Lowell D. Outland
IET307
Material Science
IET 307: Materials Science
HW 6 (based on chapter 11),
1) Explain in depth the four categories of steels, namely low, medium and high
carbon and stainless steel concentrating on its composition, properties and
typical applications. (10 points)
Low carbon steels – this classification of steels are the ones that are produced
in the greatest quantities. They generally contain less than 0.25% wt carbon and
are unresponsive to heat treatments, strengthening is accomplished by coldworking. The alloys are relatively soft and weak, nut have outstanding ductility
and toughness. They are machinable, weldable, and are the least expensive to
produce. They typically have a yield strength of 275MPa and a tensile strength
between 415 and 550MPa, with a ductility 25% EL. Included in this class is the
High Strength Low Alloy (HSLA) steels. They contain elements such as copper,
vanadium, nickel, and molybdenum. They have greater strength than plain low
carbon steels and are more resistant to corrosion,
Medium Carbon Steels – These alloys have carbon concentrates between 0.25
and 0.60 wt%, and may be heat treated to improve their properties. Plain medium
carbon steels can only be heat treated in very thin sections. Adding chromium,
nickel, and molybdenum improve the capacity of these alloys to be heat treated.
This allows for a variety of strength-ductility combinations. Heat treated alloys are
stronger than the low carbon steel, but sacrifice ductility and toughness.
High Carbon Steels – These have carbon contents between 0.6 and 1.4 wt%.
They are the hardest, strongest and least ductile of the carbon steels. They are
almost always used in a hardened and tempered state, and are especially wear
resistant and capable of holding a sharp edge. Some uses for these steels are
dies for forming and shaping metals, and cutting tools. They are also used to
make knives, razors, hacksaw blades, springs and high strength wire.
Stainless Steels – These are highly resistant to corrosion, the main alloying
element is chromium and at least 11 wt% is required. Corrosion resistance may
also be enhanced by adding nickel and molybdenum. Stainless steels are divided
into three classes, martensitic, ferritic, and austenitic. A wide range of
mechanical properties make stainless steels very versatile. Steels in the
martensitic class are capable of being heat treated in such a way that martensite
is the main constituent. Adding alloy elements in significant amounts can produce
dramatic alterations in the iron-iron carbide phase diagram. For austenitic steels,
the austenite phase field is extended to room temperature. Ferrite stainless
Lowell D. Outland
IET307
Material Science
steels are composed of the ferrite (BCC) phase field. Austenitic and ferritic
steels are hardened and strengthened by cold working and are not heattreatable. The austenitic steels are the most corrosion resistant because of the
high chromium content. They are produced in the largest quantities. Martensitic
and ferritic steels are magnetic. These steels are used in gas turbines, steam
boilers, heat treating furnaces, aircraft, missiles, and nuclear power units.
2) What is cast iron? What are the various types of cast iron? Explain each one in
detail. (10 points)
Cast irons – are a class of ferrous alloys with carbon contents above 2.14 wt%,
however most contain between 3.0 and 4.5 wt% carbons, in addition to other
alloying elements. These alloys become completely liquid at temperatures
ranging from 1150 to 1300ᵒC (2100-2300ᵒF), which is considerable lower than for
steels. Thus they are easily melted and cast.
Gray iron – the carbon and silicone contents of gay irons vary between 2.5 and
4.0 wt% and 1.0 and 3.0 wt% respectively. For most of these cast irons the
graphite exists in flakes, which are normally surrounded by an α-ferrite or pearlite
matrix. These graphite flakes cause the fractured surface to take on a gray
appearance. Gray iron is relatively weak and brittle. The tips of the flakes are
sharp and pointed and may serve as stress concentration points when an
external tensile stress is applied. Strength and ductility are greater under
compressive loads. These irons are very effective in dampening vibration. Base
structures of machines that are exposed to vibrations are frequently made of this
material. They are extremely wear resistant and in their molten state can be cast
into parts and pieces that intricate shapes where low shrinkage is desired.
