HOT- WORKING PROCESSES

HOT- WORKING PROCESSES

•Shaping of metal by deformation is a very very old tradition.

•Processes such as rolling, wire drawing etc. were common in the Middle Ages. In North America, by 1680 the Saugus Iron Works near Boston had an operating drop forge, rolling mill, and slitting mill.

•Although basic concepts of many forming processes have remained largely unchanged throughout history, details and equipment have evolved considerably.

INTRODUCTION

•Manual processes were converted into machine processes during the industrial revolution.

•The machinery then became bigger, faster, and more powerful.

•Waterwheel power was replaced by steam and then electricity.

•More recently, computer-controlled, automated operations have emerged

CLASSIFICATION OF DEFORMATION PROCESSES• Processes divide into the following :-

–Primary processes reduce a cast material into intermediate shapes, such as slabs, plates, or billets.

– Secondary processes further convert these shapes into finished or semi-finished products.

CLASSIFICATION OF DEFORMATION PROCESSES

• Bulk deformation processes –Surface area of work piece changes significantly.–Thicknesses or cross sections of material are reduced–Shapes are changed.–As volume of material remains constant, other

dimensions must change in proportion. –Enveloping surface area is altered, usually increasing

as product lengthens.

CLASSIFICATION OF DEFORMATION PROCESSES

• Sheet-forming operations

–Involve deformation of material where thickness and surface area remain relatively constant.

–Coining, for example, begins with sheet material but alters the thickness in a complex manner that is essentially bulk deformation.

HOT-WORKING PROCESSES

•Hot-working processes provide means of producing a desired shape.

•At elevated temperatures, metals weaken and become more ductile.

•With continual re-crystallization, massive deformation take place without exhausting material plasticity.

HOT-WORKING PROCESSES

Some major modern hot-working manufacturing processes are:

– Rolling– Forging– Extrusion– Hot drawing– Pipe welding– Piercing

ROLLING

• Rolling is usually first process used to convert

material into a finished wrought product.

• Stock can be rolled into blooms, billets, slabs, or

these shapes can be obtained directly from

continuous casting.

– A bloom has a square or rectangular cross

section, with a thickness greater than 6 inches

and a width no greater than twice the thickness.

ROLLING

– A billet is usually smaller than a bloom and has a

square or circular cross section. Billets are

usually produced by some form of deformation

process, such as rolling or extrusion.

– A slab is a rectangular solid where the width is greater

than twice the thickness. Slabs can be further rolled to

produce plate, sheet, and strip

ROLLING

• These hot-worked products use for subsequent processing techniques such as cold forming or for machining. – Sheet and strip fabricated into products or cold

rolled into thinner, stronger material even into foil.

– Blooms and billets rolled into finished products, such as structural shapes or railroad rail, or processed into semi-finished shapes, such as bar, rod, tube, or pipe

ROLLING• Hot Rolling :-

– is prominent among all manufacturing processes– equipment and practices are sufficiently advanced– is standardized– produce uniform-quality products at relatively low

cost– products are normally obtained in standard shapes

and sizes

Basic Rolling Process

• Heated metal is passed between two rolls that

rotate in opposite directions

• Gap between rolls is less than thickness of entering

metal.

• Rolls rotate with surface velocity that exceeds

speed of incoming metal, friction along the contact

interface acts to propel the metal forward.

Basic Rolling Process

• Metal is squeezed and elongates result in decrease of the

cross-sectional area.

• Amount of deformation in a single pass depends on the

friction conditions along the interface.

• If too much material flow is demanded, rolls cannot

advance the material and simply skid over its surface.

• Too little deformation per pass results in excessive

production cost ??

Rolling Temperatures

• Temperature control is crucial to the success of the

hot rolling process.

• If the temperature of the billet is not uniform, the

subsequent deformation will not be uniform.


• For example

If a part cools prior to working, the cooler

surfaces will tend to resist deformation.

Cracking and tearing of surface may result as

hotter, weaker interior tries to deform.


