Forging

Hot metal ingot being loaded into a hammer forge

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: '"cold," "warm," or "hot" forging. Forged parts can range in weight from less than a kilogram to 580 metric tons. Forged parts usually require further processing to achieve a finished part.

History

Forging is one of the oldest known metalworking processes.

Metal ingot after forging

Traditionally, forging was performed by a smith using hammer and anvil, and though the use of water power in the production and working of iron dates to the 12th century, the hammer and anvil are not obsolete. The smithy or forge has evolved over centuries to become a facility with engineered processes, production equipment, tooling, raw materials and products to meet the demands of modern industry.

In modern times, industrial forging is done either with presses or with hammers powered by compressed air, electricity, hydraulics or steam. These hammers may have reciprocating weights in the thousands of pounds. Smaller power hammers, 500 lb (230 kg) or less reciprocating weight, and hydraulic presses are common in art smithies as well. Some steam hammers remain in use, but they became obsolete with the availability of the other, more convenient, power sources.

Advantages and disadvantages

Forging can produce a piece that is stronger than an equivalent cast or machined part. As the metal is shaped during the forging process, its internal grain deforms to follow the general shape of the part. As a result, the grain is continuous throughout the part, giving rise to a piece with improved strength characteristics.

Some metals may be forged cold, however iron and steel are almost always hot forged. Hot forging prevents the work hardening that would result from cold forging, which would increase the difficulty of performing secondary machining operations on the piece. Also, while work hardening may be desirable in some circumstances, other methods of hardening the piece, such as heat treating, are generally more economical and more controllable. Alloys that are amenable to precipitation hardening, such as most aluminium alloys and titanium, can be hot forged, followed by hardening.

Production forging involves significant capital expenditure for machinery, tooling, facilities and personnel. In the case of hot forging, a high temperature furnace (sometimes referred to as the forge) will be required to heat ingots or billets. Owing to the massiveness of large forging hammers and presses and the parts they can produce, as well as the dangers inherent in working with hot metal, a special building is frequently required to house the operation. In the case of drop forging operations, provisions must be made to absorb the shock and vibration generated by the hammer. Most forging operations will require the use of metal-forming dies, which must be precisely machined and carefully heat treated to correctly shape the workpiece, as well as to withstand the tremendous forces involved.

Processes

A cross-section of a forged connecting rod that has been etched to show the grain flow.

There are many different kinds of forging processes available, however they can be grouped into three main classes:

Drawn out: length increases, cross-section decreases

Upset: Length decreases, cross-section increases

Squeezed in closed compression dies: produces multidirectional flow

Common forging processes include: roll forging, swaging, cogging, open-die forging, impression-die forging, press forging, automatic hot forging and upsetting.

Temperature

 Hot working and Cold working

All of the following forging processes can be performed at various temperatures, however they are generally classified by whether the metal temperature is above or below the recrystallization temperature. If the temperature is above the material's recrystallization temperature it is deemed hot forging; if the temperature is below the material's recrystallization temperature but above 3/10ths of the recrystallization temperature (on an absolute scale) it is deemed warm forging; if below 3/10ths of the recrystallization temperature (usually room temperature) then it is deemed cold forging. The main advantage of hot forging is that as the metal is deformed work hardening effects are negated by the recrystallization process. Cold forging typically results in work hardening of the piece.

Drop forging

Drop forging is a forging process where a hammer is raised up and then "dropped" onto the workpiece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and closed-die drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the workpiece, while the latter does.

Open-die drop forging

Open-die forging is also known as smith forging. In open-die forging, a hammer strikes and deforms the workpiece, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the surfaces that are in contact with the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. Therefore the operator needs to orient and position the workpiece to get the desired shape. The dies are usually flat in shape, but some have a specially shaped surface for specialized operations. For example, a die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool.

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  Edging
 
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  Fullering

Open-die forging lends itself to short runs and is appropriate for art smithing and custom work. In some cases, open-die forging may be employed to rough-shape ingots to prepare them for subsequent operations. Open-die forging may also orient the grain to increase strength in the required direction.

Cogging is successive deformation of a bar along its length using an open-die drop forge. It is commonly used to work a piece of raw material to the proper thickness. Once the proper thickness is achieved the proper width is achieved via edging.

Edging is the process of concentrating material using a concave shaped open die. The process is called edging, because it is usually carried out on the ends of the workpiece. Fullering is a similar process that thins out sections of the forging using a convex shaped die. These processes prepare the workpieces for further forging processes.

Edging

Fullering

Impression-die drop forging

Impression-die forging is also called closed-die forging. In impression-die work metal is placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. Depending on the size and complexity of the part the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities, forming what is referred to as flash. The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging the flash is removed.

In commercial impression-die forging the workpiece is usually moved through a series of cavities in a die to get from an ingot to the final form. The first impression is used to distribute the metal into the rough shape in accordance to the needs of later cavities; this impression is called an edging, fullering, or bending impression. The following cavities are called blocking cavities, in which the piece is working into a shape that more closely resembles the final product. These stages usually impart the workpiece with generous bends and large fillets. The final shape is forged in a final or finisher impression cavity. If there is only a short run of parts to be done it may be more economical for the die to lack a final impression cavity and instead machine the final features.

