Ferrous materials have made a major contribution to the development of modern technology; they span a tremendous range of properties and applications. Reflecting the industrial practices, the information provided here offers easy access to reliable processes involved in the manufacturing of Steel products like Steel Bars, Wires, Tubes, Pipes, Sheets etc that proves to be the backbone of construction and automobile industries booming worldwide.
The work closes the gap in the treatment of steel and cast iron. Each chapter takes into account the gradual transitions between the two types of ferrous materials. It demonstrates that ferrous metal and steel are versatile and customizable materials which will continue to play a key role in the future and also covers the operations performed on ferrous metals for converting them into a commodity.
The book provides a full characterization of steel, including structure, chemical composition, classifications, physical properties, production practices of different steel products, processing of ferrous metals and so on. It will prove to be a layman’s guide for the entrepreneurs who are willing to invest in the ventures related to Iron and Steel Industries, as it contains information related to processing of ferrous metals and production practices followed in Steel products manufacturing units. The text discusses the importance and objectives of processes and material used for the production of disposable products. Many examples have been provided to illustrate the concepts discussed.
The topics covered in the book are: Casting of Ferrous Metals, Heat Treatment of Ferrous Metals, Stamping Process of Ferrous Metals, Forming Process of Ferrous Metals, Machining Process of Ferrous Metals, Joining Process of Ferrous Metals, Production of Stainless Steel Wire, Production and Fabrication of Steel Bars, Steel Tube & Pipe, Stainless Steel Sheet and Different Grades of Stainless Steel.
CASTING
OF FERROUS METALS
Casting Methods
Metal casting process
begins by creating a mold, which is the ‘reverse’ shape of the part we
need.
The mold is made from a refractory material, for example, sand. The
metal is
heated in an oven until it melts, and the molten metal is poured into
the mould
cavity. The liquid takes the shape of cavity, which is the shape of the
part.
It is cooled until it solidifies.
Sand Casting
Sand casting uses
natural or synthetic sand (lake sand) which is mostly refractory
material
called silica (SiO2). The sand grains must be small enough so that it
can be
packed densely; however, the grains must be large enough to allow
gasses formed
during the metal poured to escape through the pores. Larger sized molds
use
green sand (mixture of sand, clay and some water).
Expendable-Pattern Casting (Lost foam
Process)
The pattern used in
this process is made from polystyrene (this is the light, white
packaging
material which is used to pack electronics inside the boxes).
Polystyrene foam
is 95% air bubbles, and the material itself evaporates when the liquid
metal is
poured on it.
The pattern itself is
made by molding - the polystyrene beads and pentane are put inside an
aluminum
mold, and heated; it expands to fill the mold, and takes the shape of
the
cavity.
Plaster-Mold Casting
The mold is made by
mixing plaster of paris (CaSO4) with talc and silica flour; this is a
fine
white powder, which, when mixed with water gets a clay-like consistency
and can
be shaped around the pattern (it is the same material used to make
casts for
people if they fracture a bone). The plaster cast can be finished to
yield very
good surface finish and dimensional accuracy.
One advantage of vacuum
casting is that by releasing the pressure a short time after the mold
is
filled, we can release the un-solidified metal back into the flask.
This allows
us to create hollow castings. Since most of the heat is conducted away
from the
surface between the mold and the metal, therefore the portion of the
metal
closest to the mold surface always solidifies first; the solid front
travels
inwards into the cavity. Thus, if the liquid is drained a very short
time after
the filling, then we get a very thin walled hollow object, etc.
Die Casting
Die casting is a very
commonly used type of permanent mold casting process. It is used for
producing
many components of home appliances (e.g rice cookers, stoves, fans,
washing and
drying machines, fridges), motors, toys and hand-tools - since Pearl
river
delta is a largest manufacturer of such products in the world, this
technology
is used by many HK.-based companies. Surface finish and tolerance of
die cast
parts is so good that there is almost no post-processing required. Die
casting
molds are expensive, and require significant lead time to fabricate;
they are
commonly called dies.
Casting Design and Quality
Several factors affect
the quality/performance of cast parts - therefore the design of parts
that must
be produced by casting, as well as the design of casting molds and
dies, must
account for these. You may think of these as design guidelines, and
their
scientific basis lies in the analysis - the strength and behaviour of
materials.
Shrinkage
As the casting cools,
the metal shrinks. For common cast metals, a 1% shrinkage allowance is
designed
in all linear dimensions (namely, the design is scaled p by approx 1%).
Since
the solidification front, i.e. the surface at the boundary of the
solidified
and the liquid metals, travels from the surface of the mold to the
interior
regions of the part, the design must ensure that shrinkage does not
cause
cavities.
HEAT
TREATMENT OF FERROUS METALS
Successful heat
treatment requires close control over all factors affecting the heating
and
cooling of a metal. This control is possible only when the proper
equipment is
available. The furnace must be of the proper size and type and
controlled, so
the temperatures are kept within the prescribed limits for each
operation. Even
the furnace atmosphere affects the condition of the metal being
heat-treated.
COOLING
STAGE
After a metal has been
soaked, it must be returned to room temperature to complete the
heat-treating
process. To cool the metal, you can place it in direct contact with a
COOLING
MEDIUM composed of a gas, liquid, solid, or combination of these. The
rate at
which the metal is cooled depends on the metal and the properties
desired.
The success of a
heat-treating operation depends largely on your judgement and the
accuracy with
which you identify each color with its corresponding temperature. From
a study
of table 2-1, you can see that close observation is necessary. You must
be able
to tell the difference between faint red and blood red and between dark
cherry
and medium cherry. To add to the difficulty, your conception of medium
cherry
may differ from that of the person who prepared the table. For an
actual
heat-treating operation, you should get a chart showing the actual
colors of
steel at various temperatures.
ANNEALING
In general, annealing
is the opposite of hardening, You anneal metals to relieve internal
stresses,
soften them, make them more ductile, and refine their grain structures.
Annealing consists of heating a metal to a specific temperature,
holding it at
that temperature for a set length of time, and then cooling the metal
to room
temperature.
NORMALIZING
The purpose of
normalizing is to remove the internal stresses induced by heat
treating,
welding, casting, forging, forming, or machining. Stress, if not
controlled,
leads to metal failure; therefore, before hardening steel, you should
normalize
it first to ensure the maximum desired results. Usually, low-carbon
steels do
not require normalizing; however, if these steels are normalized, no
harmful
effects result.
Thin pieces cool faster
and are harder after normalizing than thick ones. In annealing (furnace
cooling), the hardness of the two are about the same.
HARDENING
The hardening treatment
for most steels consists of heating the steel to a set temperature and
then
cooling it rapidly by plunging it into oil, water, or brine. Most
steels require
rapid cooling (quenching) for hardening but a few can be air-cooled
with the
same results. Hardening increases the hardness and strength of the
steel, but
makes it less ductile. Generally, the harder the steel, the more
brittle it
becomes.
In plain carbon steel,
the maximum hardness obtained by heat treatment depends almost entirely
on the
carbon content of the steel. As the carbon content increases, the
hardening
ability of the steel increases; however, this capability of hardening
with an
increase in carbon content continues only to a certain point. In
practice, 0.80
percent carbon is required for maximum hardness.
Case Hardening
Case hardening produces
a hard, wear-resistant surface or case over a strong, tough core. The
principal
forms of casehardening are carburizing, cyaniding, and nitriding. Only
ferrous
metals are case-hardened.
Case hardening is ideal
for parts that require a wear-resistant surface and must be tough
enough
internally to withstand heavy loading. The steels best suited for case
hardening
are the low-carbon and low-alloy series. When high-carbon steels are
case-hardened, the hardness penetrates the core and causes brittleness.
In case
hardening, you change the surface of the metal chemically by
introducing a high
carbide or nitride content. The core remains chemically unaffected.
When
heat-treated, the high-carbon surface responds to hardening, and the
core
toughens.
Flame Hardening
Flame hardening is
another procedure that is used to harden the surface of metal parts.
When you
use an oxyacetylene flame, a thin layer at the surface of the part is
rapidly
heated to its critical temperature and then immediately quenched by a
combination of a water spray and the cold base metal. This process
produces a
thin, hardened surface, and at the same time, the internal parts retain
their
original properties. Whether the process is manual or mechanical, a
close watch
must be maintained, since the torches heat the metal rapidly and the
temperatures are usually determined visually.
