1. Carbon steels are iron alloys with varying amounts of carbon. Low-carbon steel is ductile while high-carbon steel is strong. 2. Stainless steels contain at least 11% chromium which allows a protective oxide layer to form, increasing corrosion resistance. Common types are austenitic, ferritic, and martensitic stainless steels. 3. Heat treatments can be used to alter the properties of cast irons and steels. Annealing softens materials while hardening increases strength and wear resistance. Cryogenic treatment further enhances properties.

1. AIRCRAFT MATERIALS &
MANUFACTURING
BY
Dr. R. KOUSIK KUMAAR. B.E., M.Tech., Ph.D.,
ASSISTANT PROFESSOR
DEPARTMENT OF AERONAUTICAL ENGINEERING,
NMIT, BENGALURU - 64

3. Carbon Steel
Carbon steels are basically just mixtures of iron and carbon. They may contain small amounts of other elements, but
carbon is the primary alloying ingredient. The effect of adding carbon is an increase in strength and hardness.
Most carbon steels are plain carbon steels, of which there are several types.
Low-Carbon Steel
Low-carbon steel has less than about 0.30% carbon. It is characterized by low strength but high ductility. Some
strengthening can be achieved through cold working, but it does not respond well to heat treatment. Low-carbon
steel is very weldable and is inexpensive to produce. Common uses for low-carbon steel include wire, structural
shapes, machine parts, and sheet metal.
Medium-Carbon Steel
Medium-carbon steel contains between about 0.30% to 0.70% carbon. It can be heat treated to increase strength,
especially with the higher carbon contents. Medium-carbon steel is frequently used for axles, gears, shafts, and
machine parts.
High-Carbon Steel
High-carbon steel contains between about 0.70% to 1.40% carbon. It has high strength but low ductility. Common
uses include drills, cutting tools, knives, and springs.

4. Low-Alloy Steel
Low-alloy steels, also commonly called alloy steels, contain less than about 8% total alloying
ingredients. Low-alloy steels are typically stronger than carbon steels and have better corrosion
resistance.
Some low-alloy steels are designated as high-strength low-alloy (HSLA) steels. What sets HSLA
steels apart from other low-alloy steels is that they are designed to achieve specific mechanical
properties rather than to meet a specific chemical composition.
Tool Steel
Tool steels are primarily used to make tooling for use in manufacturing, for example cutting tools,
drill bits, punches, dies, and chisels. Alloying elements are typically chosen to optimize hardness,
wear resistance, and toughness.

5. Stainless Steel
Stainless steels have good corrosion resistance, mostly due to the addition of chromium as an
alloying ingredient. Stainless steels have a chromium composition of at least 11%. Passivation
occurs with chromium content at or above 12%, in which case a protective inert film of chromic
oxide forms over the material and prevents oxidation. The corrosion resistance of stainless steel is a
result of this passivation.
The table below shows the typical compositions of stainless steels:

6. Austenitic Stainless Steel
Austenitic stainless steel is the most common form of stainless steel. It has the highest general
corrosion resistance among stainless steels. It is also the most weldable of the stainless steels due
to its low carbon content. It can only be strengthened through cold work. Austenitic stainless steels
are generally more expensive than other stainless steels due to nickel content. Austenitic stainless
steels are not magnetic, although ferritic and martensitic stainless steels are. Common applications
include fasteners, pressure vessels, and piping.

7. Ferritic Stainless Steel
Ferritic stainless steel has high chromium content and medium carbon content. It has good
corrosion resistance rather than high strength. It generally cannot be strengthened through heat
treatment, and can only be strengthened via cold work.

8. Martensitic Stainless Steel
Martensitic stainless steel has high carbon content (up to 2%) and low chromium content. This
higher carbon content is the primary difference between ferritic and martensitic stainless steels.
Due to the high carbon content, it is difficult to weld. It can be strengthened through heat
treatment. Common applications include cutlery and surgical instruments.

9. Duplex Stainless Steel
Duplex stainless steel contains both austenitic and ferritic phases. It can have up to twice the
strength of austenitic stainless steel. It also has a high toughness, corrosion resistance, and wear
resistance. Duplex stainless steel is generally as weldable as austenitic, but it has a temperature
limit.

10. Precipitation-Hardenable Stainless Steel
Precipitation-hardenable stainless steel can be strengthened through precipitation
hardening, which is an age hardening process. These materials have high strength
as well as high resistance to corrosion and temperature.

11. Solid solution
A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds
of atoms in solid state and have a single crystal structure. Many examples can be found in
metallurgy, geology, and solid-state chemistry. The word "solution" is used to describe the intimate
mixing of components at the atomic level and distinguishes these homogeneous materials from
physical mixtures of components. Two terms are mainly associated with solid solutions – solvents
and solutes, depending on the relative abundance of the atomic species.
In metallurgy, solid solution strengthening is a type of alloying that can be used to
improve the strength of a pure metal. The technique works by adding atoms of
one element (the alloying element) to the crystalline lattice of another element
(the base metal), forming a solid solution.

