CHAPTER Ferrous Metals and Alloys

CHAPTER

5

Ferrous Metals and Alloys

5.1

INTRODUCTION

Ferrous metals are defined as, those which contain iron as their main constituent. Iron is abundant, easy to convert from ore to a useful form, and iron and steel are sufficiently strong and stable for most engineering application. Ferrous materials, have iron as their base and due to wide range of their properties are most useful in engineering machines and structures. Owing to the advents in steel technology and casting technique ferrous metals are cast, shaped and machined in various shapes and sizes. Several standard shapes of sections are available commercially which make the job of designer and constructor much easier. They are used for making trusses, bridges, ships and boilers. For such construction standard section and sheets or plates of steel are available. The other machine parts like shafts, gears, bearings, pulleys and bodies of machines can be made in steel through forming, cutting or casting processes or their combination. Metal cutting tools, dies, punches, jigs and fixtures are also made in ferrous metal. One of the largest consumer of steel is automobile industry. Despite the modern trend of making light cars nearly 60% of weight of a car is still due to steel and an average passenger car contains about 500 kgf of steel in India. Perhaps in countries like U.S.A. where cars of bigger size are in use this weight could be as high as 800 kgf/car. The first human effort in the direction of making tools was based upon meteoritic iron obtained from meteorite that had struck the earth. This happened more than 3000 B.C. In India the well known Ashoka Column in Delhi was constructed more than 4000 years ago. The blast furnace was invented in 1340 AD and then it became possible to produce large quantities of iron and steel.

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60

MANUFACTURING SCIENCE

5.2 CLASSIFICATION OF ENGINEERING MATERIAL

5.2.1 Iron The large family of iron alloys called steel are the most frequently used metal. Iron is believed to be the tenth abundant element in the universe. Iron makes up 5% of the earth, crust, easy to convert from ore to a useful form, and iron is sufficiently strong and stable.

FERROUS METALS AND ALLOYS

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Iron is a metal extracted from iron ore, and is almost never found in the free elemental state. In order to obtain elemental iron, the impurities must be removed by chemical reduction. Iron is used in the production of steel, an alloy or solid solution of different metals, and some non-metals, particularly carbon. Iron (as Fe2+, ferrous ion) is a necessary trace element found in all known living organisms. Iron-containing enzymes, usually containing heme prosthetic groups, participate in catalysis of oxidation reactions in biology, and in transport of a number of soluble gasses.

5.2.2 Iron Ores Iron ores are rocks and minerals from which metallic iron can be extracted. The ores are usually rich in iron oxides and vary in colour from dark grey to rusty red. The iron itself is usually found in the form of magnetite (Fe3O4), hematite (Fe2O3), limonite or siderite. Hematite is also known as “natural ore”. The name refers to the early years of mining, when certain hematite ores contained 66% iron and could be fed directly into steel-making blast furnaces. Iron ore is the raw material used to make pig iron, which is one of the main raw materials to make steel. 98% of the mined iron ore is used to make steel. Iron ore cargoes may affect magnetic compasses. Iron ore types and mining methods vary geographically. The total recoverable reserves of iron ore in India are about 9,602 million tones of haematite and 3,408 million tones of magnetite. Madhya Pradesh, Karnataka, Bihar, Orissa, Goa, Maharashtra, Andhra Pradesh, Kerala, Rajasthan and Tamil Nadu are the principal producers of iron ore in the country. History: The first iron used by mankind, far back in pre-history, came from meteors. The smelting of iron in bloomeries probably began in Anatolia or the Caucasus in the second millenium BC or the latter part of the preceding one. Cast iron was first produced in China about 550 BC, but not in Europe until the medieval period. During the medieval period, it became possible in Europe to produce wrought iron from cast iron (in this context known as pig iron). For all these processes, charcoal was required as fuel. Steel (with a smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity. New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century AD. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steel-making process, involving blowing air through molten pig iron, to produce mild steel without producing wrought iron.

5.3

PRODUCTION OF IRON AND STEEL

Reviewing the principles of the iron and steel making processes, beginning with raw materials is taken up first. This knowledge is essential to an understanding of the quality and characteristics of the steels produced by different processes.

