Ring Rolling

Ring Rolling

Ring rolling is one of the metal-forming operations which decreases the thickness (cross section) and increases the diameter (circumference) of the work-piece by squeezing effect as it passes between two rotating rolls. It is an advanced technique, extensively used to produce seamless rings which are normally being used as flanges, pipe flanges, ring gears, structural rings, gas-turbine rings, nuclear reactor parts, aero-engine casing, and different connecting flanges.

Ring rolling is an incremental bulk metal forming process used for the production of seamless rings with a wide variety of sizes and shapes as well as processable materials, which allows application of rolled rings in several industries. The process works by reducing a pre-formed cross-sectional area of a ring in two roll gaps, increasing its diameter. For achieving this, a non-driven mandrel moves in the direction of the rotationally driven main-roll in the radial roll gap, reducing the wall thickness of a ring. Also, two driven conical rolls located vertically on the opposite side of the machine shape the axial roll gap, in which the height of the ring is reduced by downwards movement of the upper roll. Two guide rolls ensure circularity as well as alignment of the ring in the machine by inducing lateral forces aiming inwards on both sides of the axis of the machine.

Rolled rings and related products are being used for several years in many applications. Input materials consist of carbon steel, aluminum, nickel, and cobalt alloys as well as copper, brass, and several types of tool steels. The use of the rolled ring mainly falls into several industry groups. Production of transportation components normally means high quantities. Lot sizes are typically in the thousands or tens of thousands. Automotive parts include bearings, ring gears, final drive gears, transmission components, clutch components, and wheel blanks. Parts for railways include wheel bearings, railway wheels, and tanker flanges. Another related market segment is off-road equipment such as bull-dozers and earth-movers. Parts for off-road equipment include under-carriage components such as drive sprockets, idler rims, and large bearings for cranes and digging equipment.

In case of aero-space industry, several jet engine and space-craft parts are rolled from hard to form, heat-resistant alloys such as stainless steel, titanium, and nickel-base superalloys. Lot sizes for these products tend to be as small as one piece. Because of the high price of this material, difficulties in machining and the strict production controls, parts in this family are typically formed as contours in as close to net-shape form as possible to reduce machining and waste. Parts used in aero-space include rotating and non-rotating rings for fans, engine casings, and engine disks.

One of the largest segments on the market is energy products. Lot sizes in this segment can be large or small, depending on application. Parts for this market segment include land-based turbine parts, flanges, spacers and blinds for the oil industry, vessel components, and nuclear reactor components. Another rapidly growing market segment is the wind power generation group of components which include bearings, tower connector flanges, and electrical generator parts. Commercial is a catch-all category for rings such as gears for large equipment, bearings for large cranes, food processing dies, and containment dies for forging.

Ring rolling is a process for creating seamless ring-shaped components using specialized equipment and forming processes. Seamless rolled rings need less input material, are produced faster, more precise, and because of the circumferential grain flow of the finished part, are stronger than those produced by alternative methods of ring production.

In its simplest form, ring rolling and its related process axial closed-die forming is an incremental forming process which exerts force on one part of the overall forging in an effort to form a better part at lower tonnages than alternative processes. Because of the unique nature of this incremental forming process, larger, more complex parts can be formed using only a fraction of the force which is needed if the entire part is formed under a die. Fig 1 shows a schematic view of the ring rolling process.

Fig 1 Schematic view of the ring rolling process

Process overview – The input blank for the ring rolling process is a donut-shaped preform which is normally formed in a hydraulic press, mechanical press, or forging hammer. Recently, tube stock which is cut to length has been successfully used as well. Blanks are normally formed hot, but warm and cold forming is also possible for smaller parts or parts which need to be formed more precisely.

The donut-shaped blank is placed on the ring roller over an undriven mandrel which is smaller than the blank, and the mandrel is forced under pressure toward a driven main roll. When the blank comes in contact with the main roll, the friction between the main roll, blank, and mandrel causes the blank and mandrel to rotate in the direction in which the main roll is turning. Centering arms are used to keep the blank centered during rolling to prevent defects from forming on the ring. The gap between main roll and mandrel is progressively reduced, hence reducing the wall thickness of the ring and simultaneously increasing its diameter because of the circumferential extrusion which occurs.

Ring height is governed either by being contained by the top and bottom of the main roll or by the use of axial rolls which simultaneously act on the top and bottom surfaces of the ring in a similar manner to the main roll and mandrel. The result is a uniform cross-section ring, disk, or contour-shaped component which can be further processed into a finished part (normally by machining).

Ring sizes and production ranges – The size range for the rolled rings is large and getting larger. Possible sizes include rings from an outside diameter (OD) of 75 mm (millimetre) to a maximum OD of 9 m (metre). Ring heights range from 12 mm to over 4 m. Weights range from 0.3 kg (kilogram) to over 90 t (ton). While the majority of rings are in the OD range between 250 mm and 1,200 mm with heights between 75 mm and 800 mm and wall thicknesses between 20 mm and 120 mm, there are a considerable number of rings which fall outside this range.

Further, because of advances in ring rolling computer control, an increasing number of extreme washer and sleeve-type rings are being rolled. For washers, wall thickness-to-height ratios of 20 to 1 are common, and using specially prepared blanks, wall-to-height ratios go as high as 28 to 1. Common sleeve-type-ring wall-to-height ratios of 1 to 25 and sleeves wall-to-height ratios of as high as 1 to 30 are possible on certain machines.

There are a large range of contoured / shaped cross sections which can be produced by ring rolling. In some cases, it is more economical and more practical to roll contoured rings as multiples of 2 to more than 8. These multiples are then slit or parted from each other by sawing or machining. The identical components are normally mirrored so as to place the thinnest wall section at the middle of the rolled ring for ease of parting. Since the ring is then symmetrical about the centre-line, such rings can frequently be rolled from a simple blank and it behaves more predictably during rolling than an asymmetrical ring behaves if rolled singly. Fig 2 shows example of contour ring shapes.

Fig 2 Example of contour ring shapes

Machines – In the mid-19th century to late-19th century, the rapid expansion of railway systems created an increasing demand for railway wheel tires. Originally, these items were forged, laboriously, using hammers. As early as 1852, however, a tire-rolling machine was built in England. The resulting increased productivity, improved product performance to put more shape into the tires before machining ensured the ring rolling technique a firm foothold in the forging industry.

Early machines were radial-pass units only (Fig 3a). i.e., they used a single roll pair and controlled height by containing the ring in a shaped tool. These machines were of two basic types, based on the plane of the ring during rolling namely (i) horizontal, in which the ring rotates around its vertical axis, and (ii) vertical, in which the ring rotates around its horizontal axis. Fig 3 shows schematics of single pass and two pass ring rolling.

Fig 3 Schematics of single pass and two pass ring rolling

The vertical machine is limited in its diameter range because of the practical considerations of floor to working height. The upper diameter limit of the horizontal machine is constrained only by the available floor space. With the horizontal machine, some means of supporting the bottom face of the ring is to be provided. Either the main roll (older machines) or mandrel of the vertical machine serves as the means of ring support. As the use of the technique encompassed a larger variety of rings and ring end uses, the fundamental shortcoming of single-pass rolling (end face defects) forced consideration of two pass rolling (Fig 3b).

