Chapter 3

Metal Works, Casting Process and Heat Treatments for Steel

3.1 Cold Work

1. Cold working of metals is permanent deformation of metals and

alloys below the temperature at which a strain-free microstructure

is produced continuously (recrystallization temperature). Usually

in room temperature.

2. Cold working causes a metal to be strain-hardened, deformed and

strengthened.

3. When a sheet metal or ingot in cold work process, crystalline

structures (lattice) are changed, distort and stretched to the

direction of the worked.

4. The metal will be hardened and increased the strength for internal

strained causing the decreasing of ductility. Change its strength

and increase the electricity resistance.

5. These cold works usually applied after hot work process :

(a) drawing

- tube drawing

- wire drawing

(b) pressing / rolling

- cold rolling

- thread rolling

(c) extrusion

- cold extrusion

- impact extrusion

6. It is a finishing process in production to produce and function as :

i. to maintain accurate dimension of the product

ii. achieve clean and smooth finishing

iii. achieve various of hardness degrees by applying various of

cold works

iv. repairing the machineability

3.1.1 Cold Rolling

1. Long lengths of metals sheet and plate with uniform cross sections

can be produced.

2. The coils of metal are usually given a reheating treatment called

annealing to soften the metal to remove internal stress introduced

during the hot-rolling operation.

3. Smaller diameter roller will be operated to thinning the metal, and

bigger roller used as support which will absorb vibration and

maintaining the thickness.

4. Lubricating material usually applied to the work piece before and

after it is rolled to refine the surface and to prevent grain

formation.

5. It is able to produce sheet metals as thin as 0.008mm – 0.009mm

called foil.

6. Advantages :

i. surface free from oxidation

ii. smoother and shinier surface

iii. fine fitting

iv. increase the tensile strength and toughness

3.1.2 Wire and Tube Drawing

1. Used to produce wire, rod and tube.

2. A process in which wire stock is drawn through one or more

tapered wire-drawing dies to the desired cross section.

3. Only annealed metal and with high ductilities metal can be

processed.

4. Friction, shear and tough pressure occurred at the joint part of the

die and the work piece and will heatened the parts.

5. Therefore, cooling elements are needed and dies has to be tough

and strong enough to resist wear and abrasive by those effects.

3.1.3 The Advantages And Disadvantages Of Cold Work

1. The advantages :

i. good surface finishing because it smoother with no oxidation

process

ii. exact measurement can be achieve with exact dimension control

because of no dimension shrinkage

iii. increase the machineability of the metal

iv. the product does not need any finishing works

v. good finishing properties

2. The disadvantages :

i. costing higher than hot work process

ii. the material become less in ductility caused by hardening work

iii. causing more brittle to the metal and lesser elasticity

iv. cold work only can be use to the elastic metal

v. bigger equipment and higher power usage

3.2 Hot Work

1. Hot working of metals is permanent deformation of metals and

alloys above the temperature at which a strain-free microstructure

is produced continuously (recrystallization temperature).

2. The recrystallization temperature for steel begins at 950°F -

1300°F.

3. If the temperature being work is sufficiently high, recrystallization

takes place as quickly as the crystals become deformed and the

metals can be heavily worked with ease without risk of cracking.

4. As the temperature falls during processing, recrystallization occurs

more slowly, not only more force is required to achieve plastic

deformation, but there is an increased risk of surface cracks

appearing.

5. If the metal temperature is rising, become burnt, oxidation of the

grain boundaries occurs and the material is severely weakened.

6. Main processes of hot work :

i. hot rolling

ii. hot forging

iii. hot extrusion

iv. hot forming

v. welding hot pressing

3.2.1 Hot Forging

1. The metal is hammered or pressed into a desired shape in the

closed-die forging.

2. The usage of closed-die forging :

i. The die cavity is the shape of finished component

ii. Both part of dies attach to hammer and anvil

iii. When force delivered, both parts combined and become one

iv. To ensure full filling of the metal in the die, material quantities

has to be more than the cavity

v. The surplus metal will run out through the die and forming the

flash

3. Applications : spanners, bolts, shafts

3.2.2 Hot Rolling

1. Hot rolling is carried out to have greater reductions in thickness of

metal ingots, until certain thickness achieve, taken by rolling pass

when the metal is hot.

