vacuum hardening hss

7/27/2019 Vacuum Hardening Hss

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HEATTREATING

PROGRESSTHE BUSINESS AND TECHNOLOGY OF HEAT TREATING

An ASM International

Publication

TM

SM

www.asminternational.org MAY/JUNE 2007 VOLUME 7, NUMBER 3

VACUUM

HARDENINGHIGH STRENGTH

STEELS

PRECISIONCARBURIZINGAEROSPACE ALLOYS

GOODTHERMOCOUPLEPRACTICE

VACUUM

HARDENINGHIGH STRENGTHSTEELS

PRECISIONCARBURIZINGAEROSPACE ALLOYS

GOODTHERMOCOUPLEPRACTICE


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Vacuum heat treatmentoffers an alternativemethod to traditionalsalt-bath and controlled-atmosphere furnacehardening techniques for

high strength steels, suchas AISI 4340M and300M. However, heattreaters must be prudentwhen choosing betweenoil and gas quenching forvacuum hardening of highstrength steels, because

each process has itsadvantages anddisadvantages.

Jeff Pritchard*

and Scott Rush**

Vac Aero International Inc.

Oakville, ON, Canada

*Member of ASM Internationaland member, ASM Heat treating Society**Member of ASM International

igh strength steel alloys, suchas 4340M, 300M, and others,are most commonly used inthe manufacture of landing

gear components. These alloys arehardened and tempered to produceultimate tensile strengths exceeding280 ksi (1,930 MPa). Vac Aero Inter-national processes landing gear com-

ponents for most of the major com-mercial and military aircraftprograms.

Prior to 1980, Vac Aero used saltbath heat treating for hardeninglanding gear components (Fig. 1).While salt bath heat treating is stillwidely used today, it has some in-herent disadvantages. For example,even though parts are thoroughlywashed after processing, there is stilla risk that corrosive salt residues canbecome trapped in narrow recessesor blind holes. In addition, thewashing process itself generates largevolumes of contaminated wasteliquid, which must be properly dis-posed of. Also, salt baths generatehazardous fumes, create operatorsafety concerns, and present a highrisk of accidental fire.

Oil quenching from controlled at-mosphere furnaces can overcomemany of the disadvantages associ-ated with processing in salt. How-

ever, while both salt bath and con-trolled-atmosphere heat treatingprocesses provide a high degree ofprotection to the steel, some partialdecarburization and high tempera-ture oxidation still occur. Therefore,for critical surfaces, substantial finalmachining envelopes must be left inplace prior to heat treating. Re-moving these envelopes after thesteel is hardened can be difficult andexpensive. These concerns led VacAero engineers to develop vacuum

HEAT TREATING PROGRESS MAY/JUNE 2007 19

H

VACUUM HARDENING HIGH STRENGTH STEELS:

OIL VERSUS

GAS QUENCHING

Vac Aero Model VAV 72126 MPOGQ in-tegral oil and gas quench vacuum furnace.

Fig. 1 Salt-bath heat treating is stillwidely used, but has several disadvantagescompared with vacuum heat treating.


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oil quenching techniques.Because vacuum furnacesare inherently leak tight,control of surface chem-istry is precise, and the de-carburization and high-temperature oxidationproblems often associatedwith salt baths and con-trolled-atmosphere fur-naces can be eliminated.

Vacuum Oil QuenchingVac Aero in 1980 put into

operation its first direct oilquenching vacuum fur-nace designed for heattreating large aircraftlanding gear components.The furnace remains in fullproduction use today. Thefurnace consists of agantry mounted heatingchamber and about a9,000-gal (34,000 liter) ca-pacity oil quench tankequipped with a load ele-

vator. A set of thermal barrier doorsshields the quench tank from the ra-diant heat of the heating chamber.The entire quenching sequence takesless than 10 seconds.