By the early 1900s, water hydraulic horizontal machines with directly operated valves were being constructed with this second pass diametrically opposite the original radial pass for the purpose of (limited) axial height reduction. These machines were termed as radial-axial mills. The first oil-hydraulic servo-valve controlled radial-axial mills appeared in the early 1960s. Since 1930, rapidly increasing use of anti-friction bearings has given rise to a demand for a particular type of seamless rolled ring. Inner and outer bearing races are produced in a wide variety of sizes, those using rolled rings predominantly ranging from 75 mm to 1,000 mm in OD, 40 mm to 250 mm in height, and up to 140 kg in weight. High-output multiple-mandrel table mills were specifically designed to meet the lighter end of this need.

Variations on these four basic mill types continue to emerge, with ever-improving machine design systems. Rapid advances in electronics have enabled the application of micro-processor and computer technology to ring rolling equipment. A variety of special-purpose machines have also been built at different times in the past 100 years. Both vertical and horizontal railway wheel rolling mills have existed since around 1900. For a time, pressure cast wheels represented the majority of the railway wheel market world-wide. However, recently, because of higher speed rail traffic and the superior quality of the rolled railway wheel, the popularity of the forged wheel is rapidly rising. Purpose-built lines now produce over 500 wheels in a single shift with tight tolerances, net surfaces, and lighter weights than cast wheels.

Of particular note is the process of axial closed die rolling (rotary or orbital forging). In this machine, a punched blank, solid disk or pre-rolled ring produced on conventional ring rolling equipment is worked between inclined-rotating dies. Annular forgings of very accurate dimensions, and in a range of complex cross sections, can be produced, using only a fraction of the force needed by competing forging processes.

Vertical rolling machines offer more rolling force and drive power for a given capital outlay than their horizontal single-pass or 2-pass counter-parts. This is because of the simplicity of their rugged construction and the minimal needs in terms of machine foundations. Vertical mills also feature smaller main roll diameters, providing a deeper bite or penetration in the rolling pass than the horizontal mills. They also provide a closer support of the rolling mandrel than 19th century majority of the horizontal machines. This allows for the use of a smaller mandrel (and corresponding smaller pierce out in the blank) for a given tonnage.

Finally, mandrels are of a simple design and are hence less expensive. Vertical rolling mills have for years been particularly favoured by producers of jet-engine rings. Modern vertical mills of today are computer numerical controlled (CNC), feature automatic mandrel retraction, and have laser-based measuring systems, making them more productive than ever before. Using profiled tooling, contoured shapes can be produced on this type of machine. Fig 4 shows a sketch of a vertical ring rolling mill.

Fig 4 Sketch of a vertical ring rolling mill

Since the rolling pass is open, vertical machines produce end-face defects (called fish tail) during rolling. Periodically, rolling is to be stopped and the ring flattened on a press or hammer before rolling can continue. There is also an OD limitation on vertical machines in that, in order for larger ODs to be rolled, the machine is to be made taller. Eventually, the ring blank as to be lifted so high in the air to load it, the design becomes impractical.

The second type of machine for ring rolling is the radial-axial horizontal rolling machine. Although several single-pass horizontal machines are presently in use, very few have been installed in recent years. The pre-dominant modern machine is the 2-pass radial-axial machine. In a radial axial ring rolling machine, the ring is rolled simultaneously in the radial pass and the axial pass, which is typically (but not always) located directly across from the radial pass. For keeping the ring in the proper position during rolling, centering rolls are used. These rolls position the ring correctly between the radial roll and axial roll during rolling.

The position of the centering rolls is carefully controlled during rolling so that there are no unwanted stresses on the ring during rolling. The result is smooth, flat surfaces on both radial and axial faces with tight tolerances all around. Since the axial frame has the capability to move during rolling, it is possible to roll rings of extremely large OD. The only limitation is the stability of the ring. Fig 5 shows a sketch of a radial-axial ring rolling mill.

Fig 5 Sketch of a radial-axial ring rolling mill

The ring blank is placed on the rolling table or over an undriven mandrel. In newer machines, this mandrel is retractable, allowing for ease of manipulation. The ring rests on table plates which form part of the radial carriage. On older machines, a backing arm (which is connected to the radial carriage) with a mandrel upper bearing is lowered to support the mandrel on top. In newer models, the mandrel comes from the top above the ring and lowered through the blank into a lower reception bearing which supports it at the bottom.

The newer design keeps the lifting and lowering mechanism out of the path of falling scale. It also allows for complex inside diameter (ID) contour tooling to be raised and lowered into the ID of the ring blank. Once in position, the mandrel and rolling table move as one toward the main roll until it makes contact. The main roll rotates at a constant, pre-selected speed. The ring begins to rotate as the mandrel squeezes the ring wall. This in turn causes the mandrel to rotate.

A separate housing, which holds a pair of conical (axial) rolls, advances until these rolls cover the end face of the ring blank. The lower conical roll is held in a fixed position such that the roll upper (horizontal) surface is typically 3 mm to 5 mm above the level of the table plates. Both conical rolls are normally driven, and the upper roll is moveable hydraulically up and down. The upper roll is guided in a slide toward the lower roll to cause axial height reduction of the ring. The axial rolls withdraw as the ring diameter increases, maintaining minimum slip rolling conditions between the conical rolls and the ring end faces and keeping the axial carriage out of the way of the growing ring.

A tracer wheel mounted on slides between the axial rolls contacts the ring OD. The ring diameter is monitored through measurement of the relative displacement of the tracer wheel and the axial roll carriage. In newer machines, the tracer roll is replaced by a laser measuring device which bounces a laser beam off the ring and measures the distance from the ring face to the laser unit. Since the laser is a non-contact measuring device, it does not affect the ring, and there are no mechanical parts to wear out. Additionally, the pin-point of the laser can measure any surface across the ring face, yielding a more accurate measurement, especially on contour rings where the ring face can be complex.

In older machines, a pair of hydraulic centering arms (Fig 6a), connected through gear segments, contact the ring OD and ensure that the ring stays round and in the correct position in relation to the longitudinal axis of the mill. Load cells in these centering arms detect differences in force against each centering roll. Through the mill control system, the load cells cause rapid, fine adjustment of axial roll speed to remove any force imbalance and hence to maintain the correct positioning of the ring during rolling. Either manually (through a potentiometer) or automatically, the centering force is reduced as rolling progresses and the stiffness of the ring decreases. Fig 6 shows the sketch of centering arm design.

Fig 6 Sketch of centering arm design

In modern machines, the centering arms are no longer connected by gear segments (Fig 6b). The CNC senses the position of the centering arm and maintains the positions of both arms through servo-valves. Since the centering arms are not physically tied together, the arms have the ability to affect the position of the ring in a variety of ways. They can hold the ring in the centre position or move it slightly out of centre for improved filling of the die in contour rolls. In fact, the arms do not even have to touch the ring when rolling. They can be programmed to gently guide it without crushing it, allowing especially thin (less than 10 mm) wall sections to be rolled.