2. The ingots will go through two big cylinder roller and then other

rollers until achieve the needed thickness.

3. Discontinuities in the ingots will be sealed or welded under huge

pressing process and gained a homogenous structure.

4. Applications : railways, construction frames

3.2.3 Hot Extrusion

1. The extrusion process is used to produce cylindrical bars or hollow

tubes.

2. Extrusion is a plastic forming process in which a material under

high pressure is reduced in cross-section by forcing it through an

opening in a die.

3. The advantages : The ability to produce varies of complicated shape

with accurate dimension and good finishing.

4. Its produce continuously but only to metals with low melting point

and with good melting ability such as bronze, brass alloys and

aluminium and its alloys.

3.2.4 The Advantages and Disadvantages Of Hot Work

1. The advantages :

i. metal are in plasticity condition. Energy and needed forces are

small. Can be worked for bigger size metals

ii. blow holes in ingots can be disappeared by compression

iii. suitable to almost all types of metal

iv. if finishing temperature are correct, smoother structure can be

achieve

2. The disadvantages :

i. better surface cannot be achieve because of corrosion by

oxidation process in high temperature

ii. higher in cost

iii. accuracy in last dimension hard to achieve because of shrinkage

factor when the hot metal are cooling

iv. life expectation for tools are lessen caused by working in higher

temperature

3.3 Casting Process

1. Casting process is a production process where the metal is formed

directly from the molten state, pouring it into a mould and

allowing it to cool and solidify, expelled from the mould to be

clean or machine for finishing.

2. The mould must be made from a material with a higher melting

point than the molten metal which the casting is to be made.

3. The mould contains a cavity in the form of the finished product

into which the molten metal is poured.

4. Types of casting process :

a) sand casting

b) lost wax/investment casting

c) pressure-die casting

d) shell casting

e) centrifugal casting

f) plaster of Paris casting

g) ceramic mould

h) evaporative pattern casting

3.3.1 The Purposes, Impotencies and Process of the Casting

1. Stages of casting process :

(a) metal is heated until its melted

(b) pouring the molten metal in a cavity mould

(c) leave the metal to solidify

(d) retrieved the solid metal from the mould

(e) clean or machine for finishing

2. The advantages :

(a) typical shapes of product which cannot be produce by other

process such as machining, forging and welding

(b) cast iron only can be worked through casting process because

of its properties and cannot be worked by other hot work to

form bars, rods or other shapes

(c) project manufacturing are simplifies, the casting process able

to poured to complete shape of a product where other process,

the product need to be heated or connected to form complete

shape

(d) small amount of wasted materials compared to machining

process

(e) casting is a cheaper process if compared to others

(f) suitable for mass production : automotive industry products,

household products and agriculture machinery

(g) wasted metal can be recycle using casting process

3.3.2 Sand Casting (Penuangan Pasir)

1. The sand casting process is usually chosen for the production of

small quantities of identical castings, complex castings with

intricate cores, large castings and structural castings.

2. A mould made by compressing or ramming the casting sand

(combination of silica sand and bantonite function as adhesive),

circling a pattern made by wood, forming a cavity in the mould.

3. The mould surrounded by a moulding box which separated into

2 parts called cope and drag. Its helps in expelling the pattern

out leaving the mould with a cavity where the molten metal are

poured in to form a product.

4. To smoother the casting works and to ensure the mould cavity is

full with molten metal, a running system including building a

runner, a riser and a gate to the mould.

5. A riser also provides surplus metal which can be drawn back

into the mould as shrinkage takes place during cooling and this

can avoids shrinkage cavities occurring in the casting.

6. The pattern has to be made oversize to allow for shrinkage of the

metal as it cools and it has called the shrinkage allowance.

