JF302 –

MATERIALS

TECHNOLOGY 1

CHAPTER 3

FERROUS MATERIAL STRUCTURE &

BINARY ALLOY SYSTEM

Understand metal production – iron and steel

Understand the process of iron and steel production

Understand plain carbon

Understanf alloy steel

Understand cast iron

LEARNING OUTLINE

IRON

Iron is an element and can be pure.

Iron can be extracted from iron ores such as rocks, mineral.

The extracted process can be done through Blast Furnace.

Basic constituent of steel.

Iron ores content: -

Magnetite

Iron Carbonate

Hematite

Limonite

Chemical Formula is (Fe 3O4).

Also called as Black Iron Ore - because the colour is black.

Percentage of Iron; 72 – 62% Fe (Iron)

MAGNETITE

Also know as sindrite

Chemical formula (FeCO3)

Its color is yellowish green

Percentage of iron; 35 – 40% Fe (Iron)

IRON CARBONATE

Chemical formula (Fe2O3)

Also known as Red Iron Ore.

Percentage of Iron; 70% Fe (Iron).

HEMATITE

Chemical formula is (FeO(OH).n(H2O))

Also known as Brown Iron Ore

LIMONITE

The purpose of use blast furnace is to reduce and convert iron

oxides into liquid iron called “hot metal”.

The appearance of blast furnace is huge, steel stack lined

with refractory brick.

Iron ores, coke and limestone are put into the top and

preheated air blown into the bottom.

Iron can be extracted by blast furnace because it can be

displaced by carbon.

It more efficient and more cost effective.

BLAST FURNACE

BLAST FURNACE

To ensure the process of extraction there are combined

mixture of subtances needed and called as charge.

Charge:

Iron ore, haematite - often contains sand with iron oxide, Fe2O3.

Limestone (calcium carbonate).

Coke - mainly carbon

The charge is placed a giant chimney called a blast furnace.

The blast furnace is around 30 metres high and lined with

fireproof bricks. Hot air is blasted through the bottom.

THE PROCESS OF BLAST FURNACE

Oxygen in the air reacts with coke to give carbon dioxide:

C(s) + O 2(g) CO2(g)

The limestone breaks down to form carbon dioxide :

CaCO3(s) CO2 (g) + CaO(s)

Carbon dioxide produced in 1 + 2 react with more coke to produce carbon monoxide:

CO2(g)

+ C(s)

2CO(g)

The carbon monoxide reduces the iron in the ore to give molten iron :

3CO(g)

+ Fe2O

3(s) 2Fe

(l) + 3CO

2(g)

The limestone from 2, reacts with the sand to form slag (calcium silicate ):

CaO (s) + SiO (s) CaSiO3(l)

REACTION OF IRON PRODUCTION

Both the slag and iron are drained from the bottom of the

furnace.

• The slag is mainly used to build roads.

• The iron whilst molten is poured into moulds and left to

solidify - this is called cast iron and is used to make railings

and storage tanks.

• The rest of the iron is used to make steel.

REACTION OF IRON PRODUCTION

It’s included in the term ferrous metal

It’s a combination of iron & carbon( 0.01 – 1%)

Contains varying amounts of manganese, phosphorus, sulfur,

silicon & 20 other alloys

Alloys added to produce steel of dif ferent characteristics .

To produce useful steel, pig iron need to be oxidized in

another furnace at about 1650°C.

Molten iron is not useful because it’s weak and brittle

although it’s very hard .

STEEL

Fastest steelmaking process – can make 250 tons of steel / hour

Melted molten iron and scrap are poured (charged) into a vessel.

Fluxing agents are added, like limestone.

The molten metal is blasted with pure oxygen. This produces iron oxide which then reacts with carbon to produce CO and CO2. The slag floats to the top of the metal.

Higher steel quality than open hearth. Used to make plate, sheet, I -beam, tubing and channel.

BASIC OXYGEN FURNACE

ELECTRIC FURNACE

Uses electric arc from electrode to metal to heat and melt it.

Can produce 60-90 tons of steel per day.

Steel is higher quality than open-hearth and BOF

IRON – CARBON PHASE DIAGRAM

PHASE IN FE-C PHASE DIAGRAM

The following phases are involved in the transformation, occurring with

iron-carbon alloys:

L - Liquid solution of carbon in iron;

δ-ferrite – Solid solution of carbon in iron.

Austenite – interstitial solid solution of carbon in γ -iron.

α-ferrite – solid solution of carbon in α-iron

Cementite – iron carbide, intermetallic compound, having fixed

composition Fe3C.

PHASE IN TRANSFORMATION OF IRON –

CARBON ALLOY

Maximum concentration of carbon in δ-ferrite is 0.09% at 2719 ºF (1493ºC) – temperature of the peritectic transformation.

The crystal structure of δ-ferrite is BCC (cubic body centered).

FERRITE

Austenite has FCC (cubic face centered) crystal structure, permitting high solubility of carbon – up to 2.06% at 2097 ºF (1147 ºC).

Austenite does not exist below 1333 ºF (723ºC) and maximum carbon concentration at this temperature is 0.83%.

Solubility is higher when compared to ferrite.

