2. Cold Work Process

3. Manufaturing processes
 Common manufaturing process categories :
 Cold working processes
 Hot working processes
 Casting processes

4. Cold-Working Processes : Cold rolling
 Reduce a plate or sheet of metal in rolling mills.
 Done at room temperatures.
 From decoiler, strip is unwound, straightened and
flattened in the pinch rolls reduces thickness.
 After thickness is reduced, it is recoiled on the right-
hand coiler.
 Then the mill rolls are reversed for further reduction.
 Finally, recoiled on the left-hand, the middle coiler.

6. Cold-Working Processes : Drawing
wire and tube
 Wire and tubes are cold drawn from stock.
 Die used are the hardened tool steel or tungsten
carbide.
 The drawing force in wire manufacture is provided by
winding the wire on to a rotating drum or ‘block’.
 Wire are work-hardened during manufacture; work
hardened brings brittleness thus need soften by
annealing process.

7. Cold-Working Processes : Drawing
wire and tube
Wire Drawing Process

8. Cold-Working Processes : Drawing
wire and tube
 Production of tubes need some form of draw bench.
 The internal bore of the tube needs support.
 Its done by drawing the tube on to a steel rod or
mandrel which passed through the die along with the
tube from which it is subsequently extracted.

9. Cold-Working Processes : Drawing
wire and tube

10. Cold-Working Processes : Advantages
& Disadvantages
Advantages Disadvantages
Good surface finish. Higher cost than hot-worked
materials. A process for material
previously hot worked. Therefore
the processing cost is added to the
hot-worked cost.
Relatively high dimensional
accuracy.
Materials lack ductility due to
work hardening, less suitable for
bending etc.
Relatively high geometrical
accuracy.
Clean surface is easily corroded.
Work hardening during the
processes: Increases strength and
rigidity. Improves the machining
characteristics of the metal, good
finish is more easily achieved
Availability limited to rods and
bars also sheets and strips and solid
drawn tubes.

11. Hot Work Process

12. Hot Working Processes : Intro
 Metals weaken and become more ductile at high
temperatures.
 Forming can take place without exhausting material
plasticity in steels at high temperatures.
 Deformation of weak austenite structure, then cools to
the stronger roomtemperature, ferrite, or much
stronger structures.
 Some of the hot working processes :
 Rolling.
 Forging.
 Extrusion.

13. Hot Working Processes :
Recrystallization
 Above the recovery range, recrystallization takes place.
 In this temperature range, the formation of new stress-
free and equiaxed grains leads to lower strength and
higher ductility.
 With less cold work there are fewer nucleuses for the
new grains, and the resulting grain size is larger.

14. Hot Working Processes : Drop Forging
 Localized compressive forces.
 Done manually anual or using power hammers,
presses or special forging machines.
 Done above the recrystallization temperature.
 The metal may be
 (1) drawn out to increase its length and decrease its cross
section
 (2) upset to decrease the length and increase the cross
section
 (3) squeezed in closed impression dies to produce
multidirectional flow.

15. Hot Working Processes : Drop Forging
(Closed Imoression Die)
 Use of a closed die, one half of which is fixed to an
anvil, whilst the other half is attached to a guided
hammer.
 Heated work piece is interposed between the die.
 Excess of metal (flash) must be available to ensure that
the die cavity is filled.

16. Hot Working Processes : Hot Rolling
 Usually is the first process
to convert material into a
finished wrought product.
 Recrystallization takes
place during process.

17.  Products of rolling process:
 A bloom - square or rectangular cross section, thickness more
than 6 inches and a width no more than twice the thickness.
 A billet is usually smaller than a bloom and has a square or
circular cross section.
 A slab is a rectangular solid where the width is more than twice
the thickness. Slabs can be further rolled to produce plate, sheet
and strip.

18. Hot Working Processes : Forward and
backward extrusion
 By extrusion, metal is compressed and forced to flow
through a shaped die.
 Hot extrusion reduces the forces required, eliminates cold-
work effect and reduces directional properties.
 Extrusion process is like squeezing toothpaste out of a
tube.
 A heated billet placed inside a confining chamber.
 A ram advances from one end, causing the billet to upset
and enter the confining chamber.
 As the ram continues to advance, pressure builds until the
material flows through the die.