Impression-die forging has been further improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging.

One variation of impression-die forging is called flashless forging, or true closed-die forging. In this type of forging the die cavities are completely closed, which keeps the workpiece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and workpiece placement.

There are other variations of part formation that integrate impression-die forging. One method incorporates casting a forging preform from liquid metal. The casting is removed after it has solidified, but while still hot. It is then finished in a single cavity die. The flash is trimmed, then the part is quench hardened. Another variation follows the same process as outlined above, except the preform is produced by the spraying deposition of metal droplet into shaped collectors (similar to the Osprey process).

Closed-die forging has a high initial cost due to the creation of dies and required design work to make working die cavities. However, it has low recurring costs for each part, thus forgings become more economical with more volume. This is one of the major reasons closed-die forgings are often used in the automotive and tool industry. Another reason forgings are common in these industrial sectors is because forgings generally have about a 20 percent higher strength-to-weight ratio compared to cast or machined parts of the same material.

Design of impression-die forgings and tooling

Forging dies are usually made of high-alloy or tool steel. Dies must be impact resistant, wear resistant, maintain strength at high temperatures, and have the ability to withstand cycles of rapid heating and cooling. In order to produce a better, more economical die the following rules should be followed:

The dies should part along a single, flat plane if at all possible. If not the parting plane should follow the contour of the part.

The parting surface should be a plane through the center of the forging and not near an upper or lower edge.

Adequate draft should be provided; a good guideline is at least 3° for aluminum and 5° to 7° for steel

Generous fillets and radii should be used

Ribs should be low and wide

The various sections should be balanced to avoid extreme difference in metal flow

Full advantage should be taken of fiber flow lines

Dimensional tolerances should not be closer than necessary

The dimensional tolerances of a steel part produced using the impression-die forging method are outlined in the table below. It should be noted that the dimensions across the paring plane are affected by the closure of the dies, and are therefore dependent die wear and the thickness of the final flash. Dimensions that are completely contained within a single die segment or half can be maintained at a significantly greater level of accuracy.

Dimensional tolerances for impression-die forgings

Mass [kg (lb)] Minus tolerance [mm (in)] Plus tolerance [mm (in)]

0.45 (1) 0.15 (0.006) 0.48 (0.018)

0.91 (2) 0.20 (0.008) 0.61 (0.024)

2.27 (5) 0.25 (0.010) 0.76 (0.030)

4.54 (10) 0.28 (0.011) 0.84 (0.033)

9.07 (20) 0.33 (0.013) 0.99 (0.039)

22.68 (50) 0.48 (0.019) 1.45 (0.057)

45.36 (100) 0.74 (0.029) 2.21 (0.087)

A lubricant is always used when forging to reduce friction and wear. It is also used to as a thermal barrier to restrict heat transfer from the workpiece to the die. Finally, the lubricant acts as a parting compound to prevent the part from sticking in one of the dies.

Press forging

Press forging works by slowly applying a continuous pressure or force, which differs from the near-instantaneous impact of drop-hammer forging. The amount of time the dies are in contact with the workpiece is measured in seconds (as compared to the milliseconds of drop-hammer forges). The press forging operation can be done either cold or hot.

The main advantage of press forging, as compared to drop-hammer forging, is its ability to deform the complete workpiece. Drop-hammer forging usually only deforms the surfaces of the workpiece in contact with the hammer and anvil; the interior of the workpiece will stay relatively undeformed. Another advantage to the process includes the knowledge of the new part's strain rate. We specifically know what kind of strain can be put on the part, because the compression rate of the press forging operation is controlled. There are a few disadvantages to this process, most stemming from the workpiece being in contact with the dies for such an extended period of time. The operation is a time consuming process due to the amount of steps and how long each of them take. The workpiece will cool faster because the dies are in contact with workpiece; the dies facilitate drastically more heat transfer than the surrounding atmosphere. As the workpiece cools it becomes stronger and less ductile, which may induce cracking if deformation continues. Therefore heated dies are usually used to reduce heat loss, promote surface flow, and enable the production of finer details and closer tolerances. The workpiece may also need to be reheated. When done in high productivity, press forging is more economical than hammer forging. The operation also creates closer tolerances. In hammer forging a lot of the work is absorbed by the machinery, when in press forging, the greater percentage of work is used in the work piece. Another advantage is that the operation can be used to create any size part because there is no limit to the size of the press forging machine. New press forging techniques have been able to create a higher degree of mechanical and orientation integrity. By the constraint of oxidation to the outer most layers of the part material, reduced levels of microcracking take place in the finished part.

Press forging can be used to perform all types of forging, including open-die and impression-die forging. Impression-die press forging usually requires less draft than drop forging and has better dimensional accuracy. Also, press forgings can often be done in one closing of the dies, allowing for easy automation.

Upset forging

Upset forging increases the diameter of the workpiece by compressing its length.Based on number of pieces produced this is the most widely used forging process. A few examples of common parts produced using the upset forging process are engine valves, couplings, bolts, screws, and other fasteners.

