Hot Extrusion Process and its Application for Steel
Hot Extrusion Process and its Application for Steel
Extrusion is a compressive deformation process in which a block of metal is squeezed through an orifice or die opening in order to get a reduction in diameter and increase in length of the metal block. The resultant product has the desired cross-section. Extrusion involves forming of axisymmetric (symmetrical about an axis) parts. Dies of circular on non-circular cross-section are used for extrusion. Extrusion normally involves high forming forces. Large hydrostatic stress in extrusion helps in the process by improving the ductility of the material.
Metals like aluminium, which are easily workable, can be extruded at room temperature. Other difficult to work metals are normally hot extruded or warm extruded. Both circular and non-circular parts can be obtained by extrusion. Channels, angles, rods, window frames, door frames, tubes, and aluminium fins are some of the extruded parts. Difficult to form materials such as steels, nickel alloys are extruded because of its inherent advantage, such as, no surface cracking because of reaction between the billet and the extrusion container. Extrusion results in better grain structure, better accuracy, and surface finish of the components. Less wastage of material in extrusion is another attractive feature of extrusion.
The applications of extrusions are constantly increasing because of the ability of the process to produce net bulk shapes in long lengths, frequently with complex cross sections. Depending on the alloy, extrusions serve the transportation, construction, mechanical, and electrical industries. Extrusions are used for durable goods, industrial equipment, heating and air conditioning applications, petroleum production, and the production of nuclear power. Practically all metals can be extruded, but extrudability varies with the deformation properties of the metal. Soft metals are easy to extrude. Hard (or high-strength) metals need higher billet temperatures and extruding pressures as well as higher-rated presses and dies.
A patent granted in 1797 by Joseph Bramah described a press in which Pb (lead) was forced through a die. This was the earliest consideration of the principle of extrusion which is hence be considered a modern process compared to other metal-forming processes like rolling and forging. Lead pipes were extruded in late 1700s in England. Later on, lead sheathing of electric cables was done by extrusion.
With the development of aluminum, which was commercially available in 1886, the extrusion process was established as an important industrial process. Today, extrusion is used in the manufacturing of several different products of different materials, but the major field of application is in the aluminum industry. In the production of complex shapes from aluminum billets, no other process can compete with extrusion.
The principle of extrusion is very simple. A billet is placed in a closed container and squeezed through a die by a ram. The design of the die opening determines the cross-section of the extruded product. When extruding tubes, a mandrel is inserted in the middle of the die. Unlike majority of the other deformation processes, all principal stresses are compressive during extrusion. Tensile stresses are only present in a small region at the exit of the die surface. When a material is plastically deformed under this state of multi-axial compression, very high strains can be reached since the workability is high at high hydrostatic pressure. The risk of metal rupture is reduced and materials, which crack in other processes, can be extruded without problems.
Extrusion, in the majority of cases, is a hot working operation but can also be carried out in cold mode. The working temperatures in hot extrusion are typically 0.7 TM to 0.9 TM, where TM is the melting temperature. This is higher than in forging and hot rolling which are normally carried out between 0.6 TM to 0.8 TM. Aluminum alloys are hot extruded at around 450 deg C to 500 deg C and steels at 1,100 deg C to 1,300 deg C. The strains involved in extrusion are large, frequently of order of 1, and the strain rate range is 0.1 per second to 102 per second.
Hot extrusion is a process in which wrought parts are formed by forcing a heated billet through a shaped die opening. As the name implies, the process is performed at high temperatures, which depend on the material being extruded. For steel the temperatures range from 1,100 deg C to 1,260 deg C. For hot working, the billet temperature is typically higher than that needed to sustain strain hardening during deformation. This is normally greater than 60 % of the absolute melting temperature of the metal.
The hot extrusion process is used to produce metal products of constant cross section, such as bars, solid and hollow sections, tubes, wires, and strips, from materials which cannot be formed by cold extrusion. Three basic categories of hot extrusion are non-lubricated, lubricated, and hydrostatic. These are schematically represented in Fig 1.
Fig 1 Three basic categories of hot extrusion process
In non-lubricated hot extrusion, the material flows by internal shear, and dead-metal zones are formed prior to the extrusion die. The sliding interface between the billet and tooling (container and die) is characterized as the condition of sticking friction, where the friction stress at the interface is the shear flow stress of the metal being extruded.
In lubricated extrusion, a suitable lubricant such as liquid glass or grease is between the extruded billet and the die and container. In this case, the sliding friction stress between the tooling and the workpiece is less than the shear flow stress of the workpiece and can be quantified as the product of the coefficient of friction and the stress normal to the tool surface.
In hydrostatic extrusion, a fluid film between the billet and the die exerts pressure on the deforming billet. The hydrostatic extrusion process is mainly used when conventional lubrication is inadequate, for example, in the extrusion of special alloys, super-conductors, composites, or clad materials. For all practical purposes, hydrostatic extrusion can be considered an extension of the lubricated hot extrusion process.
Non-lubricated hot extrusion method uses no lubrication on the billet, container, or die for the purpose of reducing friction stresses. Lubricants are, however, typically used to prevent the billet material from adhering to various tooling surfaces during the process (i.e., the ram or dummy block to the billet end). Care is to be taken such that lubricant is not introduced into the extruded product, because defects can result. However, this process has the ability to produce very complex sections with excellent surface finishes and low dimensional tolerances. Flat-face (shear-face) dies and hollow dies with flat shear faces are typically used in non-lubricated hot extrusion.
There are several different methods of extrusion but a characterization is frequently made with respect to the direction of the extrusion relative to the ram. Extrusion is classified normally into four types. They are (i) direct or forward extrusion, (ii) indirect or backward extrusion, (iii) impact extrusion, and (iv) hydrostatic extrusion. Fig 2 shows direct and indirect extrusion and idealized ram displacement load curves.
Fig 2 Direct and indirect extrusion and idealized ram displacement load curves
Extrusion is a discontinuous process and the second billet is not loaded until the first billet is extruded. During start-up of extrusion, the extrusion load increases as the material is forced to fill the container and flow out of the die. After the transient start-up phase the process is frequently considered to be steady-state. In reality, the process is never in a steady state phase since the contact conditions are changing and the temperature varies during the process. Steady-state can however be a good approximation if the friction is negligible and the temperature changes are small. The material flow has steady-state during the greater part of the extrusion process. When the billet has been extruded to a small discard there is high resistance to radial flow towards the centre and the load increases heavily. Extrusion is then interrupted. Depending on the method of extrusion, material and lubrication, considerable differences in flow behaviour can be observed during the process.
In extrusion process, the billet is placed in a container, pushed through the die opening using a ram and dummy block. Both ram and billet move. Direct extrusion and indirect extrusion are the two basic methods of working. The major difference between the methods is that there is no friction between the container and the billet surface in indirect extrusion. This means that the load needed for extrusion is decreased compared with the direct mode and the pressure is independent of the billet length. In spite of the advantages using the indirect mode, the direct process is more widely utilized. This is partly since extrusion presses for indirect extrusion are difficult to construct.
In direct or forward extrusion, the ram travels in the same direction as the extruded section, and there is relative movement between the billet and the container. The flow of material is in the same direction as the motion of the ram. In indirect or backward extrusion, the billet does not move relative to the container, and the die is pushed against the billet to form the part, and the ram which is used has a hollow shape.
Direct extrusion is a process in which is the billet moves along the same direction as the ram and punch do. Sliding of billet is against stationary container wall. Friction between the container and billet is high. As a result, higher forces are needed. A dummy block of slightly lower diameter than the billet diameter is used in order to prevent oxidation of the billet in hot extrusion. Hollow sections like tubes can be extruded by direct method, by using hollow billet and a mandrel attached to the dummy block.
