Higgins R.A. Engineering Metallurgy: Applied Physical Metallurgy
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time a temperature of 723°C has been reached the remaining austenite will
contain only 0.8% carbon. This remaining austenite will then transform to
pearlite (Fig 7.6(x)), as in the two cases already dealt with.
Any steel containing more than 0.8% carbon will have a structure con-
sisting of cementite and pearlite if it is allowed to cool slowly from its
austenitic state. Since the pearlite part of the structure always contains
alternate layers of ferrite and cementite in the correct proportions to give
an overall carbon content of 0.8% for the pearlite, it follows that any
variation in the total carbon content of the steel above 0.8% will cause a
corresponding variation in the amount of primary cementite present. (The
terms 'primary cementite' and 'primary ferrite' are used to denote that
cementite or ferrite which forms first, before the residual austenite trans-
forms to pearlite.)
A plain carbon steel which contains less than 0.8% carbon is generally
referred to as a hypo-eutectoid steel, whilst one containing more than
0.8%
carbon is known as a hyper-eutectoid steel. Naturally enough, a plain
carbon steel containing exactly 0.8% carbon is called a eutectoid steel.
7.60 So far we have been dealing only with the types of structure
produced when plain carbon steels are cooled slowly from the austenitic
condition. Such conditions prevail during industrial processes such as
normalising and annealing. By very rapid cooling from the austenitic con-
dition, such as would be obtained by water-quenching, another structure,
called Martensite, is formed. This does not appear on the equilibrium
diagram simply because it is not an equilibrium structure. Rapid cooling
has prevented equilibrium from being reached.
7.61 As most readers will already know, martensite is very hard indeed.
Unfortunately it is also rather brittle, and the steel is used in this condition
only when extreme hardness is required. To increase the steel's toughness
after quenching (at the expense of a fall in hardness) the steel can be
tempered. A modification in the structure will take place depending upon
the tempering temperature. This temperature will vary between 250 and
650
0
C according to the combination of mechanical properties required in
the finished component. Whatever temperature is used, tempering assists
the microstructure to proceed in some measure back towards equilibrium,
with the precipitation of microscopical particles of cementite in varying
amounts from the original martensitic structure. The type of structure
formed by tempering at about 400
0
C was formerly known as Troostite,
whilst that produced in the region of 600
0
C used to be called Sorbite.
Metallurgists now discourage the use of these terms for reasons which will
be explained later when the heat-treatment of steel is discussed more fully
in Chapters 11 and 12.
The Uses of Plain Carbon Steels
7.70 By varying the amount of carbon in a steel, and by selecting a
heat-treatment programme suited to that carbon content, we are able to
produce a vast range of different mechanical properties such as are avail-
Fig.
7.7 A
diagram showing
the
relationship between carbon content, microstructure
and
mechanical properties
of
plain carbon steels
in the
normalised condition. Typical uses
of
these steels
are
also indicated.
PEARLlTE °/o
ELONGATION
PER
CENT
TENSILE STRENGTH
(N/mm
2
)
HARDNESS
H
b
able
in no
other metallic alloy. Moreover, carbon steel
is a
relatively inex-
pensive alloy when compared with non-ferrous alloys generally. Small
wonder then that steel
is by far our
most important engineering alloy.
Possibly
its
most serious fault
is
that
it
rusts
and we
must often spend
considerable amounts
of
money
on
protecting
its
surface from atmospheric
corrosion (21.10).
CEMENTlTE
FERRITE
PEARLITE
CARBON
PER
CENT
screws
tinplatc
r.s.j.
structural
steels
screw
drivers
rails
wire
ropes
drills
taps
dies
saws
razors
chains
stampings
rivets
wire
nails
axles
gears
shafts
tyres
shear blades
cold chisels
some hand tools
knives
broaches
reamers
Exercises
1.
Show how the exploitation of new iron ore fields has helped to change the
balance of world power during the present century. (7.10)
2.
What advantages has 'direct reduction' over the blast-furnace process for pig
iron production? (7.25)
3.
Outline the essential chemistry common to all modern steel-making processes.
(7.33-7.36)
4.
Discuss both the economic and technical advantages of modern BOS processes
as compared with the obsolete Bessemer process. (7.36)
5.
Why has electric-arc steelmaking survived despite the high cost of electric
power? (7.37)
As shown in Fig. 7.7, the hardness of a plain carbon steel increases
progressively with increase in carbon content, so that generally the low-
and medium-carbon steels are used for structural and constructional work,
whilst the high-carbon steels are used for the manufacture of tools and
other components where hardness and wear-resistance are necessary.
