Bhadeshia H.K.D.H., Honeycombe R. Steels: Microstructure and Properties
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18 CHAPTER 2 THE STRENGTHENING OF IRON AND ITS ALLOYS
2.2 WORK HARDENING
Work hardening is an important strengthening process in steel, particularly in
obtaining high strength levels in rod and wire, both in plain carbon and alloy
steels. For example, the tensile strength of an 0.05 wt% C steel subjected to
95% reduction in area by wire drawing, is raised by no less than 550 MN m
−2
,
while higher carbon steels are strengthened by up to twice this amount. Indeed,
without the addition of special alloying elements, plain carbon steels can be
raised to strength levels above 1500 MN m
−2
simply by the phenomenon of
work hardening.
Basic work on the deformation of iron has largely concentrated on the other
end of the strength spectrum, namely pure single crystals and polycrystals sub-
jected to small controlled deformations. This approach has shown that the slip
plane in α-iron is not unique. Slip occurs on several planes, {110},{112} and {123},
but always in the close packed 111 direction which is common to each of these
planes (Fig. 2.1). The diversity of slip planes leads to rather irregular wavy slip
bands in deformed crystals, as the dislocations can readily move from one type
of plane to another by cross slip, provided they share a common slip direction.
The Burgers vector of the slip dislocations would thus be expected to be
a
2
111,
which has been confirmed by thin-foil electron microscopy.
The yield stress of α-iron single crystals is very sensitive to both temperature
and strain rate, and a similar dependence has been found for less pure polycrys-
talline iron. Figure 2.2 shows the flow stress σ
T
at temperature T, less than that
at room temperature σ
293
,plotted against T, showing that both single crystal and
polycrystalline iron of different interstitial content give values falling on the one
curve. Therefore, the temperature sensitivity cannot be attributed to interstitial
impurities. It is explained by the effect of temperature on the stress needed to
move free dislocations in the crystal, the Peierls–Nabarro stress. Direct obser-
vation of screw dislocations in iron in the electron microscope has shown that
their ease of mobility decreases strongly with decreasing temperature.
If the shear stress at any point on the stress–strain curve is considered, the
measured shear stress τ for further deformation comprises two quantities:
τ = τ
∗
+ τ
i
. (2.1)
Fig. 2.1 The slip systems in the bcc structure.
2.2 WORK HARDENING 19
Fig. 2.2 Temperature dependence of the flow stress of single crystals and polycrystals of pure
iron (Christian, Philosophical Transactions of the Royal Society 261, 253, 1967).
The effective shear stress, τ
∗
arises from the interaction of the dislocations
with short range obstacles, e.g. isolated dislocations. This stress is strongly
temperature dependent as thermal activation is helpful in moving dislocations
around short range obstacles.
1
On the other hand, τ
i
is the internal stress arising
from long range obstacles such as grain boundaries, cell walls and other com-
plex dislocation arrays.
2
In these circumstances thermal fluctuations are of no
assistance. The two component stresses are defined as follows:
τ
∗
=
1
V
H
0
+ kT ln
l˙ε
ρmAbγ
, (2.2)
τ
i
= αµbρ
½
, (2.3)
where V =activation volume
H
0
=activation enthalpy at τ =0
k =Boltzmann’s constant
T =temperature
l =length of dislocation line activated
˙ε =strain rate
m =mobile dislocation density
A =area of glide plane covered by dislocation
1
Conrad, K., Journal of the Iron and Steel Institute 198, 364, 1961.
2
Michalak, J. J.,Acta Metallurgica 13, 213, 1965.
20 CHAPTER 2 THE STRENGTHENING OF IRON AND ITS ALLOYS
b =magnitude of Burgers vector
γ =frequency of vibration of dislocation line length
α =constant
µ =shear modulus
ρ =dislocation density
The initial flow stress or yield stress with its large temperature dependence
arises primarily from τ
∗
, while the increment in flow stress resulting from work
hardening is largely independent of temperature, and is caused by the increase
in τ
i
with increasing strain as the dislocation density ρ increases.
To summarize, work hardening in conventional materials is largely due to
the creationof crystal defects, primarily dislocations,during plastic deformation.
The hardening can reach saturation once the defect creation and annihilation
rates balance.
Work hardening has an important consequence on ductility. During tensile
testing, the sample will inevitably contain features which cause the stress to
concentrate and hence to initiate necking. The reduced cross-sectional area
at the neck increases the stress in the necked region. In the absence of work
hardening to help resist local deformation, the neck becomes unstable and the
sample fractures with poor overall ductility. To encourage uniform elongation,
the work hardening rate must raise the yield strength at a rate greater than the
increase in stress due to the reduced area at the neck.