Extrusion force, which is the force needed for extrusion, in direct extrusion, varies with the ram travel. Initially the billet gets compressed to the size of container, before getting extruded. Also, initially static friction exists between billet and container. As a result, the extrusion pressure or force increases steeply. Once the billet starts getting extruded, its length inside the container is reduced. Friction between billet and container now starts reducing. Hence, extrusion pressure reduces. The highest pressure at which extrusion starts is called break-through pressure.
At the end of the extrusion, the small quantity of material left in the container gets pulled into the die, making the billet hollow at centre. This is called pipe. Beyond pipe formation, the extrusion pressure rapidly increases, as the small size billet present offers higher resistance. As the length of the billet is increased, the corresponding extrusion pressure is also higher because of friction between container and billet. Hence, billet lengths beyond 5 times the diameter are not preferred in direct extrusion. Direct extrusion can be used for extruding solid circular or non-circular sections, and hollow sections such as tubes or cups. A typical sequence of operations for the direct extrusion of a solid section is as described below.
The heated billet and the dummy block are loaded into the container. When dummy blocks are affixed to the stem of the press main ram, they are called fixed dummy blocks. The billet is extruded by the force of the hydraulic ram being pushed against it. This upsets the billet and forces the metal to flow and assume the shape of the die. During extrusion, a thin shell of material is left on the container walls. Extrusion is halted prior to complete extrusion of the billet, leaving a portion of the billet (called the discard or butt) in the container. This is to avoid extrusion of the back end of the billet and billet skin into the part, since defects result. After this the container is retracted and separated from the die, while the butt is still attached to the die face. If the dummy block is not affixed to the ram stem, it can remain adhered to the butt until it is separated. The butt is sheared from the die face as discard. The die, container, and ram are returned to their initial loading positions.
Typical ram displacement versus applied press load curves for direct and indirect extrusion are shown in in Fig 2. This shows that the load in direct extrusion increases rapidly as the ram accelerates and the billet upsets to fill the container (zone A). Extrusion can commence prior to the maximum load being reached, whereas a further increase in load is related to the increase of extrusion speed to the set speed. A somewhat conical-shaped deformation zone develops in front of the die aperture, as illustrated in Fig 1.
After the maximum load has been reached and the ram speed is constant, the extrusion pressure decreases as the billet is extruded, and contact area with the container decreases, thereby decreasing friction work. Near the end of extrusion, the load can increase again because of a change in the conical deformation zone and stress state in the shortened billet (zone D in Fig 2). This occurs when the stem / dummy block gets close enough to the face of the die. Resistance to deformation increases considerably with decreasing butt thickness, and the metal is forced to take an increasingly radial flow path. The idealized curves in Fig 2 assume a relatively constant billet temperature and a constant ram speed after initial acceleration.
Indirect extrusion (backward extrusion) is a process in which the ram moves opposite to that of the billet. Here there is no relative motion between container and billet. Hence, there is less friction and hence reduced forces are needed for indirect extrusion. For extruding solid pieces, hollow ram is needed. In hollow extrusion, the material gets forced through the annular space between the solid ram and the container. The extrusion pressure for indirect extrusion is lower than that for direct extrusion. Several components are manufactured by combining direct and indirect extrusions. Indirect extrusion cannot be used for extruding long extrudes.
In indirect extrusion, the die is placed at the end of a fixed hollow ram stem. The billet, constrained by the container, is pushed onto the die / fixed stem, with the stem moving relative to the container. There is no relative motion between the billet and the container. As a result, there is no frictional stress at the billet / container interface. Hence, the extrusion load and the increase in temperature caused by friction are reduced, as shown in Fig 2. It is to be noted that the work depicted in Fig 2 is dissipated as heat, resulting in an increase in billet, tooling, and extrudate temperatures. The sequence of operations for indirect extrusion is described below.
The die is inserted into the press at the end of the fixed stem (Fig 2). The billet is loaded into the container. The billet is extruded, leaving a butt. The die and the butt are separated from the section.
Indirect extrusion offers a number of advantages. The maximum load relative to direct extrusion is lower by around 20 % to 30 %. Extrusion pressure is not a function of billet length, since there is no relative motion between the billet and the container. Hence, billet length is limited only by the length, strength, and stability of the hollow stem needed for a given container length, not by the load. No heat is produced by friction between the billet and the container, as a result, no temperature increase occurs at the billet surface. Hence, in indirect extrusion, there is a lower tendency to generate surface defects, and extrusion speeds can be considerably higher. The service life of the tooling can be increased, especially that of the inner container liner, because of reduced friction and temperatures.
The disadvantage of indirect extrusion is that impurities or defects on the billet surface (also known as inverse segregation) affect the quality of the extrusion, since they cannot be retained as a shell or discard in the container. As a result, machined or scalped billets are used in several cases. In addition, the cross-sectional area of the extrusion is limited by the size of the hollow stem.
In hydrostatic extrusion the container is filled with a fluid. Extrusion pressure is transmitted through the fluid to the billet. Friction is eliminated in this process since there is no contact between billet and container wall. Brittle materials can be extruded by this process. Highly brittle materials can be extruded into a pressure chamber. Higher reductions are possible by this method. Pressure involved in the process can be as high as 1,700 MPa. Pressure is limited by the strength of the container, ram, and die materials. Vegetable oils such as castor oil are used.
Normally hydrostatic extrusion process is carried out at room temperature. A couple of disadvantages of the process are the leakage of pressurized oil and uncontrolled speed of extrusion at exit, because of the release of stored energy by the oil. This can result in shock in the machinery. This issue is overcome by making the ram come into contact with the billet and reducing the quantity of oil through less clearance between billet and container. Hydrostatic extrusion is used for making aluminium or copper wires-especially for reducing their diameters. Ceramics can be extruded by this process. Cladding is another application of the process. Extrusion ratios from 20 (for steels) to as high as 200 (for aluminium) can be achieved in this process.
Impact extrusion is used for producing hollow sections such as cups, and toothpaste containers. The process is a variation of indirect extrusion. The ram is made to strike the slug at high speed by impact load. Tubes of small wall thickness can be produced. Normally metals like copper, aluminium, and lead are impact extruded using this method.
Aluminum alloys are typically extruded without lubrication, but alloy steels, stainless steels, and tool steels along with copper alloys, titanium alloys are extruded with a variety of graphite and glass-base lubricants. Commercial grease mixtures containing solid-film lubricants, such as graphite, frequently provide little or no thermal protection to the die. For this reason, die wear is considerable in the conventional hot extrusion of steels and titanium alloys.
The Sejournet process is the most commonly used for the extrusion of steels and titanium alloys. In this process, the heated billet is rolled over a bed of ground glass or is sprinkled with glass powder to provide a layer of low melting-temperature glass on the billet surface. Before the billet is inserted into the hot extrusion container, a suitable lubricating system is positioned immediately ahead of the die. This lubricating system can be a compacted glass pad, glass wool, or both. The pre-lubricated billet is quickly inserted into the container, along with the appropriate followers or a dummy block. The extrusion cycle is then started.
As a lubricant, glass shows unique characteristics, such as its ability to soften selectively during contact with the hot billet and, simultaneously, to insulate the hot billet material from the tooling. The tooling is normally maintained at a temperature which is considerably lower than that of the billet. In the extrusion of steel and titanium, the billet temperature is normally 1,000 deg C to 1,250 deg C, but the maximum temperature which the tooling can withstand is 500 deg C to 550 deg C. Hence, compatibility can be attained only by using the appropriate lubricants, insulative die coating, and ceramic die inserts and by designing dies to minimize tool wear. Glass lubricants have performed satisfactorily on a production basis in extruding long lengths.
