MAKING THE MOST OF FORGING BENEFITS:
MATERIALS DEVELOPMENTS BOOST PERFORMANCE, LOWER COSTS
The inherent strength and performance benefits of forgings are being enhanced by exciting breakthroughs in new and emerging materials and by evolutionary refinements of the tried-and-true. Many of these developments have been successfully demonstrated in forged components offering higher quality, improved performance, and significant economies, making forgings more than ever the metal components-of-choice to knowledgeable designers, buyers, and other users in a broad variety of applications.
Highlighting the emerging forgeable materials are new generations of microalloys (MAs), strand-cast steels, titanium alloys, titanium aluminides, and aluminum-lithium (Al-Li) alloys. Improved superalloys, too, are hitting new heights of performance, as a result of "cleaner" processing and custom thermomechanical treatments. Farther down the road are metal-matrix composites, whose suitability for forging has already been demonstrated.
Microalloys are moving
New generations of microalloyed steels now overcome the one reservation that once prevented designers from specifying them: impact toughness. The latest microalloys are much tougher than earlier versions. In fact, depending on the microalloy selected, fracture toughness can be equivalent to quenched-and-tempered steels for forged microalloy components that require such demanding performance. Microalloyed steels, however, are not intended to replace higher-alloyed steels, whose properties surpass those of microalloys.
Typically intermediate-carbon steel alloys with a 0.3 to 0.6% C content, microalloys incorporate small additions of vanadium, columbium (niobium), titanium, or other metallic elements to achieve a property profile otherwise attainable only in higher alloy steels. Their strength comes from a precipitation hardening reaction, in which nitrides or carbonitrides are formed in the steel. Consequently, control of nitrogen levels is a key factor in most microalloyed steels.
When forged, microalloys can result in significant savings because mechanical properties are achieved "as forged" via controlled cooling. Secondary operations--post-forging heat treatments (hardening, tempering) and straightening operations, then stress relieving to eliminate distortion effects in quenched-and-tempered steels--are virtually eliminated. This saves both energy and processing time.
Although microalloy forgings have been utilized in Europe, Japan, and elsewhere since the early '70s, widespread usage in the U.S. has been deferred by forgers and end users alike, in part 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% C) microalloys with improved toughness and of "third-generation" (0.15% C) microalloys with toughness up to 6X that of initial microalloy grades.
Typical improved-toughness second-generation microalloys, when compared with initial vanadium-modified microalloy grades with high C and low Mn contents, have a lower carbon content (0.30 vs. 0.50%), higher Mn level (1.40 vs. 0.75%), and slightly less vanadium (0.10 vs. 0.15%). This combination changes the amount of ferrite and pearlite and increases the degree of precipitation strengthening. Such second-generation microalloys with a ferrite-pearlite microstructure tend to be more ferritic than highly pearlitic initial grades, yet still maintain a fairly high strength level.
Further increases in toughness have been achieved by restricting the austenite grain size through titanium and nitrogen additions to vanadium-modified steels. This approach also facilitates the forging process, permitting forging at 2200F without grain growth affecting toughness.
Third-generation microalloys with self-tempered martensitic structures possess toughness and impact strengths equivalent to those of quenched-and-tempered 4140 steel, while providing tensile strengths not previously attained in microalloys: up to 190 ksi.
Near-commercial ferrite-bainite microalloys with a low carbon content are also significant. Now being forged into prototype structural automotive components by U.S. companies, these steels are suitable for forging processes that range from cold heading through conventional hot forging.
Replacing heat-treated steels and first-generation microalloys, the Mn-Mo-Cb compositions achieve a grain size of 5 microns under controlled rolling conditions. Toughness is definitely impressive: for a water-quenched 138-ksi tensile strength steel, Charpy V-notch toughness exceeds 50 ft-lb at room-temperature. In addition, these new alloys respond very well to surface-hardening treatments, where extra fatigue- or wear-resistant properties are desired.
Higher-performance microalloys are now being forged into automotive, agricultural, truck, and heavy-equipment components, many of which have already been documented for their overall cost savings. These include truck crankshafts and connecting rods, motorcycle flywheels, truck-wheel spindles, steering knuckles, lifting hooks and related hardware, and the piston portion of railroad coupling cylinders. Among other microalloy forgings being developed is an auto transmission gear, which is ion-nitrided for increased wear resistance.
One of the most recent successes is a forged microalloy crankshaft for a supercharged auto engine. Going into production just this year, the crank is forged of a vanadium microalloy steel for Ford's 3.8L limited-production engine.
The automaker selected forged MA steel for its high-performance properties, among them minimum tensile and yield strengths of 120,000 and 72,000 psi, respectively, vs. 85,000 and 55,000 psi for the nodular iron commonly used in less demanding engine applications.
