TECHNICAL FIELD

The invention relates generally to the field of centrifugally casting metal objects, and more specifically, to the field of centrifugally casting of iron pipe.

BACKGROUND

The process of centrifugal casting of metal objects, and in particular of iron pipe, is well known and has been practiced for a century. A centrifugal casting machine includes a delivery system, such as a trough, and a rotating mold. Molten iron is poured from a machine ladle into the trough. The trough extends into the interior of the rotating mold, generally axially. One end of the mold usually includes a core, such as a sand core, to accurately shape what is called the bell of the pipe. The opposite end of the pipe is referred to as the spigot, and the elongated section in between is the barrel. The molten iron flows down the trough under the influence of gravity. The mold and trough are moved relative to one another to fill the mold with iron, typically from the bell end along the barrel to the spigot. As the mold rotates, centrifugal force disposes the iron circumferentially around the mold in a relatively even manner. Typically, the casting machine is moved via hydraulics or other mechanical means, as is known in the art, to dispose the iron as desired.

Variation in the charge mix (i.e., the source of raw material for the foundry, such as scrap iron), coke, and cupola operation results in variation in the molten iron temperature and chemical composition. The variation in content of the molten iron also manifests itself in the liquidus arrest temperature and the fluidity of the molten iron, which in turn causes variations in frictional forces, surface tension, heat diffusivity, of the molten iron from which each pipe is cast, which themselves may vary from one pour to the next based on pour temperature. All of this results in inconsistency in the flow rate of iron to the mold. The variation in molten iron content cannot be cost effectively eliminated in a facility using material from recycled or scrap sources.

The casting process can be thought of as governed by two operations, which occur roughly contemporaneously but are controlled independently of one another. One operation tilts the machine ladle to pour the iron, and the other moves the mold relative to the trough to allow distribution of the iron in the mold. The overall volume of molten iron poured for casting, and therefore the weight of the pipe, is determined by the operation of the ladle. The distribution of the iron within the mold is determined primarily by movement of the mold and secondarily by ladle operation. The applicant hereof has obtained U.S. Pat. Nos. 8,733,424; 8,910,669; 8,960,263, which are directed towards the controlling movement of the mold relative to the trough. These patents disclose an apparatus and method to control casting machine parameters and movement to account for changing fluidity of the molten iron due to variability in the charge mix and pour temperatures.

As to operation of the machine ladle operation, controls for parameters associated with tilting the machine ladle include machine ladle pouring rate, machine ladle cutback position, and spigot check position. For example, the machine ladle pouring rate sets the flow rate for the molten iron being poured from the ladle. When molten iron is poured from the machine ladle during the casting cycle, there is a delay while the iron (typically under the force of gravity) traverses a trough that terminates in a spout, from which the iron is disposed in the mold. The delay varies based on the size and length of the trough, flow rate, and other factors, but its duration is significant relative to the overall duration of the casting cycle. For example, in a process where the actual casting time is 15 seconds, the delay from pouring the iron from the ladle to its leaving the spout of the trough may be 4 seconds. This delay therefore makes any changes to controls associated with the machine ladle difficult to make “on the fly.”

Machine ladles are designed to be linear pour devices so that equal degrees of rotation of the ladle will deliver equal volumes of molten iron. Over time, however, the ladle may become non-linear in its pouring characteristics. Non-linearity in ladle pouring may be introduced by variations in the dimensions of the ladle. Slag, a viscous ceramic substance that is dissolved in molten iron, may come out of solution and deposit in the ladle. It may deposit in various locations in the ladle and cause non-linear pouring in a number of modes. Changes in pour linearity are subtle and are compounded over time. This can cause a decrease in volumetric flow rate of iron even though the ladle pour rate control (degree of tilt of the machine ladle) has not been changed at all.

One mode of non-linearity in pouring is from slag build-up in the ladle lip area. This may cause decreased volume of molten iron at the beginning of the casting cycle. The initial low volume flow would cause bell wall thickness to be less than anticipated. This can be offset by increasing the pour rate, which will also increase the pipe weight and thickness of the pipe wall throughout its length. Casting machine (mold) movement also can be adjusted to restore pipe wall uniformity.

