Steels for aircraft structures
Steel is an alloy of iron containing carbon and one or more other alloying elements. Carbon steel is the most common material used in structural engineering, with applications in virtually every industry sector including automotive, marine, rail and infrastructure. The world-wide consumption of steel is around 100 times greater than aluminium, which is the second most-used structural metal. Figure 11.1 shows the production of steel, aluminium, magnesium and composites over the course of the 20th century, and the usage of steel amounts to more than 90% of all metal consumed. Although steel is used extensively in many sectors, its usage in aerospace is small in comparison to aluminium and composite material. The use of steel in aircraft and helicopters is often limited to just 5–8% of the total airframe weight (or 3–5% by volume).
The use of steel in aircraft is usually confined to safety-critical structural components that require very high strength and where space is limited. In other words, steel is used when high specific strength is the most important criterion in materials selection. Steels used in aircraft have yield strengths above 1500–2000 MPa, which is higher than high-strength aluminium (500650 MPa), α/β titanium (830–1300 MPa) or quasi-isotropic carbon-epoxy composite (750–1000 MPa). In addition to high strength, steels used in aircraft have high elastic modulus, fatigue resistance and fracture toughness, and retain their mechanical performance at high temperature (up to 300–450 °C). This combination of properties makes steel a material of choice for heavily-loaded aircraft structures. However, steel is not used in large quantities for several reasons, with the most important being its weight. The density of steel (ρ = 7.2 g cm−3) is over 2.5 times higher than aluminium, 1.5 times greater than titanium, and more than 3.5 times heavier than carbon–epoxy composite. In addition to weight problems, most steels are susceptible to corrosion which causes surface pitting, stress corrosion cracking and other damage. High-strength steels are prone to damage by hydrogen embrittlement, which is a weakening process caused by the absorption of hydrogen. A very low concentration of hydrogen (as little as 0.0001%) within steel can cause cracking which may lead to brittle-type fracture at a stress level below the yield strength.
Aircraft structural components made using high-strength steel include undercarriage landing gear, wing-root attachments, engine pylons and slat track components (Fig. 11.2). The greatest usage of steel is in landing gear where it is important to minimise the volume of the undercarriage while having high load capacity. The main advantage of using steel in landing gear is high stiffness, strength and fatigue resistance, which provide the landing gear with the mechanical performance to withstand high impact loads on landing and support the aircraft weight during taxi and take-off. Owing to the high mechanical properties of steel, the load-bearing section of the landing gear can be made relatively small which allows storage within minimum space in the belly of an aircraft. Steel is also used in wing root attachments and, in some older aircraft, wing carry-through boxes owing to their high stiffness, strength, toughness and fatigue resistance. For similar reasons, steel is used in slat tracks which form part of the leading edge of aircraft wings.
This chapter presents an overview of the metallurgical and mechanical properties of steels for aircraft structural applications. The field of steel metallurgy is large and complex, although this chapter gives a short, simplified description of the basic metallurgy of steel. The chapter describes the steels used in landing gear and other high-strength components, namely, the maraging steels, medium-carbon low-alloy steels, and stainless steels.
Iron is alloyed with carbon and other elements, forged and then heat-treated to produce high-strength steel. Pure iron is a soft metal having a yield strength under 100 MPa, but a tenfold or more increase in strength is achieved by the addition of carbon and other alloying elements followed by work hardening and heat treatment. By control of the alloy composition and thermomechanical processing, it is possible to produce steels with yield strengths ranging from 200 MPa to above 2000 MPa. Other important structural properties such as toughness, fatigue resistance and creep strength are also controlled by alloying and thermomechanical treatment.
There are many hundreds of grades of steel, although only a small number have the high strength and toughness required by aircraft structures. Steels contain less than about 1.5% carbon (together with other alloying elements), and are categorised, rather imprecisely, on the basis of their carbon and alloying element contents. Some of the most important groups of steels are:
Mild steels (also called low-carbon steels) contain less than about 0.2% carbon and are hardened mainly by cold working. Mild steels have moderate yield strength (200–300 MPa) and are therefore too soft for aircraft structural applications.