7. A hollow casting can be made by using a core in the cavity.

8. Casting defects :

(a) scabs – these are blemishes on the surface of the casting

resulting from sand breaking away from the wall of the mould

cavity, due to lack of cohesiveness in the sand resulting from too

low clay content or from inadequate ramming, too rapid pouring

can also result in the scouring away the walls of the mould cavity

(b) cold shuts – result from casting intricate components with thin

sections from metal which is lacking in fluidity or at too low

temperature, sections of the mould may not fill completely or

the metal may flow too sluggishly and at too low temperature to

unite when separate streams meet

(c) hot tear – it is as same as part of the casting broken cause by

coherent and strained by heat shrinkage attach to unsuitable

mould design

(d) blow holes – are smooth round holes with a shiny surface

usually occur just below the surface of the casting, not normally

visible until the casting is machined, caused by steam and gases

being trapped in the mould. Results from inadequate venting,

incorrectly placed the risers, excessive moisture in the sand or

excessive ramming reducing the permeability of the sand

(permeability is the ability of the sand to allow entrapped gases

to escape between the individual sand particles)

(e) Other defects including porosity, uneven wall thickness, fins and

drawing.

9. The casting sand should have these properties :

(a) high heat temperature resistance

(b) enough adhesive strength

(c) gases permeability

(d) can be tested for grain size, compressive, tensile and shear

strength, hardness and compactability

10. The advantages of sand casting process :

i. manufacturing process for multiple usage

ii. suitable to produce one until thousand of casting units

iii. freedom in designing from weight, size and shape

iv. can be use to produce component with the weight in

grammes until tones

v. bigger size product can be cast in hollow casting technique

vi. typical shape can be make by using various of cores

vii. can be use for all kind of metal including metal that cannot

be manufactured by other process such as cast iron

viii. cost for making the mould are low because low in sand price

and reusable

11. The disadvantage of sand casting process :

i. the cast exposed to crack while cooling if the design are not

suitable

ii. limited to small quantities production if the process done

manually

iii. surface finishing quality are low and need to be machine

iv. the ira (grain) are not compacted, therefore low in

compactability and weak

v. low in ductility

vi. unsuitable for thinner casting product

11. Sand casting tools :

i. SAND MOULD – containing 85% silica sand, 8% bentonite and

7% water

ii. MOULDING BOX/ FLASK – a box where use to made a mould

in it, containing 2 parts (cope and drag), made by wood or metal

iii. PATTERN – a model or replica of product, made according to

the real shape of the product, made by wood, metal, plastic, wax

or plaster

iv. CORE – to produce a hollow product, made by plaster, metal,

ceramic or silica sand

v. BELOS – to aired the sand grain in the mould or the cavity

vi. STRIP BAR – to strip or flatten the sand on the surface of the

moulding box, made by steel

vii. LADLE – for mould finishing job as to fix broken mould, make a

groove for molten metal stream, adding or reducing mould parts

viii. GATE CUTTER & SQUIRE, MOULDING THROWEL – to fix

small damage in the cavity and create a channel for molten

metal flow

ix. RAMMER – to compact or compress the sand casting while

making the mould in the moulding box, made by wood or metal

x. VENTILATION ROD – to create ventilation holes so the heat

and air contain in the mould can be departed

xi. POWDER BAG – fill with parting powder which will be

scatter on the pattern before ramming the sand over

xii. SPRUE – to create a channel for getting system of the

molten metal (runner and riser)

xiii. DRAW PIN – to draw out the pattern from sand mould

xiv. SKIMMING LADLE – to skim the slag/ impurities floating

in the molten metal in the furnace

xv. DEGASING PLUNGER – to release the gas trapped in the

molten metal

xvi. SIEVER – to gain finer sand before ramming the sand

Figure above shows the process for preparing a mould for casting. For

that, the type of pattern use is split pattern and also using a green

sand core.

