Devin Gatherwright
10/27/2012
IET 307: Materials Science
HW 6 (based on chapter 11), Due by 11.55 PM, Wednesday, October 31’st, 2012
100 points
1. Explain in depth the four categories of steels, namely low, medium and high carbon
steels and stainless steel concentrating on its composition, properties and typical
applications. (10 points)
Answer: According to the material science textbook, steels are iron-carbon alloys that encompass
appreciable concentrations of other alloying elements. There are four types or categories of
steels: low-carbon steels, medium-carbon steels, high-carbon steels, and stainless steels.
According to the textbook, low-carbon steels are produced in the greatest amounts and generally
contain less than around 0.25 wt% carbon and are unresponsive to heat treatments that are meant
to form martensite. Instead, strengthening of a low-carbon steel is accomplished by cold work.
According to the textbook, low-carbon microstructures of ferrite and pearlite constituents, which
cause these alloys to be relatively weak and soft, but have outstanding ductility and toughness. In
addition to these properties, these alloys are weldable, machinable, and the least expensive to
produce of any of the four categories of steels. Typical applications of low-carbon steels include
structural shapes such as I-beams, and channel and angle iron; mobile body components; and
sheets that are used in buildings, bridges, tin cans, and pipelines. According to the textbook, lowcarbon steels typically have a yield strength of 275 Mpa (40,000 psi), tensile strengths between
415 and 500 Mpa (60,000 and 80,000 psi), and a ductility of 25% EL. Another group of lowcarbon steel alloys are high-strength, low-alloy steels. According to the textbook, they contain
other alloying elements such as vanadium, copper, molybdenum, and nickel in united
concentrations that are as high as 10 wt%, and also possess higher strengths than the plain lowcarbon steels. Most HSLA steels may be strengthened by heat treatment, giving tensile strengths
of 480 Mpa (70,000 psi) at the most. HSLA steels are also ductile, machinable, and formable.
According to the textbook, HSLA are more resistant to corrosion in normal atmospheres than
low-carbon steels are, which makes them ideal for many structural applications.
According to the textbook, medium-carbon steels have carbon concentrations between around
0.25 and 0.60 wt%. Medium carbon alloys can be heat-treated by austenitizing, quenching, and
then tempering to improve their mechanical properties. According to the textbook, they are often
used in the tempered condition, having microstructures of tempered martensite. The more plain
medium-carbon steels have relatively low hardenabilites, and can be heat-treated, but only in
very thin sections and with very fast quenching rates. According to the textbook, adding
chromium, nickel, and molybdenum to plain medium-carbon steels increase the variety of
strength-ductility combinations. These heat-treated alloys are therefore stronger than the lowcarbon steels, but at the cost of the ductility and toughness of the material. Applications of
medium-carbon steels include railway wheels and tracks, crankshafts, gears, and other machine
parts, as well as high-strength structural components that call for a mixture of high strength, wear
resistance, and toughness.
According to the textbook, high-carbon steels have carbon contents between 0.06 and 1.4 wt%
and are strongest, hardest, but least ductile of the carbon steels. They are most often used in a
tempered and hardened condition, and are extremely wear resistant and very capable of
maintaining a sharp cutting edge. As such, the tool and die steels are high-carbon alloys that
usually contain vanadium, tungsten, molybdenum, and chromium. These alloying elements
coalesce with carbon to from extremely hard and wear resistant carbide compounds. According
to the book, applications of high-carbon steels include tools and die used in machining, such as
drills, saws, punches, and drawing dies, as well as knives, razors, springs, and high-strength wire.
According to the textbook, stainless steels are extremely resistant to corrosion in an array of
different environments, especially the ambient atmosphere. The dominate alloying element in
stainless steels is chromium, in a concentration of at least 11 wt% or more. According to the
textbook, corrosion resistance may be improved by molybdenum and nickel additions.
According to the textbook, stainless steels are divided into three classes based on the
predominant phase constituent of the microstructure: martensitic, ferritic, and austenitic.
