Transcript DT1410 - Materials and Processes in Design

DT1410 - Materials and
Processes in Design
UNIT 4 – PROCESSING OF METALS
JEOPARDYLABS.COM/PLAY/UNIT-4663
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
Hot Working of Metals

Hot Working – Formal
Definition:
A
process of forming
metals while they are
heated above the
recrystallization or
transformation
temperatures.
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
Hot Working of Metals

Hot Working Processes:

Hot Rolling
– Forging

Swaging
– Extrusion

Drawing
– Spinning

Piercing
– Welded pipe
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
Hot Working/Heat Treating
http://www.engineeringmotion.com/videos/306/heat-treating
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
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


Slabs


6” x 6” square semi-finished steels
10” wide and 1.5” or more thick
Billets

1.5” by 1.5” squares
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
Steel Rolling
http://www.engineeringmotion.com/videos/397/steel-manufacturingrolling
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.

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.
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.
Forging
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.
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.
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.
Forging Machines

Open-die

Impression-die drop

Press

Swaging

Upset

See pages 178 – 188 in Bruce TB
Open Die Forging
http://www.engineeringmotion.com/videos/60/forging-open-die
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.
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
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
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.
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
Extrusion
http://www.engineeringmotion.com/videos/415/metal-extrusionmanufacturing
Unit 4 – Manufacturing
Processes --Metals
CHAPTER 10
COLD WORKING OF METALS
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 hairsized filaments to propeller shafts for
ships.

Almost any conceivable shape can
be made from cold-forming
processes.
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.
Cold Working of Metals

Common methods of
Cold Working:

Cold rolling
– Blanking

Pressing
– Drawing

Forming
– Extruding

Bending
– Straightening

Roll forming
– Spinning
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
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
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).
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.
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.
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)
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.
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.
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.
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.
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.
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.
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.
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
Blanking and Pressing

Piercing:


Punching:


Cutting (usually small) holes in sheet
metal.
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.
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
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.
Progressive Metal Stamping
http://www.engineeringmotion.com/videos/53/progressive-metalstamping
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.

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.
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

