Effect of Drop Rate

A 56"x16" (width by thickness) ingot was chosen to study the effect of drop rate. For these simulations, water flow rate, corner radius and aspect ratio are fixed, they are 70 gallon per minute per mold, 1.0 inch and 7/2, respectively and five different casting drop rates are chosen.

Again, the maximum principal stress level is used as the criterion for assessing the possibility of crack initiation. The temperature and stress from ABAQUS^TM post are processed by a PC program CONVERT, developed in this research to quickly obtain/assess stress distributions in the ingots.

The maximum stress levels for four transient steps and five different drop rates are listed in Table 5.1 and plotted in Fig. 5.1. The term “Stress Level 2” is used to represent the ratio of

principal stress to tensile strength at the point of interest. If stress level 2 at a point is above 1.0, micro crack is likely to initiate at this location. Table 5.2 shows the locations of maximum Stress Level 2 in four casting steps for five different drop rates. A special six-digit numbering system is used to identify the element locations in the ingot. The last two digits show the location of the element in the ingot along the x (thickness) direction. The range is from 01 to 08 with the central element to be 01 while the surface element to be 08. The two digits in the middle indicate the element location along y direction (ingot length direction). The range is from 01 to 59 with 01 at the bottom and 59 on the top. The first two digits indicate the element location along z direction (ingot width direction) with a range from 01 to 12 (center to the surface). If the first two digits are less than 10, the 0 on the left is dropped and the element number has only five digits. For example, an element numbered as 10101 indicates that it is located at bottom center of the ingot.

For drop rate of 1.0 in/min, the maximum stress level 2 does not vary much with ingot length, as shown in Figure 5.1. The reason is that the maximum stress level 2 in this case is dominated by quenching stress throughout the casting steps with the exception at 5 inches ingot length. The locations of maximum stress level 2 are in quarter area and move up along as ingot drops down.

The highest quenching stress location also moves from corner area to quarter area at ingot length of 20 in, and remains there. Thus for this case the likely crack initiation site is at the quarter area.

The principal stress direction for a typical layer is shown in Figure 5.2. As shown, the maximum principal stress in quarter area is along z (or 3) axis, or crack propagation is along the y-axis, i.e.

quarter crack.

When ingot is 5 inches long and under 1.9 in/min drop rate, the maximum stress level 2 is in center out area (i.e. center width area near outside surface) and is above 1.0, which means the maximum principal stress reached ingot tensile strength and has high probability to cause crack initiation. Figure 5.3 shows principal stress direction for this step, which indicates that at center out area the crack initiation will be along the y-axis (i.e. center crack). When ingot length is longer than 5 inches, the results show no possibility for crack initiation until it reaches 20 inches long where the maximum stress level 2 reaches1.02 and the location is inside the center area (center-in). Figure 5.4 shows maximum principal stress direction in that area, and indicates possible center crack growth. As the ingot length reaches 30 inches, it will approach steady state stage. The location of maximum stress level 2 moves from center-in position to the quarter-in area. The magnitude of stress level 2 increased to maximum. Figures 5.5 and 5.6 show the maximum principal stress direction from different viewpoint. As shown, it is noted that the maximum principal stress direction is not just alone one direction. Thus, either quarter crack or J crack may occur.

Notice that for 1.5 in/min drop rate, comparing to other drop rates, the maximum stress level 2 is relatively low throughout the casting steps and above 1.0 only when the ingot is in steady state.

For ingot length of 10 inches, the maximum stress level 2 is in center-out area as shown in Figure 5.7. The cracking mechanism will be of center crack type. For ingot length of 30 inches, maximum stress level 2 moves to the quarter area inside the ingot. Figures 5.8 and 5.9 show vector plots of maximum principal stress directions for this step. Combining Table 2 and these two figures, the possible failure mode is quarter crack.

For drop rates of 2.4 in/min and 3.0 in/min, Figure 5.1 indicates that they are not likely to cause crack initiation as long as ingot length is below 22 inches. After which, high stress level will start to build up rapidly in quarter-in area (quarter area inside the ingot), where values of maximum stress level 2 are high. Figures 5.10 to 5.13 show maximum principal stress vector plots.

Comparing these pictures to Figures 5.5, 5.6, 5.8, and 5.9, the maximum principal stress directions change to a bigger angle w.r.t. the z-axis and the high stress locations move away from the quarter area to corner area and the chance to have J crack is increased.

From above discussions, several conclusions can be made.

1. For 1.9 in/min drop rate, three types of ingot cracking may occur, i.e. center crack at the start-up stage, quarter crack or J crack at the steady state stage.

2. At steady state stage, the maximum stress level 2 increases with higher drop rate. For drop rate above 1.9 in/min, it is more likely to have J crack, below 1.9 in/min, it is likely to have quarter crack.

3. Quarter crack is the only cracking type when drop rate is slow (below 1.5 in/min). As drop rate increase center and quarter cracks may occur. For high drop rate, the probable crack type is J crack and stress level 2 is high. High drop rates (above 2.4 in/min) are not recommended for direct chill casting.

4. When drop rate is 1.5 in/min the possibility to have crack is lower comparing to other drop rates. This is the recommended drop rate.