MSE 227
Homework #9: Materials selection & Processing Solutions
Fall, 2010
1.
List the four classifications of steels. (b) For each, briefly describe the properties and
typical applications.
Solution
Low Carbon Steels
Properties: unresponsive to heat treatments; relatively soft and weak; machinable and
weldable.
Typical applications: automobile bodies, structural shapes, pipelines, buildings, bridges,
and tin cans.
Medium Carbon Steels
Properties: heat treatable, relatively large combinations of mechanical characteristics.
Typical applications: railway wheels and tracks, gears, crankshafts, and machine parts.
High Carbon Steels
Properties: hard, strong, and relatively brittle.
Typical applications: chisels, hammers, knives, and hacksaw blades.
High Alloy Steels (Stainless and Tool)
Properties: hard and wear resistant; resistant to corrosion in a large variety of
environments.
Typical applications: cutting tools, drills, cutlery, food processing, and surgical tools.
2.
Compare gray and nodular cast irons with respect to (a) composition and heat treatment,
(b) microstructure, and (c) mechanical characteristics.
Solution
Gray iron is weak and brittle in tension because the tips of the graphite flakes act as
points of stress concentration.
Solution
(a) With regard to composition and heat treatment:
Gray Iron—3.0 to 3.7 wt% C, ~0.7 wt% Mn and 2.0 to 2.55 wt% Si. No heat
treatment; however, cooling is rapid during solidification.
Nodular cast iron--2.5 to 4.0 wt% C, 1.0 to 3.0 wt% Si, and a small amount of
Mg or Ce. A heat treatment at about 700C may be necessary to produce a ferritic matrix.
(b) With regard to microstructure:
Gray iron—Ferrite + Pearlite
Nodular cast iron--Nodules of graphite are embedded in a ferrite or pearlite
matrix.
(c) With respect to mechanical characteristics:
Gray iron—good compressive strength, poor tensile strength, and brittle.
Nodular cast iron--Moderate strength and ductility.
3.
A structural member 250 mm (10 in.) long must be able to support a load of 44,400 N
(10,000 lbf) without experiencing any plastic deformation. Given the following data for brass,
steel, aluminum, and titanium, rank them from least to greatest weight in accordance with these
criteria.
Alloy
Yield Strength [MPa (ksi)] Density (g/cm3)
Brass
Steel
Aluminum
Titanium
345 (50)
690 (100)
275 (40)
480 (70)
8.5
7.9
2.7
4.5
Solution
This problem asks us to rank four alloys (brass, steel, titanium, and aluminum), from least
to greatest weight for a structural member to support a 44,400 N (10,000 lbf) load without
experiencing plastic deformation. From Equation 7.1, the cross-sectional area (A0) must
necessarily carry the load (F) without exceeding the yield strength (y), as
A0 =
F
y
Now, given the length l, the volume of material required (V) is just

V = lA0 =
lF
y
m = V =
lF
y
Finally, the mass of the member (m) is


Here  is the density. Using the values given for these alloys
m (brass) =
(8.5 g /cm3 ) (25 cm)(44,400 N)
(345


m (steel) =

m (aluminum) =
m (titanium) =
N / m2

2
)  12 m 
10 cm 
(7.9 g /cm3 ) (25 cm)(44,400 N)
 1 m 2
6
2
(690  10 N / m )  2 
10 cm 
= 273 g
= 127 g
(2.7 g /cm3 ) (25 cm)(44,400 N)
(275

10 6

10 6
N / m2

2
)  12 m 
10 cm 
(4.5 g /cm3 ) (25 cm)(44,400 N)

2
(480  10 6 N / m2 )  12 m 
10 cm 
= 109 g
= 104 g
Thus, titanium would have the minimum weight (or mass), followed by aluminum, steel, and
brass.

