01-10-2012, 04:11 PM
Design of Steel Structures
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Metallurgy of steel
When carbon in small quantities is added to iron, ‘Steel’ is obtained. Since the
influence of carbon on mechanical properties of iron is much larger than other alloying
elements. The atomic diameter of carbon is less than the interstices between iron atoms
and the carbon goes into solid solution of iron. As carbon dissolves in the interstices, it
distorts the original crystal lattice of iron.
This mechanical distortion of crystal lattice interferes with the external applied
strain to the crystal lattice, by mechanically blocking the dislocation of the crystal
lattices. In other words, they provide mechanical strength. Obviously adding more and
more carbon to iron (upto solubility of iron) results in more and more distortion of the
crystal lattices and hence provides increased mechanical strength. However, solubility
of more carbon influences negatively with another important property of iron called the
‘ductility’ (ability of iron to undergo large plastic deformation). The a-iron or ferrite is very
soft and it flows plastically. Hence we see that when more carbon is added, enhanced
mechanical strength is obtained, but ductility is reduced. Increase in carbon content is
not the only way, and certainly not the desirable way to get increased strength of steels.
More amount of carbon causes problems during the welding process. We will see later,
how both mechanical strength and ductility of steel could be improved even with low
carbon content. The iron-carbon equilibrium diagram is a plot of transformation of iron
with respect to carbon content and temperature. This diagram is also called iron-iron
carbon phase diagram (Fig. 1.2). The important metallurgical terms, used in the
diagram, are presented below.
The structural steels or ferrite – pearlite steels
The iron-iron carbide portion of the phase diagram that is of interest to structural
engineers is shown in Fig.1.2. The phase diagram is divided into two parts called
“hypoeutectoid steels” (steels with carbon content to the left of eutectoid point [0.8%
carbon]) and “hyper eutectoid steels” which have carbon content to the right of the
eutectoid point. It is seen from the figure that iron containing very low percentage of
carbon (0.002%) called very low carbon steels will have 100% ferrite microstructure
(grains or crystals of ferrite with irregular boundaries) as shown in Fig 1.2. Ferrite is soft
and ductile with very low mechanical strength. This microstructure at ambient
temperature has a mixture of what is known as ‘pearlite and ferrite’ as can be seen in
Fig. 1.2. Hence we see that ordinary structural steels have a pearlite + ferrite
microstructure. However, it is important to note that steel of 0.20% carbon ends up in
pearlite + ferrite microstructure, only when it is cooled very slowly from higher
temperature during manufacture. When the rate of cooling is faster, the normal pearlite
+ ferrite microstructure may not form, instead some other microstructure called bainite
or martensite may result.
Strengthening structural steels
Cooling rate of steel from austenite region to room temperature produces
different microstructures, which impart different mechanical properties. In the case of
structural steels, the (pearlite + ferrite) microstructure is obtained after austenitising, by
cooling it very slowly in a furnace. This process of slow cooling in a furnace is called
‘annealing’. As, mentioned in the earlier section, the formation of pearlite, which is responsible for mechanical strength, involves diffusion of carbon from ferrite to
austenite. In the annealing process sufficient time is given for the carbon diffusion and
other transformation processes to get completed. Hence by full annealing we get larger
size pearlite crystals as shown in the cooling diagram in Fig. It is very important to note
that the grain size of crystal is an important parameter in strengthening of steel .
Rapid cooling of steels
In the earlier section we saw that steel is made to under-cool by normalizing (by
giving lesser cooling time than required by the equilibrium state of the constitutional
diagram), it results in finer microstructure. However, if we cool steel very rapidly, say
quenching in cold water, there is insufficient time for the shuffling or diffusion of carbon
atoms and hence the formation of ferrite + pearlite is prevented. However, such a fast
cooling results in ‘martensite’. Slightly less rapid cooling could result in a product called
‘bainite’ which is dependent on the composition of steel. Bainite is formed above a
temperature of about 300°C and between a cooling rate of 8.4°C/sec to 0.0062°C/sec.
Martensite is formed by rapid cooling rate less than 8.4°C/sec. Very slow cooling, say
full annealing does not form both Martensite and Bainite.
Inclusions and alloying elements in steel
Steel contains impurities such as phosphorous and sulphur and they eventually
form phosphides and sulphides which are harmful to the toughness of the steel. Hence
it is desirable to keep these elements less than 0.05%. Phosphorous could be easily
removed compared to sulphur. If manganese (Mn) is added to steel, it forms a less
harmful manganese sulphide (MnS) rather than the harmful iron sulphide. Sometimes
calcium, cerium, and other rare earth elements are added to the refined molten steel.
They combine with sulphur to form less harmful elements. Steel treated this way has
good toughness and such steels are used in special applications where toughness is
the criteria. The addition of manganese also increases the under cooling before the start
of the formation of ferrite+ pearlite. This gives fine-grained ferrite and more evenly
divided pearlite. Since the atomic diameter of manganese is larger that the atomic
diameter of iron, manganese exists as ‘substitutional solid solution’ in ferrite crystals, by
displacing the smaller iron atoms. This improves the strength of ferrite because the
distortion of crystal lattice due to the presence of manganese blocks the mechanical
movement of the crystal lattices. However, manganese content cannot be increased
unduely, as it might become harmful. Increased manganese content increases the
formation of martensite and hence hardness and raises its ductile to brittle transition
temperature (temperature at which steel which is normally ductile becomes brittle).
Because of these reasons, manganese is restricted to 1.5% by weight.