Melting materials.

Specific consumable products are essential for foundry melting operations. These include items such as scrap, pig iron, refractories, fluxes and alloys. The following is a brief overview of some of these consumables.

The most important contest for foundries today is finding and developing constant sources for economical melting charge materials. This requires knowing all scrap dealers within a reasonable freight distance from the foundry, the grades of scrap that a specific process requires and securing reliable supply performance from chosen dealers. The growing competition for good scrap from mini mills makes a close relationship between a foundry and its scrap dealer(s) a cardinal one for both parties.

Scrap metals are the basic melt stock for the metalcasting industry. The unceasing discard of metals from millions of factories and families everyday includes broken, obsolete and wornout metals from scrapped autos, railroad track, steel building materials, tin cans, toys, appliances and military equipment.

Foundry buyers should be aware of scrap industry standards, and coded definitions of the common ferrous and nonferrous scrap grades as established by such organizations as ISRI and The Aluminum Association. Ferrous scrap grades are separated using an ISRI number code, while nonferrous scrap grades are designated by code words.

Ferrous Scrap

Because of the sheer amounts of scrap that foundries must consume, price and availability are vital to metalcasting economics. Foundry use of ferrous scrap in 1992 totaled nearly 46 million tons, or about 50% of all U.S. scrap--not including some 10 million tons of scrap exported around the world.

General classifications of iron and steel scrap fall into two main categories:

Prompt industrial--This premium grade scrap consists of new scrap generated by manufacturing operations (punchings, turnings, cast iron returns, etc.) that have been segregated and chemically identified.

Obsolete--This material is generated from dismantling and preparing iron and steel scrap recovered from old structures and equipment. Examples include "I" beams, rebars, and magnetically processed TABULAR DATA OMITTED and shredded steel cans and automobiles.

Important ferrous scrap factors to be considered include:

* Chemical analysis to determine the "big five" elements a given scrap contains: carbon (C), silicon (Si), manganese (Mn), phosphorous (P) and sulfur (S). For a gray iron melt, a ton of cast iron scrap has more value than a ton of steel scrap because substantial quantities of expensive C, Si and Mn must be added to a charge of steel scrap. Conversely, only small additions of C, Si and Mn are necessary to a charge of cast iron scrap. Radiation contamination of scrap is a new, but relatively uncommon, constituent that can render scrap charge material unusable. It has the potential TABULAR DATA OMITTED to harm foundry personnel and cause expensive environmental cleanup costs for foundries that unknowingly include radiation-contaminated scrap into their casting processes.

* Tramp alloys (copper, nickel, chrome, aluminum and lead) are frequently troublesome, especially in producing ductile iron. Purchasing agents should insist on a scrap analysis of tramp metals percentages. No. 1 grades of scrap contain relatively low tramp elements. As the percentages of these elements rise, the cost of the scrap decreases, eventually to be reclassified as a No. 2 grade.

* Melting yield of a charge material determines its true value. Melting yield is the weight of liquid metal delivered to the furnace well/ton of ferrous metal charged. A low delivered price/ton often is not economical when related to the molten metal at the spout. Low-cost charge materials usually contain quantities of nonmetallics and tramp alloy contaminants that generate larger percentages of unwanted slag or combustion gases.

More economical are No. 2 scrap bundles, but they contain high tramp metals and other foreign contents--such as municipal solid waste scrap, plastics and paint--that result in a poor melting yield. Inferior yield can also be caused by the tendency of iron to oxidize during melting, which is directly related to the increased surface area in a scrap charge containing turnings, unbaled thin-gauge sheet or scrap with excessive rust or heavy scale (iron oxide).

Grades of scrap with the greatest potential for creating environmental problems (always an important scrap value determinant), are those delivering low melting yields. In the end, cheaper grades of scrap are not always efficiently melted. Oils and greases, rubber, paint, plastic and cellulose products in purchased scrap often are contaminated with lead, cadmium or other metallics that run afoul of EPA standards and can prove expensive to dispose.

Specifications

A value analysis of available scrap is an important buyer precaution to assure that a ton of molten iron at the furnace spout will be produced at the lowest possible cost within foundry specifications. It is the buyer's responsibility to determine scrap standards and the dealer's responsibility to meet them.

