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Cold Extrusion Process for Steel

• satyendra
• June 19, 2024
• 1 Comment
• Backward extrusion, Cold extrusion, die, Ejector, extrusion, Extrusion pressure, Extrusion ratio, Forward extrusion, Punch, Slug, Tooling,

 Cold Extrusion Process for Steel

Cold extrusion of metals is one of the most important manufacturing processes of today and effectively involves forcing the material through a die at room temperature for creating a continuous product of consistent cross-section. Cold extrusion is so called since the billet / slug or preform enters the extrusion die at room temperature. During the process, however, the deforming material undergoes deformation heating (conversion of deformation work to heat) to several hundred degrees. Typically, a punch is used to apply pressure to the billet enclosed, partially or completely, in a stationary die.

Cold extrusion is a forging operation which has the typical advantages of material savings, work-hardening (strengthening), and grain flow or directional strengthening. Compared to other forging operations, cold extrusion is particularly attractive for several reasons such as dimensional precision, superior surface finish, net-shaped features, lower energy consumption, higher production rates, and cleaner work environment. Draw-backs of cold extrusion are higher loads, lubrication cost, limited deformation, and limited shape complexity.

Aluminum (Al) and Al alloys, copper (Cu) and Cu alloys, low-carbon (C) and medium-C steels, modified C steels, low-alloy steels, and stainless steels are the metals which are normally cold extruded. This listing is in the order of decreasing extrudability. The equipment and tooling are basically the same regardless of the metal being extruded.

Cold extrusion competes with such alternative metal-forming processes as cold heading, hot forging, hot extrusion, machining, and sometimes casting. Cold extrusion is used when the process is economically attractive because of (i) savings in material, (ii) reduction or elimination of machining and grinding operations, because of the good surface finish and dimensional accuracy of cold-extruded parts, and (iii) elimination of heat-treating operations, because of the increase in the mechanical properties of cold extruded parts. Cold extrusion is sometimes used to produce only a few parts of a certain type, but it is more commonly used for mass production because of the high cost of tools and equipment.

Cold extrusion involves backward (indirect), forward (direct), or combined backward and forward (indirect-direct) displacement of metal by plastic flow under steady, though not uniform, pressure. Backward displacement from a closed die is in the direction opposite to punch travel. Work-pieces are frequently cup-shaped and have wall thicknesses equal to the clearance between the punch and die. In forward extrusion, the work metal is forced in the direction of the punch travel. These two basic methods of extrusion are sometimes combined so that some of the work metal flows backward and some forward.

Based on the punch and die design and the resulting material flow, cold extrusion can be classified into three primary processes namely forward extrusion, backward extrusion, and lateral extrusion. In forward extrusion, the material flows in the same direction as the punch displacement. In forward extrusion, the diameter of a rod or tube is reduced by forcing it through an orifice in a die (Fig. 1a). In backward extrusion, the material flows in the opposite direction of the punch displacement. The billet is enclosed in a die and is forced to flow backward through the annular region between the punch and the die (Fig. 1b). Backward extrusion is differentiated from impact extrusion where typically a non-ferrous material is extruded backward by a rapidly moving punch and a shallow die with minimal material contact.

Forward and backward extrusion can also be simultaneously achieved through die design (Fig. 1c). In lateral extrusion, the material flows perpendicular to the direction of the punch displacement. The material is enclosed by the die and the punch and is forced through radially placed orifices (Fig. 1d). Hooker extrusion is a variation of the forward extrusion process where a tubular billet is forced through a forward extrusion die with a punch which acts as a pusher and a mandrel to reduce the outer diameter and elongate the tubular portion (Fig. 1e). Although not strictly a compressive forming operation, draw wiping or ironing is also normally used as a cold extrusion process. In ironing, the wall thickness of a tubular billet is reduced by forcing it under tension (and or compression) through a die similar to a forward extrusion die (Fig. 1f).

Fig 1 Displacement of metal in cold extrusion

Process limits – Free (open-die) forward extrusion of solid bars is limited to 30 % to 35 % reduction in area. However, if the billet is fully enclosed in a die (closed die), the reduction in area can be increased to 70 % to 75 %. Fig 1 shows the difference between open-die and closed-die forward extrusions. The ratio of length of forward extruded parts to diameter of the billet is normally limited to 8 to 1. In backward extrusion, the reduction ratio is to be kept between a minimum of 20 % to 25 % and a maximum of 70 % to 75 %. The maximum depth of extruded hole is limited to three times the hole diameter. In majority of the cases, the bottom thickness of the cup is to be equal to or higher than the wall thickness of the cup. In lateral extrusion, up to 60 % reduction in area is achievable. The width or the diameter of the extrudate has to be at least half the starting blank diameter. Draw ironing is normally limited to 30 %. However, if the tube is being pushed instead of being drawn this can be increased to 50 %.

Extrusion ratio – Extrusion ratio R is determined by dividing the original area undergoing deformation by the final deformed area of the work-piece (R = Ao/Af). Since volume remains constant during extrusion, the extrusion ratio can also be estimated by increase in length. An extrusion ratio of 4 to 1 indicates that the length has increased by around a factor of four. The metal being extruded has a large effect on the maximum ratio which is practical. Some typical approximate maximum extrusion ratios are 40 for aluminum alloy 1100, 5 for AISI 1018 steel and 3.5 for type 305 stainless steel and similar austenitic grades. Extrusion pressure increases with extrusion ratio. Figure 2 shows that extrusion ratio has a larger effect on ram pressure in the forward extrusion of C steel than either C content or type of annealing treatment.

Fig 2 Effect of C content, annealing treatment, and extrusion ratio

Extrusion pressure – In cold extrusion, a punch applies pressure to the slug or preform, causing the work-metal to flow in the needed direction. The relative motion between punch and die is achieved by attaching either one (almost always the die) to the stationary bed and the other to the reciprocating ram. The axis of the extrusion machine can be vertical or horizontal. The pressure can be applied rapidly as a sharp blow, as in a crank press or header (impact extrusion), or more slowly by a squeezing action, as in a hydraulic press. The pressure exerted by the punch can be as low as 34.5 MPa for softer metals or as high as 3,100 MPa for the extrusion of the alloy steel.