The choice between grease and glass lubricants is based mainly on the extrusion temperature. At low temperatures, lubrication is used only to reduce friction. At moderate temperatures, there is also some insulation between the hot billet and the tooling from the use of partially molten lubricants and vapour formation in addition to the lubrication effect. At temperatures above 1,000 deg C, the thermal insulation of the tooling from overheating is of equal importance to the lubricating effect, particularly with difficult-to-extrude alloys. The lubrication film can also impede oxidation.
Lubricants can be classified into two groups, as per the temperature namely (i) below 1,000 deg C is grease lubrication, such as grease, graphite, molybdenum di-sulphide, mica, talc, soap, bentonite, asphalt, and plastics (e.g., high-temperature polyimides), (ii) above 1,000 deg C is glass lubrication, such as glass, basalt, and crystalline powder.
Metal flow in hot extrusion – Metal flow varies considerably during extrusion, depending on the material, the material / tool interface friction, and the shape of the section. Experimental methods have been used to detect the various flow patterns which exist in extrusion and the flow patterns have been classified into four categories namely S, A, B and C. The maximum uniformity of flow is seen in type S. The flow is frictionless, both at the container wall and at the die, and the deformation zone is localized directly in front of the die. This type of flow characterizes very effective lubrication, for example glass lubrication in steel extrusion, or indirect extrusion using a die lubricant. Flow pattern of type A is typical for lubricated extrusion of soft alloys such as lead and tin, while B is seen in most aluminum alloys.
For type A, B and C, an area of inactive material can be seen inside the container and close to the die. This material zone is called the dead metal zone and remains still throughout the whole process. If possible, lubrication in extrusion is normally avoided. If container lubrication not results in a completely homogenous material flow, the effect of lubrication is only damaging to the surface quality of the extruded product. This is frequently the case in aluminum extrusion, where the reduction in extrusion load because of lubrication does not compensate for the surface damage which occurs. The dead metal zone is in that case utilized to produce products with high surface quality. The design of the die is important, especially when aluminum shapes are extruded. Complex shapes frequently need very complex dies with portholes, channels and welding chambers. Fig 3 characterizes four types of flow patterns which are observed in the extrusion of metals.
Fig 3 Four types of flow patterns observed in the extrusion of metals
The flow pattern S depicted in Fig 3(a) represents the maximum flow uniformity of a homogeneous material in the container because of minimal friction. Plastic deformation is localized mainly in a zone just prior to the die. Most of the non-extruded billet in the container remains undeformed, resulting in the front of the billet moving evenly into the deformation zone.
Flow pattern A in Fig 3(b) occurs in homogeneous materials where negligible friction between the container and the billet exists but where considerable friction occurs at the surface of the die and its holder. This restricts radial flow of the peripheral zones and increases the quantity of shearing in this region, thereby resulting in a slightly larger dead-metal zone than that in flow pattern S. Flow patterns of this type are seldom observed in non-lubricated extrusion. Instead, they occur during the lubricated extrusion of soft metals and alloys, such as lead, tin, brasses, and tin bronzes, and during the extrusion of copper billets covered with oxide (which acts as a lubricant).
Flow pattern B shown in Fig 3(c) occurs in homogeneous materials if considerable friction exists at both the container wall and at the surfaces of the die and die holder. The material in the peripheral zones is restricted at the billet / container interface, resulting in a velocity gradient where the material in the centre flows at a higher speed. The shear zone between the restricted region at the surface and the centre material traveling at a higher velocity extends back axially into the billet to an extent which depends on the extrusion parameters and the alloy. This produces a large dead-metal zone. As extrusion commences, shear deformation is concentrated in the peripheral regions, however, as deformation proceeds, it extends toward the centre. This increases the undesirable possibility of material flowing from the billet surface (with impurities or lubricant) along the shear zone and migrating into or under the surface of the extrusion. Also, the dead-metal zone is not completely rigid, and this can influence the flow of the metal. Flow pattern B is found in homogeneous single-phase copper alloys which do not form a lubricating oxide skin and in most aluminum alloys.
Flow pattern C shown in Fig 3(d) occurs in the hot extrusion of in-homogeneous materials when the friction is high, as in flow pattern B, and / or when the flow stress of the material in the cooler peripheral regions of the billet is considerably higher than that in the centre. This results in the surface of the billet forming a relatively stiff shell. The conical dead metal zone is much larger than the other patterns, and it extends from the front of the billet to the back. Only the material inside the funnel is plastic at the beginning of extrusion. Severe plastic deformation, especially in the shear zone, occurs as the billet flows toward the die. The stiff shell and the dead-metal zone are in axial compression during extrusion. The displaced material of the outer regions flows to the back of the billet, where it migrates toward the centre and flows into the funnel.
This type of flow is typically found in the extrusion of brasses. This is a result of peripheral cooling of the billet, which leads to an increase in flow stress in that region. The increase is since a phase has a much higher flow stress than beta phase during hot deformation. Flow pattern C however occurs when there is a hard billet shell and high friction at the container wall. It can also occur if a considerable difference in flow stress exists from a high temperature difference between the billet and container. This is known to occur in the extrusion of tin and aluminum alloys.
Extrusion speeds and temperatures – The significance of temperature in hot extrusion is so high that some have coined the term thermal management to describe the practices by which it is controlled. The temperatures developed during extrusion influence considerably the speed at which the process can proceed. This is especially true in the extrusion of hard aluminum alloys. A complex thermal situation exists as soon as the heated billet is loaded into the preheated container and extrusion begins.
The temperature distribution in the billet is influenced by several factors, including those which generate heat and those which transfer it. Heat is generated by (i) deformation of the billet from its initial diameter to the extrudate size and shape, and (ii) friction or shear stresses at the interface between the billet and the extrusion tooling which includes the container, die, or dead metal zones, as schematically depicted in Fig 1. The distribution of temperature during the process is further influenced by (i) conduction heat transfer within the billet, (ii) conduction heat transfer between the billet and the tooling (container, die, ram), (iii) heat transported with the extruded product (also called the extrudate).
These phenomena occur simultaneously and result in a complex relationship between the material and process variables, such as billet alloy and initial temperature, friction condition, tooling material and temperature, extrusion speed, shape of the extruded section, and extrusion ratio. The extrusion ratio (R) is the ratio of area of cross-section of the billet to the area of cross-section of the extrude and is mathematically defined by the equation R = (cross-sectional area of the container) / [total cross-sectional area of the extrudate(s)]. Another parameter used in extrusion is shape factor, ratio of perimeter to the cross-section of the part. An extruded rod has the lowest shape factor. The production rate can be increased by altering the extrusion ratio and the extrusion speed while maintaining the extrusion pressure at an acceptable level.
For maintaining a reasonable extrusion pressure, the flow stress of the extruded material is to be kept relatively low. As an example, by increasing the initial billet temperature and / or lowering the strain rate by lowering extrusion speed. The combination of relatively high billet temperature, large extrusion ratio, and high extrusion speed can cause a considerable rise in temperature of the extruded material, especially near the section surface. This is since essentially all of the deformation work and frictional energy is transformed into heat, which has insufficient time to dissipate because of the rapid extrusion speed. This can cause surface defects or hot shortness, especially with difficult-to-extrude aluminum alloys.
