Cold Heading Process
Cold Heading Process
Cold heading is a forming process of increasing the cross-sectional area of a blank, which is carried out at room temperature, and at one or more points along its length. The material flow over the length where the cross-sectional area increases and the length decreases. It is identical to conventional upsetting. Along with the upsetting process, cold-headed parts can also undergo other processes, such as extrusion, coining, trimming, hole punching, and thread rolling.
Cold heading process essentially involves applying force with a punch to the end of a metal blank contained in a die. The force is to exceed the elastic limit (yield strength) of the metal to cause plastic flow. It can be considered a forging operation without heat. Heading includes upsetting and extruding, and is frequently performed in conjunction with other cold forming operations such as sizing, piercing, trimming, thread rolling, blank rolling, and pointing.
Upsetting, a term used synonymously with heading, means to form a head on a fastener, or a bulge in a cylindrical part being headed. Extruding means either decreasing the diameter of the blank by pushing it through a hole, or punching a hole in the centre of the blank and allowing the metal to flow backward over the punch. In both cases the volume of the metal blank remains constant. It is merely reshaped by upsetting or extruding.
Heading is a metal-working process which goes back before the turn of the century and for several years was used only to produce simple fasteners. Today, heading is a high-speed, automated, and multi-station operation which is capable of producing not only increasingly complex metal fasteners economically, but a growing variety of other components, including some which are asymmetrical. Combined with this dramatic improvement in heading equipment is the ability to successfully cold form parts from tougher metals, including stainless steels and high temperature alloys.Heading differs considerably from machining where material is actually cut away to form a finished part. In heading there is no scrap except for a minimal quantity which can occur during secondary operations, such as trimming. Advantages of the process over machining of the same parts from bar stock include (i) almost no waste material, (ii) increased strength from cold working, (iii) controlled grain flow, and (iv) higher production rate. However, heading is not intended to replace machining. There are several cases (e.g., very complex parts, larger parts, or low production requirements) where machining is more economical. In fact, there are some materials which cannot be headed. However, heading and cold forming now enable more economical and faster production of several fasteners and other parts which previously were only be made with machining.
Cold-heading is typically a high-speed process where the blank is progressively moved through a multi-station machine. The process is widely used to produce a variety of small-sized and medium-sized hardware items, such as screws, bolts, nuts, rivets, and specialized fasteners. Cold heading is used to produce automotive components, such as gear blanks, ball studs, piston pins, spark-plug shells, valve spring retainers, engine poppet valves (intake and exhaust), and transmission shafts. Fasteners represent the single largest category of headed parts produced. The bearing industry uses cold heading to produce inner and outer races as well as precision balls and cylindrical rollers. Advancements in cold-heading machines allow parts to be formed which are longer than 300 mm and higher than 3 kg in mass. Fig 1 shows schematics of cold heading of an unsupported bar in horizontal machine.
Fig 1 Schematics of cold heading of an unsupported bar in horizontal machine
Presently, heading and cold forming machinery is much more advanced. For example, machines are now produced with five or more dies and features to allow the production of both long and short parts. One adjustment changes the cutter, feed stroke, transfer and kick-out timing functions. CNC control gives the operator instant access to production data. Quick change setups allow both punch and die components to be set up and adjusted off-line, so valuable production time is not wasted. Multi-station headers which perform a combination of upsetting, extruding, and other cold forming operations have also considerably increased heading production rates and capabilities (Fig 2).
Fig 2 Sophisticated cold heading machine with seven die station
Increasing metallurgical knowledge enables the heading and cold forming of tougher materials. Specialty alloy producers can more closely control the analysis and production of grades to meet more demanding needs for higher corrosion resistance and strength in headed parts. Where good cold forming qualities and consistent performance are desired, it is now possible to have certain grades made within controlled analysis limits to improve cold formability and subsequent secondary machining operations. In other cases, analyses which have been modified for cold forming provide a means for the economical production of certain fastener designs. Tougher tool steels extend the life of heading dies. Producers also make alloys which are versatile enough to meet fabrication operations which call for both heading and machining. Also, other heading practices, such as warm heading and hot heading, can extend the forming limits to include several of the superalloys. Fig 3 shows the sequence of steps in the production of a screw blank in three blows.
Fig 3 Cold heading of a screw blank in three blows
Process of cold heading
Cold heading equipment primarily takes round wire in a coil form and converts the wire into desired parts at a high rate of speed. Four basic steps comprise the heading process. These are (i) a length or blank of wire is cut from the wire coil, (ii) the blank is placed in line with a cavity or die, (iii) the blank is forced into a desired shape with one or more upsetting and / or extruding operations called blows, and (iv) the part is ejected.
This heading process can be part of a sophisticated cold forming machine which has additional points or stations where further operations such as trimming, piercing, or pointing, are carried out following upsetting and extruding. However, majority of the headers are of the single or double blow variety. Multi-station part formers can include up to seven die stations. The part being formed is transferred from one die to another until a completed part is produced.
The typical arrangement is horizontal, though some multi-station formers are arranged vertically, the part progresses from the first die station at the top to the last die station at the bottom in this case.
Forming parts on a heading machine using upsetting or extruding is not merely a matter of hammering the metal blank until the desired shape is reached.
The punch and die work together. The punch is a simply shaped hammer which strikes the blank on its end. This forces the other end into the die which produces, for example, a headed bolt. In a typical heading machine, the punch, carried on the gate or ram, moves toward the blank with a great deal of force, striking it with an impact of a high value of MPa.
Perhaps no operation in the cold heading sequence is more important than the wire cut-off to form the blanks. This is important since the volume of the finished part essentially equals the volume of the blank from which it has been made. Since part dimensions and part volume are inter-dependent, blanks are to be cut to consistent volume. In several cases, the upsetting of the blank is controlled by the punch and takes place outside the die. However, the head can also be formed in the die, in both the punch and die, or between the punch and die, a technique called free upsetting (Fig 4).
Fig 4 Four techniques used for the upsetting of fastener head
Types of cold heading machines
Several types of cold heading machines are available and they combine standard and special tooling for carrying out a variety of heading and cold forming operations and sequences. Some weigh as much as 250 tons and have seven die stations. Since only so much metal can be formed in one blow, the number of dies and wire diameter acceptance range are normally used to describe machine types.
Machine specifications tend to be conservative. Typical measurements cover wire diameter, blank length and heading force. Specifications for a typical solid-die heading machine can be (i) maximum wire diameter = 6 mm (force available at cut-off knife for shearing), (ii) maximum shank length = 50 mm (around 8 times wire diameter), (iii) maximum wire cut-off length = 75 mm (around 12 times rated wire diameter), (iv) maximum pieces per minute = 125 (optimum machine rotation speed), (v) maximum heading capacity = 50 tons (force needed to upset a carriage bolt of the rated wire diameter, in this case 6 mm).
Normally, each die station in the heading machine has two punches which oscillate to form the fastener head. The first punch action partially shapes the head and is called coning, while the second punch finishes the head.
A heading machine includes either solid dies or open dies. Solid dies are more common while the open dies are used when a fastener needs a very long shank which cannot be fabricated with a solid die. In solid die heading machines, the knock-out (or kick-out) pin is equally important to the inter-action of the punch and die. The knock-out pin serves as a support at the back end of the blank as the punch strikes the front end, and the knock-out pin then ejects the finished part (Fig 5a). Different combinations of upsetting and extrusion blows are possible, but upsetting is normally the first blow, with an extrusion blow following. Upsetting and extrusion can take place in the same blow.
Knock-out pin specifics – Knock-out (or kick-out) pins (Fig 5) serve two functions. They stop blanks as they enter the die at the point where upsetting is to start. For this reason, pins are to withstand some of the forming pressures. The second knock-out pin function is to eject the headed part to clear the die for the next blank. The unsupported length of the pin is not to exceed eight diameters. This is a good general rule of thumb, though some fabricators run parts with the knock-out pin equivalent to 10 diameters or 12 diameters unsupported. When the knock-out pin’s unsupported length exceeds these diameters, a supported pin assembly (Fig 5b) is suggested.
Fig 5 Knock-out pin and a supported pin assembly
Lack of support is not the only reason for pin breakage. Broken pins can result from running poorly coated wire, or wire with an incorrectly selected coating. Rough, rusted, or uncoated spots on a wire make the parts more difficult to eject and this can result in pin breakage. Also, as the end of the knock-out pin wears, it is possible for metal to extrude around the end of the pin. This can cause a tight spot in the die where the pin and work-piece overlap, resulting in sufficient additional pressure to break the pin.
A similar effect can occur when the diameter of the pin is very large. As the pin stops the work-piece when it enters the die, it frequently absorbs part of the heading process. A pin which is too large, which means it fits the die too closely, can swell, bind in the die and break from the resulting pressure. A back taper in the die (a smaller bore diameter at the die face than toward the kick-out pin) can allow the metal to upset to a larger diameter at the end of the shank than under the head. In this case, the knock-out pin is to force the larger diameter through a smaller hole during ejection. Here, excessive ejection pressure can break the pin. Machining marks left in the die can also create excessive pressure. Die bores are to be smooth and rust-free.
