Types of Metal Alloys

FERROUS ALLOYS

those in which iron is the prime constituent—are produced in larger quantities than any

other metal type. They are especially important as engineering construction materials.

Their widespread use is accounted for by three factors:

⚫ Iron containing compounds exist in abundant quantities within the Earth’s crust

⚫ metallic iron and steel alloys may be produced using relatively economical extraction,

refining, alloying, and fabrication techniques

⚫ ferrous alloys are extremely versatile, in that they may be tailored to have a wide range

of mechanical and physical properties

Steels

Steels are iron–carbon alloys that may contain appreciable concentrations of other alloying

elements; there are thousands of alloys that have different compositions and/or heat

treatments.

The mechanical properties are sensitive to the content of carbon, which is normally less

than 1.0 wt%. Some of the more common steels are classified according to carbon

concentration into low-, medium-, and high-carbon types.Subclasses also exist within each

group according to the concentration of other alloying elements.

Plain carbon steels contain only residual concentrations of impurities other than carbon and

a little manganese. For alloy steels, more alloying elements are intentionally added in

specific concentrations.

Low-Carbon Steels

Of the different steels, those produced in the greatest quantities fall within the low carbon

classification. These generally contain less than about 0.25 wt% C and are unresponsive to

heat treatments intended to form martensite; strengthening is accomplished by cold work.

Microstructures consist of ferrite and pearlite constituents. As a consequence, these alloys

are relatively soft and weak but have outstanding ductility and toughness; in addition, they

are machinable, weldable, and, of all steels, are the least expensive to produce.

Typical applications include automobile body components, structural shapes (e.g., I-

beams, channel and angle iron), and sheets that are used in pipelines, buildings, bridges,

and tin cans. They typically have a yield strength of 275 MPa (40,000 psi), tensile strengths

between 415 and 550 MPa (60,000 and 80,000 psi)

Another group of low-carbon alloys are the high-strength, low-alloy (HSLA) steels. They

contain other alloying elements such as copper, vanadium, nickel, and molybdenum in

combined concentrations as high as 10 wt%, and they possess higher strengths than the

plain low-carbon steels. Most may be strengthened by heat treatment, giving tensile strengths in excess of 480 MPa (70,000 psi); in addition, they are ductile, formable, and machinable.

Medium-Carbon Steels

The medium-carbon steels have carbon concentrations between about 0.25 and 0.60 wt%.

These alloys may be heat-treated by austenitizing, quenching, and then tempering to

improve their mechanical properties.

They are most often utilized in the tempered condition, having microstructures of tempered

martensite. The plain medium-carbon steels have low hardened abilities and can be

successfully heat-treated only in very thin sections and with very rapid quenching rates.

Additions of chromium, nickel, and molybdenum improve the capacity of these alloys to

be heat-treated giving rise to a variety of strength–ductility combinations.

These heat-treated alloys are stronger than the low-carbon steels, but at a sacrifice of

ductility and toughness. Applications include railway wheels and tracks, gears, crankshafts,

and other machine parts and high-strength structural components calling for a combination

of high strength, wear resistance, and toughness.

High-Carbon Steels

The high-carbon steels, normally having carbon contents between 0.60 and 1.4 wt%,

are the hardest, strongest, and yet least ductile of the carbon steels. They are almost

always used in a hardened and tempered condition and, as such, are especially wear

resistant and capable of holding a sharp cutting edge.

The tool and die steels are high-carbon alloys, usually containing chromium, vanadium,

tungsten, and molybdenum. These alloying elements combine with carbon to form very

hard and wear-resistant carbide compounds (e.g., Cr23C6, V4C3, and WC).

Stainless Steels

The stainless steels are highly resistant to corrosion (rusting) in a variety of environments,

especially the ambient atmosphere. Their predominant alloying element is chromium; a

concentration of at least 11 wt% Cr is required. Corrosion resistance may also be enhanced

by nickel and molybdenum additions.

