Analysis of individual metallic alloysemployed in aeronautics.

UNIT 5

Chapter 1 – SteelsChapter 2 – Aluminium alloysChapter 3 – Titanium alloysChapter 4 – Superalloys

Chapter 1 – Steels1.1) High-strength steels1.2) Maraging steels1.3) Stainless steels

Steels contribute ca. 7÷20 w% of commercial and military aircraft.

- Low-alloy steels are typically used in main landing gear, wing-root attachments and fasteners, while: - Highly alloyed “secondary hardening” steels are often used as landing gear and arresting hooks in carrier-based aircraft.- ”Precipitation (age) hardened” stainless steels are used for engine attachment fittings and for cargo handling equipment in transport aircraft.

secondary hardening (carbides)The hardening of certain alloy steels at moderate temperatures (250-650°C) by the precipitation of carbides ; the resultant hardness is greater than that obtained bytempering the steel at some lower temperature for the same time.

[ ]↓⇒↑

↑⇒↑hardness :typically

hardness

HTT

MoMo: strong carbide-former

a classical trade-off: heating required for precipitation, but high-T cause grain growth

age hardening (or: precipitation hardening) (non-C containing intermetallics)Increasing the hardness of an alloy by a relatively low-temperature heat treatment that causes precipitation of components or phases of the alloy from the supersaturatedsolid solution.

1.1) High -strength steels :specific aeronautical applications

INTRODUCTION

Target properties – primarily: yield strengths ≥1,725 MPa, high fracture toughness and resistance to stress-corrosion cracking.

Secondarily: high stiffness and wear resistance.

Control of sulfide-type inclusions, coupled with low sulfur levels, ensure the requiredtoughness.

Improving resistance to stress-corrosion cracking is typically based on addressingintergranular fracture issues (avoidance of segregating impurities and alloying additionsenhancing grain boundary cohesion).

i) Low-alloy steels (e.g., 4340, 300M), ii) More highly alloyed secondary hardening steels (e.g. HY180, AF1410, HP9-4-20, HP9-4-30) iii) Precipitation (age) hardened stainless steels (15-5PH, PH13-8) and maraging steels (C250) (see next chapters).

All grades are martensitic , subjected to hardnening (tempra) + tempering(rinvenimento) or aging during which carbides (also different from cementite) and/or other intermetallic particles are precipitated (secondary hardening).

There are numerous differences among these steels, including: the amount of martensite, the amount of carbon in solid solution, the nature of the particles precipitated on temperingand inclusion types and volume fractions.

In HY180, AF1410, HP9-4-20 and HP9-4-30 after tempering, most of the carbon precipitate as fine alloy carbides, which provide the high strength.

The martensitic precipitation hardened stainless steels (15-5PH and PH13-8) and the maraging steels (see next chapters) contain low carbon and are strengthened by the precipitation of particles during aging. These particlesare copper (15-5PH), β-NiAI (PH13-8) and Ni3Mo + Ni3Ti (maraging steels).

PHYSICAL METALLURGY

classicalaeronauticsteel

plain-strainfracturetoughess

SCCIC fracturetoughess

Aermet 100: C=0.23, Ni=11.1, Co= 13.4, Cr= 3.1, Mo= 1.2

Aermet 100: YS=1,725, KIC= 110, KISSC=33

low alloy

high alloysecondary hardening

stainlessprecipitation hardened

maraging

specifically developed for landing gear, designed to replace 300M wrt KISCC

A comprehensive selection of aeronautic high-strength steels(also including stainless and maraging grades, for comparison):

classicalaeronauticsteel

plain-strainfracturetoughess

SCCIC fracturetoughess

Aermet 100: C=0.23, Ni=11.1, Co= 13.4, Cr= 3.1, Mo= 1.2

Aermet 100: YS=1,725, KIC= 110, KISSC=33

low alloy

high alloysecondary hardening

stainlessprecipitation hardened

maraging

specifically developed for landing gear, designed to replace 300M wrt KISCC

A comprehensive selection of aeronautic high-strength steels(also including stainless and maraging grades, for comparison):