Ductile iron – Adding a small amount of magnesium and/or cerium to the gray
iron before casting produces a distinctively different microstructure and set of
mechanical properties. Graphite forms as nodules or sphere like particles instead
of flakes. The matrix phase surrounding the particles is either pearlite or ferrite
depending on the heat treatment. Castings are stronger and more ductile that
gray iron, in fact ductile iron has characteristics approaching those of steel.
Typical applications include valves, pumps, crankshaft, gears, and other
automotive parts.
Lowell D. Outland
IET307
Material Science
White Iron – for low silicone cast irons and rapid cooling most of the carbon
exists as cementite instead of graphite. The fracture surface of this material has a white
appearance giving it the name of white iron. Thick sections may have only a surface
layer of white iron, the interior may be gray iron caused by the slower cooling rate.
White iron is extremely hard but also extremely brittle, and cannot be machined. It is
highly wear resistant without a high degree of ductility.
Malleable Iron – Heating white iron at temperatures of 800 and 900ᵒC for a prolonged
period of time in a neutral atmosphere will produce a ferritic malleable iron. This iron is
similar to ductile or nodular iron, with high strength and appreciable ductility. Some uses
include connecting rods, transmission gears and differential cases for automobiles.
Compact graphite iron (CGI) - as with gray, ductile and malleable irons, carbon exists
as graphite, whose formation is promoted by the presence of silicone. Silicone content
ranges between 1.7 and 3.0 wt% and carbon between 3.1 and 4.0 wt%. Desirable
characteristics of these metals include higher thermal conductivity, better resistance to
thermal shock, and lower oxidation at elevated temperatures.
3) What is precipitation hardening? By what methods are they accomplished?
Explain the methods. (10 points)
Precipitation Hardening – the strength and hardness of some alloys may be
enhanced by the formation of extremely small uniformly dispersed particles of a
second phase within the original phase matrix; this phase must be accomplished
by phase transformation that are induced by appropriate heat-treatment. This
process is called precipitation hardening because the small particles are termed
precipitates.
Solution heat treatment – this is where solute atoms are dissolved to form a
single phase solid solution. The treatment consists of heating an alloy to a
prescribed temperature within the α phase and waiting until all of the β phase
that may have been present is completely dissolved. The procedure is followed
by rapid cooling or quenching to a temperature that is normally room
temperature, so that any diffusion and the accompanying formation of the β
phase are prevented.
Lowell D. Outland
IET307
Material Science
Precipitation heat treatment- The supersaturated α solid solution is ordinarily
heated to an intermediate temperature within α + β two phase region at which
rate diffusion rates become appreciable. The β precipitate phase begins to form
as finely dispersed particles. This process is sometimes termed aging. After the
appropriate aging time the alloy is cooled to room temperature. The character of
these β particles and subsequent strength and hardness of the alloy depend on
both the precipitation temperature and the aging time at this temperature.
4) Write a detailed study of titanium and its alloys.(10 points)
Titanium is as strong as steel and twice as strong as aluminum. It weighs 45%
less than steel and 65% more than aluminum. It is not easily corroded and is
used in airplane parts and artificial hips along with numerous of products. It is the
ninth most abundant element in the earth’s crust. Titanium oxide is used to
create white paint. Pure titanium oxide is relatively clear and is used to create
titania (an artificial gem. Titanium tetrachloride is used to make smoke screens.
5) What are the five casting methods for metals? Explain each one of them. How
would you cast an engine block?
The five methods for casting metals are:
Sand: 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.
Die: liquid metal is forced into a mold under pressure and at 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 solidification is complete, the die pieces are
opened and the cast piece is ejected.
Investment: pattern is made from wax or plastic that has a low melting
temperature. Slurry is poured around the pattern, which sets up to form a solid
mold or investment; plaster of paris is usually used. The mold is then heated, so
that the pattern melts and is burned out, leaving the mold cavity that has the
desired shape.