Commercial plain carbon steels may be classified into five groups as
indicated in Table 7.1 which, along with Fig. 7.7, indicates some of the
common uses of these alloys.
Table 7.1
Type of steel
Dead mild
Mild
Medium carbon
High carbon
Tool steels
Percentage
carbon
0.05-0.15
0.10-0.20
0.20-0.30
0.30-0.40
0.40-0.50
0.50-0.60
0.60-0.70
0.70-0.80
0.80-0.90
0.90-1.00
1.00-1.10
1.10-1.20
1.20-1.30
1.30-1.40
Uses
Chain,
stampings, rivets, wire, nails, seam-welded pipes,
mattresses, hot- and cold-rolled strip for many purposes
Structural steels, RSJ, screws, machine parts, tin-plate,
case-hardening, drop-forgings, stampings
Machine and structural work, gears, free-cutting steels, shafting,
levers, forgings
Connecting-rods, shafting, wire, axles, fish-plates, crane hooks,
high-tensile tubes, forgings
Crankshafts, axles, gears, shafts, die-blocks, rotors, tyres,
heat-treated machine parts
Loco tyres, rails, laminated springs, wire ropes
Drop-hammer dies, set-screws, screw-drivers, saws, mandrels,
caulking tools, hollow drills
Band saws, anvil faces, hammers, wrenches, laminated springs,
car bumpers, small forgings, cable wire, dies, large dies for cold
presses
Cold chisels, shear blades, cold setts, punches, rock drills, some
hand tools
Springs, high-tensile wire, axes, knives, dies, picks
Drills,
taps, milling cutters, knives, screwing dies
Ball bearings, dies, drills, lathe tools, woodworking tools
Files,
reamers, knives, broaches, lathe and wood-working tools
Saws,
razors, boring and finishing tools, machine parts where
resistance to wear is essential
6. Relate the changes in microstructure to the properties of normalised steels as
the carbon content increases from 0.1 to 1.2%. Comment on the choice of
carbon content for some typical engineering applications. (7.50)
7.
Show how the mechanical properties of tensile strength, % elongation and
hardness for normalised plain-carbon steels vary with carbon content up to
1.2%.
Sketch and label the microstructures of the following normalised steels
(i) 0.2%C; (ii) 0.8%C; (iii)
1.2%C.
(Fig 7.7)
Bibliography
Peters, A. J., Ferrous Production Metallurgy, John Wiley, 1982.
United States Steel Corporation, The Making, Shaping and Treatment of Steel.
BS 970: 1973 and 1988 Wrought Steels in the Form of Blooms, Billets, Bars and
Forgings (Part
1—carbon
steels).
BS 4659: 1971 Tool Steels.
The Formation of Alloys
8.10 Pure metals are rarely used for engineering purposes except where
high electrical conductivity, high ductility or good corrosion resistance are
required. These properties are generally at a maximum value in pure
metals, but such mechanical properties as tensile strength, yield point and
hardness are improved by alloying. In most cases the manufacture of an
alloy involves melting two or more metals together and allowing the mix-
ture to solidify in a suitably-shaped mould. The main object of this chapter
is to study the type of microstructure which is produced when the alloy
solidifies.
8.11 Broadly speaking, we are only likely to obtain a metallurgically
useful alloy when the two metals in question dissolve in each other in the
liquid state to form a completely homogeneous liquid solution. There are
exceptions to this rule, as, for example, the suspension of undissolved lead
particles in a free-cutting alloy (6.64). The globules of lead are undissolved
in the molten, as well as in the solidified alloy. In general however, a useful
alloy is formed only when the metals in question are mutually soluble in
the liquid state forming a single liquid which is uniform in composition
throughout.
8.12 When such a homogeneous liquid solution solidifies one of the
following conditions is possible:
(a) The solubility ends and the two metals which were mutually soluble
as liquids become totally insoluble as solids. They therefore separate out
as particles of the two pure metals.
(h) The solubility prevailing in the liquid state may be retained either
completely or partially in the solid state. In the former case a single solid
solution will be formed, whilst in the latter case a mixture of two different
solid solutions will result.
(c) As solidification proceeds the two metals give rise to what is loosely
termed an intermetallic compound but which in many cases might more
accurately be described as an
intermediate
phase.
8.13 Any one of the separate substances referred to in 8.12 is called a
phase. Thus a solid phase may be either a pure metal or a solid solution
8
or some intermediate phase of two or more metals so long as it exists in
the microstructure as a separate entity.
A law, based on thermodynamical considerations, and known as the
Phase Rule, was formulated originally in 1876 by Josiah W. Gibbs, Pro-
fessor of Mathematical Physics at Yale. Certain principles, derived from
this Phase Rule (9.24) are of use to us in a study of physical metallurgy.