12. Iron
Iron is a chemical element and a metal. It is the most common chemical element on Earth (by
mass), and the most widely used metal. It makes up much of the Earth's core, and is the fourth
most common element in the Earth's crust. Its atomic number is 26, because each atom has 26
protons.
The metal is used a lot because it is strong and cheap. Iron is the main ingredient used to make
steel. Raw iron is magnetic (attracted to magnets), and its compound magnetite is permanently
magnetic.
In some regions iron was used around 1200 BCE. That event is considered the transition from
bronze age to iron age.

13. Iron
Physical properties
• Iron is a grey, silvery metal. It is magnetic, though different allotropes of iron have different
magnetic qualities. Iron is easily found, mined and smelted, which is why it is so useful. Pure iron
is soft and very malleable.
• There are different types of iron. Cast iron is iron made by the way described above in the
article. It is hard and brittle. It is used to make things like storm drain covers, manhole covers,
and engine blocks (the main part of an engine).
• Steel is the most common form of iron. Steels come in several forms. Mild steel is steel with a
low percentage of carbon. It is soft and easily bent, but it does not crack easily. It is used for
nails and wires. Carbon steel is harder but more brittle. It is used in tools.
• Wrought iron is easily shaped and used to make fences and chains.
• Very pure iron is soft, and can rust (oxidize) easily. It is also fairly reactive.

14. Iron
Cast Iron
Cast iron is a ferrous alloy containing high levels of carbon, generally greater than 2%. The carbon
present in the cast iron can take the form of graphite or carbide. Cast irons have a low melting
temperature which makes them well suited to casting.
Gray Cast Iron
Gray cast iron is the most common type. The carbon is in the form of graphite flakes. Gray cast iron
is a brittle material, and its compressive strength is much higher than its tensile strength. The
fracture surface of gray cast iron has a gray color, which is how it got its name.

15. Iron
Ductile Cast Iron (Nodular Cast Iron)
The addition of magnesium to gray cast iron improves the ductility of the material. The resulting
material is called nodular cast iron because the magnesium causes the graphite flakes to form into
spherical nodules. It is also called ductile cast iron. Nodular cast iron has good strength, ductility,
and machinability. Common uses include crankshafts, gears, pump bodies, valves, and machine
parts.
White Cast Iron
White cast iron has carbon in the form of carbide, which makes the material hard, brittle, and
difficult to machine. White cast iron is primarily used for wear-resisting components as well as for
the production of malleable cast iron.
Malleable Cast Iron
Malleable cast iron is produced by heat treating white cast iron. The heat treatment improves the
ductility of the material while maintaining its high strength.

16. Heat Treatment
Heat treatments for cast irons involve stress relief, annealing, normalizing, and hardening.
Stress Relief
Stress relief is required due to internal (residual) stress in the castings from cooling a
complex or intricate shape or radical changes in the cross-sectional area. In stress relief,
the time-temperature relationship plays a significant role. Higher temperatures will affect
mechanical properties and often require the use of a protective atmosphere to avoid
oxidation.
Annealing
The annealing process is applied to castings primarily to improve machinability by
softening the material. In the case of ductile iron, increases in ductility and impact
resistance often result. Various heating and cooling cycles can be used.

17. Heat Treatment
Normalizing
Iron castings are commonly normalized to obtain a microstructure of fine pearlite. The result is
increased tensile strength and wear resistance. Normalized structures respond well to induction
hardening. Cooling rates vary from still-air to fan-assisted cooling for large castings. Tempering is
done if the final hardness is too high.
Hardening (Quench and Temper)
To avoid distortion, warpage, overheating (“burning”), or quench cracking, metallurgists carefully
select and control process parameters. The lower-critical temperature for cast irons can be
calculated by sample analysis.

18. Heat Treatment
Cryogenic Processing
Many cast irons are cryogenically treated (-195˚C/-320˚F) to stabilize the
microstructure and enhance properties (e.g., dampening and wear characteristics).
A typical cycle consists of slowly reducing the temperature for 6-8 hours,
stabilizing at temperature (typically 8-12 hours), and slowly raising the
temperature back to room temperature.

19. Heat Treatment

20. Binary phase diagrams
A binary phase diagram shows the phases formed in differing mixtures of two elements over a
range of temperatures. Compositions run from 100% Element A on the left of the diagram, through
all possible mixtures, to 100% Element B on the right. The composition of an alloy is given in the
form A - x% B.