Stainless steels are divided into three classes on the basis of the predominant phase

constituent of the microstructure—martensitic, ferritic, or austenitic. Table 5.0 lists several

stainless steels by class, along with composition, typical mechanical properties, and

applications.

Martensitic stainless steels are capable of being heat-treated in such a way that martensite

is the prime microconstituent. Additions of alloying elements in significant concentrations

produce dramatic alterations in the iron–iron carbide phase diagram

For austenitic stainless steels, the austenite (or 𝛾) phase field is extended to room

temperature. Ferritic stainless steels are composed of the 𝛼-ferrite (BCC) phase.

Austenitic and ferritic stainless steels are hardened and strengthened by cold work because

they are not heat-treatable. The austenitic stainless steels are the most corrosion resistant

because of the high chromium contents and also the nickel additions; they are produced in

the largest quantities. Both martensitic and ferritic stainless steels are magnetic; the

austenitic stainlesses are not.

Cast Irons

Generically, cast irons are a class of ferrous alloys with carbon contents above 2.14 wt%;

in practice, however, most cast irons contain between 3.0 and 4.5 wt% C and, in addition,

other alloying elements.

A reexamination of the iron–iron carbide phase diagram reveals that alloys within this

composition range become completely liquid at temperatures between approximately

1150°C and 1300°C (2100°F and 2350°F), which is considerably lower than for steels.

Thus, they are easily melted and amenable to casting. Furthermore, some cast irons are

very brittle, and casting is the most convenient fabrication technique.

For most cast irons, the carbon exists as graphite, and both microstructure and mechanical

behavior depend on composition and heat treatment. The most common cast iron types are

gray, nodular, white, malleable, and compacted graphite.

Gray Iron

The carbon and silicon contents of gray cast irons vary between 2.5 and 4.0 wt% and 1.0

and 3.0 wt%, respectively.

Mechanically, gray iron is comparatively weak and brittle in tension as a consequence of

its microstructure; the tips of the graphite flakes are sharp and pointed and may serve as

points of stress concentration when an external tensile stress is applied.

Ductile (or Nodular) Iron

Adding a small amount of magnesium and/or cerium to the gray iron before casting

produces a distinctly different microstructure and set of mechanical properties. Graphitei stll forms, but as nodules or spherelike particles instead of flakes. The resulting alloy is called ductile or nodular iron.

White Iron and Malleable Iron

For low-silicon cast irons (containing less than 1.0 wt% Si) and rapid cooling rates,

most of the carbon exists as cementite instead of graphite, A fracture surface of this alloy

has a white appearance, and thus it is termed white cast iron.

NONFERROUS ALLOYS

Steel and other ferrous alloys are consumed in exceedingly large quantities because they have such a wide range of mechanical properties, may be fabricated with relative ease, and areeconomical to produce. However, they have some distinct limitations, chiefly

⚫ a relatively high density,

⚫ a comparatively low electrical conductivity, and

Fabrication of Metals

Metal fabrication techniques are normally preceded by refining, alloying, and often

heat-treating processes that produce alloys with the desired characteristics.

FORMING OPERATIONS

Forming operations are those in which the shape of a metal piece is changed by plastic

deformation. The deformation must be induced by an external force or stress, the

magnitude of which must exceed the yield strength of the material.

When deformation is achieved at a temperature above that at which recrystallization

occurs, the process is termed hot working, otherwise, it is cold working. With most of the

forming techniques, both hot- and cold-working procedures are possible. For hot-working

operations, large deformations are possible, which may be successively repeated because

the metal remains soft and ductile. Also, deformation energy requirements are less than for

cold working. However, most metals experience some surface oxidation, which results in

material loss and a poor final surface finish. Cold working produces an increase in strength

with the attendant decrease in ductility because the metal strain hardens; advantages over

hot working include a higher quality surface finish, better mechanical properties and a

greater variety of them, and closer dimensional control of the finished piece.