• Cooling from solidification is controlled to enable direct insertion into a hot-rolling operation without additional handling or reheating.

• Brought to rolling temperature, usually gas- or oil-fired soaking pits or furnaces are normally used.

• Plain-carbon and low-alloy steels soaking temperature is approximately 22000F (l2000C)


• For smaller cross sections, induction coils may be used to heat material for rolling

• Hot rolling is usually terminated when temperature falls to about 100 to 2000F (50 to 100°C) above the recrystallization temperature of material

• Finishing temperature assures the production of a uniform fine grain size and prevents possibility of unwanted hardening

• Before additional deformation, a period of reheating is required to reestablish desirable hot-working conditions

Rolling Mill Configurations

• Rolling mill stands are available in a variety of roll configurations.

• Early reductions (often called primary roughing or breakdown passes), employ two- or three-high configuration with 24- to 55-in. (600- to 1400-mm) diameter rolls

• Two-high non-reversing mill simplest design from which material can only pass in one direction

• Two-high reversing mill permits back-and-forth rolling, rolls may stop, reversed, and brought back to rolling speed between each pass


• The three-high mill eliminates need for roll reversal but requires some form of elevator on each side of mill to raise or lower material and mechanical manipulators to turn or shift product between passes– smaller-diameter rolls produce less length of

contact for a given reduction and therefore require lower force and less energy to produce a given change in shape

– smaller cross section, however, provides reduced stiffness and pressed apart by the metal passing through the middle


• Four-high and cluster arrangements use backup rolls to support the smaller work rolls– used in hot rolling of wide plate and sheets, and

in cold rolling, where small negligence would result in an unacceptable variation in product thickness

– Foil is rolled on cluster mills since small thickness requires small-diameter rolls

– In a cluster mill, the roll in contact with the work can be as small as 1/4 in. in diameter


– Pack Rolling, a process where two or more layers of metal are rolled simultaneously as a means of providing a thicker input material

– Household aluminum foil is usually pack rolled, as evidenced by the one shiny side (in contact with the roll) and one dull side (in contact with the other piece of foil)

– In rolling of non-flat or shaped products, such as structural shapes and railroad rail, the sets of rolls contain contoured grooves that sequentially form desired shape, cross section and control metal flow

Continuous Rolling Mills• When the volume of a product justifies investment,

it may be rolled on a continuous rolling mill. – Billets, blooms, or slabs are heated and fed

through an integrated series of non-reversing stands

– Continuous mills for the hot rolling of steel strip, for example, often consist of a roughing train of approximately four four-high mill stands and a finishing train of six or seven additional four-high stands.

Continuous Rolling Mills

– In a continuous structural mill, the rolls in each stand contain only one set of shaped grooves, in contrast to the multi-grooved rolls used when the product is produced by back-and-forth passes through a single stand.

– In a continuous rolling mill, same amount of material must pass through each stand in a given period of time.

– If cross section is reduced, speed must be increased proportionately.

Continuous Rolling Mills

– Thus rolls of each successive stand must turn

faster than those of preceding one by an amount

equal to change in cross-sectional area

– If this synchronization is not maintained, material

may accumulate between stands, or demand for

incoming material may place material under

excessive tension, and cause a tearing or rupture

Continuous Rolling Mills– Synchronization of six or seven mill stands is not

an easy task, especially when key variables such as temperature and lubrication may change during a single run and product may be exiting final stand at speeds in excess of 70 miles per hour (110 kilometers per hour).

– Computer control is important to successful rolling, and modern mills are equipped with numerous sensors to provide the needed information.

Ring Rolling

• In ring rolling process, one roll is placed through the hole of a thick-walled ring, and a second roll presses in from outside.

• As the rolls squeeze and rotate, wall thickness is reduced and diameter of ring increases.

• Shaped rolls can be used to produce a wide variety of cross-section profiles.

• Resulting seamless rings find application in products such as rockets, turbines, airplanes, pipelines, and pressure vessels.