22. Annealing
• The term annealing refers to a heat treatment in which a material is exposed to an elevated
temperature for an extended time period and then slowly cooled. Typically, annealing is
carried out to (1) relieve stresses; (2) increase softness, ductility, and toughness; and/or (3)
produce a specific microstructure. A variety of annealing heat treatments are possible; they
are characterized by the changes that are induced, which often are microstructural and are
responsible for the alteration of the mechanical properties.
• Any annealing process consists of three stages: (1) heating to the desired temperature, (2)
holding or “soaking” at that temperature, and (3) cooling, usually to room temperature.
Time is an important parameter in these procedures. During heating and cooling,
temperature gradients exist between the outside and interior portions of the piece; their
magnitudes depend on the size and geometry of the piece.

23. PROCESS ANNEALING
• Process annealing is a heat treatment that is used to negate the effects of cold work—that is,
to soften and increase the ductility of a previously strain-hardened metal. It is commonly
used during fabrication procedures that require extensive plastic deformation, to allow a
continuation of deformation without fracture or excessive energy consumption.
• The iron–iron carbide phase diagram in the vicinity of the eutectoid. The horizontal line at the
eutectoid temperature, conventionally labelled A1, is termed the lower critical temperature,
below which, under equilibrium conditions, all austenite has transformed into ferrite and
cementite phases. The phase boundaries denoted as A3 and Acm represent the upper critical
temperature lines for hypoeutectoid and hypereutectoid steels, respectively. For
temperatures and compositions above these boundaries, only the austenite phase
prevails.

24. PROCESS ANNEALING

25. Normalizing
Steels that have been plastically deformed by, for example, a rolling operation, consist of
grains of pearlite (and most likely a proeutectoid phase), which are irregularly shaped
and relatively large and vary substantially in size. An annealing heat treatment called
normalizing is used to refine the grains (i.e., to decrease the average grain size) and
produce a more uniform and desirable size distribution; fine-grained pearlitic steels
are tougher than coarse-grained ones. Normalizing is accomplished by heating at least
55ºC (100ºF) above the upper critical temperature—that is, above A3 for
compositions less than the eutectoid (0.76 wt% C), and above Acm for compositions
greater than the eutectoid. After sufficient time has been allowed for the alloy to
completely transform to austenite—a procedure termed austenitizing—the treatment is
terminated by cooling in air.

26. Full Anneal
A heat treatment known as full annealing is often used in low- and medium-carbon
steels that will be machined or will experience extensive plastic deformation during a
forming operation. In general, the alloy is treated by heating to a temperature of about
50ºC above the A3 line (to form austenite) for compositions less than the eutectoid,
or, for compositions in excess of the eutectoid, 50ºC above the A1 line (to form
austenite and Fe3C phases). The alloy is then furnace cooled—that is, the heat-treating
furnace is turned off, and both furnace and steel cool to room temperature at the
same rate, which takes several hours. The microstructural product of this anneal is
coarse pearlite (in addition to any proeutectoid phase) that is relatively soft and ductile.
The full-anneal cooling procedure is time consuming; however, a microstructure
having small grains and a uniform grain structure results.

27. Spheroidizing
Medium- and high-carbon steels having a microstructure containing even coarse pearlite may
still be too hard to machine or plastically deform conveniently. These steels, and in fact any
steel, may be heat-treated or annealed to develop the spheroidite structure. Spheroidized
steels have a maximum softness and ductility and are easily machined or deformed. The
spheroidizing heat treatment, during which there is a coalescence of the Fe3C to form the
spheroid particles, can take place by several methods, as follows:
• Heating the alloy at a temperature just below the eutectoid [line A1 in Figure 31, or at about
700ºC (1300ºF)] in the α+Fe3C region of the phase diagram. If the precursor microstructure
contains pearlite, spheroidizing times will typically range between 15 and 25 h.
• Heating to a temperature just above the eutectoid temperature and then either cooling very
slowly in the furnace or holding at a temperature just below the eutectoid temperature.
• Heating and cooling alternately within about ± 50ºC of the A1 line.

28. Hardening
Hardening is the first of two operations required for the development of high-strength steels by heat
treatment. Hardening consists of heating above 𝐴𝐶3, soaking at that temperature until the mass is
uniformly heated, and then quenching in brine, water, or oil. This treatment produces a fine grain,
maximum hardness and- tensile strength, minimum ductility and internal strains. In this
condition, the material is too hard and brittle for practical use.
Heating is conducted as little above 𝐴𝐶3 as is practical, in order to reduce warping and the
possibility of cracking when the material is quenched. On the other hand, large objects are heated
to the upper limit of the hardening range in order to assure thorough heating. For the materials
and sections used in aircraft work, quenching in oil is invariably the method employed. The heat
absorption of oil is slower than that of water or brine, and consequently the cooling operation is
gentler. Less warping and cracking occurs and sufficient hardness is obtained.