For example, it can be demonstrated that in a solid binary* alloy not more
than two phases can co-exist. Therefore a binary alloy may contain:
(a) Two pure metals existing as separate entities in the structure. This
is a very rare occurrence, since in most cases there is a slight solubility
of one metal in the other.
(b) A single complete solid solution of one metal dissolved in the
other.
(c) A mixture of two solid solutions if the metals are only partially
soluble in each other.
(d) A single intermediate phase of some type.
(e) A mixture of two different intermediate phases.
(J) A solid solution and an intermediate phase mixed together—and
so on.
8.14 The general procedure in making an alloy is first to melt the metal
having the higher melting point and then to dissolve in it the metal with
the lower melting point, stirring the mixture so that a homogeneous liquid
solution is formed. When this liquid solidifies the type of structure which
is produced depends largely upon the relative physical and chemical
properties of the two metals and in part on their relative positions in the
Periodic Classification (1.69). If they are very near to each other in the
Periodic Table it is probable—though not inevitable—that they will form
mixed crystals (ie a solid solution) in which the atoms of each metal are
randomly placed in a common crystal pattern. This is rather like the effect
which would be produced if a bricklayer used both red and blue bricks
indiscriminately in building a wall. If the red and blue bricks were of
approximately equal size then a strong and reasonably regular wall would
result even though it looked a bit untidy due to the lack of pattern in
placing the different coloured bricks.
If the two metals are widely separated in the Periodic Table so that their
chemical properties, and probably their atomic sizes, differ considerably
then it is possible—but again, not certain—that they will combine to form
some sort of chemical compound (an 'intermetallic compound'). Between
these two extremes the atoms of the two metals may associate with each
other in different patterns to form transitional phases known collectively
as
intermediate
phases.
We will now consider in some detail the nature of the phases and struc-
tures mentioned above.
* Containing two metals.
The Solid Solution
8.20 A solid solution is formed when two metals which are mutually
soluble in the liquid state remain dissolved in each other after crystallis-
ation. If the resulting structure is examined, even with the aid of a high-
power electron microscope, no trace can be seen of the parent metals as
separate entities. Instead one sees crystals consisting of a homogeneous
solid solution of one metal in the other, and if the metals are completely
soluble in each other there will be crystals of one type only.
8.21 It will be clear that in order that one metal may dissolve in another
to form a solid solution, its atoms must fit in some way into the crystal
lattice of the other metal. This may be achieved by the formation of either
a 'substitutional' or an 'interstitial' solid solution (Fig. 8.1).
Fig.
8.1 Ways in which solid solution can occur.
8.22 Interstitial solid solutions can be formed only when the atoms of
the added element are very small compared with those of the parent metal,
thus enabling them to fit into the interstices or spaces in the crystal lattice
of the parent metal. Not only can this occur during solidification but, also
in many cases, when the parent metal is already
solid.
Thus, carbon can
form an interstitial solid solution with FCC iron during the solidification
of steel, but it can also be absorbed by solid iron provided the latter is
heated to a temperature at which the structure is face-centred cubic. This
is the basis of carburising steel (19.20). Nitrogen is also able to dissolve
interstitially in solid steel, making the nitriding process (19.40) possible;
whilst hydrogen, the atoms of which are very small indeed, is able to
dissolve in this way in a number of solid metals, usually producing mechan-
ical brittleness as a result.
8.23 In a substitutional solid solution the atoms of the solvent or parent
metal are replaced in the crystal lattice by atoms of the solute metal. This
substitution can be either 'disordered' or 'ordered' (Fig. 8.1). Solubility of
one metal in another is governed in part by the relative sizes of their atoms.
Thus disordered solid solutions are formed over a wide range of alloy
compositions when the atoms—or, properly, their positive ions—are of
similar size but when the atomic diameters differ by more than 14% of
DISORDERED
ORDERED
SUBSTITUTIONAL
INTERSTITIAL
that of the atom of the solvent metal then solubility will be slight because
the atomic size factor is unfavourable.
The size factor effect arises from the strain which is induced in the crystal
lattice by a badly misfitting solute atom. The atoms of the solvent metal
are displaced either outwards or inwards according as the solute atom is
either larger or smaller than the atom of the solvent metal. This distortion
of the crystal lattice from the ideal shape increases the energy of the crystal
and tends to restrict solubility in it.
Electrochemical properties of the two metals also influence the extent
to which they will form a solid solution. Thus if the two metals have very
similar electrochemical properties, generally indicated by a close proximity
to each other in the Periodic Table, then they may form solid solutions
over a wide range of compositions but if one metal is very strongly electro-
positive and the other only weakly electropositive as shown by a wide
separation in the Table, then they are very likely to form some type of
compound.
