SteelDesignBook.pdf - Steel Design HandBook HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 1 HISTORICAL DEVELOPMENT AND CHARACTERISTICS

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Unformatted text preview: Steel Design HandBook HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 1 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 1.0 INTRODUCTION According to published literature, iron was primarily used for making weapons in ancient times. The great Indian epics, Ramayana and Mahabharatha, contain evidence that our forefathers knew about the usage of iron long before many other countries knew about it! Iron is thus very native to India! This is a logical conclusion because, our war-centric epics date back to several thousand years BC. The backdrop of these epics revolves around the eastern, central and southern parts of our country, where there are still huge deposits of iron ore. Not only during Vedic times but also in medieval times, our country has been an epitome of iron wonders. A review in the subsequent sections shows that in modern times too, our country has good examples of construction in steel. Under compelling reasons, both economic and strategic, the western countries brought about the industrial revolution during the last century. Possibly because our country was under the colonial rule at that time and also due to a mood of complacency, our country failed to catch up with the western industrial revolution. During the last 50 years our country has continued to lag behind in infrastructure development and consequently poor consumption of iron and steel. Published studies by the Steel Construction Institute (U.K) have established that countries which have a higher rate of growth in Gross Domestic Product (GDP), have proportionately higher consumption of iron and steel. Soon after independence, our country had to gear itself to meet the demands for development and industrial growth and in the first few Five Year plans made reasonable strides in the area of production and usage of iron and steel. Due to various reasons, steel consumption in our country has been stagnating during the past 2-3 years. Further, steel industry is facing stiff global competition through imports. There is also considerable under utilisation of installed capacity for steel production. For sustenance of steel industry, extensive usage of structural steel in the construction sector is an important requirement. Our country has to live up to the global competition as we have done in Information Technology! In this chapter, we will first discuss about the historical development of iron and steel in the world and India. Since the present days are the days of inter-disciplinary approach to engineering solutions, we will first review the metallurgical aspect of structural steels and then proceed to discuss, the mechanical properties of steel, which are very relevant to structural designers. The approach of treating the metallurgical and mechanical aspects of steel together helps the designer in structural steelwork, to use steel effectively in tune with its performance requirement. Later, we will briefly review the production process of steel, which gives an idea about the different structural steels being produced. We will also review the special variety of steels (such as stainless steels and cold rolled steels). © Copyright reserved Version II 1-1 I.exe HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 2.0 HISTORICAL DEVELOPMENT Ancient Hittis were the first users of iron some 3 to 4 millenniums ago. Their language was altered to Indo – European and they were native of Asia Minor. There is archaeological evidence of usage of iron dating back to 1000 BC, when Indus valley, Egyptians and probably the Greeks used iron for structures and weapons. Thus, iron industry has a long ancestry. Wrought iron had been produced from the time of middle ages, if not before, through the firing of iron ore and charcoal in “bloomery”. This method was replaced by blast furnaces from 1490 onwards. With the aid of waterpowered bellows, blast furnaces were used for increased output and continuous production. A century later, rolling mill was introduced for enhanced output. The traditional use of wrought iron was principally as dowels and ties to strengthen masonry structures. As early as 6th century, iron tie-bars had been incorporated in arches of Haghia Sophia in Istanbul. Renaissance domes often relied on linked bars to reinforce their bases. A new degree of sophistication was reached in the 1770 in the design of Pantheon in Paris. Fig. 1 