METALLURGICAL FACTORS CONTROLLING FRACTURE TOUGHNESSThe fracture toughness of ultrahigh-strength steels is a complex function of the inclusion spacing and inclusion volume fraction. Typically: inclusion spacing ↑ and inclusion volume fraction ↓ ⇒ KIC↑Moreover, increasing the resistance to void nucleation also improves fracturetoughness.

high-alloy, secondary hardening

see plots

La additions

sulfide & oxisulfideinclusions

La, S, O

saturating behaviour

Crack tip opening displacement vs inclusion spacing

IcIc K∝δ

inclusion spacing

inclusion

direction ofdislocation motion

force on dislocation due to stressfield modification by inclusion

high-alloy, secondary hardeningwrt AF1410: contains Mn +Ti and La additions

Mn, Ti, S, O, C

~ volume fraction ↓ & average spacing ↑ ⇒ KIC↑

case of HY810

The superior toughnesses of the two HY180 heats (grades) containing titanium-richinclusions is due to the increased resistance of these particles to void nucleation(“softer particles”, do not tend to “tear” the structure).

dislocation cutting a deformable particle

“increased resistance of these particles to void nucleation”(“softer particles”, do not tend to “tear” the structure).

undeformableparticle

dislocation climb aroundan undeformable particle

RESISTANCE TO STRESS CORROSION CRACKING

In general: σs↑⇒ KISCC↓but KISCC is a complex (very poorly known) function of alloy type anddifferent alloys of same σs can exhibit very different KISCC values.

Attempts to improve stress-corrosion cracking resistance have focused on understanding the effects of hydrogen and impurities which segregateto grain boundaries on grain boundary cohesion. Rationale: Impurities and alloying elements which segregate to grainboundaries lower the interfacial energy.

Approach: gettering impurities and eliminating alloying elements known topromote grain boundary fracture, such as phosphorus, sulphur, siliconand manganese.e.g. 1 – rapid solidification of La-containing grades allows to form LaPO4. e.g. 2 – La also provides a means of gettering the sulfur, via La2O2S.

Again, key requirement: high strength. Commercial maraging steels σs= 1,030-2,420 MPa. Experimental maraging steels have yield strengths up to 3,450 MPa.

Key concept: C virtually got rid of (max. 0.03%)

Way to replace C: hardening achieved by the addition of intermetallic-formingalloying elements: Ni (17÷19%), Co (8÷9%), Mo (3.0÷3.5%) and Ti (0.15÷0.25%).

Properties: During age hardening, there are only very slight dimensional changes. Therefore, fairly intricate shapes can be machined in the martensitic, but soft conditionand then hardened with a minimum of distortion. Weldability is excellent. Fracture toughness is considerably better than that of conventional high-strength steels.

Applications: landing gears, arresting hooks, fan shafts in engines, helicopterundercarriages, ejector seats, rocket motor cases, missile cases and variousaircraft structural forgings.

1.2) Maraging steels

σs (ksi)kilopound in-2)

1 ksi = 6.895 MPa

most common

wrought forms: forgings, plate, sheet and bar

Rationale: C, while producing hardening of conventional high tensile steels, causes brittleness and distortion. Hardening is achieved by metallurgical reactions thatdo not involve carbon: (i) formation of Fe-Ni martensite (relatively soft ~ 30÷35 HR vs 60÷70 for C steels); (ii) age hardening, i.e.the precipitation of intermetallic compounds (T ~ 500 °C). Hence the name: “MARtensite AGe hardenING”

Heat treatment: The desired microstructure is obtained by a two-step heat treatment:(i) austenitising + quench to room T to achieve the (fine-grained) martensitic matrix; (ii) ageing at modest T (typically: 3÷9 h at 455÷510 °C) to allow precipitation in the matrix .

low-C (<0.03 %)martensite

( ) BCCC

=→

tetragonallim0

PROCESSINGImportance of having a clean microstructure with low levels of deleterious elements, particularly sulfur and carbonS and C tend to form brittle carbide, sulfide and carbosulfide inclusions.These particles crack when the metal is strained, thus initiating fracture and loweringtoughness and ductility.

