choose materials for high-temperature environments

choose materials for high-temperature environments

Materials

Choose Materials forHigh-Temperature Environments

Peter Elliott,

Corrosion & MaterialsConsultancy, Inc.

Based on a paper presented at CORROSION/2000 (NACE International 55th Annual Conference and Exhibition),Mar.26–31,2000,Orlando,FL,USA. ©NACE International. All rights reserved.

aterials are selected on the basis of service requirements,no-tably strength,so corrosion resistance (stability) may not bethe primary design consideration. Assemblies need to be strongand resilient to the unique loads and stresses imparted on them,

which can include signi cant temperature changes and thermal gradients formany high-temperature applications.

In making a choice,it is necessary to know what materials are availableand to what extent they are suited to the speci c application. The decision isquite involved and the choice is signi cantly affected by the environmentand the intended use,be it a reactor vessel,tubes,supports,shields,springs,or others. Some problems may occur because of distortion and crackingcaused by thermal expansion/contraction; typically,a high-temperature alloymight change 4in./ft from ambient to 1,000°C (1,832°F).

The user or designer needs to properly understand that the environmentdictates the materials selection process at all stages of the process or applica-tion. For example,an alloy that performs well at the service temperature maycorrode because of aqueous (dew point) corrosion at lower temperatures dur-ing off-load periods,or through some lack of design detail or poor mainte-nance procedures that introduce local air draughts that cool the system (e.g.,at access doors,inspection ports,etc.).

To provide as optimum performance as possible,it is necessary for a sup-plier to be aware of the application,and for the user to be aware of the gen-eral range of available materials. Otherwise,severe problems can result. Forexample,a catastrophic failure occurred within weeks for an ignitor,madewith Type 304 stainless steel (UNS S30400,iron,19% Cr,9% Ni,0.08% C).Type 304 stainless steel would have been suitable for clean oxidizing condi-February 2001 /cep/ 75

CEP

choose materials for high-temperature environments

Materials

tions to about 900°C (1,650°F) inHigh-temperature continuous service,or 845°Calloys and uses

(1,550°F) in intermittent (temperatureHigh-temperature alloys are typi-cycling) service (1).The failure oc-cally iron-,nickel- or cobalt-based al-curred because of overheating withloys containing >20% chromium (orcontributions from sul dation (hot30% for cobalt),which is sufficient tocorrosion). The true cause of failureform a protective oxide against fur-was a material mix-up,because Typether oxidation. The basic alloys in-304 was not speci ed,but was inad-clude various additional elements thatvertently used.

aid in corrosion resistance,notablyMechanical limits of materialsaluminum (typically >4% to developan alumina scale),silicon (up to 5%In considering traditional alloys,itto develop an amorphous (glass-like)is important for the designer and userscale that is complementary to chro-to be fully aware of the mechanicalmia),and rare earth elements (typi-limits of a material. For example,thecally <1%,e.g.,yttrium,cerium,andASME Pressure Vessel Codes adviselanthanum,that improve scale adhe-that the maximum allowable stresssion). Other additions,such as the re-shall not exceed whichever is theactive metals,the refractory metals,lowest of:(i) 100% of the averageand carbon,primarily improve me-stress to produce a creep rate ofchanical properties. The bene cial0.01% in 1,000 h; (ii) 67% of the av-and detrimental roles of common al-erage stress to cause rupture afterloying elements on the anticipated100,000 h; and (iii) 80% of the mini-performance of alloys at high-temper-mum stress to cause rupture afteratures is covered by Agarwal and100,000 h.

Brill (2).