With an extrusion ratio of 40 to 1, extrusion rates (exit speeds) can be of the order of 0.6 metre per minute (m/min) to 1.2 m/min. In contrast, a relatively low extrusion speed and low container / die temperature with respect to the billet temperature provide sufficient time for heat dissipation during extrusion and result in a reduction of extrudate temperature. This is depicted in Fig 4.
Fig 4 Extrusion exit temperature as a function of extruded length
The extrusion rate depends largely on the alloy flow stress, which in turn depends on the extrusion temperature and strain rate (apart from the alloy inherent properties). Exit speeds can be relatively high for soft alloys but are quite low for harder alloys. The evolution of extrusion temperature and temperature distribution during extrusion has been investigated in several studies. One theoretical analysis has been conducted to investigate the effect of ram speed on temperature increase. In this study, a billet of infinite length was assumed, container friction was neglected, and the interior of the container was assumed to be at the same temperature as the billet. The temperature of the billet varied along its length but was assumed to be constant at any cross section. The model predicted a sigmoidal relationship between the logarithm of ram speed and the temperature rise. Based on this model, a ram speed programme was devised which gave a constant extrudate temperature.
A constant extrusion exit temperature, which is referred to as isothermal extrusion, is desired for optimal quality, yet higher extrusion speeds (which can lead to extrudate temperature rise) are desired for increased productivity. This dichotomy has led to developments which allow the extruder to maximize extrusion speed while maintaining isothermal extrusion. A system for isothermal extrusion in which ram speed was varied to maintain extrudate temperature within required limits was presented by Laue in 1960. At that time, a 60 % time-savings was claimed for the extrusion of high-strength alloys, whereas more easily extrudable alloys result in a lower savings.
In the extrusion with a billet which is cooler at the back end than at the front (die) end, the basic premise is that as the billet is extruded, the exit temperature remains constant, since the decreasing temperature (or taper) in the billet offsets the heat being generated. Taper-heated billets are achieved in an induction heater with multiple zones which can heat different sections of the billet to different temperatures. Taper-quenched billets have been heated uniformly in a gas furnace and then partially water quenched to impose the taper on the billet.
Variable speed control technique needs a considerable improvement in the level of control over extrusion ram speed. Since exit temperature increases with ram speed, one can conversely decrease the exit temperature by decreasing the extrusion speed. Hence, in this method, extrusion speed is decreased during the cycle to offset the temperature rise because of the heat generation.
Cooling of the die and support tooling using liquid nitrogen during extrusion has been used to limit heating of the die and hence the extrusion temperature.
For container cooling, containers are available with more efficient cartridge heaters which are arranged into and controlled by different zones. Integral air-cooling passages are used to limit container heating and improve process temperature control.
Equalization of heat flow is an approach to achieve isothermal extrusion. This approach involves an energy balance equation to determine the billet and container temperatures and the extrusion speed when a uniformly heated billet is used.
Presses for extrusion – Both horizontal and vertical presses are used in the hot extrusion of metals. However, horizontal presses are by far the most common. Majority of modern extrusion presses are driven hydraulically, but mechanical drives are used in some applications, such as in the production of small tubes. The two basic types of hydraulic drives are direct and accumulator designs. In the past, accumulator presses were the most widely used type, but today, direct-drive hydraulic presses are used most extensively.
The hydraulic circuit of an accumulator-drive press consists of one or more air-over-water accumulators charged by high-pressure water pumps. The accumulator bottle (or bank of bottles) is designed to supply the quantity of water needed to provide the necessary pressure requirements throughout the extrusion stroke, with a pressure drop limited to around 10 %. Limiting this decrease in pressure is frequently critical in applications which involve marginal, difficult-to-extrude shapes. In addition to this pressure decrease characteristic of accumulator drives, the high cost of high-pressure water pumps, accumulators, and valves, as well as the substantial floor space requirements, have resulted in the extensive popularity of hydraulic direct drive presses. However, a considerable advantage of accumulator water drives is higher ram speeds (up to 380 millimetre per second, mm/s), which make these units desirable for the extrusion of steel. Water is also a non-flammable hydraulic medium, an important consideration in the extrusion of very hot billets.
Fig 5 show modern direct-drive oil-hydraulic presses for hot extrusion. The widespread use of these presses stems (in part) from the development of reliable, high-pressure, variable-delivery oil pumps, some of which operate at pressures over 34.5 MPa. Direct-drive presses are self-contained, and they need less floor space and are less expensive than accumulator-driven presses. More important, direct-drive presses do not show a reduction in the maximum available force during the entire extrusion cycle. A limitation of direct drive presses is that the stem speeds are slower than those in accumulator drives. Stem speeds to 50 mm/s are typical. However, speeds to 200 mm/s can be reached by using oil-accumulators with oil-hydraulic drives.
Fig 5 Types of extrusion presses
Modern extrusion presses include simplified hydraulic circuits to facilitate trouble-shooting, manifolds to reduce leakage and maintenance, and improved valves to minimize wear. Closed loop, constant-rate speed controls enable the consistent and repeatable production of smooth finishes and extrusion properties. In addition, the presses can be configured to operate faster for increased productivity. Solid-state programmable controllers have replaced magnetic relays on majority of the presses for increased versatility, simplified trouble-shooting, and ease of interfacing with computers.
The use of computers for interfacing, monitoring, and controlling presses and auxiliary equipment in a fully integrated extruding system instantaneously provides data on production rates, downtime, and inventory.
An extrusion press is rated by the maximum force which it is able to exert on the billet during extrusion, typically in units of metric tons, or mega-newtons (MN). The alloy, size, and shape of the part to be extruded dictate the press which is ideally be suited to produce the part. The work or energy which the press can impose on the billet is directly proportional to the extrusion force and inversely proportional to the square of the container diameter. In fact, the specific pressure rating of a press is defined as the maximum force rating divided by the cross-sectional area of the container. Some people just refer to this as the ram pressure, the pressure which is exerted to the back of the billet by the ram. The specific pressure is directly related to the work or energy available to extrude.
However, a higher-strength alloy, a decrease in part size, and an increase in part complexity all result in a higher energy requirement for extrusion. Hence, it is important to consider both the press rating and its container / billet diameter for a given product. A general rule of thumb is for the specific pressure rating to be no less than 690 MPa. However, production presses range from around 450 MPa to 1,035 MPa, depending on the process requirements.
The specific pressure needed for extrusion is a principal consideration in press selection, and this varies with several factors namely (i) the alloy flow stress at extrusion temperatures, (ii) the length of the billet (for direct extrusion), (iii) the complexity of the part cross section, (iv) the speed of extrusion, (v) the extrusion ratio, and (vi) the type of extrusion press (direct versus indirect).
The type of press, direct versus indirect determines whether friction between the container and billet exists during extrusion. With direct extrusion, which is the most widely used type, higher pressures are necessary at the beginning of the extrusion cycle because of maximum friction between the billet and container. Pressure requirements then decrease as extrusion progresses and the billet length decreases, thereby decreasing friction work. The pressure then increases again as the butt of the billet is reduced to a thickness of around 12.5 mm to 25 mm.
Longer butt lengths, however (typically to 75 mm) are sometimes used to prevent extrusion defects, hence also preventing this pressure rise from occurring at the end of the extrusion cycle. The advantages of using a press with sufficient specific pressure capacity are the ability to use lower billet temperatures and faster speeds and the ability to achieve improved metallurgical properties in the extruded products. A press having insufficient capacity can result in the inability to extrude or in extrusions of poor quality.