The punch pressure in extrusion depends on the flow stress of the material being extruded, the degree of deformation (strain), billet geometry, billet / die interface friction, and die design. Punch speed available in conventional presses has little effect on extrusion pressure. Punch speed, however, affects the extrusion process in other ways such as tool heating, lubricant deterioration, and dynamic loading. Hardness of the material is representative of its flow stress, and hence softer materials need lower extrusion pressure. Thermal softening processes of annealing and normalizing also reduce extrusion pressure.

Apart from hardness, the degree of deformation has the highest impact on extrusion pressure and is expressed as either extrusion ratio ‘R’ or reduction ratio ‘r’ [r = {(A0 – Af)/A0} x 100 where A0 is the initial cross-sectional area and Af is the final cross-sectional area]. Billet geometry factors e.g., length-to-diameter ratio are significant in forward extrusion. Die design aspects such as die entry angle (forward extrusion) and punch geometry (backward extrusion) also affect extrusion pressure.

Nomograms and empirical equations have traditionally been used for calculating the extrusion pressure. More recently, however, finite-element analysis (FEA) has provided another method for estimation of extrusion pressure especially for complex shapes. Fig 2 shows that extrusion pressure increases with extrusion ratio. It also shows that extrusion ratio has a larger effect on ram pressure in the forward extrusion of carbon steel than either carbon content or type of annealing treatment.

Work hardening of metals – Metals are work hardened when they are deformed at temperatures below their recrystallization temperatures. This can be an advantage, if the service requirements of a part allow its use in the as formed condition i.e., under some conditions, heat treatment is not needed. However, work-hardening raises the ratio of yield strength to tensile strength and lowers the ductility. Hence, when several severe cold extrusion operations follow one another, ductility is to be restored between operations by annealing. Any scale formed during annealing is to be removed by shot blasting or pickling before subsequent extrusion.

In spite of the high pressure applied to it, the metal being extruded is not compressed to any measurable quantity. Except for scale losses in annealing or the inadvertent formation of flash, constancy of volume through-out a sequence of operations is ensured. For all practical purposes, volumetric calculations can be based on the assumption that there is no loss of metal. Since an increase in extrusion ratio results in a corresponding increase in the quantity of cold deformation, the effects of work hardening normally vary directly with extrusion ratio.

Extrusion quality – C steel bars are available at additional cost in two classes of extrusion quality namely cold extrusion quality A and cold extrusion quality B. The mill preparation for cold extrusion quality A is the same as that used for special-quality bars and cold extrusion quality B is a still higher quality. Higher quality refers mainly to fewer external and internal defects. Hot scarfing and more rigorous inspection of the billets are additional operations which are performed at the mill for preparing cold extrusion quality B material.

Alloy steel without a quality extra is used in applications similar to those of cold extrusion quality A for C steel. Alloy steels are also available as cold-heading quality, which parallels cold extrusion quality B for C steel. Boron (B) modified steels for heading and extrusion are also available. The advisability of paying the additional cost for cold extrusion quality B or cold-heading quality steel depends on the severity of extrusion, the quality needs of the extruded part, and the cost of rejected parts in comparison with the extra cost for these steels.

Severity of extrusion refers mainly to the extrusion ratio. If the ratio is low and the work metal is kept under compression during flow, it is unlikely that cold extrusion quality B steel is beneficial. On the other hand, if the ratio is high or if the work metal is in tension at times during metal flow, cold extrusion quality B steel is to be considered. The cold extrusion of several parts involves both extrusion and upsetting. Upsetting is the more critical of the two operations, and the severity of the upset determines the quality of steel needed. The overall quality needs of the finished part are to be considered. Minor defects are sometimes acceptable in the finished part, or they can be removable in normal machining.

Steel for cold extrusion – Steel bars are available as normal hot rolled, precision hot rolled and cold finished. Normal hot-rolled bar is made to tolerances as per standards and is the least costly form of steel for making slugs. It is also likely to have deeper surface seams and higher depth of decarburized layers. In addition, the variation in the outside diameter of hot-rolled bars causes considerable variation in weight or volume of the slug, despite close control in cutting to length. Whether or not, the surface seams and decarburization can be tolerated depends largely on the severity of extrusion and the quality requirements of the extruded part. In several applications, acceptable extrusions can be produced with slugs cut from hot-rolled bars. Precision hot-rolled bars have 50 % better tolerances on size than normal hot-rolled bars and smaller decarburization layer. These bars are made by performing a special precision-sizing operation during hot rolling.

Cold-finished bars are made by taking the hot rolled bar through a costly series of cold-drawing steps to give them tighter dimensional tolerances (25 % of normal hot-rolled bar tolerances). Hence, the size variation in cold-finished bars is considerably less than that in hot-finished bars. However, some seams and decarburization are also present in cold-finished bars unless removed by grinding, turning, or other means. Some users gain the advantage of cold-drawn bars by passing hot-rolled bars or rods through a cold-drawing attachment directly ahead of the slug-cutting operation.

Turning, peeling, or grinding of cold-finished bars eliminate the difficulties caused by decarburization and seams. For some extrusions, especially those subjected to surface treatments which cannot tolerate a decarburized layer, previously machined bars or machined slugs are to be used. Another practice is to turn and burnish normal hot-rolled bars to remove surface defects. These practices are mandatory for precision net spline / gear forming or products needing induction hardening, which cannot withstand a decarburized surface.

C steels up to 0.3 % C can be easily cold extruded. High-C steels up to 0.5 % C can also be extruded, but the extrusion ratios are limited and spheroidize annealing can be needed. Backward extrusion normally needs spheroidize annealing for both low-C and high-C steels. Alloy steels are harder than their C steels and hence need higher pressures for extrusion. They also work-harden more rapidly, hence limiting their extrudability and intermediate annealing is frequently needed for restoring the extrudability.

Alloying elements differ in their effects on strength and hardenability. If possible, it is desirable to choose alloying elements so as to minimize strengthening while achieving the needed hardenability, e.g., boron (B) increases hardenability with minimal strengthening. Steels can be cold extruded without difficulty up to 0.35 % C. Free-cutting resulphurized steels also have lower forgeability than their C steel, since they are more susceptible to rupture during cold extrusion because of their higher occurrence of sulphide inclusions.