Upset forging is usually done in special high speed machines called crank presses, but upsetting can also be done in a vertical crank press or a hydraulic press. The machines are usually set up to work in the horizontal plane, to facilitate the quick exchange of workpieces from one station to the next. The initial workpiece is usually wire or rod, but some machines can accept bars up to 25 cm (9.8 in) in diameter and a capacity of over 1000 tons. The standard upsetting machine employs split dies that contain multiple cavities. The dies open enough to allow the workpiece to move from one cavity to the next; the dies then close and the heading tool, or ram, then moves longitudinally against the bar, upsetting it into the cavity. If all of the cavities are utilized on every cycle then a finished part will be produced with every cycle, which is why this process is ideal for mass production.

The following three rules must be followed when designing parts to be upset forged:

The length of unsupported metal that can be upset in one blow without injurious buckling should be limited to three times the diameter of the bar.

Lengths of stock greater than three times the diameter may be upset successfully provided that the diameter of the upset is not more than 1.5 times the diameter of the stock.

In an upset requiring stock length greater than three times the diameter of the stock, and where the diameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupported metal beyond the face of the die must not exceed the diameter of the bar.

Automatic hot forging

The automatic hot forging process involves feeding mill-length steel bars (typically 7 m (23 ft) long) into one end of the machine at room temperature and hot forged products emerge from the other end. This all occurs very quickly; small parts can be made at a rate of 180 parts per minute (ppm) and larger can be made at a rate of 90 ppm. The parts can be solid or hollow, round or symmetrical, up to 6 kg (13 lb), and up to 18 cm (7.1 in) in diameter. The main advantages to this process are its high output rate and ability to accept low cost materials. Little labor is required to operate the machinery. There is no flash produced so material savings are between 20 and 30% over conventional forging. The final product is a consistent 1,050 °C (1,920 °F) so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging). Tolerances are usually ±0.3 mm (0.012 in), surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly double that of conventional forging because contact times are on the order of 6/100 of a second. The downside to the process is it only feasible on smaller symmetric parts and cost; the initial investment can be over $10 million, so large quantities are required to justify this process.

The process starts by heating up the bar to 1,200 to 1,300 °C (2,192 to 2,372 °F) in less than 60 seconds using high power induction coils. It is then descaled with rollers, sheared into blanks, and transferred several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary). This process can also be couple with high speed cold forming operations. Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.

Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for LP gas cylinders. Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.

Roll forging

Roll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves. A heated bar is inserted into the rolls and when it hits a stop the rolls rotate and the bar is progressively shaped as it is rolled out of the machine. The work piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves. This continues until the desired shape and size is achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the workpiece.

Examples of products produced using this method include axles, tapered levers and leaf springs.

Net-shape and near-net-shape forging

See also: Near-net-shape

This process is also known as precision forging. This process was developed to minimize cost and waste associated with post forging operations. Therefore, the final product from a precision forging needs little to no final machining. Cost savings are gained from the use of less material, and thus less scrap, the overall decrease in energy used, and the reduction or elimination of machining. Precision forging also requires less of a draft, 1° to 0°. The downside of this process is its cost, therefore it is only implemented if significant cost reduction can be achieved.

Cost implications

To achieve a low cost net shape forging for demanding applications that are subject to a high degree of scrutiny, i.e. non-destructive testing by way of a dye-penetrant inspection technique, it is crucial that basic forging process disciplines are implemented. If the basic disciplines are not met, there is a high probability that subsequent material removal operations will be necessary to remove material defects found at non-destructive testing inspection. Hence low cost parts will not be achievable.

Example disciplines are: die-lubricant management (Use of uncontaminated and homogeneous mixtures, amount and placement of lubricant). Tight control of die temperatures and surface finish / friction.

Induction forging

Main article: Induction forging

Unlike the above processes, induction forging is based on the type of heating style used. Many of the above processes can be used in conjunction with this heating method.

Equipment

Hydraulic drop-hammer

(a) Material flow of a conventionally forged disc; (b) Material flow of an impactor forged disc.

The most common type of forging equipment is the hammer and anvil. Principles behind the hammer and anvil are still used today in drop-hammer equipment. The principle behind the machine is very simple—raise the hammer and then drop it or propel it into the workpiece, which rests on the anvil. The main variations between drop-hammers are in the way the hammer is powered; the most common being air and steam hammers. Drop-hammers usually operate in a vertical position. The main reason for this is excess energy (energy that isn't used to deform the workpiece) that isn't released as heat or sound needs to be transmitted to the foundation. Moreover, a large machine base is needed to absorb the impacts.

To overcome some of the shortcomings of the drop-hammer, the counterblow machine or impactor is used. In a counterblow machine both the hammer and anvil move and the workpiece is held between them. Here excess energy becomes recoil. This allows the machine to work horizontally and consist of a smaller base. Other advantages include less noise, heat and vibration. It also produces a distinctly different flow pattern. Both of these machines can be used for open die or closed die forging.

A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks and/or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system, different forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 short tons-force). Hydraulic presses use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press are its flexibility and greater capacity. The disadvantages include a slower, larger, and costlier machine to operate.

The roll forging, upsetting, and automatic hot forging processes all use specialized machinery.

