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7 S


Ways to Avoid Shrinkage Defects


Examine your metallurgy, tooling and molding practices to improve the quality of your castings. DOUG WHITE, ELKEM METALS INC., BOCA RATON, FLA.


everal factors in ductile iron melting and pouring can be controlled


to maximize yield and reduce shrinkage. Whether dealing with a problem part or a need to improve current yields, these methods can help reduce shrink- age defects and lead to more consistent casting quality.


1. Maximize the Graphite Precipitation Expansion Effect Without Nodule Flotation


One of the first steps in avoid-


ing shrinkage problems in ductile iron melting is to select a suitable carbon equivalent (CE) that avoids the flotation of graphite nodules leading to shrink- age. Te chart in Figure 1 advises the total carbon plus one-third silicon should not exceed 4.55. Te diagram was developed as a general rule for sec- tions varying from 0.5 in. to 1.5 in. For very thin sections, such as in manifolds, the CE may be higher. For thicker sections, it must be lower to avoid nodule flotation and an increased risk of shrinkage. When


% Silicon*


1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0


Fig. 1. Use this chart to determine the preferred carbon equivalent in your ductile iron.


Table 1. Maximum % Carbon to Avoid Nodule Flotation Square Bars (mm)


Volume-to-surface area ratios (mm) (Modulii) Cooling rate thickness in large plates (mm) Cylindrical section diameters (mm)**


20


4.79 10 19


4.00 3.90 3.80 3.69 3.59 3.49 3.38 3.28 3.18


Section Size of sample (mm) 30


50


7.06 15 28


3.96 3.86 3.76 3.65 3.55 3.45 3.34 3.23 3.13


11.34 23 45


3.88 3.78 3.67 3.57 3.47 3.36 3.26 3.16 3.05


80


17.23 35 70


MAX. % Carbon to Avoid Nodule Flotation for 2,550F Pouring Temperature


3.76 3.65 3.55 3.45 3.34 3.24 3.13 3.03 2.93


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.


36 | MODERN CASTING February 2013


carbon precipitates from liquid iron during freezing, an expan- sion effect occurs. Shrinkage will be minimized at the highest pos- sible carbon content, where the iron freezes in the eutectic mode, just below the content where primary graphite precipitates and nodule flotation occurs. Te Brit- ish Cast Iron Research Associa- tion, as part of a research project for the American Foundry Soci- ety, 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 percent- age increases, provided that freezing involves simultaneous precipitation and graphite growth contained 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 precipita-


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