Sulphur (S) is typically limited to 0.02 %. Low-C resulphurized steels can be extruded, if care is taken to keep metal in compression throughout the process. Internal purity of steel is critical in cold extrusion especially at high extrusion ratios. Central segregations increase the tendency for internal fracture along the axis of the extrusion (chevrons). Killed steels are specified for cold extrusion for ensuring homogeneous structure. Al killed steel is preferred over silicon (Si) killed steel for difficult extrusions because of the reduction in strain hardening and reduction in strain aging achieved. Si is kept in the low range of 0.2 %. Si killed steels, however, have better surface quality, which can be critical in any post extrusion operations.

Seams, laps, and scratches on steel surface can be tolerated up to 1 % of the bar diameter if cold extrusions are machined at the surface. However, if net-shaped features are being cold extruded, then the seams and laps have to be removed prior to extrusion by peeling or turning the steel bars.

Although cold extrusion is a compressive process, it is typically preceded or followed by processes of cold heading and heat treatments, which need defect-free surfaces. The steel producer frequently certifies steels meeting the cold extrusion requirements as ‘cold extrusion quality’ or ‘cold working quality’. For ensuring surface quality of steel, hot-scarfing during semi-finished state (blooms) and eddy-current testing in the finished state (bars) is sometimes needed.

Effect of composition and condition on extrudability of steel – The extrudability of steel decreases with increasing C or alloy content. Extrudability is also adversely affected by higher hardness. Free-machining additives, such as S or lead (Pb), are likely to weaken extrudability. Non-metallic inclusions, particularly the silicate type, are also detrimental to extrudability.

Carbon content – The cold extrusion of steels containing up to 0.45 % C is a normal practice, and steels with even higher C contents have been successfully extruded. However, it is advisable to use steels of the lowest C content which meets service needs. Majority of C and alloy steels which are extruded contain 0.1 % C to 0.25 % C. However, in some applications, steels with more than 0.45 % (especially alloy steels) are cold extruded. Fig 2 shows the results of studies carried out in one plant for determining the effects of C content, type of annealed structure, and extrusion ratio on the ram pressure needed to forward extrude a specific shape from C steels. These data show that ram pressures are essentially the same for steels containing 0.19 % C and 0.26 % C, regardless of the other variables, but that ram pressure is markedly increased as C content reaches 0.34 % and 0.38 %. The steel slugs (Fig 2) are coated with zinc stearate (C36H70O4Zn) over zinc phosphate [Zn3(PO4)2] and are extruded under laboratory conditions at a rate of 635 mm/min.

Alloy content – For a given C content, majority of alloy steels are harder than plain C steels and are hence more difficult to extrude. Majority of the alloy steels also work harden more rapidly than their C steel counterparts, hence, they sometimes need intermediate annealing.

Hardness – The softer a steel, the easier it is to extrude. Steels which have been spheroidize annealed are in their softest condition and are hence preferred for extrusion. Fig 2 shows that spheroidized steels are extruded at lower ram pressures than hot-rolled or mill-annealed steels, regardless of other variables. The ram pressure is to be increased as tensile strength increases for steels of low-C to medium-C content at three extrusion ratios. However, operations which precede or follow extrusion can make it impractical to have the steel in its softest condition.

Extremely soft steels of low-C to medium-C content have poor shearability and machinability, hence, some extrudability is occasionally sacrificed. Annealing techniques which produce a partly pearlitic structure are ideal for several extrusion applications in which shearability or machinability is important. Free-machining steels, containing such additives as Pb and S, are not preferred for cold extrusion. Extrusions from these steels are more susceptible to defects than extrusions from the equivalent non-free-machining steels. In addition, since parts produced by cold extrusion normally need only minimal machining (this is frequently the main reason for using cold extrusion), there is much less need for free-machining additives than when parts are produced entirely by machining.

The successful extrusion of free-machining steels depends on the quantity of upset, the flow of metal during extrusion, and the quality needs of the extruded part. Free-machining steels can normally withstand only the mildest upset without developing defects. If it is under compression at all times during flow, a free-machining steel probably extrudes without defects. However, rupture is likely, if compressive force is suddenly changed to tensile force.

Non-metallic inclusions – The fewer the non-metallic inclusions, the more desirable the steel is for cold extrusion. Silicate inclusions have been found to be very harmful. Hence, some steels have been deoxidized with Al rather than Si in an attempt to keep the number of silicate inclusions at the minimum. The Al-killed steels have better extrudability in severe applications.

Equipment – Hydraulic presses, mechanical presses, special knuckle-joint presses for cold extrusion, special cold-forging machines, and cold-heading machines are used for cold extrusion. Majority of cold extrusion operations are performed on mechanical presses or cold-heading machines. Of the two, mechanical presses are used more frequently, because of their adaptability to other types of operations. Hydraulic presses and mechanical presses are specifically designed for cold extrusion with high rigidity, accurate alignment, and long working strokes. Mechanical presses are preferred because of lower maintenance and higher production rates. Mechanical presses also need higher investment and hence are preferred for large production volumes and large batch sizes. Hydraulic presses represent only a small fraction of the total number of presses used for cold extrusion. However, hydraulic presses are especially well suited for the production of parts needing long working strokes. Proper selection of the press is important for successful cold extrusion and for the prevention of excessive maintenance charges. Hydraulic presses are typically vertical, less complex, more versatile, and have longer work strokes than mechanical presses and are normally selected for long or large extrusions. They also provide full-rated tonnage throughout the stroke. Hydraulic presses operate at lower speeds and are less suitable for automation than mechanical presses, hence, they are typically used for lower production volumes.

Mechanical presses are normally more costly and are capable of higher speeds than hydraulic presses of similar capacity. A disadvantage of a mechanical press is its limited length of stroke. A cold-heading machine combines the essential features of a mechanical press with mechanisms which feed in bar stock, shear slugs, and transfer the slugs to the die and then to other dies if needed. Mechanical presses are needed to have (i) sufficient flywheel energy (insufficient energy results in overloading and heating of the motor, as well as parts which are incompletely formed), (ii) sufficient torque capacity in the drive mechanism for delivering the necessary force at the needed point above the bottom of the stroke, and (iii) rigid structural members for preventing excessive deflection under concentrated loading.

Horizontal mechanical presses with bar or coil feeds with multiple stations and integrated billet shearing are used for small forgings. These presses are capable of applying loads up to 1,000 tons and produce up to 150 parts per minute (parts/min). Vertical mechanical presses can be single station or multi-station and are typically used to make larger extrusions with loads around 1,000 tons to 2,000 tons at 25 parts/min. For annual production volumes of more than 500,000 parts, these vertical presses are typically automated with loading and transfer systems.