List of large forging presses

Force

(tonnes) Ingot size

(tonnes) Company Country

16,000 600 Erzhong China

15,000 580 China First Heavy Industries China

14,000 600 Japan Steel Works Japan

13,000 Doosan South Korea

(source:wikipedia)

Near net shape

Near net shape is an industrial manufacturing technique. The name implies that the initial production of the item is very close to the final (net) shape, reducing the need for surface finishing. Reducing traditional finishing such as machining or grinding eliminates more than two-thirds of the production costs in some industries.

Processes

The following are various near net shape process categorized by material.

Ceramics

Gel casting

Spray forming

Structural ceramic production

Composites

Lanxide process

Plastics

Injection molding

Rapid prototyping

Metals

Casting

Permanent mold casting

Powder metallurgy

Linear friction welding

Metal injection molding

Rapid prototyping

Spray forming

Superplastic forming

(source:wikipeia)

Induction forging

Induction forging refers to the use of an induction heater to pre-heat metals prior to deformation using a press or hammer. Typically metals are heated to between 1,100 °C (2,010 °F) and 1,200 °C (2,190 °F) to increase their malleability and aid flow in the forging die.

Process

Induction heating is a non-contact process which uses the principle of electromagnetic induction to produce heat in a workpiece. By placing a conductive material into a strong alternating magnetic field, electrical current is made to flow in the material, thereby causing Joule heating. In magnetic materials, further heat is generated below the Curie point due to hysteresis losses. The generated current flows predominantly in the surface layer, the depth of this layer being dictated by the frequency of the alternating field and the permeability of the material.

Power consumption

Power supplies for induction forging vary in power form a few kilowatts to many megawatts and, depending on the component geometry, can vary in frequency from 50 Hz to 200 kHz. The majority of applications use the range between 1 kHz and 100 kHz.

In order to select the correct power it is necessary to first calculate the thermal energy required to raise the material to the required temperature in the time allotted. This can be done using the heat content of the material which is normal expressed in KW hours per tonne the weight of metal to be processed and the time cycle. Once this has been established other factors such as radiated losses from the component, coil losses and other system losses need to be factored in. Traditionally this process involved lengthy and complex calculations in conjunction with a mixture of practical experience and empirical formula. Modern techniques utilise finite element analysis and other computer aided modeling techniques, however as with all such methods a thorough working knowledge of the induction heating process is still required.

Output frequency

The second major parameter to be considered is the output frequency of the power source. As the heat is predominantly generated in the surface of the component it is important to select a frequency which offers the deepest practical penetration depth into the material without running the risk of current cancellation. It will be appreciated that as only the skin is being heated time will be required for the heat to penetrate to the centre of the component and that if too much power is applied too quickly it is possible to melt the surface of the component whilst leaving the core cool. Utilising thermal conductivity data for the material and the customers specified homogeneity (physics) requirements regarding the cross sectional ∆T it is possible to calculate or create a model to establish the heat time required. In many cases the time to achieve an acceptable ∆T will exceed what can be achieved by heating the components one at a time. A range of handling solutions including conveyors, in line feeders, pusher systems and walking beam feeders are utilised to facilitate the heating of multiple components whilst delivering single components to the operator at the required time cycle.

Advantages

Process controllability - Unlike a traditional gas furnace the induction system requires no pre-heat cycle or controlled shutdown. The heat is available on demand. In addition to the benefits of rapid availability in the event of a downstream interruption to production the power can be switched off thus saving energy and reducing scaling on the components.

Energy efficiency - Due to the heat being generated within the component energy transfer is extremely efficient. The induction heater heats only the part not the atmosphere around it.

Rapid temperature rise - High power densities ensure that the component reaches temperature extremely rapidly. Scale is reduced as are surface defects and undesirable effects on the surface metallurgy.

Process consistency - The induction heating process produces extremely uniform consistent heat this improves accuracy of the forging and can in extreme cases reduce post forging machining allowances and have a positive effect on die life.

Types

Bar end heating

Bar end heating is typically used where only a portion of the bar is to be forged. Typical applications of bar end heating are

Hot heading of bolts

Anti roll bars

Mining tools

Subject to the required throughput, handling systems can vary from simple 2 or 3 station pneumatic pusher systems to walking beams and conveyors.

Billet heating

In the induction billet heater the whole of the billet or slug is heated. Normally for short billets or slugs a hopper or bowl is used to automatically present the billets in line to pinch rollers, chain driven tractor units or in some cases pneumatic pushers. The billets are then driven through the coil one behind the other on water cooled rails or ceramic liners are used through the coil bore which reduce friction and prevent wear. The length of the coil is a function of the required soak time, the cycle time per component and the length of the billet. In high volume large cross section work it is not unusual to have 4 or 5 coils in series to give 5 m (16 ft) of coil or more.

Typical parts processed by in line billet heating:

Small crankshafts

Camshafts

Pneumatic and hydraulic fittings

Hammer heads

Engine valves

Single shot

For long billets, single shot heating can be used. This process utilises similar systems to bar end heating except that the whole of the billet is driven into individual coils. As with bar end heating the number of coils is governed by ∆T required and the thermal properties of the material being heated.