High-strength low-alloy (HSLA) steels contain a small amount of carbon (under 0.2%) like mild steels, and also contain small amounts of alloying elements such as copper, nickel, niobium, vanadium, chromium, molybdenum and zirconium. HSLA steels are referred to as micro-alloyed steels because they are alloyed at low concentrations compared with other types of steels. The yield strength of HSLA steels is 250–600 MPa and they are used in automobiles, trucks and bridges amongst other applications. The use of HSLA steels in aircraft is rare because of low specific strength and poor corrosion resistance.
Medium-carbon steels contain somewhere between 0.25 and 0.5% carbon and are hardened by thermomechanical treatment processes to strengths of 300–1000 MPa. This group of steels is used in the greatest quantities for structural applications, and they are found in motor cars, rail carriages, structural members of buildings and bridges, ships and offshore structures and, in small amounts, aircraft.
Medium-carbon low-alloy steels also contain somewhere between 0.25 and 0.5% carbon but have a higher concentration of alloying elements to increase hardness and high-temperature strength. They contain elements such as nickel, chromium, molybdenum, vanadium and cobalt. Examples are given in Table 11.1 At the higher alloy contents these steels are used as tool steels (e.g. tool bits, drills, blades and machine parts) which require hardness and wear resistance at high temperature. Strength levels up to 2000 MPa can be achieved. These steels are used in aircraft, typically for undercarriage parts.
Maraging steels also have a high alloy content, but with virtually no carbon (less than 0.03%). Alloying together with heat treatment (which, unlike that for the other steels described above, includes age-hardening) produces maraging steels with the unusual combination of high strength, ductility and fracture toughness. The strength of maraging steels is within the range of 1500–2300 MPa, which puts them amongst the strongest metallic materials. Maraging steel is used in heavily loaded aerospace components.
Stainless steels are corrosion resistant materials that contain a small amount of carbon (usually 0.08–0.25%) and a high concentration of chromium (12–26%) and sometimes nickel (up to about 22%). There are several classes of stainless steels with various mechanical properties, and their yield strength covers a wide range (200–2000 MPa). Precipitation-hardening (PH) stainless steels are used increasingly in aerospace applications, particularly where both high strength and excellent corrosion resistance is required.
Of the many steels available, it is the medium carbon low-alloy steels, maraging steels and PH stainless steels that are most used in aircraft. The alloy composition and mechanical properties of several steels (but not all the grades) used in aircraft are given in Table 11.1.
Steel is an allotropic material that can occur as several microstructural phases at room temperature depending on the alloy composition and heat treatment. The main phases of steel are called austenite, ferrite, pearlite, cementite, bainite and martensite. These phases have their own crystal structure and can all exist at room temperature. The high strength steels used in aircraft usually have a martensite microstructure, and steels with another microstructure are rarely used in highly-loaded aircraft components due to their lower strength. Although martensitic steels are the steel of choice for aircraft structural applications, it is still worth examining the other microstructural phases of steel to understand the reasons for selecting martensitic steels for aircraft.
The microstructural phases of steel can be understood from the phase diagram for iron–carbon, which is shown in Fig. 11.3. This diagram shows the equilibrium phases of iron that exist at various carbon contents and temperatures. This iron–carbon diagram is only valid when changes in temperature occur gradually (i.e. the steel is cooled or heated slowly) to allow sufficient time for the formation of stable (equilibrium) phases. The diagram is not valid when steel is cooled rapidly, which is a condition when metastable phases develop that are not represented in the diagram. Also, the phase diagram is only valid for the binary system of iron–carbon, and significant changes to the diagram may occur with the addition of alloying elements.
