Ferrous metals are defined as the metals in which the element iron predominates. The earliest use of the metal was for the manufacture of implements and weapons in the Iron Age commencing in Europecirca 1200 BC. Significant developments were the use by Wren in 1675 of a wrought iron chain in tension to restrain the outward thrust from the dome of St Paul’s Cathedral, the use of cast iron in compression for the Ironbridge at Coalbrookdale in 1779, and by Paxton in the prefabricated sections of the Crystal Palace in 1851.
Steel is a relatively recent material, only being avail- able in quantity after the development of the Bessemer converter in the late nineteenth century. The first steel- frame high-rise building of 10 storeys was built in 1885 in Chicago by William le Baron Jenney.
The platform level of the Waterloo train terminal in London (Fig. 5.1) is covered by curved and tapered 3-pin steel arches, which are designed to accommodate the flexing inevitably caused by the movement of trains at this level. The steel arches, each consisting of two prismatic bow-string trusses connected by a knuckle joint, are asymmetrical to allow for the tight curvature of the site. The tops of the longer trusses are covered with toughened glass providing views towards old Lon- don, with profiled stainless steel spanning between.
The area spanned by the shorter trusses is fully glazed.
The structure is designed for a minimum lifetime of 100 years.
MANUFACTURE OF STEEL
The production of steel involves a sequence of opera- tions which are closely interrelated in order to ensure maximum efficiency of a highly energy-intensive pro- cess. The key stages in the production process are the making of pig iron, its conversion into steel, the cast- ing of the molten steel and its formation into sections or strip. Finally, coils of steel strip are cold-rolled into thin sections and profiled sheet.
Manufacture of pig iron
The raw materials for the production of iron are iron ore, coke and limestone. Most iron ore is imported from America, Australia and Scandinavia, where the
F E R R O U S A N D N O N - F E R R O U S M E T A L S 1 6 5
Fig. 5.1 Structural steelwork—Waterloo train terminal, London.Architects: Grimshaw Architects.Photographs: Courtesy of Jo Reid & John Peck, Peter Strobel
Fig. 5.2 Blastfurnace
iron content of the ore is high. Coke is produced from coking coal, mainly imported from Europe, in batteries of coking ovens. Some of this coke is then sintered with iron ore prior to the iron-making process.
Iron ore, coke, sinter and limestone are charged into the top of the blastfurnace (Fig. 5.2). A hot air blast, sometimes enriched with oxygen, is fed through the tuy`eres into the base of the furnace. This heats the fur- nace to white heat, converting the coke into carbon monoxide, which then reduces the iron oxide to iron.
The molten metal collects at the bottom of the fur- nace. The limestone forms a liquid slag, floating on the surface of the molten iron. Purification occurs as impurities within the molten iron are preferentially absorbed into the slag layer.
carbon (coke)2C + O2
oxygen→ 2CO
carbon monoxide
Fe2O3
iron ore (hematite)
+ 3CO
carbon monoxide→2Fe
iron + 3CO2
carbon dioxide
F E R R O U S A N D N O N - F E R R O U S M E T A L S 1 6 7 The whole process is continuous, as relining the
blastfurnace with the special refractory bricks is expen- sive and time-consuming. From time to time, as the molten slag level rises, excess is tapped off for sub- sequent disposal as a by-product of the steel-making industry. When hot metal is required for the subse- quent steel-making process it is tapped off into huge ladles for direct transportation to the steel converter.
At this stage the iron is only 90–95% pure with sulphur, phosphorus, manganese and silicon as impurities and a carbon content of 4–5%. Waste gases from the blastfur- nace are cleaned and recycled as fuel within the plant.
A blastfurnace will typically operate non-stop for 10 years, producing 40,000 tonnes per week.
Steel-making
There are two standard processes used within the UK for making steel. The basic oxygen process is used for the manufacture of bulk quantities of standard-grade steels, and the electric arc furnace process is used for the production of high-quality special steels and par- ticularly stainless steel. The Manchester Stadium (Fig.
5.3) built for the Commonwealth Games in 2002 and Manchester City Football Club used approximately 2000 tonnes of structural steel.