12. Processing steps :

i) Step 1 :

the drag (lower moulding box) in upside down

position and placed on top of a flat and clean plate

ensure the floor also flatted

ii) Step 2 :

lower part of the split pattern placed in the drag

the parting powder scattered over the pattern and

the plate

Fig 1: Moulding box for sand mould casting

Fig 2: Preparing the pattern for the sand ramming

iii) Step 3 :

finer sand gain from sieving process place around

and over the pattern for 3cm of thickness

by pressing with the fingers, the finer sand then

pressed to the pattern and around it compactedly

ensure that the pattern are still while the sand

compacted

iv) Step 4 :

then add the rest of the sand for ¾ into the

moulding box

use a rammer to compact the sand with slow stroke

add more sand over the moulding box and

compressed it with harder stroke

continue/ repeat this process until gaining

compacted sand over the moulding box

v) Step 5 :

by using a strip bar, stripped/flatten the surface of

the compressed sand

the bar pulled from a conner to another by moving

it to the right and left

Fig 3: Pressing the finer sand around and over the pattern

Fig 4: Adding and compressing the sand

Fig 5 : Stripped/ flatten the sand surface

vi) Step 6 :

flip the drag so that the pattern would be on top,

then place the cope on top of the drag

lock both cope and drag together

vii) Step 7 :

upper split pattern placed on top of the lower

pattern in the drag perfectly, then placed the sprues

(runner and riser) in the suitable positions

shattered the parting powder over the pattern,

sprues and the sand surface in the drag

viii) Step 8 :

sieve the sand in the moulding box to gain 3cm of

finer sand around and over the pattern

compress the sand with fingers

add and ram the sand same as the fourth step

ix) Step 9 :

use a strip bar to striped/flatten the surface of sand

in the cope

use a ladle to strip the sand surface around the

sprues

Fig 6: The position of cope and drag

Fig 9: Strippen/flatten the sand mould using a strip bar

Fig 7: The position of the pattern and the sprues

Fig 8: The sand mould after eighth step

x) Step 10 :

twist the sprues, then pull it out slowly

use a ventilation rod to make ventilation holes at

the sand mould surface

xi) Step 11 :

separate both boxes (cope and drag) and flip it to

retrieve the split pattern

before retrieve the pattern, knock it slowly so that

the pattern and the sand surface are loosen

use draw pins to retrieve both pattern sides from

the mould

the cavity will formed after retrieving the pattern

xii) Step 12 :

a channel for molten metal flow create using gate

cutter and squire

the channel should connect the sprue cavities and

the mould cavity

the channel function as a guide for the molten

metal to flow to the mould cavity through sprue

cavities

xiii) Step 13 :

core will be place in the lower cavity mould then the

upper moulding box (cope) will be place back to its

position (on top of drag)

Fig 10: Pulling out the sprue Fig 10: The ventilation rod usage

Fig 11: The boxes part (cope and drag ) are separated to retrieve the pattern

Fig 13: Core position in the moulding box

Fig 12: Channels for molten metal flow

xiv) Step 14 :

using moulding throwel, a basin for pouring the

molten metal into are made on the surface of the

cope beside a sprue cavity called the runner

the ready for pouring molten metal mould brought

closer to the furnace

the molten metal poured into the basin, flowing

through the runner and straight to the cavity

after the molten metal solidify, the product can be

retrieve by breaking the mould

3.4 Lost-wax/Investment Casting

1. In this process, molten metal are poured into a mould made by

heat resistance material which made with wax.

2. The wax pattern then will be molten and flow out, leaving a

ceramic mould, molten metal poured in the mould, filling the

cavity.

3. Generally, it is used to produce small component with

complicated shape and in need of highly accuracity such as

sawing machine component, key, guns, etc.

4. Casting metal : steel and alloys, aluminium, copper, magnesium,

cobalt and nickel.