Characteristics of Hot-Rolled Products

• Because they are rolled and finished above recrystallization temperature, hot-rolled products have little directionality in their properties and are relatively free of deformation - induced residual stresses.

• These characteristics may vary, depending on thickness of product and presence of complex sections.


• Substantial residual stresses can be induced during hot working.


• Thin sheets often show some definite directional

characteristics, whereas thicker plate (such as that

above 0.8 in. or 20 mm) will usually have very little.

• Because of the high residual stresses in rapidly

cooled edges, a complex shape, such as an T- or H-

beam, may warp noticeably if a portion of one

flange is cut away.


• As result of hot deformation and good control hot-rolled products are normally of uniform and dependable quality and reliability.

• It is quite unusual to find any voids, seams, or laminations when these products are produced by reliable manufacturers.

• Surfaces of hot-rolled products are usually a bit rough and are originally covered with a tenacious high-temperature oxide, known as mill scale.


• Removed by an acid pickling operation, resulting in

a surprisingly smooth surface finish.

• Dimensional tolerances of hot-rolled products vary

with kind of metal and size of the product.

• For most products produced in reasonably large

tonnages, tolerances are within 2 to 5% of specified

dimension (either height or width).

Flatness Control and Rolling Defects

• Rolling of flat material with uniform thickness

requires uniform gap between rolls attaining such

an objective may be difficult.

• Consider upper roll in a set that is rolling sheet or

plate material presses upward in the middle of roll

supported in mill frame.

Flatness Control and Rolling Defects• Roll is loaded in three-point bending and tends to

flex in a manner that produces a thicker center and thinner edge.

• If roll is always used to roll same material at same temperature, forces and deflections can be predicted, and roll can be designed to have a specified amount of crowning.

• When roll is subjected to a specified load, it will “deflect into flatness”.

Flatness Control and Rolling Defects• If applied load is not of designed magnitude, profile

will not be flat and defects may result.

The thinner material will try to become longer but must remain attached to the thicker. Result may be wavy edges or fractures in center. If correction is excessive, center becomes thinner and longer, and result can be a wavy center or cracking of the edges.

Thermo-mechanical Processing and Controlled Rolling

• A rolling process is generally used as being a means of changing shape of material.

• Heat may be used to reduce forces and promote plasticity while mechanical properties (heat treatments) are usually performed as subsequent operations.

• Thermo-mechanical processing consists of both deformation and controlled thermal processing to produce desired levels of strength and toughness in the working product.


• Possible goals of thermo mechanical includes :-– Production of uniform fine grain size– Controlling nature– Size and distribution of various transformation

products (such as ferrite, pearlite, bainite, and martensite in steels)

– Controlling the reactions that produce solid solution strengthening or precipitation hardening

– Producing a desired level of toughness.


• Following must all be specified and controlled:-

– Starting structure (controlled by composition and prior thermal treatments),

– deformation details,

– temperature during the various stages of deformation,

– the cool down from the working temperature.


Computer-controlled rolling facilities are almost a

necessity if thermo-mechanical processing is to be performed successfully.

Possible benefits of thermo-mechanical processing include

– improved product properties;– substantial energy savings (by eliminating subsequent

heat treatment);– Possible substitution of a cheaper, less-alloyed metal for

a highly alloyed one that responds to heat treatment.

FORGING

• Forging is term applied to a family of processes where deformation is induced by localized compressive forces.

• The equipment can be manual or power hammers, presses, or special forging machines. The term forging usually implies hot forging done above the recrystaIlization temperature.

FORGING• The forging material may be

– Drawn out to increase its length and decrease its cross section,

– Upset to decrease the length and increase the cross section,

– Squeezed in closed impression dies to produce multidirectional flow.

FORGING

•Common forging processes include:– Open-die drop-hammer forging– Impression-die drop forging

– Press forging– Upset forging

– Automatic hot forging– Roll forging

– Swaging

Open-Die Drop-Hammer Forging

• Open-die hammer forging is the same type of forging done by blacksmith. Metal is first heated to proper temperature by gas, oil, or electric furnaces.