Press accessories – Different accessories are available as standard or optional items for hot extrusion presses, some of which are shown in Fig 5. These include (i) die slides or revolving die arms to facilitate die loading and changing, (ii) indexing containers and electrical heating elements to maintain the proper container temperature, (iii) piercing units and mandrel manipulators for the extrusion of tubes and hollow parts, (iv) internal or external billet loaders, (v) cut off shears and / or butt knockers for separating the butt from the die face / die entrance, and (vi) mechanized butt and dummy block handling systems.
Several pieces of ancillary equipment are vital to the success of the process. Prior to extrusion, the billet is heated in either a billet furnace or an electrical induction heater. The heated billet is then to be expediently transferred to and loaded into the press container. As the extruded shape exits the press, it typically undergoes a water quench to cool the metal as quickly as possible. This rapid forced cooling is particularly important for alloys which can be heat treated for increased strength and where solutionizing is done at the press to save time and energy costs. A puller system and runout table, which can be 30-metre (m) to 40 m long, is used to support and guide the product during extrusion. A stretcher is used to straighten the extruded product, and a cut off saw is used to cut parts to the needed length. A handling or conveyor system is used to transport the extruded product to stretching and cutting operations.
An extrusion line ultimately has a layout to meet certain specific functional needs. In contrast, the extrusion of majority of small tubular products and some small solid shapes are wound onto coilers or winders instead of onto a runout table. Recent developments in extrusion equipment involve the extrusion of precise curved structural shapes. In lieu of heating individual billets, as shown in Fig 6, the process of heating logs measuring 3.7 m to 6.1 m in length and then cutting them to the needed length as they emerge from the heater is sometimes done to eliminate the need to store billets of varying lengths. Log shears allow the operator to tailor billet lengths to provide maximum yield from each billet with minimal scrap. Computer control ensures that the logs are sheared to the optimal billet length for the particular die being used and for the desired extrusion length. A potential downside to this approach is that the sheared ends tend to be distorted, and this distortion can result in entrapped air which causes surface defects in some extrusion applications.
Fig 6 Lay out of a typical extrusion installation
Extrusion pullers eliminate twisting of the extruded products and ensure that equal-length extrusions are obtained from multiple-strand dies (dies from which more than one extrudate emerges). Modern extrusion pullers also result in fewer manipulations of the stretcher tail stock to accommodate unequal extrusion lengths, and the need to de-twist extruded shapes prior to stretching is virtually eliminated. In several cases, stretching needs only one operator, located at the head stock. Tail stock manipulation is controlled by the same operation. Several installations are equipped with completely programmed puller-stretcher combinations. Beyond the stretcher, automatic saw tables are typically configured to cut extrusions to the desired length prior to inspection, (automatic) stacking, and transfer to subsequent heat treatment, if needed.
Enclosed water-filled chambers have been provided at the ends of several presses which are used to extrude tubing. The tubing is extruded directly into the chamber and remains submerged for the full length of the runout table. A special gate prevents back flow through the dies, and an end crimper prevents water from filling the tube. The result of this arrangement is the production of tubing with a refined grain structure and consistent grain orientation.
Process control – Several of the improvements in the extrusion process during the last 25 years have been intimately linked to the advent of the electronics and computer age. The emergence of programmable logic controllers, micro-processors, and computer-based data acquisition as well as electronic sensors and transducers provided the tools which has led to huge improvements in quality and productivity.
The introduction of programmable logic controllers in the mid-1970s has led to wide acceptance of this technology during the 1980s to control the several operations of the extrusion process. This new technology has been quickly integrated with improved hydraulic systems and components by leaders in the industry. By the mid-1980s, a highly automated press line, operated by a three-man crew, became a reality. This endeavour no doubt paved the way for others and ultimately resulted in industry-wide improvements in product quality, process efficiencies, and hence a reduction in process cost.
Tooling – The tooling for hot extrusion consists of such components as containers and liners, stems, dummy blocks, mandrels, dies of different kinds, and their associated support tooling (i.e., holders, backers, and bolsters, etc.). Flat-face and shaped dies are the two most common types for solid profile extrusions. Flat-face dies (also termed shear dies or square dies) have one or more openings (apertures) which are similar in cross section to that of the desired extruded product. Dies for lubricated extrusion (which are also called shaped, converging, or streamlined dies) frequently have a conical entry opening with a circular cross section which changes progressively to the final extruded shape needed. Fig 7a shows this attribute as choke, which is an approach angle to the bearing.
Fig 7 Aperture details and desired locations for dies
Flat-face dies are normally easier to design and manufacture than shaped dies and are normally used for the hot extrusion of aluminum alloys. Shaped dies are more difficult and costly to design and manufacture, and they are normally used for the hot extrusion of steels, titanium alloys, high-strength aluminum alloys, and other metals.
Die design is a crucial aspect of the extrusion process which embodies engineering or applied science and craftsmanship. Optimal design is influenced by such factors as the size and shape to be produced, the maximum and minimum section thicknesses, press capacity, length of the run-out table, stretcher capacity, tool-stacking limitations, properties and characteristics of the metal to be extruded, and the press operating procedures.
Extrusion dies and tooling are machined from hot work tool steels, which are special alloy steels which maintain high strength at the high temperatures experienced during hot extrusion. Dies are typically made from AISI H13 steel hardened to 45 HRC to 50 HRC (Rockwell C scale). Flat-face dies are characterized by a bearing surface which is perpendicular to the face of the die. This is sometimes referred to as a shear edge. Dies with this feature are normally used for the most common aluminum extrusion alloys.
Dies to produce solid shapes are simpler in that they are machined from one piece of steel. The dies are multi-hole or multi-strand dies since they include more than one profile aperture, thereby resulting in the extrusion of several strands of product from one billet. The position of the die apertures within the die blank is to be judiciously chosen and normally follow some basic rules. Since the centre of the billet tends to flow faster than at the periphery, the centre of gravity of the section (aperture) is positioned closer to the periphery of the die blank. Metal also tends to flow faster through the larger sections of the aperture. As the metal exits the press, a guiding tool imposes the contour, and a robotic saw cuts the part, which is supported by a special run-out table. Hence, this approach is intended to balance the metal flow through the die to achieve as much flow uniformity as possible.
Fig 7b shows some desired locations for multi-strand dies for typical T-shaped, L-shaped, and U-shaped extrusions. Another desired need consistent with Fig 6b is that the axis of symmetry of the section coincides with a line which intersects the centre of the die. The apertures are also to be arranged as symmetrically as possible within the die blank. By dividing the die blank into segments, the centres of gravity of each aperture are to be placed on the centre of gravity of the segments. However, the container size and the number of strands is also to be considered in aperture location.
If the shape or profile to be produced is a hollow or tubular profile, the die falls into one of the three categories of hollow dies shown in Fig 8. To enter the hollow die, the heated metal is forced to flow around a bridge which supports a mandrel. The mandrel forms the inner surface of the profile, while the die opening (or plate) forms the outer surface. A solid-state weld is formed where the metal flows back together at the down-stream side of the bridge. This location is referred to as the weld chamber of the die. The bridge and weld chamber are to be designed to allow the metal to form a suitable weld and provide the die with sufficient strength. If properly done, the solid-state weld is undetectable in either appearance or performance. These dies are also used to produce shapes which are characterized as semi-hollow profiles, that are shapes which are not entirely enclosed.
Fig 8 Different styles of extrusion dyes for hollow profile
The primary objectives are to develop metal flow through the die apertures which are as uniform as possible within an aperture and from aperture to aperture. It is also important that the positions of the die apertures are located to prevent the extruded strands from contacting each other as they exit the press. It is also desired that a flat surface of the profile, and not an edge of a leg or rib, runs along the runout table.