Basic oxygen process
Bulk quantities of steel are produced by the basic oxy- gen process in a refractory lined steel furnace which can be tilted for charging and tapping. A typical furnace
Fig. 5.4 Basic oxygen furnace
(Fig. 5.4) will take a charge of 350 tonnes and convert it into steel within 30 minutes. Initially scrap metal, accounting for one quarter of the charge, is loaded into the tilted furnace, followed by the remainder of the charge as hot metal direct from the blastfur- nace. A water-cooled lance is then lowered to blow
Fig. 5.3 Structural steelwork—Manchester City Stadium.Architects: Arup Associates.Photograph: Courtesy of Arup Associates
Fig. 5.5 Electric arc furnace
high-pressure oxygen into the converter. This burns off impurities and reduces the excess carbon content while raising the temperature. Argon and a small quantity of nitrogen are introduced at the bottom of the furnace.
Lime is added to form a floating slag to remove fur- ther impurities and alloying components are added to adjust the steel composition, prior to tapping. Finally, the furnace is inverted to run out any remaining slag, prior to the next cycle.
Electric arc process
The electric arc furnace (Fig. 5.5) consists of a refractory-lined hearth, covered by a removable roof, through which graphite electrodes can be raised and lowered. With the roof swung open, scrap metal is charged into the furnace, the roof is closed and the electrodes are lowered to near the surface of the metal.
A powerful electric arc is struck between the electrodes and the metal, which heats it up to melting point. Lime and fluorspar are added to form a slag, and oxygen is blown into the furnace to complete the purification process. When the temperature and chemical analysis are correct, the furnace is tilted to tap off the metal, to which appropriate alloying components may then be added. A typical furnace will produce 150 tonnes of high-grade or stainless steel within 90 minutes.
Casting
Traditionally, the molten steel was cast into ingots, prior to hot rolling into slabs and then sheet. How- ever, most steel is now directly poured, or teemed, and
cast into continuous billets or slabs, which are then cut to appropriate lengths for subsequent processing. Con- tinuous casting (Fig. 5.6), which saves on reheating, is not only more energy efficient than processing through the ingot stage but also produces a better surface finish to the steel.
Hot-rolled steel
Sheet steel is produced by passing 25 tonne hot slabs at approximately 1250◦C through a series of computer-controlled rollers which reduce the thick- ness to typically between 1.5 and 20 mm prior to water cooling and coiling. A 25 tonne slab would produce 1 km coil of 2 mm sheet. Steel sections such as univer- sal beams and columns, channels and angle (Fig. 5.7) are rolled from hot billets through a series ofstandsto the appropriate section.
Cold-rolled steel
Sheet steel may be further reduced by cold rolling, which gives a good surface finish and increases its ten- sile strength. Light round sections may be processed into steel for concrete reinforcement, whilst coiled sheet may be converted into profiled sheet or light steel sections (Fig. 5.7). Cold-reduced steel for construction is frequently factory finished with zinc, alloys including terne (lead and tin) or plastic coating. Cold-reduced sheet steel of structural quality is covered by BS ISO 4997: 2007.
F E R R O U S A N D N O N - F E R R O U S M E T A L S 1 6 9
Fig. 5.6 Continuous casting
CARBON CONTENT OF FERROUS METALS
The quantity of carbon alloyed with iron has a pro- found influence on the physical properties of the metal due to its significant effect on the microscopic crystal structure (Fig. 5.8). At ambient temperature a series of crystal forms (ferrite, pearlite and cementite) asso- ciated with different proportions of iron and carbon are stable. However, on increasing the temperature, crystal forms that were stable under ambient condi- tions become unstable and are recrystallised into the high-temperature form (austenite). This latter crys- tal structure can be trapped at room temperature by the rapid quenching of red-hot steel, thus partially or completely preventing the natural recrystallisation processes which otherwise would occur on slow cool- ing. These effects are exploited within the various heat treatments that are applied to steels in order to widen the available range of physical properties.
Wrought iron
Wrought iron contains only about 0.02% of carbon.
It was traditionally made by remelting and oxidising pig iron in a reverberatory furnace. The process was continued until virtually all the high carbon content of the pig iron had been burnt off to produce a pasty wrought iron, which was withdrawn from the furnace and then hammered out. Wrought iron is fibrous in character due to the incidental incorporation within the metal of slag residues and impurities such as mag- nesium sulphide, which are formed into long veins
by the hammering process. Wrought iron has a high melting point, approaching 1540◦C, depending on its purity. It was traditionally used for components in ten- sion due to its tensile strength of about 350 MPa. It is ductile and easily worked or forged when red hot, and thus eminently suitable for crafting into orna- mental ironwork, an appropriate use because of its greater resistance to corrosion than steel. Because of its high melting point, wrought iron cannot be welded or cast. Production ceased in the UK in 1974 and mod- ern wrought iron is either recycled old material or, more frequently, low-carbon steel, with its attendant corrosion problems.