5. The advantages :

i. an accurate measurement up to 0.005mm can be achieve

ii. smoother and no parting line appearance on the surface

iii. complicated shape can be cast

iv. no need for machining process

6. The disadvantages :

i. highly in costing process, only for component that are

little in production and complicated shape which in need

of accurate measurement

ii. unsuitable for massive casting

iii. problem occur when in need of core usage

Fig 14: Pouring the molten metal into the mould process

Products produce by lost-wax casting

7. Steps in producing lost-wax casting:

i) Step 1 :

lost-wax casting pattern made by wax

types of wax used for this process : paraffin, bee wax, acrawax

and resin (dammar).

the wax pattern then dipped into concentrated material heat

resistance coating to gain smoother surface for inside wall of the

mould

ii) Step 2 :

the wax pattern coated with heat resistance material then put

into metal mould box or a flask

molten material are inserted into the mould box

then, let it solidifies all over the box to form a mould

the molten material consist of harden material and silica sand

figure shows how a pattern posted in the mould and the molten

material poured into the box

iii) Step 3 :

the wax pattern then heated in a furnace between 100C to

200C

the wax will be melt and flow out or lost to form a cavity in the

mould

iv) Step 4 :

the mould will be retrieved from the furnace and flipped

upside down

the molten metal will be poured into the cavity

when its solidify, the casting product can be retrieved

Fig 1: A wax pattern for lost-wax

casting

Fig 2: The pattern positioned on the mould box

Fig 3: The wax melt and flow out or lost

3.5 Pressure Die Casting

1. This process is for materials which has low melting

temperature such as aluminium and zinc alloy but not for iron.

2. This process operated by injecting molten metal into metal

mould under the pressure. Molten metal or half melt metal are

pushed in or injected into mould cavity with the pressure of 20

to 2000 kg/cm2 and the pressure stays until the metal

solidifies.

3. The type of mould used is permanent mould made by metal

and consists of two parts : fixed part and moveable part, the

mould also has air ventilations to expel the air trapped in the

mould when the casting process occurs.

4. The casting machine divided into five parts/ mechanism :

i. for opening and shutting the mould mechanism

ii. for pushing or injecting the metal into the mould

mechanism

iii. for locking the mould until the metal solidifies

mechanism

iv. for insert and retrieve core automatically mechanism

v. ejector pin for ejecting the cast product from the mould

5. There are two types of casting machine : hot chamber and cold

chamber

(a) the hot chamber machine : the melting metal furnace is part of

the machine

(b) the cold chamber machine : the melting metal furnace is not

part of the machine, can be found in horizontal and vertical

position

6. Because of the mould made by metal, higher cooling rate can be

achieved compared to the sand mould. This help metals such as

aluminium alloys and zinc to produce similar crystal structures

with finer grains.

7. The mould is made by special steel and known as „die‟, tougher

metal/ alloy with higher price and cost for making the mould are

expensive. It is a permanent mould and can be use repeatedly.

8. Advantages : economical and suitable for small component with

mass production.

9. Complicated shape and thinner cross-section can be achieve

with this process, holes defect can be reduce because of there is

no air bubble trapped because it has been pressed out by

pressure.

10. There is not need for runner and riser and it also lessen the

usage of material and production cost.

11. Applications : components for refrigerator, automotive, fans and

washing machine.

Products made by pressure die casting

12. Casting metals :

(a) hot chamber process : zinc, tin (stanum), plumbum and alloy

with low melting temperature

(b) cold chamber process : aluminium, magnesium, brass alloy

and non ferrous alloy with low melting temperature

13. Steps in making products for pressure die casting :

i) Step 1 :

molten metal inserted into the chamber

ii) Step 3 :

a piston pushing/ injecting the molten metal into

the die cavity

iii) Step 3 :

retrieving the core and output die retreat backward

iv) Step 4 :

the ejector pin will eject the product out from die

14. The advantages :

(a) in need of less working area compared to other casting

processes

(b) the outputs are all similar

(c) surface finishing highly achieve compared to other

processes

(d) products or components with complicated shape can be

produce

(e) suitable for mass production because highly in

production rate which upto 8000 casting per hour

(f) job cost are low and the operator only need less

training

15. The disadvantages :

(a) cost for mould and equipment are higher

(b) the casting are limited

(c) casting size are limited

(d) limited only to metal or alloy which has low melting

temperature

(e) mould durability are lessen if the melting temperature

for metal are higher

(f) in need of expert workers for maintenance and mould

supervise

3.5 The Advantages and Disadvantages Of Casting Process

Sand

Casting

Pressure

Die

Casting

Lost-wax

Casting

Alloy/ metal that can be cast/

process

All Alloy

based of

Cu, Zn, Al

All

Comparison of mechanical

properties

Medium Better Good

Surface finishing Medium Better Better

Possibility of forming

complicated shape

Good Better Better

3.6 Heat Treatment for Steel

Heat treatment is a sequence of heating and cooling designed

to get the desired combination of properties in the steel.

