Making iron castings involves two main sub-processes, one that deals with preparation of liquid metal, with the right proportion of alloying elements dissolved into it; and another that converts this liquid back to solid in the shape and size defined by the molds. The cost of making a casting is determined primarily by the metallic content, and it is ironic that much effort is spent on counting the molds and polishing the cast parts – while the melting unit is often denied the attention it deserves.
Tempers go wild discussing delays in pouring the molds and arrows are darted if liquid metal arrives late, driving a reason to stock surplus liquid metal despite wasted energy. When profits dip, there is no denying if the finger points towards energy costs in melting and storing. The stark truth is that any talk about optimization in the process chain remains more on paper than in practice.
It would be futile to dwell on this omnipresent scenario, but it is prudent to economize on melting costs and not in holding or delivery. Melting activities in a foundry remain so remote from the production hub because of the hazards associated with bulk handling of solids and hot liquids. Nevertheless, we shall embrace this task in earnest and explore avenues that would offer quantum benefits not only in energy utilization but also in the quality of castings made.
The engineering world understands an iron foundry as a source for cast products, manufactured by melting ferrous scrap and pig iron with some alloy additions and pouring into sand molds, to produce parts in shapes and sizes as ordered. Common terms like “scrap,” “sand,” and “pig” have for ages been diminishing the status accorded to the end products, which frequently are less sophisticated in appearance or precision than many engineered parts.
However, over recent decades the true potential of cast iron in critical applications has gained some recognition, and innovations in processes and equipment have helped to elevate it to the status it deserves. Foundrymen, with the help researchers and plant/machinery builders, have seized this opportunity and striven to raise cast iron’s quality and consistency to meet ever increasing functional needs. Now, words like “scrap,” “sand,” and “pig” hold the keys to creating value for the user.
Pig iron in the charge make-up for cast iron preparation was often perceived as another metallic input, a shade superior to scrap for meeting chemistry. The demands made on iron castings and the advent of SG iron/ductile iron, as well as compacted graphite iron, have brought in many value propositions and enabled the pig iron of the last century to evolve into a niche product, with a broad portfolio to serve the specific needs of several types and grades of cast irons.
The benefits to foundrymen go beyond mere control of elements to factors that can add value to their manufacturing processes. Typically, cleaner melting with reduced slag generation can reduce energy consumption and improve productivity through shortened cycle times, besides the assurance of quality without iterations. These are just a few details that make pig iron an attractive source of metallic input in charge materials.
What follows here is a detailed discussion to explore the reach pig iron offers today and analyze the value chain for enhancing its use in foundries. Since foundries have been migrating to induction furnaces from cupolas over recent years, content and illustrations will refer to melting in induction furnaces.
Basics revisited
What is common in all cast irons is the presence of two elements – carbon and silicon – in different proportions that form the basis for sub-classification within the family, as illustrated by Figure 1.
Carbon is in excess (beyond saturation levels in solid state) when the melt is prepared and during the process of solidification this excess carbon precipitates as graphite – or else it combines with iron and other elements to form carbides, or it remains partly as graphite and partly as carbide.
The other contributing factor is the rate of solidification in the mold, rapid cooling promoting carbide formation. The mechanical properties of the solidified cast iron are influenced primarily by the shape, size, and distribution of the precipitated graphite, and by the alloying elements present together with subsequent heat-treatment processes (see Figure 2.)
The shape of precipitating graphite can be modified, from flakes to vermicular to spheroidal, by trace amounts of magnesium (Mg) and/or cerium (Ce) introduced through special melt treatment methods. The control of size and distribution of graphite is brought about by inoculating the liquid iron melt during holding / pouring when numerous active nuclei are introduced, facilitating preferential growth of graphite on these micro-nucleants.
Inoculation alters the cooling pattern of liquid iron and restricts tendencies for carbide formation, thus improving machinability in castings. To enhance the properties for specific functional needs, traditional heat-treatment methods (including austempering, ADI) are employed.
Coreless induction melting
A coreless induction furnace is a vessel lined with refractory material and surrounded by a current-carrying, water-cooled, tubular copper coil structure. Electrical current in the coil induces an electromagnetic field that couples with the charge materials magnetically, thus creating electric current in the charge. The electrical resistance offered by different magnetic materials in the charge mix will differ and so the heating by Joule’s effect (Q ∝ I2• R) also will vary.
A charge mix consisting of dense materials, and also packed densely, is preferred for quicker melting and efficient utilization of electrical energy. Pig iron is ideal for cutting energy costs in iron foundries, as Figure 3 illustrates.
Mains frequency coreless furnaces require a heel of liquid metal for efficient melting, while medium-frequency furnaces can be emptied fully and recharged without a heel. However, the best practice for any type of furnace is the tap and charge method, where a tap-out of one-quarter to one-third of capacity is followed by topping with an equivalent charge with additives. The charge absorbs the superheat in the liquid metal so that it melts quickly and the additions dissolve predictably due to low slag generation, while energy input can be set to its maximum to prepare the furnace for the next tap-out sequence.
The hidden benefit is minimal radiation heat loss because the delays on account of recharge as well as tap-out are minimized and, importantly, the final chemistry is never allowed to fluctuate wildly. If one is looking to optimize melting operations with highest recoveries of elements this is by far the best method, with fairly precise levels of energy input for the small charges that can be regulated on a time scale.
The final chemistry would indicate a process capability Cpk > 1.33, better than other operating methods, and may at times even obviate the need for a confirmatory lab report; any trend in deviation is compensated in the next melting cycle, thus reducing furnace downtime for analysis. Factors responsible for the variations found in operational results are summarized in Figure 4. “Control of chemistry”, “temperature” and “melt quality” are critical to the melting process, for meeting expected engineering specifications in the solidified castings.
Among all the variables in this mapping, one factor that impacts the process in a particularly negative way is input charge materials. If the type and quality of inputs are consistent, the operations will be reliable, with a standard operating procedure tailored to that situation, not necessarily an ideal one.
Purchased ferrous inputs like sheet off-cuts (baled) and heavy scrap (structurals, forgings, stampings, etc.) are source-dependent for chemistry. Concerted effort is required to source them from known locations, to maintain consistency. Casting returns like gating pieces, scrapped castings (internal and returned) and machining chips (mixed origins) are often the cause for indiscriminate tweaks in charge-mix ratio. Such rash decisions can cause frequent disturbances in rhythmic melt cycles and also significantly impact cast microstructures.
This article is continued in the June 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].