The relationship between radial (wall thickness) and axial (height) reduction is selected by the operator and precisely maintained by the CNC to ensure the absence of ring surface defects. Similarly, the pattern of diameter growth rate is programmed and computer controlled. The mill operator need only set blank and finished ring dimensions at the control desk and initiate the rolling cycle. Rolling is automatically stopped when finished OD, ID, or mean diameter (chosen by the operator) is reached.

One reason the 2-pass or radial / axial mills are popular is the wide range of products which can be rolled on one machine. A typical machine is with 125-ton radial force and 125-ton axial force can roll a ring weighing less than 45 kg or more than 3 tons in size. This machine can roll rings as small as 300 mm to a maximum of 3 m or more. It can roll a washer with a height of 25 mm and a wall thickness of 500 mm or a sleeve with a height of 600 mm and a wall thickness of 25 mm, and it can do it one after the other. No tool changes are needed when rolling square cross sections on the majority of the machines. Some of the latest machines can also produce solid disks (or blind flanges) by leaving the mandrel retracted during rolling and using the axial cones to roll the ring and the centering arms to keep the process in control.

Older mills and those intended for rolling less-demanding materials have normally axial force capabilities lower than their maximum radial force. Machines are normally designated as per the radial and axial rolling forces available, e.g., 100/80 indicates a radial rolling force of 100 tons and an axial rolling force of 80 tons. Several modern machines are what are referred to as square machines in that the radial force and axial force are equal to each other. Similarly, the radial torque and axial torque are roughly equal in these machines as well.

The result is a highly flexible machine which is capable of rolling an extreme washer in one heat and then roll the next ring as an extreme sleeve. This flexibility makes them a favourite in several job shops which cater to customers who need as few as one ring. They are also ideal when rolling washer-type rings (those with high wall thickness- to-height ratios) free from end-face defects.

4-mandrel mechanical table mills have been used extensively in the production of anti-friction bearing races. Although this type of machine has not been manufactured since 1980, several are still in active use today. The non-driven mandrels are supported only at their lower ends, where they are mounted in a rotating table. The driven main roll is set inside the annular table, with its centre offset from that of the table.

The blank is loaded at position 1, where the eccentricity of the table and main roll centres provide a suitable clearance between the mandrel and the main roll. The table is then rotated by an electrical drive, and the gap between the mandrel and the main roll decreases until the ring blank is contacted (position 2). As the table continues to rotate (at much slower angular velocity than the main roll), the gap between the mandrel and the main roll decreases to a minimum (position 3), causing the rapidly rotating ring to be reduced in wall thickness and to increase in diameter. The table rotates to position 4, and the ring is unloaded. The height of the ring is controlled by a closed pass between the main roll and mandrel.

3-mandrel table mills (Fig 7) are a more recent variation of the mechanical table mill. Designed for high production of rings or contour rings, this machine is primarily used as an integral part of a dedicated production line intended to produce millions of parts per year. The 3-mandrel mill uses a rotating turret to transfer the part in and out of the machine and through the rolling pass. Only one part is rolled at a time and production rates go as high as 450 parts per hour, depending on size and complexity.

Since the transfer of the part is fully automatic within the machine, material handling through simple conveyors is possible. The incoming blank is loaded on the machine by the in-feed conveyor. The part is then transferred to the rolling pass by rotating the turret 120-degree. The rolling table then raises up completely supporting the mandrel in a lower bearing. The main roll then moves toward the mandrel in a CNC curve compressing the ring into the main roll pass and forming the part. A centering arm supports the part during rolling, ensuring tight tolerances. After rolling is complete, the turret rotates the part out of the machine and onto the exit conveyor, and the mandrel proceeds to the cooling station.

Because of the heavily guided movements, CNC servo-control, and the precise tooling, machines of this type can produce near-net shape parts with tolerances less than 0.5 mm per surface. Because of the tight tolerances needed in the finished part, blanks for this type of machine are needed to have tight tolerances. Blank weights are to be uniform and precise. Since there is no flash produced in this process, a blank which is too heavy or too light produces a part with either an over-filled or under-filled surface or corner. Input weights variations are to be held to less than 1 % of input weight. Fig 7 gives a sketch of a 3-mandrel mechanical ring rolling machine showing side view and top view.

Fig 7 Sketch of a 3-mandrel mechanical ring rolling machine

Conceived in 1976, the first multiple mandrel ring mill utilized a 4-mandrel rotating turret system to produce long production runs of several product types. Designed as a production line, these machines are capable of producing roughly 300 rings per hour of the smaller sizes. Because of large quantity of equipment dedicated to a particular part size, these machines are best suited for long production runs of thousands of parts. A minimum production run for this type of machine is around 150 parts. Newer machines based on the multiple mandrel design are also used for the production of contour parts in one rolling operation.

The forming process on a multi mandrel automatic ring rolling machine used to produce contour parts for heavy equipment is described here. The mandrels are placed in a line and each mandrel has a more aggressive ID contour. The ring blank is moved into and out of the machine by a mandrel as in previously described machines. The main roll and mandrel for the first operation are flat. The part is rolled to a particular OD and height, and then rolling stops and the machine opens slightly. The mandrel is then withdrawn and a different contoured mandrel is lowered into position. At the same time, the main roll is moved up and a second contoured main roll is brought into position. The ring is then rolled in a second operation again until a pre-determined outer diameter. At that time, the second mandrel is withdrawn and a third mandrel is brought into place. The ring is then rolled into its final near-net shape. The advantage is that only one heat is needed and the near-net-shape profile of the part needs less machining and less input material. Here again, because of the complex tooling needed and the need for accurate blanks, ring mills like this are best suited for long production runs.

Axial closed-die rolling (sometimes referred to as rotary forging) combines the elements of ring rolling with the elements of closed-die forging. Axial closed-die rolling is a continuous forming process where the upper tool is only in partial contact with the work-piece during forming and hence can produce circular forgings using up 90 % less force than is needed in closed-die forging. In axial closed-die rolling, the lower tool rotates around its vertical axis, typically at 30 rpm (revolutions per minute) to 250 rpm. The work-piece is placed in the lower tool and the lower tool begins to rotate. The upper tool has an inclined axis of around 7-degree to that of the lower tool. Either the upper tool is moved down or the lower tool moves up until the part makes contact and begins to rotate. The inclined upper tool creates a semi-parabolic contact area between the part and die. Fig 8 shows the operating principle of the axial closed-die rolling process.

Fig 8 Axial closed-die rolling process

Since only a portion of the die is in contact with the part at any given time, the force needed to forge the part is much less than the force if the entire surface are under contact. Feed rates in the range of 5 mm/sec (millimetre per second) and 25 mm/sec are used. Because of the rigidly guided frame and the closed-die nature of the process, extremely close tolerance parts (some with net surfaces) can be produced in hot, warm, or cold condition. Part tolerance can be less than 0.3 mm.

Axial closed-die rolling does not need a preformed blank in all cases. Some parts start with solid blocks cut from bar. For ring-shaped final parts, a pre-rolled ring is needed and is normally rolled on the 3-mandrel ring roller. Contoured cross sections are normal for this machine and the range of parts is quite large.

