An ASM International® HEATPublication TREATING PROGRESS THE BUSINESS AND TECHNOLOGY OF HEAT TREATING ® www.asminternational.org MAY/JUNE 2008 • VOLUME 8, NUMBER 3 HEAT TREATMENT OF LANDING GEAR HEAT TREAT SIMULATION MICROWAVE HEATING SM Aircraft landing gear, such as on this U.S. Navy FA18 fighter jet, must perform under severe loading conditions and in many different environments. HEAT TREATMENT OF LANDING GEAR The heat treatment of rguably, landing gear has Alloys Used perhaps the most stringent The alloys used for landing gear landing gear is a complex requirements for perform- have remained relatively constant operation requiring ance. They must perform over the past several decades. Alloys A under severe loading con- like 300M and HP9-4-30, as well as the precise control of time, ditions and in many different envi- newer alloys AF-1410 and AerMet ronments. They have complex shapes 100, are in use today on commercial temperature, and carbon and thick sections. and military aircraft. Newer alloys like control. Understanding the Alloys used in these applications Ferrium S53, a high-strength stainless must have high strengths between steel alloy, have been proposed for interaction of quenching, 260 to 300 ksi (1,792 to 2,068 MPa) landing gear applications. The typical racking, and distortion and excellent fracture toughness (up chemical compositions of these alloys to100 ksi in.1/2, or 110 MPa×m0.5). are listed in Table 1. contributes to reduced To achieve these design and per- The alloy 300M (Timken Co., distortion and residual formance goals, heat treatments Canton, Ohio; www.timken.com) is have been developed to extract the a low-alloy, vacuum-melted steel of stress. optimum performance for these very high strength. Essentially it is a alloys. modified AISI 4340 steel with silicon, D. Scott MacKenzie* Table 1 — Typical chemical compositions of alloys Houghton International Inc. used in landing-gear applications. Valley Forge, Pa. CMnSiNiCrMoCo AF1410 0.15 0.10 0.10 10.0 2.0 1.0 14.0 300M 0.46 0.75 1.65 1.80 0.80 0.40 — 0.07 V HP-9-4-30 0.30 0.30 0.10 7.50 1.00 1.00 4.50 0.10 V *Member of ASM International and member, ASM Heat Treating Society AerMet 100 0.23 — — 11.10 3.10 1.20 13.40 HEAT TREATING PROGRESS • MAY/JUNE 2008 23 testing has shown performance equivalent to PH15-5, but with strengths up to 290 ksi (1,999 MPa). Fabrication Sequence Billets of the desired alloy are forged in complex closed dies to a general overall shape, and the forg- ings are allowed to air cool at the forge shop. Generally, these forgings are then shipped to the manufacturer for machining. The forgings can be very large. For example, the raw forging for an U.S. Navy F-18 C/D landing gear weighs about 830 lb (380 kg). To give some idea of how much machining may be required, this forging weighed about 110 lb (50 kg) after machining. In a typical commer- cial aircraft, the main landing gear is approximately 10 ft tall, with the maximum cross section of between Figure 1 – Large gantry-type furnace used for 4 and 5 in. (3 m and 100 to 127 mm), heat treating landing gear. vanadium, and slightly greater and part weight can be upwards of carbon and molybdenum content 1,500 lb (680 kg). than 4340. The alloy is governed by The forging is net shape after ma- standard AMS 6417. This alloy has a chining. No other machining other very good combination of strength than honing the central bore is per- (280 to 305 ksi, or 1,930 to 2,100 MPa), formed. The components are then toughness, fatigue strength, and heat treated, and the part is inspected good ductility. It is a through hard- using magnetic particle inspection. ening alloy to large thicknesses. A dimensional check of the distor- HP9-4-30 (Timken Co.) is a low tion from heat treatment is also mon- alloy, high strength steel with a high itored. The bore is honed, and the nickel and cobalt content for tough- part is thoroughly cleaned for sur- ness. The typical tensile strength is face plating or coating. Multiple approximately 240 ksi (1,655 MPa). stress-relieving thermal treatments It is produced by consumable elec- are applied throughout the process, trode vacuum arc remelting process. and especially after plating. The part Production is governed by standard is then inspected again, and the final AMS 6526. protective coat of paint is applied. AF1410 (Timken Co.) is a very Once a landing gear set has been high-strength steel that exhibits ex- completed, it is moved to final as- cellent fracture toughness properties. sembly for assembly into a ship-set. It is typically used in the 260 ksi (1,790 MPa) tensile strength range. It Heat Treatment is double vacuum melted (vacuum of Landing-Gear Alloys induction melted followed by Historically, the heat treatment of vacuum arc remelting), for extremely landing gear has been done in large low inclusions and