THERMAL PROCESSES

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High strength steels based on the 43XXseries are commonly used for aircraftlanding gear components because of their

desirable mechanical properties. To achieveoptimum mechanical properties, these alloys areusually heat treated by quenching in oil from thehardening temperature, then tempered. Thisproduces a microstructure with high strength andexcellent toughness, ideal for applicationsinvolving heavy loading and cyclic fatigue. Duringhardening, internal stresses created from thethermal shock of direct oil quenching almostalways cause some degree of distortion in the heattreated component. A final machining allowance isoften left on critical dimensional features tocompensate for distortion. However, due to thedifficulty of machining these alloys in thehardened condition, machining allowances mustbe minimised.

Many heat treaters have developed hotstraightening techniques to reduce quenchdistortion. However, these techniques are verylabour intensive and often involve expensivetooling. In the current environment of cost andlead-time reduction, reducing distortion duringthermal processing has become a primaryobjective for heat treaters of landing gearcomponents.

Interrupted quenching techniques – such asausbay quenching – offer good potential forreducing quench distortion. Shown in Figure 1 isan isothermal transformation (I-T) diagram for300M (4340M) alloy, the most commonly usedsteel for ultra high strength aircraft applications.The I-T diagram illustrates the microstructuraltransformations the steel undergoes over time atconstant temperature. Superimposed on thisdiagram is a curve depicting the cooling raterequired for successful ausbay quenching. Ausbayquenching involves the hardening of the steel byfirst rapidly cooling from the austenitisingtemperature to an intermediate temperature abovethat where martensitic transformation begins. Thisintermediate temperature range corresponds to theaustenite ‘bay’ region of the cooling curve. Cooling

Advanced techniques for distortioncontrol during vacuum oil quenching High strength steels based on the 43XXseries are commonly used for aircraftlanding gear components because of theirdesirable mechanical properties; thesealloys are usually heat treated byquenching in oil from the hardeningtemperature, then tempered. Duringhardening, internal stresses created fromthe thermal shock of direct oil quenchingalmost always cause some degree ofdistortion in the heat treated component.Interrupted quenching techniques - suchas ausbay quenching - offer good potentialfor reducing such quench distortion.by Jeff Pritchard of Vac Aero InternationalInc, Oakville, Ontario, Canada.

from the austenitising temperature to the ausbayregion must be sufficiently fast to avoid the “nose”of the cooling curve or the formation ofundesirable products of austenitic decompositionwill take place. The temperature of the load isallowed to stabilise in the ausbay region to ensurethat any residual thermal gradient from cooling isreduced. After stabilising, the load is quenchedthrough the martensitic transformation range toambient temperature.

By interrupting the cooling process, the ausbayquenching technique reduces the internal stressesthat are created within the material due to thermalshock. Cooling the load quickly to the stabilisingtemperature in the ausbay region ensures that anaustenitic structure is maintained. Subsequentquenching from this lower ausbay temperaturereduces thermal shock compared to the normalpractice of direct quenching from the usualhardening temperature. This results in a lowerlevel of internal stresses with correspondingimprovements in dimensional stability of the heattreated part. It should be noted that only certainsteel compositions can be successfully hardenedby ausbay quenching. These compositions containsufficient amounts of alloying elements to ensurethat cooling to the ausbay region can beaccomplished on a practical basis withoutintersecting the nose of the I-T curve.

Traditionally, most ausbay quenching processeshave been performed using salt baths. Typically,the load is heated in one salt bath maintained atthe hardening temperature. After a soak periodsufficient to ensure complete austenitictransformation, the load is quickly transferred to asecond salt bath held at an intermediatetemperature within the ausbay range. After thetemperature of the load stabilises in the secondsalt bath, it can be removed and quenched in oil.Though it offers certain benefits, salt bath heattreating presents a number of environmentalconcerns. The process generates significantamounts of undesirable emissions, includingcorrosive fumes and contaminated rinse water.Copper plating must also be used to protect part

surfaces from detrimental changes in surfacechemistry during salt bath heat treating. Theplating is stripped with solutions containingcyanide that are difficult and expensive to disposeof. Furthermore, there is a risk that any entrappedsalt residues may cause corrosion of the heattreated part when exposed to moisture in service.Alternatives to salt bath heat treating havetherefore been actively sought because of theseproblems.

VACUUM FURNACE AUSBAY QUENCHINGWith advances in computer-based control systemsand process capabilities, specially-designedintegral oil quench vacuum furnaces can be usedfor ausbay quenching. The Vac Aero model VAV72114 MPOGQ integral oil quench vacuumfurnace (Figure 2) was designed specifically forheat treating aircraft landing gear components.Though used mostly for direct oil quenching undervacuum, this furnace is also equipped with anexternal quench loop for inert gas quenching atpressures up to two bar. The furnace is verticallyoriented with a working zone size measuring1775mm in diameter by 2900mm high and canaccommodate gross load weights up to 1200kg.The control system is logic-based and fullyautomated. A programmable logic controllerinterfaced with process controllers regulates allmechanical and thermal functions of the furnace.Safety interlocks ensure that all pre-programmedconditions are met before critical process eventsare activated.

