Chapter 2: Ferrous Material

CHAPTER 2

FERROUS MATERIAL

IRON

• Production of iron : Blast furnace

• Production of steel :

a)Basic Oxygen Furnace

b) Electric Arc Furnace

Chemical symbol: Fe

Iron melts at 1353C and boils at 2700 C

The purest iron comes from the sky in the form of meteors.

Pure iron does not rust in water; iron rust because it contains impurities.

Pure iron is seldom used in industry because it is too soft (can be scratch with the fingernail).

IRON

Cast iron

Wrought iron

Steel

Iron is used in 3 forms:

• Iron ore is not pure as it comes from iron mines.

• Types of iron ore:

• a) Hematite (Fe2O3)

• b) Magnetite (Fe3O4)

• c) Pirite

• d) Limonite

• e) Taconite

Iron Ore

1. Magnetite (Fe3O4)

Containing of 72.4% ferum.

Black, grey blackist in colour.

Magnetic minerals.

In rock or sediment rock.

Iron Ores

2. Hematite (Fe2O3)

Containing of 40-65% ferum.

Grey blackist or red chocolate in colour.

Common ore for iron.

In rock.

Iron Ores

3. Limonite (Fe2O3.3H2O) Containing of 20-55% ferum with 40% water.

Yellow, orange, red chocolate, chocolate black.

Found in loose and sedimentary soil.

Known as dehydration hematite.

Iron Ores

4. Iron Carbonate

Containing ≤30% ferum.

Example: siderite

Color ranges from yellow to dark brown or black.

Found in sedimentary rock.

Iron Ores

1) Grade : composition of iron ore

2) Compactness : moderate

3) Purity : low impurity

4) Uniformity (in composition )

Characteristics of Iron Ore

Making pig iron is the first step in the purifying of iron and the making of steel.

Pig iron is produced when the impurities are burned out of iron ore in a blast furnace.

Pig Iron

Blast Furnace It is called a blast furnace because a blast of hot air is forced into it near the bottom. A blast furnace looks like a tall chimney on the outside: *15m – 30m high *6m – 9m in diameter *covered on the inside with special bricks or clay that can withstand great heat.

BLAST FURNACE PROCESS

Iron ore (2 tons) + Coke{purified form of coal} (1 ton) + limestone (0.5 ton) + hot air (4 tons) pig iron (1 ton) + slag +

gas

Iron ore – comes from iron mines

Coke – The coke is added to provide the main chemical reagents (carbon and carbon monoxide) for the iron ore reduction

Flux – The flux, limestone and/or dolomite, is added to combine with ash in the coke and gangue in the ores, to produce a slag that rises to the top of the pool of molten pig iron that collects in the crucible.

BLAST FURNACE

Blast Furnace:Equipment a) Skip car (conveyors) – are used to deliver the blast

furnace charge to the top of the furnace

b) Bosh – is an inverted conical section in which the melting starts.

c) Receiving hopper – the blast furnace charge is loaded into the receiving hopper

d) Hearth – is an intricately constructed crucible-like vessel upon which the vertical shaft portion of the furnace sits. All the molten metal and slag collect in the hearth before being drained.

e) Bells (large and small) – prevents gas from escaping from the furnace while it is being charged.

f) Bustle pipe – encircles the blast furnace and delivers the hot blast air from the hot-blast line to the furnace

g) Stack – is the upper portion of the furnace where the burden is pre-heated

h) Injection lance – is inserted into the blowpipe that leads up to the tuyeres

i) Iron and slag notches – the molten iron is removed from the hearth through the iron notches. The metal is placed into transfer ladles, while the slag may be transferred to slag pots

j) Tuyeres – the hot blast air is delivered to the furnace through water-cooled openings called tuyeres. The tuyeres are located at the top of the hearth

Blast Furnace Equipment

1) The iron ore, coke and limestone are dumped into the top of the blast furnace, which holds about 1000 tons.

2) The burning coke and the blast of very hot air melt the iron ore.

3) The limestone mixes with the ashes of the burnt coke and with the rock and earth of the iron ore. The mixture forms a waste that is called slag.

4) Because the molten iron is heavier than the slag, it drips to the bottom of the furnace.