According to the textbook, martensitic stainless steels are able of being heat-treated in a way that
makes martensitic the prime microconstituent. As such, additions of alloying elements in
significant concentrations create radical alterations in the iron-iron carbide phase diagram. In
austenite stainless steels, the phase field is extended to room temperature, which means the
material can only be hardened and strengthened by cold working. Austenite stainless steels are
also the most corrosion resistant of the three. In contrast, both martensitic and ferritic stainless
steels possess magnetic properties, while austenitic stainless does not. According to the textbook,
some stainless steels are commonly used at elevated temperatures and in extreme environments
due to their resistance of oxidation and their ability to maintain their mechanical integrity under
these conditions. According to the textbook, equipment that uses this type of stainless steel
include gas turbines, high-temperature steam boilers, heat-treating furnaces, aircraft, missiles,
and nuclear-generating power units. According to the textbook, another type of stainless steel,
which is unusually strong and corrosion resistant. In these stainless steels, strengthening is
accomplished by precipitation-hardening heat treatments, according to the textbook.
2. What is Cast Iron? What are the various types of Cast Iron? Explain each one of them in
detail. (10 points)
Answer: According to the textbook, cast iron can be defined as a “ferrous alloy, in which the
carbon content is greater than the maximum solubility in an austenite at the eutectic
temperature.” Furthermore, according to the textbook, most commercial cast irons contain
“between 3.0 and 4.5 wt% C, and between 1 and 3 wt% S.” According to the textbook, the most
common cast iron types are gray, nodular, white, malleable, and compacted graphite.
According to the textbook, mechanically speaking, gray iron is comparatively weak and brittle in
tension due to its microstructure, with the tips of the graphite flakes being sharp and pointed,
which serve as points of stress concentration when an external tensile stress is exerted on the
material. The ductility and strength of gray iron are usually much great under compressive loads.
Furthermore, gray iron are very effective in damping vibrational energy, which makes gray iron
ideal for base structures of machines and heavy equipment that might be subject to heavy,
continuous vibrations. Furthermore, according to the textbook, gray iron is highly impervious to
wear, and as a high fluidity at a molten state, which allows for casting intricate shapes and
pieces. Also, as a plus, gray cast iron is very inexpensive to produce.
According to the textbook, nodular or ductile iron as it is often known as occurs when a small
portion of magnesium and/or cerium is added to gray iron before casting, which creates a
different microstructure, as well as a different set of mechanical properties. According to the
book, graphite will still be produced but as nodules or sphere-like particles instead of flakes,
which is called nodular iron. For nodular iron, castings are much more stronger and ductile than
gray iron. In fact, nodular iron contains mechanical properties that are very similar to that of
steel. According to the textbook, typical applications of nodular iron includes valves, pump
bodies, gears, and crankshafts.
According to the textbook, white iron is a low-silicon cast iron that is subjected to rapid cooling,
which yields a fracture surface that is white in appearance. As a result of this process, white iron
is an extremely hard material, yet it is also a very brittle material, to the point where it is actually
impossible to machine without fracture. Therefore, as a result, its uses and applications are very
limited as you might imagine. One of its few uses is as a roller in typical rolling mill. Lastly,
white iron is often utilized as an intermediary in the manufacturing of malleable iron.
According to the textbook, malleable iron is created by heating white iron at temps as high as
800 and 900 degrees C for a long time period , as well as in a neutral atmosphere in order to
prevent oxidation from occurring. This delicate process causes a decomposition of the cermenite,
which forms graphite, which according to the text, exists in either cluster or rosette form and is
surrounded by a ferrite or pearlite matrix. As a result, the microstructure is close to that of
nodular iron, which explains the incredible strength and ductility of malleable iron. Applications
of malleable iron include connecting rods and transmission gears.
According to the textbook, compacted graphite iron or CGI is made up of graphite and silicon,
with silicon ranging between 1.7 and 3.0 wt%, and carbon concentrations ranging from 3.1 and
4.1 wt%. In a microstructural sense, CGI has a wormlike shape to it, which in a sense put it
between the microstructure of gray and nodular iron. Furthermore, the chemistries that make up
CGI are much more advanced than that of its cast iron family. Desirable characteristics of CGIs
include the following: high thermal conductivity, better resistance to thermal shock, lower
oxidation rates. According to the text, CGIs are generally used for diesel engine blocks and
exhausts manifolds.