World's first cast iron bridge - Coalbrookadale bridge at Shropshire, U.K (Source: John H. Stephens, The Guinness book of Structures (Bridges, towers, tunnels, dams), 1976) Version II 1-2 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Till the 18th century the output of charcoal fired blast furnaces was almost fully converted to wrought iron production, with about 5% being used for casting. The most obvious cast iron items were the cannons in the early 16th century. Galleries for the House of Commons in England were built of slender cast iron columns in 1706 and cast iron railings were erected around St. Paul’s Cathedral in London in 1710. Abraham Darby discovered smelting of iron with coke in 1709. This led to further improvements by 1780s when workable wrought iron was developed. The iron master Henry Cort took out two patents in 1783-84, one for a coal-fired refractory furnace and the other for a method of rolling iron into standard shapes. Without the ability to roll wrought iron (into standard shapes), structural advances, which we see today, would never have taken place. Fig.2 The second Hooghly cable stayed bridge Technological revolution, industrial revolution and growth of mills continued in the West and this increased the use of iron in structures. Large-scale use of iron for structural purposes started in the Europe in the later part of the 18th Century. The first of its kind was the 100 feet Coalbrookadale arch bridge in England (Fig.1), constructed in 1779. This was a large size cast iron bridge. The use of cast iron as a primary construction material continued up to about 1840 and then onwards, there was a preference towards wrought iron, which is more ductile and malleable. The evolution of making better steel continued with elements like manganese being added during the manufacturing process. In 1855, Sir Henry Bessemer of England invented and patented the process of making steel. It is also worth mentioning that William Kelly of USA had also developed the technique of making steel at about the same time. Until the earlier part of the 19th century, the ‘Bessemer process’ was very popular. Along with Bessemer process, Siemens Martin process of open-hearth technique made commercial steel popular in the 19th century. In the later part of the 19th century and early 20th century, there had been a Version II 1-3 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS revolution in making better and newer grades of steel with the advent of newer technologies. This trend has continued until now and today we have very many variety of steels produced by adding appropriate quantities of alloying elements such as carbon, manganese, silicon, chromium, nickel and molybdenum etc to suit the needs of broad and diverse range of applications. Fig. 3 Jogighopa Road-cum-rail bridge across the river Brahmaputra 2.1 Historical development of Iron and Steel in India As mentioned earlier there are numerous examples of usage of iron in our country in the great epics Ramayana and Mahabharatha. However the archaeological evidence of usage of iron in our country, is from the Indus valley civilisation. There are evidences of iron being used as weapons and even some instruments. The iron pillar made in the 5th century (standing till today in Mehrauli Village, Delhi, within a few yards from Kutub Minar) evokes the interest and excitement of all the enlightened visitors. Scientists describe this as a "Rustless Wonder". Another example in south India is the Iron post in Kodachadri Village in Karnataka, which has 14 metres tall “Dwaja Stamba” reported to have remained without rusting for nearly 1½ millennia. The exciting aspects of these structures is not merely the obvious fact of technological advances in India at that time, but in the developments of techniques for handling, lifting, erecting and securing such obviously heavy artefacts. These two are merely examples besides several others. The usage of iron in wars during Moghul era of the history is well documented. India under the British rule experienced growth of iron and steel possibly because of the fallout of technological development of steel in the U.K. We can see several steel structures in public buildings, railway stations and bridges, which testifies the growth of steel in the Version II 1-4 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS colonial past. The “Rabindra Sethu" Howrah Bridge in Calcutta stands testimony to a marvel in steel. Even after its service life, Howrah Bridge today stands as a monument. The recent example is the Second Hooghly cable stayed bridge