• Impact delivered by some type of mechanical hammer like gravity drop or board hammer.

Operation on a Rectangular Bar

Blacksmiths use this process to reduce the thickness of bars by hammering the part on an anvil. Reduction in thickness is accompanied by barreling, as in Fig. 14.3c. (b) Reducing the diameter of a bar by open-die forging; note the movements of the dies and the workpiece. (c) The thickness of a ring being reduced by open-die forging.


• Steam or air hammers use pressure to :

– give higher striking velocities,

– more control of striking force, – easier automation,

– the ability to shape pieces up to several tons.


• Computer controlled hammers

– greatly increase the efficiency of the process

– minimize amount of noise and vibration

– Operator obtain desired shape by orienting and positioning work piece between blows.


• .

• Open-die forging is usually employed to pre-shape metal for further manufacturing operations for example consider such massive parts turbine rotors and generator shafts which may be 70 ft in length and up to 3 ft in diameter.

• Open-die forging is used to minimize the amount of subsequent machining.

• Open-die hammer forging (or smith forging) is:-

– simple and flexible process, – not practical for large-scale production. – It is slow operation– size and shape of resulting workpiece are

dependent on skill of operator.• Impression-die or closed-die forging overcomes

these difficulties by using shaped dies to control the flow of metal.

• Consist of set of dies, one half of which attaches to hammer and other half to anvil.

Impression-Die Drop-Hammer Forging

• Heated metal is positioned in lower cavity and

struck one or more blows by upper die.

• Hammering causes the metal to flow to completely

fill die cavity.

• Excess metal is squeezed out around the periphery

of the cavity to form a flash.

• When final forging is completed, flash is trimmed

off by trimming die.


• Accurate workpiece sizing is required since

complete filling of cavity must be assured with no

excess material.

• Major advantage is elimination of scrap generated

during flash formation.


Impression-Die Forging

(a) through (c) Stages in impression-die forging of a solid round billet. Note the formation of flash, which is excess metal that is subsequently trimmed off (d) Standard terminology for various features of a forging die.


• Final shape and size are set by additional forging in

the final or finisher impression

• The shape of the various cavities forces the metal to

flow in the desired direction.


Board hammers, steam hammers, and air

hammers are all used in impression die

forging.


–After forging, the flash is trimmed

–The part is quenched to room temperature.

Trimming Flash After Forging

Trimming flash from a forged part. Note that the thin material at the center is removed by punching.

Design of Impression-Die Forgings

• Forging dies are made of high-alloy or tool steel

– Are costly to design and construct

– Ability to withstand cycles of rapid heating and cooling

– Care is required to produce and maintain a smooth and accurate cavity.


• Better and economical results are obtained if following are observed:

1. Dies should be divided in a flat plane if possible. 2. Parting surface should be a plane through

center of forging 3. Adequate allowance should be provided-at

least 3° for aluminum and 5 to 7° for steel.4. Generous fillets and radii should be provided.5. Ribs should be low and wide.


6. Various sections should be balanced to avoid extreme temperature differences in metal flow.

7. Full advantage should be taken of material flow lines.

8. Dimensional tolerances should not be closer than necessary.

Design of Impression-Die Forgings • Computer-aided design has made notable advances and

development, expanded-memory computers has enabled accurate modeling of complex shapes.

• Good dimensional accuracy is one motivation for using impression-die forging. With care, these dimensions (for steel products) can be maintained within the tolerances.

•


Mass of Forging Minus Plus Lb kg in. mm in. mmI 0.45 0.006 0.15 0.018 0.482 0.91 0.008 0.20 0.024 0.615 2.27 0.010 0.25 0.03 0 0.7610 4.54 0.011 0.28 0.033 0.8420 9.07 0.013 0.33 0.039 0.9950 22.68 0.019 0.48 0.057 1.45100 45.36 0.07.9 0.74 0.087 2.21

Selection of a lubricant is also critical to successful forging. The lubricant not only affects the friction and wear and associated metal flow, but act as a thermal barrier (restricting heat flow from the workpiece to dies) and a parting compound (preventing part from sticking in cavities).