Most grades of maraging steel are therefore subjected to a series of meltingprocesses (both at ambient pressure and in vacuo). Premium grades of maraging steels used in critical aircraft and aerospace applications, for which minimum residual element (carbon, manganese, sulfur, and phosphorus) and gas (O2, N2, and H2) contents are required, are triple melted using one air and twovacuum steps.

FORMATION OF MARTENSITE

Metastable phase diagram Equilibrium phase diagram

austenitising

austenite tomartensite

ii) The austenite →→→→ martensite transformation & corresponding microstructureare essentially independent on cooling rate. Therefore, even very low cooling ofheavy sections produces a fully martensitic structure.

iii) Most grades have Ms temperatures of ~ 200÷300 °C and are fully martensitic at Troom. It is important to reach Troom to minimise untransformed austenite that notablysoftens the structure.

i) Austenitisation: typically 1 h / 25 mm (thickness). Protective gases (e.g. H2) are often used for surface protection.

ROLE OF ALLOYING ELEMENTS IN MARTENSITE FORMATIONIn addition to Ni, the other alloying elements Ms&Mf↓, with the exception of Co, whichraises it. The main role of Co is thus to raise the Ms temperature so that greater amounts ofother alloying elements (that will act as intermetallic formers during age-hardening,but which lower the Ms temperature) can be added while still allowing complete transformation to martensite before the steel cools to room temperature.

Thus Co does not directly participate in the age-hardening reaction, because thiselement does not form a precipitate with the other alloying elements. Some hardening results nevertheless from a short-range ordering reaction in the matrix that involves cobalt.

High-Ni (& Co) martensite: substitutional atoms

vacancysubstitutional atom larger than Fe

substitutional atomsmaller than Fe

Fe>Co>Ni

AGE HARDENING

Age hardening in maraging steels results primarily from the precipitation ofintermetallic compounds. Precipitates nucleate preferentially on dislocations within the martensite (vs at grainboundaries) to produce a uniform distribution of coherent fine particles. The major hardener is Mo, which upon aging initially forms Ni3Mo (orthorhombic). The metastable (wrt the final system of energies prevailing under prononged growthconditions) Ni3Mo phase nucleates initially because of its better lattice fit with the bccmartensitic matrix (n.b. an additional type of mechanical energy is involved in thisreckoning, that contributes to the ∆Gmix). Growth of Ni3Mo is restricted by coherencystrains (another type of mechanical work), and thus further ageing results in the transformation of Ni3Mo to the Fe2Mo phase (hexagonal), called “overageing”.

Add slide explaining formally contribution of mechanical energy on ∆Gmixand attending “pseudo-metastability”.

Fe Mo

Ti promotes additional age hardening through the precipitation of Ni3Ti (hexagonal),but can form TiC films at austenite grain boundaries.In particular, this happes if solubilisation of carbides at T>1150°C is followed by holding at T≅900÷1095°C. Prolonged annealing in this range should therefo re be avoided..

Mo plays the supplemental beneficial role of minimising grain-boundaryprecipitation by lowering the diffusion coefficients of a number of elements in solidsolution. This is a case of composition-dependent diffusion coefficient for species i: Di(Mj≠i).Of course, precipitation of grain-boundary phases impairs toughness.

Moreover, discrete particles of untransformed austenite have been shown to bepresent on the grain and subgrain boundaries in molybdenum-free grades(e.g.18Ni(300)).

It is worth noting that this T range is dangerous only during cooling, because whenheating from room temperature, stable carbides (≠ TiC) will already haveprecipitated. This temperature range should thus be avoided only when cooling.

