Cold Heading Quality Steels and Cold Heading Process
Cold Heading Quality Steels and Cold Heading Process
The term ‘cold heading’ is a generic term describing the continuous productions of fasteners of parts by upsetting from wire or wire rod in the coil form. The operation is carried out on specially designed horizontal presses equipped with means of feeding wire from coil, straightening, cutting to length and thence finally forming fastener in one or more blows. The presses range from relatively simple machines equipped with a single punch and die, forming the part in a single blow, to complex multi-die / punch machines with integral means for transferring the part through the die sequence. The process originated in the fastener industry, being originally used to upset the end of a cut-off length of wire to form a rivet or blank for a woodscrew or machine screw.
Cold heading is a process of high productivity using punch and dies to transform the steel wire rod at room temperature. It is a cold-forging process in which the force developed by one or more strokes (blows) of a heading tool which is used to upset (displace) the metal in a portion of a wire or rod blank in order to form a section of different contour or, more frequently, of larger cross section than the original. The process is widely used to produce a variety of small and medium-sized fasteners, such as bolts, nut, and rivets. A specific quenching and tempering process regularly follows cold heading in order to reach the final mechanical properties.
During the process of cold heading, a blank rod is used to produce the different fastener products by applying the external force through different kinds of dies and tools. The original volume of the work piece is not changed but the strength of the steel product after the process is improved to a certain level. Cold heading, however, is not limited to the cold deformation of the ends of a work piece nor to conventional upsetting with the metal displacement can be imposed at any point, or at several points, along the length of the work piece and can incorporate extrusion in addition to upsetting. In cold heading, the cross-sectional area of the initial material is increased as the height of the work piece is decreased.
Advantages of the cold heading process over machining of the same parts from suitable bar stock include (i) almost no waste material, (ii) increased tensile strength, hardness, toughness and fatigue resistance due to the cold working, and (iii) controlled grain flow which improves the finished part grain structure, (iv) design versatility, (v) high strength parts from non heat treatable steels, (vi) cost effective compared to milling, machining, hobbing and chemical etching, and (vii) high production rates.
The conventional cold heading process (Fig 1) starts with the production of wire rods. These wire rods are then drawn into the rod / wire of the required size and the spheroidizing annealing is done before the cold heading operation. Post cold heading operation, the fastener is normally subjected to quenching and tempering and straightening. Quenching and tempering of slender long bolts is particularly challenging, due to the distortions. A subsequent straightening is compulsory, what extends the lead time, increases the number of rejected parts and the scattering of properties between bolts with different straightening deformation. However, there are technological solutions which skip quench and tempering, but achieve the mechanical properties.
Fig 1 Process of cold heading of steels
The cold heading process generates great hardness divergences in areas with dissimilar deformation ratios. Hence, an adequate balance of deformation, wire rod diameter, and equilibrated mechanical features of the raw material is required to minimize the scattering of the fastener properties.
Low carbon steels can be quenched directly after hot rolling in a water cooling bed, obtaining a micro-structure of cubic martensite, ductile and cold formable. Subsequent tempering allows obtaining the desired strength and ductility levels. Pre-treated wire rod shows a micro-structure of tempered martensite and around 800 MPa of ultimate tensile strength.
There are advantages and disadvantages in case of direct use of wire rods foe cold heading. The advantages are (i) ability to manufacture long slender fasteners with final straightening, (ii) cost savings, (iii) reduction of operations and simplification of the manufacturing chain and (iv) lower processing time. The disadvantages are (i) higher heterogeneity of properties, (ii) higher tool wear, (iii) higher forging stresses, (iv) lower level of residual ductility, and (v) higher susceptibility to hydrogen embrittlement.
Alternative processes used for cold heading of steel include deformation hardening (equivalent reduction 30 %-60 %) of micro-alloyed steel thermo-mechanically hot rolled, and cold forming of a quench and tempered wire rod of a low C steel with high ductility, and low carbon and medium carbon steels which, by micro-alloying and thermo-mechanical hot rolling, can be cold forged to achieve the properties required for the fasteners. Deformation hardening process leads to an equivalent ductility loss and heterogeneous mechanical properties between areas with very different deformation ratios.
Good lubrication is essential for steels and for cold heading process. All cold heading operations in steel induce a general increase in mechanical properties, in particular hardness. This phenomenon is to be considered in order to obtain the specifications of the final part.
