Variables that Affect Cope Oxide Inclusions in Steel Castings.

Harrison Steel identified melting and pouring variables that affect melt cleanliness in an effort to establish procedures for the elimination of oxide macroinclusions.

In 1995, Harrison Steel Castings Co., a 750-employee carbon and low-alloy green sand steel shop in Attica, Indiana, began collecting data on its melts as part of the Steel Founders' Society of America Clean Cast Steel Technology Program. The segment of the program Harrison was involved in was geared toward identifying the heat-to-heat variables that influence the variation in cope surface oxide inclusions in steel castings. This segment featured in-p]ant experiments to: clear up the unknowns of melt cleanliness, examine the influence of melting and pouring practices on casting quality, and reduce heat-to-heat variation of casting defects.

As part of the experiment, Harrison, which uses two 20-ton and one 8.5-ton acid-lined, electric arc furnaces to melt and pour from bottom-pour ladles, analyzed more than 35 heats to isolate the most important variables that affect surface cleanliness on a specific casting, and then performed another 24 heats at optimized production to verify the effect of the variables. This article examines those experiments and provides conclusions for other foundries to adapt to their production.

EXPERIMENT PROCEDURES

The casting chosen for the study was an engine support for a large off-road vehicle that Harrison has been producing since 1976. Figure 1 shows its shape and gating system with a 1:1:1 (sprue:runner:ingate) ratio. To determine melt cleanliness, the total number of defects per casting were recorded along with the type of defect and total inches. The categories of defects were cracks, dirt (sand or oxidation products), shrinkage, misrun, gas and other. This article focuses on dirt defects.

Each time the casting was run, five castings with a gross pour weight of 2260 lb were poured in the heat. Because heat-to-heat dirt variation was being studied (rather than the within-heat dirt variation), the defect lengths from all five castings were averaged.

The project of data collection started with a list of melting and pouring variables. Molding and sand variables were considered background noise in the analysis of the data except when scabs were found on the gating system.

The furnace initially was charged with bags of graphite to increase the meltdown carbon level. A charge containing 50% foundry returns and 50% purchased scrap then was dropped into the furnace. Oxygen was injected to lower the carbon (C) to the desired aim point.

After verification that the heat was ready to block, another dip sample was taken just prior to blocking the heat. After the block additions, the temperature was adjusted and the heat was tapped into the ladle. The final deoxidation of the heat was done by adding pellets of aluminum to the tapping stream at the rate of 2 lb/ton. The chemical specification for the steel poured for the trials is listed in Table 1.

Following is a look at the five phases of the experiment.

PHASE I

Phase I of data collection was observation with little or no interference with the making and pouring of the heats. The analysis was initially started after the first 12 heats had been collected. All of the data were entered into a spreadsheet, and the heats then were ranked by average dirt severity.

Thirty-four variables remained in the study with a confidence level greater than 90%. It was decided that many of the variables remaining in the study were representing the same thing. Engineering judgment was used to reduce the list to the seven important variables:

1. Reduce the head height in the ladle when the castings are poured.

2. Reduce the temperature of the steel in the ladle when the castings are poured.

3. Increase the level of Si in the bath just prior to blocking heat.

4. Increase the level of Mn in the bath just prior to blocking.

5. Increase the number of bags of slag thinner added to the ladle.

6. Increase the rate the heat is tapped from the furnace into the ladle.

7. Decrease the number of heats on the furnace lining.

The average dirt length for these heats was 32.3 in. with a standard deviation of 8.3 in.

PHASE II

The variables identified as being important in Phase I were set at their optimum levels in Phase II to produce cleaner heats. The melting practice was modified in order to aim for higher levels of oxidizable elements after decarburization.

After data from 24 heats were collected, the final analysis was performed, which consisted of:

* a comparison for each variable between the 10 heats with the lowest dirt lengths vs. the 10 heats with the highest dirt lengths;

* a comparison between the dirt length for the highest 10 heats vs. the lowest 10 heats of each variable.

These two analyses were used as the basis for excluding or including variables. All of the variables that had a statistical confidence of over 90% in both studies are listed in Table 2.

