DT1410 - Materials and Processes in Design UNIT 4 PROCESSING OF
METALS JEOPARDYLABS.COM/PLAY/UNIT-4663
Slide 2
Hot Working of Metals Hot Working: Conventional wisdom says
that hot working means that the metal is deformed or worked at an
elevated temperature however, technically hot working means that:
The metal is worked at a high enough temperature that no plastic
deformation, strain hardening, or cold working takes place. Cold
Working discussed in Chapter 10
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Hot Working of Metals Hot Working Formal Definition: A process
of forming metals while they are heated above the recrystallization
or transformation temperatures.
Slide 4
Hot Working of Metals Hot Working: One way to check for hot
working is to measure the hardness or strength of the metal before
and after the working process; if no strengthening occurs, it is
hot working Hot working occurs such that before the metal cools,
the temperature is high enough that is anneals the metal and
removes the cold work that otherwise would have occurred
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Hot Working of Metals Hot Working Processes: Hot Rolling
Forging Swaging Extrusion Drawing Spinning Piercing Welded
pipe
Slide 6
Hot Working of Metals Purpose of Hot Working During the ingot
casting process (molten steel being formed into useful shapes)
steel cools from its surfaces toward the center This forms large,
columnar, dendritic grains That is, the grains grow parallel to the
direction of heat flow These large grains are usually lower in
strength and higher in elongation than is desired Shrinkage voids
also are formed which significantly reduce strength
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Hot Working/Heat Treating
http://www.engineeringmotion.com/videos/306/heat-treating
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Hot Working of Metals Purpose of Hot Working These undesirable
grains and internal shrinkage voids can be removed or altered by
the processes of Hot Working: Hot Rolling Drawing Extruding
Forging
Slide 9
Rolling Mill Intermediate Forms Ingots are sufficiently plastic
to be taken to a hot rolling mill to be shaped into one of the
intermediate forms called: Blooms 6 x 6 square semi-finished steels
Slabs 10 wide and 1.5 or more thick Billets 1.5 by 1.5 squares
Slide 10
Rolling Mill Intermediate Forms Blooms (and often ingots) are
further processed in structural rolling mills into other structural
shapes: Railroad rails I-Beams Angles Wide-flange Beams Zees
Tees
Slide 11
Steel Rolling
http://www.engineeringmotion.com/videos/397/steel-manufacturing-
rolling
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Recrystallization Steel ingot, with its typical grain
imperfections and voids, is unsatisfactory for applications where
high strength is required. Parts made from such steel would easily
fail under impact loads The columnar grains in a cast ingot must go
through recrystallization to give steel the required strength.
Slide 13
Recrystallization A process in which the distorted grain
structure of metals that are subjected to mechanical deformation is
replaced by a new strain-free grain structure during
annealing.
Slide 14
Anisotropy A condition created by the rolling process that
gives a material specific physical properties in different
directions. Rolled steel is strongest in the direction of
rolling.
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Forging
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Forging Processes Heating metals to forging temperatures
greatly increases their plasticity and workability. Because of
this, hot forging was and still is a very useful method of forming
steel articles, both large and small.
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Forging Processes Plasticity: The quality of material such that
it can be deformed without breaking. Clay is a completely plastic
material. Metals exhibit plasticity in varying amounts.
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Forging Processes Forging: A method of metalworking in which
the metal is hammered into the desired shape, or is forced into a
mold by pressure or hammering, usually after being heated to a
plastic state. Hot forging requires less force to form a given part
than does cold forging, which is usually done at room
temperature.
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Forging Machines Open-die Impression-die drop Press Swaging
Upset See pages 178 188 in Bruce TB
Slide 20
Open Die Forging
http://www.engineeringmotion.com/videos/60/forging-open-die
Slide 21
Hot Extrusion Extrusion: A process of forcing metal through a
die, similar to squeezing toothpaste out of a tube. Usually under
high pressure and at elevated temperatures, forcing materials
through a die containing the shape desired so that a shaped product
is produced. Metal and polymer shapes are produced using this
method.