A back taper in the die (a smaller bore diameter at the die face than toward the kick-out pin) can allow the metal to upset to a larger diameter at the end of the shank than under the head. In this case, the knock-out pin forces the larger diameter through a smaller hole during ejection. Here, excessive ejection pressure can break the pin.
Controlled upsetting – There is a limit to the quantity of material which can be upset in one blow under controlled conditions. Forming a more complex part in which more metal is moved farther is better accomplished in two stages, or blows, which is why single-die, double-stroke (two punches) heading machines are more widely used. Upsets are calculated on the basis of wire diameters. The length of the blank is divided by the diameter of the wire. Hence, a 125 mm blank of 12.5 mm diameter wire is 10 diameters long and a 250 mm blank of 25 mm diameter wire is also 10 diameters long. A rule of thumb is that in a single blow on a solid die heading machine, the maximum quantity of wire which can be upset under control is 2.25 diameters. Theoretically, one can use around 25 mm of the 125 mm blank to upset into a fastener head. However, majority of the single blow heading is within 1 diameter to 1.25 diameter range. With a two-blow heading sequence, up to 4.5 diameters can be upset.
At the moment of contact between the punch and blank, the part of the blank to be upset extends out of the die unsupported. If this unsupported length is too long, or higher than 2.25 diameters, the blank simply bends over on itself when struck, which produces what is known as a cold shut defect. With a 125 mm blank, 25 mm unsupported can be upset in one blow and 50 mm unsupported can be upset in two blows. If an attempt is made to upset 75 mm, it cannot be controlled since this is equal to 6 diameters. There are, of course, exceptions to the rule.
A sophisticated heading machine with a sliding punch which supports more of the blank allows two-blow upsets of 6.5 diameters. Also, in multi-station heading machines the number of diameters which can be upset is limited only by the available dies. This relationship between diameters of wire and upsetting is critical. Improper calculation can mean mismatching the diameter of the feed wire with the capabilities of the machine.
Extruding – Several cold headed parts are also extruded. Forward extrusion occurs when the metal blank is forced to enter a die diameter smaller than itself and because of it, length is increased, while diameter is decreased. Backward extrusion involves subjecting the blank to pressure from an angular punch. Since it has no place to go, the metal literally squirts along the outer perimeter of the punch, flowing backward. Forward extrusion is used to produce bolts, screws, or stepped shafts. Backward extrusion is useful in forming a variety of cylindrical shapes such as nuts, sleeves, and tubular rivets. Like upsetting, extrusion simply rearranges the shape of the blank and there is no loss of material.
Extrusion can be in an open or trapped (contained) manner. Open extrusion means the blank is forced into a die. Trapped extrusion means the blank is totally contained within a die prior to extrusion (Fig 6a). While controlled upsetting is based on diameters of wire, extrusions are governed by the area reduction of the blank (calculated as a percentage) and the angle of extrusion. The basic ground rules for open extrusion, which is more widely used than trapped extrusion, is that the percentage of area reduction in one blow cannot exceed 30 %. The extrusion angle (the angle the shoulder of the extrusion makes with the original blank) cannot exceed 30-degree (Fig 6b, 6c, and 6d).
Fig 6 Extrusion and trapped extrusion
Area is defined as the cross-sectional area of the blank using the standard circular area. A blank with a cross-sectional area of 645 square millimetres extruded to a cross-sectional area of 480 square millimetres is a 25 % reduction in area. It is to be remembered that diameter measurements, both before and after extrusion, are not directly used to calculate area reduction. The actual cross-sectional areas are to be calculated by using the above formula.
Successful extrusion practice also needs that the blank extends at least 3 mm (land) into the die for proper guidance as the punch strikes. These rules do not apply to trapped extrusion which typically allows for area reductions as high as 75 % in one blow.
Contained (trapped) extrusion – This practice is responsible for allowing heading machines to produce more complicated and multi-shaped parts formerly made on automatic screw machines. It is especially applicable for extruding larger diameter wire to the needed shank, a method which makes it possible to increase the ratio of the head diameter to the shank diameter. The radial extrusion die is normally preferred for contained extrusion of headed parts (Fig 6e). This type of die reduces die pressures and improves the flow pattern of the wire as it is pushed through the die.
There are general rules to follow when using the radial extrusion die. These are (i) the blank to be extruded is to be a minimum of 0.05 mm smaller than the die entrance, (ii) the entrance length of the die is to be a minimum of one-quarter blank diameter (maximum depth is determined by the volume of stock in the upset portion and geometry of the finished part), (iii) the radius is normally equal to ‘C’ in Fig 6e, and this varies with the percentage area reduction, (iv) the extrusion land ‘B’ is normally 10 % of dimension ‘A’, and this varies with die material, (v) extrusion relief is normally 0.5 % of ‘C’, and (vi) break corner ‘D’ with around a 45-degree angle leading into the extrusion land.
With any contained extrusion die, best results are frequently achieved by forming wire which is somewhat larger in diameter than the finished shank. Wire size is selected so that the upset dimension of the head is around twice the original wire diameter. This wire is extruded to the shank diameter and upset to the head dimensions in the customary manner.
Combining upsettings and extrusion – Multi-station heading machines frequently combine upsetting and extruding operations to form large-head, small-shank parts. The reason is that upsets can involve 6 diameters to 10 diameters, rather than the 4.5 diameters maximum. However, upsetting and extruding are separate operations, hence maximum deformation of a blank is to be figured separately for upsetting and extruding limits even with multi-station heading machines. By starting with wire stock larger in diameter than the needed shank, then extruding the shank, and finally upsetting the head, maximum deformations can be reached for both extruding and upsetting based on initial stock size.
For example, a 12.5 mm blank trap extruded to a reduction in area of 80 %, and then upset 6 diameters, results in an actual upset having 70 diameters of the extruded shank size. Since single-die, double-stroke heading machines (one die, two punches, two blows) are the norm in heading machines, combining upsetting and extrusion is a normal practice. The first blow extrudes the shank and partially forms the head, the second blow finishes the head. An important rule for combining these two operations is that parts being formed in solid dies cannot have a shank length which exceeds 8 diameters (Fig 6f, and 6g).
Since solid dies include a knock-out pin, the knock-out pin is to overcome the high friction between the shank and the die as it kicks out the finished part. If more than 8 diameters of the knock-out pin are unsupported outside the die, the knock-out pin normally bends as it pushes against the blank. Shank lengths over 8 diameters are produced using an open die, a two-part die which is spread apart by a cam mechanism as the part is finished. The next blank pushes out the finished part. No knock-out pin is used with open dies.
Process parameters in cold heading process
Upset length ratio – The ratio of initial length being upset to the initial diameter of the blank is called the upset length ratio. The upset length ratio determines the number of blows and the form of the upset needed to prevent buckling. Unsupported lengths of up to 2 to 2.5 times the blank diameter can be upset in one blow (ratio of 2 to 2.5) in steel. By enclosing the blank in a die cavity of 1.5 times the blank diameter, more than 2.5 times the blank diameter can be upset in one blow (Fig 7a). A cone-shaped cavity in the heading tool (punch) also can be used to upset a length of more than 2.5 times the blank diameter in one blow (Fig 7b). If the buckle point of the blank cannot be contained within a cone-shaped heading tool (punch), a sliding cone-shaped die cavity has to be used to support the blank (Fig 7c). Otherwise, multiple blows are needed to upset more than 2.5 diameters. Lengths up to 4.5 diameters can be upset with two blows (ratio of 4.5). Lengths up to eight diameters can be upset with three blows (ratio of 8). Again, punches and dies can be designed to increase the upset lengths. For example, using a cone-shaped heading cavity, lengths of up to 6 diameters can be upset in two blows (upset ratio of 6). Fig 5 gives die designs to overcome buckling of the blank.
Fig 7 Die designs to overcome buckling of the blank
Upset diameter ratio – The ratio of final upset diameter to the initial blank diameter is called the upset diameter ratio. The upset diameter ratio limit is sensitive to the type of material, material condition, lubrication, and shape of the upset. Finished diameters from two times blank diameter to 2.5 times blank diameter can be achieved (ratio of 2 to 2.5). Larger diameters (ratio of less than or equal to 3) can be achieved in closed-die upsetting and upsetting of shapes such as carriage bolts.
Upset strain – The upset strain is the true strain in the material, expressed as strain = Ln(L0/L1). An upset strain limit of 1.6 is normally used as a rule of thumb. Spheroidize annealing heat treatment of blanks is normally used to increase the upset strain limit beyond 1.6. The cold heading process limits are sensitive to the type of material, material condition, lubrication, equipment, and shape of the upset. Hence, there are limits, which need to be considered as general recommendations and not as definitive rules.