In case of shaped dies, two basic types of metal flow occur during the extrusion of titanium and steel with lubrication. The first is parallel metal flow, in which the surface skin of the billet becomes the surface skin of the extrusion. The second is shear metal flow, in which the surface skin of the billet penetrates into the mass of the billet, and a stagnant zone of metal at the die shoulder is created. This stagnant zone is retained in the container as discard. Shear flow in these metals is undesirable, since it prevents effective lubrication of the die and can cause interior laminations and surface defects in the extruded product.
In extrusion with grease lubricants, the common practice is to use modified flat-face dies having a small angle (choke) and a radius at the die entry. In the extrusion process with glass lubricant, the die is to be designed not only to produce parallel metal flow but also to provide a reservoir of glass on the die face. A flat-face design with a generous radius at the entry into the die opening is normally used in this situation. During extrusion, the combination of the glass pad on the die and uniform metal flow produces nearly conical metal flow toward the die opening. Glass lubrication tends to produce a better surface finish and die life than grease and graphite. The issue with grease and graphite is also in maintaining sufficient lubrication over the full length of the extrusion.
Conical dies seem to perform similar to flat-face dies when glass lubrication is used. Laminar (metal) flow can be achieved with both die types. A disadvantage of conical dies with glass lubrication is the loss of much of the glass pad early in extrusion. With grease-based lubrication, shear-type flow can occur with both conical and flat-face die types. Conical dies do improve the metal flow but tend not to retain the glass for proper lubrication.
In the Sejournet process, it is normally assumed that the primary function of the die / glass pad is to lubricate the die. In a study conducted on extruding ‘T’ sections of steel, it has been determined that the glass pad placed in front of the die does not lubricate the surface of the extrusion and is not necessary to produce an acceptable surface finish. The function of the die / glass pad is to provide a smooth flow pattern for the billet material. If that is the case, then better extrusions can be achieved by streamlined dies, even without a glass pad.
It is interesting to note that in the optimized die / glass pad design, the quantity of glass used is very much reduced, and the design of the shape of the glass pad is primarily for providing streamlined flow. This conclusion, however, apparently does not apply to all situations. In extruding a complex thin H-section of tantalum alloy, better and more consistent results have been achieved with the conical H-dies than the modified flat dies.
Review of several studies shows that, basically, two types of dies are used for extruding steel and titanium namely (i) the flat-face die, or modified flat-face die with radius entry, and (ii) the conical-entry die. It seems that flat-face dies, or modified flat-face dies, are used with glass lubrication, with the glass pad forming the die contour at the entrance. The conical-entry die is mostly used with grease lubrication, although there is evidence, at least in extrusion of other high-strength alloys, that conical-contoured dies are also used with glass lubrication. It is also important that there is a uniform distribution of lubricant on the surface of the billet.
Design and manufacture – The design and manufacture of extrusion dies is supreme to the extrusion operation, and over the past several years, die designers have made full use of the latest in technology, including computer-aided design and computer-aided machining (CAD / CAM). Multi-axis computer numerical control (CNC) machining centres and CNC electrical discharge machines have all but eliminated time-consuming handwork and have greatly improved precision and decreased lead times. Designers today make use of the latest three-dimensional CAD software to design dies for the majority of the complex profiles.
In short, improvements in die design and manufacturing have allowed extruders to greatly improve what can be extruded, the level of quality, and productivity. Apart from improvements in steel quality and heat-treating practices, much progress has been made in improving die life. Majority of the dies today are nitrided, which is a process by which atomic nitrogen is adsorbed into the surface of the steel to produce a hard, wear-resistant surface. For smaller profiles where large quantities of product are needed, dies can be successfully coated with hard thin-film coatings applied by chemical vapour deposition (CVD) or physical vapour deposition (PVD). Such coatings are typically multi-layered and provide superior wear resistance, with claims of over 1,000 billets being made for a single CVD-coated die.
Since metals contract considerably on cooling from hot extrusion temperatures, an allowance is to be provided in the design of the dies. Thermal expansion of the die itself at extrusion temperatures is to be considered for the room-temperature dimensions of the die. Potential deformation of the die under high pressures can also need to be considered in die design. Another important consideration is the tendency for metal to flow faster through a larger opening than a smaller one within the same die. Compensation is to be made for this in the design of dies to be used in extruding certain sections. As an example, when a section to be extruded has a thick wall and a thin wall, different means are used to retard metal flow through the thick section and to increase the flow rate through the thin section of the die to equalize metal flow.
The geometry of the die aperture at the front and back of the bearing surface is termed the choke and relief, respectively (Fig 7a). A choke can be provided on certain portions of the bearing surface if the die designer anticipates difficulty in filling sharp corners or completing thin sections of the extruded product. This slows the rate of metal flow and consequently fills the die aperture. Increasing the quantity of back relief at the exit side of the bearing surface increases the rate of metal flow.
Tool materials – The hot extrusion of aluminum is similar in several ways to that of magnesium with the principal difference is the pressure needed. The same tool materials are frequently used for the extrusion of either aluminum or magnesium. The dies used for the extrusion of aluminum alloys and copper alloys are normally made from AISI H11, H12, or H13 tool steels. For the extrusion of copper alloys, some organizations specify tungsten hot work steels such as H14, H19, and H21. For the extrusion of steel, H13 solid dies or H13 dies with cast H21 inserts are frequently used.
Dummy blocks, backers, bolsters, and die rings are routinely made from H11, H12, and H13 tool steels. For the extrusion of copper, brass, and steel, H14, H19, and H21 are occasionally used. Nickel alloy 718 and other superalloys are sometimes used for dummy blocks, where use of these alloys frequently results in extremely long tool life. Mandrels are normally made of either H11 or H13, regardless of the material being extruded. Mandrel tips and inserts for the extrusion of aluminum are normally made of T1 or M2. Nickel alloy 718 mandrel tips and inserts are normally used in the extrusion of copper and brass, but H11, H12, H13, H19, or H21 tips and inserts can be used for the extrusion of steel.
Container liners used in extruding aluminum or steel are normally made of H11, H12, or H13. Liners for the extrusion of copper and brass are normally made of a nickel or iron-base superalloy. Ram stems are normally made of H11, H12, or H13. Containers for the extrusion of aluminum or copper products are normally made of 4140, 4150, or 4340 alloy steel. Containers for the extrusion of steel can also be made from alloy steels, however, H13 is normally preferred.
In addition to the above materials, special insert materials and surface treatments are routinely used for applications needing better resistance to wear at high temperatures. Special insert materials include special grades of cemented tungsten carbide (sub-micron grade), nickel-bonded titanium carbides, and alumina ceramics. Special surface treatments include nitriding, aluminide coating, and application of proprietary materials by vapour deposition or sputtering.
Extrusion of steel
The high melting point of iron alloys (pure iron, 1,535 deg C, density 7.9 grams per cubic centimetre) corresponds to a high recrystallization and hot-working temperature. Hot working normally is carried out in the austenite region, depending on the alloy, between 1,000 deg C and 1,300 deg C. This is associated with high tool wear because of the high thermal and mechanical stresses and, depending on the composition, a more or less severe oxide formation. As a result, it was relatively late before these alloys have been successfully extruded.
The oil graphite lubricants known from copper alloys, and which were used for steel extrusion around 1930, were not really suitable. They were initially replaced with mixtures of oil, graphite, and cooking salt, which could be effective only with very short contact times between the hot material and the shape-forming tooling. This method of lubrication is used only rarely today for the production of mild steel tubes on vertical mechanical presses.