Given the origin of work hardening,microstructures in which the dislocation
density does not substantially increase during deformation should lack ductility.
A pertinent exampleof such amicrostructure, consisting ofextremely fine grains,
is discussed in Section 2.5.
2.3 SOLID SOLUTION STRENGTHENING BY INTERSTITIALS
Carbon and nitrogen have a disproportionate influence on the strength of fer-
ritic iron and a relatively minor effect on that of austenitic iron. Solid solution
strengthening occurs when the strain fields around misfitting solutes interfere
with the motion of dislocations. Atoms which substitute for iron cause local
expansions or contractions; these strains are isotropic and therefore can only
interact with the hydrostatic components of the strain fields of dislocations. In
contrast, an interstitial atom located in the irregular octahedron interstice in
ferrite causes a tetragonal distortion (Fig. 2.3) which has a powerful interaction
with the shear which is the dominant component of a dislocation strain field.
This is why interstitial solid solution strengthening is so potent in ferrite. The
corresponding interstitial site in austenite is the regular octahedron. An inter-
stitial atom in austenite therefore behaves like a substitutional solute, with only
hydrostatic strains surrounding it. This is why carbon is much less effective in
strengthening austenite.
2.3 SOLID SOLUTION STRENGTHENING BY INTERSTITIALS 21
(a) (b)
Fig. 2.3 (a)The regular octahedron interstice in austenite. (b) Octahedral interstice in ferrite –
notice that two of the axes are longer than the third (vertical axis). This leads to a tetragonal
distortion when the site is occupied by carbon.
Ferritic iron has such a small solubility for carbon that there is an over-
whelming tendency for the carbon to segregate to defects. This leads to another
significant effect in α-iron, that carbon and nitrogen can promote heterogeneous
deformation by making it difficult to initiate plastic flow. This is the yield point
effect, described next.
2.3.1 The yield point
Carbon and nitrogen, even in concentrations as low as 0.005 wt%, in iron lead
to a sharp transition between elastic and plastic deformation in a tensile test
performed on ferritic iron (Fig. 2.4a). Decarburization of the iron results in the
elimination of this sharp transition or yield point, which implies that the solute
atoms are in some way responsible for this striking behaviour. Frequently the
load drops dramatically at the upper yield point (A) to another value referred to
as the lower yield point (B). Under some experimental conditions, yield drops
of about 30% of the upper yield stress can be obtained. Following the lower
yield point, there is frequently a horizontal section of the stress–strain curve
(BC) during which the plastic deformation propagates at a front which can
move uniformly along the specimen. This front is referred to as a Luders band
(Fig. 2.4b), and the horizontal portion, BC, of the stress–strain curve as the
Luders extension. The development of Luders bands can be much less uniform
and, e.g., in pressings where the stress is far from uniaxial, complex arrays of
bands can be observed. These are often referred to as stretcher strains, but they
are still basically Luders bands. When the whole specimen has yielded, general
work hardening commences and the stress–strain curve begins to rise in the
normal way. If, however, this deformation is interrupted, and the specimen
allowed to rest either at room temperature or for a shorter time at 100–150
◦
C,
22 CHAPTER 2 THE STRENGTHENING OF IRON AND ITS ALLOYS
Fig. 2.4 (a) Schematic diagram of yield phenomena as shown in a tensile test, (b) Luders bands
in deformed steel specimens (Hall,Yield Point Phenomena in Metals and Alloys, Macmillan, London,
1970), (c) Luders bands in a notched steel specimen.
2.3 SOLID SOLUTION STRENGTHENING BY INTERSTITIALS 23
on reloading a new yield point is observed (D). This return of the yield point is
referred to as strain ageing.
2.3.2 The role of interstitial elements in yield phenomena
The sharp upper and lower yield point in iron is eliminated by annealing in wet
hydrogen, which reduces the carbon and nitrogen to very low levels. However,
substantial strain ageing can occur at carbon levels around 0.002 wt%, and as
little as 0.001–0.002 wt% N can result in severe strain ageing. Nitrogen is more
effective in this respect than carbon, because its residual solubility near room
temperature is substantially greater than that of carbon (Table 1.3).
Cottrell and Bilby first showed that interstitial atoms such as carbon and
nitrogen would interact strongly with the strain fields of dislocations. The inter-
stitial atoms have strain fields around them, but when such atoms move within
the dislocation strain fields, there should be an overall reduction in the total
strain energy. This leads to the formation of interstitial concentrations or atmos-
pheres in the vicinity of dislocations, which in an extreme case can amount to
lines of interstitial atoms along the cores of the dislocations (condensed atmos-
pheres), e.g. in edge dislocations at the region of the strain field where there is
maximum dilation (Figs 2.5a, b). The binding energy between a dislocation in
iron and a carbon atom is about 0.5 eV. Consequently dislocations can be locked
in position by strings of carbon atoms along the dislocations, thus substantially
raising the stress which would be necessary to cause dislocation movement.