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5.3.1 Raw Materials The three basic materials used in iron and steel making are iron ore, limestone and coke. The principal iron ores are taconite (a black flintlike rock), hematite (an iron oxide mineral), and limonite (an iron oxide containing water). After it is mined, the iron ore is crushed into fine particles, the impurities are removed by various means (such as magnetic separation), and it is formed into pellets, balls, or briquettes using binders and water. Typically, pellets are about 65 per cent pure iron and 25 mm in diameter. The concentrated iron ore is referred to as beneficiated. Some iron rich ores are used directly without palletizing. Coke is obtained from special grades of bituminous coal, which are heated in vertical coke ovens to temperatures of 1150°C and cooled with water in quenching towers. Coke has several functions in steelmaking. One is to generate the high level of heat required for chemical reactions to take place in iron making. Second, it produces carbon monoxide (a reducing gas) which is then used to reduce iron oxide to iron. The chemical by products of coke are used in making plastics and chemical compounds. Coke oven gases are used as fuel for plant operations, and power generation. The function of limestone (calcium carbonate) is to remove impurities from the molten iron. The limestone reacts chemically with impurities, acting as a flux which causes the impurities to melt at a low temperature. The limestone combines with the impurities at a low temperature. The limestone combines with the impurities and forms a slag which is light and floats over the molten metal. Slag is subsequently removed. Dolomite (an ore of calcium magnesium carbonate) is also used as a flux. The slag is later used for making cement, fertilizers, glass, building materials, rock wool insulation, and road ballast.

5.3.2 Contaminants Ideally iron ore contains only iron and oxygen. In nature this rarely is the case. Typically, iron ore contains a host of deleterious elements which are unwanted in modern steel. 1. Silica: Iron ore typically contains silicates, usually in the form of quartz. Silica is undesirable because silicon does not bond with carbon during the smelting process and can remain in the iron after it is refined. Historically, siliceous iron ore created wrought iron, a malleable and strong form of iron used by blacksmiths throughout history. Modern steel-making techniques generally use lime and other fluxes to help remove the silica from the molten iron ore, and form a slag on the surface of the molten metal. This slag can then be removed. 2. Phosphorus: Phosphorus is deleterious because it makes steel brittle, even at concentrations of as little as 0.5%. Phosphorus cannot be easily removed by fluxing or smelting, and so iron ores must generally be low in phosphorus to begin with. The iron pillar of India which does not rust, however, is protected by a phosphoric composition. Phosphoric acid is used as a rust converter because phosphoric iron is less susceptible to oxidation.

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3. Aluminium: Aluminium is generally present in iron ores as clay. This is usually removed by washing the iron ore, and by fluxing. However, again, iron oxide deposits must be relatively low in aluminium in order to be considered ore. 4. Sulphur: Sulphur is unwanted because it produces undesirable sulphur dioxide gases in the flue emissions from a smelter and interferes with the smelting process.

5.4

MAIN TYPES OF IRON

The following are some important types of iron:

5.4.1 Pig Iron The first product in the process of converting iron ore into the useful metal is pig iron. Pig iron is the product of blast furnace. The raw materials which go into the blast furnace are iron ore, coke, limestone and air. The iron ore which is most frequently used, and consists of about 50% iron and 50% earthy matter, known as gangue. The gangue is composed of silica, alumina, calcium oxide and magnesium oxide, water, phosphorus and often some sulphur. The presence of these elements in the iron ore affect the end product. The various composition in pig iron are as follows: Iron (Fe)

......... 90 to 92%

Carbon (C)

......... 3.5%

Silica (Si)

......... 2.8%

Maganese (Mn)

......... 0.65%

Sulphur (S)

......... 0.05%

Phosphorus

......... 0.8%

5.4.2 Blast Furnace A blast furnace is a type of furnace for smelting iron ore. The combustion material and ore are supplied from the top while an air is supplied from the bottom of the chamber, so that the chemical reaction takes place throughout the ore, not only at the surface. This type of furnace is typically used for smelting iron ore to produce pig iron, the raw material for wrought and cast iron. The principle of this furnace was developed in Central Europe, and the first furnace began operating in 1621. The first steel plant in India began its operation in the early part of twentieth century. The blast furnace is basically a large steel cylinder lined with refractory bricks and has the height of about a ten storey building. The charge mixture is melted in a reaction at 1650°C with air preheated to about 1100°C and blasted into the furnace through nozzles or tuyeres. Although a number of reactions

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may take place, basically the reaction of iron oxide with carbon produces carbon monoxide, which in turn reacts with the iron oxide, reducing it to iron. Preheating the incoming air is necessary because the burning coke alone does not produce sufficiently high temperatures for the reactions to occur.