29. Hardening
Quench cracking is a result of non-uniform or too rapid cooling of the steel. 'The
transition from austenite to martensite results in an increase of volume. When a
piece is quenched, the external surface will cool rapidly and become a hard,
brittle martensitic shell. As the internal austenite cools and becomes martensite
it increases in volume and internal stresses are set up which may crack the
earlier-formed outer shell.

30. NONFERROUS ALLOYS

31. HEAT TREATMENT OF ALUMINIUM ALLOYS
There are two types of heat treatment applicable to aluminum alloys. One is called
solution heat treatment, and the other is known as precipitation heat
treatment. Some alloys, such as 2017 and 2024 develop their full properties as a
result of solution heat treatment followed by about 4 days aging at room
temperature. Other alloys, such as 2014 and 7075, require both heat
treatments.

32. HEAT TREATMENT OF ALUMINIUM ALLOYS
Solution heat treatment is so named because during this treatment the
alloying constituents enter into solid solution in the aluminum. It
has been found that these alloying elements which increase the
strength and hardness are more soluble in solid aluminum at high
temperatures than at low. After the metal is held at a high
temperature for a sufficient time to complete the solution, it is
quenched rapidly in cold water to retain this condition.

33. HEAT TREATMENT OF ALUMINIUM ALLOYS
Precipitation heat treatment consists of aging material previously subjected to
solution heat treatment by holding it at an elevated temperature for quite a
long period of time. During this treatment, a portion of the alloying constituents
in solid solution precipitate out. This precipitation occurs at ordinary room
temperatures in the case of 2017 and 2024 material. The precipitate is in the form
of extremely fine particles which, due to their "keying" action, greatly increase the
strength. The "natural aging" of 2017 and 2024 material at room
temperatures is 90% to 98% complete after 24 hours, and fully complete
after four days. Alloy 2024 develops greater strength than 2017 immediately after
quenching, ages more rapidly, and is considerably less workable.

34. HEAT TREATMENT OF ALUMINIUM ALLOYS

35. HEAT TREATMENT OF MAGNESIUM ALLOYS
Solution heat treatment improves strength and results in maximum
toughness and shock resistance. Precipitation heat treatment subsequent to
solution treatment gives maximum hardness and yield strength, but with
some sacrifice of toughness. As applied to castings, artificial aging without
prior solution treatment or annealing is a stress-relieving treatment that also
somewhat increases tensile properties. Annealing of wrought products lowers
tensile properties considerably and increases ductility, thereby facilitating
some types of fabrication.

36. HEAT TREATMENT OF MAGNESIUM ALLOYS
Modifications of these basic treatments have been developed for specific alloys,
to obtain the most desirable combinations of properties.
The basic temper designations for magnesium alloys, the same as those
applied to aluminum alloys, are used.

37. HEAT TREATMENT OF COPPER ALLOYS
Copper-Nickel-Phosphorus Alloys. Alloys containing about 1% nickel and about
0.25% phosphorus, typified by C19000, are used for a wide variety of small parts
requiring, high strength, such as springs, clips, electrical connectors and
fasteners. C19000 is solution treated at 700° to 800°C. If the metal must be
softened between cold working steps prior to aging, it may be satisfactorily
annealed at temperatures as low as 620°C. Rapid cooling from the annealing
temperature is not necessary. For aging, the material is held at 425° to 475°C for
1 to 3 h.

38. HEAT TREATMENT OF COPPER ALLOYS
Chromium coppers. Chromium coppers containing about 1% Cr, such as C18200,
C18400 and C18500, are solution treated at 950° to 1010°C and rapidly
quenched. Solution treating usually is done in molten salt, but may be done in a
controlled-atmosphere furnace to prevent surface scaling and internal oxidation.
Solution treated chromium copper is aged at 400° to 500°C for several hours to
produce the desired mechanical and physical properties. A typical aging cycle is
455°C for 4 h or more.

39. HEAT TREATMENT OF TITANIUM ALLOYS
Titanium and Titanium Alloys are heat treated in order to:
• Reduce residual stresses developed during fabrication (stress relieving)
• Produce an optimum combination of ductility, machinability, and
dimensional and structural stability (annealing)
• Increase strength (solution treating and aging)
• Optimize special properties such as fracture toughness, fatigue strength,
and high-temperature creep strength

40. HEAT TREATMENT OF TITANIUM ALLOYS
Various types of annealing treatments (single, duplex, (beta), and
recrystallization annealing, for example), and solution treating and aging
treatments, are imposed to achieve selected mechanical properties. Stress
relieving and annealing may be employed to prevent preferential chemical
attack in some corrosive environments, to prevent distortion (a stabilization
treatment) and to condition the metal for subsequent forming and fabricating
operations.