11.3 Iron–carbon phase diagram showing the microstructural phases during slow cooling (equilibrium conditions) of hypoeutectic and hypereutectic steels. reproduced with permission (from D. R. Askeland, The science and engineering of materials, Stanley Thornes (Publishers) Ltd., 1996)
The phase diagram for iron–carbon is complex, although there are just a few simple things we need to understand from this diagram. The different phase regions of the diagram indicate different stable microstructures that exist for the range of temperatures and carbon contents bounded by the phase lines. The main phases in Fig. 11.3 are austenite (also called γ-iron), ferrite (α-iron) and cementite (Fe3C).
Austenite is a materials science term for iron with a face-centred-cubic (fcc) crystal structure, and this phase occurs in the Fe–C system above the eutectoid temperature of 723 °C. The eutectoid temperature is the minimum temperature at which a material exists as a single solid solution phase or, in other words, when the alloying elements are completely soluble in the matrix phase. Alloying elements present in austenite are located at interstitial or substitutional sites in the iron fcc crystal structure, depending on their atomic size and valence number. Austenite becomes unstable when Fe–C is cooled below 723 °C, when it undergoes an allotropic transformation to ferrite and cementite (α-Fe + Fe3C) during slow cooling. However, the addition of certain alloying elements, such as nickel and manganese, can stabilise the austenite phase at room temperature. On the other hand, elements such as silicon, molybdenum and chromium can make austenite unstable and raise the eutectoid temperature.
One phase formed during slow cooling of steel from the austenite phase is ferrite, which is a solid solution of body-centred-cubic (bcc) iron containing interstitial elements such as carbon and substitutional elements such as manganese and nickel. Carbon has a powerful hardening effect on ferrite by solid solution strengthening, but only a very small concentration can be dissolved into the interstitial lattice sites. The maximum solubility of carbon is about 0.02% at 723 °C, and the soluble concentration drops with the temperature to 0.005% at room temperature. Despite the low solubility of carbon, its presence in iron increases the strength at room temperature by more than five times. This is because the carbon atoms, which are about twice the diameter of the interstitial gaps in the ferrite crystal, induce a high elastic lattice strain which causes solid solution hardening. However, ferrite is soft and ductile compared with other phases of steel (i.e. cementite, bainite, martensite).
When the carbon content of steel exceeds the solubility limit of ferrite, then the excess carbon reacts with the iron to form cementite (or iron carbide) during slow cooling from the austenite phase region. Cementite is a hard, brittle compound with an orthorhombic crystal structure having the composition Fe3C (Fig. 11.4). Upon slow cooling from the austenite phase, cementite and ferrite form as parallel plates into a two-phase microstructure called pearlite. The microstructure of pearlite is shown in Fig. 11.5, and it consists of thin plates of alternating phases of cementite and ferrite. The plates are very narrow and long, which gives pearlite its characteristic lamellae structure. Pearlite is essentially a composite microstructure consisting of cementite layers (which are hard and brittle) sandwiched between ferrite layers (which are soft and ductile).
The microstructure of steel at room temperature is controlled by the carbon content. Steel containing less than 0.005% C is composed entirely of ferrite, whereas above 6.67% C it is completely cementite. The Fe–C phase diagram shows that ferrite co-exists with cementite at room temperature over the carbon content range of 0.005 to 6.67%. However, the microstructure of the steel changes over this composition range even though both ferrite and cementite are always present. At a carbon content of about 0.8% the steel is eutectic, which means the microstructure consists of an equal amount of ferrite and cementite (i.e. 100% pearlite). A eutectic material has equal amounts of two (or more) phases and, for steel, it is composed of 50% ferrite and 50% cementite.
Steel containing more than 0.8% carbon is hypereutectic, which means the carbon content is greater than the eutectic composition. Hypereutectic steel contains grains of cementite and pearlite, with the volume fraction of cementite grains increasing with the carbon content above 0.8%. Steel is hypoeutectic when the carbon content is below 0.8%, and the microstructure consists of ferrite grains and pearlite grains (i.e. lamellae of ferrite and cementite within a single grain), as shown in Fig. 11.6. The volume fraction of pearlite grains increases, with a corresponding reduction in ferrite grains, when the carbon content is increased to the eutectic composition.