Process sequence design – The design of forming process sequences has historically been achieved by a combination of empirical and calculation methods. Skilled designers, using creativity, intuition, and experience have created majority of the guidelines for forming sequence design. The guidelines as described below can be used for combining upsetting with extrusion operations, as well as for combining forward-backward extrusion and multiple extrusion operations.
Backward extrusions need to have a minimum reduction of 20 % to 25 % and a maximum of 70 % to 75 %. Bottom thickness during backward extrusion is not to be less than 1 to 1.5 times the extruded wall thickness. Open forward extrusions are to be a maximum of 25 % reduction for aluminum, 35 % for carbon steels, and 40 % for alloy steel. Trapped forward extrusion can have reductions as high as 70 % to 75 %.
Multiple forward extrusion is needed to have the highest reduction first, because of the removal of lubricants and coatings which impair subsequent forming. The number of extrusions over the same axial portion is to be limited to three.
Double forward extrusion is to be limited to a maximum reduction of 30 %, and the distance between extrusions is to be at least one blank diameter. It is preferred to upset before open extrusion, unless there is a need to extrude the diameter immediately under the head.
The use of computers and simulation software for modelling cold-heading processes has increased to the point where they are used daily. Simulations are used to develop forming progressions, analyze tooling stresses, and predict stresses and damage in the work-piece.
Steels for cold heading – Cold heading is normally performed on low-carbon steels having hardness values of 60 HRB to 87 HRB. Stainless steels, and some nickel alloys can also be cold headed. Carbon steels containing up to around 0.2 % carbon are the easiest to cold head. Medium-carbon steels containing up to 0.4 % carbon to 0.45 % carbon are fairly easy to cold work, but formability decreases with increasing carbon and manganese content. Alloy steels with more than 0.45 % carbon, as well as some grades of stainless steel, are very difficult to cold head and result in shorter tool life than that achieved when heading low-carbon steels.
Cold-heading-quality steels are subject to mill testing and inspection designed for ensuring internal soundness, uniformity of chemical composition, and freedom from detrimental surface defects. Majority of the cold-heading-quality alloy steels are low-carbon and medium-carbon grades. Typical low-carbon alloy steel parts made by cold heading include fasteners (cap screws, bolts, and eyebolts), studs, anchor pins, and rollers for bearings. Examples of medium-carbon alloy steel cold-headed parts are bolts, studs, and hexagon-headed cap screws.
Special cold-heading-quality steels are produced by closely controlled steelmaking practices to provide uniform chemical composition and internal soundness. Also, special processing (such as grinding) is applied at intermediate stages to remove detrimental surface defects. Typical applications of alloy steel bars of this quality are front suspension studs, socket screws, and some valves.
Cold-heading-quality alloy steel rod is used for the production of wire for cold heading. Severe cold-heading-quality rod, for single-step or multiple-step cold forming where intermediate heat treatment and inspection are not possible, is produced with carefully controlled manufacturing practices and rigid inspection procedures for ensuring the needed degree of internal soundness and freedom from surface defects. A fully killed fine-grained steel is normally needed for the most difficult operations. Normally, the wire made from this quality rod is spheroidize annealed, either in process or after drawing finished sizes.
Dual-phase steels featuring ferrite-martensite or ferrite-pearlite micro-structures have been developed specifically for cold-heading applications, especially for high-strength fasteners which do not need subsequent quenching and tempering treatments for attaining the mechanical property requirements. These steels achieve their strength from a combination of thermo-mechanical processing at the steel plant and strain hardening during wire drawing and cold heading. The strength of these alloy steels can be increased further by strain aging which occurs during low temperature heating after forming. The high strength and work hardening of these alloy steels result in higher forming loads and hence can affect tool life.
Some stainless steels, such as the austenitic types 302, 304, 305, 316, and 321 and the ferritic and martensitic types 410, 430, and 431, can be cold headed. Precipitation-hardening alloy steels provide higher strength levels than conventional stainless steels, but the higher initial strength decreases the headability. All stainless steels work-harden more rapidly than carbon steels and are hence more difficult to cold head. More power is needed, and cracking of the upset portion of the work metal is more likely than with carbon or low-alloy steels. These issues can be alleviated by preheating the work metal. Cold-heading wire is produced in any of the various types of stainless steel. In all cases, cold-heading wire is subjected to special testing and inspection for ensuring satisfactory performance in cold-heading and cold-forging operations.
Of the chromium-nickel group, types 305 and 302Cu are used for cold-heading wire and normally are necessary for severe upsetting. Other grades normally cold formed include 304, 316, 321, 347, and 384. Of the 4xx series, types 410, 420, 430, and 431 are used for a variety of cold-headed products. Types 430 and 410 are normally used for severe upsetting and for recess-head screws and bolts. Types 416, 416Se, 430F, and 430FSe are intended primarily for free cutting and are not recommended for cold heading. Cold-heading wire is produced using a closely controlled annealing treatment which produces optimal softness and still permits a very light finishing draft after pickling. The purposes of the finishing draft are to provide a lubricating coating which aids the cold-heading operation and produces a kink-free wire coil having more uniform dimensions.
Cold-heading wire is produced with a variety of finishes, all of which have the function of providing proper lubrication in the heading machine dies. The finish or coating is to be suitably adherent to prevent galling and excessively rapid die wear. A copper coating, which is applied after the annealing treatment and just prior to the finishing draft, is available. The copper-coated wire is then lime coated and drawn, using soap as the drawing lubricant. Coatings of lime and soap or of oxide and soap are also used. Some specialty alloys based on iron, nickel, or cobalt are used when conventional steels and stainless steels do not provide sufficient strength, corrosion resistance, or high-temperature properties (creep, oxidation, etc.). A common grade includes A-286 (UNS K66286), an iron-base precipitation hardening alloy, alloy 718 (UNS N00718) is a precipitation-hardenable nickel-base superalloy. These alloys are all very difficult to cold head, and warm-heading and hot-heading techniques frequently are used to improve the formability.
Workability and defects – Metal formability is affected by (i) the chemical composition, (ii) micro-structure, (iii) surface condition, including coatings and / or lubricants, and (iv) the presence of internal defects. Issues which pertain to all steels include chemical segregation, variability in grain size / aspect ratio, presence of other phases, texture (preferred orientation), and surface defects such as seams, laps, pits, voids, slivers, scratches, and rolled-in scale or oxides.
Carbon and alloy steel grades are normally categorized in the following application variations such as cold heading, recessed head, socket head, scrap-less nut, and tubular rivet. These wire variations are produced to meet specific requirements for chemical composition, mechanical properties, surface quality, and internal soundness. Surface defects are required to be less than 75 micrometres or 0.5D (finished wire diameter), whichever is higher. Ferrite decarburization is limited to 25 micrometres, with partial decarburization (total average affected depth) limits between 130 micrometres and 250 micrometres and worst-location depths between 200 micrometres and 380 micrometres.
Cold-heading wire which has been direct drawn from low-carbon steel wire rods can be used for simple two-blow upsets or for standard trimmed hexagon-head cap screws, but more demanding applications need a suitably annealed micro-structure for optimal workability. Annealed-in-process (AIP) or spheroidize annealed-in-process (SAIP) wire is produced by drawing rods or bars to wire, followed by thermal treatment, cleaning, and coating, and then a final drawing operation.
Recessed-head wire is used in forming fasteners with features such as crossed or square recesses. Improved-surface-quality billets and rods are used to provide better formability. This type of wire is normally produced as SAIP or spheroidize annealed at finish size (SAFS), which consists of spheroidize annealing after final cold reduction. The SAFS is the most ductile condition and is needed to be finally drawn in front of the header before forming.
Socket-head wire is similar to recessed-head wire but has superior formability needed for extruding deep internal hexagon or Torx (It is trademark for a type of screw drive characterized by a 6-point star-shaped pattern. A popular generic name for the drive is star, as in star screwdriver or star bits. The official generic name, standardized by the International Organization for Standardization as ISO 10664, is hexalobular internal) features. Upsetting tests are normally specified when applications need consistent workability, along with 100 % non-destructive eddy current inspection. The upsetting test uses a test-piece with end sections flat and parallel to each other and with an initial length (height) h = 1.5d, where d is the test-piece diameter. During the test, the length (height) of the test-piece is reduced to one-third of its initial value without cracking.
Scrap-less nut wire is used in the most severe forming operations, where both upsetting and backward extrusion are needed. Low-carbon and medium-low-carbon direct-drawn wire or wire drawn from annealed rods is used for non-heat-treated nuts (property classes 4 to 8, ISO 898-2), depending on the severity of deformation. Medium-carbon wire used for heat treated nuts (property class 9 or 10) is normally drawn from annealed or spheroidize-annealed bars or rods or is produced AIP.