These recommendations may beRefractory metals— Molybde-better appreciated by extracting typi-num,which is a bene cial additioncal data for Type 304 intended for usefor resisting aqueous chloride-in-in a pressure vessel up to 815°Cduced pitting corrosion (found in(1,500°F). Based upon ASME tables,Types 316 and 317 stainless steels,for a load of 17 MPa (2.5 ksi) atand the 6%-Mo alloys),is prone to760°C (1,400°F),the expected designcatastrophic oxidation as tempera-life would be 24 yr; at 788°Ctures exceed about 700°C (1,292°F),(1,450°F),the life falls to 7 yr; and atthe point above which MoO815°C (1,500°F),it is only 2.2 yr.3formseutectic mixtures with iron,nickel,Thus,a short-term temperature excur-and chromium oxides. The oxidesion can have a signi cant effect onMoOequipment life. Also to be noted isCatastrophic oxidation rapidly ren-3melts at 795°C (1,462°F°). that a small increase in loading,forders a metal into a useless powderyexample,from 2.5 to 3 ksi at 760°Coxide. Damage is worse in stagnant(1,400°F),can markedly reduce theconditions and appears to be exacer-life expectancy,here,from 24 to 9 yr.bated when sodium oxide is presentOverheating is the most common(e.g.,from insulation). All of the re-cause of high-temperature corrosionfractory metals (tungsten,tantalum,failure,but the temperature in uenceniobium,and molybdenum) may ex-on mechanical properties is of equalperience catastrophic oxidation. Sili-or even more signi cance in thatcide coatings have shown some tomany failures occur because of creepoffer some resistance to this catas-deformation (creep voids) and ther-trophic (“pest”) oxidation.

mal fatigue. Overheating can arise forCoatings— High-temperaturevarious reasons,including an unex-coatings or surface modi cations arepected accumulation of tenacious de-generally based on chromium,alu-posits that can foul tubes in a heat minum,or silicon,which,at highexchanger.temperatures,form protective oxides

76

/cep/ February 2001 CEP

rich in chromia,alumina,or silica,re-spectively. In more recent years,therehave been developments in applyingso-called alloy coatings,for example,the use of MCRALY (metal,chromi-um,aluminum,and yttrium) on steelsor other high-temperature alloy sub-strates. Efforts have also continued inweld overlay work,where a strongbase metal can support a corrosion-resistant surface-coated layer.

Applications— In consideringmaterials options,a thorough knowl-edge of the service applications(stress-bearing service; cyclic loadingor not; frequency of cycling; impactor erosion effects; thermal expansionand contraction) is needed.

Different high-temperature corro-sion processes are simultaneously in-volved in many common service ap-plications. Some of these are syner-gistic,which creates a formidablechallenge for users and alloy produc-ers. Some examples of the forms as-sociated with various applications aregiven in Table 1 (3,4).

Types of high-temperaturecorrosion

There are certain distinguishingfeatures about the morphology ofhigh-temperature corrosion that aid indeciding upon the cause of damage.Some typical indications includethick scales,grossly thinned metal,burnt (blackened) or charred surfaces,molten phases,deposits of variouscolors,distortion and cracking,andmagnetism in what was rst a non-magnetic (e.g.,austenitic) matrix.Damage varies signi cantly basedupon the environment,and will bemost severe when a material’s oxida-tion limits are exceeded,notablywhen an alloy sustains breakaway at-tack by oxygen/sulfur,halogen/oxy-gen,low-melting uxing salts,moltenglasses,or molten metals,especiallyafter res.

Oxidation

Many industrial processes involveoxidation,i.e.,a metal reacts in air toform and sustain a protective oxide.

choose materials for high-temperature environments

There can be several oxide products,some of which are less desirable,forexample,wustite,a defective oxide ofiron that forms rapidly at about540°C (1,000°F) on steel.

Most high-temperature alloys areoxidation resistant,so price,availabil-ity,experience,and the type of appli-cation usually dictate choice. Thereare no signi cant problems up to400°C (750°F),few up to 750°C(1,380°F),but the choice of success-ful alloys becomes somewhat limitedabove about 800°C (1,470°F).

Simple iron-chromium (or iron-chromium-molybdenum) alloys be-come less useful as service tempera-tures increase,which is where theType 300 series austenitic stainlesssteels,(304,309,310,314,330,333,etc.) and certain ferritic stainlesssteels (410 and 446) nd many appli-cations. For more arduous serviceconditions at higher temperatures,these alloys are surpassed by nickel-or cobalt-based formulations,includ-ing many of the more robust alloysthat are mechanically alloyed to im-

prove strength and to control (that is,minimize) grain growth at elevatedtemperatures.

