Civil Engineering Reference
In-Depth Information
listed 30 different grades of the material. The
Smithells Metals
Reference Book
(Gale and Totemeier, 2003) provides a com-
prehensive listing of almost all available steels.
Steel superseded cast and wrought iron towards the end of the
nineteenth century. In 1877 the Board of Trade (BoT) approved
the use of steel for bridges and Dorman Long rolled the first
joist section in 1885. It is thought that one of the first steel-
framed buildings in the UK was the Ritz Hotel in Piccadilly
London. Strangely, this was also the first known British use of
steel sections in metric units as they were of German origin. At
that time, the traditional way of connecting steel sections was
by the use of cleats and rivets. A helpful topic giving guidance
on the early use of structural steel is the
Historical Structural
Steelwork Handbook
by W. Bates (1984).
One great advantage of steel is that it is easily recycled.
Corus and other industry leaders claim that almost 100% of
steel is recycled (Corus Group, 2004b). The success of this
from the viewpoint of quality is dependent on the correct iden-
tification and selection of the material to be recycled. Good
quality assurance (QA) systems should ensure that impurities
(tramp metal) are kept to a level consistent with the required
quality of the recycled material.
The essential qualities for steel are that it should be:
by heating the ore with coke and lime in a blast furnace. Some
of this brittle material is then reprocessed to produce steel by
removing some of the carbon by blowing oxygen through the
metal in a convertor.
The electric arc furnace method (EAF) uses, as its feed-
stock, mainly scrap iron and steel. For the more critical grades
of steel it is customary to use palletised iron. The industry rec-
ognises that a system that uses largely scrap material may be
subject to injurious tramp material (copper, nickel and tin) so,
as with the BOS system, selection of the scrap material must
be consistent with the required quality of the finished product.
EAF accounts for perhaps 30% of the total output.
14.4.3 Corrosion and other potential defects
14.4.3.1 Atmospheric corrosion
When exposed to moisture and oxygen, steel, being a ferrous
material (containing iron) converts to hydrated iron oxide in
the form of rust. This is an electrochemical process and must
be countered by protecting the steel with paint or other coating.
Prior to treatment it is essential to adequately prepare the metal
by blasting, wire brushing or other means to remove mill scale.
Red lead paints used in the past to protect the steel have been
largely replaced by more sophisticated materials such as alkyds,
chemical resistant or bituminous paints, epoxides or urethanes.
Metallic coatings such as hot dip galvanising, electroplating or
sheradising are also available for particular applications.
In certain environments that are free from chloride contam-
ination it is possible to use weathering steels as an alternative
to more conventional steels and which do not require anti-
corrosion coatings. The specific, alloying elements produce a
stable oxide layer that adheres to base metal and is less por-
ous than the rust on other steels. Chloride contamination might
occur from seawater spray, salt fogs or salts used to clear snow
and ice from roads in winter. Corus recommend that weath-
ering steel should
not
be used within 2 km of a coastline. A
striking use of weathering steel is the
Angel of the North
sculp-
ture near Gateshead in the UK. A limited number of structures
have been constructed using this material and, in the dry clean
atmosphere of Tenerife, a footbridge has been built.
Resistance to atmospheric corrosion may be enhanced by
coating the steel with resistant coatings or galvanising. If
galvanising is used, reference to the section on liquid metal
assisted cracking (LMAC - see section 14.4.3.3 below) should
be made.
available in large quantities at acceptable cost;
■
suitable to be fashioned into suitable sectional shapes;
■
of adequate strength, toughness, durability and ductility;
■
suitable for joining by welding or other devices;
■
suitable for recycling.
■
Structural steel is available in many grades with yield strengths
varying from 185 N/mm
2
(for Grade S185) to 360 N/mm
2
(for
Grade E360) with a Young's modulus of 210 000 N/mm
2
. A
full explanation of these grades and their relevant mechanical
properties may be found in European structural steel standard
EN10025:2004. Further information on weathering steel is to
be found in Corus Group (2004a).
14.4.2 Manufacture
From about 1856, steel was produced using the Bessemer pro-
cess. Bessemer takes its name from Sir Henry Bessemer (1813-
98). In this system, molten pig iron is loaded into a tilting furnace
(known as a Bessemer converter) at about 1250°C. Air is blown
into the converter from the base and spiegel (a pig iron contain-
ing a
high
content of manganese and carbon) is added. The lining
of the converter acts to remove impurities to form a slag. In a
final operation, the furnace is tilted to drain off the molten steel.
The Bessemer process has been largely superseded by
the basic oxygen system (BOS). This is a two-stage process
responsible for most of the steel currently being produced. This
is an incremental process in which usable iron is first produced
some of which is reprocessed into steel. The basic feedstock
is iron ore and up to 25% scrap steel. Liquid iron is produced
14.4.3.2 Bi-metallic corrosion
The corrosion of metals is basically electrochemical in nature
and takes place in the presence of an electrolyte (a solution
containing ions). Although pure water is not a good electrolyte
it is, in practice, often polluted by small amounts of salts, acids
or alkalis which considerably increase the number of ions.
When two metals of differing galvanic potential are in close
contact with an electrolyte, an electric current passes between
