Table 1. Maximum % Carbon to Avoid Nodule Flotation Section Size of sample (mm) Square Bars (mm) Volume-to-surface area ratios (mm) (Modulii) Cooling rate thickness in large plates (mm) Cylindrical section diameters (mm)** % Silicon*
1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0
20 4.79 10 19 30 7.06 15 28 50 11.34 23 45 80 17.23 35 70 MAX. % Carbon to Avoid Nodule Flotation for 2,550F Pouring Temperature
4.00 3.90 3.80 3.69 3.69 3.49 3.38 3.28 3.18
3.96 3.86 3.76 3.65 3.65 3.45 3.34 3.23 3.13
Carbon contents should be decreased by 0.05% for each 90F (50C) increase in pouring temperature. *Silicon contents must include additions made in magnesium treatment and inoculation. **Plus lengths greater than 5x the diameter.
3.88 3.78 3.67 3.57 3.57 3.36 3.26 3.16 3.05
3.76 3.65 3.55 3.45 3.45 3.24 3.13 3.03 2.93
The British Cast Iron Research Association, as part of a research project for the American Foundry Society, produced a table providing the maximum carbon for various silicon values to avoid flotation of graphite nodules for various section thicknesses of different shapes (Table 1).
2. Time the Carbon Precipitation Expansion Effect Correctly
Generally, shrinkage is reduced as the carbon
percentage increases, provided that freezing involves simultaneous precipitation and graphite growth con- tained within austenite shells. If the percent carbon becomes too high and primary graphite starts the solidification process, a great deal of the expansion effect available from graphite precipitation is con- sumed early during freezing. The rapid precipitation of graphite as it floats in the liquid metal can result in insufficient graphite expansion effect during the later stages of freezing, within the last isolated pools of iron to freeze. Figure 2 shows plots of the maximum carbon equivalent data for square bars, flate plates and modu- lii, with three curves shown for three silicon levels. The lines have been extended to the CE eutectic of 4.3. This shows all thicker sections that must be at or below 4.3% CE to avoid primary graphite precipita- tion, nodule flotation and higher shrinkage. Carbon precipitation as graphite nodules is required at the start of freezing to ensure the carbon does not take the iron carbide form as edge chill. Too much early graphite precipitation must be avoided, or too little graphite precipitation will occur during the end of freez- ing, when the gating system and risers can no longer
进行的研究项目的一部分，美国铸铁研究协会制作了 一个表格，就不同的硅含量给出了最大碳含量，以避 免不同形状、不同壁面厚度的个件体发生石墨球漂浮 （表1）。
2.掌握石墨析出膨胀的时间
一般来说，如果凝固过程中析出石墨与包在奥氏体 壳内的石墨生长同时发生，收缩缺陷随着碳含量的增 加而减少。如果碳含量过高，由初生石墨启动凝固过 程，石墨化膨胀的效果在凝固的早期就消耗了。石墨 的快速析出而漂浮在金属液中，因此可能会导致凝固 后期。与石墨分离了的铁液最终凝固时石墨膨胀效应 不足。
图2表示了就方棒、平板和不同模数的铸件，给出 了最大碳当量数据，图中所示的三条线代表三种不同 的硅含量。标线一直延伸到共晶碳当量4.3。这就说 明，所有更厚的件的碳当量都必须等于或低于4.3％， 以避免初生石墨析出、石墨球漂浮和较大的收缩。 凝固初期，碳元素应以石墨球形式析出，以确保碳 元素不在铸件边缘激冷的条件下形成渗碳体。应避免 凝固过程中早期析出过多的石墨，否则，在凝固末期 浇注系统和冒口不能提供更多的铁液以补偿收缩时析 出的石墨就太少。
高的硅含量直接导致较多的石墨球数、较多的铁素 体和石墨早期析出。为了尽量减少收缩，碳含量应该
May 2013
FOUNDRY-PLANET.COM | MODERN CASTING | CHINA FOUNDRY ASSOCIATION | 31
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