The majority of steels used in engineering structures, including aerospace, are hypoeutectic. Eutectic and hypereutectic steels lack sufficient ductility and toughness for most structural applications owing to the high volume fraction of brittle cementite. The mechanical properties of hypoeutectic steels are controlled by their carbon content. Figure 11.7 shows that raising the carbon content increases the strength properties and lowers the ductility and toughness. This is because the volume fraction of pearlite, which is hard and brittle, increases with the carbon content.
The microstructure and mechanical properties of the hypoeutectic steels used in aircraft structures is controlled by heat-treatment as well as the carbon and alloy contents. Ferrite and cementite form when hypoeutectic steel is cooled slowly from the austenite phase. Heat treatment processes that involve different cooling conditions can suppress the formation of ferrite and cementite and promote the formation of other phases which provide the steel with different mechanical properties such as higher hardness and strength. The principal transformational phases are bainite and martensite. Bainite is a metastable phase that exists in steel after controlled heat treatment. Bainite can form when steel is cooled from the austensite phase at an intermediate rate which is too rapid to allow the formation of ferrite and cementite but too slow to promote the formation of martensite. Bainite is generally harder and less ductile than ferrite and is tougher than martensite. Steels with a bainite microstructure are used in engineering structures, but not in aerospace applications.
When hot steel in the austenite-phase region is cooled rapidly it does not change into ferrite and cementite, but instead transforms to a metastable phase called martensite. Ferrite and cementite can only form from austenite when there is sufficient time for the iron and carbon atoms to move into the bcc crystal structure of α-iron and the orthorhombic structure of Fe3C. When the cooling rate is rapid then the iron and carbon atoms in austenite do not have time to form these phases, and instead the material undergoes a diffusionless transformation to martensite, which has the body centred tetragonal structure (bct) of iron as shown in Fig. 11.8. The bct martensite structure is essentially the bcc austenite structure distorted by interstitial carbon atoms into a tetragonal lattice. The carbon atoms remain dissolved in the crystal structure because there is insufficient time during quenching to form a carbon-rich second phase (Fe3C). As a result, martensite is supersaturated with interstitial carbon atoms, which cause severe distortion and induce high strains in the crystalline lattice. Other alloying elements are dissolved in interstitial or substitutional sites and this also increases distortion of the lattice. This distortion increases the strength of martensite steel, with the yield stress being as high as 1800–2300 MPa. This is much higher than the strength of a steel with the same composition but consisting of ferrite and cementite (250–600 MPa). Unfortunately, the distortion of the crystal lattice makes martensite brittle and prone to cracking at low strain, and therefore as-quenched martensitic steel is not suitable for aircraft structures that require high toughness and damage tolerance.
Martensitic steels are tempered after quenching to increase ductility and toughness. Tempering involves heating the steel to a temperature below 650 °C to release some of the carbon trapped at the interstitial sites of the bct crystal and thereby relax the lattice strain. The freed carbon atoms react with the iron to produce iron carbides within the tempered martensite microstructure. However, tempering at too high a temperature causes the martensite to transform into ferrite and pearlite, and produces a large loss in strength. Figure 11.9 shows the effect of tempering temperature on the mechanical properties of a medium-carbon steel. The strength decreases whereas the ductility improves with increasing temperature owing to the lattice strain relaxation of the martensite. Tempered martensitic steels are used extensively in high hardness engineering structures, including many aircraft components.
Maraging steel is used in aircraft, with applications including landing gear, helicopter undercarriages, slat tracks and rocket motor cases – applications which require high strength-to-weight material. Maraging steel offers an unusual combination of high tensile strength and high fracture toughness. Most high-strength steels have low toughness, and the higher their strength the lower their toughness. The rare combination of high strength and toughness found with maraging steel makes it well suited for safety-critical aircraft structures that require high strength and damage tolerance. Maraging steel is strong, tough, low-carbon martensitic steel which contains hard precipitate particles formed by thermal ageing. The term ‘maraging’ is derived from the combination of the words martensite and age-hardening.
