MODULE 6 Flashcards
material that is typically hard, opaque, shiny, and has
good electrical and thermal conductivity.
metal
they can be
hammered or pressed permanently out of shape without breaking or cracking
malleable
able to be fused or melted
fusible
able to be drawn out into a thin wire
ductile
Metal alloys, by virtue of composition, are often grouped into two classes:
ferrous and
nonferrous.
iron is the principal constituent
Ferrous alloys
not iron based
nonferrous are alloys
They are especially important as engineering construction
materials.
FERROUS ALLOYS
Their widespread use is accounted for by three factors:
(1) iron-containing compounds exist in abundant quantities within the earth’s crust;
(2) metallic iron and steel alloys may be produced using relatively economical extraction,
refining, alloying, and fabrication techniques; and
(3) ferrous alloys are extremely versatile; in that they may be tailored to have a wide range
of mechanical and physical properties.
The principal disadvantage of many ferrous alloys is
susceptibility to corrosion.
iron–carbon alloys that may contain appreciable concentrations of other
alloying elements;
Steels
Some of the more common steels are classified according to carbon concentration namely:
low, medium, and high-carbon types.
contain less than 0.25%C.
Low-carbon steels
it is not very responsive to heat treatments
and strengthening is accomplished by cold work.
Low-carbon steels
It is soft, weak, tough, ductile, machinable,
weldable and not expensive.
Low-carbon steels
They typically have a yield strength of 275 MPa (40,000 psi),
Low-carbon steels
tensile
strengths between 415 and 550 MPa (60,000 and 80,000 psi),
Low-carbon steels
ductility of 25%EL
Low-carbon steels
25%EL.Typical
applications include automobile body components, structural shapes (I-beams, channel and angle
iron), and sheets that are used in pipelines, buildings, bridges, and tin cans.
Low-carbon steels
It can also be seen that the
composition of steel is mainly
carbon ang manganese
It contains alloying elements such as copper, vanadium, nickel, and molybdenum in
combined concentrations of >10 wt%.
High-strength, Low-alloy (HSLA) steels
It is stronger than plain low-C steels. Most
High-strength, Low-alloy (HSLA) steels
Most may be
strengthened by heat treatment, giving tensile strengths in excess of 480 MPa (70,000 psi).
High-strength, Low-alloy (HSLA) steels
They
are ductile, formable and machinable.
High-strength, Low-alloy (HSLA) steels
are more resistant
to corrosion than the plain carbon steels
the HSLA steels
contain 0.25-0.60 wt.% of carbon.
Medium-Carbon Steels
It is
the hardest, strongest, and yet least ductile of the carbon steels. They
High-Carbon Steels
They are almost always used in
a hardened and tempered condition, wear resistant and capable of holding a sharp cutting edge.
High-Carbon Steels
The tool and die steels are high-carbon alloys, usually
containing chromium, vanadium,
tungsten, and molybdenum.
These steels are used as cutting tools and dies for forming and shaping materials, as well as in knives, razors, hacksaw
blades, springs, and high-strength wire.
High-Carbon Steels
The stainless steels are highly resistant to corrosion (rusting) in a variety of environments,
especially the ambient atmosphere.
Stainless steels
Their predominant alloying element is chromium;
Stainless steels
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.
are capable of being heat treated in such a way that martensite
is the prime microconstituent.
Martensitic stainless steels
the austenite (or ɣ) phase field is extended to room
temperature.
austenitic stainless steels
composed of the α-ferrite (BCC) phase. Austenitic
Ferritic stainless steels
are hardened and strengthened by cold work because they are not heat treatable.
Austenitic and ferritic stainless steels
Theoretically, it contains > 2.14 wt.% of carbon.
Cast Irons
Usually contains between 3.0-4.5 wt.% C, hence it is very brittle.
Cast Irons
They become liquid easily between 1150 0C and 1300 0C.
Cast Irons
They are easily
melted and amenable to casting.
Cast Irons
It is Inexpensive, machinable and wear resistant.
Cast Irons
The most
common cast iron types are
gray, nodular, white, malleable, and compacted graphite
The carbon and silicon contents of gray cast irons vary between 2.5 and 4.0 wt% and 1.0 and 3.0 wt%,
Gray Iron
is comparatively weak and brittle in tension.
gray iron
Strength and ductility are much higher under compressive loads.
Gray Iron
They are very effective in
damping vibrational energy.
Gray Iron
gray irons exhibit a high resistance to wear and the
least expensive of all metallic materials
Gray 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.
Ductile (or Nodular) Iron
It has mechanical characteristics approaching those of steel.
Ductile (or Nodular) 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.
White Iron
A fracture surface of this alloy has a white
appearance, and thus it is termed
white cast iron
As a consequence of large amounts of the cementite phase, _________
is extremely hard but also very brittle, to the point of being virtually unmachinable.