The tensile strength of an alloyed steel of a certain carbon content can be increased by (i) grain size refinement, (ii) precipitation of carbo-nitrides of micro-alloying elements (boron, titanium, vanadium, and niobium), and plastic deformation. A high deformation during cold heading allows rising noticeably the yield and tensile strength hence it is necessary to have (i) as much homogeneous deformation as possible between extruded and stamped zones, and (ii) a narrow scatter of metallurgical and mechanical properties in the as-rolled wire rod.
Although cold heading is mainly used for the production of heads on rivets or on blanks for threaded fasteners, a variety of other shapes can also be successfully and economically formed by the process. Fig 2 shows the schematic diagrams of the cold heading on an unsupported wire rod or bar in a horizontal machine.
Fig 2 Schematic diagrams of the cold heading on an unsupported bar in a horizontal machine
Headability in a cold heading process is sometimes expressed as the heading limit, which is the ratio of the diameter of the largest possible headed portion to the diameter of the work piece. There is normally a direct relationship between reduction of area in a tensile test and heading limit. Steels are rated for cold heading on the basis of the length of the work piece, in terms of diameter, which can be successfully upset. Equipped with flat-end punches, majority of the cold-heading machines can upset to around two diameters of low carbon steel wire per stroke. If the unsupported length is increased beyond around two diameters, the work piece is likely to fold onto itself, as shown in Fig 3. Punches and dies can, however, be designed to increase the headable length of any work piece. For example, with a coning punch (Fig 3) or a bulbing punch, it is possible to head as much as 6 diameters of low carbon steel work piece in two strokes.
Fig 3 Cold heading of work piece with unsupported length more than two times wire diameter
Cold heading equipment
Standard cold heading machines are categorized as per two characteristics namely (i) whether the dies open and close to admit the work piece or are solid, and (ii) the number of strokes (blows) the machine imparts to the work piece during each cycle. The die in a single-stroke machine has one mating punch while in a double-stroke machine, the die has two punches. The two punches normally reciprocate so that each contacts the work piece during a machine cycle.
Single stroke solid die cold heading machines –These machines are made in diameters of around of 3 mm to 25 mm of the work piece which can be cold headed. Since these machines are 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 cold heading machines – These machines are available in the same sizes as single stroke solid die heading machines. These machines can make short-to-medium length products (normally 8 to 16 diameters long), and they can make heads which are as large as three times the diameter of the work piece. 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 – These machines are made for smaller-diameter parts of medium and long lengths and are limited to heading 2 diameters of the work piece 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 – These 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 diameter of the work piece. These machines cannot be used for extrusion, but these can pinch fins on the work piece, when needed. These machines normally pinch fins or small lines under the head of the work piece when these are not required. If these fins or lines are objectionable, these are to be removed by another operation.
Three blow heading machines – These machines utilize 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 machines are also ideal for making parts with stepped diameters in which the transfer of the work piece is accomplished with great difficulty.
Transfer and progressive heading machines – These machines are solid-die machines with two or more separate stations for various steps in the forming operation. 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 the work piece different diameters headed in these machines range from 150 mm to 255 mm. These machines can produce heads of five times diameter of the work piece or more.
Bolt making machines – These machines are solid die heading machines similar to transfer and progressive heading machines, but these machines can trim, point, and roll threads. Bolt-making machines normally consist of a cut-off station, two heading stations, and one trimming station which are 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. Bolt making machines are made for bolt diameters ranging from 4.5 mm to 32 mm.
Rod heading machines – These machines are open die heading machines having either single or double stroke. They are used for extremely long work piece (8 times to 160 times work piece diameter). The work piece is cut to length in a separate operation in another machine and fed manually or automatically into the rod heading machine.
Reheading machines – These machines are used when the work piece is required to be annealed before heading is completed. As an for example, when the amount of cold working needed cause the work piece 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.
Nut forming machines – These machines normally have four or five forming dies and a transfer mechanism which rotates the blank 180 degrees 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.
Cold heading quality steels
Cold heading quality steel is the raw material which is used for the production of fasteners such as bolts, screws, nuts, rivets, nails, and other similar complex parts. Traditionally fasteners have been manufactured using the thread cutting or by hot working method. But now the trend is moving towards using the cold working process to enhance productivity and to keep the cost down. It also provides good surface finish and dimensional accuracies to the fasteners.
Cold heading is normally carried out on low carbon steels having hardnesses in the range 75 HRB to 87 HRB. Steels containing upto around 0.2 % carbon are the easiest materials to cold head. Medium carbon steels containing around 0.4 % to 0.45 % carbon are fairly easy to cold work, but their 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 normally achieved when heading low carbon steels.