Fig 5.1

The molten metal accumulates at the bottom of the blast furnace, while the impurities float to the top of the metal. At intervals of four to five hours, the molten metal is tapped, into ladle cars. Each ladle car can hold as much as 160 tons of molten iron. The molten metal at this stage has a typical composition of 4 per cent carbon, 1.5 per cent silicon, 1 per cent manganese, 0.04 per cent sulphur, and 0.4 per cent phosphorus, with the rest being pure iron. The molten metal is referred to as pig iron. Use of the word pig comes from the early practice of pouring molten iron into small sand molds, arranged like a litter of small pigs around a main channel. The solidified metal is called pig and is used in making iron and steels. The blast furnace is shown in Fig. 5.1.


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The blast furnace remains an important part of modern iron production. Modern furnaces include Cowper stoves to pre-heat the blast air to high temperatures in order to avoid cooling (and thus having to re-heat) the mix, and use fairly complex systems to extract the heat from the hot carbon dioxide when it escapes from the top of the furnace, further improving efficiency. The largest blast furnaces produce around 60,000 tonnes of iron per week, enough for about four cars per minute. This is a great increase from the 18th century, when charcoal blast furnaces averaged 400 tons per year.

5.5 CAST IRON Cast iron is made by remelting pig iron, often alongwith substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such as phosphorus and sulphur. Carbon and silicon content are reduced to the desired levels, which may be anywhere from 2% to 3.5% for carbon and 1% to 3% for silicon depending on the application. Other elements are then added to the melt before the final form is produced by casting. Iron is most commonly melted in a small blast furnace known as a cupola. After melting is complete, the molten iron is removed or ladled from the forehearth of the blast furnace. This process was devised by the Chinese, whose innovative ideas revolutionized the field of metallurgy. Despite this, the principles of cast iron solidification are understood from the binary iron-carbon phase diagram, where the eutectic point lies at 1154°C and 4.3% carbon. Since cast iron has nearly this composition, its melting temperature of 1150 to 1200°C is about 300 degrees lower than the melting point of pure iron. Casting is a process in which molten metal is poured in a mould and on solidifying, the casting of the shape of mould is obtained. Cast iron, as already stated, is a good material for casting. General properties of cast iron are: 1. Cheap material. 2. Lower melting point (1200°C) as compared to steel (1380-1500°C). 3. Good casting properties, e.g. high fluidity, low shrinkage, sound casting, ease of production in large number. 4. Good in compression but cast iron with ductility are also available. 5. Cast iron is machinable in most cases. 6. Abrasion resistance is remarkably high. 7. Very important property of cast iron is its damping characteristic which isolates vibration and makes it good material for foundation and housing. 8. Alloy cast iron may be good against corrosion. Contents of Cast Iron: Cast iron is prepared from melting pig iron in electric furnace or in cupola furnace. Electric furnace gives better quality. The composition of cast iron is as follows:

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Iron

92% to 94%

Manganse

0.5% to 1%

Carbon

2% to 3.5%

Sulphur

0.02% to 0.15%

Silicon

1% to 3%

Phosphorus

1%

With its low melting point, good fluidity, castability, excellent machinability, wear resistance, cast iron is used to make serval machine parts subjected to compressive loads. It is however, weak in tension. Cast irons have become an engineering material with a wide range of applications, including pipes, gears, cams, pump body, cylinders valves, piston and clutch plate.

5.6

TYPES OF CAST IRON

Cast iron may be classified as follows:

5.6.1 White Cast Iron With a lower silicon content and faster cooling, the carbon in white cast iron remains as Fe3C which is very hard. Most of the carbon present in it is, in the combined form which forms carbides. It is the formation of carbide which renders the colour of the structure as white. White cast iron is too brittle for most uses, but has good hardness and abrasion resistance and relatively low cost. The composition of white cast iron is: Carbon

1.8% to 3.6%

Phosphorous 0.18% Silicon

0.5% to 2%

Sulphur

0.10%

Manganese

0.2% to 0.8%

White cast iron is very hard with BHN of 350-500. Its UTS is low in the range of 140-180 MPa. Uses: Balls for rolling element bearings, the wear surfaces of slurry pumps, grinding balls and the teeth of a backhoe’s digging bucket. It is normally sand cast to produce above parts.

5.6.2 Malleable Cast Iron It is made by changing all the combined carbon in a special white cast iron to free or temper carbon by suitable heat-treatment at about 900°C. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles

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rather than flakes. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron. The composition of malleable cast iron is as follows: Carbon

2% to 3%

Silicon

0.6% to 1.3%

Manganese

0.2% to 0.6%

Phosphorus

0.15%

Sulphur

0.10%

Uses: Malleable cast iron is used in automotive industry, rail road, agricultural implements, conveyor chain links, etc.