Tubular rivet wire has similar requirements for both heading and extruding but is normally supplied from low-carbon aluminum-killed steel in the SAIP condition, where the final drawing reduction is somewhat heavier than normal. This heavy draft strengthens the wire in order to prevent buckling of the shank during the extrusion operation. The SAFS wire also can be used, with the final drawing reduction in front of the header.
Formability considerations for steels – When steel is ordered spheroidized according to ASTM F 2282, the requirement is a minimum rating of G2 or L2 (around 60 % spheroidized, with some lamellar carbides and grain boundaries still present). This is really only suitable for light heading and not for operations needing heavy upsetting or extrusion. The carbide aspect ratio at this amount of spheroidization is still as high as 8 to 15. Improved formability, suitable for recessed-head or socket-head wire, is achieved when the spheroidization is higher than 90 %, with an aspect ratio pre-dominantly below 5. Optimal performance is achieved when the carbides are uniformly distributed and have an aspect ratio of 1 to 2.
Other important considerations for maximizing the cold formability of steels include silicon and oxygen concentrations, non-metallic inclusions, grain size, and banding / orientation. Silicon is a ferrite strengthener and hence increases flow stress. Silicon content is to be 0.2 % maximum and 0.1 % for unalloyed steels. Oxygen promotes the formation of non-metallic inclusions, but it is not as detrimental to fracture ductility as sulphur, since the oxides present in aluminum-killed steels tend to be globular and not long stringers, such as MnS (manganese sulphide). Sulphur levels are to be maintained below 0.01 % for recessed-head and socket-head applications and below 0.006 % for scrap-less nut grades. Inclusion sizes higher than 10 mm impair ductility, especially when AlN (aluminum nitride) particles are present. Grain size number, according to ASTM E 112, is to be higher than 5. Banding of ferrite-pearlite structures (not spheroidize annealed) contributes to excessive variability in flow stress and fracture strain and is to be maintained below 50 micrometres in width.
Thermo-mechanical processing (TMP) can improve formability by producing a more homogeneous distribution and finer ferritic grain size at lower hot rolling temperatures. Lower strength and improved ductility are achieved in medium-carbon unalloyed and alloyed steels because of the elimination of bainitic and martensitic phases and because of the higher ferrite-phase fraction. Heading can be performed on as-rolled rods because of the improved deformation characteristics, or TMP wire can be spheroidized in shorter process cycles because of the improved annealing response.
Workability of stainless steels can be characterized by the cold work-hardening (CWH) factor and the Md30 factor. The CWH factor varies between 80 and around 150 and is indicative of the strain-hardening behaviour (higher numbers strain harden more). The Md30 factor is the temperature (in deg C) at which 0.3 true strain, or around 25 % area reduction, leads to the transformation of 50 % of the austenite to deformation martensite. A higher Md30 temperature denotes higher martensite formation during deformation and hence higher strain hardening and reduced cold formability. The Md30 temperature can be calculated from the following equation Md30 = 551 – 462(C+N) – 9.2Si – 8.1Mn – 13.7Cr – 20(Ni + Cu) – 18.5Mo – 68Nb – 1.42(ASTM grain size – 8). Typical CWH values for type 302 are 122 to 139, and 89 for type 305. Typical Md30 temperature for 302 is -8 deg C to 6 deg C, with type 305 being -41 deg C.
Defects in formed parts include folds, bursts, adiabatic shear bands, flow localization, and several different types of cracks. Fig 8a shows an example of proper grain flow after forming through the important under-head fillet region of a fastener, while Fig 8b displays a fold in the same area. Another example of a defect produced during cold / warm heading appears in Fig 8c, in this case, an internal shear crack is visible. Adiabatic shear bands (Fig 8d) are a typical defect when heading alloy steels with low formability at high strain rates and low temperatures. Fig 8 shows uniform grain flow and defects in the formed parts.
Fig 8 Uniform grain flow and defects in the formed parts
Cold-heading machines
Cold heading is done on horizontal mechanical presses sometimes called headers. Multi-station heading machines with automatic transfer of work-piece between stations are called transfer heading machines or progressive heading machines. Bolt makers, nut formers, and parts formers are specialized types of transfer heading machines. Majority of the cold-heading machines used in high production are fed by coiled wire stock.
In the conventional method, stock is fed into the machine by feed rolls and passes through a stationary cut-off quill. In front of the quill is a shear-and-transfer mechanism. When the wire passes through the quill, the end butts against a wire stop or stock gauge to determine the length of the blank to be headed. The shear actuates to cut the blank. The blank then is pushed out of the shear into the transfer, which positions the blank in front of the heading die.
A newer technique consists of a linear feed mechanism which grips and ungrips the wire surface while reciprocating on guide shafts. The large clamping surface of the grippers minimizes damage to the wire surface / coating, especially when working with soft materials such as aluminum. The heading punch moves forward and pushes the slug into the die. At the same time, the transfer mechanism releases the slug and moves back into position for another slug. In the die, the slug is stopped by the ejector pin, which acts as a back-stop and positions the slug with the correct quantity protruding for heading. The heading operation is completed in this die, and the ejector pin advances to eject the finished piece.
In a cold-heading machine with progressive dies, the transfer mechanism has fingers in front of each of several dies. After each stroke, the ejector pin pushes the work-piece out of the die. The transfer mechanism grips it and advances it to the next station. Some of the progressive cold-heading machines have special die stations for performing finishing operations such as trimming, pointing, thread rolling, and knurling. Cold-heading machines vary in terms of the number of dies / stations, forging load, blank or wire size, blank cut-off length, speed, transfer mechanism, and special finishing capabilities, such as thread forming and knurling.
Heading machines are classified as single stroke, double stroke, or three stroke, based on the number of blows (number of punches) they deliver to the work-piece. Single-stroke and double-stroke heading machines have only one die, while the three-stroke heading machines have two dies. The punches in multi-stroke machines normally reciprocate so that each contacts the work-piece during a machine cycle. They are also further classified as open-die heading machines or solid-die heading machines, based on whether the dies open and close to admit the work metal or are solid.
In single-stroke machines, product design is limited to less than two diameters of stock to form the head. Single-stroke extruding can also be done in this type of machine. These machines are used to make rivets, rollers and balls for bearings, single-extruded studs, and clevis pins. Double-stroke solid-die heading machines can make short to medium-length products (normally 8 diameters to 16 diameters long), and they can make heads which are as large as three times the stock diameter. These machines can be equipped for relief heading, which is a process for filling out sharp corners on the shoulder of a work-piece or a square under the head. Some extruding can also be done in these machines. Because of their versatility over single-stroke cold heading machines, double-stroke solid-die heading machines are extensively used in the production of fasteners.
Single-stroke open-die heading machines are made for smaller-diameter parts of medium and long lengths and are limited to heading 2 diameters of stock because of their single stroke. Extruding cannot be done in this type of machine, but small fins or a point can be produced by pinching in the die, if desired. Similar machines are used to produce nails. Double-stroke open-die heading machines are made in a wider range of sizes than single-stroke open-die heading machines and can produce heads as large as three times the stock diameter. They cannot be used for extrusion, but they can pinch fins on the work-piece, when needed. They normally pinch fins or small lines under the head of the work-piece when these are not needed and if these fins or lines are objectionable, they are to be removed by another operation.
Three-blow heading machines use two solid dies along with three punches and are classified as special machines. Having the same basic design as double-stroke heading machines, these machines provide the additional advantage of extruding or upsetting in the first die before double-blow heading or heading or trimming in the second die. Three-blow heading machines combine the process of trapped extrusion and upsetting in one single machine to produce special fasteners having small shanks but large heads. These heading machines are also ideal for making parts with stepped diameters in which the transfer of the work-piece is accomplished with great difficulty.
Rod heading machines and reheading machines are two special types of heading machines. Rod heading machines are open-die heading machines having either single or double stroke. They are used for extremely long work (8 times to 160 times stock diameter). The work-piece is cut to length in a separate operation in another machine and fed manually or automatically into the rod heading machines.
Reheading machines are used when the work-piece is to be annealed before heading is completed, for example, when the quantity of cold working needed causes the work metal to fracture before heading is complete. Reheading machines are made as either open-die or solid-die machines, single or double stroke, and can be fed by hand or hopper. Punch presses are also used for reheading.
Transfer heading machines are solid-die machines with two or more separate stations for different steps in the forming operation. Each station has its own punch-and-die combination. The work-piece is automatically transferred from one station to the next. These machines can perform one or more extrusions, can upset and extrude in one operation, or can upset and extrude in separate operations. Maximum lengths of stock of various diameters headed in these machines range from 150 mm with 10 mm diameter to around 250 mm with 20 mm diameter. These machines can produce heads of five times stock diameter or more.
