METALS
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.
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.