Press Forging

• Required when larger pieces or thicker products must be formed. Deformation is analyzed in terms of forces or pressures. Produce a more uniform deformation and flow.

• Problems can arise because of longer time of contact between the dies and work-piece.

Press Forging

• Heated dies are generally used to:-

–Reduce heat loss

–Promote surface flow

–Enable production of finer details and closer

tolerances

Press Forging

• Forging presses are of two basic types:-

–Mechanical presses

•Use means such as cams, cranks etc

•Because of their mechanical drives, production presses are capable of up to 50 strokes per minute

Press Forging

Hydraulic presses are :-–Slower–More massive–More costly to operate. –usually more flexible–Have greater capacity. –Can be programmed to have different strokes for

different operations and even different speeds within a stroke.

Press Forging

• Hydraulic presses with capacities up to 50,000 tons

(445 MN) are in operation in the United States.

• Press forgings have higher dimensional accuracy

and can often be completed in a single closing of

dies and the process can be readily automated.

Cost-per-piece in Forging

Figure 14.18 Typical (cost-per-piece) in forging; note how the setup and the tooling costs-per-piece decrease as the number of pieces forged increases if all pieces use the same die.

Upset Forging

• Upset-forging involves increasing diameter of material by compressing its length

• It is the most widely used of all forging processes. Parts can be upset forged both hot and cold on special high- speed machines

• Work-piece is rapidly moved from station to station.

Automatic Hot Forging

• Highly automated upset equipment in which mill-length steel bars (typically, 24 ft long) are fed into one end at room temperature and hot-forged products emerge from the other end at rates of up to 180 parts per minute( i.e. 86,400 parts per 8-hour shift).

• These parts can be solid or hollow, round or symmetrical, up to 12 Ib (6 kg) in weight, and up to 7 in. (180 mm) in diameter.


– Small parts can be produced at up to 180 parts per

minute, with rates for larger pieces on the order of 90

parts per minute

Automatic Hot Forging• The process has a number of attractive features.

– Low-cost input material – High production speeds – Minimum labor is required, and since no flash is

produced, – Material savings can be as much as 20 to 30%

over conventional forging.


• The benefits of the combined operations include

– high-volume production at low cost,

– Precision,

– Surface finish,

– Characteristic of a cold finished material.

Roll Forging

• In roll forging, round or flat bar stock is reduced in

thickness and increased in length to produce such

products as axles, tapered levers, and leaf springs.

• Roll forging is performed on machines that have

two cylindrical or semi-cylindrical rolls, each

containing one or more shaped grooves.

Roll Forging

• When the bar encounters a stop, the rolls rotate,

and the bar is progressively shaped as it is rolled

out.

• The piece can be reinserted between the next set of

grooves and the process repeated to produce the

desired size and shape.

Swaging

• Swaging generally involves the hammering of a rod or tube to reduce its diameter where the die itself acts as the hammer

• Term swaging is also applied to processes where material is forced into a confining die to reduce its diameter.

Swaging

Schematic illustration of the rotary-swaging process. (b) Forming internal profiles on a tubular workpiece by swaging. (c) A die-closing swaging machine showing forming of a stepped shaft. (d) Typical parts made by swaging. Source: Courtesy of J. Richard Industries.

Swaging with and without a Mandrel

(a) Swaging of tubes without a mandrel; note the increase in wall thickness in the die gap. (b) Swaging with a mandrel; note that the final wall thickness of the tube depends on the mandrel diameter. (c) Examples of cross-sections of tubes produced by swaging on shaped mandrels. Rifling (internal spiral grooves) in small gun barrels can be made by this process.

EXTRUSION• In the extrusion process, metal is compressed and

forced to flow through a suitably shaped die to form a product with reduced but constant cross section.

• Extrusion may be performed either hot or cold, hot extrusion is commonly employed for many metals

to reduce the forces required.