The drive mechanisms on mechanical presses also vary e.g., rank, knuckle, link, and eccentric. Knuckle-joint presses offer lower and more constant velocities during work stroke than crank-drive presses, reducing dynamic loads. However, the working stroke and loads available above bottom dead centre with knuckle drives are lower than with a crank press. Link-drive presses have similar forming velocities to knuckle-joint presses and have longer strokes. Eccentric presses fill the gap between knuckle and crank presses. It is important to analyze the force-displacement curve of the press and of the process for ensuring that sufficient deformation energy is available during the cycle. In multi-station transfer presses, careful study of time-displacement curves of the press, transfer, part, and ejector is necessary for ensuring proper transfer.

Power needs – Because of work metal and tool variables, data resulting from laboratory studies of power needs for cold extrusion are normally not applicable to shop practice. The rules which can be used as guidelines in estimating pressure, force, and power needs are as described here. The first is to determine the effective contact area of the forming tool. In backward extrusion, this area is the cross-sectional area of the punch tip and for forward extrusion, the effective contact area is the annular area of the die shoulder. The second is to determine the extrusion ratio and ascertain that the ratio is within practical limits. The third is to consider the tool materials used. Properly supported punches and dies made of tool steel can be operated at peak pressures as high as 2,400 MPa, carbide punches can be operated at peak pressures to 2,750 MPa, and carbide dies at 3,100 MPa. Peak extrusion forces can be safety estimated as the product of effective contact area (as determined in the first item in this list) and peak allowable stress (as indicated in the third item in this list). The condition of the press equipment, tools, and work material, the design of the tools, and the lubricant used, all affect the maximum extrusion ratio obtainable in a particular operation. The fourth is the energy needed which is calculated as the product of extrusion force and distance over which it is to act to form the part. The power needed can be calculated from this energy and the frequency at which the energy is to be delivered. The fifth is that at operating speed, flywheel energy is to be 4 times to 10 times which is needed per stroke for extrusion, the exact multiple depends on cycle time and type of motor.

Power needs can be estimated on the basis of extrusion ratio. Other methods for determining power needs, are normally more complex, and consider the influence of several interrelated variables, including the properties of the steel to be extruded, the size and shape of the part, the thickness of the wall to be produced (or reduction of area), the temperature, the effect of lubrication, the blank shape and thickness, and the grain size and orientation.

Estimation of load – Knowledge of the forces or pressures needed for forward extrusion or backward extrusion is necessary in the design for determining tool stresses and for selecting suitable press equipment. The pressure to be applied is a function of the deformation resistance and degree of deformation. The deformation resistance, in turn, is affected by (i) the composition, mechanical properties, and condition of the work material, (ii) the external frictional forces applied, and (iii) the size and shape of both the initial slug and the finished work-piece. Practical experience has shown that for the tool steels and carbides presently in use, the specific forming pressure at the punch is not to exceed around 2,400 MPa and the die internal pressure is not to exceed around 1,900 MPa. If the estimated pressures exceed these limits, either the degree of deformation is to be reduced or a considerably shorter tool life is to be accepted.

Tooling – Knowledge of the forces acting on tool components is not always a matter of certainty. Tool design is critical in cold extrusion not only for the success of the process, but also for the safety of the operator. The design of tools is more frequently dictated by the dimensions of the part to be formed. Other factors which are also to be considered in tool design are alignment, excess volume, friction (load, heating, and wear), lubricant availability, balanced metal flow, uniform metal flow velocity, ease of assembly, stress concentration, load distribution, and elastic deflection.

Cold extrusion tooling can be separated into consumable and non-consumable. The consumable toolings are typically in direct contact with metal flow and are highly stressed. These include punches, forming dies, guide sleeves, and mandrels. The non-consumable toolings are used to support the consumable tooling and are not directly exposed to the extrusion pressure. These include shrink rings, back-up plates, spacers, and retainer rings. Non-consumable toolings are designed to be flexible and are used across different tooling set-ups, whereas consumable tooling tends to be part specific.

Although several engineering components are, or can be, designed to last indefinitely, this is seldom true in the design of highly stressed, consumable tools for cold extrusion in which a tool life of 100,000 pieces is likely to be considered above average. On the other hand, conventional design criteria are applicable to the less highly stressed, non-consumable tools for extrusion. Hence, it is convenient to distinguish between consumable tooling components, such as punches and dies, and non-consumable ones, such as shrink rings and pressure pads.

The consumable tools (punch, die, and ejector) make direct contact with the steel to be extruded. These tools are exposed to a specific load and to wear. Their design is required to incorporate features which conforms to the design needs of the work-piece while minimizing specific load and wear. It is normally possible to design tools which satisfy both objectives by facilitating the flow of metal and reducing losses because of the internal and external friction.

Tool assembly components – The components of a typical tool assembly used for the backward extrusion of steel parts are identified in Fig 3a. There is considerable variation in the tooling practice and design details of tool assembly components. Some of the principal factors affecting the design of punches and dies for backward extrusion and for forward extrusion are described below.

Fig 3 Tools and tools components for cold extrusion

Tooling set-ups – Metals can be cold extruded by different tooling set-ups, depending mainly on the size and shape of the work-piece, the composition of the work metal, and the quantity requirements. The principal types of tooling used and examples of products formed by each type are described here. Single-station tooling forms the part in one stroke of the press. Additional operations can be needed for finishing. Closed-end containers, such as tooth-paste tubes, are formed in this manner. Multiple-station tooling involves a series of separate dies arranged so that the rough blank is made into a preform, which then proceeds through successive operations until the needed form is produced.

Multiple-station tooling is frequently used for semi-continuous operations because of the need for annealing, pickling, and lubrication between operations, although it is also adaptable for continuous operations which use a transfer mechanism. This procedure has also been used in the cold forming of 75 mm and 155 mm shell bodies involving backward extrusion and forward extrusion.

Transfer presses are similar in concept to multiple-station tooling, i.e., they can perform several operations in succession, e.g., a transfer press can shear, preform, extrude, and finish draw the part in consecutive operations. Mechanical fingers transfer the work-piece from one operation to the next. Pole pieces for alternator rotors have been produced in transfer presses.