Data for: 18Ni(250) (Ni 18%, Co 8.5%, Mo 5.0%, Ti 0.4%, Al 0.1%, C<0.03%)

Possible heat-treatment problems

With prolonged ageing, the structure tends to revert to the equilibrium phases: primarily ferrite and austenite (“austenite reversion”).

But the kinetics of the hardening precipitationreactions allow considerable age hardening(+20 HRC i.e. +1,035 MPa) before the onsetof austenite reversion.

With long aging times or high temperatures, however, hardness will reach a maximum and then will start to drop: softening by precipiatate coarsening.

hardeninginitiallyvery rapidLog(t)

Dimensional changes during age hardening

Typically contraction in length of 0.04% in 18Ni(200), 0.06% in 18Ni(250), and 0.08% in both 18Ni(300) and 18Ni(350), that can be regarded as negligible for most uses.

Thus, these very small dimensional changes during aging allow many maraging steel components to be hardened after machining.

Impact of age hardening behaviour on welding

Premise on steel metallurgy of welding

softening of work-hardened materialcaused by welding

In addition, a hardness surge is possible if some sort of “hardening” takesplace as a result of cooling rate

One of the main virtues of maraging steels is their excellent weldability .

This is the result of a combination of factors.

A weld heat-affected zone in maraging steels can be divided into three regions. (a) The region closest to the fusion line contains coarse martensite produced by solutionannealing. The low carbon content produces a soft, ductile martensite on cooling, thatdoes not give rise to residual stresses, distorsion and brittleness (weld-cracking) problems.(b) Next is a narrow region containing reverted austenite produced by heating into the 595 to 805 °C range. (c) Finally, there is a region where the maximumtemperature reached during welding ranges fromambient temperature up to 595 °C: this regioncontains martensite that has been age hardenedto various extents.

Subsequent localised aging brings the hardness ofthe weld zone up to that of the base metal, essentially fully restoring the mechanical propertiesof the welded joint.Thus, the toughness of the heat-affected zone after age hardening usually matches that of the unaffected base metal.

Mechanical properties : typical features

(A) Solution heating 1h (2.5cm) @ 820°C, ageing 3h @ 4 80°C (B) Solution heating 1h (2.5cm) @ 820°C, ageing 12h @ 480°C(C) Annealing 1h (2.5cm) @ 1150°C, ageing 1h @ 595°C

Mechanical properties, temperature dependence : can be used up to ~400°C

KIC

Mechanical properties: toughness. One of the distinguishing features ofmaraging steels is superior toughness compared to conventional steels. As noted above, the toughness of maraging steels is sensitive to purity level, and carbon and sulfur levels in particular should be kept low to obtain optimum fracturetoughness.

Mechanical properties: fatigue. Maraging steels are comparable to other high-strength steels. The bulk of the fatigue life in maraging steels is in the crack initiationstage. In general, cracks tend to initiate at incoherent intermetallic inclusions. As withother steels, improved fatigue properties can be obtained by shot peening and bynitriding.

Coherent vs incoherent precipitates

Two crystals match perfectly at the interface plane so that the two lattices arecontinuous across the interface. This can only be achieved if, disregardingchemical species, the interfacial plane has the same atomic configuration(orientation, interplane distance) in both phases, and this requires the twocrystals to be oriented relative to each other in a special way.

Coherent precipitates

The strains associated with a coherent interface with lattice mismatch raise the total energy of the system, and for sufficiently large atomic misfit it becomesenergetically more favorable to replace the coherent interface with a semicoherentone, in which the lattice mismatch is periodically taken up by misfit dislocations.

Nevertheless, coherent interfaces can exhibit different degrees of lattice mismatch.

Semicoherent precipitates

When the interface plane has a very different atomic configuration in the twoadjacent phases, there is no possibility of good matching across the interface. The pattern of atoms may either be very different in the two phases or, if it is similar, the interatomic distances may differ by more than 25%.