As might be expected, the lattice structures of the two metals will influ-
ence the extent to which they are likely to form substitutional solid sol-
utions. If the structures are similar then it is reasonable to expect that one
metal can replace the other satisfactorily in its lattice. Thus two FCC metals
like copper and nickel will form a continuous series of solid solutions—
assisted by the fact that atom sizes of these two metals are almost the
same.
The valency of one metal relative to the other will also affect the tend-
ency to form a solid solution. A metal of lower valency is more likely to
dissolve one of higher valency than vice versa. Thus copper will dissolve
considerable amounts of many other metals giving useful ranges of solid
solution alloys.
To summarize, a high degree of substitutional solid solubility between
two metals is likely to be obtained when:
1.
The difference in atomic radii is less than 14%.
2.
Electrochemical properties are similar.
3.
The two metals have similar crystal structures.
Even though the size factor of the two metallic atoms may favour their
forming a solid solution they are more likely to form compounds if one is
much more strongly electropositive than the other.
When a disordered solid solution crystallises from the liquid state there
is a natural tendency for the core of the dendrite to contain rather more
atoms of the metal with the higher melting point, whilst the outer fringes
of the crystal will contain correspondingly more atoms of the metal of
the lower melting point. Nevertheless, within such a 'cored' crystal, once
formed, diffusion does take place; and this diffusion tends to produce
uniform distribution of each kind of atom. Diffusion is only appreciable at
relatively high temperatures, for which reason a rapidly cooled alloy will be
appreciably cored and a slowly cooled alloy only slightly cored. Prolonged
annealing will remove coring completely by allowing diffusion to take place
and produce a uniform solid solution. The driving force behind diffusion
is the reduction in strain energy produced by the fact that as solute atoms
THE
CORING
OF
SOLID
SOLUTIONS.
DIAGRAMS
INDICATING THE RELATIVE DISTRIBUTIONS OF ATOMS OF
METALS<g)4®
AT
VARIOUS
STAGES
DURING THE SOLIDIFICATION AND
ANNEALING OFA SOLID SOLUTION CONTAINING 5O% OFEACH METAL.
ATOMS OF
(g)-THE
METAL WITH THE
HIGHER
MELTING POINT.
ATOMS
OF ®- THE METAL
WITH
THE
LOWER
MELTING POINT.
4. ANNEALING HAS
CAUSED
DIFFUSION
TO
TAKE
PLACE RESULTING IN A
HOMOGENEOUS SOLID SOLUTION OF
METALS
® & (D .
Fig. 8.2 The
variations
in
composition
which
are
possible
in a
solid
solution.
3.
COMPLETELY SOLID — THE CORE OF
THE
CRYSTAL
IS RICH IN
METAL®
AND
THE OUTER FRINGES RICH IN
METAL®.
2. DENDRITES RICH IN
METAL®
BEGIN
TO
GROW
MAKING THE REMAINING
LIQUID CORRESPONDINGLY RICH IN
METAL®.
I. A HOMOGENEOUS LIQUID SOLUTION
OF METALS ®&®.
move away from regions of high concentration the overall distortion of the
crystal is reduced. The effects of the phenomena of coring and diffusion
are illustrated in Fig. 8.2.
8.24 In an interstitial solid solution diffusion can take place with rela-
tive ease, since the solute atoms, being small, can move readily through
the crystal lattice of the parent metal, but in substitutional solid solutions
the mechanism of diffusion is still open to speculation. It was stated earlier
(4.15) that faults can occur in crystals, giving rise to dislocations which
facilitate slip. Similarly, it is suggested that what are called 'vacant sites'
can exist in a metal crystal. These are positions in the crystal lattice which
are not occupied by an atom (Fig. 8.3).
Since there is always some difference in size between the solute atoms
and the atoms of the parent metal, the presence of a solute atom will cause
some distortion in the lattice of the parent metal. This distortion will be
minimised by the association of a solute atom with a vacant site as shown
in Fig.
8.4A.
The associated pair of vacant site with solute atom can migrate
easily through the crystal in stages as indicated in Fig. 8.4
B-F.
The rate of
diffusion will depend to some extent upon the number of vacant sites, but
also upon the concentration gradient of the solute atoms within the struc-
ture of the solvent metal (Fig. 8.6); and upon the temperature at which
diffusion is taking place. The general result will be for solute atoms to
diffuse away from regions of high concentration towards regions of low
Fig.
8.3 A 'vacant site' in a crystal lattice.
Fig.
8.4 The diffusion of a solute atom which is associated with a vacant site.