Abstract
The process of the formation of ferrite and pearlite in cast iron was investigated. Specimens used for the experiments were pure Fe-C, Fe-C-Si, Fe-C-Si-Mn alloys and commercial charcoal pig iron 40 grs. of each alloy was melted in a Tammann tube in a silit furnace and cooled with a rate of 7∼10°C/min. (furnace cooled) or 2∼5°C/min. (slow cooled) near the eutectoid temperature while the cooling curve was plotted. At any point of the cooling curve between eutectic and eutectoid temperatures the alloy was quenched into cold water and the structure was observed. The alloy quenched at each step of cooling showed the structure developed to the time and which would show the process of the transformation.
From the experiments mentioned above, the fallowing evidences were observed.
1) For iron-carbon alloy, the acicular or lump cementites precipitate in austenite matrix along the Acm-line of the iron-carbon diagram while cooling. At the eutectoid temperature, the pearlite is neucleated by the cementites. The acicular cementites appear on the boundaries of flake graphite and austenite, and the lumpy one appears on those of SASANOHA6) austenites.
2) In iron-carbon-silicon (3∼4%Si) alloy, the lump cementites precipitate on the boundaries of SASANOHA austenites and then they decompose to graphite and austenite on cooling. At the eutectoid temperature, the austenite transforms firstly to very fine pearlite, and then the pearlite or residual cementite neucleates normal pearlites in austenite matrix.
3) It is hard to get a matrix coexisted with ferrite and pearlite in pure iron-carbonand in iron-carbon-silicon alloy, unless the small amount of manganese is contained. The promotion of ferrite might be attributed to its awn effect of manganese besides desulphurization effect of it. If sulphur is containd in iron, it requires more manganese to get ferrite because of its consumption of farming manganese sulphide.
4) When iron-carbon-silicon-manganese alloy or commerical pig iron cools, it is not seldom that both ferrite and pearlite are seen. In this case, many thin flake graphites precipitate on cooling succeeding to the precipitation of original flake graphites along the Agr-line of the diagram, and ferrites crystallize out attaching to them at the eutectoid temperature followd by the pearlite formation. Pearlite appears on the boundary of ferrite and austenite or that of SASANOHA austenites by the process previously mentioned.
5) When steadite existes, pearlite is also neucleated by it.
6) The cooling curves of iron which contains both ferrite and pearlite indicate breaking and arresting points, the former corresponds to ferrite formation and the latter pearlite formation. It is suggested that the amount of ferrite could be related to the temperature interval between breaking and arresting points.
7) Iron containing a small amount of tin promotes pearlite formation remarkably and makes less ferrite matrix. The iron contained more than 1.5% of tin rystallizes out some angular particles of white phase at the end of the eutectic reaction, which have strong power of pearlite neucleation at the eutectoid temperature.