Railway wheel rolling – In the early 1900s, the first wheel rolling machines appeared for the production of the complete railway wheel. Present-day modern CNC 13-axis machines integrated into fully automatic production lines are capable of producing fully formed railway wheels complete with some net surfaces. The wheel blank is formed in either one or two 8,000-ton preform presses into a basic shape. In some processes, a hole is pre-punched in the hub. In others, the hub is rolled solid.

The blank is then transferred to the wheel rolling machine die area by a robot or a manipulator. The machine then closes, moves the individual rolling dies toward the part, and rolling begins in this sequence (i) a pressure roll moves onto the outer rim, forming the rim and expanding the outer diameter of the wheel, (ii) 2-web rolls move toward the centre of the wheel, reducing the web thickness and creating a uniform wall thickness, (iii) 2-edge rolls move toward the centre of the wheel rim, keeping the faces of the wheel flat, smooth, and uniform, (iv) a centering roller contacts the outside rim on the other side of the pressure roll to keep the part centered correctly in the rolling area, and (v) guide rolls keep the part steady during the rolling process.

During rolling, all areas of the wheel are formed at the same time. The web is reduced while the out-side rim is formed and the edges are rolled. A laser measuring device monitors the OD increase of the part, and when it has reached the correct dimension, rolling stops and a robot removes the part from the wheel-rolling machine and transfers it to a dishing press, where the final shape of the part is achieved. For solid hub wheels, the hub is pierced with the dishing press as well. When rolling is complete, the web and wheel edges are net surfaces and are not further processed.

The hub and outer rim are machined. Production rates for this type of equipment can be as high as 500 wheels per shift depending on line configuration. For an average wheel, the blank weight is around 440 kg. The slug weighs around 11 kg, and the dished part weighs around 430 kg. Part tolerances average to + 3 mm. The OD increase for the part is around 110 mm. Fig 9a and 9b shows an example of a wheel being rolled and the part profile.

Fig 9 Rolling of railway wheel and process technology

Product and process technology – Ring rolling is a deceptively simple process, but is exceedingly complex and as yet not fully understood or fully predictable. For several years, largely by experience or trial and error, producers of ring rolling equipment and the users of the equipment have developed production techniques which allow production of consistently dimensioned, and frequently complexly shaped, rings in a wide variety of forgeable materials. Even today, there are several ring rolling mills in operation which rely heavily on the skill of the operator and the skill to produce a satisfactory product.

However, the ever-increasing understanding of the fundamental behaviour of materials during rolling has led to the incorporation of this knowledge as well as the latest prevailing process control technology into each successive generation of rolling equipment. By the early 1980s, the first computer radial-axial ring rolling machines became operational. These machines were capable of rolling with extremely high height-to-wall thickness ratios at speeds considerably higher than those possible with manual control.

Early investigative work concentrated on the displacement of individual zones of material because of the ring rolling. Deformation was found to occur across the entire cross section of the ring if the slip fields (Fig 9c) overlapped. Slip fields were created by the roll indentation of the metal being worked. Considerable displacement of material was found at the ID, with less displacement occurring at the OD, both in the direction of rolling, in relation to the relatively undisturbed material at the ring mean diameter (Fig 9d). The grain flow was confirmed as circumferential.

Since the beginning of ring rolling, efforts have been made to predict the outcome of rolling using mathematical models which emulate the rolling process. Much of this work is aimed at improving the accuracy of mathematical models of the process so that increasingly realistic computer simulations can be carried out. The ability to roll difficult ring configurations on machines of given characteristics can hence be better predicted, and the direction machine design is to take to roll particular ring types and materials can also be determined. Studies of ring rolling are ongoing in several countries. Because of the availability of ever-increasing computer computation power, dramatic gains have been made in the area of finite-element analysis (FEA). While the entire rolling process cannot be simulated, successful single revolution analysis has been performed.

In typical finite-element modelling (FEM) analysis, a wire frame model of the part is developed. The quality of the analysis is determined by the size of the wire frame or mesh. The smaller the mesh size, the better and more accurate is the result of the simulation. Because of the large quantity of variables in ring rolling and the transformation the ring undergoes in each revolution, a fine mesh simulation of a single rotation of even a simple design ring takes days to simulate (simulation time is proportional to the cube of the size of the mesh). For predicting the outcome of a ring, which sometimes takes 300 revolutions or more to complete, takes months to simulate.

Another issue is the cumulative error associated with multiple passes (or revolutions) made during ring rolling. Typical FEM models consider a straight forging pass. In reality, the forging pass in a ring mill is curved, meaning the volume reduction through a curved pass is slightly different from that in a straight pass. This volume error is not significant in a single revolution, but in multiple revolutions (such as occurs during a rolling of a ring), this error is cumulative and causes errors in the results which can become excessive.

For addressing the time issue, designers have developed a set of rapid design tools which make some assumptions in the interest of saving processing time. Using a combination of geometrical mapping and upper-bound elemental techniques, a rapid reverse and a more detailed forward simulation have been developed. The reverse simulation maps the transformation from the final shape to the initial shape while maintaining the proper volume consistency. A forward simulation then uses this data to FEM analyze a small section of the ring through the rolling pass and predict the intermediate shape of the part as a result of the deformation which takes place in that pass. The next section (perhaps 20-degree or so) is then simulated, and so on, until the one entire ring revolution has been simulated. After taking into account the elongation of the part in the pass and other factors such as cooling of the section before it comes around to the rolling pass again, the results from the previous simulation are then used as a starting point for the next revolution, and so on, until the ring reaches its final shape.

The result is the ability to simulate a ring which typically rolls in 2 minutes to 3 minutes on the machine in around 8 hours to 10 hours of computer time. But the important factor is the ability to predict how that part is going to roll and whether it is going to develop any defects. For testing the results, rings are rolled on CNC ring rolling mills and the actual rolling is recorded on a data logger. Additionally, any imperfections are noted. The outcome is encouraging. Rolling times, volume distributions, and needed forces of the simulation track very closely with the actual data plots. More importantly, the software is able to accurately predict defects which can occur during the rolling, giving the engineer an important tool in reducing the number of trials and intermediate rolling tests which typically occur in a part development cycle.

Process control technology – The majority of ring rolling machines installed world-wide since 1960 have originated in Germany. Not surprisingly, German organizations have been responsible for much of the theoretical and practical development which has occurred in this specialized area of forging. In particular, a researcher at one German organization has developed a combination of theoretical and empirical relationships which has been successfully applied to ring mill design. Following his work, a simulation programme has been developed which assists the engineer in predicting how a ring rolls on a mill. It has been refined over the years and has become a useful tool to predict rolling forces needed, rolling curve, rolling temperature, blank shape, and rolling time.

