Recovery of Lead During the Cupola Melting of Iron.
The degradation of gray cast iron microstructures due to trace amounts of lead frequently has been reported in the past, however, there has not been sufficient information for foundrymen to determine:
* if a given amount of lead in the metallic charge will result in a given amount of lead in castings;
* the effect lead's presence in the charge has on lead recovery;
* the residual lead composition that will be detrimental in a casting;
* the influence of other factors affecting the recovery of lead.
A research consortium composed of gray iron foundries and steel scrap processors was formed to document the effect of lead in cast iron melts and investigate factors affecting lead recovery from a gray iron cupola charge. This article will report the results of one aspect of this study.
The detrimental effect of trace amounts of lead in gray iron results in the formation of graphite "appendages" on the pre-existing graphite flakes. These graphite flake appendages have been called "sooty," "spiky," mesh and Widmanstatten. Their formation is believed to be associated primarily with the lead content of the iron, but it is apparent that other factors also are operative.
Widmanstatten graphite (Fig. la) develops as a result of the precipitation of graphite along preferred crystallographic planes within the austenite at elevated temperatures, most likely after the completion of solidification. Widmastatten graphite first was reported in the late 1930s, but it was not until the 1950s that it was realized that small amounts of lead caused a severe reduction in mechanical properties of gray iron due to the formation of Widmanstatten and other detrimental graphite shapes. Unusual failures occurring in heavy gray iron castings were attributed to abnormal graphite accompanying the presence of lead and tellurium. It was estimated that these abnormal graphite forms reduced the tensile strength by 50%.
These graphite platelets can be exceptionally fine and even difficult to see during microstructural examinations at 100X The specimen must be well-polished and viewed at 400-500X to discern these platelets. Since they form within solid austenite on preferred crystallographic planes, these platelets will assume an angular relationship (in three dimensions) of 120[degrees] to each other within a given austenite grain. Their influence on mechanical properties appears to be more significant in higher-silicon content alloys prone to ferrite formation, but in conventional pearlitic gray cast iron, the effect on hardness is minimal as compared to the reduced tensile strength that results.
Another deteriorated form of graphite related to lead content is that of mesh graphite (Fig. lb). This graphite form often appears in lead-contaminated irons, but its mode of formation (and the factors that induce its formation) are less known. Mesh graphite is more likely in thinner casting sections while Widmanstatten graphite forms in sections solidifying at a slower rate. The effect on mechanical properties, however, is more severe than that obtained from Widmanstatten graphite.
The lead that may be present in a given gray cast iron can come from a wide variety of sources, however, the principle concern in this study is the lead in ferrous scrap that goes undetected. These experiments were designed to better understand the method that lead uses to enter the molten iron in cupola melting.
A series of six experimental cupola heats was conducted to document variations in cupola iron chemistry occurring when lead was added intentionally to the cupola charge. These heats were carried out in a 27-in-diameter refractory-lined front-slagging cupola. Each heat consisted of three 300-lb charges. The charges were made up of 200-lb structural steel and 100-lb pig iron. Since the primary concern of these experiments was that of lead recovery, no attempt was made to control melt composition.
Fig. 2 summarizes the melting procedure employed for each of the heats studied. Two identical cupolas were employed with heats 1, 3 and 5 produced in one cupola and heats 2, 4 and 6 in the other. Lead was introduced into the heats in a number of different ways as well as in different amounts for the specific purpose of determining the extent to which this characteristic may be significant in cupola melting operations. The following variations were considered:
Heat A--A 9-lb bundle of sheet lead was prepared and placed in the center of the cupola on top of the second metallic charge.
Heat B--Sheet lead (4.5 lb) was placed inside of a steel tube that was welded shut on both ends, after which holes were drilled into the ends to permit air to escape. The tube was placed in the center of the cupola on top of the second charge.
Heat C--Similar to Heat B except 9lb of lead was added.
Heat D--Similar to Heat A except 4.5 lb of lead was used.
Heat E--A 4.5-lb bundle of lead was placed into a pocket formed in the cupola sand bottom. The bed was burned in, and melting commenced after the lead was in place.
Heat F--Similar to Heat E except 9 lb of lead was used.
As noted in Fig. 2, the cupola bed was burned in and the bed height adjusted with a suitable coke addition. Limestone (20 ib) then was placed on top of the coke bed. The 300-lb metal charge was added, covered with a 50lb coke split and a 10-lb limestone addition. This is repeated for each charge, although the stone is omitted on the last charge. The air blast then was turned on. After 8 min, molten metal appeared at the cupola tuyeres, and metal and slag accumulated in the cupola well. After another 8 min, the cupola was tapped, and iron flowed freely through the cupola spout into the ladle receiving it, while the slag was held back in the cupola until the cupola bottom was dropped.