EXTRUSION

• Extrusion process is like squeezing toothpaste out of a tube. In the case of metals, a common arrangement is to have a heated billet placed inside a confining chamber. As ram continues to advance, pressure builds until material flows plastically through the die.

EXTRUSION

• Lead, copper, aluminum, magnesium, and alloys of these metals are commonly extruded, because of relatively low yield strengths and low hot-working temperatures.

• Steels, stainless steels, and nickel-based alloys are far more difficult to extrude.

EXTRUSION

• Almost any cross-sectional shape can be extruded from nonferrous metals..

Extrusions and Products Made from Extrusions

Extrusions and examples of products made by sectioning off extrusions. Source: Courtesy of Kaiser Aluminum.

EXTRUSION

• Extrusion has a number of attractive features.

• Extrusion dies can be relatively inexpensive, and one die only be required to produce a product. Conversion from one product to another requires only a single die change, so small quantities of a desired shape can be produced economically.

• Major limitation of process is a requirement that cross section be uniform for entire length of product.

• Extruded products have good dimensional precision.

EXTRUSION

• For most shapes, tolerances with a minimum of + 0.003in. are easily attainable.

• Grain structure is typical of other hot-worked metals, but strong directional properties (longitudinal versus transverse) are usually observed.

• Standard product lengths are about 20 to 24 ft, but lengths in excess of 40 feet have been produced.

Extrusion Methods

• Extrusions is produced by various techniques and equipment configurations. Hot extrusion is usually done by either the direct or indirect method

• Direct extrusion, a solid ram drives entire billet to and through a stationary die, must provide additional power to overcome frictional resistance between surface of moving billet and confining chamber.

Extrusion Methods• Indirect extrusion, a hollow ram drives die back

through a stationary, confined billet.

Direct-Extrusion

Schematic illustration of the direct-extrusion process.

Types of Extrusion

Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral;

Extrusion Methods

• Lubrication is another primary concern. • Lubricant applied to billet must thin considerably as

material passes through die. • At all stages of process, it will be expected to

function both as a lubricant and a barrier to heat transfer.

Extrusion of Hollow Shapes• Hollow shapes, and shapes with more than one

longitudinal cavity, can be extruded by several methods. For tubular products, the stationary or moving mandrel processes of are quite common.

• For products with multiple or more-complex cavities, a spider-mandrel die (also known as a porthole, bridge, or torpedo die) may be required.

•

Extrusion of Hollow Shapes

• Process is limited to materials that can be extruded without lubrication and that can easily be pressure welded.

• Hollow extrusions will obviously cost more than solid ones as additional tooling is required

• However, a wide variety of shapes can be produced that cannot be made economically by any other process.

Metal Flow in Extrusion

• The flow of metal during extrusion is often quite complex and some care must be exercised to prevent surface cracks, interior cracks, and other flow-related defects.

• Metal near center of chamber can often pass through die with little distortion, while metal near surface undergoes considerable shearing.

• In direct extrusion, friction between forward- moving billet and stationary chamber and die serves to further impede surface flow.

Metal Flow in Extrusion

• If surface regions of billet undergo excessive

cooling, the deformation is further impeded and

cracks tend to form on the product surface.

• If quality is to be maintained, process control must

be exercised in the areas of design, lubrication,

extrusion speed, and temperature.

Types of Metal Flow in Extrusion with Square Dies

Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction or in indirect extrusion. (b) Pattern obtained with high friction at the billet-chamber interfaces. (c) Pattern obtained at high friction or with coiling of the outer regions of the billet in the chamber. This type of pattern, observed in metals whose strength increases rapidly with decreasing temperature, leads to a defect known as pipe (or extrusion) defect.

HOT DRAWING OF SHEET AND PLATE

• Drawing is a plastic deformation process in which a flat sheet or plate is formed into a recessed, three-dimensional part with a depth more than several times the thickness of the metal.

• As a punch descends into a mating die (or the die moves upward over a mating punch), the metal assumes the desired configuration.

• Hot drawing is used for forming relatively thick-walled parts of simple geometries, usually cylindrical.