Incoherent precipitates

Uniformly distributed coherent precipitates of the appropriate dimensionsgive rise to an effective strengthening mechanism

Incoherent precipitates exhibit a notable dislocation-stopping activity, thus favouring brittleness.

Interaction of coherentvs. incoherent precipitateswith disolocations

SHAPING OF MARAGING STEELS

Cold working. Can be performed by any conventional technique when in the solution-annealed (unaged or as-transformed) condition. They have very low work-hardening rates and can be subjected to very heavy reductions (>50%) with onlyslight accompanying gains in hardness. Heavily cold worked structures can be softened by austenitizing or solutionannealing at temperatures of about 815 °C.Cold-worked pieces can be directly age hardened, in which case the total strengthincludes the increase in strength produced by cold working plus that produced byprecipitation hardening.

Machining. Maraging steels can be machined by any conventional technique whenin the solution-annealed or age-hardened condition. Machinability is generally asgood as or better than that of conventional steels of the same hardness.

Powder metallurgy. Limited experience: σs≅1600 MPa, combined with reasonableDuctility (i.e. facility of shaping by conventinal approaches), have been made by sintering elemental powders.

Surface treatments

i) Cleaning - Grit blasting is the most efficient technique for removing oxide filmsformed by heat treatment. Maraging steels can be chemically cleaned by pickling in sulfuric acid or by duplex pickling in hydrochloric acid and then in nitric acid plus hydrofluoric acid. Grease and oils can be removed by cleaning in trichloroethane-type solutions.

ii) Coating - Maraging steels can be nickel plated in chloride baths provided thatproper surface-activation steps are followed. Heavy chromium deposits can beplated on top of nickel electrodeposits. Maraging steels are less susceptible to hydrogen embrittlement during plating thanconventional quenched and tempered steels of comparable hardness. They are notimmune to hydrogen, however, and baking (e.g. 200°C, 5h) after plating isrecommended.

iii) Nitriding - Considerable surface hardening in dissociated ammonia. Hardnesslevels equivalent to 65 to 70 HRC can be achieved at depths of up to 0.15 mm afternitriding for 24 to 48 h at 455 °C. Nitriding at this te mperature allows age hardeningto occur during nitriding; therefore, the two processes can be accomplishedsimultaneously . Both fatigue strength and wear resistance of maraging steels are improved bynitriding.

Resistance to Corrosion and Stress Corrosion Cracking

In general, maraging steels have slightly better corrosion resistance than temperedmartensitic alloy steels. - In industrial and marine atmospheres, the corrosion rates of maraging steels are about half those of conventional steels. - In static and flowing seawater, maraging and conventional steels have essentiallythe same corrosion rates. - In saline and acidic solutions, maraging steels show somewhat better corrosionresistance. - Hot oxidation resistance is noticeably better than that of tempered martensiticalloy steels because of the nickel and cobalt contents of these materials (betteroxide films).

Maraging steels are susceptible to stress-corrosion cracking in most aqueousenvironments. Maraging steels have better resistance to stress-corrosion cracking than tempered martensitic steels of comparable strength.

(See also below wrt plating) Maraging steels appear to be more resistant tohydrogen embrittlement than low-alloy steels. Cathodic protection can also be used, but must be employed very carefully toavoid hydrogen charging (that, of course, could initiate cracking).

maraging

PH stainlesshigh-alloysecondaryhardening

1.3) Stainless steelsN.B. The key reason for selecting stainless steels is corrosion resistance,In this section we shall chiefly focus on structural aspects – though with somereference to phenomenological corrosion performance – strictly corrosion-relatedaspects are postponed to the Section of this course dedicated to corrosion.

Crucial ingredient: Cr ~≥ 12%

They achieve their stainless characteristics through the formation of an invisible and adherent chromium-rich oxide surface film (~Cr2O3).This oxide forms and heals itself in the presence of oxygen (or more in general, oxidising conditions).