5.6.3 Grey Cast Iron It is the iron which is most commonly used in foundary work. Silicon is essential in making of grey cast iron as opposed to white cast iron. If a piece of this iron is broken, its fractured surface shows the greyish colour, after which it is named as grey iron. The metal expands slightly on solidifying as the graphite precipitates, resulting in sharp castings. The graphite content also offers good corrosion resistance. Graphite acts as a lubricant, improving wear resistance. It has much higher thermal conductivity. Grey cast iron tends to “damp” mechanical vibrations (including sound), which can help machinery to run more smoothly. Grey cast iron differs from white cast iron in percentage of Si while carbon percentage is almost same. Graphite flakes in grey cast iron precipitate from liquid alloy and addition of Si, Al or Ni accelerates graphatization. The graphite flakes vary in length from 0.01 to 1.0 mm. The flakes provide an easy passage to cracks, thus not allowing softer microstructure to deform plastically. The best properties are obtained with flakes oriented randomly. Inoculant agents like metallic Al, Ca, Ti, Zr, SiC and CaSi when added in small amount cause smaller graphite flakes and random orientation. Grey cast iron’s high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors. Addition of strong inoculating agent like CaSi to liquid metal before casting provides high strength to grey cast iron. UTS as high as 400 MPa is obtained which can be increased to 520 MPa by oil quenching treatment. Such cast iron is called Meehanite iron. The various composition of grey cast iron are as follows: Carbon 3% to 3.5% Silicon 1% to 2.75% Manganese 0.40% to 1% Phosphorus 0.15% to 1% Sulphur 0.02% to 0.15%

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Grey cast iron has hardness varying between 149 and 320 BHN and UTS of 150 to 400 MPa. Uses: Grey cast iron is used in machine tool structures, sanitary work, piston rings, rolling mills, manhole cover and household appliances.

5.6.4 Nodular cast iron A more recent development is nodular or ductile cast iron. Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron but parts can be cast with larger sections. The various composition in Nodular cast iron are as follows: Carbon

3.2% to 4.2%

Silicon

1% to 2.8%

Manganese

0.5% to 1%

Phosphorus

0.10%

Sulphur

0.03%

Uses: Nodular cast iron is used in power transmission equipments, valve and fitting, pumps, I.C. engine, paper industries and construction machinery, etc.

5.6.5 Alloy Cast Iron Ni and Cr are added to cast iron to produce alloy cast iron with improved properties. Nihard C.I. is obtained by addition of 3 to 5% Ni and 1 to 3% Cr. The hardness of this alloy is 650 BHN. The Ni-hard is further modified to have improved impact and fatigue resistance with 4.8 Ni and 4.15 % Cr. Good corrosion and heat resistant cast iron is obtained by addition of 14 to 30% Ni and 1 to 5% Cr. This alloy is called Ni-resist cast iron.

5.7

WROUGHT IRON

Wraught iron is a ferrous material aggregate from a solidifying mass of pasty particles of highly refined metallic iron in which a minutely and uniformly distributed quantity of slag is incorporated without subsequent fusion. The various composition in wrought iron are as follows: Carbon 0.02% Silicon

0.12%

Sulphur

0.018%

Phosphorus

0.02%

Slag

0.07%


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Wraught iron is very pure containing at least 99.5% iron, in which particles of silicon slag are distributed as fibres. This result in a ductile iron that has certain properties particularly corrosion resistance, which make it of importance as an engineering material. Earlier it was the most important structural metal. It has been almost entirely replaced by steel. Uses: Wrought iron is used in building construction such as waste vent, drawn pipe, gas collection hood, coal handling equipment, cooling tower and spray pond piping, etc. It is also used in the manufacturing of chains as it can be easily welded and possesses good impact strength.

5.8 STEEL The most important engineering material today is steel. Iron is the base of all steels which are widely used for making machines and structures. Ferrous materials are cast, shaped and machined in various shapes and sizes. Serval standard shapes and sections are available commercially making designing and constructing easy. The standard sections, sheets and plates are often used in making boilers, ships, bridges and trusses. Machine parts like shafts, gears, pulleys, bearings and bodies of machines are made in steel through such technologies as forming, forging, cutting welding or casting. Steel is used for making punches dies, jigs, fixtures and cutting tools. Automobile industry is a large consumer of steel. The increased use of steel in industry is because of its capacity to be produced in various alloyed forms and response to heat treatment. Plain carbon steel contains 0.1-0.8% carbon used for general engineering purposes and 0.9%-1.2% carbon used for wear resistance. Between 1.3 and 2.2% C ferrous material is normally not used, whereas between 2.4 and 4.2% C it is cast iron. Steel is a metal alloy whose major component is iron, with carbon content between 0.02% and 1.2% by weight. Carbon is the most cost effective alloying element for iron, but many other alloying elements are also used. Carbon and other elements act as hardening agent, preventing line defects in the iron crystal lattice from sliding. Varying the amount of alloying elements and their distribution in the steel controls qualities such as the hardness, ductility, and tensile strength. Steel with increased carbon content can be made harder and stronger than iron, but is also more brittle. The maximum solubility of carbon in iron is 1.7% by weight, occurring at 1130° Celsius; higher concentrations of carbon or lower temperatures will produce cementite which will reduce the material strength. Steel is also to be distinguished from wrought iron with little or no carbon, usually less than 0.035%.