homogeneity. gantry-type atmosphere furnaces AerMet 100 (Carpenter Technol- (Fig. 1). Endothermic atmosphere ogy Corp., Reading, Pa.; www.cartech. produced by a gas generator, or by com) is a high strength, high tough- in-situ production of endothermic ness alloy, with excellent resistance gas by dissociation of methanol have to stress corrosion cracking and fa- been used, and both methods are still tigue. It is usually used in the 875 and in use today. The dissociation of 925ºF (470 and 495ºC) aging treat- methanol using an exterior generator ment condition. also has been used. Because of the de- Ferrium S53 (Questek Innovations sign of the furnaces, it is difficult to LLC, Evanston Ill.; www.questek. place an internal fan inside the fur- com) is a martensitic, carbide- nace, and stratification of the atmos- strengthened, secondary-hardened phere can become a problem. corrosion resistant steel developed Control of the atmosphere is crit- to serve as a drop-in replacement for ical. Because the heat treatment of 300M steel. Stress corrosion cracking landing gear is a neutral process (nei- 24 HEAT TREATING PROGRESS • MAY/JUNE 2008 ther carburizing nor decarburizing), precision carbon control is essential. Because of the nature of the alloys, and the high strength levels required, regions of decarburization serve as a weak spot and can be the cause of the initiation of low-cycle fatigue. Car- burization also can be an initiation site for fatigue or stress-corrosion cracking. In general, decarburization is limited to a partial decarburization of 0.003 in. (0.076 mm) maximum. Complete decarburization is prohib- ited. Carburization is limited to 20 KHN increase over the core hardness. Because of the difficulty and expense of testing individual landing gear for decarburization or carburization, multiple test specimens accompany the parts throughout the process. Test slugs for carbon measurement, and test blocks for fracture toughness and tensile specimens typically accom- pany each part. The test blocks are serialized so they can be identified with a specific landing gear compo- nent, so that if a problem occurs, the landing gear can be identified and quarantined for disposition. The successful control of the at- mosphere produced for the neutral hardening process depends on the accurate prediction and regulation of the carbon potential of the atmos- phere inside the furnace. This bal- ances the effective carbon in the steel with the carbon potential of the at- mosphere so that no decarburization or carburization occurs on the parts. In production furnaces, the exact composition is difficult to predict or ening is to be accomplished. In gen- Figure 2 – Typical heat treated load of landing calculate because of air infiltration, eral, silicon, manganese, cobalt, and gear racked for vacuum oil quenching. oily work, and residual coolant on nickel tend to cause the activity of work pieces, etc. By experience, it is carbon in steel to increase, while the possible to achieve good carbon con- elements chromium, molybdenum, trol. A properly calibrated carbon or and vanadium tend to decrease the oxygen probe will provide an accu- activity of carbon in steel. For 300M, rate representation of the carbon po- the carbon potential to achieve neu- tential if the atmosphere is stable or tral hardening tends to be approxi- near equilibrium. mately 0.08% higher at the typical For the atmosphere to be neutral austenitizing temperature of 1600ºF to the steel, the activities of the carbon (870 ºC). This would mean that the in the atmosphere and the steel must carbon potential in the furnace be equal. It is the activity of the at- would have to be approximately mosphere that is directly indicated higher by 0.08% than the carbon con- and controlled by the carbon probe. tent in the steel would indicate. The carbon activity in steel depends Routine weekly testing with alloy on the temperature and the concen- test slugs are used to validate the at- tration of carbon, as well as the con- mosphere, along with routine daily- centration of alloying elements. Be- by-shift shim stock analysis. Shim cause aerospace alloys contain stock testing and the test slug are typ- significant concentrations of alloying ically logged in a statistical process elements, it is important to consider control chart, with variations imme- the various effects of the various diately corrected and verified. The elements if successful neutral hard- oxygen probe is calibrated monthly HEAT TREATING PROGRESS • MAY/JUNE 2008 25 Table 2 — Typical heat treatments for aerospace steel alloys used for landing gear Alloy Preheat, ºC Austenitize, ºC Oil Quench, ºC Refrigerate, ºC Temper, ºC Temper, ºC 300M 500 870 80 300 300 HP9-4-30 500 840 80 -75 900 900 AF1410 500 840 80 -75 496 480 AerMet100 500 885 66 -75 482 — using an infrared analyzer.