Certain alloys (usually those withrare earth additions) are more re-silient to oxidation under thermal cy-cling (shock) conditions. Some appli-cations do not allow an alloy to fullydevelop its steady-state condition,thus,performance is dictated by thetransient (not-so-protective) surfacescales. Transient effects will becomeapparent should failure analysis beperformed.

Caution should be given to iron-chromium-nickel alloys that can beprone to sigma-phase formation be-tween 540–800°C (1,000–1,470°F),which results in premature brittle fail-ure. Molybdenum-containing alloys(Types 316 and 317 stainless steelsand the 6%-Mo alloys) can be proneto catastrophic oxidation above about680°C (1,256°F).

Sul dation

Sulfurous gases are common tomany applications,including fuel

combustion atmospheres,petrochemi-cal processing,gas turbines,and coalgasi cation. Sul des (e.g.,sulfurvapor,hydrogen sul de) can be verydamaging,because metal sul desform at faster rates than do metal ox-ides. Sul des have low melting pointsand produce voluminous scales (scalespallation).

With mixed corrodant environ-ments (oxygen and sulfur),alloy per-formance is based upon a subtle inter-play between oxide and sul de for-mation. Oxides are more stable; sul- des form more rapidly (due to kinet-ics). Thus,oxides,sul des,or bothmay form. If deposits are also pre-sent,then conditions at the metal sur-faces are reducing compared to areasexternal to the deposits. Damage canbe extensive.

Mixed sulfur-and-oxygen gasescan invoke very high corrosion ratesdue to breakaway attack,typicallyabove about 600°C (1,110°F) fornickel-based alloys,920°C (1,688°F)for cobalt-based,and 940°C (1724°F)for iron-based formulations. Break-

February 2001 /cep/ 77

CEP

choose materials for high-temperature environments

Materials

Above: Tube failure due to local overheating.Left: Burst tube walls due to overheating.

away attack is commonly associatedperformance is dictated by the uniquewith sulfur and excess air. Once theproperties of the halides,including rst-formed oxide is lost or de-high vapor pressures,high volatilitystroyed,sul des can invade the(vaporization),low melting points,chromium-depleted substrate,thus,mismatched expansion coefficientscausing accelerated attack to occur. with metal substrates,and the effectsStainless steels and iron-based al-of displacement reactions wherebyloys are preferred over high-nickel al-oxide or sul de are thermodynami-loys,because nickel is prone to form-cally favored over the halides.

ing the low-melting nickel-nickel sul-Alloy performance is greatly af- de eutectic,Ni-Ni3S2,which melts atfected by oxidizing or reducing con-635°C (1,175°F). Eutectics of cobaltditions. For oxidizing atmospheres orand iron occur at higher temperatures,for vapors jointly present with oxy-880°C (1,616°F) and 985°Cgen (or air),there is an opportunity(1,805°F),respectively.

for reduced corrosion rates (kinetics)Alloys can be weakened by inter-associated with oxide formation,al-nal corrosion,most noticeably whenthough the scale may later be disrupt-mobile species are present,such ased by the volatile halides,especiallylow-melting sul des,which are typi-if iron-based alloys are used. Nickel ed by localized dull uniform grayalloys are generally favored for halo-phases within the alloy matrix. Atgen atmospheres,since iron-based al-times,liquid-appearing phases areloys are more vulnerable,due to theirfound in the metallurgy.

volatile products,e.g.,FeClAlloys containing aluminum,sili-3. Siliconadditions are useful if oxidizing envi-con,and cobalt are useful in sul diz-ronments prevail,but not for reducinging environments. Many alloys classi-conditions. Preoxidation is not nor- ed as candidates for sul dation domally a bene t for reducing halogenwell only if oxides are rst able toattack.

form. Preoxidation can be of value.

