The cost of making iron castings is determined primarily by the metallic content of the material charged to the furnace, and purchased ferrous inputs like baled or heavy scrap are source-dependent for chemistry. A concerted effort is required to source scrap from known suppliers, to maintain consistency. Casting returns like gating pieces, scrapped castings, and machining chips often are the cause for indiscriminate tweaks in charge-mix ratio. This article continues the report presented in the May 2022 issue of FM&T.
Pig iron in charge mix
Pig iron has always been among metalcasters’ choices for ferrous inputs, because it requires less energy to melt, and its elements are recoverable without significant loss. It is prudent to have pig iron in the charge mix to minimize losses, substituting for part of steel scrap and the additions that go with it. Figure 1 illustrates the values that pig iron can ensure. It makes sense to maximize the use of pig iron, to address the requirements of melt chemistry and inoculation.
Density brings value
The fact that pig iron is essentially similar to cast iron, albeit in a crude form, makes it a preferred input for preparing cast iron melts, along with dilutants like low-carbon steel scrap. The densities of both irons are nearly the same, but in a charge mix consisting of pig iron and foundry returns the difference is significant.
Cast iron foundry returns are never uniform in shape and the odd dimensions of gating pieces and broken castings do not permit close packing when charged into the furnace. Further they are contaminated with adhering sand and dirt, often not well-cleaned in a tumbling process.
On the other hand, pig iron can be packed to much higher bulk densities due to uniformity in shape and size. This aspect creates value both in bulk storage and furnace charging. The same argument goes for scrap steel, either as baled sheets or as heavy offcuts, but these alternatives cannot measure up to pig iron in providing good bulk densities. There are no contaminants in pig iron, so long as it is stored in clean bunkers out of contact with water. It makes sense to maximize pig iron input along with steel scrap for improving recoveries.
Melting point is an advantage
The melting point of scrap steel is around 1,500° C while that of pig iron is considerably lower, around 1150° C, as evident from the familiar phase diagrams shown in Figure 2.
Low bulk densities with plenty of exposed surfaces make steel scrap the primary originator of liquid slags, while pig iron in comparison offers substantial value with minimal slag generation. Figure 3 illustrates the differences between the two.
Final chemistry is clear
The fact that pig iron is a cast product from a homogeneous melt and that its carbon and silicon contents are close to those of cast irons make it an ideal choice to blend with input materials in the charge mix. The recovery of elements in pig iron is predictable due to minimal losses and thus takes out uncertainties associated with final chemistry when the melt is almost ready to tap.
Unlike scrap steel from multiple sources inviting undesirable elements in uncertain amounts, pig iron as an alloy is well suited to the chemistry of cast iron. The concentration of carbon, silicon, sulphur, phosphorus, titanium, and calcium found in raw pig irons would call for controlled proportions in a charge mix, while specially processed pig irons would offer much better values in modified chemistry for a variety of foundry processes and resultant properties in castings.
Pig iron composition is more uniform from a batch and therefore sourcing from reliable manufacturers would make process compliance effective with batch control for consistency.
Apart from the above, the value that pig irons bring is through the absence of elements like chromium, vanadium, tin, copper, molybdenum, tungsten, lead, antimony, etc., usually found in steel scrap and its coatings. In many instances uncontrolled influx of one or more such elements from purchased steel scrap can result in undesirable microstructures and hard phase constituents that hamper machinability and/or mechanical properties.
Casting rejects are not just a drain on resources but also can downgrade the capabilities of a foundry in maintaining quality consistently. Pig iron helps to dilute these elements while being friendly to cast iron chemistry and one would definitely add value by making iron castings with a degree of relief from disruptions in supplies. However, dilution of unwanted elements through pig iron should not be considered the solution to the problems that follow from accepting all types of steel scrap indiscriminately.
Chemistry trimming is easier
Trimming additions are made to the liquid metal for adjustments in final chemistry before superheating for tap-out. These are done after slag removal on a clean bath for maximum recoveries. While pure metals like copper and tin, and ferroalloys like FeMn, FeSi, FeMo, FeCr, etc., would go into solution easily, dissolution of carbon would be delayed since the metal is already saturated with carbon.
Carbon additives, low in density, keep floating and will be partly burned out as gases. Fine carbon dust also would escape through the vortex of heated air above the furnace.
Needless to say, recovery of carbon is inconsistent while there is every likelihood of overshooting tapping temperature besides loss of precious energy. Here is a case where pig iron wins the battle as a carbon raiser, so long as there is room to increase bath volume, or when such accommodation is practiced as a standard procedure.
Gaining from low slag volume
Slag is undesirable not only in the castings that are produced but also during the melting process. Process slag is a combination of non-metallics charged into the furnace and the products of reaction at elevated temperatures.
It is possible to clean foundry returns by shotblasting to remove adhering molding sand. Unfortunately, steel scarp and oily casting pieces do not undergo any cleaning process, the only exception being burn-out of oil and coating material where preheating is a practice prior to charging.
Due to the large surface areas exposed, sheet steel scrap gets rusted during storage and the oxides end up as slag during melting. Pig iron is not contaminated either by oil or by coatings. The rusting tendency is significantly less for pig iron due to its low surface-to-volume ratio.
Being lighter than liquid iron, slag tends to float – but the heat it carries is drawn from the metal beneath, so thermal energy is lost to the process. For quick and efficient melting, slag volumes must be minimized. Pig iron in the charge mix minimizes slag formation and conserves energy. When slag generation is reduced it takes less time to clean the liquid metal, and hence productivity improves. The effort and fatigue associated with slag removal are lessened considerably, while radiation losses through the open lid also will be minimized.
Melt rate and cycle time
Efficiency in induction melting is achieved by maximizing power input in the shortest possible time to reach desired tap-out temperatures in a given charge burden. The power drawn by the furnace is influenced by the weight of metallic charge materials contained within the furnace volume and therefore, as material density increases the transfer of energy for melting will increase, too.
Increased packing density thanks to pig irons will improve melt rates through accelerated heating effect. Besides energy consumption and cycle time, metrics like conduction and radiation heat loss, slag generation, carbon burn-out, chemistry disturbance, lining wear, etc., can highlight the value of using pig iron.
Metallurgical considerations
Melting consistency alone does not guarantee metallurgical quality in a cast structure. The cast iron melt prepared in induction furnaces is continuously changing its nucleation status when held at temperatures required for tapping, and then sent for pouring into molds.
Micro-nucleates present in the hot metal steadily lose their active status by a phenomenon called Ostwald Ripening, whereby coarsening of nuclei takes place. This necessitates rejuvenation of status by a process of inoculation prior to the pouring event.
In a rhythmic production environment where tap-to-tap times are stable, the process of inoculation can be standardized to create a desired metallurgical structure in castings. Base level nucleation status would depend on charge mix, charging sequence, cycle time, tapping temperature, holding time, etc., and appropriate standard operating procedures have to be set in place.
This presentation is an attempt to describe melting process for cast irons in coreless induction furnaces and elucidate positive values that can be experienced through enhancing, and then optimizing the use of pig iron in the charge mix. Foundries that make a careful review of their melt shop operations will find opportunities to implement the pig iron route – and to improve productivity accordingly.
This article concludes the report presented in the May 2022 issue of FM&T.
Sundaram Subramanian is a veteran engineer with expertise in cast iron, ductile iron, and aluminum diecasting. Contact him at [email protected]