It was not until 1950 that the metal forming process of extrusion was successfully applied to the steel industry. The possibility to extrude steel arose with the introduction of the Ugine-Sejournet process. Sejournet discovered that steels can be extruded if molten glass is used as lubrication. The molten glass not only protects the heated billet from oxidation and acts as a lubricant between the material and the tooling, but also acts as thermal insulation so that the die and container heat more slowly than with the lubricants previously used. It is now possible to produce alloy steel tubes and steel sections on horizontal extrusion presses with a lubricated container. Today the Ugine-Sejournet process is the most important method for steel extrusion.
Steels are extruded using the direct extrusion process with lubricated containers without a shell. With this process, it is possible to achieve short contact times with fast extrusion speeds, which is important for the necessary high extrusion temperatures. The use of conical dies results in a material flow in which the billet surface forms the surface of the extruded product.
The growth in the extrusion of steel in the 1950s and 1960s was followed by a continuous decline. The production of seamless tubes in mild steels and low-alloy structural steels on vertical presses has largely been replaced by more economic continuous rolling processes. Seamless tubes in these steel grades are now replaced whenever possible by the less-expensive longitudinally welded tubes.
Today, the extrusion of steel tubes and steel sections is used only when the material, the section shape, or the low volume required cannot be produced by other processes or only with significant expense. The reasons for extruding steel tubes are as below.
Crack-free production of long products even in materials which are difficult to hot work and that tend to crack during rolling
Production of small volumes. If unusual dimensions or materials are involved, then frequently the setting and operation of rolling processes designed for mass production is uneconomic. The tooling costs can also be very high. In contrast, extrusion can be viable for quantities as low as three billets.
Experimental or pilot production of tubes and sections which are going to be produced in large quantities more economically by rolling.
Around only 30 % of all steel tubes are produced as seamless and of these, less than 10 % are produced by extrusion. Extrusion process is used for three product groups namely (i) mild steel tubes, (ii) alloy steel tubes, and (ii) steel sections.
In the manufacturing of seamless tubes, direct extrusion is frequently used in combination with other processes like forging, piercing and rolling. Steel extrusion is performed at high temperature and associated with high thermal stresses in the tools, leading to wear problems. The extrusion speed is to be high since the billet loses heat rapidly. The exit speed is typically 1 meter per second (m/s) to 2 m/s for high alloyed steels.
In glass-lubricated extrusion, there is a layer of glass between the billet and the container, between the billet and the mandrel, and between the billet and the die. Each billet is heated to the extrusion temperature and then rolled in a powder of glass during transportation to the extrusion chamber. Glass powder is also applied inside the billet to assure good lubrication between billet and mandrel. Lubrication through the die is provided by a thick disc of compacted glass, the glass pad, which is placed between the billet and the die.
Glasses which are optimally suited for all steels with their different extrusion temperatures and flow resistances do not exist. Hence, glasses with different compositions and thus a different viscosity temperature dependence are desired for the various material groups. The criterion for the upper limit of the temperature application of glasses in extrusion is the so-called ‘hemi-spherical’ temperature. The viscosity is then around 20,000-poise. The lower limit is characterized by the compressive softening temperature (CST) where the glass is extremely tough (around 10 to the power 10 poise) but no longer breaks in a brittle manner. The extrusion temperature is to be at least 200 deg to 300 deg above the CST.
Standardized glasses are available from different manufacturers. Today only a few types are used in extrusion with SiO2 as the main constituent. They contain oxides of sodium, potassium, calcium, magnesium, aluminum, and boron. Barium oxide is also sometimes added. These additions lower the melting point and stabilize the glasses.
The billet from the preheating furnace is rolled over a sloping table covered in the glass powder and also has powder spread in the bore using a spoon. The powder immediately melts and protects the billet from further oxidation and can even dissolve any oxide which has formed. If the billet is not pierced before it is loaded into the press, the procedure for applying the glass powder has to be repeated (after the reheating). Different glasses are sometimes used for internal and external application.
The grain size of the glass also plays a role since fine grain-size powder melts more quickly than coarse grain. In front of the die, there is a pressed disc of the same glass powder or fibre glass (with water glass as the binding agent) which slowly melts during the extrusion process and encloses the front of the extrusion. The thickness of the glass layer is of the order of 10 micro-meters.
Careful matching of the type of glass, the extrusion temperature, and the extrusion speed, as well as the quantity of glass is important to produce a perfect glass, coating on the extrusion. If too little glass flows through the die, grooves form and if the quantity is too high, the surface of the section exhibits bulges or a so-called ‘orange peel surface’ corresponding to the individual grains.
During extrusion, the glass pad is pressed against the die by the hot metal. The glass pad deforms with the billet and melt progressively to surround the extrusion with a lubricant glass film. The principle of tube extrusion by the Ugine-Sejournet process is shown in Fig 9. The glass layer on the finished product is very thin and is easily removed after cooling.
Fig 9 Glass-lubricated steel tube extrusion
Attempts have been made to understand the film formation of the glass lubricant. A theoretical model of the lubrication mechanism has been proposed which include progressive melting, hydrodynamic flow, and stability of the film. Still, the behaviour of the glass lubricant, and especially the glass pad, during extrusion is not fully understood. A general observation, however, is that the metal flow is almost frictionless when glass is used as lubrication.
Compared to aluminum extrusion, no dead metal zone is observed in glass-lubricated extrusion. Another important lubricating effect of the glass is the thermal insulation which prevents the tooling from overheating. The main goal in tube extrusion is to manufacture consistent products with minimal dimensional variation. One particular dimensional issue is referred to as eccentricity, i.e., the hole in the extrudate is not centered along the centre-line of the billet outer diameter.
A high extrusion speed is possible for the majority of iron alloys. Depending on the material and the extrusion ratio, ram speeds in the range of 20 mm/s to 300 mm/s are achieved so that the billets are extruded within a few seconds. High extrusion speeds are necessary to keep the stresses on the tooling to a minimum. The tooling temperature is not to exceed 500 deg C. The melting properties of the glass used for lubrication prevents the use of the maximum extrusion speeds so that in practice, the minimum ram speed is 50 mm/s and the upper limit is 200 mm/s. High extrusion speeds can be achieved only with hydraulic accumulator drives. In the steel industry, direct oil operating systems are not used. The construction of the horizontal hydraulic tube extrusion presses is basically similar to those used for copper tube extrusion.
Some quantity of eccentricity is always produced when tubes are manufactured but the dimensional variations of the extrudate can be minimized, for example by tight control of process parameters and material flow in the process. In one of the studies, it has been proposed that the major causes of eccentricity in steel tubing are billet temperature gradients, billet preparation, equipment misalignment, and improper lubrication. Eccentricity can be because of either one or a combination of these variables. Good quality of the die is also essential to achieve tubes with tight dimensional tolerances and good surface quality. Since the die is subjected to very high temperature and pressure, a new die has to be inserted for each extrusion. The used dies can in most cases be grinded and reused.
By using hollow billet and a mandrel at the end of the ram, hollow sections such as tubes can be extruded to closer tolerances. The mandrel extends up to the entrance of the die. Clearance between the mandrel and die wall decides the wall thickness of the tube. The mandrel is made to travel along with the ram in order to make concentric tubes by extrusion.