Up-setters or headers are used for continuous operation, frequently incorporating both backward and forward extrusion and cold heading. Fasteners such as hexagonal socket-head cap screws are typical examples of parts produced in up-setters. Rotating dial or indexing can be applied for manual or automatic production. In operation, the table of the press holding the dies indexes, and the head containing the punches remains stationary except for vertical movement. Slugs can be fed automatically, and one or more parts can be formed with each stroke of the press. Instrumentation stops the operation immediately in the event of misalignment, punch breakage, or a wrong-size slug. Gear extrusions are representative examples of parts produced in this type of tooling, at the rate of two extrusions for each press stroke.

Tool materials – Recommended materials for extrusion punches include M2 and M4 high-speed tool steels and tungsten carbide (WC). Tool steel punches are to be heat treated to a hardness of 62 HRC (Rockwell hardness C-scale) to 66 HRC, and they are to have a high compressive yield strength. Die inserts are normally fabricated from such alloy tool steels as D2, M2, and M4, and are heat treated to 58 HRC to 64 HRC, depending on the steel.

Tungsten carbide is also used when high loads and stiffness are needed. It is extensively used since it provides good die life, high production rates, and good dimensional control. Tungsten carbide frequently finds application as a punch material in backward extrusion. Retainer rings or housings used for tungsten carbide dies are required to have sufficient strength and toughness for preventing splitting and failure of the working tools. Shrink rings are to be fabricated from hot-work die steels such as H11 or H13 heat treated to 46 HRC to 48 HRC. Outer housings are frequently made from H13 die steel or from 4340 alloy steel.

Punch design – A major issue problem in punch design consists of assessing the nature and magnitude of the stresses to which the punch is subjected in service. Since the stresses are dynamic, fatigue effects arise, and these fatigue effects, in conjunction with the inherently brittle nature of hardened tool steels, necessitate care in avoiding design features likely to produce stress concentrations. The stability issues which can arise when slender punches are used, are affected by the accuracy of alignment provided by the tool set or the press itself, or by factors in the extrusion operation, such as punch wander, initial centering, and use of distorted slugs.

The backward extrusion punch (Fig 3b) is subjected to high pressures approaching 3,000 MPa. At these high loads, it is also important to limit the effective length-to-diameter ratio of the extruded hole to around 3 to 1 for rigidity and since it also affects stability. The punch nose contour controls the metering of the lubricant during the process. The design of the punch nose has a considerable effect on extrusion pressures and tool life. In backward extrusion acceptable results are achieved with a nose profile consisting of a truncated cone having an included angle of 170-degree to 180-degree, with an edge radius of 0.5 mm to 2.5 mm, and a land length of 1.25 mm to 1.9 mm with the shank relieved 0.1 mm to 0.2 mm on the diameter. Although these values reduce initial punch stresses, small cone angles or large radii are undesirable, because of rapid lubricant depletion and the risk of metal-to-metal contact.

Design of the punch nose to distribute the lubricant properly during extrusion is necessary for minimizing the pressures developed. The area ratio between punch shank and head is also an important design factor. A large ratio has the effect of spreading the punch load over a large area of pressure pad. On the other hand, it needs a wider block of metal for its fabrication with a resultant cost increase. Since pressure pads are less expensive than punches, it is normally advisable to favour the smaller ratios. The pressure pad, which transmits the load from the back of the punch to the die set, are to be designed for economy, ease of replacement, and efficiency in reducing the number of punch failures.

A hemi-spherical nosed punch, although desirable from a load point of view, results in rapid depletion of the lubricant. A tapered punch nose with a 170-degree included angle is found to be optimal for controlling the lubricant escape for avoiding lubricant depletion before the end of the process and preventing punch-splitting failures. The edge radius of the punch controls the material overshoot as it negotiates the corner. This overshoot affects the extruded diameter and the tendency to form folds. Typically, a water-fall radius of 5 % of bearing land diameter is used. The bearing land of the punch is to be minimal for reducing friction, yet long enough to impart dimensional control to the extruded diameter. The normal practice is to use 1.5 mm to 4.5 mm long land. The punch stem and shoulder are to be designed with gradual angles and radii for decreasing stress concentration. Surface finish on the punch is also critical. Grinding marks are to be removed, and working surfaces are to be lapped in the direction of metal flow.

A 0.125 micrometer finish has been found to be satisfactory. In forward extrusion, the punch design is simpler since the stress seen is much lower and the metal flow against the punch is minimal. Issues of stress concentration and strength have to be considered, and the punch nose is not as critical as in backward extrusion. It is typically flat with a bearing land of 3.25 mm and diameter which is very close to the bore of the die (0.025 mm to 0.125 mm, or smaller) for preventing metal squirt between the punch and die. The closing-in of the die bore when the die is assembled in shrink rings, is to be considered when designing this diameter.

Die design – The high forming pressure in cold extrusion leads to high hoop stress in the cylindrical dies. Rather than increase the quantity of material in the die for resisting these loads, shrink-fit or press-fit rings are used to induce favourable compressive stresses in the die. Shrink-fit assemblies are made by heating the outer ring and allowing it to cool around the die. Normally, the die insert is force fitted mechanically into the shrink ring, using tapered interference surfaces and molybdenum di-sulfide (MoS2) as a lubricant. In general, no further advantage is gained by making the outside diameter of shrink-ring more than four times to five times the die diameter. The holding power of a shrink fit is typically higher than that of press-fit since higher interference can be achieved.

The taper angle which is normally used is 0.5-degree to 1-degree. The conditions for achieving the specified advantages of the taper force-fit are careful preparation of the taper shell surfaces and exact agreement between taper angles of corresponding contact faces. If the shell surfaces do not provide uniform support over the entire die length, the pre-stresses are unequal, and the reinforcement is not fully effective. In some set-ups, the first reinforcement is applied by taper force-fit and the second (outer) reinforcement by shrinking-on.

Extrusion dies are normally inserted in one or more shrink rings for providing reinforcement. These rings pre-stress the die in compression by providing interference fits between the rings and the die. This results in lower working stress and hence longer fatigue life of extrusion tools. A similar technique is used to shrink radially segmented die inserts together for preventing the segments from separating under load. Permanent shrink-fit assemblies are sometimes made by heating the outer ring to facilitate assembly.

In forward extrusion, the die is under maximum pressure, and this pressure is not distributed uniformly. The external pressure from the shrink ring on the die balances the internal pressure from extrusion on the die. The design of the interferences and the diameters for the rings is based on Lame’s theory of thick compound cylinders. The tool designer is required to calculate the hoop stresses on the inner die wall and to provide adequate reinforcement with shrink rings. Ordinarily, pressures of less than around half the yield strength of the die do not need reinforcement, while those in excess of this value do need reinforcement.