The changes in the properties of steel after heat treatment are

due to the phase transformations and structural changes that

occur during the heat treatment.

Heat treatment process :

1. treatment for stable structure / soften the structure :

i) Annealing

ii) Normalizing

2. treatment for unstable structure / harden the structure :

i) Quenching

ii) Tempering

3.6.1 Purpose of Steel Heat Treatment

1. Increase strength and hardness

2. Repairing the ductility

3. Changing the grain size and chemical composition

4. Repairing the machine-ability

5. Stress relieving

6. Hardening

7. Changing the electricity and magnetic properties

3.6.2 Recrystallization

Recrystallization process

(a) Before working

(b) After cold working- the grain of the metal becomes distorted

and internal stresses are introduced into the metal.

(c) Nucleation commences at recrystallization temperature

(d) Crystals commence to grow as atoms migrate from the

original crystals and attach themselves to the nuclei

(e) After annealing is complete the grain structure is restored

1. Full Annealing

2. Stress Relieving Annealing

3. Spheroidizing Annealing

3.6.3 Heat Treatment Process and Its Effects to Steel

3.6.4 Annealing

Annealing is heating the steel over the upper critical

temperature and then cooling slowly through the

transformation range.

Slow cooling is generally achieved in a closed furnace by

switching-off the supply.

The purposes of annealling :

i. to reduce hardness

ii. to improve machine-ability

iii. to relieve internal stresses

iv. to produce the necessary microstructure

3.4.4.1 Full Annealling

Full annealing is heating and soaking (2 hours) the material,

depends on the thickness of the component and followed with

slow cooling process in the furnace.

i. Steel :0.83% carbon (<0.83% C), heated to 25 – 50 oC

above the upper critical temperature

ii. high carbon steel (>0.83% C) the temperature are 50 oC

above the lower critical temperature (723C)

3.6.4.2 Stress Relieving Annealing

It is a low temperature (about 500°C) annealing treatment

applied to cold worked steels. In practice, it is carried out

between 630°C and 700°C to speed up the process and limit

the grain growth.

It results in lowering of the residual stresses, thereby lessening

the risk of distortion in machining.

This process only for steel with less than 0.4% carbon.

The advantages of this process compared to full annealing:

i. lessen fuel cost because the process only used low

temperature

ii. lessen the maintenance cost because the furnace and

charging material operate in lower temperature

iii. no oxidation to steel at low temperature

iv. quicker processed than the full annealing with less ira

(grain) growth and mechanic properties can be

repaired

3.6.4.3 Spheroidizing Annealing

Heating and cooling to produce a spheroidal form of carbide in

steel called spheroidizing.

Desired for minimum hardness, maximum ductility and

highest machine-ability.

Applied to high carbon steels.

Material is heated to certain

temperature

Soaked to enough time

(medium) and let the

changing happen

Cooled to certain rate

Lamelar Pearlite Pearlite commences to Spheriodization of “ball up” “Balling up” completed Pearlite Cementite

Finer grain/ira and simplify spheroidising process is used to

soften plain carbon steels which have been work

hardened/quench hardened.

3.6.5 Normalizing

Defined as heating the steel 50oC above the upper critical

temperature and cooling it in the air.

Purpose : to gain the fine grain structure to improved strength

and toughness but reduce its ductility and malleability.

The temperature and timing are controlled to avoid grain

growth.

3.6.6 Quenching

Heating the steel upto upper critical temperature followed by

rapid cooling (steel is immersed in a liquid bath such as water

or oil).

