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Today: high-tech materials, crankshafts, and open dies.

Today: High-tech materials, crankshafts, and open dies

Beyond the classic hot-forming advantages, we're experiencing a materials revolution. The Forging Industries Association (FIA) speaks through its Forging Solutions 1 newsletter: "Materials developments and refinements on existing alloys are delivering higher quality, improved part performance, and significant economies for forged parts versus alternative metal components. At the top of the emerging materials list are microalloys, strand-cast steels, new titanium alloys, and aluminum-lithium (Al-Li) alloys. In the high-temperature area, improved superalloys are hitting new performance standards, thanks to special thermo-mechanical treatments. Not to be forgotten, metal-matrix composites may one day be a forging-material option."

Microalloys on the move

FIA reports the success of microalloy materials for forging. These include intermediate carbon-steel alloys (0.3 to 0.6 percent C) that incorporate small amounts of vanadium, niobium, or other elements. They offer potential economies because mechanical properties are achieved as-forged via controlled cooling.

A myriad of high-strength, forged components have been documented for their overall cost savings. These include crankshafts, connecting rods, motorcycle fly-wheels, truck-wheel spindles, steering knuckles, lifting hooks and related hardware, and railroad coupling cylinders. And, among other new microalloy forgings in development is an automobile transmission gear, which gets its wear resistance from an ionitriding treatment.

Although microalloy forgings have been used in Europe, Japan, and elsewhere since the early 1970s, widespread application in the US has been deferred by forgers and end users alike, according to FIA. In part, this is because of concerns for adequate toughness and, consequently, product liability. Today, however, that situation has changed with the introduction of second-generation, lower-carbon-content (0.1 to 0.3 percent C) microalloys with improved toughness and third-generation (0.15 percent C) microalloys with toughness up to six times that of earlier versions.

Truck crankshafts forged

"Forged crankshafts, made from vanadium microalloy, are edging nearer to commercialization in the US for truck engines," says FIA. "After full testing and economic evaluation by major truck manufacturers is completed, full production is expected to begin. The driving force, of course, is economics - based on an estimated 10 percent or more overall cost savings to be achieved by eliminating heat treatments that have been required for conventional quenched-and-tempered steels. Improved machinability will grant additional savings in many cases.

"The microalloy steel selected is said to be ideal for medium-strength forging applications like crankshafts, which do not experience severe impact loads in service. Characterized as a low-carbon, higher-manganese version, the vanadium-modified microalloy comprises 0.3 percent C, 1.50 percent Mn, and 0.11 percent V. It possesses strength, hardness, and induction-hardening characteristics that are enhanced by the high Mn level and the microalloying element. Compared with earlier medium-carbon versions, this microalloy can achieve higher toughness because of lower carbon and higher manganese levels."

The association reports the key to developing the crankshaft application was the refinement of a laboratory forging procedure that optimizes heating, hot working, and cooling of constant-volume, cylindrical steel billets. This was accomplished experimentally by selecting forging parameters that simultaneously improve both strength and toughness. For example, lowering the forging temperature and increasing the forging reduction results in a finer austenite grain size, maximizing the ideal property combination.

Crankshafts forged and then fan cooled during trial production runs confirmed what laboratory experiments had predicted: Microalloy steel can yield acceptable strength and toughness combinations for high-volume applications. Most important, both strength and hardness values are virtually identical from the surface to the center of the crankshaft. Ductility and toughness properties are slightly higher at the surface because of finer grain size, and fatigue strength is estimated to be equivalent to that of quenched-and-tempered plain carbon steel.

Forged microalloyed steel

In another application reported by FIA, a forged microalloyed-steel crankshaft for Ford's high-performance supercharged engine - the 3.8L SC - delivers properties that surpass those of a conventional nodular-iron crankshaft, which is the industry standard for most engines.

Although the original design specified ADI (austempered ductile iron), that material was incapable of achieving engineering targets for property consistency and machinability. This traces back to the complexities of melting and casting this material to achieve consistent response in heat treatment.

The best solution to this problem was the selection of a forging whose strength, modulus, and fatigue properties fulfilled the higher-performance criteria required for the 210-hp, V-6 engine. And, because a microalloyed steel was selected, properties are achieved as-forged, eliminating the expense of quenching and tempering operations that forged carbon-steel cranks routinely require.