Often also Ni is added in order to achieve:- better aqueous corrosion resistance- very good resistance to oxidation at high temperatures

5 main types:Ferritic, martensitic, austenitic, duplex (austeno-ferritic) and precipitation hardenable.

SELECTION OF AERONAUTIC APPLICATIONS

Stainless steel is used for several structural components of aircraft fuselages, engine mounts, engine oil lines, landing gear components, landing-hooks, valves, fuel tanks, hydraulic lines, hydraulic fittings, compressor casings, compressorblades, spacers, frames and casings for gas turbines and engine parts and is findingincreasing application in parts for hydraulic, fuel injection, exhaust, and heatingsystems as well as rocket casings.

Stainless steel aircraft tubing, produced from various chromium-nickel types, hasmany structural and hydraulic applications in aircraft construction because of its high resistance to both heat and corrosion.

Work-hardened tubing can be used in high-strength applications, but it is notrecommended for parts that may be exposed to certain corrosive substances or tocertain combinations of corrosive static or fluctuating stress.

Low-carbon tubing: types or compositions stabilised by titanium, niobium or tantalum are commonly used when welding is to be done without subsequent heattreatment.

also

N (

part

ial

Ni r

epla

cem

ent)

Phase dominance as a function of composition

The σσσσ phase problem (a range of compositions, mainly Cr-related)

TTT of σ phase in 25% Cr, 3% Mo, 4% Ni

If Cr very high σ phase can form at high-T (both treatment and service) ⇒ brittleness

dangerous T range

dangerous holding time

nucleation

growth

tetragonal

FERRITIC (some “4XX” grades) 11.5-30% Cr + Si & Mn, low CLow ductile-brittle transition temperature (⇐BCC) (if C+N<0.015%, Ttrans<Troom)Mechanical properties ~C-steels

Section at 0.01% C

MARTENSITIC (some “4XX” grades) typical 11-18% Cr, 0.1-1.2% CExcess carbides may be present to increase wear resistanceEasily hardenable: with 12% Cr hardenability Is so high that rapid quenchingis not necessary.

Section at 0.1% C

TTT for a typical martensitic stainless steel

“high”

⇓slow coolingtrajectories

allowed

T=650°C

γγγγ

αααα

18/8:just inside

the α/γ region

AUSTENSITIC (2XX, 3XX) 17-25% Cr + 8-35% Ni, low CCan be hardened only by cold-workingn.b. at T~Tamb, γ should transform into α, but the transformation is sluggish and has no practical impact, but cold working can provide enough energy to give rise to a martensitic transformation

Austenitic

Specific uses of austenitic grades:Types 309 and 310 are used for parts such as aircraft cabin heaters and jet engine burner liners.

DUPLEX: austeno-ferritic ~50%The exact amount of each phase is a function of composition and heat treatment. The corrosion resistance of duplex stainless steels is like that of austenitic stainlesssteels with similar alloying contents. However, duplex stainless steels possesshigher tensile and yield strengths and improved resistance to stress-corrosioncracking than their austenitic counterparts. The toughness of duplex stainless steels is between that of austenitic and ferriticstainless steels.

PRECIPITATION HARDENINGChromium-nickel alloys containing precipitation-hardening elements such ascopper, aluminium or titanium. Precipitation-hardening stainless steels may be either austenitic or martensitic inthe annealed condition. Those that are austenitic in the annealed condition are frequently transformable tomartensite through conditioning heat treatments, sometimes with a subzero(cold-)treatment. In most cases, these stainless steels attain high strength byprecipitation hardening of the martensitic structure.

SPECIAL AERONAUTIC APPLICATIONSPrecipitation-hardening martensitic stainless steels are used for short-time elevated-temperature exposures for which resistance to corrosion and high mechanicalproperties at temperatures up to 425 °C are important.The higher-carbon grade (0.13%) is used for compressor blades, spacers, framesand casings for gas turbines.Stainless steel grades 15-5PH and PH13-8Mo have been used in structural parts, and PH3-8Mo stainless steel has served as fasteners. Parts in cooler sections of the engine have been fabricated from type 410 stainless steel. Custom 455, 17-4PH, 17-7PH, and 15-5PH stainless steels have been used in the space shuttle.