A primary objective in two-pass (radial-axial) ring rolling is to achieve diameter growth through cross-sectional reduction (with freedom from surface defects) quickly enough to allow profitable operation. A potential source of end surface defects and ovality arises in the axial roll pass. For avoiding slipping and scuffing at the ring end faces, conical roll pairs are necessary for height reduction. In this way, roll and ring surface speeds are matched across the ring faces. For maintaining this no-slip condition, the axial roll carriage is needed to withdraw horizontally during rolling at the same speed at which the ring centre moves. Another benefit of this operational principle is that higher vertical rolling forces can be applied for a given motor power since less power is wasted through slippage. Hence, flat cross sections with height-to-wall thickness ratios exceeding 1 to 20 can be rolled.

Older ring mills were force controlled, meaning the operator adjusted the force applied to the rolling pass and then received a resulting growth rate of the ring. Later, with the advent of CNC control and improved hydraulic components, majority of the ring mills now use feed rate control, meaning the operator selects a growth rate of the ring and the CNC uses whatever force is needed (within the capability of the machine) to achieve it. The computer applies different feed rates (both radial and axial) at given times to follow a pre-determined rolling pattern or rolling curve. The objective with both control systems is to (i) change the cross section of the ring in a specific manner to avoid the surface defects, and (ii) control the diameter growth rate in phases to minimize rolling time, but to complete the rolling process with a stable and round end product.

With regard to the changes in cross section, the ratio between radial (wall thickness) reduction is to be constantly maintained as per the relationship (Vieregge relationship) represented by the equation ‘delta b/delta h = h/b’, where delta b is the wall reduction increment, delta h is the height reduction increment, h is the ring height, and b is the ring wall thickness (equation 1). This equation is derived from consideration of the spread which occurs when rolling with an open pass.

At the relatively low deformation rates per revolution which occur in ring rolling, plastic deformation takes place in the outer layers of the material, but the centre tends to remain rigid / elastic. In the radial pass, this causes beads of material (Fig 10) to form because of the lateral spread where the rolls and the ring are in contact. When these beads are rolled by the axial pass (1/2 revolution later), higher circumferential growth takes place at the inner and outer diameters than in the region of the ring mean diameter. The material in this region is stretched and further reduction in height results, continuing the formation of hollows (or defects) in the ring faces.

Fig 10 Formation of beads during radial-axial rolling

Increased axial rolling removes this defect, but leads to the same type of defect on the inside and outside diameter surfaces of the ring. This chicken and egg imbalance is difficult for an operator to control and balance. However, a CNC control has no problem monitoring both radial and axial wall reduction simultaneously as well as OD growth rate. This gives the CNC control considerable advantages over manual rolling techniques.

A secondary effect of bead formation caused by excess radial rolling is that ring height on the exit side of the radial pass is considerably higher than that on the ingoing side. Contact between the beaded ring bottom face and the table plate on the exit side of the radial pass causes the ring to lift from the horizontal plane, and it attempts to spiral up the radial pass. The ring then either goes out of control and rolling is to be halted, or then ring is held down (especially washer-type rings), and the cross section is distorted (takes on a dish like shape). Maintenance of the Vieregge relationship (equation 1) between incremental wall-to-height reduction and instantaneous ring-height to wall-ratio prevents these defects from forming. Here again, the CNC control monitors and prevents this type of defect from forming.

The relationship which results from the equation 1 is given by the equation ‘h-square – b-square = constant’ (equation 2), i.e., a hyperbolic relationship exists between wall thickness and ring height. In addition, given a constant volume of material, a hyperbolic relationship is to be maintained between instantaneous ring height and diameter. Typical cross-sectional rolling curves derived using equation 1 and equation 2 are shown in Fig 11. The critical nature of starting blank design is highlighted by equation 1 and equation 2 since there is only one theoretically ideal starting blank cross section for any ring.

Fig 11 Rolling strategies for sleeve type and washer type rings

However, in practice, it has been found that considerable license can be taken with respect to blank configuration. This is frequently necessary because of limitations imposed by the equipment used, both for forming the blanks and for rolling the rings. The present-day modern rolling mills allow selection of the shape of the height-to-wall reduction curve, enabling the operator to compensate for less-than ideal blanks and other process variables.

The speed at which the cross-section is reduced directly affects the OD growth rate and (depending on ring stiffness), the stability of the ring (roundness) during rolling. Typically, modern rolling mills provide for up to 6 sequential ring growth control phases, although 3 are normally adequate. In the initial phase, the rate of cross-sectional reduction increases from soft contact between rolls and blank to maximum in a few seconds. The second (normally main) phase of rolling involves decreasing cross-sectional reduction rate resulting in near-constant diameter growth rate. The third phase involves a steadily decreasing diameter growth rate to maintain ring stability with decreasing ring rigidity (cross section versus diameter). The final reduction phase needs very low-cross-section reduction rates and, hence, low diameter growth rates. Final dimensions are achieved in this phase.

Contour ring rolling – With contoured cross sections (Fig 2), the behaviour of the material being worked is even more difficult to predict than with rectangular cross sections. Some experimental and analytical work has been done in several places. In addition, a combination of theoretical and empirical relationships has been developed which gives reasonably accurate results when applied to the preforming of blanks and predicting the degree of success in achieving a desired contour from a given blank shape.

The first commercially produced contoured rings were railway wheels made in the first ring rolling machine, which was built in 1852. Then, blanking design and rolling technique were a matter of trial and error. One of the most important qualities of a successful, modern contour ring rolling organization is still the practical experience gained from producing a wide range of shapes in a variety of materials over several years.

Several contours can be rolled from regular rectangular blanks, especially axisymmetric shapes with thinner wall sections at the centre (double flanged OD or ID). However, once the height of the groove exceeds 50 % of the total ring height, the depth of the groove which can be rolled without considerable overall shape distortion is progressively reduced. As an example, with groove height at 80 % of total height, successful groove depth is limited to around 20 % of final ring wall thickness. This assumes closed-pass rolling and sufficient diameter expansion from blank to ring.

When it is found that a rectangular or open-die blank does not yield the desired contour, blank preforming is to be used. Typically, the starting point for a new contour shape (from a preformed blank) is the application of a simple volume distribution calculation from ring to blank. The ring is divided into a number of axial slices, or disks, and the volume of each slice is calculated. By knowing the size of the rolling mandrel to be used, and hence the ID of the blank, a theoretical blank OD can be calculated for each of the corresponding slices (assuming no height change). The theoretical blank OD shape is generated by the aggregate of the individual slice ODs.

The resulting blank shape is unlikely to be successful in practice since it does not take into account the axial flow of the material and since it assumes that each slice is being rolled throughout. For the latter to occur, the shape of the contoured rolls has to change continually, initially corresponding with blank shape and finishing at ring shape. In practice, a crude but frequently effective solution to this requirement consists of two-stage rolling, first using a roll shape intermediate between blank and ring. Some allowance for axial flow of material, when using a radial closed pass, is made by having a blank height lower than the finished ring height.

When using a radial-axial rolling mill, either the blank design is to be such that height is reduced to final height before material enters the upper section of the pass or the upper axial roll is to operate in reverse of conventional mode and move up during rolling, as is the case with flange rolling. The practical blank has a less pronounced flange than that of the simple, theoretical blank, but still has the necessary volume of material. A deeper (theoretical) flange, only partially enclosed by the corresponding groove in the main roll, results in the folding and lapping of material at the junction of the upper flange face and tapered OD. This is because of the localized deformation fields at the junction of the flange and the taper and at the ID, with the core of the ring remaining essentially elastic. The practical blank is designed to allow for axial flow of material toward the thinner upper section of the ring.

