Industrial Engineering Chapter 5 The Low Duc Ing The Fine Surface Detail The Cast Product Tility

Chapter 5

Metal-Casting Processes and

Equipment; Heat Treatment

Questions

5.1 Describe the characteristics of (1) an alloy, (2)

pearlite, (3) austenite, (4) martensite, and (5)

cementite.

tal lattice. This structure also possesses a

5.2 What are the effects of mold materials on fluid

flow and heat transfer?

The most important factor is the thermal con-

ductivity of the mold material; the higher the

conductivity, the higher the heat transfer and

the greater the tendency for the fluid to solid-

ify, hence possibly impeding the free flow of the

5.3 How does the shape of graphite in cast iron af-

fect its properties?

(b) Nodules. Graphite can form nodules or

spheroids when magnesium or cerium is

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sically similar to flakes.

(d) Compacted flakes. These are short and

5.4 Explain the difference between short and long

freezing ranges. How are they determined?

Why are they important?

scribed in Section 5.3.2 on p. 196, the freezing

5.5 We know that pouring molten metal at a high

rate into a mold has certain disadvantages.

Are there any disadvantages to pouring it very

the mechanical properties of castings? Which

physical properties are also affected adversely

by porosity?

5.7 A spoked hand wheel is to be cast in gray iron.

In order to prevent hot tearing of the spokes,

would you insulate the spokes or chill them?

pansion of 2.5%. Although this fact may sug-

gest that hot tearing cannot occur, considera-

5.8 Which of the following considerations are im-

Both (1) and (3) would result in a situation

contrary to a riser’s purpose. That is, if a riser

solidifies first, it cannot feed the mold cavity.

ets of air to be trapped, or increased dissolu-

tion of air into the metal, leading to defects in

the cast part. For these reasons, the size and

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5.10 Explain why gray iron undergoes expansion,

rather than contraction, during solidification.

5.11 How can you tell whether a cavity in a casting

are due to porosity or to shrinkage?

Swiss cheese) and is often, though not always,

generally spherical in shape. Shrinkage poros-

5.12 Explain the reasons for hot tearing in castings.

5.13 Would you be concerned about the fact that

a portion of an internal chill is left within the

casting? What materials do you think chills

should be made of, and why?

because any gas evolved when the molten

metal contacts the chill may not readily

escape.

(c) The chill may not fuse with the casting,

5.14 Are external chills as effective as internal chills?

Explain.

mold can impede the free flow of molten metal

into the mold? Explain.

this alloy will also be quite large (see Fig. 5.6).

Since the mushy condition consists of interlac-

freezing and long freezing ranges, respectively?

Explain.

In an alloy with a long freezing range, the pres-

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5.17 It has long been observed by foundrymen that

low pouring temperatures (that is, low super-

reasons for these phenomena.

5.18 What are the reasons for the large variety of

casting processes that have been developed over

5.19 Why can blind risers be smaller than open-top

risers?

Risers are used as reservoirs for a casting in re-

gions where shrinkage is expected to occur, i.e,

areas which are the last to solidify. Thus, risers

must be made large enough to ensure that they

are the last to solidify. If a riser solidifies before

the cavity it is to feed, it is useless. As a result,

an open riser in contact with air must be larger

to ensure that it will not solidify first. A blind

riser is less prone to this phenomenon, as it is

5.20 Would you recommend preheating the molds in

permanent-mold casting? Also, would you re-

mine the time at which you would remove the

casting from the mold?

molten metal, the metal chills rapidly, develop-

ing a fine-grain hard skin with higher strength.

As a result, the strength-to-weight ratio of die-

cast parts increases with decreasing wall thick-

ness.

5.23 We note that the ductility of some cast alloys

is very low (see Fig. 5.13). Do you think this

should be a significant concern in engineering

applications of castings? Explain.

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(a) affect properties, such as toughness and fa-

tigue,

(b) have a significant influence on further

ensure the material remains elastic and where

5.24 The modulus of elasticity of gray iron varies

significantly with its type, such as the ASTM

class. Explain why.

5.25 List and explain the considerations involved in

material requirements. Often, they are ma-

5.26 Why is the investment-casting process capable

of producing fine surface detail on castings?

The surface detail of the casting depends on

5.27 Explain why a casting may have a slightly dif-

ferent shape than the pattern used to make the

mold.

5.28 Explain why squeeze casting produces parts

mold processes.

The squeeze-casting process consists of a com-

dimensional accuracy and surface finish are typ-

5.29 Why are steels more difficult to cast than cast

irons?

5.30 What would you recommend to improve the

surface finish in expendable-mold casting pro-

cesses?

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5.31 You have seen that even though die casting pro-

duces thin parts, there is a limit to the mini-

mum thickness. Why can’t even thinner parts

be made by this process?

5.32 What differences, if any, would you expect in

the properties of castings made by permanent-

mold vs. sand-casting methods?

5.33 Which of the casting processes would be suit-

5.34 Why are allowances provided for in making pat-

terns? What do they depend on?

tions for the shrinkage that occurs upon solidifi-

mold casting.