Flame hardening may be
either manual or automatic. Automatic equipment produces uniform
results and is
more desirable. Most automatic machines have variable travel speeds and
can be
adapted to parts of various sizes and shapes. The size and shape of the
torch
depends on the part.
When the cutting end
has cooled, remove the chisel from the bath and quickly polish the
cutting end
with a buff stick (emery). Watch the polished surface, as the heat from
the
opposite end feeds back into the quenched end. As the temperature of
the
hardened end increases, oxide colors appear. These oxide colors
progress from
pale yellow, to a straw color, and end in blue colors. As soon as the
correct
shade of blue appears, quench the entire chisel to prevent further
softening of
the cutting edge.
Caustic Soda
A solution of water and
caustic soda, containing 10 percent caustic soda by weight, has a
higher
cooling rate than water. Caustic soda is used only for those types of
steel
that require extremely rapid cooling and is NEVER used as a quench for
nonferrous metals.
DRY
QUENCHING
This type of quenching
uses materials other than liquids. Inmost cases, this method is used
only to
slow the rate of cooling to prevent warping or cracking.
Air
Air quenching is used
for cooling some highly alloyed steels. When you use still air, each
tool or
part should be placed on a suitable rack so the air can reach all
sections of
the piece. Parts cooled with circulated air are placed in the same
manner and
arranged for uniform cooling. Compressed air is used to concentrate the
cooling
on specific areas of a part. The airlines must be free of moisture to
prevent
cracking of the metal.
STAMPING
PROCESS OF FERROUS METALS
Compound Die
A compound die blanks
and perforates a part at the same time in the same station. In most
cases this
operation perforates a hole or holes down, while the part blanks up.
This
allows slugs from those holes to fall through the die. This method
leaves the
part in the die, requiring some means of part removal.
Compound dies commonly
run as single-hit dies. They can run continuously with a feeder,
provided you
can remove the part in a timely manner. Open Back Inclinable (OBI)
presses - in
the inclined position along with an air blow-off - aid in part removal.
A disadvantage of a
compound blank die is its limited space that tends to leave die
components thin
and weak. This concentrates the load and shock on punches and matrixes,
resulting in tooling failures.
Progressive Die
Progressive dies
provide an effective way to convert raw coil stock into a finished
product with
minimal handling. As material feeds from station to station in the die,
it
progressively works into a completed part.
Progressive dies
usually run from right to left. The part material feeds one progression
for
each press cycle. Early stations typically perforate holes that serve
as pilots
to locate the stock strip in later stations.
There are many
variations of progressive die designs. The design shown here
illustrates some
common operations and terminology associated with progressive dies.
Fixed strippers have
several drawbacks. They do not hold the stock strip flat and are unable
to
absorb impact and snap-thru shock. The result is poor part flatness and
premature punch failure.
We generally do not
recommend fixed strippers for high-volume or high-precision jobs. A
typical
clearance under the stripper is 1½ times the material thickness - 1/16"
to
1/8" is common clearance on the sides of the stock strip.
Clearance under a fixed
stripper is commonly 1½ times the part material. This allows for
variations in
part material thickness and for stock strip deformation.
This deformation
allowance under the punch point results in punch point chipping. That
deformation can also cause lateral movement of both part and punches,
resulting
in punch point breakage and poor part quality.
At snap-thru there is a
sudden unloading of pressure on the punches and part material. This
generates
shock, which can lead to punch head breakage.
Note the buckling of
the part material throughout the press cycle, as seen in. This can lead
to
dimensional and functional problems in the finished part.
The buckling effect
binds the part on the ends of the punches, which increases stripping
pressure
and potentially chips the punch face.
Urethane Stripper
Urethane strippers are
inexpensive and simple to use. They slide over the end of a punch with
a slight
press fit, which prevents the stripper from falling into the die during
operation.
Through use, urethane
strippers fatigue and become loose on the punches. You must continually
monitor
them to prevent them from falling into and damaging the die. Some
urethane
strippers are molded with a head designed to fit a standard urethane
retainer.
This greatly enhances urethane stripper life and reliability.
Deformation and
movement of the urethane strippers can move the stock strip or part
laterally,
creating punch-to-matrix alignment problems.
A urethane stripper
strips the part off the ends of the punches as it returns to its
original
shape. Due to the urethane’s pliable nature, the part material may
distort
during the perforating and stripping process.
Some urethane strippers
have a steel washer attached to the end to minimize part distortion.
Exercise
caution when using this type of urethane stripper on shaped punches or
applications where large amounts of pre-load are required. Catastrophic
punch
failure can occur if the punch face catches the steel face prior to
hitting the
part material.
The optimum urethane
stripper should have a combination of two different grades of urethane:
a high
hardness grade of urethane for the face and a medium hardness grade for
the
body. This helps maintain part flatness without sacrificing durability
and
elasticity.
Spring Stripper
Spring strippers offer
superior performance.
Their main advantage is
that as the die closes, they hold the stock strip or part flat and in
place
during perforating. A spring stripper prevents the part material from
lifting
or hanging up on the punches at withdrawal.
Continuous pressure
throughout the working portion of each press cycle provides superior
performance in tool reliability, part quality and press life.
Over-entry or closing a
die below its recommended shut height can have catastrophic
consequences.
To calculate tonnage
requirements for perforating, multiply the part material thickness
times the
length of the cut, or perimeter of the hole, times the material shear
strength.
Determine the perimeter of a round hole by multiplying pi times the
hole
diameter.
It is important to
include the stripper pressure when calculating die tonnage
requirements.
Stripper pressure should be at least 8% of the perforating force. Some
die
manufacturers require stripper pressure as high as 25% of the
perforating
pressure.
Punch Stagger
Stagger punch lengths
to minimize impact and snap-thru shock. You can split punch lengths
into two or
three groups, reducing impact and snap-thru shock by half or third.
Common practice is to
stagger the different groups of punches by an amount equaling stock
thickness.
Although this reduces the initial shock, it does not reduce the total
shock.
Each punch, or group of punches, is exposed to both impact and
snap-thru shock.
Making stagger equal to
or slightly less than burnish length in the hole being perforated
greatly
reduces impact and snap-thru shock. This amount of stagger allows the
next
group of punches to contact the material before the first group snaps
through.
The snap-thru energy from the first group of punches is absorbed and
used to
drive the next group of punches through the part material.
Using burnish length
instead of material thickness as the amount of stagger is extremely
important
in high-speed stamping applications. It reduces punch entry to minimize
punch
wear and slug pulling. Because the punches withdraw from the stock
strip
sooner, you also gain more feed time.
Piercing
Piercing makes a hole
without removing a slug. A sharp or pointed punch tears open a hole,
leaving a
ragged edge that has been formed down.
A food grater is a good
example of what pierced holes look like in a finished product.
Perforate and Shave
Shaving achieves a high
percentage of burnish or shear in a hole. Shaving occurs in a
two-station
operation.
The first station
resembles most perforating operations using optimum engineered die
clearance.
This optimizes tool life while minimizing work hardening of the part
material.
The second station cuts
the hole to size using tight die clearance.
Determining punch and
matrix sizes starts in the shave station. The shave punch point size
equals the
desired finished hole size. The shave station matrix hole has 1% to 1½%
of the
material thickness clearance per side (2% to 3% of the material
thickness total
clearance). Too much clearance in a shave station results in a shear
and
rebreaking of the hole.
Once you know the shave
station component dimensions, you can determine the perforating station
component sizes. The perforating matrix equals or is slightly larger
than the
shave station matrix size. Perforating clearance is as much as possible
without
generating an excessive burr. This clearance is achieved by reducing
the punch
point size.
Piloting
Pilots locate the stock
strip or part. The pilot working length extends beyond the perforating
punches
and a fully extended stripper.
The pilot nose picks up
an existing hole and moves the stock strip or part into proper location
before
the stripper makes contact.
Pilot point diameters
are commonly dimensioned .001" smaller than the punch point diameter
used
to perforate the locating hole. This prevents the stock strip or part
from
sticking.
Proper die clearance
for pilots is subject to debate. Many designers maintain a very tight
clearance
of .0005" or less, incorporating the matrix as a guide below the part
material. This offers additional lateral support that results in better
part
location when forming or working with thick material.
