SOLIDCast UNIT 6: Customizing Curves for Cast Irons
For ductile irons and gray irons, the system allows you to customize the cooling curves based on the specific composition of Carbon, Silicon and Phosphorus in your alloy. The system also calculates the CFS point and overall amount of shrinkage during solidification and makes automatic adjustments to the shrinkage curve, based on the cooling of an unrigged casting.
Cast Iron is one of the most complex alloys in terms of how it solidifies and how volume changes affect the likelihood of shrinkage porosity. Following is a description of current methodology
in regard to modeling iron castings in SOLIDCast, and how to best customize the data for your process.
In the standard SOLIDCast database, there are two base alloy selections for ductile iron, and three for gray iron, as follows:
CI DI Ferr
CI DI Pearl
CI GI 3.5 CE
CI GI 4.0 CE
CI GI 4.4 CE
These represent generic cast irons, and should be modified, as explained below, for use in your own shop. It is possible to customize these alloys for specific compositions, casting types and processes.
As an example, suppose that we have a Ferritic Ductile Iron hub casting(shown below) with the following composition:
We would first go to Model Materials List, click on the Casting Materials Tab, then click on the From DB… button. If necessary, scroll down the list of alloys and select the CI DI Ferr material from the SOLIDCast database. Once the material is highlighted click the button that says Use This Casting Material. You will be asked if you want to:
Click Yes. Then adjust the Initial Temperature to an appropriate pouring temperature, in this case 2500F. The entry should look something like this:
At this stage you do not need to make further adjustments to the material, since we first need to run a simulation of the unrigged model to obtain cooling rate information. Additional steps to set up the simulation are given in units 7, 8, 13 and 16.
Once this naked simulation is complete, double-click the simulation icon in the Project Tree to load the simulation results. You should get a summary screen like this:
Click the Close button to close this window, then select Simulation Riser Design Wizard. This will allow us to calculate the effective casting modulus for the part, which will be used to help adjust the curves.
From the introductory screen, click the Next> button to begin calculations.
Click the button that says Calculate and Display Casting Modulus, then click the Next> button. This will bring up a screen for plotting the modulus values. Just click Next>. The next screen will show the range of Modulus Data. If you are in English units, the modulus will be in inches. If you are in Metric units, the modulus will be in centimeters.
At present, we are interested in the maximum modulus, which is 0.478 inches.
Since the modulus information is all we need, click the Cancel button to cancel the plot, then click on the Finish button to exit the Riser Design Wizard. We have the information we need to adjust the curves for our iron alloy.
To get back to the Materials List, we need to reload the model. Do this by clicking on the model icon in the Project Tree on the left side of the screen. Then click on Model Materials List. Then click on the Iron Calculation Tab. This will bring up the following screen, which will have data on it from the previous calculation.
Here, we enter the C, Si and P contents as percentages.
This approach recognizes that castings of different section modulus will behave differently in regard to volume changes. The modulus is the maximum calculated by the Riser Design Wizard. In this case the value is 0.478 inches.
The calculations, which are based on the German Foundry Society s VDG Nomograms also require an estimate of the average temperature of the iron in the mold. This is NOT the pouring temperature. In general, it could be estimated that iron loses, say 75 F to 100 F during the pouring process. This means that the entry here should generally be pouring temperature less 75 F to 100 F. So if we have a pouring temperature of 2500F the estimate of metal temperature in the mold might be 2425F.
There are two slider bars, one for Metallurgical Quality and one for Mold Rigidity. Values can range from 1 to 100 for both factors. In general, the better the process control, inoculation, etc., the better the metallurgical quality. And the harder and stronger the mold, the higher the mold rigidity. This example uses middle values of 50 for each factor.
After entering the above information, we click Calculate Iron Properties and see a screen similar to this:
The software shows a thumbnail of the adjusted solidification and shrinkage curves, and allows you to go directly into the cast iron riser design function, or you can click Close. When you exit this function, the new curves are automatically saved in the Materials List.
You can now go to the Casting Material screen, enter a unique name for this alloy, and click the Add to DB button to add this to your database as a customized alloy.
Interpretation of Results
Interpreting the results of a cast iron simulation generally involves looking at two different kinds of shrinkage porosity: Primary Shrinkage and Secondary Shrinkage.
Primary shrinkage is due to the liquid contraction of the alloy during the initial stages of solidification, and is predicted in SOLIDCast by using the Material Density function. This shows formation of primary shrink cavities as well as piping of risers. Critical values of Material Density are usually considered to be in the range of around 0.995. In other words, if a part of the casting has a Material Density value of less that 0.995, there should be some discernable shrinkage in the casting at that point.
Secondary shrinkage generally occurs late in solidification, and is mainly the result of expansion pressures opening cavities within the casting. These cavities are usually found in the thermal centers of the casting. The quality of the iron will generally determine whether secondary shrinkage is a problem or not. If the iron is well inoculated and within spec for chemistry, then graphite expansion will normally continue long enough during solidification to prevent the occurrence of secondary shrinkage. However, if the iron quality is less than desirable, secondary shrinkage is more likely to occur as graphite expansion in thermal centers may not continue long enough to counteract the expansion pressure. Fluctuating iron quality usually will result in intermittent appearance of shrinkage in castings.
SOLIDCast can be used to indicate thermal centers where secondary shrinkage is likely to appear. The best indication of thermal centers can be seen with the FCC Criterion. To use this after a simulation has been run, from the Simulation menu select Calculate FCC Custom Criterion , then plot the Custom-High output. When you select Custom-High, the system will display the range of FCC values, which may be different for each casting. We have found that, to establish a starting point, a good procedure is to plot about 40% of the total range as a critical value. For example, if the range is 0 to 3.6, you might start plotting at 3.6 X 0.4 = 1.44. Plotting above this number shows more severe indications, below this number would be less severe. For subsequent design iterations, use the same critical value to compare the result of one simulation to the next in order to evaluate changes.
Riserless castings are even more complex than risered castings when it comes to modeling and prediction of shrinkage. We have found that in many cases, feed metal flows from thermal centers rather than under the influence of gravity.
It is possible to adjust SOLIDCast so that feed metal flows from thermal centers instead of downward under gravity influence. This can be done by running the SOLIDCast Simulation Parameters utility and checking the box labeled Thermal Center Feeding (for Riserless Iron Castings). Once you check this box, click the Apply button. Then click Exit. Once you have done this, all subsequent simulations will be done using Thermal Center Feeding.
When Thermal Center Feeding is active, all feed metal flow would be considered to be from thermal centers rather than downward under the influence of gravity.
When using this approach, the range of Material Density values is generally different than normal. You may end up with a range of something like 0.954 1.0. To plot the predicted areas of shrinkage, plot values that are at the lower end of this range.
In order to go back to normal feeding, go back into the SOLIDCast Simulation Parameters utility, clear the Thermal Center Feeding box, click Apply, then click Exit.