Heat Treating Titanium And its Alloys | Heat Treating | Annealing (Metallurgy)

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Heat Treating Titanium And its Alloys
  How to ensure success instress relieving, annealing,solution treating andaging, and other heat  processing operations. The overview is based on achapter in the author’s new ASM book,  Titanium: A Technical Guide. by Matthew J. Donachie Jr.* ConsultantWinchester, N.H. * Fellow and Life Member of ASM International T itanium and titaniumalloys are heat treatedfor several reasons:ã To reduce residualstresses developedduring fabrication(stress relieving)ã To produce the most acceptablecombination of ductility, machin-ability, and dimensional and structuralstability, especially in alpha-beta al-loys (annealing)ã To increase strength by solutiontreating and agingã To optimize special properties,such as fracture toughness, fatiguestrength, and high-temperature creepstrengthStress relieving and annealing may be used to prevent preferential chem-ical attack in some corrosive environ-ments, to prevent distortion, and tocondition the metal for subsequentforming and fabricating operations.Hot isostatic pressing, a specializedheat treatment process (Fig. 1 and 2),can help narrow the fatigue propertyscatter band and raise the minimumfatigue life of cast components.Typical stress relieving, annealing,and solution treating and aging cyclesare given in the Datasheet in this issueof  Heat Treating Progress . Beta transustemperatures for commercially pure(CP) titanium and selected titaniumalloys also are included. Response to heat treatment   HEAT TREATINGTITANIUM AND ITS ALLOYS Fig. 1 — Aft engine mount bulkhead for the Pratt & Whitney-powered Boeing 777 aircraft isthe first cast titanium alloy component to be used in a fracture-critical aerospace application. It re- placed a fabricated assembly after passing a series of FAA-mandated static tests last year. The sub-stitution was enabled by technical developments in the investment casting process combined withadvanced hot isostatic pressing (HIP) techniques. Photo courtesy of Howmet Castings. ã  The basic alpha, near-alpha, alpha- beta, and beta alloys have heat treat-ment responses attuned to the mi-crostructure (phases and distribution)that can be produced, which is a func-tion of chemical composition. Alpha, near-alpha: Because alphaalloys undergo little in the way of phasechange, their microstructure cannot bemanipulated much by heat treatment.Consequently, high strength cannot bedeveloped in the alpha alloys by heattreatment. However, some near-alphaalloys, such as Ti-8Al-1Mo-1V, can besolution treated and aged to develophigher strengths. Both alpha and near-alpha titanium alloys can be stress re-lieved and annealed. Alpha-beta: The alpha-beta alloysmake up the largest class of titaniumalloys. Microstructures can be sub-stantially altered by working (forging)and/or heat treating them below orabove the beta transus. Compositions,sizes, and distributions of phases inthese two-phase alloys can be manip-ulated within certain limits. As a re-sult, alpha-beta alloys can be hardened by heat treatment, and solutiontreating plus aging is used to producemaximum strengths. Other heat treat-ments, including stress relieving, alsomay be applied to these alloys. Beta alloys: In commercial (meta-stable) beta alloys, stress relieving andaging treatments can be combined.Also, annealing and solution treatingcan be identical operations. Beta transus: The beta transus tem-perature (Datasheet, Part 1) of a tita-nium alloy — the minimum temper-ature above which equilibrium alphaphase does not exist — is very signifi-cant for heat treating purposes, espe-cially when the heat treatment in-volves heating near or above thistemperature.When the heat treatment tempera-ture is near the beta transus, thetransus of each heat in a lot must beaccurately determined, because thevalue will vary from heat to heat dueto small differences in composition,particularly oxygen content. Titaniumproducers generally certify the betatransus for each heat they supply.Note that hardness testing is notrecommended for checking the effec-tiveness of heat treating titanium al-loys. The correlation between strengthand hardness is poor in these mate-rials. Whenever verification of a prop-erty is required, the appropriate me-chanical test should be used. Stress relieving of titanium Stress relieving is probably the mostcommon heat treatment given to tita-nium and titanium alloys. It is used todecrease the undesirable residualstresses that result from nonuniformhot forging deformation, nonuniformcold forming and straightening,asymmetric machining of plate(hogouts) or forgings, welding of wrought, cast, or powder metallurgy(P/M) parts, and cooling of castings.Stress relieving helps maintainshape stability and also can eliminateunfavorable conditions such as loss of compressive yield strength — theBauschinger effect — that can be par-ticularly severe in titanium alloys.Stress relieving can be performedwithout adversely affecting strengthor ductility.Typical