Cast iron
Cast iron contains in excess of 2% carbon in iron. It is manufactured by the carbonising of pig iron and scrap with coke in a furnace. The low melting point of around 1130◦C and its high fluidity when molten, give rise to its excellent casting properties but, unlike wrought iron, it cannot be hot worked and is gener- ally a brittle material. The corrosion resistance of cast iron has been exploited in its use for boiler castings, street furniture and traditional rainwater goods. Mod- ern foundries manufacture castings to new designs and as reproduction Victorian and Edwardian com- ponents.
Differing grades of cast iron are associated with different microscopic crystal structures. The com- mon grey cast iron contains flakes of graphite, which cause the characteristic brittleness and impart the grey
Fig. 5.7 Hot-rolled and cold-rolled sections (after Trebilcock 1994) colour to fractured surfaces. White cast iron contains the carbon as crystals of cementite (iron carbide, Fe3C) formed by rapid cooling of the melt. This material may be annealed to reduce its brittle character. A more ductile cast iron (spheroidal cast iron) is produced by the addition of magnesium and ferrosilicon and annealing which causes the carbon to crystallise into graphite nodules. This material has an increased tensile strength and significantly greater impact resistance. All
Fig. 5.8 Effect of carbon content on the properties of wrought iron and steels
cast irons are strong in compression. The designation system for cast irons is described in BS EN 1560: 1997.
Road iron goods, such as manhole covers, made from largely recycled grey cast iron are heavy but brit- tle. Where increased impact resistance is required for public roads, lighter and stronger ductile iron compo- nents are used. Traditional sand-cast rainwater goods are usually manufactured from grey cast iron, while cast iron drainage systems are manufactured from both grey and ductile iron. Unlike steel, cast iron does not soften prematurely in a fire, but may crack if cooled too quickly with water from a fire hose. Cast iron drainage systems in both grey and spheroidal cast iron are covered by the standard BS EN 877: 1999. Cast iron drainage systems are particularly appropriate in heritage and conservation areas.
Steels
A wide range of steels are commercially available reflecting the differing properties associated with car- bon content, the various heat treatments and the addition of alloying components.
Carbon contents of steels range typically between 0.05% and 1.7% and this alone is reflected in a wide spectrum of physical properties. The low- carbon (0.05–0.15%) and mild steels (0.15–0.25%) are
F E R R O U S A N D N O N - F E R R O U S M E T A L S 1 7 1 relatively soft and can be subjected to extensive cold
working. Medium-carbon steels (0.25–0.5%), which are often heat treated, are hard wearing. High-carbon steels (0.5–0.9%) and carbon tool steels (0.9–1.7%) exhibit increasing strength and wear resistance with increasing carbon content. In addition, extra-low- carbon steel (<0.02%) and ultra-low-carbon steel (<0.01%), with carbon contents similar to that of tra- ditional wrought iron, are used for high formability and drawing applications.
HEAT TREATMENT OF STEELS
The physical properties of steels can be modified by various heat treatments which involve heating to a particular temperature followed by cooling under con- trolled conditions. The full range of heat treatments is described in BS EN 10052: 1994. Construction steels for the manufacture of component parts requiring quenching and tempering (+QT) or normalising (+N) are covered by BS EN 10343: 2009.
Hardening
Rapidly quenched steel, cooled quickly from a high temperature in oil or water, thus retaining the high- temperature crystalline form, is hard and brittle. This effect becomes more pronounced for the higher car- bon content steels, which are mostly unsuitable for engineering purposes in this state. Quenching is often followed by tempering to reduce excessive hardness and brittleness.
Annealing and normalising
These processes involve the softening of the hard steel, by recrystallisation, which relieves internal stresses within the material and produces a more uniform grain structure. For annealing, the steel is reheated and soaked at a temperature of approximately 650◦C, then cooled slowly at a controlled rate within a furnace or cooling pit. This produces the softest steel for a given composition. With normalising, the steel is reheated to 830–930◦C for a shorter period and then allowed to cool more rapidly in air. This facilitates subsequent cold working and machining processes.
Tempering
Reheating the steel to a moderate temperature (400–600◦C), followed by cooling in air, reduces the brittleness, allowing some recrystallisation of the
metal. The magnitude of the effect is directly related to the tempering temperature with ductility increasing, and tensile strength reducing, for the higher process temperatures.