The drawback with tight
clearance around a pilot is when a misfeed causes a pilot to perforate
a hole.
The extreme stripping force created by the tight clearance galls the
pilot,
possibly pulling it from the retainer. Ball lock pilots are
particularly
vulnerable to pulling due to misfeeds.
Another practice
employed by designers is to use material thickness as the clearance per
side
around pilots. The intent is to allow enough room around the pilot for
the part
material to extrude down into the matrix without grabbing the pilot.
The
problem is that when the material pierces and extrudes down, it tends
to spring
back resulting in excessive stripping force.
Coining
Coining leaves an
impression in the part surface. You can apply this process to one or
both sides
of the part. In many cases coining is used to thin or displace
material. No
slug is removed in coining operations.
Embossing
Embossing deforms a
shape within the part, but without intentional thinning of the part
material.
A punch is used to form
material into a blind hole. The punch bottoms out to produce a flat
surface at
the bottom of the form.
FORMING
PROCESS OF FERROUS METALS
Rolling
In metalworking,
rolling is a metal forming process in which metal stock is passed
through a
pair of rolls. Rolling is classified according to the temperature of
the metal
rolled. If the temperature of the metal is above its recrystallization
temperature, then the process is termed as hot rolling. If the
temperature of
the metal is below its recrystallization temperature, the process is
termed as
cold rolling. In terms of usage, hot rolling processes more tonnage
than any
other manufacturing process and cold rolling processes the most tonnage
out of
all cold working processes.
Hot and Cold
Rolling
In smaller operations
the material starts at room temperature and must be heated. This is
done in a
gas- or oil-fired soaking pit for larger workpieces and for smaller
workpieces
induction heating is used. As the material is worked the temperature
must be
monitored to make sure it remains above the recrystallization
temperature. To
maintain a safety factor a finishing temperature is defined above the
recrystallization temperature: this is usually 50 to 100°C (122 to
212°F) above
the recrystallization temperature.
If the temperature does
drop below this temperature the material must be re-heated before more
hot
rolling.
Hot rolled metals
generally have little directionality in their mechanical properties and
deformation induced residual stresses. However, in certain instances
non-metallic inclusions will impart some directionality and workpieces
less
than 20 mm (0.79 in) thick often have some directional properties.
Also,
non-uniformed cooling will induce a lot of residual stresses, which
usually
occurs in shapes that have a non-uniform cross-section, such as l-beams
and
H-beams. While the finished product is of good quality, the surface is
covered
in mill scale, which is an oxide that forms at high-temperatures. It is
usually
removed via pickling or the smooth clean surface process, which reveals
a
smooth surface. Dimensional tolerances are usually 2 to 5% of the
overall
dimension.
Flat Rolling
Flat rolling is the
most basic form of rolling with the starting and ending material having
a
rectangular cross-section. The material is fed in between two rollers,
called
working rolls, that rotate in opposite directions. The gap between the
two
rolls is less than the thickness of the starting material, which causes
it to
deform. The decrease in material thickness causes the material to
elongate. The
friction at the interface between the material and the rolls causes the
material to be pushed through.
Ring Rolling
Ring rolling is a
specialized type of hot rolling that increases the diameter of a ring.
The
starting material is a thick-walled ring. This workpiece is placed on
an idler
roll, while another roll, called the driven roll, presses the ring from
the
outside. As the rolling occurs the wall thickness decreases as the
diameter
increases. The rolls may be shaped to form various cross-sectional
shapes. The
resulting grain structure is circumferential, which gives better
mechanical
properties. Diameters can be as large as 8 m (26 ft) and face heights
as tall
as 2 m (79 in). Common applications include rockets, turbines,
airplanes,
pipes, and pressure vessels. Structural shape rolling Cross-sections of
continuously rolled structural shapes, showing the change induced by
each
rolling mill.
Shape
The term shape is used
to describe the flatness and the profile of the workpiece. The profile
consists
of the how the thickness of the workpiece varies across the width of
the
workpiece and can be measured in units of length. The flatness of the
workpiece
is based on how the fiber elongation varies across the width of the
workpiece
and it typically measured in I-Units.
Another way to overcome
defection issues is by decreasing the load on the rolls, which can be
done by
applying an longitudinal force: this is essentially drawing. Other
method of
decreasing roll defection include increasing the elastic modulus of the
roll
material and adding back-up supports to the rolls.
Equipment
A horizontal hydraulic
press for hot aluminum extrusion (loose dies and scrap visible in
foreground).
Indirect Extrusion
In indirect extrusion, also
known as backwards extrusion, the billet and container move together
while the
die is stationary. The die is held in place by a “stem” which has to be
longer
than the container length. The maximum length of the extrusion is
ultimately
dictated by the column strength of the stem. Because the billet moves
with the
container the frictional forces are eliminated.
Drives
Accumulator water
drives are more expensive and larger than direct-drive oil presses, and
they
lose about 10% of their pressure over the stroke, but they are much
faster, up
to 380 mm/s (15 psi). Because of this they are used when extruding
steel. They
are also used on materials that must be heated to very hot temperatures
for
safety reasons.
Hydrostatic extrusion presses
usually use castor oil at pressure up to 1400 MPa (200 ksi). Castor oil
is used
because it has good lubricity and high pressure properties.
Production forging
involves significant capital expenditure for machinery, tooling,
facilities and
personnel. In the case of hot forging, a high temperature furnace
(sometimes
referred to as the forge) will be required to heat ingots or billets.
Owing to
the massiveness of large forging hammers and presses and the parts they
can
produce, as well as the dangers inherent in working with hot metal, a
special
building is frequently required to house the operation. In the case of
drop
forging operations, provisions must be made to absorb the shock and
vibration
generated by the hammer. Most forging operations will require the use
of
metal-forming dies, which must be precisely machined and carefully heat
treated
to correctly shape the workpiece, as well as to withstand the
tremendous forces
involved.
Press Forging
We specifically know what kind
of strain can be put on the part, because the compression rate of the
press
forging operation is controlled. There are a few disadvantages to this
process,
most stemming from the workpiece being in contact with the dies for
such an
extended period of time. The operation is a time consuming process due
to the
amount of steps and how long each of them take. The workpiece will cool
faster
because the dies are in contact with workpiece; the dies facilitate
drastically
more heat transfer than the surrounding atmosphere. As the workpiece
cools it
becomes stronger and less ductile, which may induce cracking if
deformation
continues. Therefore heated dies are usually used to reduce heat loss,
promote
surface flow, and enable the production of finer details and closer
tolerances.
Roll Forging
The work piece is then
transferred to the next set of grooves or turned around and reinserted
into the
same grooves. This continues until the desired shape and size is
achieved. The
advantage of this process is there is no flash and it imparts a
favorable grain
structure into the workpiece.
Bottoming
In bottoming, the sheet is
forced against the V opening in the bottom tool. U-shaped openings
cannot be
used. Space is left between the sheet and the bottom of the V opening.
The
optimum width of the V opening is 6 T (T stands for material thickness)
for
sheets about 3 mm thick, up to about 12 T for 12 mm thick sheets. The
bending
radius must be at least 0.8 T to 2 T for sheet steel. Larger bend
radius
require about the same force as larger radii in air bending, however,
smaller
radii require greater force—up to five times as much—than air bending.
Advantages of bottoming include greater accuracy and less springback. A
disadvantage is that a different tool set is needed for each bend
angle, sheet
thickness, and material. In general, air bending is the preferred
technique.
MACHINING
PROCESS OF FERROUS METALS
Chucking the
Workpiece
We will be working with
a piece of 3/4" diameter 6061 aluminum about 2 inches long. A workpiece
such as this which is relatively short compared to its diameter is
stiff enough
that we can safely turn it in the three jaw chuck without supporting
the free
end of the work.
For longer workpieces
we would need to face and center drill the free end and use a dead or
live
center in the tailstock to support it. Without such support, the force
of the
tool on the workpiece would cause it to bend away from the tool,
producing a
strangely shaped result. There is also the potential that the work
could be
forced to loosen in the chuck jaws and fly out as a dangerous
projectile.
Turning with Power
Feed
One of the great
features of the 7x10 is that it has a power lead screw driven by an
adjustable
gear train. The leadscrew can be engaged to move the carriage under
power for
turning and threading operations. Turning with power feed will produce
a much
smoother and more even finish than is generally achievable by hand
feeding.