Bolt makers can trim, point, and roll threads. Bolt makers normally have a cut-off station, two heading stations, and one trimming station served by the transfer mechanism. An ejector pin drives the blank through the hollow trimming die to the pointing station. The trimming station can be used as a third heading station or for extruding. In bolt makers, the last station in the heading area is a trimming station. The trimming die (which is on the punch side) is hollow, and the die ejector pin drives the trimmed work-piece completely through the die and, by an air jet or other means, through a tube to the pointing station.
Pointers are of two types. Some have cutters which operate much like a pencil sharpener in putting a point on the work-piece (hence producing some scrap), others have a swaging or extruding device which forms the point by cold flow of the metal. The pointed work-piece is placed in a thread roller. A bolt maker has a thread roller incorporated into it. The rolling dies are flat pieces of tool steel with a conjugate thread form on their faces. As the work-piece rolls between them, the thread form is impressed on its shank, and it drops out of the dies at the end, frequently as a finished bolt.
Nut formers have a transfer mechanism which rotates the blank 180-degree between one or two dies or all the dies. Hence, both ends of the blank are worked, producing work-pieces with close dimensions, a fine surface finish, and improved mechanical characteristics. A small slug of metal is pierced from the centre of the nut, which amounts to 5 % to 15 % waste, depending on the design of the nut.
Parts formers are flexible multi-station machines designed for making a variety of cold-formed parts. These machines can have up to six stations or seven stations, versatile transfer mechanisms, and punch kick-outs allowing them to make complex parts. Parts formers are also equipped with quick tool changing and can also handle wire feed or slug feed. With these machines, different additional operations, such as extrusion, notching, coining, and undercut forming, can be performed on either end of the blank to produce complex net or near-net shaped parts at a very high rate.
Tools – The tools used in cold heading consist principally of punches and dies. The dies can be made as one piece (solid dies) or as two pieces (open dies). Solid dies (also known as closed dies) consist of a cylinder of metal with a hole through the centre. They are normally preferred for the heading of complex shapes. Solid dies can be made entirely from one material, or can be made with the centre portion surrounding the hole as an insert of a different material. The choice of construction depends largely on the length of the production run and / or complexity of the part. For extremely long runs, it is sometimes desirable to use carbide inserts, but it can be more economical to use hardened tool steel inserts in a holder of less expensive and softer steel.
When a solid die is made in one-piece, common practice is to drill and ream the hole to within 0.076 to 0.13 mm of finish size before heat treatment. After heat treatment, the die is ground or honed to the desired size. Surface roughness for cold-heading tools is to be around Ra (average roughness) = 0.1 micrometres to 0.2 micrometres, with Rz (peak-to-valley height measurement) = 1 micrometre maximum.
Solid dies are normally quenched from the hardening temperature by forcing the quenching medium through the hole, making no particular attempt to quench the remainder of the die. By this means, maximum hardness is attained inside the hole, the outer portion of the die is softer and hence more shock resistant. Since the work metal is not gripped in a solid die, the stock is cut to length in one station of the heading machine and the cut-to-length slug is then transferred by mechanical fingers to the heading die. In the heading die, the slug butts against a back-stop as it is headed. Ordinarily, the back-stop also serves as an ejector.
Open dies (also called two-piece dies) consist of two blocks with matching grooves in their faces. When the grooves in the blocks are put together, they match to form a die hole, as in a solid die. The die blocks have as many as eight grooves on various faces so that as one wears, the block can be turned to make use of a new groove. Since the grooves are on the outer surface of the blocks, open-die blocks are quenched by immersion to give maximum hardness to the grooved surfaces. Open dies are normally made from solid blocks of tool steel, because of the difficulty involved in attempting to make the groove in an insert set in a holder. Open dies are made by machining the grooves before heat treating, then correcting for any distortion by grinding or lapping the grooves after heat treating.
In open-die heading, the dies can be permitted to grip the work-piece, similar to the gripper dies in an upsetting machine. When this is done, the backstop needed in solid-die heading is not necessary. However, some provision for ejection is frequently incorporated into open-die heading.
Design – The shape of the head to be formed in the work-piece can be sunk in a cavity in either the die or the punch, or sometimes partly in each. The decision on where to locate the cavity frequently depends on possible locations of the parting line on the head. It is possible to extract the work-piece from both the punch and the die. It is normally useful, but not entirely necessary, to design some draft in the work-piece head for ease of ejection. An important consideration in the design of cold-heading tools is that the part is to stay in the die and not stick in the punch. Hence, it is particularly difficult to design tooling for midshaft upsets. Where possible, the longest part of the shank is left in the die.
There is less of a problem with open dies which use a special die closing mechanism. Some punches are equipped with a special synchronized ejector mechanism for ensuring that the work-piece comes free. Cold heading imposes severe impact stress on both punches and dies. Minor changes in tool design frequently register large differences in tool life.
Other important factors to consider in cold heading tools are pre-stressing and venting. Die inserts are placed into radial compression by an interference fit between the outside surface of the insert and the inside surface of the case (stress ring). Multiple inserts can be used for the highest pre-stressing applications. Venting is necessary to prevent trapping of air and lubricants in tools, which can lead to increased tool pressures (decreased tool life) and problems with underfill.
Tool materials – Materials selection for dies and punches in cold heading is similar to cold extrusion. Wear resistance and toughness are the main properties used in choosing cold-heading tools. Other properties which are to be considered include hardness, compressive strength, fatigue strength, and stiffness. Also, it is important to understand the nature of the tool loading and ultimate failure mode when selecting materials, since this guides the selection process. For example, tools which routinely fail by abrasion or galling need improved wear resistance, whereas tools which fail by chipping or fracture need additional toughness.
Punches – Shallow hardening tool steels, such as W1 or W2 quenched and tempered to 58 HRC to 62 HRC, can be used for cone and finish punches as well as heading punches which are not highly loaded. Air-hardening grades A2 and D2 (59 HRC to 61 HRC) or high-speed steels such as M2 or M4 (61 HRC to 63 HRC) provide improved hardness and wear resistance while maintaining adequate toughness. For highs-peed part formers where considerable heat is generated and the coolant time is frequently limited, grades such as T15, HS 10-4-3-10, or HS 6-5-3-8 are superior to conventional tool steels.
Applications which entail high loads and wear, such as extrusion punches and mandrels, indenting punches for recessed-drive fastener features, and piercing pins, are frequently produced from cemented carbides because of their much higher compressive strength and wear resistance. Optimal performance for carbides in these applications is provided by micro-structures consisting of moderate grain size (1.5 micrometres to 4.5 micrometres) and cobalt binder levels of 6 % to 12 %, which have been consolidated using hot isostatic pressing. Shock-resistant tool steels such as S1 and S7, are also used for the cold heading of tools, especially for the heading of intricate shapes when tool materials such as W1 and carbide have failed by cracking. The shock-resistant steels are normally lower in hardness than preferred for maximum resistance to wear, but it is frequently necessary to sacrifice some wear resistance to gain resistance to cracking.
When producing bolts which have square portions under the heads, or dished heads, or both, the right tool steel selection is important for preventing tool failure. Newer-generation matrix high-speed steels and cold work tool steels with 8 % chromium have been successfully used as substitutes for grades such as D2 (12 % chromium) or M2 (high-speed steel) when improved toughness is needed.
Tool steels produced by the powder metallurgy (P/M) process provide superior performance compared to conventional wrought products because of lack of segregation, smaller primary carbide size, uniform distribution of carbides, and fine grain size. Proprietary P/M grades are available from a number of tool steel producers which offer substantial improvements in wear resistance while maintaining toughness equal to conventional grades, such as M2 or D2. The combination of very high carbon levels (frequently higher than 2 %) and considerable quantities of chromium, molybdenum, tungsten, cobalt, and vanadium result in a large volume fraction of carbide particles dispersed in the steels, which results in exceptionally high strength and wear properties.
Dies and Inserts – W1 and W2 tool steels can be used for simple heading dies made without inserts. Inserts are normally made from high-alloyed steels, such as D2, M2, and M4 (60 HRC to 64 HRC), or from tungsten carbide having a relatively high percentage of cobalt (13 % to 25 %) for higher toughness. Improved tool steel performance is achieved with P/M grades such as A11, M4, and T15, with the two latter grades especially useful for operations with low lubrication and high working temperatures. Carbides are preferred for high-volume production and for cold heading of difficult-to-form steels (high forming loads). Extrusion dies typically feature 12 % to 25 % Co binder, with upsetting dies needing 20 % to 25 % cobalt for maximum toughness.
Support tooling – Kick-out pins (ejectors) are typically made from O1 or A2 hardened to 59 HRC to 61 HRC. Pins which support large loads during heading or extrusion can be produced from M2 hardened to 61 to 64 HRC. Pressure pads can use a number of steels, depending on the specific environment. Cutters and quills can be fabricated using tool steels, but carbides have better resistance to wear and dulling. In assemblies, inner stress rings are normally made from H13, D2, or M2, and outer stress rings and cases are made from H13 or L6. Designs which need significant press fits in order to generate the desired preloads use maraging steels heat treated to around 54 HRC.