Most of the variation in cleanliness on the castings could be explained using three variables. The first two variables were related to where the castings were poured in the heat. Pouring location (from the start of the heat to the end) sets the level of the head height and the pouring temperature in a bottom-pour ladle. The third variable was the level of residual silicon in the bath after decarburization (referred to as block silicon). The block silicon (Si) was correlated with almost all of the variables during the making of the heat.

The importance of pouring temperature is shown in Table 3. High pouring temperatures corresponded with dirty heats. One theory is that the higher temperatures superheat the mold, causing the steel to freeze more slowly, thus giving the inclusions time to agglomerate and float toward the cope surface. Another theory is that the inclusions formed at the higher temperatures may be more detrimental to cope surface cleanliness than inclusions formed at lower temperatures.

The importance of head height in steel cleanliness is shown in Table 4. Previous work with the clean steel project used water modeling to show that the air entrainment was proportional to the head height (raised to the 2.5 power) multiplied by pouring time. With other items such as inspection criteria and gating system constant, the combination of head height and pouring temperature was the largest contributor to steel cleanliness for a casting.

The second largest contributor to steel cleanliness was the biggest surprise in the study-residual Si and manganese (Mn). Table 5 shows that higher levels of residual Si (after the blow) are associated with cleaner castings.

Due to the large variation attributed to these three variables--pouring temperature, head height and block Si--an analysis of variance was run on the 59 trials collected. Dirt length was the dependent variable and head height, pouring temperature and block silicon were the independent variables. All factors were significant.

The correlation between the chemistries also was taken from the furnace. The meltdown chemistries were highly correlated with one another. If the charges were built with higher alloy materials, the levels of C, Mn, Si and chromium (Cr) were elevated at meltdown. The melt chemistry also was highly correlated to the block chemistry. Higher meltdown chemistries had higher block chemistries. Cleaner castings were obtained from heats with higher levels of oxidizable elements after decarburization.

The temperature after oxygen injection also was related to the meltdown Mn and Si. As the meltdown Si was increased, the temperature after oxygen increased. The meltdown C did not correlate well with the temperature after oxygen.

Residual Cr had an effect on the level of residual Si and Mn after the injection process because of the inter-relationship between meltdown Mn, Si and Cr. The level of block Cr for the grade studied set the level of the final Cr since there was no Cr addition. When final Cr showed up in the list of 25 important variables, it related back to heats that finished with a high block Si.

In regard to the correlation between chemistries taken from the furnace and the percent recoveries for the elements, the block Mn and Si are correlated with the recoveries of these elements. The block chemistry and recovery for these elements were directly related to the temperature of the bath after oxygen injection. The only recoveries that had an influence on the dirt length were the recoveries of Mn and Si. As the recovery of the elements increased, the dirt length decreased.

The recovery of C showed a correlation with the level of C just prior to blocking the heat. The recovery of Al was mildly correlated with the level of C in the bath, the tap temperature, the type of calcium wire used (solid calcium vs. Ca-Si) and the temperature during the wire injection.

Oxidizable Elements

The experiments spawned the belief that the correlation between a high block chemistry and low amount of dirt must relate back to the oxygen availability in the bath after the blow. As the available O was lowered (at higher block chemistries), the recovery of the elements increased and dirt length decreased.

From the data collected, one conclusion has to be drawn from the following two choices: the active O in the bath was dependent on more than C content; and the O available that affected the recoveries did not come from the steel bath.

The levels of residual Si and Mn were not set by the level of the C in the bath. They were directly related to the temperature of the bath after the O injection. Clean heats with high recoveries were made at all levels of block C (0.085-0.195%).

In order to further verify the importance of block Si, all of the heats collected in Phases I and II were divided into two groups based on the block Si level. The first group included all heats with 0.045 block Si and higher (specified as a good melting practice). The second group included all heats with less than 0.045 block Si (specified as a bad melting practice). The primary difference between a good and bad melting practice was the temperature of the bath during and after oxygen injection.