White Iron
Its use is
limited to applications that necessitate a very hard and wear-resistant surface, without a high
degree of ductility—for example, as rollers in rolling mills
White Iron
Heating white iron at temperatures between 800 and 900 oC for a prolonged time period
and in a neutral atmosphere (to prevent oxidation) causes a decomposition of the cementite,
forming graphite, which exists in the form of clusters or rosettes surrounded by a ferrite or
pearlite matrix, depending on cooling rate. The
Malleable Iron
Silicon
content ranges between 1.7 and 3.0 wt%,
Compacted Graphite Iron
carbon concentration is normally between
3.1 and 4.0 wt%
Compacted Graphite Iron
Tensile and yield strengths for _______ are comparable
to values for ductile and malleable irons, yet are greater than those observed for the higher
strength gray irons.
Compacted Graphite Iron
are intermediate between values for gray
and ductile irons; also, moduli of elasticity range between 140 and 165 GPa ( and psi).
Compacted Graphite Iron
desirable characteristics of ______ include the
following: higher thermal conductivity, better resistance to thermal shock (i.e., fracture resulting
from rapid temperature changes) and lower oxidation at elevated temperatures.
Compacted Graphite Iron
are now being used in a number of important applications—these include: diesel
engine blocks, exhaust manifolds, gearbox housings, brake discs for high-speed trains, and
flywheels.
Compacted Graphite Iron
are metals that do not have any iron in them at all.
NONFERROUS ALLOYS
It is not attracted to
the magnet and do not rust easily when exposed to moisture.
NONFERROUS ALLOYS
It is highly resistant to corrosion in diverse environments including the ambient
atmosphere, seawater, and some industrial chemicals.
Copper and Its Alloys
most common copper alloys
brasses
are alloys of copper and several other elements, including tin, aluminum, silicon,
and nickel.
bronzes
most common heat-treatable copper alloys
beryllium coppers.
tensile strengths as high as 1400 MPa (200,000
psi),
Copper and Its Alloys
Applications include jet aircraft landing gear
bearings and bushings, springs, and surgical and dental instruments.
Copper and Its Alloys
are characterized by a relatively low density (2.7 g/cm3 as compared to 7.9 g/cm3 for steel), high electrical and thermal conductivities, and a resistance to
corrosion in some common environments,
Aluminum and its alloys
The chief limitation of _________ is its low
melting temperature 660 oC.
aluminum
aluminum alloys are classified as either
cast or wrought.
more common applications of aluminum alloys include aircraft structural parts, beverage
cans, bus bodies, and automotive parts (engine blocks, pistons, and manifolds).
Aluminum and Its Alloys
most outstanding characteristic of _____ is its density, 1.7 g/cm3, which is the
lowest of all the structural metals.
magnesium
are relatively unstable and especially susceptible to corrosion in
marine environments.
magnesium alloys
have
replaced engineering plastics that have comparable densities in as much as the magnesium
materials are stiffer, more recyclable, and less costly to produce.
magnesium alloys
The pure metal has a relatively low density (4.5 g/cm3),
a high melting point [1668 oC ], and an elastic modulus of 107 GPa ( psi).
Titanium and Its Alloys
are
extremely strong; room temperature tensile strengths as high as 1400 MPa (200,000 psi) are
attainable, yielding remarkable specific strengths.
Titanium alloys
major limitation of _______ is its chemical
reactivity with other materials at elevated temperatures and quite expensive.
titanium
the corrosion resistance of _____ at normal temperatures is
unusually high; they are virtually immune to air, marine, and a variety of industrial environments.
titanium alloys
They are commonly utilized in airplane structures, space vehicles, surgical implants, and in the
petroleum and chemical industries.
Titanium and Its Alloys
Metals that have extremely high melting temperatures are classified as
refractory
metals.
Included in this group are niobium (Nb), molybdenum (Mo), tungsten (W), and tantalum
(Ta).
The Refractory Metals
are utilized for extrusion dies and structural parts in space vehicles;
incandescent light filaments, x-ray tubes,
Molybdenum alloys
welding electrodes
tungsten
alloys
is immune to chemical attack by virtually all environments at temperatures below
150 oC and is frequently used in applications requiring such a corrosion-resistant material.
Tantalum
have superlative combinations of properties.
superalloys
Most are used in aircraft
turbine components, which must withstand exposure to severely oxidizing environments and high
temperatures for reasonable time periods.
superalloys
These materials are classified according to the
predominant metal(s) in the alloy, of which there are three groups—
iron–nickel, nickel, and cobalt.
are a group of eight elements that have some physical
characteristics in common.
The Noble Metals
They are expensive (precious) and are superior or notable (noble) in
properties, that is, characteristically soft, ductile, and oxidation resistant.