Microstructure also influences the upsettability of the steels. The work material can sometimes be cold worked during the wire-drawing process, resulting in an increase in tensile strength and difficulty in cold heading. Large deformations or difficult-to-work materials frequently need process or spheroidization annealing before cold heading.
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. These materials work harden more rapidly than carbon steels and hence are 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 problems can be reduced by preheating the work piece.
Many different low carbon, medium carbo, and alloy steel grades are used to make all the various strength grades and property classes of steel fasteners suitable for service between -50 deg C and 200 deg C. In addition to the effects of steel composition on corrosion resistance and high temperature properties, the hardenability of the steels used for threaded fasteners is important when selecting the chemical composition of the steel. As strength requirements and section size increase, hardenability becomes a major factor. Ductility and strength required for cold heading are obtained by a wide range of low carbon, alloyed and boron grades. Bainitic grades are also used in specific applications.
Steel grade 1022 is a popular low carbon steel for the fasteners, although the low carbon content limits hardenability and therefore confines 1022 steel to the smaller diameter product sizes. For many product diameter sizes, this steel grade is one of the most widely used steels for the fasteners upto the level of combined size and proof stress at which inadequate hardenability precludes further use. This medium carbon steel has achieved its popularity because of excellent cold-heading properties, low cost, and availability. Steel grade 1541 steel is extensively used for applications requiring hardenability greater than that of 1038 steel, but less than that of alloy steels.
The low carbon aluminum killed steel for cold heading requires good forgeability properties. In such steels it is important to minimize the amount of free carbon and nitrogen in solution which is not forming carbides and nitrides. This is because the carbon and nitrogen in the steel cause strong work hardening when the steel temperature rises. In order to fix the free nitrogen, addition of titanium is more effective than aluminum. An addition of chromium is also helpful for deceasing of the carbon in solution since chromium forms carbides. Other methods for fixing the free carbon and nitrogen are control rolling and the aging treatment.
The steel grade can also be alloyed with elements such as manganese, chromium, boron and molybdenum depending on the requirements of the final product. The chemical analysis is a trade-off between the necessary ductility prior to processing and the final properties obtained after quenching and tempering. Specific grades of steels have also been developed for engine bolts with an ultimate tensile strength of over 1500 MPa and improved hydrogen resistance.
The selection of the right cold heading quality steel depends on the requirements of the process to which it is being subjected. Principally it is based on the deformation required to produce the final product and mechanical properties to be achieved. In some cases magnetic properties and mainly its corrosion behaviour are also needed to be considered. Optimum cold heading properties require adjustment of the chemical compositions of the steels, low level of non-metallic inclusions as well as the right mechanical properties. A key characteristic is the material surface which is to be completely free of defects as well as suitable and appropriate for coating, in order to lubricate the metal-metal contact at the time of deformation to minimize the effort. The diameter tolerance is another set of parameters necessary for cold heading application.
A good quality feed stock for cold heading is to be a subtle combination of a uniform microstructure and a phosphate coating without any presence of roll seam and/or rust. Steel material containing inclusions of MnS-stringers, around 30 micrometers long, and aligned along its length can badly affect the quality of a fastener, and can contribute to an early failure when employed in service. However, small inclusions of sulphides and oxides, less than 6 micrometers long, have little influence on product quality.
A low-carbon, precipitation strengthened bainitic steel has been specially designed for the production of cold-headed products without heat-treating operations. The chemical composition of the bainitic steel has been developed using high Ti content, in the range of 0.1 % to 0.2 %. This steel contained 0.06 % to 0.08 % carbon, around 1.9 % manganese, around 0.3 % nickel + copper, around 0.002 % boron and around 0.1 % titanium. The low-carbon cementite-free granular bainite, in which the precipitation of brittle cementite is replaced by the finely dispersed MX-type carbides and a ductile second phase, is the most suitable micro-structure, which fulfils the cold headability requirements. This steel has the exceptional workability of wire rod, as well as the high strength and ductility of the final products, which can be achieved by developing in the wire rod during TMCP either non-recrystallized or, alternatively, dynamically recrystallized austenite grains with an average size of less than 15 micrometers, followed by accelerated cooling at rates in the range 3 deg C to 6 deg C per second to around 400 deg C to 500 deg C. After accelerated cooling, the wire rod is slowly cooled in coil, which allows for intense precipitation of titanium carbide.