• Because the material is hot, there is often considerable thinning as it passes through the dies.


• In contrast, cold drawing uses relatively thin metal, changes thickness very little or not at all, and produces parts in a wide variety of shapes.

• A punch then descends, pushing metal through die, converting circular blank to a cylindrical cup.

• Height of cup walls is determined by difference between the diameter of original blank and diameter of punch.

• This dimension is limited by formation of several defects.


• Wrinkles can appear in cup walls as circumference is reduced, or punch can act as a piercing tool, tearing blank around punch perimeter.

• There are several means of producing parts with taller walls.

• If gap between punch and die is less than thickness of incoming material, cup wall is thinned and elongated simultaneously (a process often called ironing or wall ironing).

HOT DRAWING OF SHEET AND PLATE• If this thinning is objectionable, an intermediate

shape can be produced, with the further reduction in diameter (and concurrent increase in wall height) being taken in a subsequent redrawing with a smaller punch and die,

• Some drawn products are designed to utilize part of original disk as a flange around the lip of the cup.

• In this case the punch does not push the material completely through the die, but descends to a predetermined depth and then retracts.

• The partially drawn product is then ejected upward, and the perimeter of the remaining flange is trimmed to the desired size and shape

PIPE WELDING• Large quantities of steel pipe are made by two processes

that use hot forming of steel strip coupled with deformation-induced welding of its free edges.

• Both of these processes, utilize steel in the form of skelp-long strips with specified width, thickness and edge configuration.

• Because the skelp has been hot rolled previously and welding process produces further compressive working and recrystallization, pipe welded by these processes tends to be very uniform in quality.

PIPE WELDINGButt-Welded Pipe • In the butt-welding process for making pipe, steel

skelp is heated to a specified hot-working temperature by passing it through a furnace.

• Upon exiting the furnace, it is pulled through forming rolls that shape it into a cylinder and bring the free ends into contact.

• The pressure exerted between the edges of the skelp is sufficient to upset the metal and produce a welded seam.

• Additional sets of rollers then size and shape the pipe and it is cut to standard, preset lengths.

• Product diameters range from 1/8 in. (3 mm) to 3 in. (75 mm), and speeds can approach 500 ft/min.

PIPEWELDING

Lap-Welded Pipe• Lap-welding process for making pipe differs from butt-

welding technique in that skelp now has beveled edges and the rolls form the weld by forcing lapped edges down against a supported mandrel.

• This process is used primarily for larger sizes of pipe, from about 2 in. (50 mm) to 14 in. (400 mm) in diameter.

• Because product is driven over a supported mandrel, product length is limited to about 20 to 25 feet.

PIERCING

• Thick-walled seamless tubing can be made by rotary piercing,

• A heated billet is fed longitudinally into the gap between two large, convex-tapered rolls.

• These rolls are powered to rotate in the same direction, but axes of rolls are offset from axis of billet by about 6°, one to right and the other to left.

• Clearance between rolls is preset at a value less than diameter of billet.

PIERCING

• As billet is caught by rolls, it is simultaneously rotated and driven forward.

• The reduced clearance between rolls forces billet to deform into a rotating ellipse.

• Rotation of the elliptical section causes the metal to shear about the major axis.

• A crack tends to form down the center axis of billet, and cracked billet is then forced over a pointed mandrel that enlarges and shapes the opening to form a seamless tube.

PIERCING

• The result is a short length of thick-walled seamless

tubing, which can then be passed through a reeler

and sizing rolls to straighten it and reduce the

diameter and/or wall thickness.

• Seamless tubes can also be expanded in diameter

by passing them over a larger mandrel.

• As the diameter and circumference increase, the

walls correspondingly thin.

PIERCING

• Mannesmann mills used in hot piercing can produce

tubing upto 12 in. (300 mm) in diameter.

• Larger-diameter tubes can be produced on Stiefel

mills, which use same principle but replace convex

rolls of Mannesmann mill with larger-diameter

conical disks.

HOT- WORKING PROCESSES

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