The reduction of stress in the die because of the shrink rings also prolongs the fatigue life of the dies. Frequently an intermediate shrink ring is added to the assembly for a more efficient design. Commercially available multiple-ring shrink rings utilizing strip winding are also used in severe forming applications. Outer shrink rings are made from 4340 alloy steel or H13 die steel heat treated to 46 HRC to 48 HRC. Care is to be taken for ensuring that the yield strength of the outer ring is not exceeded during die assembly. Premium grades of H13 die steel are typically used for providing the highest fatigue life and safe assembly.

Fig 3a shows a typical backward extrusion die in a shrink ring assembly. The die is designed so that the anvil absorbs the axial load, and the load on the die is minimized. This is needed for preventing cracking at the die corners. The length of anvil is designed to allow for elastic deflection under extrusion load. The forward extrusion die design involves more factors (Fig 3c). The internal diameters are prescribed by the product requirements.

Interchangeable die inserts are normally force fitted mechanically, using a tapered press fit and molybdenum di-sulphide as a lubricant. Of the two methods, shrinking-on by heating is normally preferred, since a cylindrical hole and shaft are easier to fabricate than a tapered hole and shaft. However, a taper fit has several advantages which include (i) the hardness and yield strength of the different die components are not lowered (as they are lowered by heating) and can be measured with dependable accuracy, (ii) the pre-stress value is ensured by strict control of the input measurements, (iii) release and exchange of the inner die bushings is quick, easy, and inexpensive, (iv) die parts can be standardized, and (v) hot-working die steels are not used.

The design of the extrusion die angle (Fig 3d) is dependent on the extrusion ratio. Typically, the extrusion die angle (half angle) varies from 5-degree to 30-degree. Standards are normally used for designing the extrusion die angle. When using higher angles, care has to be taken for preventing chevrons (a type of cracking) during multiple extrusions. Chevrons or central bursts are internal arrow-shaped defects occurring along the axis of a forward extrusion. Typically, lower angles are preferred for higher reductions.

The safe zone decreases with work hardening. Hence, extra care is needed in designing multiple forward extrusions. For high reductions (higher than 35 %), a radius is preferred in place of the angle since it needs lower extrusion loads. The die land is typically 0.75 mm to 3.25 mm long. For high reductions (higher than 35 %), the billet has to be fully contained within the die and hence, a long lead die is used for this purpose. An angle of 30-degree to 60-degree is preferred for extruding hollow parts, the angle varying inversely with wall thickness.

Extrusion dies are normally made from M2 tool steel (60 HRC to 62 HRC). Tungsten carbide is also used for high volume or critical extrusions. Ejection pressure on the work increases with decreasing die angle, since higher friction is to overcome because of the larger surface area. The ejection pressure also increases with an increase in the length of the part. Extrusion pressure also causes elastic expansion of the die, which shrinks when the pressure is discontinued. Hence, very high wall pressures are developed, and these needs correspondingly high ejection pressures. This is to be considered when designing ejector pins. Ejector pins are typically made from S7 tool steel heat treated to 54 HRC to 56 HRC, and M2 tool steel heat treated to 60 HRC to 62 HRC can be used in higher-pressure applications.

It is advisable to standardize on the size of reinforcing elements. In general, no further advantage is gained by making the outside diameter of a reinforcement more than four times to five times the die diameter. In forward extrusion, die angles are determined by the shape of the work-piece and by the operating sequence. In general, an angle of 2 alpha = 24-degree to 70 degree (Fig 3d) is selected for the forward extrusion of solids, and an angle of 2 alpha = 60-degree to 125-degree is preferred for extruding hollow parts, the angle varying inversely with wall thickness. Ejection pressure on the work increases with decreasing die angle, since the higher friction is to be overcome. This pressure also increases with an increase in the length of the part. Extrusion pressure causes elastic expansion of the die, which shrinks when the pressure is discontinued. Hence, very high wall pressures are developed, and these need correspondingly high ejection pressures.

Preparation of slugs -The preparation of slugs frequently represents a substantial fraction of the cost of producing cold-extruded parts. Despite the loss of metal, sawing, and cutting off in a machine, such as an automatic bar machine, are widely used methods of producing slugs. The advantages of these methods include dimensional accuracy, freedom from distortion, and minimal work hardening. The disadvantages are material loss as saw kerf and slower production rates. The use of circular saws instead of band saws and double cuts per cycle has considerably improved the production rates. The cycle time and material losses increase with billet diameter. The quality and roughness of the cut has an important effect on the quality of the extrusion.

Shearing is a chip-less process and is a more economical means of producing billets because of much higher production rates. It is an economical means of producing slugs. Variation in the sizes of the slugs is a major disadvantage of shearing. Extrusion process design has to allow for this variation. If precise shape is needed, then the billets have to be coined to desired dimensions in a press. In shearing, the ends of the billet are work hardened and hence their ductility is reduced.

Another billet-cutting method is the adiabatic cut-off process involving high-speed / high-energy impact processing. The cycle time with this process is around a milli-second, so the production rates can be very high. Production rates of several hundred parts per minute are possible, based on the capabilities of the material-handling system. This process produces precision cut-off blank which is burr-free, with minimum pull-down and end distortion and is also capable of cutting steels in hardened condition.

If slugs are allowed to vary in size, die design is required to allow for the escape of excess metal in the form of flash. An alternative to die adjustment in some applications is to compensate for the distortion and other discrepancies in sheared slugs by coining the slugs to desired dimensions.

Hot-rolled bar is normally the least costly form of steel for making slugs, but hot-rolled bars are likely to have deeper surface seams and higher depth of decarburized layers than cold finished bars. In addition, the variation in the outside diameter of hot-rolled bars cause considerable variation in weight or volume of the slug, despite close control in cutting to length. Whether or not the surface seams and the decarburization can be tolerated depends largely on the severity of extrusion and the quality needs of the extruded part. In several applications, acceptable extrusions can be produced with slugs cut from hot-rolled bars.