19. Hot Working Processes : Forward and
backward extrusion
 In direct extrusion, a solid ram drives the entire billet
to and through a stationary die.
 In indirect extrusion a hollow drives the die back
through a stationary, confined billet.

20. Hot Working Processes : Advantages
& Disadvantages
Advantages Disadvantages
Low cost. Poor surface finish – rough and scaly.
Grain refinement from cast structure. Dimensional accuracy of a low order due to
shrinkage during cooling.
Materials are left in a fully annealed
condition and are suitable for cold working.
Due to distortion on cooling; leads to geometrical
inaccuracy.
Scale gives some protection against corrosion
during storage.
Poor finish when machined.
Availability as sections (girders) and forgings
as well as the more usual bars, rods, sheets
and strip and butt-welded tube.
Low strength and rigidity for metal considered.

21. Casting process

22. Casting Process
 Pouring molten metal into a mould patterned after the part
to be manufactured.
 The important factors in casting operation are :
 Flow of the molten metal into the mould cavity
 Heat transfer during solidification and cooling of metal
 Type of mould material.
 Solidification of the metal from its molten state.
 There are a few ways of casting prosess which are :
 Sand casting
 Investment casting
 Pressure die casting

23. Casting Process : Sand Casting
 Sand casting consists of:
 placing a pattern in sand to make an imprint,
 filling the resulting cavity with molten metal,
 allowing the metal to cool until it solidifies,
 breaking away the sand mould and removing the
casting.

25. Casting Process : Sand Casting
 Typical parts - machine-tool bases, engine blocks,
cylinder heads and pump housings
 Sand moulds are characterized by the types of sand
that comprise them :
 Green-sand - the sand in the mould is moist or damp
while the metal is being poured into it
 Cold-box mould - various organic and inorganic
binders are blended to bond the grains for greater
strength
 No-bake mould - a synthetic liquid resin is mixed with
the sand

26. Casting Process : Investment Casting
 Wax patterns produced in precision metal moulds and then assembled
to a ‘tree’.
 The wax assembly is then ‘invested’ with a mixture of powdered
sillimanite and ethylsilicate.
 By heating it form a strong silica bond between the particles.
 The heating process also melts out the wax pattern leaving the mould
cavity.
 Extremely complex shapes can be cast since the pattern is not
withdrawn.
 Extreme precision in dimensions is possible.
 Importance for making small components from very hard, strong
materials.
 Blades for gas-turbines and jet-engines can be cast by this process.
 High operational cost.

27. Casting Process : Investment Casting

28. Casting Process : Pressure Die Casting
 Molten metal is injected into the mould cavity under
pressure.
 Much more accurate impression of the mould cavity is
obtained.
 Pressure die casting is the more common process and
‘cycling’ is rapid.
 As soon as the casting is solid the die is parted.
 Detached by a system or ejector pins.

29. Casting
Process :
Pressure Die
Casting

30. Casting Process : Comparison
Item Advantages Limitations
Sand Casting Low cost. Poor surface finish
Accurate dimension Weak brittle
structure
Investment
Casting
Complex shape can
be cast.
High operational
cost.
Precession in
dimensioning
Difficult to build the
patterns
Pressure Die
Casting
High productivity High operational
cost.
Good surface finish Need high pressure.

31. Heat Treatment
Procecesses

32. Heat Treatment
 Heat treatment - controlled heating and cooling of
metals
 Change properties to improve performance or for
further processing.
 Consist of
 heating,
 soaking
 cooling
 Vary the heating, soaking & cooling of p.c.s, different
combinations of mechanical properties can be
obtained.

33. Heat Treatment : Annealing
 Rendering soft and ductile steel for further cold work
and machining.
 Three annealing processes :
 stress-relief annealing
 spheroidising
 Full-annealing
 Chose process depends on
 carbon content
 pretreatment processing
 subsequent process and use

34.  Fig :
Temperature
bands of
annealing
processes on
iron carbon
phase
equilibrium
diagram.
 Cooling rate
is as slow as
possible.

36. Annealing : Full Annealing
 P.c.steels solidify above the heat
treatment temperatures.
 Large castings, insulated by the
sand mould, take a very long time
to cool down.
 Large forgings processed at
temperatures above their upper
critical temperatures for long
period.