Below is general information about ferrous scrap, according to ISRI (Institute of Scrap Recycling Industries) specifications:

* Cleanness--all grades shall be free of dirt, nonferrous metals, foreign material of any kind and excessive rust and corrosion;

* Off-grade material--the inclusion of negligible amounts of metallic material exceeding (to a minor extent) applicable size limits, failure (to a minor extent) to meet foundry quality or material requirements, shall not change the classification of the scrap if the inclusion is unavoidable in the customary preparation and handling of the grade involved;

* Residual alloys--or "free alloys," are any alloys contained in the scrap that have not been added and, in the case of steel, shall not exceed 0.45% nickel, 0.20% chromium, 0.10% molybdenum and 1.65% manganese;

* Deviation--any change in iron or steel scrap classifications must be made only by mutual agreement between the buyer and the seller.

Nonferrous Scrap

Major foundry scrap markets exist for nonferrous metals, particularly for aluminum and copper.

The aluminum industry consists of two major sectors, the primary aluminum producers and the secondary smelters. They differ in raw materials, processing technologies and market demand.

The primary aluminum industry consists of 12 producers with an annual production capacity exceeding 4 million tons of wrought and semifabricated products. The secondary aluminum industry comprises 34 producers with a capacity of about 1.25 million tons a year.

In contrast to primary aluminum, which requires bauxite as its raw materials, secondary aluminum smelters rely on aluminum scrap that they collect, separate and smelt to produce specification aluminum alloys. They market hot metal and/or ingots to the casting industry.

Secondary smelters vary from small (about 3000 short tons annually) to complex operations (up to 120,000 short tons/year). In addition to ingot, they produce deoxidized alloys in the form of notched bar and shot for steel foundries. Secondary aluminum producers recycle 40-45% of new and old aluminum scrap generated each year in the U.S.

The critical factor in secondary aluminum production is an adequate supply of scrap. Availability is determined by geographic location of secondary producers, the price of various ingot specifications and the types of equipment and production techniques employed.

The recovery and reuse of aluminum scrap is rising as a percentage of the total national usage. More than 3 million tons of aluminum were recycled in 1992, or 40% of the whole U.S. aluminum market.

New scrap generally includes can manufacturing trimmings, chips and turnings from machine operations; it represents about 60% of total recycled aluminum.

Old scrap is the major growth area in recyclable aluminum scrap. Most of it comes from recycled cans, automotive shredding and appliance recovery sources.

Secondary operators--dependent on sufficient quantities of suitable scrap and smelting dross they collect, separate and prepare for smelting--produce 18 aluminum alloys, each with a specific chemical composition. About 65% of the aluminum industry's output is used by the auto industry.

The distinction between the two materials (primary and secondary aluminum) is underscored by the physical fact that primary wrought products require a lower impurity level than do cast products because some common alloying agents (copper and silicon) reduce aluminum ductility. It is difficult and costly to remove metallic impurities (except for manganese) from aluminum by common melting and refining techniques.

The quality and type of aluminum scrap largely determines the alloy produced. Secondary aluminum is almost universally limited to the casting industry.

Copper Scrap

Copper conjures up the image of quality more than any commonly used metal today. From copper plumbing to bronze bearings and artwork, a copper product still is thought of as the best that money can buy.

After a steady decline that began in the 1960s (reflecting escalating costs and the high lead content in some of its alloys), copper turned around in the last decade and consumption has risen consistently. In 1992, recyclers reclaimed more than 1500 tons of copper scrap. Foundries, consistent users TABULAR DATA OMITTED of reclaimed copper ingot, produced nearly 400 tons of copper and copper-base castings.

Principal castings produced from recycled copper scrap are plumbing fixtures, electrical fastener components and, to a minor extent, cast bronze and brass building supplies.

Copper scrap recovery last year topped 1.4 million short tons, a reuse rate exceeding 42% of all available copper.

Pig Iron

The least-cost melting charge is achieved by the dealer's ability to assemble and deliver scrap in terms of a foundry's requirements (chemical and physical qualities in proper relationship). Because this relationship is not always possible, high-purity pig iron addition to melts often can provide the difference. Its use as a charge supplement, however, remains relatively low.

Steel scrap is the major source of iron units for ferrous foundries but foundries can improve fluctuating quality iron units with pig iron to maintain consistent casting quality.