Cold-finished bars are more expensive than hot-rolled bars. The size variation in cold-finished bars is considerably less than that in hot-finished bars. However, some seams and decarburization are also present in cold-finished bar stock unless removed by grinding, turning, or other means. Some extrusion plants gain the advantage of cold-drawn bars by passing hot rolled bars or rods through a cold-drawing attachment directly ahead of the slug-cutting operation.

Machined or ground bars are more costly than cold-drawn bars, but eliminate the difficulties caused by decarburization, seams, and variation in outside diameter. For some extrusions, especially those subjected to surface treatments which cannot tolerate a decarburized layer, requirements are such that previously machined bars or machined slugs are to be used.

For surface preparation of steel slugs, phosphate coating for cold extrusion is almost universal practice. The primary purposes of this coating are, first, to form a non-metallic separating layer between the tools and work-piece and, second, by reaction with or absorption of the lubricant, to prevent its migration from bearing surfaces under high unit pressures. During extrusion, the coating flows with the metal as a tightly adherent layer. The recommended preparation of steel slugs for extrusion consists of alkaline cleaning, water rinsing, acid pickling, cold and hot water rinsing, phosphate coating, and rinsing as discussed below.

Alkaline cleaning is done for removing oil, grease, and soil from previous operations so that subsequent pickling is effective. Alkaline cleaning can be accomplished by spraying the slugs with a heated (65 deg C to 70 deg C) alkaline solution for 1 min to 2 min or by immersing them in alkaline solution at 90 deg C to 100 deg C for 5 min to 10 min. Water rinsing is done for removing residual alkali and for preventing neutralization of the acid pickling solution. Slugs are normally rinsed by immersion in over-flowing hot water, but they can also be sprayed with hot water.

In case of acid pickling, majority of the installations use a sulphuric acid (H2SO4) solution (10 % by volume) at 60 deg C to 90 deg C. Pickling can be accomplished by spraying for 2 min to 15 min or by immersion for 5 min to 30 min, depending on surface conditions (normally, the quantity of scale). Three times are normally sufficient for removing all scale and for permitting a good phosphate coating. Bright annealing or mechanical scale removal, such as shot blasting, as a substitute for pickling has proved unsatisfactory for severe extrusion, if considerable scale is present. However, the use of a mechanical scale-removing method prior to pickling can reduce pickling time, and for producing extrusions of mild severity, the mechanical (or bright annealing) methods have frequently been used without subsequent pickling or combined with cold pickling process.

Cold and hot water rinsing can be carried out by immersion or spraying for 0.5 min to 1 min for each rinse. Two rinses are used for ensuring complete removal of residual pickling acid and iron (Fe) salts. Cold water rinsing is normally of short duration, with heavy overflow of water for removing majority of the residual acid. Hot water at around 70 deg C increases the temperature of the work-piece and ensures complete rinsing. Phosphate coating is performed by immersion in zinc phosphate [Zn3(PO4)2] at 70 deg C to 80 deg C for 3 min to 5 min.

Rinsing with cold water, applied by spraying for 0.5 min or by immersion for 1 min, removes the major portion of residual acids and acid salts left over from the phosphating solution. This rinse is followed by a neutralizing rinse applied by spraying or immersion for 0.5 min to 1 min using a well-buffered solution such as sodium carbonate (Na2CO3), which is to be compatible with the lubricant. In the second rinse, the remaining residual acid and acid salts in the porous phosphate coating are neutralized so that absorption of, or reaction with, the lubricant is complete.

Stainless steels are not amenable to conventional phosphate coating (which is why stainless steels are more difficult to extrude than C steels). Oxalate [(C2O4)-2] coatings have been developed with reactive soaps. Cu plating of stainless-steel slugs is preferred. Lime (CaO) coating is sometimes substituted successfully for Cu plating. In extreme cases, the stainless steel can be zinc (Zn) plated and then coated with zinc phosphate and a suitable soap lubricant.

Lubricants for steel – A soap lubricant has traditionally provided the best results for the extrusion of steel. Slugs are immersed in a dilute soap solution, i.e., 45 milli-litre per litre (mL/L) to 125 mL/L at 65 deg C to 90 deg C for 3 min to 5 min. Some soaps are formulated to react chemically with the zinc phosphate coating, resulting in a layer of water-insoluble metal soap (zinc stearate) on the surfaces of the slugs. This coating has a high degree of lubricity and maintains a film between the work metal and tools at the high pressures and temperatures developed during extrusion.

Other soap lubricants, with or without filler additives, can be used effectively for the mild extrusion of steel. This type of lubricant does not react with the phosphate coating, but is absorbed by it. Although the lubricant got by the reaction between soap and zinc phosphate is optimal for extruding steel, its use needs precautions. If soap accumulates in the dies, the work-pieces do not completely fill out. Best practice is to vent all dies so that the soap can escape and keep a timed air blast into the dies for removing the soap, and also for keeping a coating of mineral seal oil (applied as an air-oil mist) on the dies to prevent adherence of the soap. Polymer lubricants are gaining wider use for all but the most severe applications where coating build-up in the dies is a concern.

When steel extrusions are produced directly from coiled wire (similar to cold heading), the normal practice is to coat the coils with zinc phosphate. This practice, however, has one deficiency, since only the outside diameter of the work-metal is coated, the sheared ends are uncoated at the time of extrusion. This deficiency is partly compensated for by constantly flooding the work-metal with sulpho-chlorinated oil. Since the major axis of an extrusion machine is normally horizontal, there is less danger of entrapping lubricant than when extruding in a vertical press.

New lubricants (non-phosphate coatings) are replacing soap-phosphate treatments for the cold extrusion process. Soap-phosphate treatments, although very effective for extrusion process, are not conducive to continuous processing because of the long cycle time (30 min). Another disadvantage is the waste liquid treatment and disposal needed for the solutions used in the process. Oil-based and water-based lubricants are available which can be applied through a simple process of tank-dip, air-blowing, and hot air drying, which can be used in a continuous production line. The waste liquid treatment is considerably reduced. The application of these new lubricants is gaining acceptance slowly.

Cleaning the extruded parts can be a significant item in the cost of cold extrusion. In general, the more effective is the lubricant, the more difficult it is to remove. The methods used for removing pigmented drawing compounds are normally effective for removing the lubricants used for cold extrusion.