5.9 STEEL-MAKING Steel was first produced in China and Japan in about 600-800 A.D. The process is essentially one of refining the pig iron obtained from the blast furnace. The refining of pig iron consists in reduction of the percentage of manganese, silicon, carbon, and other elements, and control of its composition by the addition of various elements. The molten metal from the blast furnace is transported into one of three types of furnaces. The steel-making furnaces

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are open hearth, electric, or basic oxygen. The name open hearth derives from the shallow hearth shape that is open directly to the flames that melt the metal. Developed in the 1860s, the open-hearth furnace is being replaced by electric furnace and by the basicoxygen process. These newer methods are more efficient and produce better quality steels.

5.9.1 Electric Furnace The electric furnace was first introduced in 1906. The source of heat is a continuous electric arc formed between the electrodes and the charged metal as shown in Fig. 5.2. Temperature as high as 1925°C is produced in this type of furnace. There are usually three graphite electrodes in direct arc electric furnace, and they can be as large as 750 mm in diameter and 1.5 to 2.5 m in length. Their height in the furnace can be adjusted depending on the amount of metal present and wear of the electrodes.

Fig. 5.2

Direct Arc Electric Furnace.

Steel scrap and a small amount of carbon and limestone are dropped into the electric furnace through the open roof. Electric furnaces can also use 100 per cent scrap as its charge. The roof is then closed and the electrodes are lowered. Power is turned on, and within a period of about two hours the metal melts. The current is shut off, the electrodes are raised, the furnace is tilted, and the molten metal is poured into a ladle, which is a receptacle used for transferring and pouring molten metal. Electric furnace capacities range from 60 to 90 tons of steel per day. The quality of steel produced is better than that of openhearth or basic-oxygen process. In the indirect arc electric furnace as shown in Fig. 5.3, the arc is struck between two electrodes close to metal.

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Fig. 5.3

Indirect Arc Electric Furnace.

5.9.2 Induction Type Electric Furnace The induction type electric furnace as shown in Fig. 5.4 is used for smaller quantities. The metal is placed in a crucible, made of refractory material and surrounded with a copper coil through which alternating current is passed. The induced current in the charge melts the metal. These furnaces are also used for remelting metal for casting.

Fig. 5.4 Induction type electric furnace.

5.9.3 Basic Oxygen Furnace The basic-oxygen furnace (BOF) is the newest and fastest steel-making process. Typically, 200 tons of molten pig iron and 90 tons of scrap are charged into a refractory lined barrel shaped vessel called converter as shown in Fig. 5.5 (a). Charging of furnace is shown in Fig. 5.5(a), (b) and (c). Pure oxygen is then blown into the furnace for about 20 minutes under a pressure of about 1250 kPa, through a water-cooled lance, as shown in Fig. 5.5(d). Fluxing agents, such as burnt lime are added through a chute.

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Fig. 5.5

Basic oxygen process of steel-making, illustrated through various operations.

The vigorous agitation by the oxygen refines the molten metal through an oxidation process in which iron oxide is first produced. The oxide reacts with the carbon in the molten metal, producing carbon monoxide and carbon dioxide. The iron oxide is reduced to iron. The lance is retracted and the furnace is tapped by tilting it. The opening in the vessel is so provided that the slag still floats on the top of the molten metal as seen in Fig. 5.5 (e). The s!ag is then removed by tilting the furnace in the opposite direction. The BOF process is capable of refining 250 tons of steel in 35 to 50 minutes. Most BOF steels, which are of better quality than open-hearth furnace steels and have low impurity levels, are processed into plates, sheets, and various structural shapes, such as I-beams and channels. Steel may also be melted in induction furnaces from which the air has been removed, similar to the one shown in Fig. 5.4. The vacuum melting produces high quality steels because the process removes gaseous impurities from the molten metal.