Draft is provided to allow the removal of the

pattern without damaging the mold. If the

mold material is sand and has no draft, the

ter. Under each type of material, list the cast-

ing processes that are used, and explain why

low cost. Used for sand, shell, expanded-

ing for the production of relatively small

5.37 Explain why carbon is so effective in imparting

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the crystal lattice. Because it is higher, the

5.38 Describe the engineering significance of the ex-

istence of a eutectic point in phase diagrams.

The eutectic point corresponds to a compo-

Hardness represents the material’s resistance to

2.6 starting on p. 51), while hardenability is

p. 236).

5.40 Explain why it may be desirable or necessary for

castings to be subjected to various heat treat-

out to optimize the grain structure of castings.

5.41 Describe the differences between case hardening

5.42 Type metal is a bismuth alloy used to cast type

When one considers the use of type or for preci-

sion castings such as mechanical typewriter im-

pressions, one realizes that the type tool must

have extremely high precision and smooth sur-

5.43 Do you expect to see larger solidification shrink-

age for a material with a bcc crystal structure

or fcc? Explain.

The greater shrinkage would be expected from

Since the APF for fcc is 0.74 and for bcc it is

a material with a fcc structure. This can also

5.44 Describe the drawbacks to having a riser that

are: the material in the riser is eventually

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5.45 If you were to incorporate lettering on a sand

casting, would you make the letters protrude

from the surface or recess into the surface?

What if the part were to be made by invest-

ment casting?

5.46 List and briefly explain the three mechanisms

by which metals shrink during casting.

The microstructures are as follows:

viating from these rules.

Refer to Figure 5.10 for terminology used in this

problem.

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ing, leading to aspiration and associated

defects. If the well is much deeper, then

5.50 Describe the characteristics of thixocasting and

5.51 Sketch the temperature profile you would ex-

pect for (a) continuous casting of a billet, (b)

the liquid through the continuous addition of

molten metal. The die walls extract heat, and

the coolant spray at the die exterior removes

heat even more aggressively. Thus, a sketch of

Conduction

boundary

5.52 What are the benefits and drawbacks to hav-

ing a pouring temperature that is much higher

melting temperature?

the pouring temperature is close to the melt-

ing temperature, there will be less likelihood

5.53 What are the benefits and drawbacks to heating

the mold in investment casting before pouring

in the molten metal?

5.54 Can a chaplet also act as a chill? Explain.

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5.55 Rank the casting processes described in this

chapter in terms of their solidification rate.

For example, which processes extract heat the

fastest from a given volume of metal and which

is the slowest?

There is, as expected, some overlap between the

ing, die casting (hot chamber), permanent mold

casting, shell mold casting, investment casting,

sand casting, lost foam, ceramic-mold casting,

and plaster-mold casting.

5.56 The heavy regions of parts typically are placed

in the drag in sand casting and not in the cope.

5.57 Referring to Fig. 5.3, estimate the following

quantities for a 20% Cu-80% Ni alloy: (1) liq-

with no solids present since the alloy is not be-

5.58 Determine the amount of gamma and alpha

We determine the following quantities from

Fig. 5.6 on p. 197: (a) At 750◦C, the alloy is

just in the single-phase austenite (gamma) re-

gion, thus the percent gamma is 100% (10 kg),

and alpha is 0%. (b) At 728◦C, the alloy is

in the two-phase gamma-alpha field, and the

weight percentages of each is found by the lever

rule (see Example 5.1):

0.77 −0.022×100%

of alpha present:

5.59 A round casting is 0.3 m in diameter and 0.5 m

in length. Another casting of the same metal

is elliptical in cross section, with a major-to-

minor axis ratio of 3, and has the same length

and cross sectional area as the round casting.

Both pieces are cast under the same conditions.

What is the difference in the solidification times

of the two castings?

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sic geometry text for the area equation deriva-

Eq. (5.11) on p. 205, we thus have

5.60 Derive Eq. (5.7).

We note that Eq. (5.5) on p. 200 gives a rela-

tionship between height, h, and velocity, v, and

Eq. (5.6) on p. 201 gives a relationship between

5.61 Two halves of a mold (cope and drag) are

weighted down to keep them from separat-

Fig. 5.10) is 20 in. by 20 in. and 15 in. deep. The

parting line is at the middle of the part. Esti-

The force exerted by the molten metal is the

product of its cross-sectional area at the parting

line and the pressure of the molten metal due to

the height of the sprue. Assume that the sprue

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lem 5.61 influence your answer? Explain.

5.63 Plot the clamping force in Problem 5.61 as a

the casting increases, the cross-sectional area

of the molten metal increases, hence the buoy-

0 5 10 15 20

-500

1000

5.64 Sketch a graph of specific volume vs. temper-

ature for a metal that shrinks as it cools from

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Arect = 2(70,680) + (1228)(500)

5.66 A 75-mm thick square plate and a right circu-

lar cylinder with a radius of 100 mm and height

of 50 mm each have the same volume. If each

is to be cast using a cylindrical riser, will each

part require the same size riser to ensure proper

= 0.0942 m2

h= 0.075 m, and the same volume as the cylin-

0.0847 2

Therefore, the cylindrical casting will take

of 1 in. at its bottom.

From Eq. (5.7) on p. 201 and substituting for

the area, it can be shown that

4 = C(ho)−0.25 or C= 4h0.25

o

that C= 1.863. These values are substituted

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