The high interest of aerospace cus-tomers with the companys vacuumoil quenching process led to the needfor a second furnace to satisfy the de-mand. The second unit (Fig. 2) incor-

porated some new design features toenhance the process based on expe-rience gained over the ten-year op-erating period of the first vacuum oilquench furnace. For example, theunit incorporates a vacuum tight sealdoor between the heating chamberand quench tank; isolation of thequench tank from the heating

chamber is complete when the sealdoor is closed. The furnace also hasseparate mechanical pumping sys-tems to evacuate the heatingchamber and quench tank, which re-sults in lower vacuum levels and re-duced impurities in the heatingchamber creating an even cleaner en-vironment in which to process theload. The furnace can accommodateworkloads up to 73 in. diameter and114 in. high (1,830 and 2,900 mm) andgross load weights up to 3,500 lb

(7,700 kg).The system consists of a heating

chamber, gas quench system, insula-tion (thermal barrier) door chamber,and seal door chamber mounted ona moveable gantry. In a pit below areoil quench and reservoir tanksequipped with oil heaters andcoolers. Aschematic of the furnace isshown in Fig. 3. The oil quench tankhas a capacity of 10,770 gal (44,500liters) and contains a high speed el-evator and impellors to agitate theoil.

Automated CycleA typical hardening cycle is fully

automated. For loading, the chamberassemblies are raised several inchesfrom the quench tank flange bymotor driven screw jacks, oil ispumped from the quench tank to thereservoir tank, and the gantry ismoved away from the quench tank.The quench tank elevator is raised,the load is placed on it, and the ele-vator and load are lowered into thequench tank. The gantry is movedback into position and lowered untilthe bottom of the seal door chamberis sealed on the quench tank flange.The seal door and insulation doorsopen and the elevator raises the loadinto the heating chamber. As the ele-vator descends back into the quenchtank, the insulation doors, which alsoserve as the furnace hearth, close andreceive the load. After the elevator is

20 HEAT TREATING PROGRESS MAY/JUNE 2007

Fig. 2 Vac Aero Model VAV 72114 MPOGQ vacuum oil quench furnace.

Fig. 3 Schematic illustration of vacuumoil quench furnace in Figure 2.


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withdrawn, the seal door closes toisolate the heating chamber from thequench tank, the heating chamber isevacuated, and heating begins.

While these processes are un-derway, the quench oil is pumpedfrom the reservoir tank back into thequench tank. The quench tank isevacuated and backfilled with a par-

tial pressure of nitrogen to slightlybelow atmospheric pressure. Theload is preheated then austenitizedaccording to specification require-ments. At the beginning of thequenching sequence, the heatingchamber is backfilled with nitrogento a pressure equal to that in thequench tank. Power to the heating el-ements remains on during backfill toprevent heat loss in the load. The sealdoor opens and the elevator ascendsto raise the load slightly above the

hearth upon which the insulationdoors open and the elevator quicklylowers the load into the quench tank.Quench delay in this process for a full114 in. high load is less than 10 sec-onds. While the load is cooling in thequench tank, the gas quench system

activates to cool the heating chamberin preparation for accepting the nextload.

Athird, larger vacuum oil quenchfurnace, commissioned in Decemberof 2005, was built for hardening verylarge components such as those beingdeveloped for the landing gear in theAirbus A380. The unit operates

mostly on the same principles as itspredecessor.

Control of DistortionThe thermal shock induced by

quenching hot steel in oil almost al-ways causes some degree of dimen-sional distortion in the steel. Distor-tion can also be compounded bystress-inducing manufacturingprocesses such as forging or ma-chining that take place prior to heattreating. The primary advantage of

Vac Aeros vacuum oil quenchingtechnology is the precise control ofsurface chemistry that allows leavingminimal or no machining envelopeon the component prior to heattreating. The challenge in vacuumhardening then becomes counter-

acting distortion in the absence of afinal machining envelope to yield aheat treated component within therequired dimensional tolerances.