Power feed is also a lot more convenient than hand cranking when you
are making
multiple passes along a relatively long workpiece.
The power feed is
engaged by the knurled tumbler gear lever on the back of the headstock.
To
change the lever setting you must pull back on the knurled sleeve with
considerable force. With the sleeve pulled back you can move the lever
up and
down to engage its locking pin in one of three positions. In the center
position the lead screw is not engaged and does not turn. In the upper
position
the lead screw rotates to move the carriage towards the headstock and
in the
lower position the lead screw moves the carriage away from the
headstock. For
turning, you will generally want to cut towards the headstock, so move
the
lever to the upper position and release the sleeve to engage the
locking pin.
Grinding is a subset of
cutting, as grinding is a true metal-cutting process. Each grain of
abrasive
functions as a microscopic single-point cutting edge (although of high
negative
rake angle), and shears a tiny chip that is analogous to what would
conventionally be called a “cut” chip (turning, milling, drilling,
tapping,
etc.).
Cylindrical
Grinding
Cylindrical grinding
(also called center-type grinding) is used in the removing the
cylindrical
surfaces and shoulders of the workpiece. The workpiece is mounted and
rotated
by a workpiece holder, also known as a grinding dog or center driver.
Effects on
Workpiece Materials
Mechanical properties
will change due to stresses put on the part during finishing. High
grinding
temperatures may cause a thin martensitic layer to form on the part,
which will
lead to reduced material strength from microcracks.
Threading
Threading is the
process of creating a screw thread. More screw threads are produced
each year
than any other machine element.
Single-Point
Threading
Single-point threading,
also colloquially called single-pointing (or just thread cutting when
the
context is implicit), is an operation that uses a single-point tool to
produce
a thread form on a cylinder or cone. The tool moves linearly while the
precise
rotation of the workpiece determines the lead of the thread.
The cutter geometry
reflects the thread pitch but not its lead; the lead (thread helix
angle) is
determined by the tool path. Tapered threads can be cut either with a
tapered
multiple-form cutter that completes the thread in one revolution using
helical interpolation,
or with a straight or tapered cutter (of single- or multiple-form)
whose tool
path is one or more revolutions but cannot use helical interpolation
and must
use CAD/CAM software to generate a contour-like simulation of helical
interpolation.
Thread Grinding
Then the blank is
slowly rotated through approximately 1.5 turns while axially advancing
through
one pitch per revolution. Finally, the centerless thread grinding
process is
used to make head-less set screws in a similar method as centerless
grinding.
The blanks are hopper-fed to the grinding wheels, where the thread is
fully
formed. Common centerless thread grinding production rates are 60 to 70
pieces
per minute for a 0.5 in (13 mm) long set screw.
Additive Methods
Many, perhaps most,
threaded parts have potential to be generated via additive
manufacturing, of
which there are many variants, including fused deposition modeling,
direct
metal laser sintering, 3D printing, solid free form fabrication,
layered object
manufacturing, and stereolithography. Most additive technologies are
still on
the laboratory end of their historical development, but further
commercialization is picking up speed.
Uses
For instance, a 15-inch
drilling machine can center-drill a 30-inch-diameter piece of stock.
Other ways
to determine the size of the drill press are by the largest hole that
can be
drilled, the distance between the spindle and column, and the vertical
distance
between the worktable and spindle
Care
of Drilling Machines
Lubrication
Lubrication is important
because of the heat and friction generated by the moving parts. Follow
the
manufacturer’s manual for proper lubrication methods. Clean each
machine after
use. Clean T-slots, grooves, and dirt from belts and pulleys. Remove
chips to
avoid damage to moving parts. Wipe all spindles and sleeves free of
grit to
avoid damaging the precision fit. Put a light coat of oil on all
unpainted
surfaces to prevent rust. Operate all machines with care to avoid
overworking
the electric motor.
Power-Feed
The power-feed drilling
machines are usually larger and heavier than the hand-feed. They are
equipped
with the ability to feed the cutting tool into the work automatically,
at a
preset depth of cut per revolution of the spindle, usually in
thousandths of an
inch per revolution.
Common twist drill
sizes range from 0.0135 (wire gage size No. 80) to 3.500 inches in
diameter.
Larger holes are cut by special drills that are not considered as twist
drills.
The standard sizes used in the United States are the wire gage numbered
drills,
letter drills, fractional drills, and metric drills. Twist drills can
also be
classified by the diameter and length of the shank and by the length of
the
fluted portion of the twist drill.
The margin is the
narrow surface along the flutes that determines the size of the drill
and keeps
the drill aligned.
The portion of the
drill body that is relieved behind the margin is known as the body
clearance.
The diameter of this part is less than that of the margin and provides
clearance so that all of the body does not rub against the side of the
hole and
cause friction. The body clearance also permits passage of lubricants
around
the drill.
Drill
Point
When grinding the lip
angle, use the drill point gage and grind one lip perfectly straight
and at the
required angle (usually 590). Then flip the drill over and grind the
other lip.
Once the angle is established, then the lip clearance angle and lip
length can
be ground. If both lips are not straight and of the same angle, then
the chisel
edge will not be established. It is it important to have a sharp and
centered
chisel edge or the drill will not rotate exactly on its center and the
hole
will be oversized. If the drill point is too flat, it will not center
properly
on the workpiece.
Drill
Drifts
Drill drifts are flat,
tapered keys with one rounded edge that are designed to fit into a
spindle
chuck’s slot to force a tapered shank drill loose. The rounded top of
the small
end of the drill drift is designed to face upward while inserting the
drift
into the slot. There are two types of drill drifts, the standard type
and the
safety type. The standard drift must be inserted into the chuck’s slot
and then
struck with a soft hammer to jar the taper shank drill loose. The drill
will
fall quickly if not held by the hand and could break or cause injury.
The
safety drill drift has a sliding hammer weight on the drift itself to
allow for
a free hand to stay constantly on the drill as it comes loose.
Table or Base
Mounting
When a workpiece is
table or base mounted, the strap clamps must be as parallel to the
table or
base as possible. All bolts and strap clamps should be as short as
possible for
rigidity and to provide for drilling clearance.
Parallel bars should be
set close together to keep from bending the work. Washers and nuts
should be in
excellent condition. The slots and ways of the table, base, or vise
must be
free of all dirt and chips. All work holding devices should be free of
burrs
and wiped clean of oil and grease. Work holding devices should be the
right
size for the job. Devices that are too big or too small for the job are
dangerous and must be avoided.
Drilling Round
Stock
When drilling shafts,
rods, pipes, dowels, or other round stock, it is important to have the
center
punch mark aligned with the drill point. Use V-blocks to hold the round
stock
for center punching and drilling. Align the center of the round stock
with a
square or by lining the workpiece up with the twist drill point.
Another method
to drill round stock is to use a V-block drill jig that automatically
centers
the work for drilling.
JOINING
PROCESS OF FERROUS METALS
Until relatively
recently, structural steel connections were either welded or riveted.
High-strength bolts have completely replaced structural steel rivets.
Indeed,
the latest steel construction specifications published by AISC (the
13th
Edition) no longer covers their installation. The reason for the change
is
primarily due to the expense of skilled workers required to install
high
Strength structural steel rivets. Whereas two relatively unskilled
workers can
install and tighten high strength bolts, it took a minimum of four
highly
skilled riveters to install rivets in one joint at a time.
Blind rivets, also
known as pop rivets, are tubular and are supplied with a mandrel
through the
center. The rivet assembly is inserted into a hole drilled through the
parts to
be joined and a specially designed tool is used to draw the mandrel
into the
rivet. This expands the blind end of the rivet and then the mandrel
snaps off.
These types of blind rivets have non-locking mandrels and are avoided
for
critical structural joints because the mandrels may fall out, due to
vibration
or other reasons, leaving a hollow rivet that will have a significantly
lower
load carrying capability than solid rivets. Furthermore, because of the
mandrel
they are more prone to failure from corrosion and vibration. Unlike
solid
rivets, blind rivets can be inserted and fully installed in a joint
from only
one side of a part or structure, “blind” to the opposite side.