5) The furnace is emptied (called tapping) through a tap hole at the bottom. The melted iron flows out into a ladle for transfer to a steel furnace or into a trough and then into iron pig molds.

6) The slag which floats on top of the melted iron in the blast furnace, is drained off through a separate hole. The blast furnace works continuously, and is tapped every 5 to 6 hours.

Blast Furnace: Process Description

The oxygen in the iron oxides is removed by a series of chemical reactions:

1) 3Fe2O3 + CO = CO2 + 2 Fe3O4 Begins at 450 C

2) Fe3O4 + CO = CO2 + 3 FeO Begins at 600 C

3) FeO + CO = CO2 + Fe Begins at 700 C

or

FeO + C = CO + Fe

The coke is ignited by this hot blast and reacts to generate heat:

C + O2 = CO2 + Heat

Since the reaction takes place in the presence of excess carbon at a high temperature, the CO2 is reduced to carbon monoxide:

CO2 + C = 2CO

* The product of this reaction, carbon monoxide is necessary to reduce the iron ore.

Blast Furnace: Chemical Reaction

The limestone descends in the blast furnace and remains a solid while going through its first reaction as follows:

CaCO3 = CaO + CO2

This reaction requires energy and starts at about 875C.

The CaO formed from this reaction is used to remove sulphur from the iron which is necessary before the hot metal becomes steel. This sulphur removing reaction is:

FeS + CaO + C = CaS + FeO + CO

* The CaS becomes part of the slag.

Blast Furnace: Chemical Reaction

BASIC OXYGEN FURNACE •To make steel from pig iron, it is necessary to remove carbon and other impurities.

•In the BOP, pure oxygen is blown into the molten iron through a water-cooled lance that enters through the top of the furnace.

The use of pure oxygen enables the chemical composition of the steel to be controlled very accurately. Thus, BOP steel is of very high quality.

The use of pure oxygen results in higher furnace temperatures, thereby shortening the time required to convert the pig iron to steel.

(a BOP furnace can produce about 300 tons of steel an hour)

Advantages of using pure oxygen

Basic Oxygen Furnace (BOP)

BOP: Process Description

Basic Oxygen Furnace (Relau Asas Oksigen)

The BOP furnace is tilted on its side for charging. Molten iron and scrap are charged into its mouth.

It is then tilted to an upright position, and pure oxygen is blown into the furnace under high pressure, thus burning out

the impurities

Burned lime, converted from limestone, is also added to the furnace with the oxygen to increase the removal of impurities.

When the impurities have been burned out of the molten iron, the necessary elements (such as carbon, nickel, chromium and others) are added to meet the specifications for the steel required.

Cont.

The furnace is then emptied by tilting it on its side and pouring the molten steel into a large ladle

ELECTRIC ARC FURNACE

ELECTRIC ARC FURNACE

Electric Arc Furnace

The Electric Arc Furnace Process

Advantages of Electric Arc Furnace

•The electric arc process is used when close control of temperature and exact amounts of alloying elements are important.

•Higher temperatures can be reached than with other steel-making furnaces.

•Electric arc furnaces are good for making high-carbon steel, steels alloyed with metals that have high melting points, and stainless steels.

Powerful electric arcs bridge the air gap between large carbon electrodes and the metal to be melted.( The metal serves as the other electrodes)

The arcs produce the heat required to melt the metal.

100% iron and steel scrap can be used.

Temperature control is very good. This makes possible very close control of the grain structure of the steel.

Cont.

Furnace charging

Melting

Refining

De-slagging

Tapping

Furnace Operations

The roof and electrodes are raised and are swung to the side of the furnace to allow the scrap charging crane to move a full bucket of scrap into place over the furnace.

The scrap falls into the furnace and the scrap crane removes the scrap bucket.

The roof and electrodes swing back into place over the furnace. The roof is lowered and then the electrodes are lowered to strike an arc on the scrap.

Furnace Charging

The roof of an arc furnace removed, showing the three electrodes

Furnace Charging

Melting is accomplished by supplying energy to the furnace interior. This energy can be electrical or chemical. Electrical energy is supplied via the graphite electrodes and is usually the largest contributor in melting operations.