The increased levels of oxidizable elements at the block were believed to be related to lower levels of active O in the system available to oxidize the alloy additions. As a consequence of the lower O availability, the recoveries were higher. All of the block alloy additions had to be lowered to keep the chemistry near the aim points due to these higher recoveries.

Two approaches were used to achieve a better melting practice with higher temperatures during the O injection. The first was to raise the amount of oxidizable elements in the bath such that the excess heat from oxidation of these elements ensured that the bath temperature was high enough. The second was to make sure the bath temperature was high enough before oxygen injection was started. Both practices have worked successfully, however, a combination of both seemed to work the best to guarantee the proper temperature was reached.

A good melting practice was achieved with the following methods:

1. Start with higher meltdown Si (increased by 0.11%) and Mn (increased by 0.15%).

2. Start oxygen injection with a higher bath temperature to promote oxidation of C. The results of a good melting practice were:

1. Higher block Si (by 0.016%) and Mn (by 0.038%).

2. Higher recovery of Si (by 7.5%) and Mn (by 3.4%).

3. A reduction of 26.2 lb FeSi, 46.5 lb Si-Mn and 17.2 lb Fe-Mn.

4. Higher hardenability due to Mn recovery.

Ladle Treatment Variables--Early in the course of these trials, there was a change from steel-clad calcium wire to steel-clad Ca-Si powder wire for cost reasons. It was found that the Ca-Si powder wire produced a less violent reaction than did the solid calcium wire. As a result of less steel exposed to the air during the injection process, the Al recovery improved from 38.2 to 40.1% (90% confidence level). The Mn and Al losses per minute also were reduced with the change in wire type (99.95% confidence level). The Si recovery went from 87.6 to 94.7% (99% confidence level). By adding the last part of the Si addition to a fully killed steel in the ladle, the recovery was much higher.

Pouring Practice Variables--From earlier analyses, it was found that head height, pouring temperature and block Si account for a large percentage of the variation in casting quality. The effects of the other pouring variables such as flow rate and visual stream also were measured. The conclusions from this data were that the most important variables were still the pouring temperature, head height and block silicon. The highlighted conditions show that the best castings were poured with a good quality stream into the pouring cup and low flow rates into the mold cavity.

Slag Thinner Variables--The number of slag thinner bags added to the ladle was found to have a significant effect on dirt length. When the ladles were dumped at the end of the heat, this addition allowed nearly all of the slag to be removed. This variable initially appeared due to differences in the addition practice between the day and night shifts, however, further analysis showed that the slag thinner affects the dirt length regardless of where the castings are poured in the heat.

Refractory Variables--The effect of the number of heats on the furnace lining was found to be significant in the original observation trials. As more data were collected, this variable dropped out of the picture.

Effects of Tapping Rate Variables--The tapping rate has an effect on other variables but does not have an effect on the dirt found on the castings.

The average dirt length for Phase II heats was 19.8 in. with a standard deviation of 7.5 in.

PHASE III

Phase III was performed to separate the effect of head height from pouring temperature. Since bottom-pour ladles were used, castings poured at the end of the heat had a low head height and were also the coolest doe to the longer times in the ladle. By putting a pouring basin on all of the molds, the effect of head height in the ladle was reduced. A better comparison then was made between castings poured at the start of the heat (hotter) and castings poured at the end of the heat (colder).

In order to see if this was feasible, one heat was poured with a pouring basin on two of the five castings in the heat. The pouring time doubled on the molds with the basins, and the castings were severely misrun. In order to get a similar pouring time, the foundry engineers had to open up the choke in the gate. The problem was that the choke was the entire gate system.

The casting was rerigged using a streamlined gate with a 1:3:3 gating ratio (Fig. 2). Castings then were poured without using pouring basins. Nine of these heats were indexed, and the average dirt length dropped to 2.2 in.

PHASE IV

After data from nine heats with the new gate were collected and the improvement was verified (2.2-in. average dirt length), the calcium wire injection requirement was removed. After data from four heats with the new gate without wire treatment were collected, the requirement of calcium wire was reinstated. The average dirt length for the four nonwired heats was 9.3 in. Also, the head height and pouring temperature were found to have the same effects found earlier.