Joint
Analysis
The stress and shear in
a rivet is analyzed like a bolted joint. However, it is not wise to
combine
rivets with bolts and screws in the same joint. Rivets fill the hole
where they
are installed to establish a very tight fit (often called interference
fit). It
is difficult or impossible to obtain such a tight fit with other
fasteners. The
result is that rivets in the same joint with loose fasteners will carry
more of
the load—they are effectively more stiff. The rivet can then fail
before it can
redistribute load to the other loose fit fasteners like bolts and
screws.
The welding processes
covered in this chapter are gas welding, arc welding which includes
manual
metal arc welding (MMA), tungsten inert gas shielded arc welding (TIG),
gas
metal arc welding (MIG, MIG/CO2), submerged arc welding is (SAW), etc.
High
energy density processes like electron beam welding, laser beam
welding, plasma
welding are also dealt with. Pressure welding and some special welding
techniques like electro-slag welding etc. are also discussed in detail.
The
broad classification of the welding processes.
Submerged arc
welding (SAW)
Submerged arc welding
is a method in which the heat required to fuse the metal is generated
by an
electric current passing through between the welding wire and the work
piece.
The tip of the welding wire, the arc and the weld area are covered by a
layer
of granular flux. A hopper and a feeding mechanism are used to provide
a flow
of flux over the joint being welded.
MIG welding (gas
metal arc welding)
Gas metal arc welding
is a gas shielded process that can be effectively used in all
positions. The
shielding gas can be both inert gas like argon and active gases like
argon-oxygen mixture and argon-carbon-di-oxide which are chemically
reactive.
It can be used on nearly all metals including carbon steel, stainless
steel,
alloy steel and aluminium. Arc travel speed is typically 30-38 cm
minute and weld
metal deposition rate varies from 1.25 kg/hr when welding out of
position to
5.5 kg/hr in flat position.
Plasma
Welding
Since too powerful a
jet would cause a turbulence in the molten puddle, the jet effect on
the work
piece is softened by limiting gas flow rates through the nozzle. Since
this
flow alone may not be adequate to protect the molten puddle from
atmospheric
contamination, auxiliary shielding gas is provided through an outer gas
cup on
the torch.
The molten metal
flowing around the keyhole forms a reinforced weld bead. Square butt
joints
upto 6 mm thick can be welded in a single pass by this method.
For heavier plates
which require multi-pass welding partial beveling is done and the root
pass of
the largest size is deposited with the key hole technique without using
filler
wire. The rest of the passes are then carried out with normal melt-in
technique
with filler wire addition. PAW process is limited to around 25 mm thick
plates.
PRODUCTION OF
STAINLESS STEEL WIRE
Melting
Process
The involved process of
manufacturing precision stainless steel wire begins at the melt shop.
The
initial melt is composed of controlled scrap, processed ores and virgin
pure
metallic elements. When charged into a furnace, melting is accomplished
by
high-power electric arcs transferred from graphite electrodes. Once
molten, the
entire batch or heat is given a unique alphanumeric identity and tapped
into a
waiting preheated ladle for transfer to a secondary refining operation.
Once cooled into a
straight billet, each length of the billet is identified to maintain
necessary
trace-ability from melt all the way through to finished wire. Each
billet is
often times visually inspected for surface consistency to ensure that
no
abnormal surface irregularities were formed during the
continuous-casting
process. Depending on the condition found, billets may be spot surface
ground
to blend or remove irregularities that could otherwise contribute to
rolling
defects.
Production of
Spring Wire
A majority of spring
wire products begins with hot-rolled and solution-annealed wire rod
that has
been de-scaled and acid cleaned. The resultant consistent white pickled
finish
is now ready for conversion into high-quality drawn wire.
Spring manufacturing is
also a demanding process. It requires a wire starting stock that is
manufactured with the utmost consistency in size, and mechanical and
physical
properties, as specified as part of an order or specification.
By keeping variations
to a minimum, the springmaker can realize a much greater degree of
consistency
in the coil winding operation, where free length and coil OD are held
in close
control.
Spring wire can be
produced in a wide variety of specialty alloys. Applications for these
special
grades may include springs for high heat resistance, corrosion
resistance and
other high-performance attributes needed in the automotive, aerospace,
chemical
and process industries.
In addition to common
stainless steel types 302/304, 316 and 17-7PH, other exotic stainless
grades
can bring added benefits. Duplex stainless steel UNS S32205 or 2205
alloy is a
grade that combines the properties of austenitic and ferritic stainless
steels.
Wire
Drawing
Drawing is usually
performed at room temperature, thus classified as a cold working
process, but
it may be performed at elevated temperatures for large wires to reduce
forces.
More recently drawing has been used with molten glass to produce high
quality
optical fibers.
Process
The American wire gauge
scale is based on this. This can be done on a small scale with a draw
plate, or
on a large commercial scale using automated machinery. The process of
wire
drawing improves material properties due to cold working.
The areal reduction of
small wires is 15-25% and larger wires are 20-45%. Very fine wires are
usually
drawn in bundles. In a bundle, the wires are separated by a metal with
similar
properties, but with lower chemical resistance so that it can be
removed after
drawing. If the reduction in diameter is greater than 50%, the process
may
require annealing between the process of drawing the wire through the
dies.
Commercial wire drawing usually starts with a coil of hot rolled 9 mm
(0.35 in)
diameter wire. The surface is first treated to remove scales. It is
then fed
into either a single block or continuous wire drawing machine.
The block is also
tapered, so that the coil of wire may be easily slipped off upwards
when finished.
Before the wire can be attached to the block, a sufficient length of it
must be
pulled through the die; this is effected by a pair of gripping pincers
on the
end of a chain which is wound around a revolving drum, so drawing the
wire
until enough can be coiled two or three times on the block, where the
end is
secured by a small screw clamp or vice. When the wire is on the block,
it is
set in motion and the wire is drawn steadily through the die; it is
very
important that the block rotates evenly and that it runs true and pulls
the
wire at a constant velocity, otherwise “snatching” occurs which will
weaken or
even break the wire. The speeds at which wire is drawn vary greatly,
according
to the material and the amount of reduction.
Often intermediate anneals
are required to counter the effects of cold working, and to allow
further
drawing. A final anneal may also be used on the finished product to
maximize
ductility and electrical conductivity.
An example of product
produced in a continuous wire drawing machine is telephone wire. It is
drawn 20
to 30 times from hot rolled rod stock.
While round
cross-sections dominate most drawing processes, non-circular
cross-sections are
drawn. They are usually drawn when the cross-section is small and
quantities
are too low to justify rolling. In these processes, a block or
Turk’s-head
machine are used.
Lubrication in the
drawing process is essential for maintaining good surface finish and
long die
life.
Mechanical
Properties
The strength-enhancing
effect of wire drawing can be substantial. The highest gruel strengths
available on any steel have been recorded on small-diameter cold-drawn
austenitic stainless wire. Tensile strength can be as high as 400 ksi
(3760
MPa).)
Drawing dies are typically made of
tool steel, tungsten carbide, or diamond, with tungsten carbide and
manufactured diamond being the most common. For drawing very fine wire
a single
crystal diamond die is used. For hot drawing, cast-steel dies are used.
For
steel wire drawing, a tungsten carbide die is used. The dies are placed
in a
steel casing, which backs the die and allow for easy die changes. Die
angles
usually range from 6-15º and each die has at least 2 different angles:
the
entering angle and approach angle. Wire dies usually are used with
power as to
pull the wire through them. There are coils of wire on either end of
the die
which pull and roll up the wire with a reduced diamet.
PRODUCTION OF
STEEL BARS
Steel is a metal alloy
containing iron, carbon and other metals. Steel bars, which are used to
reinforce concrete work, come in different shapes, strengths and sizes.
They
are built through different methods. Steel is non-combustible, but it
starts to
lose strength when temperatures reach 750 degrees Fahrenheit. Common
types of
steel bars include hot rolled bars, cold twisted deformed bars and TMT
bars.
Hot
Rolled Bars
Hot rolled bars are
round, have a smooth surface and are made by a method called hot
rolling, which
consists of transforming a piece of steel into a cylindrical bar by
rolling it
when still hot. Hot rolled bars can also have ribbed surfaces, which
increases
the bond strength of the pieces.
Cold
Twisted Deformed Bars
These bars are first
hot rolled with three or more parallel ribs or indentations. When
cooled, the
bars are twisted, thus straining the steel’s elastic limit this helps
to make
the bar stronger. However, cold twisted deformed bars corrode much more
quickly
than other bars because of the hot-cold method by which they are
produced.