After the cupola was tapped, pin samples and a chilled disc sample were obtained at periodic intervals from the continuous molten iron stream for chemical analysis. The iron was collected in a single ladle and was poured into pig molds. Sections removed from selected pigs also were used for chemical analysis. Last, pieces of iron present in the cupola bottom drop also were subjected to chemical analysis.
Discussion of Results
Samples were submitted to two laboratories for chemical spectrographic analysis (carbon was determined by a combustion method). There were differences in the reported values of lead in these irons. One of the apparent difficulties existing in the literature is that the accuracy and repeatability of the lead content present may be questionable so that comparison of lead content from one reference to another is difficult. Another difficulty arises from research studies that report only lead added and have no report of lead recovered.
Recovery of lead appears to be a function of a number of variables, not the least of which is the melting procedure. In one study involving cupola melting of dirty cylinder block scrap (estimated to have 0.07% lead), no lead contamination was observed in the resulting iron. When the same charge was melted in a coreless induction furnace, 50% of the lead in the charge was recovered in the iron. When leaded steel containing 0.17% lead was melted in a cupola, up to 10% lead was recovered.
The highest lead content was obtained when the added lead did not pass through the cupola melting zone but had been placed in the cupola well. Even then, there is no correlation to the amount of lead placed in the cupola well. The lead content of the iron is exceptionally high (although values this high have occasionally been reported in commercial foundries) in these two cases. Where the lead addition was placed on the second cupola charge (but had been encapsulated in a steel tube), the residual lead analysis spiked, however, the maximum values recorded were less than 0.01%. Low lead recoveries were obtained when unencapsulated lead was in the cupola above the melting zone.
Within the melting zone, lead is readily vaporized or oxidized so that the recovery of lead in the iron is considerably less than otherwise would be expected. When encapsulated in a simple steel tube, the lead is protected from the cupola atmosphere until the tube melts. It would be expected that heavier-walled encapsulation would be more effective and may result in a substantial amount of lead being deposited in the cupola well. When that lead enters the cupola well, its recovery not only is greater, but lead contamination of the melt persists for a longer period of time.
All iron tapped from the cupola was collected in one ladle, from which a number of pigs were poured. It may be assumed that much of the variation in residual lead content observed during the course of melting would be buffered in the ladle. In those cases where lead bundles had been placed on top of the second metal charge (Heats A and D), the residual lead content of the gray iron was 0.001 and 0.0006%, respectively. This is an exceptionally low recovery, considering that 1% lead was added in Heat A and 0.5% lead added in Heat D.
Encapsulating the lead resulted in an increased recovery of lead (0.0032% lead from a 9-lb addition). Encapsulation increased lead recovery by a factor of three. The highest lead content was achieved when lead was placed in the cupola well. Heat E resulted in iron having 0.023% lead from an addition of 0.5% lead, a recovery of only 4.6% of that added.
In addition, a high lead content was observed in the cupola bottom drop where lead had been placed in the cupola well. As would be expected, the lead content in the bottom drop was higher than that found in the pigs.
These experiments demonstrated that lead recovery in cupola melting is inefficient, and lead easily can be contained in molten iron. Recovery of lead is greatly affected by the way in which lead enters the cupola charge. The lead that is most difficult to identify in metallic charges also results in the highest recovery and the greatest metallurgical damage to the iron. For example, lead that is well-encapsulated so that it is not released until its part of the charge descends far into the melting zone resulting in lead that is maintained in the cupola well.
Because of its high density and low solubility in molten iron, lead in the cupola well can be expected to settle to the bottom of the well, contaminating molten iron for an extended length of time. A single occurrence of a large amount of well-encapsulated lead will affect the cupola-melted iron. On the other hand, lesser quantities of lead, particularly when present as "free" lead, will be largely vaporized or oxidized in the upper portions of the cupola and will not accumulate in the cupola well.
Past research, however, suggests that the recovery of lead is a much more complex phenomenon. This research program intends to delve further into the effects of lead in cast iron and arrive at a means to counteract the detrimental effects of its presence. Future work will strive to obtain a reasonable and reproducible analytical determination of the amount of lead that is present in a given casting.
This article was adapted from a paper (98092) presented at the 1998 AES Casting Congress and is available from the AFS Library at 800/537-4237.
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|Date:||Aug 1, 2000|
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