Once the final scrap charge is fully melted, flat bath conditions are reached. At this point, a bath temperature and sample will be taken. The analysis of the bath chemistry will allow the melter to determine the amount of oxygen to be blown during refining.

Melting

Refining operations in the electric arc furnace have traditionally involved the removal of phosphorus, sulfur, aluminum, silicon, manganese and carbon from the steel.

These refining reactions are all dependent on the availability of oxygen. Oxygen was lanced at the end of meltdown to lower the bath carbon content to the desired level for tapping. Most of the compounds which are to be removed during refining have a higher affinity for oxygen that the carbon. Thus the oxygen will preferentially react with these elements to form oxides which float out of the steel and into the slag.

Refining

De-slagging operations are carried out to remove impurities from the furnace. During melting and refining operations, some of the undesirable materials within the bath are oxidized and enter the slag phase.

The furnace is tilted backwards and slag is poured out of the furnace through the slag door.

De-slagging

Once the desired steel composition and temperature are achieved in the furnace, the tap-hole is opened, the furnace is tilted, and the steel pours into a ladle for transfer to the next batch operation (usually a ladle furnace or ladle station).

During the tapping process bulk alloy additions are made based on the bath analysis and the desired steel grade.

Tapping

An electric arc furnace (the large cylinder) being tapped

Tapping

KINDS OF STEEL

There are two main kinds of steels: a) plain carbon steels b) alloy steels

Plain carbon steels are divided into three main groups:

1) Low-carbon steel (mild steel)

2) Medium-carbon steel

3) High-carbon steel

Plain carbon steel

Fe-C Phase Diagram

Ferrite : is an interstitial solid solution of carbon in alpha iron. Ferrite dissolves considerably less carbon than austenite, with maximum amount being 0.025%C at 723ºC. Ferrite is nearly pure iron.

Cementite: Fe3C is a compound of iron and carbon referred to as iron carbide. Cementite contains 6.67% C and 93.3% Fe. Cementite is so hard it can be machined only by grinding.

definition:

Pearlite : is a lamellar aggregate of ferrite and cementite formed from the eutectoid decomposition of austenite during slow cooling. Pearlite has the microstructural appearance of fingerprints.

Austenite : (γ-Fe) is an interstitial solid solution of carbon in gamma iron. The solid solubility of carbon in austenite is a maximum of 2.08% at 1148ºC and decreases to 0.8% at 723ºC.

definition:

Upper critical temperature (point) A3 is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypoeutectoid alloys.

Upper critical temperature (point) ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hypereutectoid alloys.

Lower critical temperature (point) A1 is the temperature of the austenite-to-pearlite eutectoid transformation. Below this temperature austenite does not exist.

definition:

Plain Carbon Steels

Contains between 0.05% to 0.30% carbon

Good weldability and machinability.

Low tensile strength, very ductile and soft.

Application : body for washing machine, wire, steel pipe, screw and nuts.

Low carbon steel is also used for forge work, rivets, chains and machine parts that do not need great strength

Low-Carbon Steel

Contains 0.3% to 0.6% carbon.

Medium carbon steel is stronger than low carbon steel. It is also more difficult to bend, weld and cut than low carbon steel.

Balances ductility and strength and has good wear resistance

Medium carbon steel is used for bolts, shafts, car axles, rails and other parts or tools that require strong metal.

Medium-Carbon Steel

Also known as carbon tool steel

Contains between 0.6% to 1.5% carbon

The best grades of this steel are made in electric furnaces

It is used to make such tools as drills, taps,dies,reamers,files,cold chisels,crowbars and hammers.

It is hard to bend, weld and cut

High-Carbon Steel

1) Silicon

- a chemical element found in rocks. It gives hardness to iron.

2) Sulfur

- a yellow, flammable nonmetal. Too much sulfur makes these metals weak and brittle, causing cracks. A little sulfur is added to some steels for better machinability.

Other Elements in Steel

3)Phosphorus

- a poisonous, active nonmetal. It causes brittleness and coarse grain in iron and steel.

4) Manganese

- a grayish white metal, hard and brittle. It resembles iron but is not magnetic. It is used in making steel.

Cont

Alloy Steels

cont

•Alloys steels are made by combining steels with one or more other elements.

•These elements are usually metals.

•They are intentionally added to obtain properties that are not found in plain carbon steels.