A particular attraction of this theory is that only a very small concentration of
interstitial atoms is needed to produce locking along the whole length of all
dislocation lines in annealed iron. For a typical dislocation density of 10
8
lines
cm
−2
in annealed iron, a carbon concentration of 10
−6
wt% would be sufficient
to provide one interstitial carbon atom per atomic plane along all the disloca-
tion lines present, i.e. to saturate the dislocations. Consequently, this theory can
explain the observation of yield phenomena at very low carbon and nitrogen
concentrations.
The formation of interstitial atmospheres at dislocations requires diffusion
of the solute.As both carbonand nitrogendiffuse verymuch more rapidlyin iron
Fig. 2.5 Interstitial atoms in the vicinity of an edge dislocation (a) random atmosphere
(b) condensed.
24 CHAPTER 2 THE STRENGTHENING OF IRON AND ITS ALLOYS
than substitutional solutes, it is not surprising that strain ageing can take place
readily in the range 20–150
◦
C. The interstitial concentration, c, in a dislocation
strain field at a point where the binding energy is U is given by:
c = c
0
e
U/kT
, (2.4)
where c
0
is the average concentration. In general, this approach leads to a
Maxwellian distribution of solute about the dislocation, but for carbon and
nitrogen in steel,the elastic interaction energy U between solute and dislocation
is so large that U ≥kT. Consequently the atmosphere condenses to form rows
of interstitial atoms along the cores of the dislocations.
The critical temperature T
crit
below which there is condensation of the
atmosphere, occurs when c =1 and U =U
max
:
T
crit
=
U
max
k ln
1
c
0
. (2.5)
Therefore, if c
0
=10
−4
and U
max
≃10
−19
J, T
crit
=700 K. Thus the yield point
would be expected at temperatures below 700 K, but it should disappear at
higher temperatures when dislocations can escape from their atmospheres as
a result of thermal activation. This corresponds approximately with results of
experiments showing the temperature dependence of the stress–strain curve of
mild steel (Fig. 2.6). As expected, the zone of yielding is well defined at the
lower testing temperatures, becoming less regular as the temperature is raised,
until it is replaced by fine serrations along the whole stress–strain curve. This
phenomenon is referred to as dynamic strain ageing, in which the serrations
represent the replacement of the primary yield point by numerous localized
yield points within the specimen. These arise because the temperature is high
enough to allow interstitial atoms to diffuse during deformation, and to form
atmospheres around dislocations generated throughout the stress–strain curve.
Steels tested under these conditions also show low ductilities, due partly to the
high dislocation density and partly to the nucleation of carbide particles on the
dislocations where the carbon concentration is high. The phenomenon is often
referred to as blue brittleness, blue being the interference colour of the steel
surface when oxidized in this temperature range.
The break away of dislocations from their carbon atmospheres as a cause
of the sharp yield point became a controversial aspect of the theory because it
was found that the provision of free dislocations, e.g., by scratching the surface
of a specimen, did not eliminate the sharp yield point. An alternative theory
was developed which assumed that, once condensed carbon atmospheres are
formed in iron, the dislocations remain locked, and the yield phenomena arise
from the generation and movement of newly formed dislocations (Gilman and
Johnston). The velocity of movement v of these fresh dislocations is related to
2.3 SOLID SOLUTION STRENGTHENING BY INTERSTITIALS 25
Fig. 2.6 Typical stress–strain curves for mild steel at elevated temperatures (Hall,Yield Point
Phenomena in Metals and Alloys, Macmillan, 1970).
the applied stress as follows:
v =
σ
σ
r
m
, (2.6)
where σ
r
is a reference stress, σ is the yield stress and m is an index characteristic
of the material, varying between 1 and 60. The strain rate ˙ε can be defined in
terms of the movement of dislocations, as
˙ε = nvb, (2.7)
where n is the number of mobile dislocations per unit area, v is their average
velocity and b is Burgers vector.
Using Equation (2.7) the strain rates at the upper yield point (˙ε
U
) and the
lower yield point (˙ε
L
) can be defined as follows:
˙ε
U
= ρ
U
v
U
b,
where ρ
U
is the mobile dislocation density at the upper yield point and
˙ε
L
= ρ
L
v
L
b,
26 CHAPTER 2 THE STRENGTHENING OF IRON AND ITS ALLOYS
where ρ
L
is the mobile dislocation density at the lower yield point, so
v
U
v
L
=
ρ
L
ρ
U
,
and using Equation (2.6)
σ
U
σ
L
=
ρ
L
ρ
U
1
m
. (2.8)
Thus the ratio σ
U
/σ
L
will be large, i.e. there will be a large yield drop, where m
is small, and when ρ
L
is much larger than ρ
U
. Consequently, if at the upper yield
stress the density of mobile dislocations is low, e.g. as a result of solute atom
locking, a large drop in yield stress will occur if a large number of new disloca-
tions is generated. Observations indicate that the dislocation density just after
the lower yield stress is much higher than that observed at the upper yield point.