Slide 22
Hot Extrusion Extrusion: Stronger non-ferrous metals (aluminum,
copper, brass, magnesium) and ferrous metals usually need to be at
a forging temperature to make them plastic enough for extrusion.
Square and round tubular products, structural shapes (tees, zees,
angles) and round, square, or hexagonal solid shapes are common
extruded pieces
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Hot Extrusion Advantages of Extrusion: Allows for the
manufacture of intricate shapes Example: Inexpensive bronze gears
can be extruded into 20 ft or 40 ft lengths and later cut off to
the desired widths. For short runs of product, extrusion is more
economical due to lower tooling costs
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Hot Extrusion Two basic types of Extrusion: Direct Extrusion
Metal placed in a container A Ram is pushed The work billet is
forced to flow through one or more openings in a die.
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Hot Extrusion Two basic types of Extrusion: Indirect Extrusion
A die is mounted into a ram As the ram is pushed against the work
billet material flows through the die opening and through the
ram
Unit 4 Manufacturing Processes --Metals CHAPTER 10 COLD WORKING
OF METALS
Slide 28
Cold Working of Metals Historically, cold working was
accomplished by hammering on soft metals (gold, silver, copper,
etc) for jewelry and other ornamentation. Today, cold-formed
products range from very fine hypodermic needles to huge pipeline
tubes and from tiny hair- sized filaments to propeller shafts for
ships. Almost any conceivable shape can be made from cold-forming
processes.
Slide 29
Cold Working of Metals Cold Working: Deforming a metal
plastically at a temperature below its lowest recrystallization
temperature. Cold working usually occurs at room temperature Strain
hardening occurs as a result of this permanent deformation.
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Cold Working of Metals Common methods of Cold Working: Cold
rolling Blanking Pressing Drawing Forming Extruding Bending
Straightening Roll forming Spinning
Slide 31
Cold Working of Metals Advantages of Cold Working: Better
surface finish Closer dimensional tolerances Better machinability
Superior mechanical properties Better strength-to-weight ratios
Enhanced directional properties
Slide 32
Cold Working of Metals Disadvantages of Cold Working: Metal is
more brittle Metal is less workable Annealing is required to
continue the process The metal may contain residual stresses that
can cause warping or distortion Subsequent heating (welding) will
change the cold working structure and reduce its strength
Slide 33
Factors in Cold Working Properties that enable metals to be
Cold Worked 1. Plasticity: The ability of metals to be deformed
permanently in any direction without cracking or splitting. The
higher temperatures used in hot working make metals more plastic,
but recrystallization at high temperatures prevents them from
increasing in strength. To achieve higher strengths and hardnesses,
the metal must be deformed at normal temperatures (below the
recrystallization temperature).
Slide 34
Factors in Cold Working When metals are cold worked to a
certain point, the next operation requires forces greater than
those previously applied to deform the metal further. Each
operation brings the particular metal closer to its ultimate
strength and point of rupture. As the amount of cold work increases
the yield strength it gets closer to the maximum stress or tensile
strength, and the strain at failure decreases. The objective in
cold working, therefore, is to stop well short of failure.
Slide 35
Factors in Cold Working The degree of deformation (amount of
cold working) determines the level of toughness, strength,
hardness, and remaining ductility. If more deformation is needed,
then a process anneal is used to restore plasticity. Process anneal
on the cold-worked steel is often carried out in a closed container
of inert gas to avoid scaling problems a technique called a bright
anneal.
Slide 36
Factors in Cold Working Properties that enable metals to be
Cold Worked 2. Ductility and Malleability: Ductility the property
of a metal that allows it to deform permanently or to exhibit
plasticity without rupture while under tension. Steel, aluminum,
gold, silver, nickel Malleability the ability of a metal to deform
permanently when loaded in compression. Metals that can be rolled
into sheets or upset cold forged are malleable. Most ductile metals
are malleable, but some malleable metals are not very ductile
(lead)
Slide 37
Factors in Cold Working Properties that enable metals to be
Cold Worked 2. Ductility and Malleability: Some metals and alloys
such as stainless steel, high-manganese steel, titanium, and
zirconium tend to work harden, that is, to quickly increase
hardness as cold working progresses. Work hardening is often a
troublesome difficulty in machining operations.