3. What is Precipitation hardening? By what methods are they accomplished? Explain the
methods. (10 points)
Answer: According to the textbook, precipitation hardening can be defined as the “hardening and
strengthening of a metal alloy by incredibly tiny and uniformly dispersed particles that
precipitate from a super saturated solid solution.” According to the textbook, there are two
methods from which precipitation hardening is achieved: solution heat treating and precipitation
heat treating.
According to the textbook, solution heat treatment is where all the solute atoms are dissolved to
create a single-phase solid solution. For example, if you take a certain alloy and heat the alloy to
a certain temperature with the 𝛼 phase field in order to complete dissolve all the 𝛽 phase that is
present, then you will only be left with the aforementioned 𝛼 phase. This process is then
followed by rapid cooling or quenching to room temperature, which further prevents the
diffusion and/or the accompanying formation of the 𝛽 phase. Thus creating a non-equilibrium is
created in which the 𝛼 phase solid solution that is saturated with the B atoms is present at room
temperature, which makes the alloy soft and weak.
According to the textbook, precipitation heat treatment is when the supersaturated 𝛼 solid
solution is heated to a intermediate temperature with the 𝛼 𝑝𝑙𝑢𝑠 𝛽 two-phase region, which
causes temperature diffusion rates to become appreciable. Furthermore, the 𝛽 precipitate phase
commences to form as finely dispersed particles of composition 𝐶𝛽 , which is known as “aging”.
After a specific amount of aging time has occurred, the alloy is cooled to room temperature.
However, with increasing time, both the strength and hardness of the material increases to its
maximum and then decreases. This is known as a overaging, in which a reduction in both
strength and hardness occurs.
4. What is a shape memory alloy? Explain. What kinds of metals are called shape memory
alloys? (10 points)
Answer: According to the textbook, shape memory alloys possess the remarkable ability to
return to their pre-deformed shapes and sizes after being deformed due to an appropriate heat
treatment that enables the alloy to “remember” its previous size and shape. Furthermore,
deformation is usually carried out at a low temperature, whereas shape memory for these alloys
occurs upon heating. Alloys that are capable of recovery from deformation via this process are
nickel-titanium alloys, as well as some copper-based alloys.
5. Explain the process of purifying iron from iron ore in a few sentences and then draw a
schematic of this process (10 points)
Answer: According to innovateous.com, the process of refining iron from iron ore utilizes heat
and another substance, usually coal or “coke” as it is also known as. This is due to the fact that
oxygen molecules can bond to the substance in question and separate from the iron. Coal is used
in this circumstance due to the fact that carbon can bond with oxygen atoms at high temperatures
that are capable of melting iron, according to innovateous.com. At these high temperatures,
carbon and oxygen form gases such as carbon monoxide and carbon dioxide, which escape
through a chimney in a furnace where the heating is taking place. According to innovateous.com,
a temperature of 900 degrees C is required to heat the iron ore along with coke in a furnace to
achieve this phenomenon. The following is a diagram of the process:
6. Explain ‘Tempering’ of steel. What properties does it improve and what is the phase
change happening here? (10 points)
Answer: According to the textbook, the tempering of steel is required to relieve the internal
stresses that are created by quenching via heat treatment. Also, tempering of steel also improves
both the ductility and strength of martensite steel, which as you know, is both very hard and very
brittle. According to the textbook, tempering is accomplished by heating a martenitic steel to a
temperature that is below the eutectoid for a finite time period. The temperature can be as low as
200 degrees C to relieve the internal stresses. Furthermore, according to the textbook, the
tempering heat treatment enables, by diffusional processes, the creation of tempered martensite
steel. The phase change that occurs between the martensite, which is BCT, single phase, to
tempered martensite, which is 𝛼 + 𝐹𝑒3 𝐶 𝑝ℎ𝑎𝑠𝑒𝑠 respectively.
7. Give 5 examples of components or products that were originally made from steel alloys
and are now made from aluminum alloys. In each case give reasons as to why this change
took place. (10 points).
Answer:
1. Bicycle Frames. Why: Makes for a more lightweight frame, yet maintains the sturdiness and
strength.