stress-relief cycles are listed in the Datasheet, Part 2. Selection decision: When symmet-rical shapes are machined in the an-nealed condition, using modest cutsand uniform stock removal, stress re-lieving may not be required. However,the greater the depth of cut and/or themore nonuniform the cut, the morelikely it is that stress relieving will beneeded either to successfully completethe machining and fabrication cycleor to ensure maximum service life of the component.It may be possible to omit a sepa-rate stress relief if the manufacturingsequence can be adjusted so that anannealing or hardening operation alsoserves to relieve residual stresses. Forexample, forging stresses can be re-lieved during the annealing operationrequired prior to machining. Example:Large, thin forged rings have beenprocessed with minimum distortion by rough machining material in theannealed condition. Subsequent op-erations include solution treating,quenching, partial aging, finish ma-chining, and final aging. The partialaging operation also relieves quench-ing stresses, while the final aging re-lieves stresses developed during finishmachining. Time/temperature and cooling: More than one combination of timeand temperature can yield a satisfac-tory stress relief. Cooling rate from thestress-relieving temperature is not crit-ical for titanium alloys. However, uni-formity of cooling is. This is particu-larly true in the 480 to 315°C (900 to600°F) temperature range. Furnace orair cooling is preferred. Oil or waterquenching should not be used to ac-celerate cooling after stress relieving.These faster quenchants can promotenonuniform cooling, which can induceresidual stresses. Metallurgical response: The met-allurgical response of the alloy in-volved plays a major role in the selec-tion of stress-relief cycles. To reducestresses in a reasonable time, the max-imum temperature consistent withlimited change in microstructure isused.The treatment involves holding ata temperature sufficiently high to re-lieve stresses but not cause an unde-sirable amount of precipitation orstrain aging in alpha-beta and beta al-loys, or undesirable recrystallizationin single-phase alpha alloys that relyon cold work for strength.Beta alloys and the more highly al-loyed alpha-beta compositions rely onmicrostructural control via heat treat-ment to optimize strength properties.Consequently, they are best stress re-lieved using a thermal exposure thatis compatible with the recommendedannealing, solution treating, stabiliza-tion, or aging process. Note, however,that the stress relief treatment per seis not used to control microstructure. Quality control: The only way tonondestructively gage the effective-ness of a stress-relief cycle is by X-raydiffraction. Stress relieving producesno significant changes in microstruc-ture that can be detected by light op-tical microscopy.Although X-ray stress measurementcan be used to assess the degree of stress reduction, the method is im-perfect. Very limited data are avail-able, most of which were generated inthe first two decades following thecommercial development of titanium.The shapes of residual stress-vs.-timecurves at each stress-relief tempera-ture are likely to differ for every alloy.They also are a function of prior pro-cessing. Nevertheless, relative stressreduction as a function of time at tem-perature is routinely treated as an in-variant function, and the relative stresscurves are applied to alloys for whichactual measurements are limited ornonexistent. Process annealing methods “Annealing” is a generic term andmay be applied differently by differentproducers. For example, solutiontreating is frequently considered anannealing process, and the stress re-lief heat treatment is often called stressrelief annealing. Techniques that serve ã  primarily to increase toughness, duc-tility at room temperature, dimen-sional and thermal stability, and,sometimes, creep resistance are con-sidered “process annealing” or just“annealing” methods. Annealing treatments: Commonannealing treatments include mill, du-plex, recrystallization, and beta an-nealing. Selected cycles are listed inthe Datasheet, Part 3.ã Mill annealing is a general-pur-pose treatment given to all mill prod-ucts. It is not a full anneal, and canleave traces of cold or warm workingin the microstructure of heavilyworked product (particularly sheet).ã Duplex annealing is an exampleof the multiple-anneal processes thatsometimes are specified. Triplex an-nealing also has been practiced. Suchtreatments frequently are used in thecontext of solution treating and aging.ã Both recrystallization and beta an-nealing are used to improve tough-ness. Recrystallization annealing hasreplaced beta annealing for fracture-critical airframe components. In thismethod, the alloy is heated into theupper end of the alpha-beta range,held for a predetermined time, andthen very slowly cooled.