The complete ring cross-section is to be acquired at the same moment the final diameter is reached for the following reason. Immediately after the pass is filled, the thinner-wall section attempts to grow faster circumferentially than the thicker-wall sections for a given decrease in roll the gap. The thicker sections are stretched by the more rapid circumferential growth of the thin sections, and the contour begins to deteriorate in the thicker-wall sections. When preparing to roll an unfamiliar contour shape, blanks are sometimes machined from rough forgings, enabling trial rolling to be carried out without expenditure on possibly inappropriate preforming tools.

Roll diameters are an important consideration in contour forming. The relative curvature between the mandrel and the ring increases throughout rolling, while that between the main roll and the ring decreases. Hence, as rolling progresses, the penetration of the mandrel into the ring increases, and that of the main roll decreases. Conventional rolling mill design, hence, lends itself to ID contouring, with mandrel diameters which are small in relation to main rolls.

Vertical ring mills which are used for contour rolling, are normally designed to allow use of much smaller diameter main rolls when OD contouring. For a limited extent, the same effect can be achieved by using a large-diameter mandrel sleeve and 2-stage rolling on conventional ring mills.

The benefits of contour rolling are reduced material input and reduced machining to finished product. Typically, a weight savings of 1.5 % to 30 % can be achieved by using contoured versus rectangular rings. For determining whether the additional cost of tooling and extended setup time is justified, the break-even point against reduced material and machining cost and minimum order quantity is to be determined. Even on lower-cost materials, this quantity can be only 25 pieces to 50 pieces, especially with repeating orders. Where higher cost materials, such as super-alloys, are involved, production of only three or four pieces can justify contouring.

Rolling forces, power, and speeds – Economical production of seamless rings by the radial-axial rolling process needs rings to be rolled as quickly as possible in a manner which is consistent with dimensional accuracy and metallurgical integrity. A primary factor is the resistance of the material to deformation. This is related to the flow stress of the material at a given temperature and the conditions existing in the rolling pass (roll diameters, frictional resistance, and so on).

With typical ring mill configurations, rolling speeds, and rates of cross-sectional reduction at temperatures of 1,050 deg C to 1,100 deg C, the resistance to deformation of a plain carbon steel is found to be around 160 MPa and that of a bearing steel around 196 MPa (Fig 12a). For these materials, a decrease in temperature of 100 deg C increases resistance to deformation by around 50 %. Quite obviously, rolling force requirements can be minimized by operating at the maximum temperature allowable metallurgically. This needs consideration of both the temperature losses because of the radiation and conduction as well as the temperature increase caused by plastic deformation. Fig 12 shows resistance to deformation and the excessive indentation by the mandrel.

Fig 12 Resistance to deformation and the excessive indentation by the mandrel

Rolling forces cannot be dealt with in isolation. The combination of roll force and resistance to deformation determines the extent to which the rolls indent the ring. With increasing indentation, the drive power needed increases and can reach the mill motor limit well before the maximum roll force has been applied. Further, with very heavy indentation, the relatively small diameter, non-driven mandrel can exert so much circumferential resistance which the driven main roll is unable to overcome it, the driven roll then slips, and the ring fails to rotate (Fig 12b).

Modern mills apply the principle of adaptive control to avoid such problems, i.e., forces and torques are monitored continually by computer, and if they approach the upper limits of the mill and are changing in such a way that these limits are about to be exceeded, then they are automatically reduced in such a way as to maintain a pre-determined relationship between height reduction and the OD growth.

A further limiting factor in the speed with which a ring can be rolled is the stability of the ring during rolling. A ring rotating at too high a speed, with excessive speed changes because of the extrusion in each rolling pass, can lack the rigidity needed to accommodate the different forces and moments acting on it. Gross out of roundness and / or out of flatness can result. In practice, circumferential speeds to 3.6 m/sec (metre per second) are used on smaller mills, and 1 m/sec to 1.6 m/sec on larger mills. Diameter growth rates to 35 mm/sec are normally achieved during the main ring expansion phase, growth rates of 1 mm/sec are used during the rounding or calibration phase.

Blank preparation – In the present-day modern ring rolling lines, economic blank preparation is more important than ever. Preventing defects in the finished product starts with proper blank preparation. Simply put, the first objective of blank making is to put a hole in the work-piece which is of sufficient diameter to allow the blank to fit over the rolling mandrel. The diameter of the mandrel has to be such that sufficient force can be applied to reduce the ring wall section at an acceptable rate. The smaller is the hole, the less is the material wasted.

There are two methods of producing ring blanks. The first method is to forge the blanks needed for the day’s production all at once and then place them in a reheating furnace for later rolling. The blanking press can be close to the ring mill or it can be far removed from the forging cell. In some cases, the forge crew operates the press or hammer to forge the blanks and then moves to the mill and rolls them. In other cases, there are separate forge and roll crews to get maximum equipment utilization. In both cases, blanks are forged and placed in the reheating furnaces in groups to make rolling easier and simplify job tracking. This method is preferred when there are varied lot sizes and with aero-space materials which is to be reheated after blanking or is to be cold inspected and defects ground out prior to the next step.

The second method is the forge and roll method. Using this method, the forged blank is transferred directly to the rolling mill after it is finished. The obvious advantage of this method is that the blank is not reheated or manipulated twice after blanking. There is a substantial savings in time, man-power, and energy, making this an attractive alternative. In fact, majority of the small rings are produced using some form of the blank and roll method. The disadvantage is that not all parts can be rolled in one heat. Sometimes, they need to be reheated or, because of a process error, cannot be rolled after blanking. An extra reheating furnace is normally necessary to have for those times when forge and roll simply does not work.

In several ring rolling applications, blank preparation is carried out on an open die forging press or hammer. Using loose tools such as punches, containment rings, saddles, and bars, these methods have been successfully applied for years to produce forged rings and are extremely versatile. However, these methods are not economical in several cases. Production of ring blanks using these methods needs more time, more man-power, and a wide range of tooling. Hand-made blanks also suffer from forging errors such as off-centre punching and pierce defects or rags. While the initial investment is somewhat lower (majority of the forge shops already have a press), in the long run, an automated ring blanking press is the preferred approach.

In an automated ring blanking press, the table, indent punches, and piercing punch are staged on the press in such a fashion that it is not necessary to manually manipulate these elements to form a blank. They can be remotely and automatically brought into play as needed for the blanking process. The operator supervises the blanking process, but the machine does the work. When complete, the finished blank is either moved to the ring mill or to a reheating furnace by a conveyor, a lift truck, or a robot.

In a typical open die ring blanking press, starting material is normally round, although round- cornered square or octagonal billets can be used. When non-round billet is used, initial working is needed to convert it to round stock. The heated block is placed under the main ram in the centre of a flat upper and lower die. The part is then pre-upset to form a pan-cake and the upper ram returns. A pair of centering arms mounted on the sides of the press then move in and centre the part, ensuring it is in the centre of the press. A swing arm containing a tapered indent punch is then swung in above the piece. The upper ram moves down and forces the indent punch into the part until the top of the indent punch is only around 25 mm above the press bottom platen.