Long declining in foundry use because of its cost and the ready availability of inexpensive common scrap, pig iron offers technical and economic benefits that are becoming more significant for producing high-quality iron castings where markets and customer demand can absorb the cost premium.

Three types of pig iron are manufactured:

* basic pig iron;

* a primary grade used in gray iron production;

* ductile grade pig (higher purity, less residual materials, lower P, Mn and S, controlled levels of Si and C) for high-quality ductile iron production.

Pig iron grades, most imported from Brazil, depend on the ore, the reduction process, refining treatments and/or alloying practices employed by the manufacturer. Classification of standard grades is based primarily on P content. The C content in pig iron may vary between more than 4% down to 2.6%.

The foundry buyer must consider the chemical consistency of the pig iron that guarantees the repeatability of melt chemistry and the acceptability of the TABULAR DATA OMITTED resultant castings; the reliability of suppliers and their inventories of required pig grades; and the basic price and shipping charges.

Inoculants

Graphite flakes occur in various shapes and sizes in gray iron, and it is known that mechanical properties as well as matrix structures of gray iron castings are closely related to types of graphite flakes present in castings. Consequently, several products have been developed to control the types and sizes of graphite flakes in gray iron castings. These products are called inoculants and the addition process is known as inoculation.

Inoculation is used in the production of gray, ductile and compacted graphite irons to control the mechanical properties of the resulting castings by influencing the formation of graphite types. Inoculants establish nucleation centers for intended kinds of graphite precipitation.

Successful inoculation entails several secondary interactions that trigger the graphite flake movement toward nucleation sites during solidification of the molten metal. Inoculation also reduces chill-forming tendencies and increases the vital eutectic cell count, prevents undercooling and supports metal matrix consistency during successive pours.

The common denominator to all techniques for adding inoculants to cast iron is consistency. Inoculants should be added to the molten iron immediately before pouring (late stream inoculation) to assure thorough mixing of the inoculant with the molten metal before it enters the mold. Late stream inoculation prolongs the effects of inoculants and minimizes the debasing effects on successive castings caused by the gradual but inevitable inoculant fade.

The trend now is to inoculate by placing pellets or granulated inoculant in the pouring cup, in the sprue, in an enlarged chamber in the sprue base or by a cored wire containing an inoculant that is fed continuously into the sprue during pouring.

The replacement of ladle inoculation by metal stream inoculation often requires less inoculant to achieve the same result but at less cost. Other benefits include the reduction or elimination of heat treatment in as-cast structures free of carbides and a reduction in pinhole and dross defects.

Like cast irons, aluminum also can be modified by a process known as grain refining. The most widely practiced method is chemical grain refinement in which additions are made to the melt immediately before casting. The most common grain refiners are titanium, boron or zirconium, none of which changes the composition of the base alloy material.

Fine grain size is promoted by enhancing the number of nuclei. The greater number of nuclei allows more aluminum grains to form, resulting in a smaller grain size and improved aluminum properties.

Structural modification prevents the excessive growth of aluminum grains and is accomplished by adding grain-refining elements such as titanium, boron and calcium that induce the formation of nuclei, or seeds. These seeds of insoluble material nucleate fine, equiaxed grains as the aluminum solidifies.

Aluminum also can be modified to improve its strength and predictable fracture properties. Modification refines the silicon flake morphology in an aluminum-silicon alloy, producing a fine-grained, uniform, fibrous structure that improves ductility. Modification elements are strontium or sodium.

Refractories

Refractories are special nonmetallic materials that foundries use as thermal barriers and insulation to confine the intense heat of molten metals. They maintain enough physical and chemical identity to resist harsh foundry processing conditions safely and economically, and are suitable for structural purposes.

Before furnaces, ladles or pouring systems can be used as effective melting, holding or conveying media for molten metals, they must be properly lined with a refractory material. The refractory forms a container within a container, able to withstand destructive forces of mechanical stress, abrasion, pressure, erosion, corrosion and rapid thermal cycling.

The process of selecting a refractory involves:

* an engineering or data phase where foundry melting equipment and pouring practices are examined in terms of suitable refractory materials;

* a judgment phase where the refractory selection decision is based on the thermal and mechanical parameters determined by the engineering phase.