DETAILS OF PRECIPITATION HARDENING PROCESSESThe precipitation-hardening stainless steels are of two general classes: single-treatment alloys and double-treatment alloys.

Single-treatment alloys (e.g. Custom 450, 17-4 PH, 15-5 PH) are - solution annealed at about 1040 °C to dissolve the hardening agent. - Upon cooling to room temperature, the structure transforms to martensite that issupersaturated with respect to the hardening agent . - A single tempering treatment at about 480 to 620°C is all that is required toprecipitate a secondary phase to strengthen the alloy.

Double-treatment alloys (e.g. 17-7 PH) are - solution annealed at about 1040°C and -then water quenched to retain the hardening agent in solution in an austeniticstructure. The austenite is conditioned by: (i) heating to 760°C to precipitate carbides and thereby unbalance the austenite so that it transforms to martensite upon cooling to a temperature below 15°C. (ii) Alternatively, the austenite may be conditioned at a higher temperature, 925 °C, at which fewer carbides precipitate, and then may be transformed to martensite bycooling to room temperature, followed by refrigerating to -75°C. (iii) Transformation can also be effected by severe cold work (about 60 to 70% reduction). - Once the structure has been transformed to martensite by one of these threeprocesses, tempering at 480 to 620°C induces precipitation of a secondarymetallic phase , which strengthens the alloy.

All grades with C>~0.03% are subjected to “sensitisation” @ 600-650°C

Sensitisation ⇒ grain-boundary precipitation of (Fe,Cr)23C6⇒ intergranular corrosion.Carbides are dissolved at T>800°C

precipitation of Cr-carbide at grain boundaries depletes the adjacent regions fromCr to less than the critical 12% for passivity. Moreover, crabides induce local galvanic coupling.

Ti and Nb sequestrate C (“stabilised stainless steels”) and inhibit (Fe,Cr)23C6 formation

NOTES ON CORROSION: metallurgy & phenomenology (for elchem. details, see later)

Corrosion PropertiesStainless steels are susceptible to several forms of localized corrosive attack. The selection of a grade of stainless steel for a particular application involves the consideration of many factors, but always begins with corrosion resistance . Once the grades with adequate corrosion resistance have been identified, it is thenappropriate to consider mechanical properties.

Effects of Composition on corrosion performance

Chromium is the essential element in the formation of the passive film . Other elements influence chemical & mechanical film properties.And are needed because eccessively high chromium adversely affect mechanicalproperties.

Nickel is effective in promoting repassivation , especially in reducing environments. Increasing nickel content to about 8 to 10% decreases resistance to stress-corrosioncracking (SCC), but further increases begin to restore SCC resistance. Resistance to SCC in most service environments is achieved at about 30% Ni.

Manganese ~Ni. High-manganese steels have some special mechanicalproperties. MnS adversely affects pitting resistance.

High-Mn steels: owing to their attitute to form martensite under mechanical action, these materials are very resistant to abrasion and will achieve up to three times theirsurface hardness during conditions of impact, without any increase in brittleness whichis usually associated with hardness. This behaviour is connected to two facts:(i) under impact, a hard surface layer forms on top of a tough substrate;(ii) if the hardened layer is removed by adrasion, a new hardened layer forms undersubsequent impacts.

Brodie helmet

Molybdenum stabilises the passive film in the presence of chlorides . It is especiallyeffective in increasing resistance to the initiation of pitting and crevice corrosion.

Carbon permits hardenability by heat treatment, which is the basis of the martensiticgrades,and provides strength in the high-temperature applications of stainlesssteels, but it is detrimental to corrosion resistance through its reaction with chromium(sensitisation). In the ferritic grades, carbon is also extremely detrimental totoughness (σ phase).