The main purpose is to improve its mechanical properties such as:

Improve tensile strength

Improve toughness

Wear resistance

Corrosion resistance

Resistance to heat

Improve properties by heat treating

The purpose for alloying the steel

1) Chromium (chrome)

- gives hardness to steel, and toughens it. It also makes the steel’s grain finer and causes the steel to resist rust, stains, shocks, and scratches.

- chromium is the basis for stainless steel (contains 11% to 26%)

It has a lasting, bright, silvery gloss.

Effects of Alloying Element

Some important uses for stainless steel:

-sinks

-tableware

-pots and pans

-cutting tools

-dental tools

-automobile parts

-fine measuring tools and instruments

cont

2) Cobalt

- an important metal used in making cutting tool alloys (these alloys include high-speed steels, cast alloys and cemented carbides)

-its ability to improve the hardness of cutting tools when they are hot or even red-hot.

cont

3) Manganese

- a hard, brittle, grayish-white metal.

- It purifies and adds strength and toughness to steel.

- Manganese steel remains hard even when cooled slowly.

- It is so very hard that it is difficult to cut, so it is usually cast into shape.

Cont.

4) Molybdenum

-a silvery white metal, it adds strength and hardness to steel and allows it to stand heat and shocks.

-molybdenum steel is used for automobile parts, high grade machinery, wire as fine as 0.01 mm in diameter, ball bearings and roller bearings.

Cont.

5) Nickel

- Adds strength and toughness to steel.

- Nickel steel does not rust easily and is very strong and hard.

- It is used for wire cables, shafts, steel rails, automobile and railroad car axles and armor plate.

Cont.

6) Tungsten

-a rare, heavy, white metal that has a higher melting point than any other metal.

-tungsten adds hardness to steel.

-it gives steel a fine grain and allows steel to withstand heat.

- Tungsten steel is used extensively for making cutting tools. These tools need no special hardening treatment and will withstand high temperatures.

Cont.

7) Vanadium

- a pale, silver-gray metal. It is brittle and resists corrosion.

- Vanadium steel can withstand great shocks.

- It is used for springs, automobile axles and gears and other parts that vibrate when in use.

- Vehicles used in heavy construction commonly used vanadium steel.

Cont.

8) Chromium-vanadium steel

- is hard and has great tensile strength.

- It can be bent double while cold and is easy to cut.

- is used for automobile parts such as springs, gears, connecting rods and other parts which must be strong and tough but not brittle.

Cont.

Types of alloy

steel

Magnetic steel

Tool and

mould steel

Structure steel

Corrosion

resistance steel

Heat resistance

steel

Alloy steel

Alloy elements: Cr, Ni, P, Si, Mn, Mo, Cu is added to increase strength solid solubility hardening.

Increase ability to metal forming and weldable.

High strength, high toughness and resistance to stress.

Application : beams, shaft

Structure Steel

Alloy elements : Al, Si, Cr

High Cr ~12% Cr resistance to high temperature corrosion

Cr reacts with oxygen to form chromium oxide and prevent the steel from corrode.

Application : gas turbine, engine casing and aircraft frame, cutlery.

Corrosion Resistance Steel

Forming of carbide and nitride.

Carbide : V, W, Cr forms highly hardness carbide such as Fe4W2C – hardness and strength increase.

High Speed Steel (HSS)- contains14-22%W, 4%Cr and 1%V forms hard carbide heat resistance.

Nitrida : Ti and Al forms Ni2Ti, NiAl, Ni3Ti – improves ductility and toughness.

Maraging steel – rocket and high speed air craft.

Heat Resistance Steel

High hardness and wear resistance.

Use for cutting, shaping or forming such as rolling, drawing.

Have high carbon contents 0.6-1.3%.

Contain elements such as Cr, Co, Mn, Mo, Ni, Si, W, and V to form various alloy carbides and help increase the hardness, wear resistance and elevated-temperature softening resistance.

Tool Steel

Contain Cr and Ni as the principal alloying elements.

Have low-to-medium carbon content and a total alloy content of 1.5-5%.

It is special tool steel which is to be heat treated.

Mould Steel

Ferromagnetic metals in room temperature are Fe, Co and Ni.