Coatings have become a significant part of the tool engineering process. Thin-film coatings deposited by the chemical vapour deposition (CVD) and physical vapour deposition (PVD) processes are normally applied to all types of tool steel and carbide components, with the most frequently used being the PVD coatings of TiN, TiCN, and TiAlN. Wear-critical parts, such as extrusion punches and recessed-drive indenting punches, have such significantly improved tool life that they are always be coated. The advantages for the PVD technique are low process temperature results in no dimensional changes after final tempering, thin coatings do not impair tolerances during forming, and polishing after coating is not always necessary. However, carefully controlled polishing after coating further improves the friction properties and reduces galling.
Tool steels which are tempered at low temperatures, such as W1, O2, and S7, normally cannot be coated for improved surface wear properties. The CVD and PVD processes and the subsequent heat treatment methods are to be carefully controlled to prevent grain growth and carbide coarsening in tool steels, which considerably degrade wear resistance and fatigue strength.
Preparation of work metal
The operations needed for preparing stock for cold heading can include heat treatment, drawing to size, machining, descaling, cutting to length, and lubricating.
Heat treatment – The cold-heading properties of the majority of the metals are improved by some form of thermal treatment after hot rolling. The steel cold-heading industry has developed the conventions for describing wire which include, (i) DD, direct drawn from wire rod or bar, (ii) DFAR or DFAB, drawn from annealed rod or bar, (iii) DFSR or SFSB, drawn from spheroidize-annealed rod or bar, (iv) AFS or SAFS, drawn to size and annealed or spheroidize annealed, and (v) AIP or SAIP, drawn, annealed, or spheroidize annealed in process and finally lightly drawn to size.
Regular annealing is performed by heating wire near or below the lower critical temperature (Ac1) holding for a suitable time, and then slow cooling. This process does not produce a specific micro-structure or surface finish. Spheroidize annealing consists of prolonged heating near or slightly below the Ac1 temperature, followed by slow cooling, and produces a micro-structure consisting of spheroidal (globular) cementite distributed throughout the ferrite matrix. Spheroidizing has conventionally been performed using either batch (bell or car bottom furnaces) or long continuous (pusher-type or roller-hearth) furnaces, with batch roller-hearth furnaces (short time cycle) having been more recently introduced.
Typical processing time for spheroidizing is from 12 hours to 24 hours, making it by far the most time-consuming stage in steel part production. Both inter-critical and subcritical process cycles are used, with the inter-critical process being more susceptible to decarburization and high energy consumption because of the higher temperatures involved. Hyper-eutectoid steels such as SAE 52100 need inter-critical annealing in order to break up coarse pro-eutectoid cementite. Both ferrous and non-ferrous precipitation hardening alloys are frequently solution heat treated prior to forming and then subsequently age hardened.
Drawing to size produces stock of uniform cross section which performs as predicted in dies which have been carefully sized to fill out corners without flash or die breakage. Tolerances for diameter and out-of-roundness are important factors in controlling the volume of metal to be worked and are included in different industry standards. Out-of-round wire can cause localized die wear showing up as wear rings in the drawing die. The elliptical cross section produces non-uniform cold work around the circumference of the wire, which contributes to distortion of the product and causes strength and ductility variation through the cross section. Producers frequently produce wire and wire rod with reduced tolerances. Some steel producers have developed precision rolling techniques which can produce wire rod with diameter tolerances as low as 0.2 mm. Drawing to size also improves strength and hardness when these properties are to be developed by cold work and not by subsequent heat treatment. This is very important for nuts and other threaded fasteners, since insufficient strain hardening during wire drawing can result in failure to meet specification requirements.
While a fully spheroidized micro-structure is desirable for formability, steel wire is rarely used in the as-spheroidized condition, because of the poor coil configuration, the formation of a shear lip during cut-off, and the potential for undesirable bending of long sections during upsetting. For these reasons, almost all materials are given a light wire-drawing reduction (skin passing), normally in the range of 3 % to 8 % but frequently as high as 20 %, after the thermal treatment. This wire drawing can be performed either by the wire producer or in front of the forming operation, depending on the wire size and application. Wire drawing in front of the heading machine can decrease costs and reduce strain aging.
Precipitation-hardening alloys, such as MP35N or A-286, which are used for high-strength threaded fasteners can be processed with large cold reductions after solution treating, as high as 20 % to 36 % for MP35N and 50 % to 60 % for A-286. The strains in the cold-worked areas promote increased precipitation hardening and higher strength. Metallurgical defects, such as central bursting and redundant work produced during drawing, can degrade workability.
Turning and grinding – Drawn wire can have defects which carry over into the finished work-piece, exaggerated in the form of breaks and folds. Seams in the raw material which cause these defects are not to be deep enough to be objectionable in the shank or body of a bolt but can cause cracks in the head during cold heading or subsequent heat treatment. Surface seams, laps, and other defects can be removed by turning, grinding, or shaving at the wire mill or by machining the headed product. A typical quantity of removal is 0.2 mm to 0.5 mm, for a total diameter reduction of 0.4 mm to 1 mm). Applications which use shaved wire include aerospace and specialty fasteners, bearing races, and engine poppet valves (intake and exhaust).
Descaling – Work metal which has been heat treated normally needs to be descaled before cold heading. Scale can cause lack of definition, defects on critical surfaces, and dimensional inaccuracy of the work-piece. Methods of descaling include abrasive blasting, water-jet blasting, pickling, wire brushing, and scraping. Selection of method depends largely on the quantity of scale present and on the needed quality of the surfaces on the headed work-pieces. Acid pickling is normally the least expensive method for complete removal of heavy scale. Improved descaling during batch pickling of coils is achieved when the individual loops are separated on the hook in order to allow complete surface contact with the acid. Vibration or oscillation during pickling of larger-weight coils can be used instead of unbanding them. Also, rinsing the coils with a high-pressure spray after an immersion rinse removes residual acid and smut.
Cutting to length – In a heading machine which has a shear-type cut-off device as an integral part of the machine, cutting to length by shearing is a part of the sequence. In applications in which cutting to length is done separately, shearing is the method normally used for bars up to around 50 mm in diameter. For larger diameters, sawing is normally used. Gas cutting and abrasive-wheel cutting are considered obsolete and no longer used in contemporary applications. Because of the very high cutting speed, there is little deformation or damage to the end of the wire, even when cutting short blanks from spheroidized material. When combined with a linear feed mechanism which eliminates the need for a wire stop, improved volume control and minimal work hardening of the slug ends occur. In several cases, this eliminates the need to square up the blank in the first station.
Coating and lubrication – Although some of the more ductile metals can be successfully cold headed to moderate severity without lubrication, majority of the metals to be cold headed are lubricated to reduce forming loads, prevent galling and sticking in the dies, and avoid excessive die wear. There are a number of coatings applied to heading stock, depending on alloy type and the nature of the forming, such as lime or borax coatings, zinc phosphate or oxalate conversion coatings, film layers of stearate soaps or molybdenum disulphide (MoS2), and plating with softer metals such as copper.
Newer dry-film coatings consisting of acrylic and polyolefin polymers have gained acceptance because of their ease of cleaning and environmental friendliness, in addition to their lubricating qualities. Lubricants are typically mineral oils, or synthetic oils, since majority of water-emulsifiable compounds have inadequate film strength or wettability to prevent contact between the work-piece and the tools, especially during heavy extrusion operations. Lubricant oils are compounded with polar additives, such as fatty acids and esters, in order to wet the metal surfaces, as well as other modifiers, such as extreme-pressure (EP) additives, anti-foaming and anti-bacterial agents, detergents, and anti-oxidants. Because of the environmental concerns with EP additives based on chlorinated paraffins, olefins, or fatty oils, lubricants are being replaced by newer formulations, normally with EP additives based on sulphur and including increased quantities of friction and anti-wear modifiers.
Coatings for carbon and alloy steel bars, wire rods, and wire which are thermally treated at finished size (AFS, SAFS) include lime (CaO) or borax (hydrated sodium borate), zinc phosphate plus lime, zinc phosphate plus reactive or non-reactive stearate, or zinc phosphate plus lime and polymer. If cold drawing is the final operation (AIP, SAIP), a drawing compound consisting of calcium or aluminum stearate, possibly with an addition of MoS2 for severe upsetting or extrusion, is applied in the die box. This produces a dry, hard, non-gummy film which minimizes slippage during the feeding process at the heading machine and reduces the potential for die clogging.