What makes halogens differentHalogenation

from other oxidants is their high mo-bility and diffusivity into a metal,re-Halogen attack is commonly man-sulting in internal damage of theifested as a combination of scale spal-alloy matrix. Fluorine can penetratelation with internal alloy damage in-twice the distance of chlorides,cluding voids that form as a result ofwhich means that the predominanthighly volatile species (5).Materialmode of damage in uorine-contain-

78

/cep/ February 2001 CEP

ing gases is by means of internal attack. Halide products are also hygro-scopic (3),so it is not unusual to dis-cover local protrusions on a metalthat have been removed during ser-vice. In laboratory studies,it is com-mon to nd that a surface apparentlyfree from chlorides (removed duringmetallographic preparation) is laterfound to show them. This is becausethe chlorides have been leached outfrom deep under the voided areas inthe metal.

Carburization

Several environments are synony-mous with carburization,includingpyrolysis and gas-cracking processes,reforming plants,and heat-treating fa-cilities that involve carbon monoxide,methane,and hydrocarbon gases.Damage is usually manifested as in-ternal carbides,notably in grainboundaries and is generally worstabove 1,050°C (1,922°F). When car-burizing conditions alternate with ox-idizing ones,carbides can becomeoxidized to oxides,which yields car-bon monoxide that can weaken thegrain boundaries in an alloy. Such analloy fails by “green rot,”a name thatdescribes the green fractured surfacethat results (chromium oxide).

Strongly carburizing atmospheres(i.e.,those that have a carbon activity>1) can cause a metal to form coke-like layers,often of a dusty form.This form of attack,termed metaldusting,commonly occurs between425–800°C (790–1,470°F) and can bevery rapid (in days not months).Damage is either general or localized(pitting),as dictated by the ability ofthe alloy to form a surface oxide (6).

choose materials for high-temperature environments

Above: Thermal fatigue crack in boiler tube.

Top: Tube fouling in anincinerator plant due tocarryover of deposits.Bottom: Through-metalperforation in tubing from a

carbon black plant.

Carbon steels and alloy steels arenormally uniformly thinned by metaldusting; more highly alloyed materi-als usually display local outgrowthsof coke emerging through small pitsthat broaden with time.

Cast iron-nickel-chromium alloysare widely used for carburizing appli-cations,including the more recent al-loys containing 1–2% silicon and1.5% niobium (the HP Mod alloys)(4,6).High-nickel alloys (with lowsolubility for carbon) nd many ap-plications for carburizing conditions.Stronger nickel-based alloys withhigh chromium and silicon contentsare useful in more demanding envi-ronments. Highly alloyed ferriticstainless steels (that are able to morerapidly form a thin oxide lm) tend tooutperform austenitic steels.

base metals. Iron tends to be detri-mental,as do aluminum and titaniumin low concentrations. Silicon forms abrittle intermetallic compound withnitrogen and can contribute to scalespallation,especially in applicationsat low oxygen concentrations (poten-tials),where thin oxides can form,and during thermal cycling.

Nitriding

Relatively little is reported aboutnitridation other than material perfor-mance is weakened (embrittlement)as a result of the formation of internalnitrides in the alloy (4).It is commonto expect damage with nitrides at700–900°C (1,290–1,650°F). Nitridesappear generally as needle-like pre-cipitates in the alloy matrix.

Nickel- and cobalt-rich alloys ap-pear to be rst-choice candidates forresisting nitride attack,because of thelow solubility of nitrogen in these

Molten products

Deposits are a common product inmany high-temperature applications,including boilers,waste incinerators, uidized-bed combustors,and gasturbines. A whole series of reactionsis possible should deposits becomemolten and no single mechanism canbe applied generally to characterizesuch damage (7).

The mechanisms of molten prod-uct corrosion are complex. The typesof damage include fuel-ash corrosion— sulfates,including acid and basic uxing reactions (8),and vanadicslag attack (9)— molten salt corro-sion (chlorides,nitrates,and carbon-ates) (4),and molten glass corrosion.Liquid metal attack is yet anotherspecial category (4).

Fuel-ash or ash/salt-deposit cor-

rosionstems from high-temperaturecorrosion processes associated withfuel combustion products in boilers,waste incinerators,and gas turbines.Thus,products can include variousdeposits (oxidizing or reducing) withactive contributions from oxygen,sulfur,halogens,carbon,and nitrogen(4,7).Typically,alloy matrices dis-play intergranular attack (oxides andchlorides) beneath disturbed oxidelayers possibly fused with molten de-posits and internal sul des within thealloy-affected zone.