Purpose : to increase hardness, strength and wear resistance.

Rapid cooling : austenite has no time to change into pearlite

but forming the body-centered-tetragonal crystals as the

supersaturated solid solution of carbon in iron called

martensite.

Caused by distorts lattice, the structures appears as a cicular

(needle-shaped).

It becomes very hard and brittle depends upon

1. the carbon contents

2. heating temperature

3. heating timing

4. cooling starting temperature

5. cooling rate.

Quenching media :

o salt water

o cool water/ pipe

o oil solution

3.6.7 Case Hardening

A process for hardening a ferrous material. The surface layer

(case), is substantially harder than the remaining material,

known as the core.

Carbon is added to the surface layers of a low carbon steel or

low alloy steel component to a carefully regulated depth.

Following by heat treatment process to harden the case and

refine the core.

There are 2 case hardening processes :

1. Carburizing / Surface Hardening

2. Nitriding

3.6.7.1 Carburizing

Carbon content at the surface of a ferrous material is increased

by heating process above 910oC.

Purpose : to obtain hard martensite phase at the surface.

There are two methods used :

a) pack carburizing

b) gas carburizing

Pack Carburizing

1. Parts to be carburized are packed with carburizing compounds

in steel boxes, then heated to the carburizing temperature

followed by cooling in air.

2. Carburizing compounds = carburizing agents and energizers

3. Carburizing agents : hardwood charcoal and coke

4. Energizers : barium and sodium carbonates, helps in

producing higher amounts of carbon monoxide and more

active carbon.

Gas Carburizing

Part to be carburized is heated in gaseous medium rich in

carbon.

Commonly used gases : natural gas, oven gas, butane, propane

and liquid hydrocarbon.

Case hardening

Case Core

Carbon Content 1.0%C 0.3%C

Temperature Hardening

Temperature : 760C

Annealing Temperature : 870C

Grain growth

Quenched Medium Air quenched – Reheating

– Air quenched

Water quenched to gain fine grain

3.6.7.2 Nitriding

A case hardening process by increasing the nitrogen content at

the surface of steel.

Nitrogen gas is absorbed into the surface of the metal to form

very hard nitrides.

Heating the components in ammonia gas at between 500 -

600oC for over 40 hours.

At this temperature, the ammonia gas breaks down and the

nitrogen atomic is readily absorbed into the surface of the

steel.

Examples of components : mould block, pump shaft, printing

die, and brake drum.

The advantages :

i. Cracking and distortion are eliminated since the

processing temperature is relatively low

ii. Corrosion resistance of the steel is improved

iii.The treated components retain their hardness when the

temperature is increased up to 500oC

iv. Surface harnesses as high as 1100 HV

v. Suitable for treated large amount of components

The disadvantages :

i. Capital cost for plant are higher

ii. Alloy steel for this process are highly cost

iii. A long time process and in need of neat monitoring

3.6.8 Tempering

Heating previous hardened steel to a temperature (below the

lower critical temperature) and cooling back to room

temperature. All hardened steels must be tempered

immediately after hardening/quenching.

Purposes :

i. Relief of internal stresses occurred after quenching

ii. Increasing the toughness and ductility

iii. Reduced the hardness and strength

Even though this process softened the steel, tempering is

different from annealing because the last structure achieved

named Tempered Martensite.

The temperature above the lower critical temperature allowing

the grain growth and causing the grain to be rougher which

will affect the strength. Suggested temperatures as shown in

the next table.

Tempering temperature (oC) Usage

220 saw blade

240 drill bit, milling cutting tool

250 mould, puncher

280 chisel

Activity 1 :

Tempering

Temperature ( C )

TROOSITE

SORBITE

230-400C

Hard and brittle martensite

transforms into fine pearlitic

structure in granular shape.

Tougher but less hard than

martensite.

Carbon steel cutting tool.

400-600C Cementite particles “ball up”.

Tougher and more ductile than

troosite.

Components subjected to shock

loads; spring.

Similarity

Similar in the original form and only different in grain size and they called

TEMPERED MARTENSITE.