Hot Forging . 
Warm Forging .
Cold Forging .
Open Die Forging .
Closed Die Forging .
Roller Ring Forging .

Forging defined

At its most basic level, forging is the process of forming and shaping metals through the use of hammering, pressing or rolling. The process begins with starting stock, usually a cast ingot (or a “cogged” billet which has already been forged from a cast ingot), which is heated to its plastic deformation temperature, then upset or “kneaded” between dies to the desired shape and size.

What is forging? 

When we need to select a process for the production of a critical metal component, they face an enormous array of possible alternatives. Many metalworking processes are now available, each offering a unique set of capabilities, costs and advantages. The forging process is ideally suited to many part applications; however, some people may be unaware of the exclusive benefits available only from this form of metal forming. In fact, forging is often the optimum process, in terms of both part quality and cost, especially for applications that require maximum part strength, custom sizes or critical performance specifications.

There are several forging processes available, including impression or closed die, cold forging, and extrusion. However, here we will discuss in detail the methods, application and comparative benefits of the open die and seamless rolled ring forging processes. We invite you to consider this information when selecting the optimum process for the production of your metal parts.

A historical perspective on metal forging

To meet the changing needs of industry, forging has evolved to incorporate the tremendous advances in equipment, robotics, computers and electronic controls that have occurred in recent years. These sophisticated tools complement the creative human skills which, even today, are essential to the success of every metal forging made. Modern forging plants are capable of producing superior-quality metal parts in a virtually limitless array of sizes, shapes, materials and finishes.

During this hot forging process, the cast, coarse grain structure is broken up and replaced by finer grains. Shrinkage and gas porosity inherent in the cast metal are consolidated through the reduction of the ingot, achieving sound centers and structural integrity. Mechanical properties are therefore improved through reduction of cast structure, voids and segregation. Forging also provides means for aligning the grain flow to best obtain desired directional strengths. Secondary processing, such as heat treating, can also be used to further refine the part.

Forging can create a myriad of sizes and shapes with enhanced properties when compared to castings or assemblies.

What is open die forging?

Open-die forging is also known as smith forging. A hammer strikes and deforms a metal on a stationary anvil. In this type of forging, the metal is never completely confined in the dies—allowing it to flow except for the areas where it is in contact with the dies. It is the operator’s responsibility to orient and position the metal to achieve the desired final shape. Flat dies are used, with some having specially shaped surfaces for specialized operations. Open-die forging is suitable for simple and large parts, as well as customized metal components.

Advantages of open-die forging:

  • Better fatigue resistance and strength
  • Reduces chance of error and/or holes
  • Improves microstructure
  • Continuous grain flow
  • Finer grain size

Closed-die forging (impression-die)

Closed-die forging is also known as impression-die forging. The metal is placed in a die and attached to an anvil. The hammer is dropped onto the metal, causing it to flow and fill the die cavities. The hammer is timed to come into contact with the metal in quick succession on a scale of milliseconds. Excess metal is pushed out from the die cavities, resulting in flash. The flash cools faster than the rest of the material, making it stronger than the metal in the die. After forging, the flash is removed.

In order for the metal to reach the final stage, it is moved through a series of cavities in a die:

  1. Edging impression (also known as fullering or bending)
    The first impression used to mold the metal into a rough shape.
  2. Blocking cavities
    The metal is worked into a shape that more closely resembles the final product. The metal is shaped with generous bends and fillets.
  3. Final impression cavity
    Final stage of finishing and detailing the metal into the desired shape.

Advantages of closed-die forging:

  • Produces parts up to 25 tons
  • Produces near net shapes that require only a small amount of finishing
  • Economic for heavy production

Difference between open die forging and closed die forging

While impression or closed die forging confines the metal in dies, open die forging is distinguished by the fact that the metal is never completely confined or restrained in the dies. Most open die forgings are produced on flat dies. However, round swaging dies, V-dies, mandrels, pins and loose tools are also used depending on the desired part configuration and its size.

Although the open die forging process is often associated with larger, simpler-shaped parts such as bars, blanks, rings, hollows or spindles, in fact it can be considered the ultimate option in “custom-designed” metal components. High-strength, long-life parts optimized in terms of both mechanical properties and structural integrity are today produced in sizes that range from a few pounds to hundreds of tons in weight. In addition, advanced forge shops now offer shapes that were never before thought capable of being produced by the open die forging process.


The production of seamless forged rings is often performed by a process called ring rolling on rolling mills. These mills vary in size to produce forged rings with outside diameters of just a few centimeters to over 7.5 meters and in weights from a single kilogram up to over 135 metric tons.

What is Rolled Ring Forging?

The ring rolling process starts with a circular preform of metal that has been previously upset and pierced (using the open die forging process) to form a hollow “doughnut.” This doughnut is heated above the recrystallization temperature and placed over the idler or mandrel roll. This idler roll then moves under pressure toward a drive roll that continuously rotates to reduce the wall thickness, thereby increasing the diameters (I.D. and O.D.) of the resulting ring.