Tough and durable, inexpensive to produce, maintains strong magnetism when miniaturized and can produce a stable magnetic force in spite of temperature changes or vibration.

Magnetic steel is used to form magnet induction

Mashima & Alnico

Magnetic Steel

CAST IRONS

Cast iron is pig iron that has been remelted and poured into a mold.

Iron cools in the shape of a useful machine part or article.

An object made this way is called a casting.

Basic elements in cast iron

Iron

Carbon : 2.8 to 3.6%

Silicon :1.0 to 3.0%

Manganese : 0.4 to 1.0%

Magnesium : 0.03 to 0.05%

Phosphorus : 0.05 to 1.0%

Sulfur : 0.1 to 0.35%

Cast Iron

Properties of cast iron:

1. Tensile strength moderate180-350MPa

2. High compression strength.

3. Immersion capacity good.

4. High wear resistance

5. Good machinability

6. Low impact resistance and ductility

Cast Iron

1. Cooling rate:

High - stable cementite

- white cast iron

- hard and brittle

Low - cementite forms to ferrite and graphite

- gray cast iron

- ductile

2. Carbon content– increase of carbon lower the melting temperature and more graphite are formed→ gray cast iron

Cast Iron Properties Effects

3. Cross-section Size Wide - low solidify rate - graphite - gray cast iron Small - vs wide cross-section 4. Content of other elements: silicon - high liquidity → graphite → gray cast iron sulfur - cementite stable, forms MnS → white cast iron Manganese – combine with sulfur to form MnS Fosforus - liquid steel→ graphite → gray cast iron

Cast Iron Properties Effects

A) Gray Cast Iron

B) White Cast Iron

C) Malleable Iron

D) Ductile Cast Iron

Kinds of Cast Iron

Types of cast

iron

Nodular cast iron

White malleable

Cast iron

Gray cast iron

White cast iron

Black malleable

cast iron

Most carbon is in a free state.

It is scattered in the form of graphite flakes throughout the crystalline grain structure of the metal.

This arrangement of carbon makes the cast iron brittle. Thus, it fractures easily from sharp blows.

It has a gray crystalline color where fractured.

Gray Cast Iron (1.7%C to 4.5%C)

Microstructure of Gray Cast Iron

It is so named because of a white, crystalline color at the fracture when broken.

White cast iron is made by rapid cooling the molten pig iron.

Most of the carbon in white cast iron is in chemically combined state.

It forms a hard substance called cementite,Fe3C.

White Cast iron (2%C to 3.5%C)

White cast iron is so hard that it cannot be machined except by grinding.

Its direct use is limited to castings requiring the surfaces to withstand abrasion and wear.

The major use of white cast iron,however, is making malleable cast iron.

Cont.

Microstructure of White Cast Iron

Malleable iron is cast iron that has been made soft, tough, strong and malleable.

When white cast iron is heated at a high temperature for 100-120 hours, it is converted to malleable iron.

This heat treatment process, called malleableizing, takes place in heat treatment furnace.

Malleable Cast Iron (2%C to 2.6%C)

Metal is malleable, it can be hammered into different shape without cracking.

Heat treatment changes the arrangement of the carbon from its combine state as cementite to free carbon.

At the prolonged high temperature, the free carbon comes together to form clusters or globule.

The surrounding iron then becomes soft and machinable.

They are used for making tough castings for automobiles, tractors and many kinds of machinery parts.

cont

Microstructure of White Malleable Cast Iron

Microstructure of Black Malleable Cast Iron

Also known as nodular iron or spheroidal graphite iron.

The carbon is in the free state.

It is in small, rounded lumps carbon clusters called nodules.

The iron surrounding the tiny balls of graphite is so tough and machinable.

Ductile Cast Iron

Ductile cast iron is produced very much like gray cast iron.

Magnesium alloys and certain other elements are added to a ladle of gray iron before it is poured into molds to make castings.

These additives and proper heat treatment cause the carbon in the molten iron to form balls or nodules as it cools and becomes solid.

Cont.

Ductile cast iron has properties similar to malleable iron.

It is tough, machinable, and possesses many of the characteristics of steel.

It is used for making tough castings for automobiles, farm machinery and many other kinds of machinery.

Cont.

Microstructure of Ductile Cast Iron