Typical parts processed by single shot billet heating:

Lorry axles

Marine camshafts

(source:wikipedia)

Power hammer

A power hammer

Power hammers are mechanical forging hammers that use a non-muscular power source to raise the hammer preparatory to striking, and accelerate it onto the work being hammered. Also called "Open Die Power Forging Hammers." They have been used by blacksmiths, bladesmiths, metalworkers, and manufacturers since the late 1880s.

Design and Operation

A water-powered trip hammer, predecessor to the power hammer

A typical power hammer consists of a frame, an anvil, and a reciprocating ram holding a hammer head or die. The workpiece is placed on the lower anvil or die and the head or upper die strikes the workpiece. The power hammer is a direct descendant of the trip hammer, differing in that the power hammer stores potential energy in an arrangement of mechanical linkages and springs, in compressed air, or steam, and by the fact that it accelerates the ram on the downward stroke. This provides more force than simply allowing the weight to fall. Earlier designs like trim hammer, steam drop hammers, board or strap hammers, use the power source to raise the ram, but let its fall be propelled solely by gravity.

Power hammers are rated by weight of moving parts that act directly on the work piece. This includes the weight of the parts that may consist of upper die, ram, mechanical linkage arms and spring(s) or ram, piston, and associated connecting rod(s). Specific design elements are dictated by the power source. The largest power hammer was powered by steam and was rated at 125 short tons (113 t).

Types

Power hammers are generally categorized by their power source.

Steam

Read More: Steam hammer

Steam hammers use steam to drive the hammer. These tended to be the largest models as the great energy of steam was needed to operate them. Locomotive works was one location such large hammers were needed. The workpieces were sometimes so large it required an overhead crane and several men to position the piece in the hammer, and a man to operate the machine.

Mechanical

These hammers tended to smaller and were operated by a single man both holding the workpiece and operating the machine. The majority of these mechanical linkage machines were powered by line shaft flat belt systems or later electric motors that rotated a crank on the machine that drove the ram.

Air

Air-power hammers use pneumatics to drive the hammer.

History

Steam and mechanical power hammers were made into the middle of the 20th century in the USA. At the end of the 19th century the mechanical power hammer became popular in smaller blacksmith and repair shops. These machines were typically rated between 25 and 500 pounds of falling weight. Many may still be seen in use in small manufacturing and artist-blacksmith shops today. In the middle of the 20th century power hammers driven by compressed air began to gain popularity and several manufacturers are currently producing these hammers today.

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(source:wikipedia)

Steam hammer

Hammering out a draw bar on the steam drop hammer
 in the blacksmith shop, Santa Fe R.R. shops

A steam hammer is a power-driven hammer used to shape forgings. It consists of a hammer-like piston located within a cylinder. The hammer is raised by the pressure of steam injected into the lower part of a cylinder and falls down with a force by removing the steam. Usually, the hammer is made to fall faster by injecting steam into the upper part of the cylinder. Steam hammers that fall by their own weight are called steam drop hammers. Steam hammers vary greatly in weight from 45 kilograms to 90 metric tons.

The steam hammer was invented around 1837 by the Scot James Nasmyth, in Manchester, England and produced in his Patricroft foundry which he built adjacent to the (then new) Liverpool and Manchester Railway and the Bridgewater Canal. From the first, Nasmyth's steam hammer could vary the force of the blow within a very wide range. He was fond of showing how he could break an egg placed in a wineglass without breaking the glass, which was followed by a blow that shook the building.

Blacksmiths using a small steam drop hammer at the Atchison,
Topeka and Santa Fe Railway shops in Topeka, Kansas, 1943

An original Nasmyth hammer stands facing his foundry buildings (now a 'business park'). A larger Nasmyth & Wilson steam hammer stands in the campus of the University of Bolton.

The intended first use of the steam hammer lay in forging the paddle shaft of the SS Great Britain. However, the paddle technology was replaced with the screw propeller, and implementation of the hammer was left to the Schneider Electric, Creusot foundry in Le Creusot, France.

The steam hammer was one of many machine tools invented around this time which allowed for large scale industrialisation and the use of machines to build machines. Using the same principles of operation, Nasmyth also developed a revolutionary steam powered pile-driving machine. At its first use at Devonport, a dramatic contest was carried out. His engine drove a pile in four and half minutes compared to the twelve hours that the conventional method required.

(source:wikipedia)

Danieli Automation

Danieli Automation is an Italian company, part of the Danieli Group, located in Buttrio (UD). It is a world leader in the field of automation process for the metals industry.

Brief history

Danieli Automation SpA has been established in Buttrio in 1969, based on an intuition of Mr. Luigi Danieli. Born as CEDA (Costruzioni Elettromeccaniche Dispositivi Automazione), the company changed the name in Danieli Centro Automation during the 1990s, to finally take the actual denomination in 2004. At first, the main function was to assist the mother house to design, develop and realize the electrical and automation systems for the machines produced by Danieli Officine Meccaniche SpA. Over the years, the company grown and expanded, becoming today a world reference in the metals industry sector.