The furnace heating chamber is mounted on amovable gantry. During loading (Figure 3a), thegantry moves aside and the load is placed on anelevator in the quench tank. To avoid contact withthe load prior to heating in the vacuum chamber,the quench oil is transferred to a reservoir tank.The elevator lowers the load into the emptyquench tank and the heating chamber rolls backinto position over the quench tank, then lowersand seals to the quench tank flange. A vacuumseal door and hearth doors within the heatingchamber open and the elevator raises the load into

Figure 2- VAC AERO Model VAV 72114 MPOGQintegral oil quench vacuum furnace.

Figure 1 - Isothermal transformation diagram for 300Msteel showing load cooling rate during ausbayquenching.

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the heating chamber (Figure 3b). As the elevatorwithdraws into the quench tank, the hearth doorsclose to support the load and the seal door closesto isolate the heating chamber. After the loadingsequence is complete the quench oil is pumpedback into the quench tank from the reservoir tank.

During processing, the furnace chamber isevacuated to a pressure of approximately 10-2mb.The oil quench tank is also evacuated to apressure of approximately 400mb. When chamberevacuation reaches a certain preset pressure,heating of the load starts. The recommendedaustenitising temperature for most high strengthsteels is usually about 870°C. Heating ramp ratesare selected to minimise thermal gradients withinthe load without creating excessive cycle times.Control of the ramp rate during heating is animportant factor in relieving stresses that mayhave been created during machining of thecomponents being heat treated. During heating,the load is also held at a stand-off temperature of620°C for a short period of time to further improvetemperature uniformity. Maintaining temperatureuniformity throughout the cycle is essential toprevent the build up of thermal stresses and toensure that all sections of the load undergo themicrostructural transformations needed to achievethe desired mechanical properties.

After the load is soaked for a sufficient period oftime at the austenitising temperature, the ausbayquenching process can begin. The first step in thisprocess is the backfilling of the furnace chamberwith nitrogen gas. Nitrogen is a very economicalquench gas with good capacity for heat removaland no detrimental reactivity with the base metal.The chamber is backfilled to a pressure of 400mb,equalising it to the pressure in the oil quenchtank. During backfill the quench blower isactivated. The blower circulates the gas through amanifold, plenum and series of quench nozzleslocated 360° around the circumference of the load.Hot gas exits the chamber and passes through aheat exchanger where it is cooled before beingrecirculated by the blower through the load(Figure 3c).

Cooling by gas quenching continues until theload has reached a temperature of about 537°C, inthe middle of the ausbay region. The rate ofcooling during the gas quenching stage is carefullycontrolled so that it is fast enough to avoid thenose of the cooling curve but not so fast that theload is cooled below the required ausbaytemperature. The amount of quench gas blownthrough the load is regulated by a damping valvein the manifold. Normally open for maximumcooling rate, this valve will begin to close if theausbay temperature is at risk of being undershot.As the nose of the cooling curve is passed, heatingelement power is also increased to further slow thecooling rate. Both heating and quench gas flow areautomatically controlled through themicroprocessors in the furnace control system.

When the ausbay temperature is reached, thedamping valve is fully closed and the gas quenchstage of the process is complete. Sufficient poweris applied to the furnace heating elements tomaintain the load at 537°C. After a reasonablesoak period, the temperature of the load isstabilised and the oil quench stage begins. First,the vacuum seal door is opened and the quenchelevator is raised to a point where it again supportsthe weight of the load. The hearth doors are thenopened and the elevator and load rapidly descendinto the quench tank (Figure 3d). Power to theheating elements is maintained during oilquenching to ensure that the load remains at the

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ausbay temperature until immersion in the quenchoil. After the load is lowered into the quench tank,power to the heating elements is shut off and thevacuum and hearth doors are closed. The quenchblower is then re-activated to cool the heatingchamber to ambient temperature.

The quench tank contains more than 40,000litres of quench oil. Oil temperature is regulatedby heaters and coolers to ensure sufficient quenchspeed while preventing excessive thermal shock.Careful monitoring of quench oil conditions isanother important factor in distortion control.

PRACTICAL APPLICATIONSCase Study 1: Main landing gear piston. A pistontypical of those used in the main landing gear of alarge commercial aircraft is illustrated in Figure 4.Manufactured from 300M material, this piston hasa barrel height of approximately 1350mm andmeasures about 1000mm wide across the axle.Both barrel and axle are tubular in design withsection thicknesses ranging from a minimum of2.5mm throughout most of the tube wall to 20mmwhere barrel and axle meet. The material is heattreated to an ultimate tensile strength of 19,700 -21,100 kg/cm2 (280-300ksi).