ã Beta annealing is done at a tem-perature only slightly higher than the beta transus, to prevent excessivegrain growth. Annealing time de-pends on section thickness and should be long enough to permit completetransformation to beta. Time at tem-perature after transformation to betashould be held to a minimum to con-trol grain growth of the beta phase.Beta annealing can be followed by anair cool, although larger sections mayneed to be fan cooled or even waterquenched to prevent the formation of detrimental alpha phase at grain boundaries.The cooling method used afterhigher-temperature annealing can af-fect tensile properties. For example,air cooling of Ti-6Al-6V-2Sn from themill annealing temperature results ina tensile strength lower than that ob-tained by furnace cooling. Regardlessof the method used, if distortion is aproblem, the cooling rate should beuniform down to 315°C (600°F).Because process annealing treat-ments usually are less closely con-trolled than solution treating andaging, more property variability or“scatter” will occur in annealed alloys.Nevertheless, many titanium alloysare placed in service in the annealedcondition. Phase stability: In beta and alpha- beta titanium alloys, thermal insta- bility is a function of beta-phase trans-formations. In alpha-beta alloysduring cooling from the annealingtemperature, or in isothermal expo-sure of beta alloys, beta can transformto the undesirable (brittle) interme-diate phase, omega.Beta alloy chemical compositionsare controlled to prevent omega for-mation, and alpha-beta alloys aregiven a stabilization anneal. This an-nealing treatment produces a stable beta phase capable of resisting furthertransformation when exposed to ele-vated temperatures in service. In thecase of alloys that are solution treatedand aged, the aging treatment may beable to double as the stabilization heattreatment.Alpha-beta alloys that are lean in beta, such as Ti-6Al-4V, can be aircooled from the annealing tempera-ture without impairing their stability.Furnace cooling (slow cooling), how-ever, may promote formation of Ti 3 Al,which can degrade the alloy’s resis-tance to stress corrosion.A duplex anneal is used to obtainmaximum stability in the near-alphaalloys Ti-8Al-1Mo-1V and Ti-6Al-2Sn-4Zr-2Mo. First step is a solution an-neal at a temperature high in thealpha-beta range, usually 25 to 55°C(50 to 100°F) below the beta transus forTi-8Al-1Mo-1V alloy, and 15 to 25°C(25 to 50°F) below the beta transus forTi-6Al-2Sn-4Zr-2Mo. Forgings areheld for one hour (nominal) and thenair or fan cooled, depending on sec-tion size.The solution anneal is followed bystabilization annealing for eight hoursat 595°C (1100°F). The final annealingtemperature should be at least 55°C(100°F) above the anticipated use tem-perature so that no further micro-structural changes will occur duringservice.Note that maximum creep resis-tance can be developed in Ti-6Al-2Sn-4Zr-2Mo by beta annealing or betaprocessing (and by adding silicon). Distortion: Straightening, sizing,and flattening operations are oftennecessary to correct distortion re-sulting from annealing, particularly of close-tolerance thin sections. Becausetitanium alloys exhibit excessivespringback, the straightening of bar toclose tolerances and the flattening of sheet present major problems for pro-ducers and fabricators. Straightening,sizing, and flattening can be stand-alone processes or can be combinedwith annealing (or stress relief) by use ã Fig. 2 — In hot isostatic pressing, or HIP, elevated temperatures and isostatic pressure are si-multaneously applied to components for a precisely controlled time. The heat- and pressure-treating process can be used to reduce or eliminate voids in castings through creep and diffusion bonding. TheseTi-6Al-4V seal housings are being loaded into a HIP vessel at Howmet Castings, Whitehall, Mich. HIP’ing will reduce microshrinkage in the castings. Photo courtesy of Howmet Castings.  of appropriate fixtures.Unlike aluminum alloys, titaniumalloys are not easily straightened whencold. Springback and resistance tostraightening at room temperaturemake it necessary to employ an ele-vated temperature process. Creepstraightening is the method of choice.Creep straightening takes advan-tage of the low creep resistance of many titanium alloys at annealingtemperatures. Thus, with proper fix-turing and, in some instances, judi-cious weighting, many sheet metalfabrications and thin, complex forg-ings can be satisfactorily straightenedduring annealing. Again, uniformcooling to below 315°C (600°F) afterstraightening can improve results.In creep flattening, titanium sheetis heated while being held betweentwo clean, flat sheets of steel in a fur-nace containing an oxidizing or inertatmosphere. Vacuum creep flattening,a variation, is used to produce stress-free flat plate for subsequent ma-chining. The plate is placed on a large,flat, ceramic bed that has integral elec-tric heating elements. Insulation isplaced on top of the plate, and a plasticsheet is sealed to the frame. The bedis slowly heated to the annealing tem-perature while a vacuum is pulledunder the plastic. Atmospheric pres-sure creep-flattens the plate. Solution treating and aging Maximum strength in titanium al-loys is achieved by solution annealing(commonly