Lost Foam: foam that can be formed by compressing polystyrene beads into the
desired shape and then bonding them together by heating. As the molten metal
is poured into the mold, it replaces the pattern which vaporizes. Upon
solidification, the metal assumes the shape of the mold.
Lowell D. Outland
IET307
Material Science
Continuous: the refined molten metal is cast directly into a continuous strand
that may have either a rectangular or circular cross section. Solidification occurs
in a water cooled die having the desired cross sectional geometry.
You would cast an engine block with sand casting
6) From the following list of applications, select the most appropriate metals and
their alloys that suit them and cite at least one reason for your choice.
Applications- (a) engine block, (b) electric cable, (c) automobile body, (d) watch
strap, (e) kitchen knife, (f) thermocouple, (g) hip transplant, (h) beverage can, (i)
jet aircraft landing gear bearing, (j) dental restoration ; metals are – magnesium,
aluminum, stainless steel, platinum, copper, gray cast iron, titanium, silver, and
carbon steels. (10 points)
Application
Metals
engine block
Cast Iron
electric cable
Copper
automobile body
carbon steels
watch strap
kitchen knife
Stainless steel or titanium
carbon steels
Reason
Strength, corrosion
resistance and ability to
make by casting
Because of electrical
conductivity
Can be easily fabricated and
formed.
Strength and resistance to
corrosion.
Because of their ability to
hold a sharp edge.
thermocouple
Made from two dissimilar
metals. Depending on the
intended use
Depends on the environment
and other conditions.
hip transplant
Titanium or stainless steel
Strength, and durability
beverage can
aluminum
Ability to hold carbonated
products
Lowell D. Outland
IET307
Material Science
jet aircraft landing gear
bearing
dental restoration
Aluminum alloy
Can withstand shock
Silver
Ease of work
7) What is the Jominy end quench test? Explain with a schematic. (10 points)
This is a test that is widely used to determine hardness.
With this procedure all factors that may influence the depth to which a piece
hardens are maintained constant. A cylindrical specimen 25.4 mm in diameter
and 100 mm long is austenitized at a prescribed temperature for a prescribed
time. After removal from the furnace it is quickly mounted to a fixture and the
lower end is quenched by a jet of water at a specified flow rate and temperature.
Thus the cooling rate is maximized at this end and lessens as the distance from
this point increases. After the piece has cooled to room temperature shallow flats
are ground along the length of the specimen and Rockwell hardness
measurements are taken at precise intervals.
Schematic obtained from(University of Minisota Duluth) website.
8) Discuss whether it would be advisable to hot work or cold work the following
metals and alloys on the basis of melting temperature, oxidation resistance yield
Lowell D. Outland
IET307
Material Science
strength, and degree of brittleness: aluminum alloys, magnesium alloys, titanium
alloys, copper alloys, and tungsten. (10 points)
Aluminum alloys - Both hot and cold work may be used based on the above
criteria.
Magnesium Alloys –Hot working Magnesium alloys is recommended, although
some minor forming may be accomplished at room temperature.
Titanium Alloys – Should be hot worked, however cold working can be done for
certain applications.
Copper alloys – Both Hot and cold working is acceptable for these alloys.
Tungsten – Should be hot worked.
9) Cite the advantages and disadvantages of forming metals by extrusion as
opposed to rolling. (10 points)
Rolling is the most widely used deformation process. It 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 high quality surface finish. Circular shapes as well as I-beams and railroad rails are
fabricated using grooved rolls.
Extrusion is when 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 products include rods and tubing that have
rather complicated cross sectional geometries; seamless tubing may also be extruded.
10) What is shape memory a alloy? Explain. What kinds of metals are called shape
memory alloys? (10 points)
Shape Memory Alloy: polymorphic, it may have 2 crystal structures, and the
shape memory effect involves phase transitions between them
Nickel-titanium alloys and some copper base alloys are shape memory alloys.
Lowell D. Outland
IET307
Material Science