Maraging steel contains an extremely low amount of carbon (0.03% maximum) and a large amount of nickel (17–19%) together with lesser amounts of cobalt (8–12%), molybdenum (3–5%), titanium (0.2–1.8%) and aluminium (0.1–0.15%). Maraging steel is essentially free of carbon, which distinguishes it from other types of steel. The carbon content is kept very low to avoid the formation of titanium carbide (TiC) precipitates, which severely reduce the impact strength, ductility and toughness when present in high concentration. Because of the high alloy content, especially the cobalt addition, maraging steel is expensive.
Maraging steel is produced by heating the steel in the austenite phase region (at about 850 °C), called austenitising, followed by slow cooling in air to form a martensitic microstructure. The slow cooling of hypoeutectic steel from the austenite phase usually results in the formation of ferrite and pearlite; rapid cooling by quenching in water or oil is often necessary to form martensite. However, martensite forms in maraging steel upon slow cooling owing to the high nickel content which suppresses the formation of ferrite and pearlite. The martensitic microstructure in as-cooled maraging steel is soft compared with the martensite formed in plain carbon steels by quenching. However, this softness is an advantage because it results in high ductility and toughness without the need for tempering. The softness also allows maraging steel to be machined into structural components, unlike hard martensitic steels that must be tempered before machining to avoid cracking.
After quenching, maraging steel undergoes a final stage of strengthening involving thermal ageing before being used in aircraft components. Maraging steel is heat-treated at 480–500 °C for several hours to form a fine dispersion of hard precipitates within the soft martensite matrix. The main types of precipitates are Ni3Mo, Ni3Ti, Ni3Al and Fe2Mo, which occur in a high volume fraction because of the high alloy content. Carbide precipitation is practically eliminated owing to the low carbon composition. Cobalt is an important alloying element in maraging steel and serves several functions. Cobalt is used to reduce the solubility limit of molybdenum and thereby increase the volume fraction of Mo-rich precipitates (e.g. Ni3Mo, Fe2Mo). Cobalt also assists in the uniform dispersion of precipitates through the martensite matrix. Cobalt accelerates the precipitation process and thereby shortens the ageing time to reach maximum hardness. Newer grades of maraging steel contain complex Ni50(X,Y,Z)50 precipitates, where X, Y and Z are solute elements such as Mo, Ti and Al.
The precipitates in maraging steel are effective at restricting the movement of dislocations, and thereby promote strengthening by the precipitation hardening process. Figure 11.10 shows the effect of ageing temperature on the tensile strength and ductility of maraging steel. As with other age-hardening aerospace alloys such as the 2XXX Al, 7XXX Al, β-Ti and α/β-Ti alloys, there is an optimum temperature and heating time to achieve maximum strength in maraging steel. When age-hardened in the optimum temperature range of 480–500 °C for several hours it is possible to achieve a yield strength of around 2000 MPa while retaining good ductility and toughness. Over-ageing causes a loss in strength owing to precipitate coarsening and decomposition of the martensite with a reversion back to austenite. The strength of maraging steels is much greater than that found with most other aerospace structural materials, which combined with ductility and toughness, makes them the material of choice for heavily loaded structures that require high levels of damage tolerance and which must occupy a small space on aircraft.
Medium-carbon low-alloy steel contains 0.25–0.5% carbon and moderate concentrations of other alloying elements such as manganese, nickel, chromium, vanadium and boron. These steels are quenched from singlephase austenite condition, and then tempered to the desired strength level. Aircraft applications for this steel include landing gear components, shafts and other parts. There are numerous grades of medium-carbon low-alloy steel, and the most important for aerospace are Type 4340, 300 M and H11 which cover the range from moderate-to-high strength, and provide impact toughness, creep strength and fatigue resistance.