Seamless rolled rings can be produced in configurations ranging from flat, washer-like parts to tall, cylindrical shapes, with heights ranging from less than (25.4mm) to more than (3m) . Wall thickness to height ratios of rings typically range from 1:16 up to 16:1, although greater proportions can be achieved with special processing. The simplest and most commonly used shape is a rectangular cross-section ring, but shaped tooling can be used to produce seamless rolled rings in complex, custom shapes with contours on the inside and/or outside diameters.


Part integrity

Directional strength:

By mechanically deforming the heated metal under tightly controlled conditions, forging produces predictable and uniform grain size and flow characteristics. Forging stock is also typically preworked to refine the dendritic structure of the ingot and remove porosity. These qualities translate into superior metallurgical and mechanical qualities, and deliver increased directional toughness in the final part.

Structural strength:

Forging also provides a degree of structural integrity that is unmatched by other metalworking processes. Forging eliminates internal voids and gas pockets that can weaken metal parts. By dispersing segregation of alloys or nonmetallics, forging provides superior chemical uniformity. Predictable structural integrity reduces part inspection requirements, simplifies heat treating and machining, and ensures optimum part performance under field-load conditions.

Impact strength:

Parts can also be forged to meet virtually any stress, load or impact requirement. Proper orientation of grain flow assures maximum impact strength and fatigue resistance. The high-strength properties of the forging process can be used to reduce sectional thickness and overall weight without compromising final part integrity.

Grain flow comparison

Forged bar

Grain flow is oriented to improve ductility, toughness, and increase fatigue resistance.

Machined bar

Uni-directional grain flow has been cut when changing contour, exposing grain ends. This renders the material more liable to fatigue and more sensitive to stress corrosion cracking

Cast bar 

Cast bars typically do not have a desired grain structure.

Part flexibility 

Variety of sizes 

Limited only to the largest ingot that can be cast, open die forged part weights can run from a single pound to over 200 metric tons. In addition to commonly purchased open die parts, forgings are often specified for their soundness in place of rolled bars or castings, or for those parts that are too large to produce by any other metalworking method.

Variety of shapes

Shape design is just as versatile, ranging from simple bar, shaft and ring configurations to specialized shapes. These include multiple O.D./I.D. hollows, single and double hubs that approach closed die configurations, and unique, custom shapes produced by combining forging with secondary processes such as torch cutting, sawing and machining. Shape designs are often limited only by the creative skills and imagination of the forging supplier.

Metallurgical spectrum

Metallurgical properties can be greatly varied through alloy selection, part configuration, thermal mechanical working and post-forming processes.

Quantity and proto type options

Virtually all open die and rolled ring forgings are custom-made one at a time, providing the option to purchase one, a dozen or hundreds of parts as needed. An added benefit is the ability to offer open die prototypes in single-piece or low-volume quantities. No better way exists to test initial closed die forging designs, because open die forging imparts similar grain flow orientation, deformation and other beneficial characteristics. In addition, the high costs and long lead times associated with closed die tooling and setups are eliminated.


Material savings

Forging can measurably reduce material costs since it requires less starting metal to produce many part shapes.

Machining economies

Forging can also yield machining, lead time and tool life advantages. Savings come from forging to a closer-to-finish size than is capable by alternative metal sources such as plate or bar. Less machining is therefore needed to finish the part, with the added benefits of shorter lead time and reduced wear and tear on equipment.

Reduced rejection rates

By providing weld-free parts produced with cleaner, forging-quality material and yielding improved structural integrity, forging can virtually eliminate rejections.

Production efficiencies

Using the forging process, the same part can be produced from many different sizes of starting ingots or billets, allowing for a wider variety of inventoried grades. This flexibility means that forged parts of virtually any grade can be manufactured more quickly and economically.


Carbon, Alloy & Tool Steel
1010105017 CrNiMo643209310A182F12MIL-S 23284CL1
1018105523174330V52100A182F22MIL-S 23284CL2
1020106033104340ABS Grade 2 A 182F91NITRALLOY “N”
1021106541304350EX55A336F1NITRALLOY 135
1029111741426150A 105A350LF3L-6 
1030 114141458620A182FIA508CL2 
1035 114641508622A182F5A508CL3 
10401541V4150 RES.8630 A182F9A723GR2 
104515B2243B17 8822A182F11A723GR3 
Bearing QualityCalcium Treated
Aircraft Quality
4130 (AMS 2301)
4140 (AMS 2301)
4150 (AMS 2301)
4320 (AMS 2301)
4330V (AMS 6427)
4340 (AMS 2301)
4340 (AMS 6415)
4340 (MIL-S 5000)
4820 (AMS 2301)
9310 (AMS 6260)
NIT. 135 (AMS 6470)
4340 (AMS 6414)
4340 (AMS 8844)
9310 (AMS 6265)
Stainless Steel 
304/304L34743115-5PHFERRALIUM 255 ® 
310405440A17-4PHFERRALIUM SD40 ® 
316/316L410440C254 SMO ®  
317/317L416A182F51/DUPLEX 2205NITRONIC GRADES  
321 420 13-8PH 
Copper BaseNickel BaseAluminumTitanium 
Custom Grades 
CaterpillarGeneral ElectricWestinghouse   
Highlander 613™