Final properties of the microalloyed-steel forgings surpass those of typical nodular iron. For the forged crank, minimum yield strength is 72,000 psi versus 55,000 psi for nodular iron; minimum tensile strength, 120,000 psi versus 85,000 psi. In addition, steel's modulus of elasticity is 30 million psi, compared to a minimum 22 million psi for nodular iron.

Higher stiffness further enhances performance under higher in-service stresses. Equally important, fatigue strength of the forged crank (without additional operations to improve fatigue properties in selected areas) is estimated at 55,000 psi, exceeding that of nodular iron at 32,000 psi to 35,000 psi.

To further boost fatigue life, engineers opted for a shot-peening operation and not the usual deep-rolling process to increase fatigue properties on main journals. This operation is estimated to increase fatigue strength by 30 percent or more. Basically, shot peening of the steel crank's fillets induces compressive stresses (on the surface), which must be overcome before tensile stresses can affect those part areas.

Open-die-options

Solutions 10 reports open-die forging is still viable. The process employs a hammer, press, or ring roller to progressively work starting stock into a desired shape, most commonly between flat-faced dies. "It's undoubtedly the most flexible method for producing complex, high-strength components that routinely fulfill or surpass design criteria in virtually any structural application," says FIA.

"Improvements in processing, equipment, and controls have created parts that are closer to finish dimensions, parts with more complex configurations, greater part-to-part uniformity, and parts that deliver material savings. All at competitive cost."

The report concludes, "A major advantage is design versatility. Open-die forging handles a wide range of part sizes and shapes. Size and weight capabilities are much greater than most alternatives, including impression-die (closed-die) forging. For open dies, forgeable materials include superalloys and Inconel 625 for high-performance rotating parts.

"Open-die forgings are limited only to the largest ingot that can be cast. Shapes range from simple solid shafts to contour-formed pressure vessels, hollow forgings, and large-diameter rings. Of course, controlled deformation eliminates cast defects.

"When as-cast ingot is used as starting stock, a typical 3-to-1 forging reduction (less with hollows) yields maximum consolidation, or densification, by deforming metal to the center of the workpiece, thereby eliminating porosity and breaking up inclusions. In rolling, reductions of 7-to-1 are required for full consolidation because only the surface is heavily deformed."

The sequence diagram shows how an open-die forging is often the best way to produce an integral gear blank and hub. It's cost effective and offers flexibility of size change. If it becomes necessary to increase the gear-blank diameter or thickness, open-die forging readily accomodates such a change. For 10 or 20 pieces, open-die forging is ideal, achieving the optimum property combination. For 1000 pieces, closed-die forging would be more practical.

In this case, hot forging consists of a series of cross-section reductions and then upsetting the gear-blank section to the proper size. A 4000-lb steam hammer is used to provide good depth of deformation (i.e., to the center of the workpiece) and to eliminate the as-cast grain structure.

Hot forging refines and densifies the characteristic cast structure of the starting stock, thereby eliminating inherent cast porosity, redistributing segregation more evenly via metal flow, and reducing the size of as-cast large inclusions. By controlling the reduction ratio throughout the forging process, the extensive working or deformation imparted achieves structural integrity, a uniform microstructure, and improved mechanical properties.

The report states that overall properties far surpass those that can be obtained with a cast, machined, or welded component. The steel of choice was 4140, selected for its combination of strength, ductility, toughness, and fatigue properties.

PHOTO : Forged chain link features true grain flow to yield maximum strength potential of material. In contrast, grain flow in link made from plate is broken by machining, and cast link has no grain flow.

PHOTO : Forged microalloyed-steel crankshaft for Ford limited-production, supercharged engine. Courtesy FIA.

PHOTO : Induction heated 6"-round-cornered billet was press forged to create this microalloy crankshaft with 5"-dia main bearing. Forging was then fan cooled in trial production run for truck engine. Source: FIA member company.

PHOTO : Sequence of open-die forging operations for an integral gear blank and hub. First operation on the steel stock consists of drawing or solid, forging, and then further drawing to reduce cross section and elongate the material into cylindrical shape. In operation 3, the bolster retains the workpiece while the larger-diameter portion of the cylinder is upset. Further upsetting in operations 4 and 5 produces 14" stem.
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Title Annotation:Forging Still a Contender!
Publication:Tooling & Production
Date:Dec 1, 1989
Words:1496
Previous Article:To forge or not to forge.
Next Article:Gaging hot forgings.
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