37. Annealing : Stress-Relief Annealing
 Steels below 0.4% carbon content.
 Such steels will not fully quench harden, but, they are
frequently cold worked and become work hardened.
 Grain structures are distorted during working,
recrystallisation can initiate at 5000C.
 In practice, annealing is done at 630 to 7000C to speed
up the process and limit grain growth.

38. Annealing : Speroidising Annealing
 Crystals of pearlite have a laminated structure
consisting of alternate layers of cementite and ferrite.
 If steels (>0.4 %C) are heated below the critical
temperature (650-700 °C), the cementite tends to 'ball
up'.
 Aspheroidisation of pearlitic cementite takes place.
 No phase changes happen at sub critical temp.
 Spheroidisation of the cementite is a surface tension
effect.

39. Normalising
 Resembles full annealing but cooling rate is accelerated.
 Work is taken out from the furnace for cooling in free air.
 Process temperature is the same as full annealing.
 After 'soaking' the steel at the process temperature, more
rapid cooling is performed.
 Grain transform from fine grain austenite into fine grain
ferrite + pearlite.
 Rapid cooling avoids grain growth associated with
annealing.

40. Normalising
 Frequently used for
stress relieve between
rough machining and
the finish machining of
large castings to avoid
'movement' due to slow
release of internal
stresses and loss of
accuracy.

41. Quenching
 Heat steel to its hardening temperature it becomes
austenitic.
 Cooled it quickly, not enough time for transforming
austenite into pearlite and ferrite or pearlite and
cementite.
• Instead, a structure called
martensite is formed.
• It is the hardest structure to
produce in a P.C.S
• Under microscope, it
appears as a needle-shaped
(acicular) crystal.

43. Quenching : Quenching Media
 In order of severity:
 Compressed air blast - high-speed steel tools and
components of small section
 Oil - high-carbon steels (1.2-1.4 %) and alloy steels
 Water – P.C.S & alloy steels
 Brine (10 per cent solution) – max hardness
 Care must be taken to ensure that distortion is kept to a
minimum.
 Selection of media depends on the type of steel and the
required properties.

45. Tempering Process
 Quenched P.C.S is hard but brittle and hardening stresses
are present - Little practical use.
 Reheating, or tempering, to relieve the stresses and
reduce the brittleness.
 Tempering transforms martensite into less brittle
structures.
 Increase in toughness is accompanied by decrease in
hardness, unfortunately.
 Transform unstable martensite into more stable pearlite.
 Tempering temperatures
 Below 200oC only relieve the hardening stresses.
 Above 220oC, martensite transforms into troostite.
 Tempering above 400oC giving a structure called sorbite.
 Troostite (for most cutting tools) is much tougher but less
hard.
 Sorbite is tougher and more ductile then troostite; used in
shock loadcomponents e.g springs.

46. Case Hardening
 Often components require a hard case to resist wear &
tough core to resist shock loads.
 Hardness & toughness do not exist in a single steel since,
 Core should not exceed 0.3-0.4% carbon for toughness, while
 Surface should have 1.0%C for adequate hardness.
 Case hardening is the answer.
 Carburising - Add carbon to the surface layers of L.C.S to a
depth.
 Heat-treatment - Harden the case and refine the core.
 Case harden by
 Carburising
 Nitrogen hardening (nitridization)

47. Case Hardening : Carburising
 L.C.S (approximately 0.1 per cent carbon) absorb
carbon when heated to austenitic condition.
 Various carbon containing materials used:
 Solid media (pack carburising) - bone charcoal or
charred leather, together with sodium and/or barium
carbonate as energiser.
 Gaseous media (Gas carburising) – ‘Natural' gas
(methane C2H6).

48. Case Hardening : Nitrogen Hardening
 Hard, wear-resistant coating on component made
from special alloy steel, e.g drill bushes.
 Absorption of nitrogen gas into the surface of the
metal to form very hard nitrides.
 Components are heated in ammonia gas at 500~600o C
in 40 hours.
 Ammonia gas breaks down and the atomic nitrogen is
absorbed into the surface of the steel.

49. Case Hardening Process
Case Hardening
process
Advantages Disadvantages
Solid media Lowest processing cost 6-8 hours to heat
the crucible.
Gaseous media Widely available
because the using of
natural gas (methane).
Need more
advance process
plant
Nitrogen
Hardening
Corrosion resistant of
steel is improved.
No subsequence
grinding or
finishing process
possible.