Several techniques can be used tocontrol distortion during heattreating. For components that be-come highly stressed from operationsprior to the hardening treatment,

such as forging and/or machining, aseparate stress relief cycle may be re-quired. However, in most cases, theseresidual stresses can be relievedduring the hardening cycle byslowing the heating rate.

Fixturing techniques are also im-portant for control of distortion. Longcomponents are usually hung froma fixture to take advantage of theforce of gravity as a stabilizing influ-ence. Components incorporatingmajor differences in section thick-

nesses are especially prone to distor-tion due to different rates of heatingand cooling between thin and thicksections. For these components, heatsinks are sometimes attached to thethinner sections to provide a betterthermal balance. Oil composition,


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temperature and agitation can alsoaffect distortion. Some oil composi-tions are designed to achieve higherquenching speeds than others. High-speed oils are preferred for hardeningvery thick sections and achievingmaximum strength in steels havinglower hardenability. However, theycan also cause greater levels of dis-

tortion in thin sections or in steelshaving high hardenability. Cooler,more highly agitated oil also createsmore thermal shock. Oil temperatureand agitation are carefully regulatedin specific hardening applications byusing controls device built into thequench tank.

Interrupted quenching techniquessuch as ausbay quenching also offergood potential for reducing distor-tion in certain grades of high strengthsteel. Vac Aero designed such a capa-

bility into its latest model vacuum oilquench furnace. Ausbay quenchinginvolves rapidly cooling the steelfrom the austenitizing temperatureto an intermediate temperatureabove that where martensitic trans-formation begins. The intermediatetemperature range corresponds to awide austenitic bay region on theisothermal transformation diagramsfor the steel. Cooling from the austen-itizing temperature to the ausbay re-

gion must be sufficiently fast to avoidthe nose of the transformation curveand corresponding formation of un-desirable products of austenitic trans-formation. This is accomplished byactivating the gas quench system.

With heating element power off,the gas quenching process continuesuntil the temperature of the load

reaches about 1000F (540C). Therate of cooling during the gasquenching stage must be carefullycontrolled so that it is fast enough toavoid any decomposition of austenitebut not so fast that the load is cooledbelow the required ausbay tempera-ture. The flow of gas circulatedthrough the load during thequenching stage is regulated by adamping valve in the inlet of themanifold. Normally open for max-imum cooling rate, this valve starts

to close as the ausbay temperature isapproached. Power to the heating el-ements is also gradually returned.Both quench gas flow and heating el-ement power are automatically reg-ulated through the use of micro-processors in the furnace controlsystem. Input from thermocouplesin the heating chamber provides themicroprocessors with real-time dataso cooling rate corrections are madeinstantaneously.

When the gas quench

system is deactivated,sufficient power is ap-plied to the heating ele-ments to maintain theload at the ausbay tem-perature, where it is heldat the soaking tempera-ture until a uniform tem-perature is achievedthroughout the work-load. Many highstrength steels can beheld in the ausbay tem-perature range for sev-eral hours without anyaustenitic transforma-tion. After a sufficientsoak, the load isquenched in oil usingthe same sequence ofevents as described pre-viously. By interruptingthe cooling process, theausbay quenching tech-nique reduces thermal

shock and the stresses it creates. Thisresults in corresponding improve-ments in dimensional stability of theheat treated component.

Vac Aero uses this technique onseveral critical landing gear compo-nents fabricated of 300M steel. Com-prehensive analyses show thatausbay quenching can reduce

quench-related distortion in thesecomponents by more than 50% com-pared with direct oil quenching.

