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Applications of Austempered Cast IronsK.L. HayrynenK.R. BrandenbergJ.R. KeoughApplied Process Technologies Division, Livonia, MICopyright 2002 American Foundry SocietyABSTRACTThe Austempering process is a high performance heat treatment that, when applied to cast iron, produces components that, in many cases, have properties superior to those processed by conventional means. Cast irons that can be Austempered include: Ductile Iron, Gray Iron and Carbidic Ductile Iron.This paper will provide examples of applications of Austempered Cast Irons. Product examples will be discussed along with the specific material properties that resulted in their choice over competitive materials. These examples will illustrate why austempered cast irons are being chosen for high performance applications.INTRODUCTIONAustempering is a specialty, isothermal heat treatment process that can be applied to cast iron to increase strength and toughness. Figure 1 shows a schematic isothermal (I-T) diagram that illustrates the austempering process. The castings are initially heated to an austenitizing temperature (A to B), typically in the range of 1550 – 1750°F (840-950°C). The material is then held at the austenitizing temperature for a time sufficient to achieve a uniform matrix of austenite (B to C). This is followed by rapidly cooling to avoid pearlite formation to the austempering temperature (C to D). Isothermal heat treatment at the austempering temperature (450-750°F or 230-400°C) continues until the transformation to ausferrite occurs (D to E). The castings are subsequently cooled to room temperature (E to F).Figure 1: Schematic Isothermal Transformation Diagram illustrating theAustempering Process for Cast IronsFive different grades of ADI can be produced depending upon the choice of heat treatment parameters. Table 1 contains the ASTM 897-90 and 897M-90 specifications for ADI. At this writing, SAE Standard J2477 for Automotive ADI is undergoing balloting but no, recognized standards/specifications for AGI and CADI are available. In the case of AGI and CADI, the desired mechanical properties (and volume of carbide for CADI) are typically specified by the customer.Table 1: ASTM 897-90 and ASTM 897M-90 Specifications for ADI (Properties are minimum values)GradeTensileStrength(ksi/MPa)YieldStrength(ksi/MPa)Elongation(%)RT ImpactEnergy(ft-lbs/J)TypicalHardness(HBW)1125 / 85080 / 5501075 / 100269 – 3212150 / 1050100 / 700760 / 80302 – 3633175 / 1200125 / 850445 / 60341 – 4444200 / 1400155 / 1100125 / 35388 – 4775230 / 1600185 / 1300N/A N/A444 - 555Austempered Ductile Iron has an exceptionally high strength-to-weight ratio with good fatigue strength and fracture toughness. The density of ductile iron is 10% less than that of steel so ADI parts can replace steel forgings and castings at a weight savings. In addition, with strength three times greater than that of aluminum with only two and a half times the density, ADI can replace aluminum at equal weight for a substantial cost savings.Austempered Gray Iron (AGI) has been applied sporadically since the 1930’s. It was found that Austempering did not crack gray irons while quenching and tempering them did. Therefore, the primary application of Austempering of gray iron, until lately, has been to produce crack-free, hardened gray iron components. Today, however, AGI is receiving renewed interest for its good combination of noise damping, strength and wear resistance.Carbidic Austempered Ductile Iron (CADI) is a family of ductile cast irons produced with carbides, (both thermally and mechanically introduced), that are subsequently Austempered to exhibit adequate toughness and excellent wear resistance. The current numbers of applications for CADI are limited, but growing. Agricultural components have been produced in CADI with as-cast carbides since the early 1990’s. A Sandvik licensee has produced limited production quantities of CADI parts with cast-in, crushed carbides as well. In addition, research into chill-carbide applications is ongoing. APPLICATIONS OF ADI, AGI AND CADIADI BRACKET FOR INDEPENDENT TRAILER SUSPENSIONThe Australian trucking industry has interesting challenges in terms of hauling freight over rough and isolated distances that can be exceptionally long. Independent suspensions on large over-the-road box trailers can allow for flat floors and increased cargo space. For this application, the original independent suspension design was a fabrication made from low carbon steel. An on-road test resulted in the welded components failing after approximately 1200 km of service. A second iteration of the welded steel bracket allowed for travel up to 4000 km. As a result, a ductile iron casting was designed and austempered to Grade 2 ADI as shown in Figure 2. This ADI bracket is approximately 900 mm long and 1200 mm high, with a weight of 105 kg. The typical Brinell hardness is 300, along with an un-notched Charpy impact strength in excess of 100 J.Figure 2: ADI Independent Truck Trailer Suspension BracketTable 2 compares the properties of ADI to those of forged steel. ADI has the advantage over steel in tensile strength, yield strength and hardness. However, it has a lower stiffness than steel, which must be addressed in designing the component.Table 2 –Typical Properties of Steel vs. Grade 2 ADI2Forged Steel ADITensile Strength,MPa (KSI)779.1 (113)1034.2 (150)Yield Strength,MPa (KSI)510.2 (74)792.8 (115)Modulus,GPa (MSI)205.4 (29.8)166.8 (24.2)Elongation, %109Hardness, BHN262280These ADI brackets have successfully traveled over 322,000 km with no problems. Along with the added 20 cubic meters of storage space inside the truck trailer, it is expected that these new brackets will also increase tire life by over 80,000 km. CRANKSHAFT FOR TVR TUSCAN SPEED SIX SPORTS CARThe original material of choice for the crankshaft in the TVR inline 6 –cylinder engine was forged steel. However, the high cost to manufacture soon lead to the consideration of other materials. An 800/2 ductile iron was tested, but failed on a bench dynamometer. ADI (Figure 3) became the obvious next choice. This crankshaft is rough machined, austempered and then finish machined.304.8 mm (12 in)Figure 3: ADI Crankshaft for the Tuscan Speed SixMechanical test specimens were machined from steel, ductile iron and ADI crankshafts. The test results are presented in Table 3. Both the steel and ADI crankshaft outperform the ductile iron component. Note that the Grade 1 ADI crankshaft exhibits the best fatigue strength which was the critical property for this application. Other benefits to using ADI over a steel forging included a lower manufacturing cost and a reduction in weight.Table 3: Test Results for the Steel, Ductile Iron and ADI CrankshaftsSteel Ductile Iron ADIYield Strength738 (107)538 (78)827(120)MPa (KSI)Tensile Strength910 (132)903 (131)1083(157)MPa (KSI)Fatigue Strength400 (58)324 (47)427 (62)MPa (KSI)Impact Energy325 (240)75 (55)141(104)Joules (ft – lbs)Elongation (%)23.210.813.7Hardness226-266262-277300BHNADI RAILCAR WHEELSADI has been studied as a suitable alternative material for railcar wheels. When compared to steel, ADI exhibits three times higher damping and, thus, promises a decrease in traveling noise. A further advantage is that ADI has a 10% lower density compared to steel due to the presence of graphite nodules. These graphite nodules also positively influence the wear characteristics by acting as a lubricant between the contacting parts.Figure 4 shows the mass loss after 140,000 cycles for wheel/rail material combinations. Note that the ADI wheel shows the lowest mass loss compared to the steel wheels. Additionally, the steel rail in combination with the ADI wheel has the lowest mass loss versus steel wheel and rail combinations.Figure 4: Mass loss for various wheel/rail material combinations at 3% slip and F N= 1410NFurther work has shown that the mass loss at higher contact forces than 1410 N can be reduced considerably through the use of ADI, especially in the rail sample. The cause of this is primarily believed to be the lubricating action of the graphite; although, the strain-hardening tendency of the ausferrite matrix also plays an important role.Standard railcar wheels are currently produced very cost effectively in steel using a well developed, semi-permanent, graphite mold casting process. It is doubted that the ADI process could cost effectively replace the current process. Furthermore, the Ausferrite microstructure in ADI cannot withstand the overheating that can occur at the brake/wheel interface during hard braking with full contact (block-type) brakes. Therefore, ADI would only be suitable for low speed applications or for those using disc brakes.With the aforementioned limitations ADI may find increased application for wheels in such applications as commuter railcars, overhead cranes, rail repair vehicles and material handling applications such as mining and equipment transfer cars.Figure 5 shows ADI wheels that are used in material handling cranes.Figure 5: ADI Flanged wheels for overhead cranes.ADI CYLINDER LOCK CASEManufacturers of trailers and train cars are continually seeking new ways to keep vandals and thieves out of the secured trailer or boxcar. Thieves are very creative in their approach to breaking locks. They freeze them with fire extinguishers. They also saw them open with hack saws or smash them with sledge hammers.One door manufacturer saw the opportunity for ADI to replace ductile iron in their cylinder lock cases. ADI appealed to the designer for its good strength, strain hardening when being worked and its low temperature properties. The strain hardening results when the carbon stabilized austenite in the Ausferrite transforms locally to martensite when acted upon by a high normal force (such as filing or sawing). At lower temperatures, ADI maintains a rather high percentage of its room temperature toughness. Figure 6 shows that ADI retains nearly 70-80% of its room temperature toughness at –40°C.Figure 6: Impact transition curves for ADI Grades 1.0 and 1.5.The manufacturer provided the tester a sledge hammer and five minutes to break the cylinder lock case. The ductile iron lock case was easily pried open. However, the ADI cylinder lock case resisted penetration. Figure 7 shows an ADI cylinder lock case.Figure 7: ADI cylinder lock case for Postal Service vehiclesADI TOW HOOKSWhen GM switched to hydro-formed rails on their new light truck models, they created a need for a new design of tow hook. The precedent hook had been constructed of bent square steel wire. A new ADI design (Figure 8) allowed the engineers to slide the hook between the frame rails for attachment without the need for a second bracket. The ADI hooks passed all pull testing requirements, (actually doubling the pull capability over bent wire hooks), and