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Forging trends update.

Six major technical trends in forging were identified in a recent survey by the Forging Industry Association's (Cleveland, OH) technical committee. Among them is continuing investment by North American forgers in equipment capable of generating net-shape and near-net-shape products. Also, electronic feedback mechanisms are being used on a larger scale to provide more precise control of presses, rolling mills, and other forming equipment. Other important trends include quality testing, bottom pouring of forging-quality testing, bottom pouring of forging-quality ingots, expanded use of strand-casting to produce forging materials, and advanced aluminum alloys for forging.

Process trends

Screw presses are being used to produce net-shape forgings in increasingly complex shapes, Figure 1. Forgings for steam-turbine and jet-engine blades, for example, are finish forged with no need for extensive machining. Development work on increasingly sophisticated screw presses, which has been under way for 8 to 10 years, has accelerated in production practice during the last five.

An advanced 880-ton screw press was installed last year at Battelle Columbus Laboratory to support development efforts stimulated by growing interest among US and Canadian forgers. Work will focus on basic equipmeent evaluation, tooling requirements, and control systems. Presently, North American forgers use approximately 75 screw presses, dedicated principally to forging brass products and jet-engine parts.

Closer tolerance control enables another US producer of heat-resistant alloy rings to deliver parts for aircraft turbines with up to 30 percent less weight, requiring significantly less finish machining. Last year a new ring-rolling mill went into production. It was equipped with a television camera and digital electronics to monitor and display tolerance information as the ring is made. Greater control over inner and outer diameters and cross section results in a near-net-shape workpiece with significant material savings, Figure 2.

Materials innovation

Modified forging practices often require greater control over raw material, particularly with superalloys. For instance, lowering the forging temperature range of nickel-based alloys from 2000-2100 F to 1850-1950 F results in better control over grain size (important for improved mechanical properties).

Among changes in refining techniques is substitution of electroslag remelting for vacuum-arc remelting, used in conjunction with vacuum induction melting. benefits include better material purification by eliminating inclusions, which improves low-cycle fatigue properties. Developmental work also is being done with electron-beam remelting.

Another growing use of electron-beam technology is testing alloy samples for oxides, nitrides, and other impurities. This consists of electron-beam melting of small samples in a crucible. Impurities collect at the surface in a rafting effect, providing a precise quantitative measure of purity.

On another front, many forging companies are adopting statistical process controls. To facilitate application of SPC and to promote information exchange, FIA has formed a new SPC subcommittee (for examples of firms applying SPC, see last month's Advanced Manufacturing Technology section).

Intense interest from the forging industry helped encourage a stepped-up production timetable for lightweight aluminum-lithium alloys for aerospace forgings. The new alloys will become generally available for forgings, in plate, and for extrusions in a few months.

Working with the forging industry and other fabricators, producers developed the aluminum-lithium alloy to meet goals for lighter weight replacements for certain alloys now used in the aerospace industry--2024-T3X, 7075-T6X, and 7075-T73X. To date, aluminum-lithium alloy ingots have been rolled in sizes from 8500 to 10,000 lb to develop rolling and finishing procedures. Forging billets are being produced for similar purposes now.

In a continuing search for more cost-effective methods to produce high-quality forgings, many companies are moving to bottom-poured ingots for forging material, Figure 3. It's estimated that bottom-poured ingots have captured 40 percent of the market for forging quality steel.

Bottom pouring eliminates many surface and subsurface defects because pouring rates are slow and nonturbulent, and suitable fluxing prevents oxxidation during teeming. Rotary nozzles and load cells with digital readouts are among the newer concepts used to control pouring rates. This process can be used for a range of carbon and alloy steels. In addition, multiple ingots now are produced from a single mold.

One producer of heavy shafts using the open-die process reports a shift from 90-percent rolled billets and 10-percent top-poured ingots to a present mix of 90-percent bottom-poured ingots and 10-percent billets. The result is a five-to seven-percent improved yield with less hot scarfing required to remove surface imperfections. Also, cleanliness is improved and internal soundness is comparable to rolled round-cornered square blooms, particularly in larger sizes.

Also, there is interest in adapting the process to produce hollow bottom-poured ingots of carbon and alloy steels for rolled sleeves, rings, flanges, and other hollow round shapes. Some interest also is shown by bar producers for special cast-to-shape bottom-poured ingots to be used for rerolling into bar sizes. The objective is reduced cost.

The desire for reducing cost in forged parts with equivalent mechanical properties is driving the use of strand-cast steels in lieu of ingot castings for certain applications, Figure 4. Two trends are apparent in strand casting. The first is the general increase in strand-cast capacity--with large steel producers joining in expansion--and greater use of strandcast steel by the forging industry and other steel fabricators.

The second is growing interest among steel producers in horizontal strand casting. Successes achieved during 1980 and 1981 with horizontal casting in small dimensions (32-mm and 60-mm round, and 75-mm and 96-mm round) have led to larger horizontal casting units producing sizes up to 205-mm round.

The horizontal method is employed in casting cobalt-and nickel-based alloys as well as stainless and high-temperature alloys. Mechanical properties of forged strand-cast steels are comparable to properties from billets produced from ingot casting. And the education process continues. Producers of strand-cast materials are advising forgers to ensure sufficient reductions to fully refine the cast structure. For example, with ingot casting, partial refinement occurs when a 30" square ingot is reduced to a 4" semi-finished billet for forging.

By contrast, for many ordinary forging applictions, a 6:1 reduction of strand-cast material is acceptable. For severe applications (e.g., torsion fatigue properties required in an axle), a 12:1 forging reduction may be required.

Forgers are finding that significant cost savings are possible, despite the greater working required. One producer of forged oil-field products recently shifted his consumption from 60-percent billets formed from ingot-cast steel to 80-percent strand-cast steel.
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Publication:Tooling & Production
Date:Oct 1, 1985
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