The Noble Metals
are most
common and are used extensively in jewelry.
silver, gold, platinum,
are highly resistant to corrosion in many environments, especially
those that are basic (alkaline).
Nickel and its alloys
are
mechanically soft and weak, have low melting temperatures, are quite resistant to many corrosion
environments, and have recrystallization temperatures below room temperature.
Lead, tin, and their alloys
also is a relatively soft metal having a low melting temperature and a
subambient recrystallization temperature.
Unalloyed zinc
are ductile and have other mechanical characteristics that are
comparable to those of titanium alloys and the austenitic stainless steels.
Zirconium and its alloys
the primary
asset of these alloys is their resistance to corrosion in a host of corrosive media, including
superheated water.
Zirconium and its alloys
are those in which the shape of a metal piece is changed by plastic
deformation;
FORMING OPERATIONS
is mechanically working or deforming a single piece of a normally hot metal; this
may be accomplished by the application of successive blows or by continuous squeezing.
Forging
Forgings are classified as:
closed die
open die
a force is brought to bear on two or more die halves having the finished shape
such that the metal is deformed in the cavity between them
closed die
-two dies having simple geometric shapes (e.g., parallel flat, semicircular) are
employed, normally on large workpieces.
open die
is 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.
Rolling
may be used in the production of sheet, strip, and foil with high quality surface finish.
Cold rolling
Circular shapes as well as I-beams and railroad rails are fabricated
grooved rolls.
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 cross-
sectional area.
Extrusion
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.
Drawing
is a fabrication process whereby a totally 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
ordinary sand is used as the mold
material.
Sand Casting
A two-piece mold is formed by packing sand around a pattern that has the shape of the
intended casting.
Sand 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.
Die Casting
However, this technique lends itself only to relatively small pieces and to alloys of zinc,
aluminum, and magnesium, which have low melting temperatures.
Die Casting
(sometimes called lost-wax) casting,
Investment Casting
This technique is employed when high dimensional accuracy, reproduction of fine detail,
and an excellent finish are require,
Investment Casting
expendable pattern is a foam that can be formed by compressing polystyrene beads into the
desired shape and then bonding them together by heating.
Lost Foam Casting
Metal alloys that most commonly use this technique are cast irons and aluminum alloys;
furthermore, applications include automobile engine blocks, cylinder heads, crankshafts, marine
engine blocks, and electric motor frames.
Lost Foam Casting
At the conclusion of extraction processes, many molten metals are solidified by casting
into large ingot molds.
Continuous Casting
compaction of powdered metal, followed by a
heat treatment to produce a denser piece.
Powder Metallurgy
This method is especially suitable for metals having
low ductilities, since only small plastic deformation of the powder particles need occur.
Powder Metallurgy
Furthermore, parts that require very close dimensional tolerances (e.g., bushings and gears)
may be economically produced using this technique.
Powder Metallurgy
two or
more metal parts are joined to form a single piece when one-part fabrication is expensive or
inconvenient.
Welding
variety of
welding methods exist,
including arc and gas welding, as well as brazing and soldering.
is a heat treatment process in which a material is exposed to an elevated
temperature for an extended time period and then slowly cooled.
Annealing
is carried
out to relieve stresses; to increase softness, ductility, and toughness; and/or to produce a specific
microstructure. Annealing
Annealing
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.
a heat treatment process used to refine the grains and produce a more uniform
and desirable size distribution.
Normalizing
is the process for making material harder.
Hardening
is a heat treatment method mostly used to increase the yield
strength of malleable metals.
Ageing or Precipitation Hardening
produces uniformly dispersed particles within a metal’s grain
structure which bring about changes in properties.
Ageing or Precipitation Hardening
is especially common for boiler parts, air bottles, accumulators, etc. This
method takes the metal to a temperature just below its lower critical border.
Stress relieving
Tempering carried out by preheating previously quenched or normalized steel to a
temperature below the lower critical temperature (often from 205 to 595 ̊C), holding, and then
cooling to obtain the desired mechanical properties.
Tempering
The higher the temperature in the tempering process,
the lower the hardness.
is the process of hardening the surface of steel while
leaving the interior unchanged.
Case hardening or Surface hardening
The principal forms of casehardening are :
Carburizing
Cyaniding
Nitriding
It is process of increasing the carbon content on the surface of steel. It is a heat
treatment process in which iron or steel is heated in the presence of another material (in the
range of 900 to 950 °C ) which liberates carbon as it decomposes
Carburizing
It is a process of producing hard surfaces by immersing low carbon steel in cyanide
bath maintained at 800°C – 850°C. The parts are then quenched in water or oil. This
process helps to maintain bright finish of the parts.
Cyaniding