2. Cylinder Block. Why: low density makes more a more lightweight part. Resistance to
corrosion as well as high thermal conductivity also plays a part.
3. Crank Case. Why: low density makes the part more lightweight. Resistance to corrosion.
4. Aerospace manufacturing. Why: aluminum alloy is not flammable and has a low density,
which makes it ideal for aircraft and aircraft parts.
5. Piping. Why: high ductility, low density, resistant to corrosion, and good conductor of both
electrical and thermal energy.
8. 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 – magnesium, aluminum, stainless steel, platinum, copper, gray
cast iron, titanium, silver, carbon steels (10 points)
Answer:
Aluminum, wrought, nonheat-treatable: beverage can. Why: highly resistant to corrosion,
ductile, generally lightweight.
Gray Iron: Engine Block. Why: resistant to vibration and wear, capable of being cast into
intricate shapes.
Copper: Electric Cable. Why: ductile and a good conductor of thermal and electrical energy.
Magnesium Cast Alloy: Aircraft Landing Gear Bearing. Why: low density makes more a
lightweight part.
Titanium: Hip Transplant: low density yet high-strength makes it ideal for being used in the
human body.
Platinum: Thermocouple: capable of withstanding elevated temperatures.
Plain High-Carbon Steel and Martensitic Stainless Steel: Kitchen Knife. Why: High-Carbon
Steel maintains its sharp cutting edge and is wear resistant. Martensitic Stainless Steel is
corrosion resistant.
Magnesium Cast Alloy: Automobile body. Why: low in density makes the frame lightweight, yet
the high strength makes the frame strong.
Silver: dental restoration. Why: soft and ductile, also oxidation resistant makes it ideal for dental
work.
Platinum and Silver: Watch Strap: soft and ductile. Both can be used in jewelry pieces due to
their properties and appearance.
9. What is the Jominy end-quench test? Explain in a paragraph and then draw a schematic.
(10 points)
Answer: According to the materials science textbook, a Jominy end-quench test is a
“standardized laboratory test that is used to assess the hardenability of ferrous alloys.” In this
test, all of the factors that influence the hardenability of a piece are kept constant. For example, a
cylindrical piece of a certain diameter and length is austenized at a certain temperature for a
finite amount of time. Once the piece is removed from the furnace, it is then mounted in a
fixture, then the lower end of the piece is quenched via water of a certain temperature and flow
rate, creating a maximum cooling rate at the quenched end. Once the piece has cooled, shallow
flats are imprinted along the length of the specimen and Rockwell Hardness readings are
observed at plotted a function of distance from the end of the specimen that was quenched.
10. 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 strength, and
degree of brittleness: aluminum alloys, magnesium alloys, titanium alloys, copper alloys
and tungsten (10 points)
Answer:
For aluminum alloys, both hot and cold work is possible. However, cold work decreases
aluminum’s resistance to corrosion, so it is more advisable to do hot work. It should be further
noted that aluminum has a low melting temperature of around 1,220 degrees F, which limits the
maximum temperature at which it can be subjected to. Despite this, most aluminum alloys can be
heat treated, which makes them capable of precipitation hardening, according to the textbook.
For magnesium alloys, both hot and cold work is possible, though due the fact that magnesium is
difficult to deform at low and room temperatures, only so little cold work can be done without
annealing the magnesium alloy material. Therefore, it is advisable to use hot work, and hot work
is used in most cases. In titanium alloys, cold work by annealing is usually used in alpha titanium
alloys due to the fact that 𝛼 is the stable phase, according to the textbook. In contrast, alpha +
beta titanium alloys are capable of heat treatment, while near-alpha alloys are very similar to the
alpha titanium alloys, which therefore makes cold work by annealing the desired treatment. For
copper alloys, cold work is advisable due to the fact that copper alloys are both soft and ductile,
which makes copper alloys very ideal for any amount of cold work. Also, copper alloys are very
corrosion resistant, which makes them further susceptible to cold work. According to the
textbook, copper cannot be hot worked at all.
Works Cited
“How is Iron Ore Formed?” innovateous. Retrieved October, 29th, 2012, from:
http://www.innovateus.net/science/how-iron-refined-ore