During hardening, the piston is hung in avertical position. Past experience has shown thathanging vertically produces far less quenchdistortion than processing in a standing orhorizontal orientation. Despite these fixturingprecautions, distortion of the axle section by

Figure 3d - After stabilising at the ausbaytemperature, the furnace doors open, the elevatorraises to support the load, then descends quickly intothe quench tank to complete the process.

Figure 3c - Cooling to the ausbay temperature isaccomplished by backfilling the furnace chamber withnitrogen and circulating the gas through the furnacequench loop.

Figure 3b - The elevator raises the load into thefurnace chamber, then hearth doors and seal doorclose as the elevator withdraws. Quench oil pumpsback into the quench tank from the reservoir.

Figure 3a - Loading sequence of integral oil quenchvacuum furnace. Heating chamber rolls aside toexpose load elevator mounted in quench tank.

upward deflection during quenching is an inherentoutcome of the hardening cycle. In a sample oftwenty pistons subject to direct oil quenching,distortion of the right and left hand sides of theaxle averaged 0.58mm from the centreline. Fifteenof the twenty pistons in the sample exceeded theallowable axle distortion limit of .50mm andrequired hot straightening for dimensionalrecovery.

A second sample of twenty pistons washardened using the ausbay quenching technique.Though upward deflection of both right and lefthand sides of the axle again occurred, thedistortion was much less than experienced withdirect oil quenching. With ausbay quenching, axledistortion averaged only .28mm and none of thetwenty pistons exceeded the allowable axledistortion limit. Final inspection of each loadconfirmed that mechanical properties achieved byausbay quenching met all specificationrequirements and were very similar to thosemeasured in samples hardened by direct oilquenching.Case Study 2: Main fitting. A second comparisonof direct oil quenching versus ausbay quenchingwas performed on a sample of main fittings, acomponent of the landing gear from a smallerregional jet aircraft. The fitting is againmanufactured from 300M material and requiresheat treatment to the same mechanical propertiesas the piston discussed earlier. During thehardening cycle, the fitting is fixtured in a

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horizontal position as shown in Figure 5. In thiscase, the most serious distortion occurs when thetrunnion arm of the fitting deflects inward duringoil quenching. In direct oil quenching of twentyfittings, deflection of the trunnion arm from thecentreline averaged .68mm, well above theallowable limit of .50mm. When ausbayquenching was used for hardening of these parts,trunnion arm distortion was reduced to an averageof only .17mm. Again, all required mechanicalproperties were met.

In these two examples, ausbay quenchingreduced average quench related distortion bymore than 50% compared to direct oil quenchingwithout any loss in mechanical properties. Theseimprovements in distortion yielded significantsavings in hot straightening labour and tooling.

HIGH PRESSURE GAS QUENCHINGMany aerospace heat treating specifications nowpermit vacuum hardening of limited sectionthicknesses of certain high strength steels by gasquenching at pressures of 5 bar or higher. Thedevelopment of high pressure gas quenching wasbased on the principle that the denser the coolingmedium, the more heat will be extracted from theload. By pressurising, the quench gas becomesdenser, with improved heat transfer properties. Inaddition to the pressure of the quench gas, thecooling efficiency of the system is influenced byother factors such as the type of quench gas used,the velocity of the gas and design factors related tothe size and shape of the chamber in which thequenching is taking place.

In terms of distortion control, it is widelyassumed that because gas quenching creates less

distortion caused by oil quenching to that causedby high pressure gas quenching. Surprisingly, itwas determined that for many components,distortion caused by gas quenching was greaterthan caused by oil quenching. With gasquenching, cooling rate can vary significantly atdifferent points on the surface of the component.Differential cooling rates within an individualcomponent can cause stresses that lead todistortion. Oil quenching provides more uniformcooling as all surfaces of the load are quicklyenveloped by the quenching medium.

CONCLUSIONSPractical studies confirm the benefits of ausbayquenching in reducing distortion during hardeningof complex components. While the use of ausbayquenching is limited to certain alloy compositions,many of the steels used in high strengthapplications can be successfully hardened by thistechnique. The use of vacuum furnaces for ausbayquenching is a relatively recent advance madepossible by microprocessor-based furnace controlsystems and specialised equipment design. Bycombining gas and oil quenching in an interruptedquenching process, high strength steels can beheat treated to achieve full mechanical propertieswhile minimising quench distortion. Ausbayquenching reduces heat treating costs by avoidinglabour and tooling expenses associated with hotstraightening. The use of vacuum furnaces forperforming this process provides further costsavings over older heat treating technologies byeliminating several expensive environmentalconcerns.www.vacaero.com

Figure 5 - Schematic illustrationof Main Fitting showing directionof distortion of trunnion armduring quench.

Figure 4 - Schematic illustration of Main LandingGear Piston. Arrows show direction of axledistortion occurring during quench.

thermal shock than oil quenching it shouldproduce less quench-related distortion. In 1998,Vac Aero commissioned a large bottom-loadingvacuum gas quench furnace with a 10 bar pressurequenching capability. A number of trials wereperformed on different components to compare

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