4.
Some of our modern kitchen cookware is made of ceramic materials.
(a) List at least three important characteristics required of a material to be used for this
application.
(b) Make a comparison of three ceramic materials as to their relative properties and, in
addition, to cost.
(c) On the basis of this comparison, select the material most suitable for the cookware.
Solution
(a) Important characteristics that are required of a ceramic material to be used for kitchen
cookware are: (1) it must have a high resistance to thermal shock (Section 17.5) in order to
withstand relatively rapid changes in temperature; (2) it must have a relatively high thermal
conductivity; 3) it must be relatively strong and tough in order to endure normal kitchen use;
and 4) it must be nontoxic.
(b) Possible materials worth considering are a common soda-lime glass, a borosilicate
(Pyrex) glass, and a glass ceramic. These materials and some of their characteristics are
discussed in this chapter. Using Equation 17.9 a comparison of the resistance to thermal shock
may be made. The student will need to obtain cost information.
(c) It is left to the student to make this determination and justify the decision.
5.
Write short description of polymeric materials that are used in the packaging of food
products and drinks. Include a list of the general requisite characteristics of materials that are
used for these applications. Now cite a specific material that is utilized for each of three
different container types and the rationale for each choice.
Solution
There are three primary requirements for polymeric materials that are utilized in the
packaging of food products and drinks; these are: (1) sufficient strength, to include tensile, tear,
and impact strengths; (2) barrier protection--that is, being resistant to permeation by oxygen,
water vapor, and carbon dioxide; and (3) being nonreactive with the food/drink contents--such
reactions can compromise the integrity of the packaging material, or they can produce toxic byproducts.
With regard to strength, poly(ethylene terephthalate) (PET or PETE), poly(lactic acid)
(PLA), polystyrene (PS), and oriented polypropylene (OPP) have high tensile strengths, linear
low-density polyethylene (LLDPE) and low-density polyethylene (LDPE) have high tear
strengths, while those polymers having the best impact strengths are PET and poly(vinyl
chloride) (PVC). Relative to barrier characteristics, ethylene vinyl alcohol (EVOH) and
poly(vinylidene chloride) (PVDC) copolymers are relatively impermeable to oxygen and carbon
dioxide, whereas high-density polyethylene (HDPE), PVDC, polypropylene, and LDPE are
impervious to water vapor.
Most common polymers are relatively nonreactive with food products, and are considered
safe; exceptions are acrylonitrile and plasticizers used in PVC materials.
The aesthetics of packaging polymers are also important in the marketing of food and
drink products. Some will be colored, many are adorned with printing, others need to be
transparent and clear, and many need to be resistant to scuffing.
Biodegradable plastics and those based on renewable materials are increasingly gaining
importance in packaging. For example, poly(lactic acid) is completely biodegradable and is
produced using from corn or other biomass. Its uses can range from plastic forks and spoons to
wrapping individual pieces of candy.
On the basis of the preceding discussion, examples of polymers that are used for specific
applications are as follows:
PET(E) for soda pop containers;
PVC for beer containers;
LDPE and HDPE films for packaging bread and bakery products.
6.
(a) Cite advantages of forming metals by extrusion as opposed to rolling. (b) Cite some
disadvantages.
Solution
(a) The advantages of extrusion as opposed to rolling are as follows:
(1) Pieces having more complicated cross-sectional geometries may be formed.
(2) Seamless tubing may be produced.
(b) The disadvantages of extrusion over rolling are as follows:
(1) Nonuniform deformation over the cross-section.
(2) A variation in properties may result over a cross-section of an extruded piece.
7.
Contrast compression, injection, and transfer molding techniques that are used to form
plastic materials.
Solution
This question requests that we compare polymer molding techniques. For compression
molding, both heat and pressure are applied after the polymer and necessary additives are
situated between the mold members. For transfer molding, the solid materials (normally
thermosetting in nature) are first melted in the transfer chamber prior to being forced into the die.
And, for injection molding (normally used for thermoplastic materials), the raw materials are
impelled by a ram through a heating chamber, and finally into the die cavity.
8.
In your own words, briefly describe what happens as a glass piece is thermally tempered.
Thermal tempering of glasses is described in Section 14.7.