Selection of procedure – The shape of the part is normally the primary factor which determines the procedure used for extrusion. For example, typically short cup-like parts are produced by backward extrusion, while solid shaft-like parts and thin-walled hollow shapes can normally be produced more easily by forward extrusion. Semi-hollow shapes and thick-walled hollow shafts are made with both forward and backward extrusion. Also, for several shapes, both forward extrusion and backward extrusion are used. Other factors which influence procedure are the composition and condition of the steel, the needed dimensional accuracy, quantity, and the cost.

The procedures used to extrude a given shape from highly extrudable steels are simpler than those used for more difficult-to-extrude steels. For difficult steels, it is necessary to incorporate more passes and one or more annealing operations into the process. Some shapes are not completely extrudable from a difficult-to-extrude steel and in such case one or more machining operations are needed.

Normal extrusion procedures are associated with certain ranges of dimensional accuracy. Special procedures and controls can provide higher-than-normal accuracy at increased cost. Cold extrusion is ordinarily not considered unless a large quantity of identical parts is to be produced. The process is

seldom used for fewer than 100 parts, and more frequently, it is used for hundreds of thousands of parts or continuous high productions. Quantity requirements determine the degree of automation which can be justified and frequently determine whether the part is to be completed by cold extrusion (assuming it can be if tooling is sufficiently elaborate) or whether, for low quantities, a combination of extruding and machining is more economical.

Cost per part extruded normally determines (i) the degree of automation which can be justified, (ii) whether a combination of extruding and machining is to be used for low-quantity production, and (iii) whether it is more economical to extrude parts for which better-than-normal dimensional accuracy is specified or to achieve the needed accuracy with secondary operations. It is sometimes possible to extrude a given shape by two or more different procedures. Under these conditions, cost is normally the deciding factor. Several procedures for extruding specific steel parts are categorized mainly by part shape as described below.

In case of cup-like parts, the basic shape of a simple cup is frequently produced by backward extrusion, although one or more operations such as piercing or coining are frequently included in the operations sequence. For cup-like parts which are more complex in shape, a combination of backward extrusion and forward extrusion is more frequently used. In case of tubular parts, backward extrusion and forward extrusion, drawing, piercing, and sometimes upsetting are frequently combined in a sequence of operations to produce different tubular parts.

In case of stepped shafts, three methods are normally used to cold form stepped shafts. If the head of the shaft is relatively short (length little or no larger than the headed diameter), it can be produced by upsetting (heading). For a head more than around 2.5 diameters long, however, upsetting in a single operation is not advisable since buckling results because of the excessive length-to-diameter ratio of the unsupported portion of the slug. Under these conditions, forward extrusion or multiple-operation upsetting is to be considered.

Forward extrusion can be done in a closed die or an open die. In a closed die, the slug is completely supported, and the cross-sectional area can be reduced by as much as 70 %. Closed die extrusion gives better dimensional accuracy and surface finish than the open-die extrusion. However, if the length-to-diameter ratio of the slug is more than around 4 to 1, friction along the walls of the die is so high that the closed die method is not feasible, and an open die is to be used. In an open die, reduction is to be limited to around 30 % to 35 %, or the non-supported portion of the slug buckles. Stepped shafts can, however, be extruded in open dies using several consecutive operations.

The combination of cold extrusion and cold heading is frequently the most economical means of producing hardware items and machinery parts which need two or more diameters that are widely different. Such parts are normally made in two or more passes in same type of heading machine, although presses are sometimes used for relatively small parts. Presses are needed for the heading and extruding of larger parts.

Parts which have a large difference in cross-sectional area and weight distribution cannot be formed economically from material equivalent in size to the smallest or largest diameter of the completed part. The most economical procedure consists of selecting material of an intermediate size, achieving a practical quantity of reduction of area during forward extrusion, and forming the large sections of the part by heading.

In case of extrusion of hot upset pre-forms, although the use of symmetrical slugs as the starting material for extrusion is a normal practice, other shapes are frequently used as the starting slugs or blanks. One or more machining operations sometimes precede extrusion in order to produce a shape which can be more easily extruded. The use of hot upset forgings as the starting material is also a normal practice. Hot upsetting followed by cold extrusion is frequently more economical than alternative procedures for producing a specific shape. Axle shafts for cars and trucks are regularly produced by this practice. The advantages include improved grain flow as well as low cost.

In case of extrusion of large parts, although the majority of cold extrusion of steel is confined to relatively small parts (starting slugs rarely weigh more than 11.5 kg), much larger parts have been successfully cold extruded. For press operations, the practical extremes of part size are governed by the availability of equipments and tool materials, the plasticity of the work material, and economical production quantities. Bodies for large-caliber ordnance shells have been successfully produced by both hot and cold extrusion processes.

Dimensional accuracy – In cold extrusion, the shape and size of the work-piece are determined by rigid tools which change dimensionally only from wear. Since tool wear is normally low, successive parts made by cold extrusion are nearly identical. The accuracy which can be achieved in cold extrusion depends largely on the size and shape of the given section. Accuracy is also affected by tool material, die compression, die set design, tool guidance, and the rigidity of the press drive. Tolerances for cold extrusion are normally denoted as close, medium, loose, and open. Definitions of these tolerances, as well as applicability to specific types of extrusions, are described below.

Close tolerance is normally considered to be +/- 0.025 mm or less. Close tolerances are normally restricted to small (less than 25 mm) extruded diameters. Medium tolerance denotes +/- 0.15 mm. Extruded diameters of larger parts (up to 100 mm), headed diameters of small parts, and concentricity of outside and inside diameters in backward extruded parts are typical of dimensions on which it is practical to maintain medium tolerance. Loose tolerance denotes +/- 0.35 mm. This tolerance normally applies to short lengths of extruded parts less than around 90 mm long. Open tolerance is normally considered to be higher than +/- 0.35 mm. This tolerance applies to length dimensions of large, slender parts (up to 500 mm, and sometimes longer).

Variation – With reasonable maintenance of tools and equipment, the quantity of variation of a given dimension is normally small for a production run. Some drift can be expected as the tools wear and work metal properties vary from lot to lot.

Causes of problems – The problems which are normally encountered in cold extrusion are (i) tool breakage, (ii) galling (adhesive wear which is caused by the microscopic transfer of material between metallic surfaces during transverse motion, sliding), or scoring (a process in which a groove gets cut into rigid material) of tools, (iii) work-pieces sticking to the dies, (iv) work-pieces splitting on outside diameter or cupping in inside diameter, and (v) excessive build-up of lubricant in dies. Tab 1 lists the most likely causes of these problems.