Products

Danieli Automation’s products cover the entire range of metals industry activities. The product’s line includes automation systems for:

> Electric steelmaking

> Casting - hot/cold rolling - finishing for flat products

> Casting - rolling - finishing for long products

> Rolling - finishing for seamless tube lines

And:

> Supply chain management

> Electrical & power systems

> Detection and quality control instrumentation

(source:wikipedia)

Rolling (metalworking)

A rolling schematic

In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process and cold rolling processes the most tonnage out of all cold working processes.

There are many types of rolling processes, including flat rolling, foil rolling, ring rolling, roll bending, roll forming, profile rolling, and controlled rolling.

 Temperature

Hot rolling

A coil of hot-rolled steel

 Hot working

Hot rolling is a metalworking process that occurs above the recrystallization temperature of the material. After the grains deform during processing, they recrystallize, which maintains an equiaxed microstructure and prevents the metal from work hardening. The starting material is usually large pieces of metal, like semi-finished casting products, such as slabs, blooms, and billets. If these products came from a continuous casting operation the products are usually fed directly into the rolling mills at the proper temperature. In smaller operations the material starts at room temperature and must be heated. This is done in a gas- or oil-fired soaking pit for larger workpieces and for smaller workpieces induction heating is used. As the material is worked the temperature must be monitored to make sure it remains above the recrystallization temperature. To maintain a safety factor a finishing temperature is defined above the recrystallization temperature; this is usually 50 to 100 °C (122 to 212 °F) above the recrystallization temperature. If the temperature does drop below this temperature the material must be re-heated before more hot rolling.

Hot rolled metals generally have little directionality in their mechanical properties and deformation induced residual stresses. However, in certain instances non-metallic inclusions will impart some directionality and workpieces less than 20 mm (0.79 in) thick often have some directional properties. Also, non-uniformed cooling will induce a lot of residual stresses, which usually occurs in shapes that have a non-uniform cross-section, such as I-beams and H-beams. While the finished product is of good quality, the surface is covered in mill scale, which is an oxide that forms at high-temperatures. It is usually removed via pickling or the smooth clean surface process, which reveals a smooth surface. Dimensional tolerances are usually 2 to 5% of the overall dimension.

Hot rolling is used mainly to produce sheet metal or simple cross sections, such as rail tracks.

Cold rolling

A coil of cold-rolled steel

Cold working

Cold rolling occurs with the metal below its recrystallization temperature (usually at room temperature), which increases the strength via strain hardening up to 20%. It also improves the surface finish and holds tighter tolerances. Commonly cold-rolled products include sheets, strips, bars, and rods; these products are usually smaller than the same products that are hot rolled. Because of the smaller size of the workpieces and their greater strength, as compared to hot rolled stock, four-high or cluster mills are used. Cold rolling cannot reduce the thickness of a workpiece as much as hot rolling in a single pass.

Cold-rolled sheets and strips come in various conditions: full-hard, half-hard, quarter-hard, and skin-rolled. Full-hard rolling reduces the thickness by 50%, while the others involve less of a reduction. Quarter-hard is defined by its ability to be bent back onto itself along the grain boundary without breaking. Half-hard can be bent 90°, while full-hard can only be bent 45°, with the bend radius approximately equal to the material thickness. Skin-rolling, also known as a skin-pass, involves the least amount of reduction: 0.5-1%. It is used to produce a smooth surface, a uniform thickness, and reduce the yield-point phenomenon (by preventing Luder bands from forming in later processing). It is also used to breakup the spangles in galvanized steel. Skin-rolled stock is usually used in subsequent cold-working processes where good ductility is required.

Other shapes can be cold-rolled if the cross-section is relatively uniform and the transverse dimension is relatively small; approximately less than 50 mm (2.0 in). This may be a cost-effective alternative to extruding or machining the profile if the volume is in the several tons or more. Cold rolling shapes requires a series of shaping operations, usually along the lines of: sizing, breakdown, roughing, semi-roughing, semi-finishing, and finishing.

Processes

Flat rolling

Flat rolling is the most basic form of rolling with the starting and ending material having a rectangular cross-section. The material is fed in between two rollers, called working rolls, that rotate in opposite directions. The gap between the two rolls is less than the thickness of the starting material, which causes it to deform. The decrease in material thickness causes the material to elongate. The friction at the interface between the material and the rolls causes the material to be pushed through. The amount of deformation possible in a single pass is limited by the friction between the rolls; if the change in thickness is too great the rolls just slip over the material and do not draw it in. The final product is either sheet or plate, with the former being less than 6 mm (0.24 in) thick and the latter greater than; however, heavy plates tend to be formed using a press, which is termed forming, rather than rolling.

Oftentimes the rolls are heated to assist in the workability of the metal. Lubrication is often used to keep the workpiece from sticking to the rolls. To fine tune the process the speed of the rolls and the temperature of the rollers are adjusted.

Foil rolling

Foil rolling is a specialized type of flat rolling, specifically used to produce foil, which is sheet metal with a thickness less than 200 µm (0.0079 in).[citation needed] The rolling is done in a cluster mill because the small thickness requires a small diameter rolls.To reduce the need for small rolls pack rolling is used, which rolls multiple sheets together to increase the effective starting thickness. As the foil sheets come through the rollers, they are trimmed and slitted with circular or razor-like knives. Trimming refers to the edges of the foil, while slitting involves cutting it into several sheets.Aluminum foil is the most commonly produced product via pack rolling. This is evident from the two different surface finishes; the shiny side is on the roll side and the dull side is against the other sheet of foil.