The ram is then moved up again and the top of the blank is re-flattened (because of the defects which are induced on the top when indenting the piece) and pressed to its final height. The part is then lifted off the bottom die by the centering arms and the bottom die moves under the part until the pierce hole is directly under the indented portion of the part. The centering arms then lower the part onto the table again, and the table moves back to its original position, which is under a pierce punch.  The pierce punch then pierces out the web, and this is the only material wasted during the process. The part is then moved to the mill or reheating furnace. These functions are pre-programmed into the programmable logic controller (PLC) by the operator before-hand and are more or less the same for any size blank which are to be made.

Although a wide variety of rings can be rolled from blanks made by this simple process, alternative methods are to be used when large ring height-to-wall ratios are needed and for severely contoured rings with limited rolling reduction (and little diameter growth). With thin-wall sleeves, and even with square cross section rings whose mass is very small in relation to the physical dimensions of the mill, the diameter of the indenting tool can approach that of the upset preform. The indent punch then behaves less like a pre-piercing tool and more like a flat die. The result is a grossly distorted and unacceptable blank with a height less than that of the rolled ring.

This issue can be overcome either by employing slow open-die forging techniques or by indenting the work-piece in a container. The former needs a loose small diameter punch to be pressed into the piece. The blank with punch entrapped is then turned onto its OD and forged incrementally so that the ID expands and the height increases. This method is slow and severely limits output but avoids the cost of containment dies.

By using a larger-capacity press and container dies, excellent blanks can be produced at a rate sufficient to maintain full ring mill production. As an example, a mill which is rolling rings weighing up to 2 tons and using open-die forming blanks from a 1,500-ton hydraulic press needs the use of at least a 2,500-ton press using container dies to maintain full production on this type of ring. A fundamental need here is the ease in ejecting the work-piece from the die, using a hydraulic cylinder housed in the lower portion of the press frame.

On smaller, high-speed ring mills, a 3-station or 4-station blanking press with an integral work-piece transfer system or robot transfer is needed for maintaining an adequate supply of blanks. A press of this type is almost always used in line with the ring roller. The finished blanks are transferred directly to the ring roller through robots and conveyors. Presses of this type can produce open-die blanks, container die blanks, and split-die contoured blanks. Production rates vary from 250 pieces per hour for larger rings to 450 pieces per hour for smaller rings.

Using a modular bottom bolster and top tool holder, tooling can be set up outside the press, and tool sets exchanged in around 20 minutes, hence maximizing the production time available. A wide range of complexly shaped blanks, which can be necessary for rolling rings with complex contours, can be produced using split dies by combining different top and bottom tools at the centre station of the press.

One other method which is used today for blank preparation is the use of hollow round stock. This hollow stock is more expensive than solid stock and comes in a variety of ODs and wall thicknesses. For creating a blank, one simply saws off the needed portion of hollow bar and places it into the reheating furnace. As mentioned earlier, there is only one ideal blank for every ring size. However, because of the ability of the CNC control to adapt for variations in blank dimensions and still make an acceptable product, it is possible to supplement production using this method.

Ancillary operations

In addition to the rolling mill and the blanking press, a variety of additional equipment is needed.

Cutting of billets – Some method for accurately cutting of the raw material to the needed input weight is necessary. Cold shearing and hot shearing are used, the latter is normally used when an integrated production line is involved. Circular saws, which are sometimes carbide tipped, and band saws are frequently used, particularly on stainless steels. Abrasive saws are used on titanium alloys and superalloys.

Heating – Reheating of cut blocks is normally done in box or rotary fossil-fuel fired furnaces. Induction heating is sometimes used for smaller stock and has the advantage of minimum scale formation. Different methods of hot block descaling are used, both mechanical (as an example, flailing cable, chains, or rotating brushes) and high-pressure (14 MPa to 90 MPa) water spray, which is particularly effective.

Other operations – Some shops use devices for sizing rings immediately after rolling. These can be straight forward hydraulic presses, in which the ring is forced through a circular sizing die, or ring expanders, which stretch a ring by applying force to multiple, appropriately shaped segments acting on the ID of a ring. Expanders are frequently used in aero-space applications.

Appropriate heat treatment facilities are necessary, whether to render the product more easily machinable or to achieve the mechanical properties specified for the end product. Shot blasting is frequently used to remove scale formed during hot working. The resulting surface is easier to inspect and to machine.

Blanking tools and work rolls – Although hot-work tool steels such as H11 and H13 are frequently used for blanking and rolling tools, especially when working heat-resistant alloys, less-expensive alloy steels such as AISI 4140 and AISI 4340 find wide application for the less-demanding work materials. When blanks are open-die forged on hammers or presses, simple tapered indenting punches are driven into the preform. The preform is then turned over, allowing the punch to fall out, and the punch is then used to cut out the slug remaining from indenting, hence forming the doughnut-shaped blank.

A wide range of punch diameters and lengths are typically available to accommodate the several different blank dimensions needed. With several punches in each size and each cooled in water immediately after use, AISI 4140 or AISI 4340 are quite acceptable in terms of life and cost. If special-purpose ring blank presses are used, tool duplication is normally not feasible, and short periods of cooling between each blanking operation cannot be sufficient to allow the use of the regular alloy steels mentioned previously.

Fig 13(b) shows a 3-degree tapered swing arm mounted indent punch typically used in blanking presses. A low-alloy steel such as ASM 6F2 at 38 HRC to 43 HRC (350 HB to 400 HB) can be necessary for withstanding the higher tool working temperature. Fig 13(c) and Fig 13(d) show the type of piercing punch and support ring which is used on a 2-station or 3-station blanking press to shear out the slug created by indenting. Almost invariably, the punch is either solid H13 or has an exchangeable tip in H13 heat treated to around 49 HRC (460 HB). The support ring is also normally made of H13. Typically, the radial clearance between the punch and the support ring is of the order of 2 mm to 5 mm for punches 125 mm to 220 mm in diameter. Fig 13 shows blanking and rolling tools used in ring rolling.

Fig 13 Blanking and rolling tools used in ring rolling

On high-speed blanking presses, the indenting punch in the centre station is so heavily used that even when it is made of H13, continuous internal water cooling is necessary, along with inter-cycle external water-spray cooling. Container dies used on a slower-speed-, larger press (e.g., 2,500-ton capacity) can frequently be made from AISI 4140 or AISI 4340 if the duty cycle is long enough and inter-cycle water cooling is adequate. Inserts fabricated from H13 tool steel can be necessary on smaller blanks with shorter cycle times.

On presses where no means are available for stripping blanks off (indenting) punches, these punches typically have a taper of 3-degree per side. Powdered coal or water-borne graphite lubricants are normally used for ensuring release of the punch from the blank. Where stripping mechanisms (depending on the type) are available for ejecting the blank, release tapers of around 1-degree can be used for both punches and containers.