improved the crash testing performance of the vehicles equipped with them. These hooks have been in production since the beginning of the 2000 model year and are produced at a rate exceeding two million parts per year.Figure 8: ADI Tow HookAGI CYLINDER LINERAs engine manufacturers are pressed to reduce emissions, diesel engine manufacturers are designing engines to operate at ever increasing injection pressures. As this happens, the traditional gray iron cylinder liner has been pushed beyond its fatigue capabilities.Highly alloyed gray irons have been employed, but the very alloys that produce the higher “as-cast” strength also produce fine, hard metal carbides that reduce machinability and score the piston rings in service.Induction hardening has also been utilized to increase the strength and wear properties of cylinder liners. However, it is difficult to obtain a consistent induction hardened layer in the flange areas of the liner. One after market supplier of cylinder liners supplied AGI cylinder liners for rebuilt diesel engines. Their testing indicated an improved performance of austempered liners versus ones that were induction hardened. The AGI cylinder liners had strength throughout the liner as opposed to a surface layer and exhibited better wear properties as well.In one test, the fatigue strength for Class 30 Gray Iron at 10 million cycles increased from 16 ksi to 24 ksi after being austempered at 600°F (316°C). Thus, AGI liners can give the engine designer the fatigue strength, and wear resistance required without the damaging metal carbides in the matrix. Detroit Diesel has used this process for many years and today produces over one million AGI cylinder liners per year. Other manufacturers are now investigating this method. A typical AGI cylinder liner is shown in Figure 9.Figure 9: Typical AGI diesel engine cylinder linerCARBIDIC ADI IN AGRICULTURAL APPLICATIONSCADI is a relatively new engineering material. However, its visibility has greatly increased of late with the public launch of CADI in programs at John Deere. In the February 2000 issue of SAE Off Highway Magazine, John Deere announced the use of CADI elements in its revolutionary new rotary combine shown in Figure 10. Another public announcement followed in March 2000 in the John Deere’s Owners Circle Magazine regarding the use of CADI in their Lazer Rip ripper points.Figure 10 : John Deere’s new, high performance, rotary combine uses CADI in its critical thrashingelements. (Courtesy of SAE Off Highway Magazine Feb 2000)CADI was chosen for the ripper point application because of its wear properties in combination with adequate toughness. The wear and toughness combination of CADI have also resulted in it being the material of choice for the plow points pictured in Figure 11.In Figure 12, the abrasive wear resistance of CADI is shown versus as-cast and austempered cast irons. Note that the volume loss is the lowest for CADI. Table 4 lists typical un-notched Charpy impact values at room temperature. CADI has an impact strength of 10 ft-lbs (14 J), which is competitive with carburized and hardened steels.Figure 11: Carbidic ADI plow point.Figure 12: Abrasive wear resistance of CADI vs. as-cast and Austempered gray and ductile irons.Table 4: Typical un-notched Charpy impact values.(Tested at 72°F or 22°C)Material Un-notched Charpy Impact Energy(ft-lbs / J)Carb & Hard 8620 Steel13 / 1830 – 45% Carbide 500 CADI10 / 14Pearlitic Malleable Iron13 / 187003 Ductile Iron38 / 52Grade 5 ADI40 / 545506 Ductile Iron45 / 61Grade 3 ADI70 / 95Grade 1 ADI90 / 1224512 Ductile Iron95 / 129SUMMARYAustempered Cast Irons are providing cost effective solutions in many high performance applications. The desired properties of the various Austempered irons can be achieved by modifying the graphite morphology and the metal matrix to produce the desired result for the end user. Creative applications are being found in many market segments. ACKNOWLEDGMENTSThe authors would like to acknowledge the hard work and dedication of the employees of Applied Process Inc, AP Westshore, AP Southridge, ADI Engineering Process and Heat Treatment Ltd., Steele and Lincoln Foundry and ADI Treatments Ltd. Individuals who contributed to the information presented include: Terry Lusk, Doug Maxwell, Mel Ostrander, John Wagner and Jerry Wurtsmith. Additionally, the contributions of the following companies to this paper are sincerely appreciated: Carroll Agricultural Products, John Deere, Machining Enterprises, TVR Engineering Ltd., Waupaca Foundry, and Whiting Door Manufacturing.REFERENCESBoelan, R, “Impact Properties and Plane Strain Fracture Toughness of Two Grades ofAustempered Ductile Iron,” Research Note Monash University, August 2001.Brandenberg, K., Keough, J., Lee, I., Maxwell, D., and Newman, P., “Independent TrailerSuspension Utilizing ADI Bracket,” submitted for publication for 2002 SAE World Congress.Brandenberg, K., Ravenscroft, J., Rimmer, A., and Hayrynen, K., “An ADI Crankshaft Designedfor High Performance in TVR’s Tuscan Speed Six Sports Car,” Automotive Casting Processesand Materials, SAE World Congress March 2001.Keough, J., “Austempered Materials and their Applications to Drive Line and SuspensionComponents,” SAE International Off-Highway & Powerplant Congress & Exposition, Sept2000.Mädler, K., “On the Suitability of ADI as an Alternative Material for (Railcar) Wheels,” EnglishTranslation,GIFA, June 1999, Dusseldorf, Germany.。