Forging

Forging is mechanically working or deforming a single piece of a usually hot metal; this

may be accomplished by the application of successive blows or by continuous squeezing.

Rolling

Rolling, the most widely used deformation process, consists of passing a piece of metal

between two rolls; a reduction in thickness results from compressive stresses exerted by

the rolls. Cold rolling may be used in the production of sheet, strip, and foil with a high-

quality surface finish. Circular shapes, as well as I-beams and railroad rails, are fabricated

using grooved rolls.

Extrusion

For extrusion, a bar of metal is forced through a die orifice by a compressive force that is

applied to a ram; the extruded piece that emerges has the desired shape and a reduced

complicated cross-sectional geometry; seamless tubing may also be extruded.

Drawing

Drawing is the pulling of a metal piece through a die having a tapered bore by means of a

tensile force that is applied on the exit side. A reduction in cross section results, with a

corresponding increase in length. The total drawing operation may consist of a number of

dies in a series sequence. Rod, wire, and tubing products are commonly fabricated in this

way.

CASTING

Casting is a fabrication process in which a completely molten metal is poured into a mold

cavity having the desired shape; upon solidification, the metal assumes the shape of the

mold but experiences some shrinkage.

Casting techniques are employed when

(1) the finished shape is so large or complicated that make any other method would be

impractical;

(2) a particular alloy is so low in ductility that forming by either hot or cold working would

be difficult; and

(3) in comparison to other fabrication processes, casting is the most economical. The final

step in the refining of even ductile metals may involve a casting process.

Sand Casting

With sand casting, probably the most common method, ordinary sand is used as the mold

material. A two-piece mold is formed by packing sand around a pattern that has the shape

of the intended casting. A gating system is usually incorporated into the mold to expedite

the flow of molten metal into the cavity and to minimize internal casting defects. Sand-cast

parts include automotive cylinder blocks, fire hydrants, and large pipe fittings.

Die Casting

In die casting, the liquid metal is forced into a mold under pressure and at a relatively high

velocity and allowed to solidify with the pressure maintained. A two-piece permanent steel

mold or die is employed; when clamped together, the two pieces form the desired shape.

When the metal has solidified completely, the die pieces are opened and the cast piece is

ejected.

Investment Casting

For investment (sometimes called lost-wax) casting, the pattern is made from a wax or

plastic that has a low melting temperature. Around the pattern a fluid slurry is poured that

sets up to form a solid mold or investment; plaster of Paris is usually used. The mold is

then heated, such that the pattern melts and is burned out, leaving behind a mold cavity

having the desired shape.

Lost-Foam Casting

A variation of investment casting is lost-foam (or expendable pattern) casting. Here, the

expendable pattern is a foam that can be formed by compressing polystyrene beads into the

desired shape and then bonding them together by heating.

Continuous Casting

These casting and rolling steps may be combined by a continuous casting (sometimes

termed strand casting) process. Using this technique, the refined and molten metal is cast

directly into a continuous strand that may have either arectangular or circular cross section;

solidification occurs in a water-cooled die having the desired cross-sectional geometry.

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 55°C (100°F) above the upper critical

temperature.

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.

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.

HEAT TREATMENT OF STEELS

Conventional heat treatment procedures for producing martensitic steels typically involve

continuous and rapid cooling of an austenitized specimen in some type of quenching

medium, such as water, oil, or air.

The optimum properties of a steel that has been quenched and then tempered can be

realized only if, during the quenching heat treatment, the specimen has been converted to

a high content of martensite; the formation of any pearlite and/or bainite will result in other

than the best combination of mechanical characteristics. During the quenching treatment,

it is impossible to cool the specimen at a uniform rate throughout—the surface always cools

more rapidly than interior regions.

The successful heat treating of steels to produce a predominantly martensitic

microstructure throughout the cross section depends mainly on three factors:

(1) the composition of the alloy,

(2) the type and character of the quenching medium,

(3) the size and shape of the specimen. The influence of each of these factors is now

addressed.