Tubes can also be made using solid billet and using a piercing mandrel to produce the hollow. The piercing mandrel is made to move independently with the help of hydraulic press. It moves along with the ram coaxially. First the ram upsets the billet, keeping the mandrel withdrawn. Next the mandrel pierces the billet and ejects a plug of material from central. Then the ram and mandrel together are moved in and extrude the billet. Plug rolling and Mannesmann processes are the two methods of producing seamless tubes which are shown in Fig 10.
Fig 10 Extrusion of tubes
Port hole extrusion is another method of producing tubes and hollow sections in aluminium, magnesium etc. In this method, a die with a number of ports and a central mandrel supported by a bridge is used. The billet is squeezed through the ports and flows in separate streams. After the die section the extruded streams are joined together by welding in the welding chamber.
As with alloy steel tubes, for steel sections only horizontal hydraulic extrusion presses are used. The laminar extrusion takes place with glass lubrication and without a shell. In this case the competing processes, which are economically more competitive for suitable quantities and specific sections, again have a higher market share than the extrusion process. In principle, all steel grades which can be hot worked can be extruded to sections. However, the tool wear increases drastically with increasing extrusion temperature. The higher the flow stress at the extrusion temperature, the lower the possible extrusion ratio is and also, as a rule, the length of the extrusion. The processes used to produce sections are given below.
Hot rolling is in which the dimensional range falls within a circumscribing circle of 250 mm diameter. The possible weight per meter is 1 kilogram (kg) to 7 kg with minimum wall thickness of 3 mm. The minimum wall thickness tolerance is +/- 0.3 mm. In the hot-rolling process, cross-sectional undercuts are impossible and hollow sections cannot be produced. A relatively large tonnage is needed to justify the cost of the manufacture of the profiled roll pairs needed for the sequential tools.
In case of machining from solid material, the high material loss and the expensive process ensure that this process normally follows a non-machining deformation and only when other methods of producing the final shape have to be excluded.
Cold profile forming from steel strip is another process. This process needs material which can be cold bent and the section is required to have a uniform wall thickness. The final shape is produced from the flat sheet using several roll sets in the bending machine. The form-shaping tool pairs are expensive, so the process is economic only for large quantities.
Joining of part sections by longitudinal welding, riveting, or bolting in which the extruded sections are frequently used to produce more complex sections or larger cross-sectional areas.
In case of extrusion, the possible dimensional range falls within a circumscribing circle of around 250 mm diameter with a weight per meter of 1.5 kg/m to 100 kg/m and a wall thickness of at least 3.5 mm. The thickness tolerance which can be achieved is 0.5 mm. Complicated sections and also hollow sections can be produced. Obviously, given the severe thermal stressing of the form-producing tooling, the degree of complexity and the range of sections which can be produced cannot be compared with aluminum sections.
Sharp edges are impossible because of the risk of tooling failure and the thermo-mechanical localized stresses. External edges, hence, have a minimum radius of at least 1.5 mm and internal edges on hollow sections a minimum of 4 mm. Tolerances which are too wide for the application can frequently be reduced by subsequent cold drawing.
Extrusion is used even for sections which can be produced by hot rolling when it is not economical to produce the roll sets needed for the multi-stand mills because of the small volume needed. Extrusion can then be used for the production of prototypes and first series. Extrusion is also preferred for sections which can be more economically produced by welding but which cannot have any weld for safety reasons. The average lot size is 10 billets per order and is hence normally small. If larger quantities are needed, an alternative method of production is normally sought for cost reasons, e.g., hot rolling, even if the shapes have to be slightly modified and simplified. Of the steel sections produced in a section mill, only 8 % to 10 % are extruded. Around 65 % of sections are hot rolled and the others are produced using other processes.
Extrusion produces the same mechanical properties as hot rolling. In both processes the hot-working temperature is higher than the recrystallization temperature, and a fine-grain recrystallized structure is produced. Structural steels are processed along with heat treatable steels and alloy steels (stainless, heat resistant as well as tool grades), as well as nickel-base alloys and, more rarely, cobalt-base alloys and titanium alloys. The starting material for carbon steels is continuously cast and supplied in around 10 m lengths from the steel plant cleaned by pickling or shot blasting. The starting material for alloy steels has to be hot rolled or forged in the same way as for tube production for homogenizing and to achieve a fine-grain, crack-free structure. The extrusion process with a lubricated container and without a shell resembles which is used for tube production. The large billets used for heavy sections and, in the case of hollow sections, pre-bored billets also have to be chamfered on the end surfaces to achieve the uniform flow of the glass lubricant.
As soon as an order is passed to production, the bars delivered to billet production are crosscut, peeled, turned or ground and chamfered. The billets for hollow sections have also to be bored. The hole is larger than the circumscribing circle of the mandrel cross section. The billets are heated to 1,000 deg C to 1,300 deg C in a rotary hearth furnace with a reducing protective gas burner or in an induction furnace similar to steel tubes. Salt bath heating is also used. After the billets have been removed from the furnace, they are rolled down a table with glass powder. The glass film formed from the molten powder prevents high thermal losses and oxidation of the billet surface and can even dissolve the oxide that has formed. A 4 mm-thick glass powder disc bonded with water glass is placed in front of the conical die. The type of glass used is the same as for the extrusion of steel tubes.
The tube and solid extrusion presses used have a capacity of 15 MN to 25 MN. The diameter of the billets varies from 150 mm to 250 mm, depending on the section cross-sectional area, and the length can extend to 900 mm. Extrusion ratios up to 100 are possible but rarely used. Hollow sections are extruded over round or profiled mandrels which are not internally cooled and which move with the stem during the extrusion process. The thickness is to be at least 20 mm to be able to withstand the large thermal stresses.
It is not possible to use bridge dies as with aluminum and copper because of the high extrusion temperature, the relatively high flow stress, and the resultant thermo-mechanical stresses on the tooling. The billet loaded into the container is extruded to a discard length of 10 mm to 20 mm. After the container has been opened, the discard is cut from the section with a hot saw. The section is pulled back through the die and then removed on a powered roller conveyor. After pushing out the discard together with the dummy block, the die is changed for a new or reworked die using a rotating arm or a slide. The die has to be checked for the dimensional tolerances after each extrusion and, if necessary, reworked because of the high thermo-mechanical stresses.
The length of the section which is extruded as quickly as possible has a maximum length of 20 m so that the die which is deformed by the temperature effect during the extrusion does not exceed the extrusion tolerances toward the end of the extrusion. The needed high ram speeds of up to 300 mm/s can only be achieved by a water hydraulic system. The sections cool in free air but bend and twist considerably in the longitudinal direction and deform in the transverse direction because of the different flow behaviours of different cross-sectional areas in the die and the faster cooling of thin legs after extrusion. To avoid accidents from moving sections, the runout table is occasionally covered to form a tunnel. In multi-hole extrusion, the extruded sections normally have varying lengths because of the different quantities of lubricant in the die openings.
The extruded sections have to be straightened and de-twisted by stretching 2 % to 3 % on stretchers with rotating stretcher heads and capacities of up to 3000 kN. Hot stretching is also used for the higher-strength alloys (alloy steels, Ni, and Ti alloys). This stretching process is sufficient with carbon steels to break off the 10-micrometer to 20-micrometer thick glass film from the lubrication. Alloy steels, however, have to be pickled and sometimes first shot blasted. After stretching, it can be necessary to carry out further straightening on a roller correction machine or even on a straightening press with profiled tools. This straightening process adds considerably to the costs of producing steel extruded sections. If a coarse grain structure or, in the case of alloy steels, excessive mechanical properties are detected, a subsequent annealing treatment is needed. Tight tolerances are achieved by bright drawing. If necessary, sections can also be ground.