As deformed bars are
rods of steels provided with lugs, ribs or deformation on the surface
of bar,
these bars minimize slippage in concrete and increases the bond between
the two
materials. Deformed bars have more tensile stresses than that of mild
steel
plain bars. These bars can be used without end hooks. The deformation
should be
spaced along the bar at substantially uniform distances.
To limit cracks that
may develop in reinforced concrete around mild steel bars due to
stretching of
bars and some lose of bond under load it is common to use deformed bars
that
have projecting ribs or are twisted to improve the bond with concrete.
These
bars are produced in sections from 6 mm to 50 mm dia. In addition the
strength
of bonds of deformed bars calculated should be 40 to 80% higher than
that of
plain round bars of same nominal size. And it has more tensile stress
than that
of plain round bars of same nominal size. Cold twisted deformed (Ribbed
or Tor
Steel Bars) bars are recommended as best quality steel bars for
construction
work by structural Engineer.
Various Grades of
Mild Steel Bars
Some of manufacturers
stamped MS bars grade with their make/name and also give certification
of test
and grade. On the basis of the above information you can store mild
steel bars
grade-wise at the site of work.
Steel Bars for RCC
Work
All finished steel bars
for reinforced work should be neatly rolled to the dimension and
weights as
specified. They should be sound, free from cracks, surface flaws,
laminations,
rough, jagged and imperfect edges and other defects. It should be
finished in a
work manlike manner.
Weight of Different
Steel Bars
When we want to
purchase Mild steel members from the market, the shopkeeper quotes the
price of
steel members in weight. When any type of steel members for use in
house
construction is required, we calculate the length of steel member in
feet or
meter but we are ignorant about the weight of steel.
Here are details of
weight per meter for various types of steel members :
Types of Cold
Finished Bars
Cold drawn bars are
widely used in mass production of parts due to their excellent
mechanical and
dimensional properties, with machining characteristics in excess of the
hot
rolled bar condition. Round, hexagonal and square bars can be produced
by cold
drawing.
Turned and polished
round bars have similar mechanical properties to those of equivalent
hot rolled
bar, but exhibit a smooth, bright surface finish and improved
dimensional
accuracy. They are widely used where a surface free of decarburisation
is
required, for example in induction hardening and when the surface must
be free
from surface defects, such as for use in cold forming.
Production Flow
To meet the diversified
end-use requirements, cold-rolled coil is designed to provide specific
attributes such as high formability, deep drawability and good
paintability.
How those good attributes come will be explained by the following steps.
Pickling
Continuous
Pickling Line
The cold reduction
operation induces very high strains into the sheet, thus, the sheet not
only
becomes thinner, but also becomes much harder, less ductile. However,
after the
cold-reduced product is annealed, it becomes very soft and formable. In
fact,
the combination of cold reduction and annealing lead to a refinement of
the
steel that provides very desirable and unique forming properties for
subsequent
use by the customers.
The pickling operation
must be well-controlled to assure that all the oxides formed during hot
rolling
are removed. The thickness of the hot-rolled strip is important in that
the
properties of the final cold rolled and annealed product is influenced
by the
cold reduction. This means that the thickness of each hot-rolled coil
is
carefully controlled to provide the mill with a specific thickness to
achieve
the proper cold reduction. Among other things, cold reduction affects
the
forming behavior of the product after annealing.
After the steel is
batch annealed, the specific properties of the steel sheet depends on
the steel
chemistry, the temperatures used during hot rolling, the amount of cold
reduction, and the annealing cycle.
In the method of
annealing, the steel is maintained under a protective (non-oxidizing)
atmosphere using hydrogen and nitrogen to prevent oxidizing the steel
while it
is at high temperature. In addition to preventing oxidation, the
protective
atmosphere is designed to clean the steel breaking down the oils that
are
present after cold rolling and entraining the oil vapors in the
hydrogen/nitrogen gases that are passed through the furnace.
Skin Pass
After annealed, the
steel coil is most often processed by passing it through a set of rolls
that
appear similar to the rolls used during cold rolling, in fact, skin
pass does
impart a small amount of cold reduction, typically between 0.25 and 1.0
percent.
Warehousing
After those processes are finished, the
following work
will involve packing steel coils after being done in skin pass mill.
Those
coils are moved into the warehouse waiting for loading.
PRODUCTION OF
STEEL TUBE AND PIPE
In the case of seamless
tube and also in the case of the Fretz-Moon welding process, the
production
stage invariably involves a heating operation, in which case the
product may
also be referred to as hot-formed tube or pipe. Downstream facilities
for hot drawing
or hot expanding occur relatively rarely; on the other hand, hot-formed
tubes
are extensively used as starting products for a downstream cold forming
process. The latter is used in order to extend the product mix of a
plant
toward smaller diameters and wall thicknesses (DIN 2391), to reduce
wall
thickness and diameter tolerances, and to achieve special surface
finishes or
mechanical/thermo-mechanical properties in the tube.
As the requirements
imposed on tubular products continued to increase, not only were the
associated
manufacturing processes constantly improved, but also appropriate
systems for
effective production control and quality assurance were introduced.
Nowadays,
tube and pipe manufacturers of renown all have a system in place
enabling the
production process from the steelworks to the finished tube to be
continuously
monitored and documented for total traceability, and effectively
controlled on
the basis of quality criteria. The mechanical and nondestructive tests
stipulated in the relevant technical specifications are carried out by
personnel operating independently from the production control
department so as
to guarantee product of a constantly high quality.
Another possibility for
the production of seamless tube was invented by H. Ehrhardt. By
piercing a
solid square ingot in a round die, he was able to produce a
thick-walled hollow
shell with a closed bottom. This shell was subsequently stretched on a
mandrel
bar through tandem-arranged ring dies to produce the final tube
dimensions. This
so-called push bench process in its modified form has remained viable
to this
very day.
Pierce and Pilger
Rolling Process
The pierce and pilger
method for the production of seamless pipe is also referred to as the
Mannesmann process after its inventors, the Mannesmann brothers. Pipe
diameters
above the rolling range indicated can also be produced by expansion. To
this
end, the largest rolled pipes are reheated and then expanded either by
pulling
through a plug - a process often performed in several passes to
gradually
increase the outside diameter - or by rolling on a becking mill.
Whichever of
the two processes is employed, the wall thickness is, of course, also
reduced.
With small pilger mills
for manufacturing the lower size range, the two-stage rolling process
is still
employed today. The starting material takes the form of round rolled
steel
blooms, although round ingots are still frequently used. Round
conticast
billets measuring between 100 to approx. 300 mm in diameter are also
being
increasingly employed.
The piercing mill
features two specially contoured work rolls which are driven in the
same
direction of rotation. Their axes are inclined by approx. 3 to 6° in
relation
to the horizontal stock plane. Generally, the roll gap is closed by a
non-driven
support roll at the top and a support shoe at the bottom. Located at
the centre
of the roll gap is a piercing point which functions as an internal tool
and is
held in position by an external thrust block via a mandrel.
Push Bench Process
This process is also
known as the rotary forge process, and - in Germany - after the name of
its
inventor, as the Ehrhardt process. It is employed for the manufacture
of tube
in the diameter range from approx. 50 to 170 mm with wall thicknesses
from 3 to
18 mm and lengths up to 18 m. Modern push bench plants usually only
produce one
(large) hollow bloom size, leaving a downstream stretch-reducing mill
to
convert this into all the usual tube dimensions down to a smallest
outside
diameter of approx. 20 mm.
Arranged in the foundation
bed of the push bench are up to 15 roll stands. The roll stands usually
comprise three (sometimes four) circumferentially distributed,
non-driven
grooved rollers. The gradually decreasing cross sections of the roller
passes
produce reductions which, in the main work passes, can amount to up to
25%.
During this process, between 6 and 7 roll stands are simultaneously in
operation at any one time. The push force is applied to the mandrel bar
by a
rack-and-pinion arrangement, and operating speeds can be up to 6 m/s.
Pierce and Draw
Process
This process, also
developed by H. Ehrhardt, is similar to the push bench variant but,
unlike the
technologies described so far, is not suitable for mass production.
Consequently, therefore, the number of plants employing this process is
quite
small. These, however, are specially designed for the manufacture of
seamless
hollow components combining large diameters with large wall thicknesses.