AUSTENITE

α-ferrite has BCC

crystal structure

low solubility of

carbon – up to

0.025% at 1333 ºF

(723ºC).

α-ferrite exists at

room temperature.

FERRITE

It is an intermetallic

compound

Cementite is a hard and

brittle substance,

influencing on the

properties of steels and cast

irons.

Low tensile strenght and

high comprehensive

strength.

Crystal structure is

orthormobic.

CEMENTITE

T h e f o l l o w i n g p h a s e t r a n s f o r m a t i o n s o c c u r w i t h i r o n - c a r bo n a l l o y s :

A l l o y s , c o n t a i n i n g u p t o 0 . 5 1 % o f c a r b o n , s t a r t s o l i d i f i c a t i o n w i t h f o r m a t i o n o f c r y s t a l s o f δ - f e r r i t e .

C a r b o n c o n t e n t i n δ - f e r r i t e i n c r e a s e s u p t o 0 . 0 9 % i n c o u r s e s o l i d i f i c a t i o n , a n d a t 2 7 1 9 º F ( 1 4 9 3 º C )

r e m a i n i n g l i q u i d p h a s e a n d δ - f e r r i t e p e r f o r m p e r i t e c t i c t r a n s f o r m a t i o n , r e s u l t i n g i n f o r m a t i o n o f

a u s t e n i t e .

A l l o y s , c o n t a i n i n g c a r b o n m o r e t h a n 0 . 5 1 % , b u t l e s s t h a n 2 . 0 6 % , f o r m p r i m a r y a u s t e n i t e c r y s t a l s i n

t h e b e g i n n i n g o f s o l i d i f i c a t i o n a n d w h e n t h e t e m p e r a t u r e r e a c h e s t h e c u r v e A C M p r i m a r y c e m e n t i t e

s t a r s t o f o r m .

I r o n - c a r bo n a l l o y s , c o n t a i n i n g u p t o 2 . 0 6 % o f c a r b o n , a r e c a l l e d s t e e l s .

A l l o y s , c o n t a i n i n g f r o m 2 . 0 6 t o 6 . 6 7 % o f c a r b o n , e x p e r i e n c e e u t e c t i c t r a n s f o r m a t i o n a t 2 0 9 7 º F

( 1 1 4 7 º C ) . T h e e u t e c t i c c o n c e n t r a t i o n o f c a r b o n i s 4 . 3 % .

I n p r a c t i c e o n l y h y p o e u te c t i c a l l o y s a r e u s e d . T h e s e a l l o y s ( c a r b o n c o n t e n t f r o m 2 . 0 6 % t o 4 . 3 % ) a r e

c a l l e d c a s t i r o n s . W h e n t e m p e r a t u r e o f a n a l l o y f r o m t h i s r a n g e r e a c h e s 2 0 9 7 º F ( 1 1 4 7 º C ) , i t

c o n t a i n s p r i m a r y a u s t e n i t e c r y s ta l s a n d s o m e a m o u n t o f t h e l i q u i d p h a s e . T h e l a t t e r d e c o m p o s e s b y

e u t e c t i c m e c h a n i s m t o a f i n e m i x t u re o f a u s t e n i t e a n d c e m e n t i t e , c a l l e d l e d e b u r i t e .

A l l i r o n - c a r bo n a l l o y s ( s t e e l s a n d c a s t i r o n s ) e x p e r i e n c e e u t e c t o i d t r a n s f o r m a t i o n a t 1 3 3 3 º F

( 7 2 3 º C ) . T h e e u t e c t o i d c o n c e n t ra t i o n o f c a r b o n i s 0 . 8 3 % .

W h e n t h e t e m p e r a t u re o f a n a l l o y r e a c h e s 1 3 3 3 º F ( 7 3 3 º C ) , a u s t e n i t e t r a n s f o r m s t o p e a r l i t e ( f i n e

f e r r i t e - c e m e n t i t e s t r u c t u re , f o r m i n g a s a r e s u l t o f d e c o m p o s i t i o n o f a u s t e n i t e a t s l o w c o o l i n g

c o n d i t i o n s ) .

PHASE TRANSFORMATIONS OCCUR WITH

IRON - CARBON ALLOYS

Classification. Three types of ferrous alloys :

Iron: less than 0.008 wt % C in α−ferrite at room T

Steels: 0.008 - 2.14 wt % C (usually < 1 wt % ) α-ferrite + Fe3C at

room T (Chapter 12)

Cast iron: 2.14 - 6.7 wt % (usually < 4.5 wt %)

TYPES OF FERROUS ALLOYS

Critical temperatures

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.

Magnetic transformation temperature A2 is the temperature

below which α -ferrite is ferromagnetic.

CRITICAL TEMPERATURES

TO BE CONTINUED

NEXT CLASS

Alloy steels are iron-carbon alloys, to which alloying elements

are added with a purpose to improve the steels properties as

compared to the carbon steels.

Purpose of alloying

Increase hardenability

Improve strength at ordinary temperature

Improve mechanical properties at either high or low temperatures

Improve toughness at any minimum hardness or strength

Increase wear resistance

Increase corrosion resistance

Improve magnetic properties

ALLOY STEELS