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5.10 CLASSIFICATION OF STEEL According to Indian Standard (IS : 7598–1974) steel is classified as:

5.10.1 Plain-Carbon Steel This is a metal alloy, a combination of two elements, iron and carbon, where other elements are present in quantities too small to affect the properties. The only other alloying elements allowed in plain-carbon steel are: Manganese (1.65% max.), silicon (0.60% max.), and copper (0.60% max.). Steel with a low carbon content has the same properties as iron, soft but easily formed. As carbon content rises the metal becomes harder and stronger but less ductile and more difficult to weld. Higher carbon content lowers steel’s melting point and its temperature resistance in general. Plain carbon steel are divided into the following types, depending upon the carbon content: 1. Low Carbon Steels

(0.07-0.25% C)

2. Medium Carbon Steel

(0.25-0.55% C)

3. High Carbon Steel

(0.55-0.9% C)

4. Carbon Tool Steel

(0.9-1.6% C)

1. Low Carbon Steel: It is the most common form of steel as its price is relatively low while it provides properties that are acceptable for many applications. Mild steel is a low carbon steel. It becomes malleable when heated and so can be forged. It is largely used for making structures when carbon percentage is 0.1 to 0.25. Dead mild steel is used to make rivets, nails, chain links, seam welded pipes, etc. Density of this metal is 7861 kg/m3 (0.284 lb/in3) and the tensile strength is a maximum of 500 MPa (72500 psi). 2. Medium Carbon Steel: Medium carbon steels which can successfully undergo heattreatment have a carbon content in the range of 0.25 to 0.55% by weight. Trace impurities of various other elements can have a significant effect on the quality of the resulting steel. Trace amounts of sulphur in particular make the steel red-short. It is stronger and tougher than mild steel, possesses better hardness and tensile strength but is less ductile. These steels are easily rolled, forged, welded and machined. It is suitable for deep forging, boiler drums, casting, automobile engine components, helical springs, locomotive tyres, wire ropes, clutch plates, large forging dies, hammers, snaps for pneumatic rivets, shafts, axles, gears, etc. 3. High Carbon Steel: The steel containing 0.55 to 0.9% C is high carbon steel. High strength and wear resistance are main characteristics. Drop hammers, dies, saws (0.60.7% C), hammers, anvils, wrenches, leaf springs (0.7-0.8% C), shear blades, punches, rock drills chisels (0.8-0.9% C) are some products made in high carbon steels.

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The plain carbon steels are also divided into three groups, as follows: (a) Eutectoid Steels: Ideally these steels contain 0.83% carbon and structure is entirely lamellar pearlitic. However, fully pearlitic structure exists in steels containing 0.8%C. Mn to an extent of 1% reduces carbon in eutectoid steel to 0.7%. (b) Hypo Eutectoid Steels: Steel containing carbon between 0.008% and 0.83% are hypoeutectoid. The structure is made of grains of pearlite and ferrite. The increasing carbon increases proportion of pearlite and thus strength (pearlite is stronger) but ductility reduces. (c) Hyper Eutectoid Steels: Steel containing carbon in excess of 0.8% fall in this class. Pearlite and cementite are present in structure. The strength is high but ductility is low. 4. Carbon Tool Steel: The carbon percentage varies between 0.9 to 1.6. High hardness can be achieved through treatment. They are very resistant to wear and hence make good tooling material.

5.10.2 Alloy Steel Alloy steel is steel alloyed with other elements in amounts between 1 and 50% by weight to improve its mechanical properties. Alloy steels are broken down into two groups: low alloy steels and high alloy steels. Low alloy steels are defined as having alloy contents between 1 and 5% and high alloy steels have 5 to 50% alloying contents. However, low alloy steels are most common. All commercial steels contain varying amounts of Mn, Si, S and P and frequently varying amounts of Cr, Ni, Mo and V. If alloying elements other than carbon are present in small amounts (e.g. Mn upto 0.8%, Si upto 0.3% etc) the steel is still plain carbon steel. S and P when more than 0.05%, make steel brittle, hence they are reduced to at least this value during steel making. Si if less than 0.2% does not affect strength and ductility. Upto 0.4% Si helps improve strength without affecting ductility but more than this impairs strength upto 1%. Mn provides strength to steel but if it exceeds 1.5% steel becomes brittle. Mn is added during steel making as it acts as deoxidizer. Many elements are intentionally added to steel. When Mn in excess of 1% and Si in excess of 0.3% make alloy steels. One particular effect of alloying is that martensite is formed with low rates of cooling. This permits larger sections to harden more than in plain carbon steel. The important elements used to alloy steel are Ni, Cr, Mo, W, Mn and Si. The b.c.c metals like Cr, W and Mo tend to form carbides and thus reduce Fe3C. The f.c.c metals like Ni, Al, Cu and Zr do not form carbides. These steels have greater strength, hardness, hot hardness, wear resistance, hardenability, or toughness compared to carbon steel. However, they may require heat treatment in order to achieve such properties. The effects of alloying elements in terms of various desired properties are summerised below: • Hardenability—Si, Mn, Ni, Cr, Mo, W, B • Toughness—Ni, Si • High temperature resistance—Cr, Mo, W