Upsets of low to moderate severity can be produced with material using lime and soap coatings, sometimes augmented by oils or greases applied to the wire upon entering the forming machine. Lime is applied after pickling by dipping the coils in a 2 % to 12 % suspension of lime in water at around 80 deg C. Thicker coatings are achieved by dipping and drying up to three times. The roughness of the coating promotes adhesion of lubricants such as drawing compounds and dry-film polymers.
While these finishes are widely used, phosphate coatings are frequently used for the more demanding applications. Phosphate coatings have a wide range of crystal shapes and coating weights / thicknesses, with majority of the cold forming applications based on fine-grained, iron-free crystals yielding coating weights between 5 grams per square metre (g/sq-m) and 15 g/sq-m and layer thickness of 2 micrometres to 15 micrometres. The coating is to be dense and completely cover the surface, with no bare spots. Typical upsetting and light extrusions use lower coating weights, with backward or multiple forward extrusions needing coating weights of at least 10 g/sq-m in order to provide separation between the tools and work-piece even after considerable surface expansion has occurred.
The AFS / SAFS wire is normally coated with a reactive sodium stearate lubricant after phosphating, with the reacted layer forming an insoluble zinc stearate. This reacted layer typically has a coating weight of around 0.8 g/sq-m to 2.2 g/sq-m, and the excess sodium stearate layer is typically 1.1 g/sq-m to 2.2 g/sq-m. The total layer thickness is normally less than 10 micrometres thick. This product is intended to be drawn in front of the cold header prior to forming.
Liquid lubricants are extremely important when any forming of the sheared ends takes place, especially during multiple extrusion operations. The lubricants are applied to the work-pieces and tools by means of flooding and spraying. They are particularly necessary during heading and extrusion of difficult-to-form metals such as work-hardened precipitation-hardening stainless steels and nickel-base superalloys. Stainless steels and specialty ferrous alloys such as A-286 are normally electroplated with 2 micrometres to 2.5 micrometres of copper (applied over a nickel strike) and then lubricated with oil / grease, soap, or molybdenum disulphide.
Simple upset heads can be accomplished with either pre-coat or lime coatings which have been drawn in soap or grease. Pre-coat is an aqueous dip of potassium sulphate or sodium sulphate salt which forms a crystalline coating on the metal surface. The MoS2 coatings (drawn in soap or grease) are used for typical upsets and when sharper corners need to be filled. The most severe forming needs a thicker copper layer (3 micrometres to 10 micrometres) plus MoS2 which is either applied to AFS wire or to AIP wire which is subsequently skin passed (3 % to 5 % reduction) in a drawing soap.
Induction heating of stainless and specialty alloys for warm forming needs copper plating plus MoS2, since the higher temperature exceeds the capabilities of other lubricants. Normal hot alkaline cleaning does not remove electroplated copper, which necessitates the use of nitric acid immersion. The other coatings can be normally removed with alkaline cleaners. The presence of a passive oxide film on these alloys means that they are not easily phosphated, so oxalate coatings are used instead. A typical application consists of 5 micrometres to 8 micrometres of oxalate with a stearate soap or MoS2 lubricant layer on top. The use of oxalate coatings is on the decline because of the environmental reasons.
In all cold heading, the best practice is to use the simplest and the least lubricant which provides the acceptable results, for two reasons namely (i) excessive quantities of lubricant can build up in the dies, resulting in scrapped work-pieces or damaged dies, and (ii) removal of lubricant is costly (the cost of removing lubricant normally increases in proportion to the effectiveness of the lubricant).
Complex work-pieces
Cold-headed products which have more than one upset portion need not be formed in two heading operations. Several can be made in one operation of a double-stroke heading machine. The length of the work-piece which can be partly upset is normally limited to five times the diameter of the wire. The only other limitation is that the heading machine is to be able to accommodate the diameter and length of wire needed for the work-piece. Fig 9 shows typical part with centre upsets and upsets at both ends.
Fig 9 Typical part with centre upsets and upsets at both ends
Three pieces, each with two end upsets which have been made completely in one operation in a double-stroke open-die heading machine, are shown in Fig 9a. These parts have been made at a rate of 80 pieces / min. Production rate is limited only by the speed of the machine used, not by the item being produced. The product becomes more expensive when the upsetting operation has to be performed twice, as in production of the 710 mm long axle bolt shown in Fig 9b. This part has needed two upsetting operations, since the die in a standard double-stroke cold header has not been long enough to form both upsets in the machine at the same time. One or more additional operations can be needed for work-pieces which need pointing as well as a complex upset.
Centre upsetting – Majority of the cold heading involves forming an upset at the end of a section of rod or wire. However, the forming of upsets at some distance from the end is a normal practice. The trailer-hitch-ball stud shown in Fig 9c is representative of an upset performed midway between the ends of the wire blank. This stud has been upset and extruded in two strokes in an 18 mm solid-die machine. The diameter of one end section is smaller than that of the original wire, and the round centre collar is flared out to more than 2.5 times the wire diameter. The centre-collar stud shown in Fig 9d is another example of a centre upset. Both ends of the stud have been extruded below wire size, while the centre collar has been expanded to more than three times the original wire diameter. This stud has been formed in three strokes in a progressive header.
Control of the volume of work metal to prevent formation of flash and to prevent excessive loads on the tools is important in the majority of the cold heading operations. In centre upsetting, control of metal volume is normally even more important, not only to prevent flash and tool overload but also to prevent folds.
Segmented dies, multiple upsets, and blank rotation – Segmented die forming is capable of producing parts with multiple upsets, notches, grooves, formed or pierced holes on different axes, and non-symmetrical features with offset axes. The latter two variations still use standard machine motion and transfer, since the secondary axis is parallel to the primary axis. Fig 10 shows how multiple upsets can be produced using segmented dies and an air-loaded punch pin.
Fig 10 Process sequence for upsetting a second head on a caster stem
The air-loaded punch pin pushes the blank into the die while the rest of the punch tooling is moving toward the die. The inserts are closed by axial movement along the tapered grooves, because of the punch case making contact with the stationary die. Once the moving die segments reach their fully closed position, the punch tooling is now fixed, and a cavity is formed in the desired shape of the new upset. Continued advancement of the punch pin now upsets the secondary head. The cycle is completed as the punch case begins to recede from the stationary die, the insert segments are moved into the open position by the ejector(s) in the punch, and the punch pin remains in the forward position to keep the blank pressed into the die. Blank rotation is a tooling feature which rotates the work-piece from a horizontal to a vertical orientation, allowing forming operations to be performed at 90-degree to the original axis.
Fig 11 shows the cold forming progression used to produce an M6 eyebolt blank. The process starts with a conventional cut-off blank, followed by forward extrusion at the first operation and upsetting / forward extrusion at the second operation. The third operation consists of a 90-degree rotation of the blank, with no forming taking place. The fourth hit flattens the eye section, with the last hit piercing the centre of the eye.
Fig 11 Process sequence used to cold head an M6 eyebolt
Economy in cold heading
Cold heading is an economical process because of high production rates, low man-power costs, and material savings. Production rates range from around 2,000 pieces per hour to 50,000 pieces per hour, depending on the part size. Fewer machines are needed to meet production requirements than with other processes, resulting in reduced costs for equipment, maintenance, and floor space. Man-power costs are minimal, since majority of the operations are performed automatically, needing man-power only for setup, supervision, and parts handling.
Material savings results from the elimination or reduction in chips produced. Typical scrap losses are 1 % to 3%, with the only waste coming from piercing and trimming. When cold heading is combined with other operations, such as extrusion, trimming, and thread rolling, the savings is considerable. Subsequent machining or finishing of the cold headed parts is normally not necessary. This can be especially beneficial when relatively expensive work materials are used.
Reverse forming
Reverse forming consists of forming a shape by upsetting or extruding into a die mounted on the moving press ram with a punch tool mounted on the stationary segment. Reverse forming is used frequently to increase forming speeds on short or complex parts which are difficult to grip and transfer (Fig 12). It is also used to convert 180-degree transfer processes into straight-across processes when upsetting and extrusion operations are needed. Tooling arrangements are used on machines with an appropriate working stroke which allow parts to be held between the punch and die kick-out pins, fully supported, as the press slide withdraws.
Fig 12 Conventional and reverse forming methods
Dimensional accuracy
Part tolerances which can be achieved in cold heading are dependent on a number of variables, including the type of forming (open heading, contained heading, and combined operations), the severity of upset or extrusion, the length-to-diameter ratio of the blank, the type of steel being formed, and the quality of the tooling and machine. Work can be produced to much closer tolerances in cold heading machines than in hot heading machines. Tolerances on parts produced by single-stroke heading machines need to be higher than on parts given two or more blows. Rivets, frequently formed in single-stroke machines, have tolerances of +/- 0.38 mm, except where otherwise specified. Shanks for rolled threads frequently are allowed only +/- 0.038 mm. Small-parts can normally have closer tolerances than larger parts.