Aircraft components are made using stainless steel when both high strength and corrosion resistance are equally important. Stainless steel contains a large amount of chromium (12–26%) which forms a corrosion-resistant oxide layer. Chromium at the steel surface reacts with oxygen in the air to form a thin layer of chromium oxide (Cr2O3) which protects the underlying material from corrosive gases and liquids. The Cr2O3 layer is a very thin and impervious barrier which protects the underlying steel substrate.
There are several types of stainless steel: ferritic, austenitic, martensitic, duplex and precipitation-hardened stainless steels; although only the latter is used for structural aerospace applications because of its high tensile strength and toughness. Precipitation-hardened stainless steel used in aircraft has a tempered martensitic microstructure for high strength with a chromium oxide surface layer for corrosion protection. Like maraging steel, precipitation-hardened stainless steel is age-hardened by solution treatment, quenching and then thermal ageing at 425–550 °C. The most well known precipitation-hardened stainless steel is 17–4 PH (ASTM Grade A693), which contains a trace amount of carbon (0.07% max) and large quantity of chromium (15–17.5%) with lesser amounts of nickel (3–5%), copper (3–5%) and other alloying elements (Mo, V, Nb). The nickel is used to improve toughness and the other alloying elements promote strengthening by the formation of precipitate particles.
An early application of stainless steel was in the skins of super- and hypersonic aircraft, where temperature effects are considerable. Stainless steel was used in the skin of the Bristol 188, which was a Mach 1.6 experimental aircraft built in the 1950s to investigate kinetic heating effects. Stainless steel was also used in the American X-15 rocket aircraft capable of speeds in excess of Mach 6 (Fig. 11.11). The skin of these aircraft reaches high temperature owing to frictional heating, and this would cause softening of aluminium if it were used. Steel is heat resistant to 400–450 °C without any significant reduction in mechanical performance. Stainless steel is no longer used in super- and hypersonic aircraft owing to the development of other heat-resistant structural materials which are much lighter, such as titanium. Stainless steel is currently used in engine pylons and several other structural components which are prone to stress corrosion damage, although its use is limited.
There are many types of steels used in structural engineering, but only maraging steel, medium-carbon low-alloy steel and precipitation-hardened stainless steel are used in aircraft structures. Steel is used in structures requiring high strength and toughness but where space is limited, such as landing gear, track slats, wing carry-through boxes, and wing root attachments. The percentage of the airframe mass that is constructed of steel is typically 5–8%.
Advantages of using steel in highly-loaded aircraft structures include high stiffness, strength, fatigue resistance and fracture toughness. Stainless steel provides the added advantage of corrosion resistance. Problems encountered with steels include high weight, potential hydrogen embrittlement, and (when stainless steel is not used) stress corrosion cracking and other forms of corrosion.
Maraging steel is used in aircraft components because it combines very high strength (about 2000 MPa) with good toughness, thereby providing high levels of damage tolerance. These properties are achieved by the microstructure consisting of a ductile martensite matrix strengthened by hard precipitate particles formed by thermal ageing.
Medium-carbon low-alloy steel also combines high strength and toughness owing to a microstructure of tempered martensite containing hard carbide precipitates. This type of steel has similar mechanical properties to maraging steel, but is more susceptible to stress corrosion cracking. It is used widely in aerospace structural components.
Precipitation-hardened stainless steel is characterised by high strength and excellent corrosion resistance, and is used in aircraft structures prone to stress corrosion. The microstructure consists of martensite and precipitation particles. High corrosion resistance occurs by the formation of a chromium oxide surface layer which is impervious to corrosive gases and fluids.
Bainite: Metastable phase of steel that occurs as a range of microstructures comprising fine carbide precipitates in an acicular (needle-shaped) ferrite matrix. The precise structure is a function of the temperature at which the carbides precipitate.
Martensite: Microstructural phase of steel formed by a diffusionless shear mechanism when the material is cooled at a fast rate from the austensite phase. The fast cooling rate retains carbon in supersaturated solid solution of body-centred-tetragonal (bct) iron.