Another mode of non-linear variation in pouring is from slag build-up in the teapot area of the ladle. This may cause decreased volume of molten iron at the end of the cast cycle. Insufficient volume of molten iron at the end of the cast cycle would cause the pipe wall to be thin at the spigot. The spigot wall thickness may be increased by changing the ladle cutback position control, which will allow the ladle to pour for a longer period of time. The longer pour time will increase the volume of molten iron to the spigot end of the pipe, increasing its thickness there.

Non-linear variation in pouring may be generated by erosion of the ceramic ladle lining material. This erosion may cause the ladle to change dimension in any area. As the non-linearity increases, the pipe wall thickness uniformity may vary in a number of ways. Often the only remedy for pipe wall thickness that is not sufficiently uniform is to replace the machine ladle.

In sum, with variability introduced by chemical content of molten iron, changes in pour temperature, and changes in linearity of ladle pouring characteristics, uniformity of results and adherence to specifications can be difficult to achieve even with hydraulic systems controlled by programmable logic controllers (PLCs). The casting operator simply cannot detect changes in the iron or pour characteristic that affect wall thickness uniformity in a timely manner in order to adjust the casting machine controls. Instead, until now when an iron pipe is cast in a centrifugal casting machine, no feedback for its wall thickness uniformity is available for the casting machine operator for hours after casting, because the pipe typically must complete annealing and processing (a period of hours) before it can be measured, usually with an ultrasonic thickness instrument.

Although casting apparatus may operate at different speeds, in one example, the pipe is cast at a rate of 0.75 seconds per foot. Therefore, a 20-foot length of pipe may require only 15 seconds to cast. It will be apparent that hundreds of pipe may be cast before the first pipe has been annealed and sufficiently cooled for the ultrasonic measurement of wall thickness. Further, when one of the casting parameters is altered in an attempt to correct the error, it will require several more hours before it can be determined if the alteration has resulted in the desired correction or has increased variation in pipe wall thickness.

Thus, there is a need for an apparatus and method that provides an accurate estimate of uniformity of pipe wall thickness proximate to completeness of the casting process.

SUMMARY

Embodiments of the present invention satisfy these needs, but it should be understood that not all embodiments satisfy each need. One embodiment comprises a method of estimating wall thickness of an object formed in centrifugal casting process in which molten metal is poured in a stream from a ladle into a casting machine having a trough that is movable linearly with respect to a longitudinal axis of a rotating cylindrical mold. In this embodiment, a volume of molten metal associated with a sample of the stream poured from the ladle is determined and correlated to a longitudinal position on the mold. Images of the molten stream as poured from the ladle are captured by a machine vision device, and the cross sectional area of each sample is determined. The overall volume of the object is determined from its weight immediately after casting and the density of the material from which it was cast. This volume is divided by the sum of the areas of each sample to determine the length of the stream. Multiplying this length with the area of one sample yields the volume of molten iron associated with that sample. The volume for each sample is determined in this way and correlated to its position on the mold. The correlation is based on position data obtained from a controller controlling and recording the movement of the casting machine. Using the incremental volumes and their associated positional data, wall thickness over the length of the object may then be calculated and displayed.

In another embodiment, the diameter of the stream samples or their volumes may be correlated with velocity of the casting machine over time and displayed. The relationship between these variables is indicative of uniformity of wall thickness.

Another embodiment of the present invention comprises an apparatus for determining an estimate of uniformity wall thickness of an object centrifugally cast by from molten metal poured from a ladle into a trough that is positioned relative to a rotating mold for disposing the molten metal from the trough into the mold. In this embodiment, the apparatus includes an image capture device positioned to capture an image of a stream of molten metal poured from the ladle, a drive system coupled to at least one of the trough or mold, a controller for controlling the drive system and receiving data indicative of movement of the trough relative to the mold; and a processor programmed to use image data from the image capture device to correlate over time a volume of molten metal poured from the ladle with position data of the mold for where that volume was cast and determine an estimate of wall thickness uniformity from the correlation. In a preferred embodiment, the apparatus includes a graphical display in operative communication with the processor, where the processor is further programmed to provide an indicator of uniformity on the display. The apparatus may also include a sensor positioned to detect when molten metal exits the trough, where the sensor is in operative communication with the processor.)