Diameter tolerances as close as +/- 0.013 mm can be achieved on solid sections by using precision sizing (ironing) dies, although maintenance of a tolerance this close increases (i) product cost, (ii) needs careful control of machines, tools, and work metal, and (iii) is unusual in practice. More typical values for trap-extruded diameters are +/- 0.05 mm to 0.08 mm. Diameters produced by open heading can normally be controlled to a tolerance of +/- 0.18 mm to 0.38 mm, whereas a tighter tolerance of +/- 0.08 mm to 0.13 mm is achievable if the head can be contained at least partially in the die.
State-of-the-art production techniques can produce parts with feature-to-feature length variations of only +/- 0.05 mm, but more typical values for smaller parts (less than 50 mm) are +/- 0.13 mm. Total part length variations down to +/- 0.25 mm are possible for smaller parts and between +/- 0.38 mm and 0.80 mm for longer parts. Concentricity as measured by total indicated runout (TIR) can be as low as 0.03 mm to 0.07 mm for parts using advanced reverse forming methods with conventional practices produce values of higher than and equal to 0.15 mm. For longer shafts and bolts (length higher than 200 mm and length / diameter ratio around 25 to 1), TIR is of the order of 6 micrometres per millimetre of length or more without subsequent straightening.
Surface finish
Surfaces produced by cold heading are normally smooth and seldom need secondary operations for improving the finish. However, surface roughness can vary considerably among different work-pieces or among different areas of the same work-piece, depending on (i) surface of the wire or bar before heading, (ii) quantity of cold working in the particular area, (iii) lubricant used, and (iv) condition of the tools.
Cold drawing of the wire before cold heading improves the final surface finish. The best finish on any given work-piece is normally where direct contact has been made with the tools, such as on the top of a bolt head or on an extruded shank portion where cold working is severe. The lubricant is likely to have a higher effect on the appearance of a headed surface than on surface roughness as measured by instruments. For example, heavily limed or stearate-coated wire produces a dull finish, but the use of grease or oil results in a high-lustre finish.
The condition of the tools is very important in controlling the workpiece finish. Rough surfaces on punches or dies are registered on the work-piece. Hence, the best surface finish is produced only from tools which are kept polished. The ranges of finish shown on the square-necked bolt are typical Ra (average roughness) values for such a part when headed from cold-drawn steel, using ground and polished tools. The best finish is on the top of the head and on the extruded shank, while the poorest finish is on the outer periphery of the round head. Using the peak-to-valley height measurement, Rz, values of 10 micrometres to 63 micrometres are typical for extrusion operations, while values of 4 micrometres can be achieved using specially designed processes. Constrained forming processes such as cold coining can produce Rz values of less than 10 micrometres.
Combined heading and extrusion
It is a normal practice to combine cold heading with cold extrusion, and this frequently permits the selection of a work metal size which greatly lessens forming severity and prolongs tool life. Two parts shown in Fig 9, a trailer-hitch-ball stud (Fig 9c) and a centre-collar stud (Fig 9d), reflect the flexibility in design achieved by combining centre upsetting and extrusion. In addition to increased tool life, other advantages can sometimes be achieved by combining cold heading and cold extrusion.
Warm heading and hot heading
Warm heading and hot heading techniques involve the heating of wire or blanks during certain stages of the heading process and allow forming of more heavily alloyed metals, including precipitation hardening stainless steels and high temperature alloys. To assist in forming parts such as recessed-head screws from tougher metals, wire can be heated before it enters the heading machine. This reduces yield and tensile strengths to improve forming characteristics.
Warm heading in the metallurgical sense is really cold heading since the metallurgical structure is not affected. The material is simply made more ductile. However, with hot heading the metallurgical structure of the material is frequently altered. In both the cases, less pressure is needed to make the metal flow plastically since warm and hot heading techniques lower material strength and increase ductility. Warm heading has been applied successfully in forming stainless steels and high temperature alloys.
Heating is very effective in the 175 deg C to 230 deg C temperature range. While warm heading or hot heading is not normally needed for highly headable stainless grades such as type 430, it can be used with these metals to improve metal flow and avoid stress cracking in severe upsets. Warm heading is accomplished by heating the wire before it enters the feed rolls or, when possible, between the feed rolls and the heading machine. Three types of heating methods are normally used (i) resistance, (ii) gas, or (iii) induction. Hot heading, on the other hand, means heating the wire to the 590 deg C to 650 deg C temperature range. It is almost equivalent to forging. Proper choice of lubricants is necessary for effective warm heading and hot heading. Temperatures for hot heading can be as high as around 980 deg C, depending on the characteristics of the work metal.
In warm heading (a variation of the cold heading process), the work metal is heated to a temperature high enough to increase its ductility yet still below the recrystallization temperature. A rise in work metal temperature normally results in a marked reduction in the energy needed for heading the material, with tooling loads reduced by as much as 50 % compared to cold forming. These lower tool forces normally reduce die breakage, but the higher contact temperatures can result in increased wear.
Applications – Warm heading is occasionally used to produce an upset which has needed a larger machine if the upsetting is to be done cold, but by far the most extensive use of warm heading is for the processing of difficult-to-head metals, such as stainless steels, titanium alloys, and nickel-base alloys. Typical examples include the manufacture of high-strength fasteners from alloys such as A-286, Ti-6Al-4V, and alloy 718, as well as inner and outer bearing races using martensitic stainless steels. Warm heading allows for high-speed production of parts which otherwise have to be forged on vertical presses, frequently eliminating the need for reheating and relubricating the work-piece.
Warm heading is especially useful for forming titanium alloys, because of the limited cold ductility of alpha-beta alloys and the high yield strength of all alloys in the annealed condition. Heating to a temperature of 430 deg C results in around a 40 % reduction in yield strength. Warm forming of relatively simple shapes can be performed in the range of 430 deg C to 590 deg C, with more complicated shapes being headed in the range of 650 deg C to 850 deg C. Titanium alloys are very sensitive to heading speed and readily form adiabatic shear bands at high strain rates.
Machines and heating devices – Warm heading machines are essentially the same as cold-heading machines, except that warm heading machines are designed to withstand the higher temperature of the work metal. Induction heating coils or resistance heating elements can be used as auxiliary heating equipment. Vertical presses can also be used for higher temperature heading operations but are very frequently used on large-diameter parts which are hot headed, meaning above the recrystallization temperature. Induction heating is the method normally used to heat work material for warm heading, although direct resistance heating is also used in some applications.
The wire is normally heated before it enters the feed rolls, but it is advantageous to use a set-up with the induction coil between the feed rolls and the heading machine frame. The main draw-back of induction heating is the high initial cost of the power supply. Hence, its use is normally restricted to continuous high production. Direct resistance heating, on the other hand, has the advantages of simplicity of equipment, accuracy of control, safety (because voltage is low), and adaptability to heating of a continuous length of work metal.
The normal set-up for resistance heating uses a second feeder-roll stand similar to that already on the heading machine. The second stand is positioned around 1.5 metres behind the first, and the wire stock (work metal) is fed through both sets of rolls. Leads from the electrical equipment are attached to the two sets of rolls, and the circuit is completed by the portion of the wire which passes between them. The wire (work metal) then becomes the resistance heater in the circuit.
Temperature control – Close control of wire surface and core temperature are important, since uneven heating causes variable temperature distribution and hence variable deformation resistance. This results in poor workability and difficulty in maintaining dimensional capability. Also, the lubricity of the wire coating can be altered, with the coating smearing onto the tooling at the cut-off station and impairing subsequent formability.
Tools – Whether or not the same tools can be used for warm heading as for cold heading depends entirely on the temperature of the tools during operation. Although the tools normally operate at a temperature considerably lower than that of the work metal, it is important that the tool temperature be known. Tool temperature can be checked with sufficient accuracy by means of temperature-sensitive crayons. Under no circumstances the tool is to be allowed to exceed the temperature at which it gets tempered after hardening. Tools such as die inserts made from a high-alloy tool steel, such as D2, ordinarily are not to be permitted to operate above 260 deg C.
When tool temperatures exceed those discussed previously, the use of tools made from a hot-work tool steel, such as H12 or H13, is appropriate. However, the lower maximum hardness of such a steel somewhat limits its resistance to wear. More typical is the use of high-speed tool steels such as M2 or M4 (60 HRC to 63 HRC) for die inserts, which provide the high hardness and the resistance to tempering needed for long tool life. Standard grades for recessed punches include M1 and M2, with advanced tooling materials such as P/M M4, T15, and A11 being used when conditions warrant.
Other advantages of warm heading – As the heading temperature of a work-hardenable material increases, the resulting hardness decreases. Hence, if a material is warm headed, the hardness remains low enough to permit such secondary operations as thread rolling, trimming, drilling, and slotting. In cold heading, the upset head of a work hardening metal is very hard, a rolled thread is moderately hard, and the undeformed shoulder is relatively soft. These variations can be minimized by warm heading.
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