at Calcutta (Fig. 2), which involves 13,200 tonnes of steel. Similarly the Jogighopa rail-cum-road bridge across the river Brahmaputra (Fig. 3) is an example of steel intensive construction, which used 20,000 tonnes of steel. There are numerous bridges, especially for railways built, exclusively using steel. As far as production of steel in India is concerned, as early as in 1907, Jamsetji Nusserwanji Tata set up the first steel manufacturing plant at Jamshedpur. Later Pandit Jawaharlal Nehru realised the potential for the usage of steel in India and authorised the setting up of major steel plants at Bhilai, Rourkela and Durgapur in the first two five year plans. In Karnataka Sir Mokshakundam Visweswarayya established the Bhadravati Steel Plant. Today we also have a number of private sector steel plants in India. The annual production of steel in 1999-2000 has touched about 25 million tonnes and this is slated to grow at a faster rate. However, when compared to countries like USA, UK, Japan, China and South Korea the per capita consumption of steel in India is extremely low at 27.5 kg/person/year. By way of comparison, rapidly growing economies like China consume about 80 kg/person/year. 3.0 METALLURGY OF STEEL There is a definite need for engineers involved in structural steelwork to acquaint themselves with some metallurgical aspects of steel. This will help the structural engineer to understand ductile behaviour of steel under load, welding during fabrication and erection and other important aspects of steel technology such as corrosion and fire protection. To this end, in the following sections, we shall discuss briefly the metallurgical composition of steel, its effect on heating and cooling and the effects of alloying elements such as carbon, manganese and other additive metals. 3.1 The crystal structure and the transformation of iron Pure iron when heated from room temperature to its melting point, undergoes several crystalline transformations and exhibits two allotropic modifications such as (i) body centred cubic crystal (bcc), (ii) face centred cubic crystal (fcc) as shown in Fig.4. When iron changes from one modification to the other, it involves the ‘latent heat of transformation’. If iron is heated steadily, the rise in temperature would be interrupted when the transformation starts from one phase to the other and the temperature remains constant until the transformations are completed. The flat portion of the heating/cooling curve in Fig. 5 exemplifies this. On cooling of molten iron to room temperature, the transformations are reversed and almost at the same temperature when heated as shown in Fig. 5. Iron upto a temperature of 910°C remains as ‘ferrite’ or ‘α-iron’ with ‘bcc’ crystalline structure. Iron is ferromagnetic at room temperature, its magnetism decreases with increase in temperature and vanishes at about 768°C called the Curie point. The iron that exists between 768°C and 910°C is called the ‘β-iron’ with a ‘bcc’ structure. However, in the realm of metallurgy, this classification does not have much significance. Version II 1-5 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Between 910°C and 1400°C, iron transforms itself into ‘austenite‘ or ‘ γ -iron’ with ‘face centred cubic’ (fcc) structure. When temperature is further increased, austenite reverts itself back to ‘bcc’ structure, called the ‘δ-ferrite’. Iron becomes molten beyond 1539°C. The different phases of iron are summarised in Table 1. (a) Body centred cube (bcc) (b) Face centred cube Fig.4 Crystal structure of Iron Temp 0 C 1539 0 C 1600 1400 bcc 1200 Heating γ fcc 1000 800 1400 0 C Non-Magnetic δ Cooling 910 0 C β 768 0C 600 400 200 α Magnetic bcc 0 Time Fig.5 Allotropy of Iron Version II 1-6 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Table 1: Various forms of Iron Stable Temp. Range C Form of matter Phase Identification symbol >2740 Gaseous Gas Gas 1539-2740 Liquid Liquid Liquid 1400-1539 Solid bcc δ-ferrite 910-1400 Solid fcc γ-austenite <910 Solid bcc α-ferrite 0 It is interesting to note that a given number of atoms when arranged as fcc crystals occupy slightly less volume than when arranged as bcc. Due to this reason, there would be a slight volume reduction when iron transforms itself from ferrite to austenite. As shown in Fig. 4, both bcc and fcc structures have interstitial hole positions (inter atomic spaces) which are at mid point of the cube for bcc and at mid point of the cube edges for fcc. In γ-iron or austenite, more volume fraction of interstitials can be accommodated than in α-iron or ferrite. Atoms of elements such as carbon, nitrogen, hydrogen and boron, whose atomic diameter is smaller, would occupy these inter atomic spaces. Such an arrangement is called an ‘interstitial solid solution’ as shown in Fig. 6. In other words the solute atoms are -Carbon -Ferrite Fig.6 Interstitial solid solution of Carbon in Iron accommodated in the interstices (inter atomic spaces) of the crystal lattice of the solvent. If we take the example of carbon, since the interstices of fcc are larger than the bcc, the solubility of carbon in austenite would be more than its solubility in ferrite. 