To summarize, the occurrence of a sharp yield point depends on the occur-
rence of a sudden increase in the number of mobile dislocations. However, the
precise mechanism by which this takes place will depend on the effectiveness
of the locking of the pre-existing dislocations. If the pinning is weak, then the
yield point can arise as a result of unpinning. However, if the dislocations are
strongly locked,either by interstitial atmospheres or precipitates, the yield point
will result from the rapid generation of new dislocations.
Under conditions of dynamic strain ageing, where atmospheres of carbon
atoms form continuously on newly generated dislocations, it would be expected
that a higher density of dislocations would be needed to complete the deform-
ation, if it is assumed that most dislocations which attract carbon atmospheres
are permanently locked in position. Electron microscopic observations have
shown that in steels deformed at 200
◦
C, the dislocation densities are an order
of magnitude greater than those in specimens similarly deformed at room tem-
perature. This also accounts for the fact that increased work hardening rates are
obtained in the blue-brittle condition as compared to tests at room temperature.
2.3.3 Strengthening at high interstitial concentrations
Austenite can take into solid solution up to 10 at% carbon which can be retained
in solid solution by rapid quenching. However,in these circumstances the phase
transformation takes place, not to ferrite but to a body-centred tetragonal struc-
ture referred to as martensite (see Chapter 5). This phase forms as a result of a
diffusionless shear transformation leading to characteristic laths or plates, which
normally appear acicular in polished and etched sections. If the quench is suffi-
ciently rapid, the martensite is essentially a supersaturated solid solution of car-
bon in a tetragonal iron matrix,and as the carbon concentration can be greatly in
excess of the equilibrium concentration in ferrite,the strength is raised very sub-
stantially. High carbon martensites are normally very hard but brittle, the yield
strength reaching as much as 1500 MN m
−2
;much of this increase can be directly
2.5 GRAIN SIZE 27
attributed to increased interstitial solid solution hardening, but there is also a
contribution from the high dislocation density which is characteristic of marten-
sitic transformations in iron–carbon alloys. Martensite will be dealt with in more
detail in Chapter 5, which shows that by subjecting it to a further heat treat-
ment at intermediate temperatures (tempering), a proportion of the strength is
retained, with a substantial gain in the toughness and ductility of the steel.
2.4 SUBSTITUTIONAL SOLID SOLUTION
STRENGTHENING OF IRON
Many metallic elements form solid solutions in γ- and α-iron. These are invari-
ably substitutional solid solutions, but for a constant atomic concentration of
alloying elements there are large variations in strength. Using single crystal data
for several metals, Fig. 2.7 shows that an element such as vanadium has a weak
strengthening effect on α-iron at low concentrations (<2 at%), while silicon
and molybdenum are much more effective strengtheners. Other data indicate
that phosphorus, manganese, nickel and copper are also effective strengtheners.
However, it should be noted that the relative strengthening may alter with the
temperature of testing,and with the concentrations of interstitial solutes present
in the steels.
The strengthening achieved by substitutional solute atoms is, in general,
greater the larger the difference in atomic size of the solute from that of iron,
applying the Hume-Rothery size effect. However, from the work of Fleischer
and Takeuchi it is apparent that differences in the elastic behaviour of solute
and solvent atoms are also important in determining the overall strengthening
achieved. In practical terms, the contribution to strength from solid solution
effects is superimposed on hardening from other sources, e.g. grain size and dis-
persions. Also it is a strengthening increment, like that due to grain size, which
need not adversely affect ductility. In industrial steels, solid solution strengthen-
ing is a far from negligible factor in the overall strength, where it is achieved by
a number of familiar alloying elements, e.g. manganese, silicon, nickel, molyb-
denum, several of which are frequently present in a particular steel and are
additive in their effect. These alloying elements are usually added for other
reasons, e.g. Si to achieve deoxidation, Mn to combine with sulphur or Mo to
promote hardenability. Therefore, the solid solution hardening contribution can
be viewed as a useful bonus.
2.5 GRAIN SIZE
2.5.1 Hall–Petch effect
The refinement of the grain size of ferrite provides one of the most important
strengthening routes in the heat treatment of steels. The first scientific analysis
of the relationship between grain size and strength, carried out on ARMCO