Carburising
Components may be case hardened to produce a higher carbon content on the outer surface, whilst leaving the core relatively soft, thus giving a hard wearing surface without embrittlement and loss of impact resistance to the centre. Usually, this process involves heating the components surrounded by charcoal or other carbon-based material to approximately 900◦C for sev- eral hours. The components are then heat treated to develop fully the surface hardness.
SPECIFICATION OF STEELS
Steels within the European Union are designated by a series of European Standards, BS EN 10025: 2004.
Hot rolled structural steels:
BS EN 10025-1: 2004 General technical data BS EN 10025-2: 2004 Non-alloy structural steels BS EN 10025-3: 2004 Weldable fine-grained
structural steels BS EN 10025-4: 2004 Rolled weldable fine-
grained structural steels BS EN 10025-5: 2004 Steels with improved atmo-
spheric corrosion resistance BS EN 10025-6: 2004 High yield strength struc-
tural steels
In addition, BS EN 10210-1: 2006 and BS EN 10219- 1: 2006 relate to hot- and cold-formed structural hollow sections, respectively. The standard grades and their associated characteristic strengths are illustrated inTables 5.1–5.5. In the standards, S refers to structural steel and the subsequent coding numbers relate to the minimum yield strength. The sub-grade letters refer to impact resistance and other production conditions and compositions, such as W for weather-resistant steel.
Steel numbers for each grade of steel are defined by BS EN 10027-2: 1992.
The following example illustrates the two coding systems for one standard grade of steel:
Table 5.1 Steel designations for standard grades to BS EN 10025-2: 2004 (hot rolled products of non-alloy structural steels)
Designation Properties
BS EN 10027-1: 2005 and BS EN 10027-2 : 1992 BS EN 10025-2: 2004 limits
Grade Number Ultimate tensile strength (MPa) Minimum yield strength (MPa)
S185 1.0035 290–510 185
S235JR 1.0038 360–510 235
S235JO 1.0114 360–510 235
S235J2 1.0117 360–510 235
S275JR 1.0044 410–560 275
S275JO 1.0143 410–560 275
S275J2 1.0145 410–560 275
S355JR 1.0045 470–630 355
S355JO 1.0553 470–630 355
S355J2 1.0577 470–630 355
S355K2 1.0596 470–630 355
Notes:
Sub-grades JR, JO, J2 and K2 indicate increasing impact resistance as measured by the Charpy V-notch test.
K has a higher impact energy than J, the symbols R, O and 2 refer to the impact test at room temperature, 0 and−20◦C, respectively.
Data is for thicknesses of 16 mm or less.
Table 5.2 Steel designations for higher grade structural steels to BS EN 10025-3: 2004 (hot-rolled products in weldable fine grain structural steels)
Designation Properties
BS EN 10027-1: 2005 and BS EN 10027-2 : 1992 BS EN 10025-3 : 2004 limits
Grade Number Ultimate tensile strength (MPa) Minimum yield (strength MPa)
S275N 1.0490 370–510 275
S275NL 1.0491 370–510 275
S355N 1.0545 470–630 355
S355NL 1.0546 470–630 355
S420N 1.8902 520–680 420
S420NL 1.8912 520–680 420
S460N 1.8901 550–720 460
S460NL 1.8903 550–720 460
Notes:
Sub-grade N (normalised or normalised rolled) relates to the physical state of the steel and L (low temperature impact) to high impact resistance.
Data is for thicknesses of 16 mm or less.
S275JR (BS EN 10027-1: 2005) 1.0044 (BS EN 10027-2: 1992)
S275JR S refers to structural steel.
The yield strength is 275 MPa.
J is the lower impact strength at room temperature R.
1.0044 The first digit is the material group num- ber with steel 1.
The second pair of digits is the steel group number with 00 referring to a non-alloy base steel.
The final digits refer to the particular grade of non-alloy steel.
F E R R O U S A N D N O N - F E R R O U S M E T A L S 1 7 3 Table 5.3 Steel designations for higher grades to BS EN 10025-4: 2004 (hot rolled products in thermomechanical-rolled
weldable fine-grain structural steels)
Designation Properties
BS EN 10027-1: 2005 and BS EN 10027-2: 1992 BS EN 10025-4 : 2004 limits
Grade Number Ultimate tensile strength (MPa) Minimum yield strength (MPa)
S275M 1.8818 370–530 275
S275ML 1.8819 370–530 275
S355M 1.8823 470–630 355
S355ML 1.8834 470–630 355
S420M 1.8825 520–680 420
S420ML 1.8836 520–680 420
S460M 1.8827 540–720 460
S460ML 1.8838 540–720 460
Notes:
Sub-grade M (thermomechanical rolled) relates to the physical state of the steel and L (low temperature impact) to high impact resistance.