Initially, the company had planned to produce ADI (austempered ductile iron) cranks in its foundry but, unfortunately, the ADI version could not be produced within the required specifications on the existing production line. The narrow processing window, particularly temperature control, made it difficult to adapt to ADI from nodular-iron production.
Properties of most MA steels for forging are equivalent to those of Q&T grades, except for toughness. Importantly, many medium-strength forging applications do not experience severe impact loads in service and therefore do not require the ultimate in toughness. A prime example is a truck crankshaft forged of a vanadium-microalloyed steel, whose properties from surface to center are extremely uniform. In this instance, cost saving was a prime driver in selecting a microalloy forging. Another was fatigue strength, estimated to be equal to that of a quenched-and-tempered plain carbon steel.
While toughness improvements have made forged MAs more appealing to OEMs and designers, and even with these new commercial applications, an industry specification is definitely needed to make designers more comfortable with MA forgings. To this end, Forging Industry Association's (FIA) technical committee has prepared a draft specification, which will be submitted to professional engineering and standards organizations (including both ASTM and SAE) as a first step in developing detailed property requirements, processing parameters, and compositional limits.
Strand-cast steels for economy, quality
Use of strand-cast steel bar--practically all carbon and low-alloy steel grades--for forged components is "the wave of the future," say major steel suppliers. Streamlined processing in going from cast billet or bloom to forging stock saves: (1) considerable energy, since there is much less reheating, and (2) time, since only one rolling process is required vs. the usual two. As a result, cost reductions of up to 10% have been documented for forging-quality steels.
Just as important is the improved quality now routinely achieved in strand-cast products for forging. Compared with earlier strand-cast products in which unsound centers (porosity, segregation, etc.) had to be pierced out of forged parts, the latest materials possess significantly improved internal structures, thanks to the incorporation of clean-steel practices, improved shrouding, and electromagnetic stirring.
|Tensile Strength||827 MPa (120 psi)||827 MPa (120 ksi)|
|Yield Strength||579 MPa (84 ksi)||572 MPa (83 ksi)|
|Reduction in Area||33%||27%|
|19J (14 ft-lb)||10J (7 ft-lb)|
|-30C Charpy V-notch||11J (8 ft-lb)||8J (6 ft-lb)|
Shrouding the teeming stream to prevent oxides from forming has produced much cleaner steel that boosts fatigue performance, increases transverse toughness, and facilitates machinability of forged parts. Other benefits include forgings with improved surface finishes and highly uniform chemical compositions throughout.
Although bottom-poured forging ingots also can achieve these quality improvements, strand-cast stock has the economic edge. Cost savings combined with mechanical properties that are equivalent to or sometimes better than cast ingot material have led to the commercialization of many forgings that make use of strand-cast product, among them: connecting rods, crankshafts, yokes for U-joints, and trolley brackets, with many others in development.
In addition, certain microalloy grades can be strand cast. This is expected to produce further cost reductions in specific microalloy forgings.
Emerging materials for high temperatures, aerospace
Introductions of new alloys and research on emerging materials are expanding the scope for precision forging of aerospace structural components. Performance-wise, precision forgings are getting a boost from both new materials and processing developments. Improved properties are now offered by new titanium alloys and by emerging materials such as Al-Li, titanium aluminides, and metal-matrix composites.
Together, materials development and refinements of the forging process--hot-die, isothermal, and conventional forging technology--have broadened the range of net-shape structural components in terms of size, complexity of shape, and performance.
In titanium developments, for example, the relatively new Ti 10-2-3 near-beta alloy permits forging at lower temperatures and increases forgeability over alpha-beta alloys like Ti 6-4. As a result, Ti precision forgings now in production development have extended PVAs (plan view areas) into the 300 to 350 sq. in. range. The next step will be to evaluate alternative ways (i.e., other than ingot melting) to produce high-quality superalloy die materials necessary to push PVAs up to 450 sq. in.
Other new Ti alloys are also expected to have an impact on future forged components. With an 1100F service temperature, one new wrought Ti alloy (Ti-1100) can operate at about 100F higher than existing Ti alloys. Destined for compressor components of gas-turbine engines (blades, discs, and cases), the alloy's 100F temperature advantage translates into a 25X longer creep service life in existing engines or improved performance in newly designed engines. The latter would entail running the same engine configuration hotter or at a higher speed than would be possible with the existing baseline alloy, Ti-6242-Si.
Running an engine hotter with existing alloys could mean a switch to a nickel-base alloy, which would incur weight penalties and require design modifications. The new material (Ti-6Al-2.75Sn-4.0Zr-0.40Mo-0.45Si-0.07O2-0.02Fe) is said to be a direct substitution and is easy to forge, not requiring isothermal techniques. Creep and stress-rupture properties surpass those of the baseline alloy, while tensile properties are equivalent.
The latest Al-Li alloys, although expensive, promise to produce strengths equivalent to those of existing aluminum alloys, but at a reduced density. At the same thickness, significant weight reductions can thus be achieved.