Hot corrosionis generally regardedas attack in the joint presence of sulfurand oxygen. Typically,attack is consid-ered to be triggered by molten alkalimetal salts that melt above 700°C(1,290°F). Sodium sulfate,with a melt-ing point of 884°C (1,620°F),derivedfrom sodium chloride and sulfur fromthe fuel,is considered to be closely in-volved in the mechanism of hot corro-sion (8).This mechanism is consideredto have four stages:oxidation (incuba-tion); mild sul dation; oxide failure;and catastrophic attack (internal sul- des via a porous voluminous complexoxide/deposit layer). Hot corrosion isan irreversible autocatalytic process.

February 2001 /cep/ 79

CEP

choose materials for high-temperature environments

Materials

Table 2. Guide to candidate materials.

Corrosion ModeOxidation

Basic Alloy Types

Fe-Ni-(Co) >20% (30%) Cr.Stabilized to minimize

sensitization. Al, Si bene cial. Rare earth additions aid scale retention.

Candidates*

304, 321,309, 310, 800(HT), 803,430, 446,HR120, 330, 85H, 333,600, 601(GC), 602CA, 617, 625,253MA, 353MA, DS, 214, MA956,MA754, X, etc.

Notes and Cautions

Wide choice dictated byapplication and function;Mechanical properties; Thermal cycle (shock);Transient vs. steady state;Internal oxides. Beware σ;

W, Mo — catastrophic oxidation.Sulfur vapor, H2S, etc. — no oxides. Beware of Ni/Ni3S2eutectic; coatings can help. SO2, SO3 , etc. — risk of

breakaway attack with oxidesand sul des; Al coatings. (See hot corrosion.)

Internal carbides withintergranular attack; Casttubes bene t from smoothI.D. surfaces.

Metal dusting (at lowertemperatures). Green rot (with intermittent O2-C). Internal nitrides (e.g.,AlN) can weaken alloy; Thin oxide at lowoxygen partial pressure reduces nitridation.

Volatile products; Internal attack with voids;Hygroscopic products

(e.g.,chlorides); Scale spallation.Applications dictate alloy,or coating: Gas turbines(strong + CRA ); vanadicslag (high-Cr + Si).Intergranular attack,internal voids, and

probable embrittlement. Complex uxing reactions;oxidation; sul dation;

chlorination; uorination, etc.Dissolution or alloying effects;Intergranular attack;Depends on system.

Seek input from suppliers;

Consider online tests/monitoring.

Sul dation

(Reducing gasesno oxides)Sul dation

(Oxidizing gases)

Fe- with high Cr (Al) alloys.

9–12%Cr steels: 309, 310, 330,800(HT), 803, HR120, 85H,

253MA, 353MA, MA956, 446, 671,6B, 188, etc.

As above with 153MA, 601, HR160,MA754, MA956, 333, 556, etc.

Fe-Cr-based alloys. Oxideformation a bene t. Preoxidization may help.Wide use of cast alloys.For worse conditions, usehigh-Ni alloys with Cr, Si. (Low solubility of C in Ni isbene t for Ni alloys.)

Carburization

HH, HK, HPMod, 309, 310, 330, 333,85H, 800(HT), 803, DS, HR160, 600,601, 253MA, 602CA, 617, 625, 690,MA754, MA956, X, 556, 706, 718, 750, etc.

Nitridation

Ni-alloys rather than Fe. Avoidhigh Cr levels. Use low Al andlow Ti levels (nitride formers).Si promotes scale spalling.Ni alloys generally betterthan Fe. Bene ts: Cr (not HF),Al, Si (with oxygen).

Preoxidation not bene cial.FeCrMo alloys at lower

temperatures; CRAs for S, O,C — subject to application.High Cr, Al, Si useful (also as coatings).

Ni alloys generally favored;some high-Cr alloys;

NiCrMoW alloys for moltenchlorides.