Nitrogen is beneficial to austenitic stainless steels in that it enhances pittingresistance, retards the formation of the chromium-molybdenum σ phase, and strengthens the steel. Nitrogen is essential in the newer duplex grades for increasingthe austenite content, diminishing chromium and molybdenum segregation, and raising the corrosion resistance of the austenitic phase. Nitrogen is highly detrimentalto the mechanical properties of the ferritic grades and must be treated as comparableto carbon when a stabilizing element is added to the steel.

Forms of Corrosion of Stainless Steels

General (uniform) corrosion when the environment is capable of stripping the passive film from the surface and preventing repassivation (e.g. lower-chromium ferritic stainlesssteel to moderate concentration of hot H2SO4).

304 bolt

316 pipe

Galvanic corrosion results when two dissimilar metals are in electrical contact in a corrosive medium. If the stainless steel is passive in the environment, galvanic interaction with a more noble metal is unlikely to produce significant corrosion. If the stainless steel is active, galvanic interaction with a more noble metal producessustained rapid corrosion of the stainless steel without repassivation. The most important aspect of galvanic interaction for stainless steels is the need toselect fasteners and weldments of adequate corrosion resistance relative to the bulk material, which is likely to have a much larger exposed area.

Pitting is a localised attack that produces the penetration of a stainless steel with almostnegligible weight loss to the total structure. It is associated with a local discontinuity ofthe passive film (mechanical imperfection, inclusion (MnS), surface damage, localchemical breakdown, deposits). Cl¯ is the most common pitting initiator. Pits are typically self-enhancing (the local chemical environment is substantially more aggressive than the bulk environment). Pitting resistance mainly imparted by Cr and Mo. Pittingsusceptibility of favoured bt high temperatures.

upper wing detail landing gear leg

Crevice corrosion can be considered a severe form of pitting. Any crevice, whetherthe result of a metal-to-metal joint, a gasket, fouling, or deposits, tends to restrictoxygen access, resulting in attack. Higher-chromium, and especially higher-molybdenum, grades are more resistantto crevice attack.

Intergranular corrosion is a preferential attack at the grain boundaries of a stainlesssteel. It is generally the result of sensitisation.

failed aircraft component

Stress-corrosion cracking combination of a susceptible alloy, sustained tensile stress, and a particular environment leads to cracking of the metal. Stainless steels are particularly susceptible to SCC in Cl¯ environments (even very low concentrations). Temperature worsens SCC. Most ferritic and duplex stainless steels are either immune or highly resistant to SCC. All austenitic grades, especially AISI types 304 and 316, are susceptible. Although some degree of pitting or crevice precede SCC, the amount can be so smallthat it is undetectable . Stress corrosion is difficult to detect while in progress, evenwhen pervasive, and can lead to rapid catastrophic failures.It is difficult to alleviate the environmental conditions that lead to SCC. Tensile stress is one parameter that might be controlled. However, the residualstresses associated with fabrication, welding, or thermal cycling, rather than design stresses, are often responsible for SCC, and even stress-relieving heat treatments do not completely eliminate these residual stresses.

landing gear beamairplanefitting

Erosion-corrosion corrosion of a metal or alloy can be accelerated when there is anabrasive removal of the protective oxide layer.

helicopter component

Oxidation because of their high Cr contents, stainless steels tend to be very resistantto oxidation. Important factors to be considered in the selection of stainless steel grades for high-T service is the adherence of the oxide scale upon thermal cycling .Because many of the stainless steels used for high temperatures are austeniticgrades with relatively high nickel contents, it is also necessary to be alert to the possibility of sulfidation attack.History: stainless steels found applications in the construction of super- and hypersonicexperimental aircraft, where temperature effects are considerable. Stainless steel formed the primary structural material in the Bristol 188, built to investigate kinetic heating effects, and alsoin the American rocket aircraft, the X-15, capable of speeds of the order of Mach 5-6.