3.2 The Iron-Carbon Constitutional Diagram 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, we would study the fundamentals of Iron-Carbon alloy in a little elaborate way. The atomic diameter of carbon is less than the interstices between iron atoms. The carbon goes into solid solution of iron. As carbon dissolves in the interstices, it distorts the original crystal lattice of iron. The iron crystals, which were centred originally at the intersection of symmetry axes of the iron crystals, get distorted as seen from Fig. 6. Version II 1-7 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS Temperature in 0C Peritectic point 1534 0C 1493 0C 0.1% δ ferrite Liquid 1400 1200 1000 γ γ Fe3C+ Liquid 1147 0C phase Austenite 910 0C Eutectic point + Liquid γ + Fe3C Austenite + Cementite α + γ 723 0C 0.02% α ferrite 800 600 400 Eutectoid point α + Fe3C Ferrite + Cementite 200 0.0 0.0 0.8% 1.0 4.3% 2.0 Cementite 3.0 5.0 4.0 Carbon content in wt. % Fig.7 Iron – Iron-Carbide phase diagram 6.0 6.67 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 α-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 Version II 1-8 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS 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, which is a plot of transformation of iron with respect to carbon content and temperature, is shown in Fig.7. This diagram is also called iron-iron carbide diagram. The important metallurgical terms, used in the diagram, are presented below. Ferrite (α): Virtually pure iron with body centred cubic crystal structure (bcc). It is stable at all temperatures upto 9100C. The carbon solubility in ferrite depends upon the temperature; the maximum being 0.02% at 7230C. Cementite: Iron carbide (Fe3C), a compound iron and carbon containing 6.67% carbon by weight. Pearlite: A fine mixture of ferrite and cementite arranged in lamellar form. It is stable at all temperatures below 723 oC. Austenite (ϒ): Austenite is a face centred cubic structure (fcc). It is stable at temperatures above 723 oC depending upon carbon content. It can dissolve upto 2% carbon. These terms are summarised in Table 2. Temp 0 C 1200 Austenite (ϒ) i 1000 11470C j 800 Austenite + Cementite 7230C k Ferrite + Austenite 600 Eutectoid Point Ferrite (ϒ ) 400 Ferrite + Pearlite 200 l a Hypo-Eutectoid steel b c Cementite + Pearlite Hyper-Eutectoid steel d 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Weight % of Carbon Fig.8 The Eutectoid section of the Iron – Iron Carbon phase diagram The maximum solubility of carbon in the form of Fe3C in iron is 6.67%. Addition of carbon to iron beyond this percentage would result in formation of free carbon or graphite in iron. At 6.67% of carbon, iron transforms completely into cementite or Fe3C (Iron Carbide). In the iron-carbon phase diagram, there are three important points such as (1) Version II 1-9 HISTORICAL DEVELOPMENT AND CHARACTERISTICS OF STRUCTURAL STEELS eutectoid point (2) eutectic point and (3) peritectic point shown in dotted circles in Fig.7. Generally carbon content in structural steels is in the range of 0.12-0.25%. Upto 2% carbon, we get a structure of ferrite + pearlite or pearlite + cementite depending upon whether carbon content is less than 0.8% or beyond 0.8%. Beyond 2% carbon in iron, cast iron is formed. Table 2: Metallurgical terms of iron Name α - Iron Fe3C Ferrite + Cementite laminar mixture γ - Iron Metallurgical term Ferrite Cementite Pearlite Austenite % Carbon(max) 0.02 6.67 Crystal structure 0.80 (overall) 2.0 (depends on temperature) fcc bcc - 3.3 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.8. 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. 9(a). Ferrite is soft and ductile with very low mechanical strength. The microstructure of 0.20% carbon steel is shown Fig. 9(b). This microstructure at ambient temperature has a mixture of what is known as ‘pearlite and ferrite’ as can be seen in Fig. 8. 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 w...
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