• • • • • •

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Corrosion resistance—Cr, Mo, W Wear resistance—Cr, Mo, W, V Low temperature impact strength—Ni Machinability—S, P, Pb Fatigue strength—V Surface hardening—Al

5.10.3 Structural Steels Low alloy steels are used for structures such as rails, buildings and concrete reinforcement. They are also used for weldability and for making such machine parts as gears, clutches, shafts and spring. A typical low alloy steel used for structural purpose will have following composition. C—0.12%, Mn—0.75%, Si—0.25%, Cu—0.3% This steel is characterized by yield strength of 350 MPa and 15% elongation after hot rolling. Mn and Si prevent embrittlement in welding thus improving weldability. Cu improves corrosion resistance. Cr, Ni and V may be added to this steel for improving yield strength to a level of 625 MPa without affecting weldability.

5.10.4 High Speed Steels This class of steel is used for making tools to shape, cut or deform materials. Tools used for this purpose normally require high hardness, resistance to wear and ability to retain hardness at high temperature. W, Cr, Mo, V, Mn and Si are the elements which can impart desired properties and they are added in the range of 0.6 to 1.6%, carbon is restricted to 0.5% since more of it reduces shock resistance. 18:4:1 type of steel contaning 18% W, 4% Cr and 1% V in addition to its eutectoid composition develops adequate hardness, strength, toughness, wear and heat resistance. Such steel is suitable for making cutting tools, shear blades, forming and rolling dies. These steels may retain hardness upto 500°C. The hardness is provided due to formation of a complex carbide Fe4W2C. Addition of 5% to 12 % Co increases hardness which is retained at higer temperature of 600°C. High duty punching tools and dies are made in alloy steels containing large amounts of alloying elements. High-carbon, high-Cr die steel contains 2% C, 0.3% Mn and 12% Cr. This is a very hard steel and can be machined only in annealed state but with difficulty. On annealing from 800ºC hard particles of carbide, small and large size become dispersed in softer ferritic matrix. The tools and dies machined in annealed state are hardened. Creep resistant steels for pipelines (at 400 to 500ºC) contain low carbon, 0.4 to 6% Mo, 0.25 to 1% V and Cr upto 6.0%. Ball bearing steel in which rolling elements, outer and inner races are made is in alloy of C—0.9 to 1.1%, Cr—0.6 to 1.6%, Mn—0.2 to 0.4%. This steel develops high hardness and fatigue strength.

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5.10.5 High Strength Low Alloy Steel High strength low alloy steel (HSLA) is marked with balanced properties such as toughness, fatigue strength, weldability and formability, H.S.L.A contains carbon 0.07 to 1.3%; Ti, V, Al and Co less than 0.5%. Very difficult combination of such properties as resistance to abrasion and shock, high strength, toughness and ductility is required in excavating and crushing machines, rail road crossings, oil well, cement and mining industries. Hadfield Mn steel has such properties. It contains 1 to 1.4% C, 0.3 to 1% Si, 10 to 14% Mn with Fe.

5.10.6 Stainless Steel These are alloy steels used particularly for their resistance to corrosion which is obtained due to formation of thin oxide coating on the surface. The oxide coating does not allow atmospheric oxygen or other ions to react with iron and thus no rusting occurs. Cr forms the protective oxide layer maintaining the lustre and appearance of steel. Only Cr in solid state can provide oxide layer and if it forms carbide in steel it can not serve this purpose, hence it has to be in large proportion. The resistance to corrosion increases with increasing Cr. The stainless steel is divided into three sub groups. (a) Ferritic Stainless Steels contain only Cr as alloying element in addition to carbon which may vary between 0.05 to 0.15, while Cr may vary between 13% to 30%. Only α-phase exists at all temperatures but some Cr may precipitate as carbide along with ferrite at room temperature. This steel is ductile and forms in complicated shapes. It has excellent resistance to corrosion. (b) Austenitic Stainless Steel is obtained by adding Ni such that both Cr and Ni together are restricted to 24% but not less than 8% of either of them. Austenitic phase is obtained when steel is quenched from upper critical temperatures. At very slow cooling rate α-phase may separate completely. 18:8 stainless steel with 18% Cr, 8% Ni and 0.1% C is very popular for use against corrosion of all types. Austenitic stainless steel finds wide application in chemical plants, decorative pieces and household utensils. (c) Martensitic Stainless Steel contains 13% or 18% Cr which causes γ-phase to exist at high temperature and α-phase at room temperature when cooled at normal rates. It can be quenched to obtain martensitic structure and hence is heat treatable whereas other two types are not heat treatable. This steel is oil quenched from upper critical temperature. Three types of martensitic steel in use are: (i) 0.07-0.1% C, 13% Cr. (ii) 0.2- 0.4% C, 13% Cr. and (iii) 0.1% C, 18% Cr, 2% Ni. These steels are used for turbine blades, surgical instruments, springs, ball bearings, pump shafts, air craft fittings, etc.