The production range of
such facilities lies between approx. 200 and 1450 mm in outside
diameter, with
wall thicknesses ranging from approx. 20 to 270 mm. This therefore
provides an
effective complement to the product mix available in large pilger
mills. With a
maximum length of around 10 m, tube blanks and hollow sections can be
manufactured (in all steel grades), by this process for items such as
power
plant components, hydraulic cylinders, high-pressure gas cylinders and
pressure
vessels, as can products such as thick-walled square section tubes.
Assel Rolling
Process
Assel mills are used
nowadays to produce stainless tube with outside diameters ranging from
60 to
250 mm and lengths of up to 12 m. The ratio of outside diameter to wall
thickness tends to lie in the region 4 to 15. The smallest inside
diameter of
the tubes is approx. 40 mm. The tubes manufactured by this method are
characterized by their excellent concentricity and are extensively
employed in
the production of turned components (shafts, axles) and also for
medium-alloy
steel roller bearing production (general product name: mechanical tube).
The starting material
predominantly takes the form of round steel blooms of the appropriate
length
which are heated to forming temperature in a rotary hearth furnace.
Following
descaling and end face centering, the bloom is formed into a hollow
shell in
the cross roll piercing mill and then fed into the Assel mill.
Although Diescher mills
have not enjoyed wide market penetration as elongating or
finish-rolling
facilities, modern cross roll piercing mills are nowadays being
equipped more
and more with Diescher discs (see continuous mandrel process and plug
rolling/MPM process).
Cold Drawing
Seamless precision
steel tube has been standardized in DIN 2391 for the diameter range
from 4 to
120 mm and wall thicknesses from 0.5 to 10 mm. In addition, however,
non-standardized intermediate sizes, and tube up to 380 mm outside
diameter
with wall thicknesses up to 35 mm, can also be manufactured by cold
drawing.
These predominantly
take the form of draw benches equipped with a continuous chain; or draw
benches
with reversible finite drawing and return chains attached to the
drawing
carriage. Other designs include rope-type draw benches, rack and pinion
draw
benches and also draw benches with a hydraulic drive system.
Large tube lengths are
generally drawn using a floating plug on continuous-type straight-line
machines
in which two reciprocating sledges alternate in the performance of the
drawing
operation. Tube of small diameter is usually cold-drawn by the bull
block
process in which the stock is taken from a coil and the drawing power
is
applied by a capstan.
The pass design of the
two rolls consists of a circular recess, corresponding to the cross
section of
the hollow blank, which tapers over a certain portion of the roll
circumference
to provide an ideal, continuous transition to the finished tube
diameter.
Consequently, as the rolls move forward and backward, the hollow blank
is
formed in the desired manner. An essential aspect of the process lies
in the
fact that elongation of the hollow blank to produce the finished tube
is
performed by simultaneous reduction of the diameter and the wall
thickness.
This is aided by the shape of the mandrel which tapers from the hollow
blank
inside diameter to the finished tube inside diameter. Following a
forward and
backward rolling cycle, the rolls release the blank which is then
advanced by a
certain, infinitely variable feed value. The corresponding material
volume is
then elongated with the subsequent forward and backward rolling cycle
executed
by the stand.
Aside from this hot
pressure welding technique, in which the strip is heated in a furnace
to
welding temperature, several other processes were devised by the
American E.
Thomson between the years 1886 and 1890 enabling metals to be
electrically welded.
The basis for this was the property discovered by James P. Joule
whereby
passing an electric current through a conductor causes it to heat up
due to its
electrical resistance.
The starting material
can be formed into its tubular shape in either the hot or cold
condition. A
distinction is made in this respect between continuous tube forming and
the
single tube forming process.
In continuous tube
forming, uncoiled strip material is taken from an accumulator, with the
leading
end and trailing end of the consecutive coils being welded together.
In single pipe
production, the tube forming and welding process is not performed over
endless
lengths, but rather (as the name suggests) in single pipe lengths.
The main methods used
for the production of welded tube and pipe are the Fretz-Moon,
high-frequency
induction, submerged-arc and combination gas-shielded submerged-arc
processes,
plus the various gas-shielded welding methods for the production of
stainless
steel tube and pipe.
Pressure Welding
Processes
Fretz-Moon
Process
In this process, named
after its inventors, steel strip in the form of a continuous skelp is
heated to
welding temperature in a forming and welding line. The stock is
continuously
formed by rollers into an open-seam tube and then the mating edges are
pressed
together and welded by a process related to the forge-welding technique
of old.
The hot-rolled steel
strip coils used as the starting material are uncoiled at high speed
and stored
in loop accumulators. These serve as a buffer during the continuous
production
process, enabling the trailing end to be butt-welded to the leading end
of the
strip provided by the next coil. This continuous strip or “skelp” is
taken
through a tunnel furnace where it is heated to a high temperature.
Laterally arranged
burners increase the temperature at the skelp edges to a welding heat
approx.
100 to 150°C higher than the temperature prevailing at the skelp
centre.
Low-Frequency
Process
In this process,
welding is performed with alternating current frequencies from 50 to
400 Hz. An
electrode comprising two insulated discs of a copper alloy serves not
only as
the power supply but also as the forming tool and the element which
generates
the necessary welding pressure.
The electrodes
constitute the critical components of the plant, because not only must
they be
provided with a groove which matches the diameter of the tube being
manufactured, but also this radius has to be constantly monitored for
wear
during production operations.
The material extruded
during the pressure welding process forms an inner and outer flash
along the
weld zone which has to be removed inline just downstream of the welding
point
by internal and external trimmers.
The welding current can
be introduced into the open-seam tube both by conductive means using
sliding
contacts and by inductive means using single or multi-wind coils.
Consequently,
a distinction is made in the nomenclature between high-frequency
induction
(HFI) welding and high-frequency conduction welding. The strip or skelp
is shaped
in a roll forming mill or in an adjustable roll stand (natural function
forming) into the open-seam tube for the manufacture of a wide range of
products. These include line pipe and structural tube in the size range
from
approx. 20 to 609 mm OD and 0.5 to approx. 16 mm wall thickness, and
also tube
blanks as the feedstock for a downstream stretch-reducing mill. The
starting
stock is provided in the form of coiled steel strip or hot-rolled wide
strip.
Depending on the tube dimensions and application, and particularly in
the
manufacture of precision tube, the steel strip may either undergo an
upstream
pickling operation, or cold-rolled strip is used. The individual coils
are
welded together and, at high uncoiling speeds, the strip first passes
through a
loop accumulator. The tube welding machine operates continuously at a
speed
ranging from 10 to 120 m/min by drawing the strip from the loop
accumulator.
The roll forming mill
is used for tube diameters up to max. 609 mm, and generally consists of
8 to 10
largely driven roll forming stands in which the strip is gradually
shaped into
the open-seam tube-as indicated in stages 1 to 7 in Fig. 29. The three
fin pass
stands - 8, 9 and 10 - guide the open-seam tube toward the welding
table. The
forming rolls have to be precisely matched to the final tube diameter.
For the
manufacture of large-diameter pipe, the natural function forming
process may
also be applied. Fig. 30 shows the principles of this forming process
involving
a series of roll stands (roller cages).
The main features of
the roller cage is that a number of non-driven internal and external
forming
rollers, adjustable within a wide product diameter range, are
configured in a
funnel-shaped forming line which gradually bends the strip into the
open-seam tube
shape. Only the breakdown stand at the inlet and the fin pass stands at
the
exit end are actually driven. The cross-sectional details A-B, C-D and
E-F in
Fig. 30 indicate the degree of deformation and the arrangement of the
forming
rollers at various sections along the line.
Before the strip enters
the forming section, it is straightened and cut to a constant width by
a
longitudinal edge trimmer. The cut edges may be additionally bevelled
for
welding preparation. The strip is then formed into an open-seam tube as
described above, and with the gap still relatively wide, fed via three
or four
fin pass stands to the welding table. The overhead fin rolls, the width
of
which is tapered toward the welding point, determine the gap entry
angle and
control its central position in the welding table. There the converging
strip
edges are pushed against each other by shaped squeeze rolls and then
welded by
means of the high-frequency electric resistance process.
The HF pressure weld
can either be left in its as-welded condition or subsequently
heat-treated in
the normalizing range, depending on the application. Partial inductive
annealing of the weld may also be performed on the continuous tube, or
the
individual tubes may be subjected to a separate heat treatment
following
cutting to length, depending on the material flow conditions within the
plant.
