Chapter 3

1. 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.

2. 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

3. 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. 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

5. (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,

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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 100C to
200C
 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

13. 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

14. 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

15. 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

16. 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

17. 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 (723C)
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

18.  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

19. 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 : 760C
Annealing Temperature : 870C
Grain growth
Quenched Medium Air quenched – Reheating
– Air quenched
Water quenched to gain fine grain

20. 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

21. Activity 1 :
Tempering
Temperature ( C ) TROOSITE SORBITE
230-400C Hard and brittle martensite
transforms into fine pearlitic
structure in granular shape.
Tougher but less hard than
martensite.
Carbon steel cutting tool.
400-600C 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.