Ring rolling

A schematic of ring rolling

Ring rolling is a specialized type of hot rolling that increases the diameter of a ring. The starting material is a thick-walled ring. This workpiece is placed on an idler roll, while another roll, called the driven roll, presses the ring from the outside. As the rolling occurs the wall thickness decreases as the diameter increases. The rolls may be shaped to form various cross-sectional shapes. The resulting grain structure is circumferential, which gives better mechanical properties. Diameters can be as large as 8 m (26 ft) and face heights as tall as 2 m (79 in). Common applications include rockets, turbines, airplanes, pipes, and pressure vessels.

Roll bending

Roll bending

 Roll bender

Roll bending produces a cylindrical shaped product from plate or steel metal.

Roll forming

Roll forming is a continuous bending operation in which a long strip of metal (typically coiled steel) is passed through consecutive sets of rolls, or stands, each performing only an incremental part of the bend, until the desired cross-section profile is obtained. Roll forming is ideal for producing parts with long lengths or in large quantities.

Structural shape rolling

Cross-sections of continuously rolled structural shapes, showing the change induced by each rolling mill.

 Structural shape rolling

Controlled rolling

Controlled rolling is a type of thermomechanical processing which integrates controlled deformation and heat treating. The heat which brings the workpiece above the recrystallization temperature is also used to perform the heat treatments so that any subsequent heat treating is unnecessary. Types of heat treatments include the production of a fine grain structure; controlling the nature, size, and distribution of various transformation products (such as ferrite, austenite, pearlite, bainite, and martensite in steel); inducing precipitation hardening; and, controlling the toughness. In order to achieve this the entire process must be closely monitored and controlled. Common variables in controlled rolling include the starting material composition and structure, deformation levels, temperatures at various stages, and cool-down conditions. The benefits of controlled rolling include better mechanical properties and energy savings.

Mills

Rolling mills

Rolling mill for cold rolling metal sheet like this piece of brass sheet

A rolling mill, also known as a reduction mill or mill, has a common construction 

independent of the specific type of rolling being performed:

Work rolls

Backup rolls - are intended to provide rigid support required by the working rolls to prevent bending under the rolling load

Rolling balance system - to ensure that the upper work and back up rolls are maintain in proper position relative to lower rolls

Roll changing devices - use of an overhead crane and a unit designed to attach to the neck of the roll to be removed from or inserted into the mill.

Mill protection devices - to ensure that forces applied to the backup roll chocks are not of such a magnitude to fracture the roll necks or damage the mill housing

Roll cooling and lubrication systems

Pinions - gears to divide power between the two spindles, rotating them at the same speed but in different directions

Gearing - to establish desired rolling speed

Drive motors - rolling narrow foil product to thousands of horsepower

Electrical controls - constant and variable voltages applied to the motors

Coilers and uncoilers - to unroll and roll up coils of metal

Slabs are the feed material for hot strip mills or plate mills and blooms are rolled to billets in a billet mill or large sections in a structural mill. The output from a strip mill is coiled and, subsequently, used as the feed for a cold rolling mill or used directly by fabricators. Billets, for re-rolling, are subsequently rolled in either a merchant, bar or rod mill. Merchant or bar mills produce a variety of shaped products such as angles, channels, beams, rounds (long or coiled) and hexagons.

Configurations

Various rolling configurations. Key: A. 2-high B. 3-high C. 4-high D. 6-high E&F. Cluster

Mills are designed in different types of configurations, with the most basic being a two-high non-reversing, which means there are two rolls that only turn in one direction. The two-high reversing mill has rolls that can rotate in both directions, but the disadvantage is that the rolls must be stopped, reversed, and then brought back up to rolling speed between each pass. To resolve this, the three-high mill was invented, which uses three rolls that rotate in one direction; the metal is fed through two of the rolls and then returned through the other pair. The disadvantage to this system is the workpiece must be lifted and lowered using an elevator. All of these mills are usually used for primary rolling and the roll diameters range from 60 to 140 cm (24 to 55 in).

To minimize the roll diameter a four-high or cluster mill is used. A small roll diameter is advantageous because less roll is in contact with the material, which results in a lower force and energy requirement. The problem with a small roll is a reduction of stiffness, which is overcome using backup rolls. These backup rolls are larger and contact the back side of the smaller rolls. A four-high mill has four rolls, two small and two large. A cluster mill has more than 4 rolls, usually in three tiers. These types of mills are commonly used to hot roll wide plates, most cold rolling applications, and to roll foils.

Historically mills were classified by the product produced:

Blooming, cogging and slabbing mills, being the preparatory mills to rolling finished rails, shapes or plates, respectively. If reversing, they are from 34 to 48 inches in diameter, and if three-high, from 28 to 42 inches in diameter.

