This calculation is plotted in Figure 1. The figure shows the relationship of the maximum aluminum and nitrogen con- tent for four cooling rates; 5°
C/hr, 10° C/hr, 20° C/hr, 30 °
C/
hr. These cooling rates were chosen to represent the pro- cess range typically seen in sand casting. From this chart, the maximum allowable aluminum content to avoid em- brittlement can be determined for a given nitrogen level and cooling rate. For instance, if the nitrogen content is 80 ppm (0.008 wt%) and the cooling rate is 20 °C/hr then the maxi- mum allowable Al is 0.04 wt%. Similarly, this graph could be used if the composition is known and some threshold cooling rate is used to reject potentially embrittled castings.
This relationship can be compared to historical data on em- brittlement charts. Figure 2 shows the comparison to the original Hannerz curve,3
the Lorig and Elsea data,2 Roesch and Zimmerman data.1 and the The estimated cooling rates
for each curve are also shown on the data labels. Although this data is plotted at different cooling rates, the trends and limits of each reference are comparable. Of these curves, the Hannerz accurate equation is the most conservative.
The Hannerz paper also included the equation for the TTT curve. The TTT curve can be generated for any composition of aluminum and nitrogen. The equation for the TTT curve is shown as follows,
where t is time in seconds. This equation for the TTT curve was also checked against the curves published by Banks with good agreement.6
Comparing the cooling curve from
the casting against the TTT curve can be used to evaluate for embrittlement. An example of the TTT curves are shown in Figure 3 for a composition of 130 ppm (0.013 wt%) nitrogen and several aluminum compositions.
In Figure 3, cooling curves at the center of a one dimensional slab are plotted for several half-thicknesses of the slab. These cooling curves were generated using commercial casting simulation software.7
A standard database for Weldable Cast
B-Grade Carbon Steel (WCB) and for chemically bonded fu- ran sand was used with a temperature dependent heat transfer coefficient from the database. A constant sand thickness of 4” was used to surround each slab. An initial temperature of 1630°C (2966°F) was used. The influence of the superheat on these results is negligible. Thermocouple curves were gener- ated from the center of the slab, the slowest cooling rate. For a nitrogen content of 130 ppm, the aluminum content was ad- justed until the TTT curve just touched the cooling curve. This is expected to be a conservative prediction of the maximum aluminum content before AlN embrittlement would occur for that slab thickness. The final figure is plotted in a semi-log scale to allow the time scale to be compressed to show all the cooling curves and TTT curves.
Equation 2
The above procedure was used to analyze three nitrogen contents of 80, 130 and 180 ppm. In addition to the cooling curve, the maximum feeding modulus for each slab was re- corded for use in Figure 4. These curves illustrate the inverse relationship between the feeding modulus and the maximum aluminum content. This graph could be used directly to cal- culate a conservative value of the maximum aluminum con- tent for a nitrogen level from the known section thickness or
Figure 1. Recalculated Hannerz chart shows four cooling rates using the improved equation. At a particular cooling rate, compositions above the line are at risk of embrittlement.
International Journal of Metalcasting/Summer 10
Figure 2. Comparison of historical AlN chart data, (a) Hannerz original at 20°C/hr, (b) Hannerz accurate (This work) at 20°C/hr, (c) Lorig and Elsea at 55°C/hr, and (d) Roesch and Zimmerman at 28°C/hr. The Hannerz accurate equations are the most conservative prediction for AlN precipitation.
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