Refractories are classified on the basis of their chemical composition and by the forms in which they are to be used. There are many types of refractories, but most in common use are manufactured (alone or in various combinations) from oxides of silicon, aluminum, magnesium, calcium, chromium and zirconium. Many refractory materials contain impurities or accessory minerals--such as sodium or potassium compounds--that can be reactive to ferrous metals. Their use must be considered in certain foundry melts.

Careful selection and proper installation of refractories are of primary importance to successful melting, handling and pouring practices. Periodic refractory repair/replacement is required at intervals that vary according to furnace design, metal melted, refractory type and application, and foundry melting and molding practices. New refractories and crucibles contain moisture, which can add hydrogen to a melt unless it is completely driven off.

Types of Refractories

Common refractory types for ferrous metals applications differ from those used in nonferrous service. Those used for the most severe ferrous environments generally are made from high-purity tabular alumina. These exhibit exceptional hot strength, high density, low porosity, and resistance to corrosion and penetration by slag and fluxes.

Chromium oxide or chromium oxide and phosphorous are sometimes included to add protection against slag damage. Alumina-graphite refractories guard against furnace slag and thermal shock and are used in slag lines.

Alumina-silica refractories are used in abrasion-resistant applications such as arc furnace roofs, steel ladles and hot metal cars.

Common refractory types include: bauxite silicon carbide for taphole, well, breast and front slaggers; resin-bonded bauxite for slaggers on high production cupolas; high alumina formulations for corrosive slags; alumina silica carbide for continuous blast cupolas w/basic slags.

Filters

Refractories are also used to fabricate ceramic foam filters to clean ferrous and nonferrous metals during casting. The application of filters and filtered gating systems offers many clean metal advantages, and for many high alloys their added cost to produce good castings can be justified.

Filters usually are placed in runner or gating systems to trap slag, dross and other nonmetallic inclusions, reduce pouring turbulence, simplify gating system design and reduce mold filling time.

A new direct pouring system combines a proprietary ceramic filter and an insulating sleeve as a single unit. It allows the direct pouring of castings in sand and permanent molds, eliminates the gating system and reduces pouring times.

Fluxes

Foundries are relying to a greater extent now on fluxing of melts to assure the best physical and mechanical properties in cast metals. There are many commercial flux materials for both ferrous and nonferrous metals, but the trick is choosing the right flux for the best results. The chemical properties of the alloys generally determine the flux treatment used. Fluxes aid melting and casting quality, but improper use can lead to defective castings.

Fluxing processes used to improve aluminum metal quality and reduce costs fall into four broad classes:

* Covering flux--(mixtures of chlorides, KCl, NaCl, nonhygroscopic salts); prevents gas pickup and reduces dross formation by oxidation; flux becomes a liquid at molten aluminum temperatures, effectively sealing the melt surface; absorbs oxides and other nonmetallic inclusions; provides a barrier against gas absorption; and produces a dry dross.

* Cleaning flux--(fluoride compounds), removes solid, nonmetallic inclusions; an exothermic reaction to soften the dross and keep furnace walls clean.

* Degassing flux--(chlorine-generating compounds); removes entrapped gases, especially hydrogen, that cause casting porosity; after a covering flux treatment, the dross is pulled aside and a degassing flux is plunged slowly or inserted through a lance directly into the bottom of the melt. Gas bubbles or flux powder disperse and rise through the melt to clean it, causing a nucleation action that forms fine equiaxed grains of metal during solidification.

* Drossing-off flux--(-200 mesh-free aluminum put on dross, producing an exothermic reaction, melting the +200 aluminum captured in the dross, returning it to the melt); recovers aluminum from drosses, produces a dry dross that is skimmed easily from the bath.

* Sodium-free flux--for alloys with greater that 5% magnesium that may become brittle if exposed to even trace amounts of sodium.

Fluxing requires a holding period of 5-10 minutes to let the flux separate from the metal. Pouring immediately after fluxing causes inclusion problems.

Fluxes are added to the melt to collect, agglomerate and bring to the surface oxides and nonmetallic inclusions. Fluxes should not be stirred into the melt lest they lead to inclusion defects. After the waiting period, the impurities should be skimmed carefully from the melt surface before pouring.

Gaseous fluxes (such as nitrogen, chlorine or a mixture of these) are often bubbled through aluminum melts to remove inclusions.