Slide 38
Factors in Cold Working Properties that enable metals to be
Cold Worked 3. Elastic Behavior: When a metal is placed under
stress within its elastic range it will return to its former shape
when the load is removed. If the metal takes on a permanent set by
loading it beyond the elastic limit, it will be permanently
deformed, but will bounce back to some extent because of its
elastic properties. This is known as springback or elastic
recovery.
Slide 39
Factors in Cold Working Properties that enable metals to be
Cold Worked Elastic Behavior: The tendency for a metal part to
return (or relax) somewhat from the form to which it has been bent
or deformed when the forces causing the change in the form are
removed.
Slide 40
Factors in Cold Working Elastic Properties of metals can also
cause problems: The vast majority of metals are polycrystalline
they consist of many crystals or grains that were initially formed
when the metal solidified. Cold working tends to stretch out the
grain in the direction of working, yet the grains still may have
atom lattices that are not aligned. Residual stresses may be left
behind after cold working.
Slide 41
Factors in Cold Working Elastic Properties of metals can also
cause problems: Residual stresses: Stresses induced within the
structure of a metal by cold working, machining, and heat
treatments and remaining in the metal after the treatment is
completed. These stresses are alive and active within the metal.
They can cause a correctly bored hole to be oblong or a straight
piece of bar to banana when machined on one side.
Slide 42
Factors in Cold Working Residual stresses: The solution to
residual stresses are to use a stress relief or recovery anneal
Stress Relief Anneal: The reduction of residual stress in a metal
part by heating it to a given temperature and holding it there for
a suitable length of time. This treatment is used to relieve
elastic stresses caused by welding, cold working, machining,
casting, and quenching.
Slide 43
Cold Rolling in the Steel Mill Hot-rolled steel bars and plates
must be sufficiently over-size because the cold-finishing process
reduces them in cross section. The hot-rolled bars or sheets are
placed in a hydrochloric or sulfuric acid dip that removes the
scale a process called pickling. Cold rolling then flattens and
lengthens the grains in the direction of rolling.
Slide 44
Blanking and Pressing One of the most versatile forms of metal
working is that of converting flat sheet and strip metals into
useful articles. Sheet metal can be: Pierced Punched Blanked
Slide 45
Blanking and Pressing Piercing: Cutting (usually small) holes
in sheet metal. Punching: The operation of cutting a hole in sheet
metal using a die. The hole material is scrapped. Blanking: The
operation of cutting a shape with a die from sheet metal stock. The
hole material is saved and used for further operations.
Slide 46
Drawing, Forming, Extruding Metal Stamping Operations: Coining
and Embossing are stamping operations that form the surface of
metal. Impressions of letters, figures, and patterns are formed by
pressing them onto the metal
Slide 47
Drawing, Forming, Extruding Metal Stamping Operations: Coining:
Shaping a piece of metal in a mold or die, often creating raised
figures or numbers. Embossing: The raising of a pattern in relief
on a metal by means of a high pressure on a die plate.
Slide 48
Progressive Metal Stamping
http://www.engineeringmotion.com/videos/53/progressive-metal-
stamping
Slide 49
Drawing, Forming, Extruding Metal Bar, Tube, and Wire Drawing:
Bar Drawing: Figure 10.25 (p. 205) Performed on a draw bench. The
drawing process hardens the metal and gives it a smooth
finish.
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Drawing, Forming, Extruding Metal Bar, Tube, and Wire Drawing:
Seamless Tubing: Figure 10.26 (p. 205) Also cold drawn on a draw
bench This process includes a mandrel inside the tube to thin the
walls and provide internal finish.
Slide 51
Drawing, Forming, Extruding Metal Bar, Tube, and Wire Drawing:
Wire Drawing: Similar process to bar drawing, but involves much
smaller diameters of metal Is a continuous process done on rotating
equipment