The consumable tools on radial-axial ring rolling mills are principally the mandrel and, to a lesser extent, the axial (conical) rolls and the main roll. Depending on the mill design and force capability, mandrels can be as small as 30 mm in diameter (for a 30-ton ring mill) and as large as 450 mm in diameter for a ring mill with a radial capacity of 500 tons. Fig 13(e) shows a typical 165 mm diameter mandrel for a mid-size ring mill with 100 tons radial capacity. Such mandrels are normally fabricated from ASM 6F3 at 370 HB to 410 HB. Again, AISI 4340, at 300 HB to 350 HB, with adequate water-spray cooling, can be used with good results (i.e., producing up to 3,000 rings before failing through heat check-initiated fatigue). Axial rolls (Fig 13f) on older machines typically have a 45-degree included angle, along with relatively short working lengths. This severely limits the ring wall thickness they can cover and lead to rapid wear of the conical surfaces. With the resultant need to change axial rolls frequently, 2-part designs are frequently used with the working cone bolted to an installed roll shaft.

Modern machines have 30-degree to 40-degree included angle axial cones and longer working lengths. Wear is spread over the higher length, and roll changes are needed less frequently (e.g., after 600 hours to more than 1,000 hours of use). Axial rolls can be 1-piece or 2-piece design and AISI 4140, ASM 6F2, and ASM 6F3 are typical materials. These rolls are normally welded and re-worked to original dimensions several times before being discarded. Extended service life can be achieved by using a cobalt-base hard facing alloy, around 1.5 mm thick, on the working surfaces on these axial cones. Fig 13(g) shows a typical AISI 4140 main roll for a 100-ton radial capacity ring mill. Such rolls tend to wear very heavily at the point where the bottom corner of the ring is contacted. For prolonging use between roll changes, the roll and shaft assembly are periodically adjusted down-ward from maximum height setting gradually toward minimum, typically over a full range of 30 mm.

Combined forging and rolling – In several cases, the combined approach of forging and rolling offers benefits not available by one process alone. A good example of this is in the production of bevel ring gears for the automobile industry. Because of the industry pressure for tighter tolerances and lower per-part pricing, this process has become popular as a method for producing parts at a lower cost with higher quality than previous methods.

In the forming process of a typical bevel ring gear on a 2,000-ton mechanical forging press, the part is upset in the first station, transferred to a second station die where the shape is formed, and then to a third station where the centre is trimmed out. The part is then transferred to a trim press, where the OD is trimmed. Manpower needed is two persons to three persons. For a 305 mm bevel gear, the weight of the scrap is around 3 kg. The average production rate is 200 parts to 250 parts per hour. Because of the weight of the part, operator rotation is necessary as well.

When the same part is produced using a combined forging and rolling approach, a simple preform is produced on a 1,000-ton mechanical forging press (since there is no flash on the OD of the part) and transferred between stations by a robot. In the first station, the part is upset. In the second station, the part is pre-formed so that the centre is sized for the ring rolling mill mandrel, and in the third station the centre is trimmed out. The part is then transferred to a ring rolling machine where the part is rolled into a ring gear.

Because of the automated nature of the operation, only one operator is needed to supervise the operation of the line. The time of the operator is spent performing part quality checks and tooling replacement staging. The only material lost in this process is the relatively small knockout in the preform which weighs around 0.2 kg. The average production rate for this part is 300 per hour, and the line stops only for tool changes. The resulting part has much tighter tolerances as well namely (i) the OD tolerance is reduced to + 0.8 mm, (ii) runout is 1 mm, (iii) ID tolerance is + 0.8 mm, and (iv) height is + 0.5 mm.

The savings for this combined forge / rolling approach are (i) input blank weight is reduced by 21 % (ii) lighter blank needs less induction heating power, (iii) the part is more consistent, (iv) less machining time is needed, (v) less press tonnage is needed for a given part size, (vi) press dies are less complex and last longer, and (vii) fewer operators are needed. Finally, because of the circumferential grain flow in the rolled part, the finished part is metallurgically improved as well.

Hence, when compared with the conventional method of forging only, the combined approach of forging / rolling yields a cost savings of 28 % or more. There are several sources of dimensional variation in the ring rolling process. The volume of material rolled is affected by variation in the cut weight of the billet, scale loss fluctuation because of the differing heating conditions, and variation in centre-web thickness removed at the blanking stage. Additionally, final dimensions are affected by rolling temperature, machine deflection, the accuracy of the measuring instrument, ring ovality, distortion in subsequent heat treatment, and surface flaws. Cross-sectional shape inaccuracies are also to be taken into account.

Depending on age and condition, the ability of a particular machine varies widely. CNC-controlled machines have the ability to switch off with an accuracy of 0.1 mm. Older-machines have a greatly reduced ability to switch off accurately. Additionally, the mechanical condition of the machine affects its dimensional accuracy. For keeping the dimensional accuracy, some producers size their products by pressing them through a sizing die or deliberately roll them undersize and expand them to size in a segmented shoe ring expander.

For ensuring that the rolled ring have enough material in the right places to make the final product, each producer adds a machining envelope to the finished machined ring. This envelope is determined by all the factors mentioned previously, as well as a keen understanding of the particular equipment.

For maximizing potential of rolling a ring which cleans up, majority of the new ring rolling equipment has the ability to distribute the available material where it is most needed. The operator can select to have any excess material distributed to the ID, OD, or split it between by rolling to the mean diameter.

Persistent market pressure for near-net-shape rings, wider application of statistical process control (SPC) techniques, and the use of CNC ring rolling machines has generated steadily increasing dimensional precision of rolled rings. Information on allowances and tolerances are hence to be taken only as a generalized starting point, and it is to be understood that the ability of individual producer of the rolled rings to meet or improve on the standard allowances and tolerances varies greatly. Fig 14 shows the relationship between this machining allowance and dimensional tolerance.

Fig 14 Allowances and dimensional tolerances for seamless rolled rings

Alternative processes – Relatively small rings can be forged in closed dies. Maximum diameter is limited by the distance between hammer legs, or between press columns, and the available forming energy. Material waste is relatively high, and grain flow is radial unless a preform is ring rolled. Larger rings can be open-die forged using a saddle arrangement. This method is slow, labour intensive, and tends to produce polygonal rather than smooth-faced rings.

If service conditions are not too demanding, rings of a wide range of dimensions can be gas cut from plate. Contoured rings are largely impractical to produce by this approach, much material is wasted, and the longitudinal flow from the plate produces variation in mechanical properties around and in the direction of the circumference.

Rings of a wide range of diameters and cross sections can be made by the 3-roll forming of bar or plate, followed by welding of the joint. Subsequent cold rolling or warm rolling is sometimes used to form complex thin-wall cross sections. Special-purpose rolling machines have been developed for this purpose. Small rings up to around 330 mm in diameter, especially bearing rings, are sometimes machined from seamless tube. Again, the axial grain flow of the tube can be unacceptable and maximum wall thickness is quite limited.

Centrifugal casting is sometimes used to produce circular components, and it has its own peculiar advantages and disadvantages. Non-rotating gas-turbine parts are routinely made in heat-resistant materials by this method.

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