries, precipitated carbide in grain and graphite. Pearlite was found around eutectic carbide in some specimens, however, all pearlite was decomposed by the annealing treatment. As Si content was increased, the number and size of carbide decreased and full ferrite matrix were obtained. The starting temperature of ferrite-austenite transformation and the coefficient of thermal expansion increased with the increase of Si content. The growth of Si 6.0wt% specimen during held at 1,000 oC is lower than other specimens, and it is considered that in the case of Si 6.0wt% specimen, the carbon movement is restrained due to the low solubility of carbon in matrix. In the case of annealed specimens, the contraction during ferrite-austenite transformation was not found when heating. This is because the re-distribution of the graphite was happened through the movement of carbon during annealing treatment. |