In the subsequent tube
finishing department, the tubes are further processed on straightening
machines. The straightening operation may be preceded by a heat
treatment, depending
on the tube dimensions and application. Nondestructive examination
facilities
and the performance of a visual inspection serve to monitor the
production
process. Once completed, the tubes are subjected to the relevant,
specified
acceptance procedures irrespective of the in-process tests and
inspections
performed on them.
High-Frequency
Induction Welding Process
In the high-frequency
induction welding process (HFI or Induweld process), welding speeds of
up to
120 m/min may be attained, depending on wall thickness and application.
The open-seam tube 1 to
be welded is introduced in the direction of the arrow to the welding
table
where it is engaged by the squeeze rolls 5. These initially press
together the
incoming open seam edges approaching at angle 2. The high-frequency
current
supplied by the welding generator 4 forms an electro-magnetic field
around the
induction coil 3 which induces an AC voltage in the open-seam tube
corresponding to a current travelling around the tube circumference.
Any increase in the
rate of deposition beyond this limit requires the employment of several
wire
electrodes. This then allows a higher overall current to be applied for
the
welding work without the danger of the current carrying capacity of the
flux
being exceeded at any of the individual wire electrodes. In practical
operations, increased performance is obtained by employing a multi-wire
welding
configuration with 2, 3 or 4 electrodes.
The MAG process is
being increasingly used for tack-welding in the manufacture of spiral
and
longitudinally welded large-diameter pipe. The tack weld also serves as
the
weld pool backing for the subsequent submerged-arc welding process. The
prerequisites for an optimum weld are a precise edge preparation
(double-V butt
joint with wide root faces) and a good, continuous tack weld. In
large-diameter
pipe production, the welding speeds for the tack weld range from
approx. 5 to
12 m/min.
The Production of
Longitudinally Welded Pipe (U-ing/O-ing process)
The plates employed for
longitudinally welded pipe are formed on presses featuring open dies
for the
U-ing and closed dies for the O-ing operation. The process is also
sometimes
referred to as the UOE process (U-ing, O-ing, Expanding) and is applied
in the
manufacture of longitudinally welded large-diameter pipe in individual
lengths
up to 18 m. Depending on the material and diameter, the wall
thicknesses range
from 6 to 40 mm. The starting material invariably takes the form of
steel plate
as indicated above.
Spiral Pipe
Production with Separate Forming and SAW Welding Lines
A special roller table
rotates the pipe in precise accordance with its spiral joint, so
enabling the
SAW welding heads to perform first the inside and then the outside
passes.
Precise weld centerline alignment control of the inside and outside
welding
heads is required in this operation in order to minimize weld offset.
The two- or three-wire
method is employed for the inside and outside pass welding operations.
Aside from a few
modifications, the subsequent production stages such as pipe end
machining,
hydrostatic testing and also the nondestructive examinations and
mechanical
tests, are in principle the same as those applied in the conventional
spiral
pipe manufacturing process.
Here again, a high
standard of quality is achieved by in-process quality control
activities which
are performed after every stage of production. The results of these
tests and
inspections are immediately fed back to the individual production stage
concerned in order to ensure continuous product quality optimization.
MANUFACTURING
OF STAINLESS STEEL SHEET
Manufacturing
Process
The manufacturing of
stainless steel sheet involved a series of processes. First the raw
material is
melted in a electric furnace. This step usually involves 8 to 12 hours
of
intense heat. Next the mixture is cast into slabs. Next the slabs goes
through
forming operations, beginning with hot rolling, in which steel is
heated and
passed through huge rolls.
Heat
Treatment
After the stainless
steel is formed, most types must go through an annealing steps.
Annealing is a
heat treatments in which is steel is heated and cooled under controlled
conditions to relive internal soften the metal. Some steels are heat
treated
for higher strength. Lower aging temperatures produce high strength
with low
fracture toughness, while higher-temperature aging produces a lower
strength,
tougher material.
Cutting
Nibbling is a process
of cutting by blanking out a series of overlapping holes and is ideally
suited
for irregular shapes.
Stainless steel can
also be cut using flame cutting, which involves a flame-fired torch
using
oxygen and propane in conjunction with iron powder. This method is
clean and
fast. Another cutting method is known as plasma jet cutting, in which
an
ionized gas column in conjunction with an electric arc through a small
orifice
makes the cut. The gas produces extremely high temperatures to melt the
metal.
Finishing
A bright finish is
obtained by first hot rolling and then cold rolling on polished rolls.
A highly
reflective finish is produced by cold rolling in combination with
annealing in
a controlled atmosphere furnace, by grinding with abrasives, or by
buffing a
finely ground surface. A mirror finish is produced by polishing with
progressively finer abrasives, followed by extensive buffing. For
grinding or
polishing, grinding wheels or abrasive belts are normally used. Buffing
uses
cloth wheels in combination with cutting compounds containing very fine
abrasive particles in bar or stick forms. Other finishing methods
include
tumbling, which forces movement of a tumbling material against surfaces
of
parts, dry etching (sandblasting), wet etching using acid solutions,
and
surface dulling. The latter uses sandblasting, wire brushing, or
pickling
techniques.
Manufacturing at the
Fabricator or End User
After the stainless
steel in its various forms are packed and shipped to the fabricator or
end
user, a variety of other processes are needed. Further shaping is
accomplished
using a variety of methods, such as roll forming, press forming,
forging, press
drawing, and extrusion.
There
are a variety of methods for joining
stainless steel, with welding being the most common. Fusion and
resistance
welding are the two basic methods generally used with many variations
for both.
In fusion welding, heat is provided by an electric arc struck between
an
electrode and the metal to be welded.
Bending
Process of Steel Sheet
The Air Bending
Process
The upper tool (or
punch) is pressing down on the sheet metal at the center of the bend
while the
lower tool (or vee die) is pressing up on the sheet metal.
In general, the tooling
only touches the sheet metal along these 3 lines. Thus the process is
call “air
bending”.
Flat Layouts
In general, sheet metal
stretches when it is bent. For example, if you were to take a piece of
metal
that measured exactly 2.000" in the flat and then bent it down the
middle
at 90°, when you measure the length of the 2 bends (from the outside of
the
bend), the sum of the leg lengths would be greater than 2.000" (in the
case of 16 GA (.059) cold roll steel, it would be likely to add up to
2.094").
GRADES OF
STAINLESS STEEL
A
Brief Overview of Stainless Steel
This development was
the start of a family of alloys which has enabled the advancement and
growth of
chemical processing and power generating systems upon which our
technological
society is based.
Subsequently several
important sub-categories of stainless steels have been developed. The
sub-categories are austenitic, martensitic, ferritic, duplex,
precipitation
hardening and super alloys.
Austenitic
Grades
Austenitic grades are
those alloys which are commonly in use for stainless applications. The
austenitic grades are not magnetic. The most common austenitic alloys
are
iron-chromium-nickel steels and are widely known as the 300 series. The
austenitic stainless steels, because of their high chromium and nickel
content,
are the most corrosion resistant of the stainless group providing
unusually
fine mechanical properties. They cannot be hardened by heat treatment,
but can
be hardened significantly by cold-working.
“L”
Grades
The “L” grades are used
to provide extra corrosion resistance after welding. The letter “L”
after a
stainless steel type indicates low carbon (as in 304L). The carbon is
kept to
.03% or under to avoid carbide precipitation. Carbon in steel when
heated to
temperatures in what is called the critical range (800 degrees F to
1600
degrees F) precipitates out, combines with the chromium and gathers on
the
grain boundaries.
“H” Grades
The “H” grades contain
a minimum of .04% carbon and a maximum of 10% carbon and are designated
by the
letter “H” after the alloy. People ask for “H” grades primarily when
the
material will be used at extreme temperatures as the higher carbon
helps the
material retain strength at extreme temperatures.
Ferritic
Grades
Ferritic grades have
been developed to provide a group of stainless steel to resist
corrosion and
oxidation, while being highly resistant to stress corrosion cracking.
Duplex
Grades
Duplex grades are the
newest of the stainless steels. This material is a combination of
austenitic
and ferritic material. This material has higher strength and superior
resistance to stress corrosion cracking. An example of this material is
type
2205. It is available on order from the mills.
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