Just recently introduced, one promising Al-Li alloy (IncoMAP alloy AI-905XL) starts with a powder, which is compacted, then extruded into round bar stock for forging. A direct replacement for 7075-T73 aluminum, the mechanically alloyed material contains both magnesium and lithium (Al-4.0Mg-1.3Li) to boost strength and reduce density. It can be forged conventionally and reaches full strength as processed. Consequently, economies are anticipated by elimination of heat treatment and straightening operations.
Elastic modulus is about 15% greater than 7075-T73; yield and tensile strengths are slightly higher. Like strength, fracture toughness is not only greater, but clearly more isotropic. Klc fracture toughness of the new Al-Li-Mg alloy is 30 ksi in.1/2 (both long and short transverse directions) vs. 20 or less for most aluminum alloys. General corrosion resistance surpasses that of 7075-T73, with a stress-corrosion-cracking threshold of 50,000 vs. 42,000 psi. Weight savings are 8%.
Although forged Al-Li components are not yet in production, aircraft manufacturers are testing prototypes, such as forged 2090 Al-Li tow fittings on Boeing 747-200 and -300 aircraft. At a 7% weight reduction, the Al-Li forgings withstand higher stress than their 7075-T73 predecessors and exhibit improved resistance to environmental corrosion.
Research in titanium aluminides is likewise being fueled by potential applications, such as the National Aerospace Plane and high-performance turbine engines. While commercial alloys will probably take another five years to develop, they will possess elevated-temperature performance equal to nickel-base superalloys, but at an impressive 60% density reduction.
Metal-matrix composites, based on either aluminum or titanium reinforced with ceramic fibers or whiskers, may still develop for forging applications. However, existing forging alloys, superalloys included, can meet almost all current performance requirements when the optimum forging process is utilized. Forged turbine discs for jet engines, for instance, are made via isothermal forging to yield the ultimate in service life.
Improved superalloys (nickel-cobalt-based alloys) are moving toward higher performance goals, particularly in terms of service life. To this end, the production of "cleaner" nickel-base alloys is being explored. One approach is to improve refining practices--at present, VIM (vacuum induction melting), then VAR (vacuum arc remelting); or VIM followed by ESR (electroslag remelting)--by implementing cold-hearth refining. This involves using electron-beam or plasma melting as an intermediate processing step (i.e., after VIM but before VAR) to further improve cleanliness. While the cold-hearth route does help, additional thermomechanical processing, homogenization, and forging are needed to produce a uniform microstructure.
Other processes like ESR are also being considered to improve VIM quality. Here, defects characteristic of VIM (primarily nitrides and oxides, along with sulfides and ceramic particles) can be removed via ESR. Today, some superalloy grades go through VIM, ESR, then VAR to achieve a high level of cleanliness.
Current VIM practices can also be improved by selecting more pure raw materials for melting and by using furnace refractories that will not contaminate the melt with ceramic particles.
Another route, of course, is wrought P/M (powder metal) superalloys, which offer the highest performance. Basically, atomized powder is extruded into billets, then forged. This yields a much finer, more uniform microstructure and eliminates segregation characteristic of conventional ingot product. In critical high-performance engine components, this translates into higher strength at higher temperatures.
New thermomechanical treatments boost performance
Many existing materials have reached their performance limits with incremental changes, sometimes achieved by optimizing thermomechanical processes to boost performance. Customizing thermomechanical treatments to boost strength, toughness, and stress-corrosion resistance of existing alloys is becoming a trend in forging today. Modifying the forging process is now becoming a more common practice to enhance a particular property like fracture toughness, narrow tolerance distribution, or reduce property spread within a forging. This approach is equally successful with aluminum, steel, titanium, and practically all other forgeable materials.
Typically, selecting a narrow range of forging parameters and/or combining special deformation practices with the optimum heat treatments can be used to achieve such goals. Such is often the case when existing alloys must meet never-before-attained property values. A case in point is the 1,000-lb spreader beams that secure and launch Space Shuttle payloads. Here, the forger exactingly fine-tuned the thermomechanical working sequence to deliver unusually high properties in 8-in.-thick sections. Using a PH stainless grade developed for thin and small sections, the forged beams achieved an ultimate tensile strength of 215,000 psi, Klc fracture toughness of 45 ksi in.1/2, and service temperatures as low as -65F.
Other examples abound. For instance, designing a special forging process and optimizing the tempering of ultra-high-strength steels made it possible to attain maximum fracture toughness without sacrificing strength in large forgings with section thicknesses up to 10 in. Similarly, combinations of custom forging and thermal treatments have produced optimum grain size, strength, and toughness in heat- and corrosion-resistant materials like A286 and Inconel 625.
Inconel is a registered trademark of Inco Alloys International, Inc.
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