HOT WORKING PROCESSES Shaping of metal by deformation

HOT- WORKING PROCESSES • Shaping of metal by deformation is a 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 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 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.

Rolling Temperatures • 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.

Rolling Temperatures • 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 oilfired soaking pits or furnaces are normally used. • Plain-carbon and low-alloy steels soaking temperature is approximately 22000 F (l 2000 C)

Rolling Temperatures • 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 2000 F (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 hotworking 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

Rolling Mill Configurations • 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

Rolling Mill Configurations • 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

Rolling Mill Configurations – 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-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.

Characteristics of Hot-Rolled Products • Substantial residual stresses can be induced during hot working.

Characteristics of Hot-Rolled Products • 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 Hbeam, may warp noticeably if a portion of one flange is cut away.

Characteristics of Hot-Rolled Products • As result of hot deformation and good control hotrolled 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.

Characteristics of Hot-Rolled Products • 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.

Thermo-mechanical Processing and Controlled Rolling • 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.

Thermo-mechanical Processing and Controlled Rolling • 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.

Thermo-mechanical Processing and Controlled Rolling 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 recrysta. Ilization 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 Blacksmiths use this process to reduce thickness of bars by hammering the part on an anvil. Reduction in thickness is accompanied by barreling, as in Fig. 14. 3 c. (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.

Open-Die Drop-Hammer 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.

Open-Die Drop-Hammer Forging • 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 Drop-Hammer Forging • 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.

Impression-Die Drop-Hammer Forging • 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.

Impression-Die Drop-Hammer Forging • 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.

Impression-Die Drop-Hammer Forging • 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.

Impression-Die Drop-Hammer Forging Board hammers, steam hammers, and air hammers are all used in impression die forging.

Impression-Die Drop-Hammer Forging – After forging, the flash is trimmed – The part is temperature. quenched to room

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.

Design of Impression-Die Forgings • 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.

Design of Impression-Die Forgings 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. •

Design of Impression-Die Forgings Mass of Forging Lb I 2 5 10 20 50 100 Minus kg 0. 45 0. 91 2. 27 4. 54 9. 07 22. 68 45. 36 in. 0. 006 0. 008 0. 010 0. 011 0. 013 0. 019 0. 07. 9 mm 0. 15 0. 20 0. 25 0. 28 0. 33 0. 48 0. 74 Plus in. 0. 018 0. 024 0. 033 0. 039 0. 057 0. 087 mm 0. 48 0. 61 0 0. 76 0. 84 0. 99 1. 45 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 costsper-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 milllength 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.

Automatic Hot Forging – 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.

Automatic Hot 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 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. 003 in. 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, threedimensional 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 forming relatively thickwalled parts of simple geometries, usually cylindrical. • Because the material is hot, there is often considerable thinning as it passes through the dies.

HOT DRAWING OF SHEET AND PLATE • 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.

HOT DRAWING OF SHEET AND PLATE • 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 skelplong 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 WELDING Butt-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 buttwelding 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.