The average dirt length for Phase IV heats was 9.3 in. with a standard deviation of 3.5 in.

PHASE V

In the next experiments, data collection will continue with the new gate system with the calcium wire requirement to see whether the variation in cleanliness can be explained with the new gate using the same melting and pouring variables. Oxygen probe analysis also will be made on heats in order to verify the suspected relationship between residual block Si and soluble O in the bath.

CONCLUSIONS

Six general conclusions can be surmised from the experiments:

1. It appears that the magnitude of the cleanliness was set by the gating system (new vs. old). This was the most important variable that determined how clean a casting was going to be before the first casting was poured.

2. A large variation in average casting quality was seen from heat-to-heat with a particular gating system. Some heats were cleaner than others in terms of casting quality. The heat-to-heat variation of average casting quality was partially explained through both melting and pouring variables.

3. The largest variation from heat-to-heat was correlated to variations in the head height and pouring temperature.

4. The second largest variation from heat-to-heat was due to differences in the melting practice that can be associated with the residual block Si level.

5. The large within-heat variation indicated that melting practice could not account for all of the variation. Some of the variation must be explained by the pouring variables. A similar conclusion was drawn comparing acid vs. basic melting: a high variation within a heat relative to the variation between heats suggests that gating and pouring operations are the most important.

6. The large within-heat variation after controlling the head height, pouring temperature and melting practice was attributed to variables associated with the filling of the mold cavity (flow rate and stream quality).

                     Chemical Specifications of Steel
           Minimum (%) Aim Point (%) Maximum (%)
C             0.18       0.23            0.26
Mn            0.8        1.35            1.5
Si            0.3        0.45            0.6
Nickel         --        Residual        --
Chrome         --        Residual        0.5
Molybdenum     --        Residual        0.3
Al            0.02       0.045           0.1
                       Phase II Important Variables
                               Preferred Confidence (%)
Variables                      direction    Phase I     Phase II
Melt Mn                        Increase       97.5        99
Melt Si                        Increase       99.5        99.5
Melt Chrome                    Increase       97.5        97.5
Mn removed/min of O            Increase       95          99.5
Si removed/min of O            Increase       99          99.5
Block Mn                       Increase       97.5        99.5
Block Si                       Increase       99.5        99.5
Block Chrome                   Increase       95          97.5
Si-Mn addition/ton of steel    Decrease       99.5        99.5
Extra Mn addition/ton of steel Decrease       97.5        97.5
Al added/ton of steel          Increase       95          95
Time from block to tap (min)   Decrease       99.5        97.5
Number of heats on ladle walls Decrease       99          90
Number of bags of slag thinner Increase       99.5        99.5
Melt temperature at pouring    Decrease       99.5        99.5
Head heighf when pouring       Decrease       99.5        99.5
Stream quality rating          Increase       99          97.5
Flow rate into mold cavity     Decrease       99          99.5
Time to fill mold cavity       Increase       95          99.5
% recovery Mn                  Increase       99.5        99
% recover Si                   Increase       90          99.5
Ideal diameter                 Increase       95          90
Final Chrome                   Increase       97.5        95
               Effect of Pouring Temperature on Dirt Length
                   Average  Dirt
                   temp.   length
                   (F)     (in.)
10 high pour temp. 2875     36.9
10 low pour temp.  2824     18.5
10 dirtiest heats  2869.5   42.4
10 cleanest heats  2839.6   13.1
                 Effect of Pour Head Height on Dirt Length
                  Avg. head  Dirt
                   height   length
                    (in.)   (in.)
10 high
head heights        92.2     36.3
10 low
head heights        56.6     21.2
10 dirtiest heats   87.7     42.4
10 cleanest heats   62       13.1
                     Effect of Block Si on Dirt Length
                  Average   Dirt
                  block Si Length
                    (%)    (in.)
10 high block Si   0.063    22.6
10 low block Si    0.033    36.7
10 dirtiest heats  0.037    42.4
10 cleanest heats  0.051    13.1