Billet mills, three-high, rolls from 24 to 32 inches in diameter, used for the further reduction of blooms down to 1.5x1.5-inch billets, being the preparatory mills for the bar and rod

Beam mills, three-high, rolls from 28 to 36 inches in diameter, for the production of heavy beams and channels 12 inches and over.

Rail mills with rolls from 26 to 40 inches in diameter.

Shape mills with rolls from 20 to 26 inches in diameter, for smaller sizes of beams and channels and other structural shapes.

Merchant bar mills with rolls from 16 to 20 inches in diameter.

Small merchant bar mills with finishing rolls from 8 to 16 inches in diameter, generally arranged with a larger size roughing stand.

Rod and wire mills with finishing rolls from 8 to 12 inches in diameter, always arranged with larger size roughing stands.

Hoop and cotton tie mills, similar to small merchant bar mills.

Armour plate mills with rolls from 44 to 50 inches in diameter and 140 to 180-inch body.

Plate mills with rolls from 28 to 44 inches in diameter.

Sheet mills with rolls from 20 to 32 inches in diameter.

Universal mills for the production of square-edged or so-called universal plates and various wide flanged shapes by a system of vertical and horizontal rolls.

Tandem mill

A tandem mill is a special type of modern rolling mill where rolling is done in one pass. In a traditional rolling mill rolling is done in several passes, but in tandem mill there are several stands and reductions take place successively. The number of stands ranges from 2 to 18. Tandem mill can be either hot or cold rolling mill type.

Defects

In hot rolling, if the temperature of the workpiece is not uniform the flow of the material will occur more in the warmer parts and less in the cooler. If the temperature difference is great enough cracking and tearing can occur.

Flatness

Roll deflection

Maintaining a uniform gap between the rolls is difficult because the rolls deflect under the load required to deform the workpiece. The deflection causes the workpiece to be thinner on the edges and thicker in the middle. This can be overcome by using a crowned roller, however the crowned roller will only compensate for one set of conditions, specifically the material, temperature, and amount of deformation. If the flatness defect is great enough the edges will become wavy or the center may produce fractures.

Another way to overcome defection issues is by decreasing the load on the rolls, which can be done by applying an longitudinal force; this is essentially drawing. Other method of decreasing roll defection include increasing the elastic modulus of the roll material and adding back-up supports to the rolls.

Surface defects

There are six types of surface defects:

Lap

This type of defect occurs when a corner or fin is folded over and rolled but not welded into the metal. They appear as seams across the surface of the metal.

Mill-shearing

These defects occur as a feather like lap.

Rolled-in scale

This occurs when mill scale is rolled into metal.

Scabs

These are long patches of loose metal that have been rolled into the surface of the metal.

Seams

They are open, broken lines that run along the length of the metal and caused by the presence of scale.

Slivers

Prominent surface ruptures.

History

Iron and steel

The earliest rolling mills were slitting mills, which were introduced from what is now Belgium to England in 1590. These passed flat bars between rolls to form a plate of iron, which was then passed between grooved rolls (slitters) to produce rods of iron.The first experiments at rolling iron for tinplate took place about 1670. In 1697, Major John Hanbury erected a mill at Pontypool to roll 'Pontypool plates'—blackplate. Later this began to be rerolled and tinned to make tinplate.[citation needed] The earlier production of plate iron in Europe had been in forges, not rolling mills.

The slitting mill was adapted to producing hoops (for barrels) and iron with a half-round or other sections by means that were the subject of two patents of c. 1679.

Some of the earliest literature on rolling mills can be traced back to Christopher Polhem in 1761 in Patriotista Testamente, where he mentions rolling mills for both plate and bar iron. He also explains how rolling mills can save on time and labor because a rolling mill can produce 10 to 20 and still more bars at the same time which is wanted to tilt only one bar with a hammer.

A patent was granted to Thomas Blockley of England in 1759 for the polishing and rolling of metals. Another patent was granted in 1766 to Richard Ford of England for the first tandem mill. A tandem mill is one in which the metal is rolled in successive stands; Ford’s tandem mill was for hot rolling of wire rods.

Other metals

Rolling mills for lead seem to have existed by the late 17th century. Copper and brass were also rolled by the late 18th century.

Modern rolling

The modern rolling practice can be contributed to the efforts of Henry Cort of Fontley Iron Mills, near Fareham, England. In 1783 a patent was issued to Henry Cort for his use of grooved rolls for rolling iron bars. With this new design mills were able to produce 15 times the output per day than with a hammer.Although Cort was not the first to use grooved rolls; he was the first to combine the use of all the best features of various ironmaking and shaping processes known at the time. Thus the term “father of modern rolling” was giving to him by modern writers.

The first rail rolling mill was established by John Birkenshaw in 1820 where he produced fish bellied wrought iron rails in lengths of 15 to 18 feet. With the advancement of technology in rolling mills the size of rolling mills grew rapidly along with the size products being rolled. Example of this was at The Great Exhibition in 1851 a plate 20 feet long, 3 ½ feet wide, and 7/16 of inch thick, weighed 1,125 pounds was exhibited by the Consett Iron Company. Further evolution of the rolling mill came with the introduction of Three-high mills in 1853 used for rolling heavy section.

(source:wikipedia)