High Pressure Gas QuenchingMany aerospace heat treating spec-

ifications now permit vacuum hard-ening of limited section thicknessesof certain high strength steels by gasquenching at pressures of 5 bar orhigher. The process is becoming in-creasingly popular because vacuumgas quench furnaces are generally less

expensive to operate than vacuum oilquench furnaces and offer some fur-ther environmental advantages. Thedevelopment of high pressure gasquenching was based on the principlethat the denser the cooling medium,the more heat will be extracted fromthe load. By pressurizing, the quenchgas becomes more dense and has im-proved heat transfer properties. In ad-dition to the pressure of the quenchgas, the cooling efficiency of thesystem is influenced by other factors

such as the type of quench gas used,the velocity of the gas, and design fac-tors related to the size and shape ofthe chamber in which the quenchingis taking place.

In terms of distortion control, it iswidely assumed that because gasquenching creates less thermal shockthan oil quenching it should produceless quench-related distortion. In1998, Vac Aero commissioned a largebottom-loading vacuum gas quenchfurnace with a 10 bar pressurequenching capability (Fig. 4). Anumber of trials were performed ondifferent components to compare dis-tortion caused by oil quenching withthat caused by high pressure gasquenching. Surprisingly, it was de-termined that for many components,distortion caused by gas quenchingwas greater than caused by oilquenching. Further analysis indi-cated that gas velocity is one of theprimary factors determining cooling

22 HEAT TREATING PROGRESS MAY/JUNE 2007

Fig. 4 Vac Aero Model VAV 72100 MP-10 vacuum gasquench furnace with 10-bar pressure quench capability.


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efficiency during gas quenching. Gasvelocity was found to decreasesharply beyond a distance of about 18in. (460 mm) from the gas quenchnozzle. The quench nozzles are lo-cated in circumferential rows withinthe furnace. The distance from aquench nozzle to the componentbeing quenched can vary significantly

over the entire surface of the compo-nent, particularly on larger compo-nents. Hence, the gas velocity and,therefore, the cooling rate can varysignificantly at different points on thesurface of the component. Differen-tial cooling rates within an individualcomponent can cause stresses thatlead to distortion. Oil quenching pro-vides more uniform cooling as all sur-faces of the load are quickly en-veloped by the quenching medium.

A conclusion derived from this is

that cooling during high pressure gasquenching, though still fast enoughto produce full mechanical propertiesin the steel, is less uniform than thatachieved in oil quenching, thereforesometimes causes greater distortion.

Another phenomenon evident

during the companys high pressuregas quench studies involved the ten-sile properties of 300M (essentially amodified AISI 4340 steel with silicon,vanadium, and slightly highercarbon and molybdenum contentsthan 4340) steel. In controlled exper-iments, ultimate tensile strengths forgas quenched and tempered speci-

mens of 300M often exceeded 300 ksi(2,068 MPa) versus typical strengthsbetween 285 and 295 ksi (1,965 and2,034 MPa) for oil quenched and tem-pered specimens. Similar trends havealso been reported by others. Elec-tron microscopy analyses, while notcompletely conclusive, strongly sug-gest the presence of bainite in the gasquenched specimens having thehighest ultimate tensile strengths. Afully martensitic structure was evi-dent in all of the oil quenched spec-

imens. The presence of bainite in themicrostructure could have negativeeffects on the performance of aircraftstructural components.

SummaryHeat treaters must be prudent

when choosing between oil and gasquenching for vacuum hardening ofhigh strength steels. Each process hasits advantages and disadvantages.Through practical experience, VacAero has concluded that high pres-sure gas quenching is better suitedfor smaller parts where distortion isnot a primary concern. The process

should also be performed in smallervacuum furnaces where high gas ve-locities can be maintainedthroughout the working area makingcooling more uniform. The restric-tions on section thicknesses that canbe successfully hardened by highpressure gas quenching also limit itsuse, so it is likely to replace oilquenching in only a smallnumber of applications.

For more information:Jeff Pritchard,president, or Scott Rush, vice president,Vac Aero International Inc., 1371 SpeersRd., Oakville, ON L6L 2X5 Canada; tel:905-827-4171; fax: 905-827-7489; e-mail:

[email protected] ; Internet: www.vacaero.com.


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