Ni- or some Co-/high-Cralloys; Some refractories.Fe-alloys with Cr, Al, Si

usual rst choices (subject toliquid metal, e.g,. Liq. Na, K, molten Zn, Pb, etc.)Synergy of processes.

309, 800(HT), 330, 446, 188, 230,600, 602CA, 625, 253MA, etc.

Halogenation:chlorination, uorination, etc.Fuel ash corrosion

800H, 333, 200, 201, 207, 600,601, 602CA, 214, N, H242, B3, etc.

309, 310, 800(HT), 600, 601, 602CA,625, 825, 253MA, 353MA, MA754,MA758, MA956, IN657, 671, etc.

Molten salts

As with halogens, sul dation:depends on nature of salts (acidic/basic).

600, 601, 602CA, 671, 690, MA758, etc.

309, 310, 85H, 253MA, etc.

Molten glass

Liquid metals

ComplexEnvironments

CRAs or coatings

Note: This is a general, not exhaustive, guide.* Not in any preferred order.

CRAs are corrosion-resistant alloys.

80

/cep/ February 2001 CEP

choose materials for high-temperature environments

Vanadic slag corrosionoccurs fol-lowing combustion of certain low-grade or residual fuel oils that are ofhigh vanadium,sulfur,and alkalimetals. The molten sodium vanadylvanadates typically ux away protec-tive oxides and then rapidly dissolvethe metal. Many high-temperature al-loys cannot survive 100 h at 900°C(1,652°F) in vanadic slags (9).Vanadic attack can be managed bylowering temperatures (if possible),using fuel-oil additives (such as mag-nesium and calcium oxides),or byspecifying high-chromium alloys. Sil-icon-rich coatings are bene cial andappear to complement the role ofchromium (9).

Molten glasstypically induces in-tergranular attack with voids (fromvolatile halides) and sul des. Oxidesare generally fused into the glass. At-tack is commonly rapid,and highchromium-nickel-based alloys areusually employed. Iron-rich alloyscan be prone to severe attack due totheir ability to form low-meltinghalides (e.g.,FeClMolten salts,used for heat treating3).

applications,nuclear engineering,solar cells,and metal extraction,gen-erally promote intergranular attack inalloys,often with voids and internallow-melting products (halides).

A common feature in most high-temperature aggressive environmentsis the synergy of the reactants onewith each other (Table 1). Wastagecan be easily measured,but the mech-anism(s) are not as easy to determine.As a minimum,a rigorous study andanalysis of the morphologies shouldhelp to establish the rate-controllingprocess,which should help to betterde ne the type of alloy that could beconsidered as a candidate. The broadexpertise of the material suppliersshould be fully explored in the questfor a suitable choice. Monitoring trialsusing test spools are recommendedwherever they can be used.

Candidate alloys

Choice should be based on carefulconsiderations,including,primarily,

the function of the component or ves-recommended alloys and alloy typessel. As might be expected,there arefor various high-temperature environ-many candidates,yet,from these thements are summarized in Table 2.choice is often reduced to one of two,This table is intended only as a guide;once the total range of properties isno order of merit is to be interpretedfully explored. Factors to be consid-from the sequence of listings (orered include mechanical propertiesomissions) in this table. Also,the(strength, exibility,fatigue life),alloy lists are not meant to be inclu-physical properties (expansion andsive,but,rather,merely typical exam-contraction,re ectivity,magnetism,ples of what has worked in the eld.

etc.),availability (shape and form),and price (economic decision basedTo sum up

on overall costs and fabrication,etc.).Ideally,the material choice isAs a convenience,some generally

based on known data and experience,which implies communication be-tween a user and a supplier. A betterknowledge of anticipated componentrequirements in addition to corrosionbehavior provides for a better choiceand the expectation of more reliableservice.

Proper identi cation and recordingof damage from prior systems is apositive bene t in deciding upon analternative alloy or coated system.Wherever possible,and certainly fornew and complex environments,test-ing is to be recommended. x

CEP

February 2001 /cep/

81

本文来源：https://www.bwwdw.com/article/nh0m.html