called “solution heattreating” or just “solution treating”)followed by quenching and thenaging. The process can be used to ob-tain a wide range of strength levels inalpha-beta and beta alloys. The re-sponse of most titanium alloys to so-lution treating and aging srcinates inthe instability of the high-temperature beta phase at lower temperatures.In general, solution treating andaging does not mean the same thingfor titanium as it does for traditionalage-hardening systems, such as alu-minum alloys or nickel-base superal-loys. Ti-2.5Cu is a rare exception be-cause a compound (Ti 2 Cu) doesprecipitate from supersaturated alphaphase upon quenching from a high-temperature solution anneal and thenaging at an appropriate temperature.The Ti 2 Cu forms zones (as in alu-minum alloys) that increase strengthat lower temperatures. Note, however,that Ti-2.5Cu does not produce pre-cipitate particles, such as gammaprime, that characterize nickel-basesuperalloys, which are true high-tem-perature alloys.No titanium alloy of conventionalcomposition is truly age hardenable.However, an addition of silicon tonear-alpha and alpha-beta alloys willimprove high-temperature strength,presumably by formation of a silicidephase during customary solutiontreating and aging processes.Solution treating and aging (or sta- bilization) usually, but not always,follow working operations to optimizemechanical properties. Heating analpha-beta alloy to the solutiontreating temperature produces ahigher ratio of beta phase to alphaphase. This phase partitioning is main-tained by quenching; on subsequentaging, the unstable beta phase and anymartensite that may be present de-compose, increasing strength. Com-mercial beta alloys, generally suppliedin the solution-treated condition, needonly to be aged. Furnace conditions: After beingcleaned, titanium parts are loaded intofixtures or racks that permit free ac-cess to heating and cooling media.Thick and thin components of thesame alloy may be solution treated to-gether, but the time at temperature(soak time) is determined by thethickest section. The rule of thumb formost alloys: 20 to 30 minutes for every25 mm (1 in.) of thickness.A load can be charged directly intoa furnace operating at the solutiontreating temperature. Preheating is notessential, but it can be used to mini-mize distortion of complex parts. Mi-crostructural changes occurringduring heat treating can cause com-ponents to “grow.” (Designs for largeparts require allowances for growth.)The growth due to heating can be re-tained after cooling, and it can be in-creased either by longer hold times atthe solution treating temperature or by lower heating rates.Depending on the alloy, the solu-tion treating temperature is eitherslightly above or slightly below its betatransus. Solution treating cycles for se-lected titanium alloys are given in theDatasheet, Part 4. Beta alloys: Solution treating tem-peratures for beta alloys can be abovethe beta transus. Beta alloys normallyare obtained from producers in the so-lution-treated condition. If reheatingis required, soak times should be onlyas long as necessary to obtain com-plete solutioning, because grains cangrow rapidly under these conditions(since no second phase is present toprevent it). For near-beta alloys, solu-tion heat treating may have to be car-ried out below the beta transus (analpha-beta anneal). The solution-treated product will contain globularalpha plus retained beta. The mi-crostructure after aging will consist of a bimodal alpha distribution (primaryalpha plus alpha from aging). Alpha-beta alloys: Alpha-beta al-loys are solution treated at a temper-ature slightly below the beta transus.Close control of temperature is essen-tial. If the beta transus is exceeded, ten-sile properties (especially ductility) arereduced and cannot be fully restored by subsequent thermal treatment.Selection of a solution treatment foralpha-beta alloys is made after de-ciding on the combination of me-chanical properties required afteraging. A change in the solutiontreating temperature of an alpha-betaalloy alters the amount of beta phaseand, consequently, affects the alloy’sresponse to aging (Table 1).To obtain high strength with ade-quate ductility, it generally is neces-sary to solution treat at a temperaturehigh in the alpha-beta field, normally25 to 85°C (50 to 150°F) below the betatransus. If higher fracture toughnessor improved resistance to stress cor-rosion is required, beta annealing or beta solution treating may be desir- Table 1 — Effect of solution treating temperature on tensile  properties at room temperature of Ti-6Al-4V bars(a) Solution treating Tensile Yield Elongation in temperature strengthstrength(b)4D(c)°C°FMPaksiMPaksi% 8451550102514998014218870160010601549851431790016501095159995144169251700111016110001451694017251140165105515316 (a) Properties determined on 13 mm (0.5 in.) in diameter bar after solution treating, quenching, and aging. Aging treatment:8 h at 480°C (900°F), air cool. (b) 0.2% offset. (c) D = specimen diameter. ã
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