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. C Mn Si Ni Cr Mo Co 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. Weekly decarburization and high-tempera- quality quench oils are used to checks of the carbon probe accuracy ture oxidation problems often asso- achieve uniform quench perform- and the flow settings is accomplished ciated with traditional controlled-at- ance and to achieve load after load using shim stock processed through mosphere heat treating can be consistency. Proactive maintenance the typical austenitize and quench minimized. This technique also al- programs to control oxidation and cycle. The proper CO and CO2 con- lows producers of aircraft landing minimize staining and to achieve centration is also verified whenever gear to finish machine critical sur- consistent and quenching uniformity an operator suspects that the oper- faces on these components prior to are required. Typical times and tem- ating conditions are not accurate, or heat treating. This in turn, reduces peratures are shown in Table 2. when the shim stock shows that the final machining costs when the part Racking is critical. Care has to be carbon potential is incorrect. While is in the hardened condition. As a re- taken to properly balance the load errors in the probe limit the real ac- sult, many landing gear heat treaters because of the weight. In addition, curacy of the system, carbon control have adopted vacuum oil quenching care has to be taken to properly to 0.50±0.01%C is not uncommon. of landing gear components. spread out the load for uniform heat During loading of the furnace, the Though the use of high pressure transfer along the length of the parts. atmosphere is diluted with infiltrated gas quenching for hardening certain Local pockets of heat from the sup- air, and typically, the carbon probe alloys that were previously oil porting center post are often a cause would call for natural gas additions quenched has been gaining popu- of longitudinal distortion of several to maintain the carbon potential. This larity, vacuum oil quench furnaces millimeters (up to 12 or so) on a part would be computed on the basis on have a significant role to play in the length of 12 ft (3.5 m). A typical load the fixed basis of 20% CO. This influx heat treating field. Gas quenching for heat treatment is shown in Fig. 2. of natural gas yields a carbon spike is limited to the hardening of fairly inside the furnace as the atmosphere thin sections of a relatively small Conclusions and temperature recovers. This in- number of alloys, so vacuum oil The heat treatment of landing gear troduces a risk of carburization of the quench furnaces offer an important is a complex operation that requires part. To avoid this problem, a modi- advantage in a large number of heat precise control of the variables of fication of the typical atmosphere treating applications. While gas pres- time, temperature and carbon con- control scheme is necessary. Typically, sure quenching of landing gear com- trol. Understanding the interaction the reset on the atmosphere controller ponents is being done for smaller of quenching, racking, and distortion is inhibited whenever the door is components (axles), the application contributes to reduced distortion and opened. This prevents controller is limited due to the necessary sec- residual stress. The high quality heat “wind-up” and minimizes the over- tion thicknesses of large landing gear treatment of landing gear contributes shoot on the atmosphere. Atimer can components. to our flying safety and excellent be used to start the controller until the Control of residual stresses and safety record. HTP atmosphere recovers to approxi- distortion is very critical. Residual mately 20% CO. In addition, all gas stresses, if tensile, can cause prema- additions (air or natural gas) are de- ture fatigue cracking. Distortion is layed until the atmosphere recovers. difficult and expensive because of the This is usually accomplished using a size of the parts, and the heat treated time delay in-line with the atmos- strength of the material. It also can phere addition valves. Proportional introduce new residual stress fields gas addition valves are used to de- that can influence fatigue properties. liver precise gas additions, instead of The choice of the proper quenching less expensive and less precise “On- practice, quenchant, and racking are Off” type gas additions. Use of stop- critical to minimize distortion and off compounds or copper plating is residual stresses. often used as insurance to prevent de- Because the section thicknesses are carburization or carburization. large, it is often necessary use fast- to Vacuum oil quenching offers var- medium-fast oil quenchants, to For more information: Dr. D. Scott Mac- achieve the through-hardened mi- Kenzie is Technical Specialist - Heat Treat- ious environmental and economic ing Products, Houghton International benefits over traditional hardening crostructure requirements. Large Inc., Madison and Van Buren Aves. Valley techniques in controlled atmosphere variation in hardness from surface to Forge, PA 19482; tel: 610-666-4007; furnaces. Because vacuum furnaces core is not permitted. This is one fax: 610-666-5689; e:mail: smackenzie@ are inherently leak tight, control of reason that extreme hardenability al- houghtonintl.com; Web site: www. surface chemistry is precise and the loys such as 300M are used. High houghtonintl.com. 26 HEAT TREATING PROGRESS • MAY/JUNE 2008