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It may be remembered that Cr is held in solid solution upto 500°C and separates between 500°C to 700°C to precipitate as carbide at grain boundaries. Such precipitation will greatly reduce corrosion resistance. This is also the cause of weld decay. However, the carbides of Cr redissolve in solid solution when welded part is reheated to 900-1000°C and quenched. Martensitic steel can be given quenching treatment to obtain high strength but ferritic and austenitic steels can improve strength through mechanical treatment.

5.10.7 Magnetic Alloy Steel Two groups of magnetic steels viz: hard and soft magnetic steels are in use. Hard variety retains magnetism and soft variety is demagnetized easily. Earliest permanent magnetic material was 1% plain carbon steel. Later developments proved alloy steel containing W, Cr and Co as better magnetic materials. Such steels contain high proportions of Ni, Co, and Al with small amount of Al, Alnico is a common magnetic material. It contains 10% Al, 18% Ni, 12% Co, 6% Cu. The remaining is Fe. Soft iron was earlier used as soft magnetic material. Iron-Si alloys with 4.5% Si is a good soft magnetic material. Ni alloys with 78% and 75% respectively with iron are alloys, currently in use as transformer cores and shield for submarine cables. These alloys are known as Permalloy and Mumetal.

5.10.8 Tool Steel Tool steel refers to a variety of carbon and alloy steels that are particularly well-suited to be made into tools. Their suitability comes from their distinctive hardness, resistance to abrasion, their ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). With a carbon content between 0.7% and 1.4%, tool steels are manufactured under carefully controlled conditions to produce the required quality. The manganese content is often kept low to minimise the possibility of cracking during water quenching. However, proper heat treating of these steels is important for adequate performance, and tooling blanks intended for oil quenching are available. Tool steels are made to a number of grades for different applications. Choice of grade depends on, among other things, whether a keen cutting edge is necessary, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements. In general, the edge temperature under expected use is an important determinant of both composition and required heat treatment. The higher carbon grades are typically used for such applications as stamping dies, metal cutting tools, etc. Tool steels are also used for special applications like injection moulding, because the resistance to abrasion is an important criterion for a mould that will be used to produce hundreds of thousands of parts.

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The common tool steels contain C, W, Cr, Mo, V, Mn, Si in the range of 0.6 to 1.0%. For shock resistance C is restricted to 0.5%. High temperature strength is provided by 2 to 18% of W and Mo. V between 0.1 to 2% helps hardenability. Si is added to tool steels to improve toughness.

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QUESTIONS

1. Give the classification of metals. 2. What are the effects of different alloying elements on cast iron? 3. How cast iron differs from steel? 4. How will you classify steel? 5. Differentiate among low-carbon steel, medium-carbon steel, high carbon steel. 6. Differentiate between the followings: (i) White cast iron and Nodular cast iron. (ii) Grey cast iron and Alloy cast iron. 7. Write the short notes on the following: (i) Iron ore (ii) Pig iron (iii) Blast furnace 8. Define alloy steel. Why the alloying elements are added to steel? 9. What is the effect of carbon, sulphur and phosphorus on the properties of steel? 10. Classify stainless steel and mention uses of different stainless steels. 11. What are different alloying elements present in tool steels? What properties develop by their addition? Describe the applications. 12. What properties are desired in tool steels? Correlate these properties with alloying elements. 13. What are hard and soft magnetic materials? Give examples of iron base materials and other alloys used as magnetic materials. 14. What is high speed steel? Describe composition of 18:4:1 steel. For what purposes this steel is suitable? 15. Give general classification of material. 16. What is pig iron? How is it produced and what are its uses? 17. Distinguish between grey cast iron and white cast iron. Mention uses. 18. Distinguish between malleable and nodular cast irons.