Data is for thicknesses of 16 mm or less.
Table 5.4 Steel designations for weather-resistant grades to BS EN 10025-5: 2004
Designation Properties
BS EN 10027-1: 2005 and BS EN 10027-2 : 1992 BS EN 10025-5: 2004 limits
Grade Number Ultimate tensile strength (MPa) Minimum yield strength (MPa)
S235JOW 1.8958 360–510 235
S235J2W 1.8961 360–510 235
S355JOWP 1.8945 470–630 355
S355J2WP 1.8946 470–630 355
S355JOW 1.8959 470–630 355
S355J2W 1.8965 470–630 355
S355K2W 1.8966 470–630 355
Notes:
Sub-grades JO, J2 and K2 indicate increasing impact resistance, respectively.
Sub-grade W refers to weather-resistant steel.
P indicates a high phosphorus grade.
Data is for thicknesses of 16 mm or less.
Table 5.5 Steel designations for high yield strength quenched and tempered steels to BS EN 10025-6: 2004
Designation Properties
BS EN 10027-1: 2005 and BS EN 10027-2 : 1992 BS EN 10025-6: 2004 limits
Grade Number Ultimate tensile strength (MPa) Minimum yield strength (MPa)
S460Q 1.8908 550–720 460
S500Q 1.8924 590–770 500
S550Q 1.8904 640–820 550
S620Q 1.8914 700–890 620
S690Q 1.8931 770–940 690
S890Q 1.8940 940–1100 890
S960Q 1.8941 980–1150 960
Notes:
Q indicates quenched steel.
Data is for thicknesses between 3 and 50 mm.
Fig. 5.9 Structural steelwork—Wembley Stadium, London.Architects: Foster + Partners.Photograph: Arthur Lyons STRUCTURAL STEELS
Weldable structural steels, as used in the Wembley Sta- dium, London (Fig. 5.9), have a carbon content within the range 0.16–0.25%. Structural steels are usually nor- malised by natural cooling in air after hot rolling. The considerable size effect, which causes the larger sec- tions to cool more slowly than the thinner sections, gives rise to significant differences in physical proper- ties; thus, an 80 mm section can typically have a 10%
lower yield strength compared to a 16 mm section of the same steel. Whilst grade S275 had previously been considered to be the standard-grade structural steel and is still used for most small beams, flats and angles, the higher grade S355 is increasingly being used for larger beams, columns and hollow sections.
Hollow sections
Circular, oval, square and rectangular hollow sections are usually made from flat sections which are progres- sively bent until almost round. They are then passed through a high-frequency induction coil to raise the edges to fusion temperature, when they are forced together to complete the tube. Excess metal is removed from the surface. The whole tube may then be reheated to normalising temperature (850–950◦C), and hot rolled into circular, oval, rectangular or square sec- tions. For smaller sizes, tube is heated to 950–1050◦C, and stretch reduced to appropriate dimensions. The standard steel grades to BS EN 10210-1: 2006 are S275J2H and S355J2H (Table 5.6). Cold-formed hol- low sections differ in material characteristics from the hot-finished sections and conform to BS EN
10219: 2006. The lowest grade S235, with minimum yield strength of 235 MPa is imported, but the stan- dard non-alloy grades are S275 and S355. Grades S420 and S460 are designated as alloy special steels (Table 5.7).
For larger hollow sections, the rotary forge process is used to produce seamless tubes. A hot tapered ingot is pierced by a hydraulic ram, and the central void is opened up by the action of rollers and a rotating mandrel. The steel subsequently passes through a series of eccentric rollers which elongate the tube reducing the section to the required dimensions.
Bending of structural sections
Castellated beams, rolled, hollow and other sections can be bent into curved forms by specialist metal bending companies. The minimum radius achievable depends on the metallurgical properties, thickness and cross-section. Generally, smaller sections can be curved to smaller radii than the larger sections, although for a given cross-section size the heavier-gauge sections can be bent to smaller radii than the thinner-gauge sections. Normally, universal sections can be bent to tighter radii than hollow sections of the same dimen- sions. Elegant structures, such as Merchants Bridge, Manchester (Fig. 5.10), can be produced with curved standard sections and also curved tapered beams. The cold bending process work hardens the steel, but with- out significant loss of performance within the elastic range appropriate to structural steelwork. Tolerances on units can be as low as±2 mm with multiple bends, reverse curvatures and bends into three dimensions all possible. Increasingly, cold bending is replacing