Air-lift hammer

A type of gravity-drop hammer in which the ram is raised for each stroke by an air cylinder. Because the length of stroke can be controlled, ram velocity and therefore the energy delivered to the workpiece can be varied. See also Drop Hammer and Gravity Hammer.

Aircraft quality

Denotes stock of sufficient quality to be forged into highly stressed parts for aircraft or other critical applications. Such materials are of extremely high quality, requiring closely controlled, restrictive practices in their manufacture in order that they may pass rigid requirements, such as magnetic particle inspection.

Alloy steel forging

One made from a steel containing additional alloying elements other than carbon (e.g., Ni, Cr, Mo) to enhance physical and mechanical properties and/or heat-treat response.


Aeronautical Materials Specification.

As forged

The condition of a forging as it comes out of the finisher cavity without any subsequent operations.

Auxiliary operations

Additional processing steps performed on forgings to obtain properties, such as surface conditions or shapes, not obtained in the regular processing operation.

Axial rolls

In ring rolling, vertically displaceable, tapered rolls, mounted in a horizontally displaceable frame opposite from but on the same centerline as the main roll and rolling mandrel. The axial rolls control the ring height during the rolling process.

Axisymmetric forging

A forging where metal flow, during deformation, is predominately in a direction away from a common axis in a radial direction.

Backward extrusion

Forcing metal to flow in a direction opposite to the motion of a punch or die.


A section hot rolled from a billet to a form, such as round, hexagonal, octagonal, square, or rectangular, with sharp or rounded corners or edges, with a cross-sectional area of less than 16 sq in. (A solid section that is long in relation to its cross-sectional dimensions, having a completely symmetrical cross section and whose width or greatest distance between parallel faces is 3/8 in. or more).


Convexity of the surfaces of cylindrical or conical bodies, often produced unintentionally during upsetting or as a natural consequence during compression testing. See also Compression Test.

Batch/batch-type furnace

A furnace for heating materials where all loading and unloading is done through a single door or slot.

Bend or twist (defect)

Distortion similar to warpage, but resulting from different causes; generally caused in the forging or trimming operations. When the distortion is along the length of the part, it is called “bend”; when across the width, it is called “twist.”


A die impression, tool, or mechanical device designed to bend forging stock to conform to the general configuration of die impressions subsequently to be used.


A preliminary forging operation to give the piece approximately the correct shape for subsequent forming.


A semifinished, cogged, hot-rolled, or continuous-cast metal product of uniform section, usually rectangular with radiused corners. Billets are relatively larger than bars. See Bloom.


Amount of the die in contact with the workpiece throughout one entire forging reduction, e.g., heavy bite is three-quarter to full width of the die.


Raw material or forging stock (also called a “slug” or “multiple”) from which a forging is made.

Blast cleaning

A process for cleaning or finishing metal objects by use of an air jet or centrifugal wheel that propels abrasive particles (grit, sand, or shot) against the surfaces of the workpiece at high velocity.


The forging operation in which metal is progressively formed to general desired shape and contour by means of an impression die (used when only one block operation is scheduled).

Block and finish

The forging operation in which the part to be forged is blocked and finished in one heat through the use of a die having both a block impression and a finish impression in the same die. This also covers the case where two tools mounted in the same machine are used, as in the case of aircraft pistons. Only one heat is involved for both operations.

Block, first and second

Blocking operation performed in a die having two blocking cavities in the same die; the part being forged is successively blocked in each impression all in one heat. As many as three blocker dies are sometimes needed for some forgings and up to three operations are sometimes required in each die.

Block, first, second, and finish

The forging operation in which the part to be forged is passed in progressive order through three tools mounted in one forging machine; only one heat is involved for all three operations.

Blocker impression

The forging die impression which gives the forging its general shape, but omits any details that might restrict the metal flow; corners are well rounded. The primary purpose of the blocker is to enable the forming of shapes too complex to be finished after the preliminary operations; it also reduces die wear in the finishing impression.

Blocker-type forging

A forging that approximates the general shape of the final part with relatively generous finish allowance and radii. Such forgings are sometimes specified to reduce die costs where only a small number of forgings are desired and the cost of machining each part to its final shape is not exorbitant.


A semifinished product of square, rectangular, or even round cross section, hot rolled, or forged. For steel, the width of a bloom is not more than twice the thickness, and the cross sectional area is usually not less than about 36 sq. in. No invariable rule prevails for distinguishing between blooms and billets; the terms are frequently used interchangeably.

Board hammer

A type of gravity drop hammer where wood boards attached to the ram are raised vertically by action of contrarotating rolls, then released. Energy for forging is obtained by the mass and velocity of the freely falling ram and the attached upper die. See also Drop Hammer.

Bolster plate

A plate to which dies can be fastened; the assembly is secured to the top surface of a press bed. In press forging, such a plate may also be attached to the ram.


A relatively short protrusion or projection on the surface of a forging, often cylindrical in shape.


(1) An initial rolling or drawing operation, or a series of such operations, for reducing an ingot or extruded shape to desired size before the finish reduction. (2) A preliminary press-forging operation.

Brinell hardness

The hardness of a metal or part, as represented by the number obtained from the ratio between the load applied on and the spherical area of the impression made by a steel ball forced into the surface of the material tested. The Brinell Hardness Number (BHN) is determined by measuring the diameter of the impression using a low power microscope, then matching this diameter with the load on a standard table.


A bulge, bend, kink, or other wavy condition of the workpiece caused by compressive stresses. See also Compressive Stress.


Permanently damaging a metal or alloy by heating so as to cause either incipient melting or intergranular oxidation.


A thin ridge or roughness left on forgings by cutting operation such as slitting, shearing, trimming, blanking, or sawing.

Buster (preblocking impression)

A type of die impression sometimes used to combine preliminary forging operations such as edging and fullering with the blocking operation to eliminate blows.

Buster (rougher)

An impression employed in a die when considerable metal movement is required and which precedes a blocker cavity and a finisher cavity. Also known as breakdown/pancake, scalebreak, cheese.

Carbon steel

Steel containing carbon up to about 1.2%, and only residual amounts of other elements except for those added for composition control, with silicon usually limited to 0.60 % and manganese to 1.65%. Carbon steel forgings are forgings manufactured out of carbon steel.


Also known as sub-bolster, die assembly, trim and pierce assembly. An assembly of top and bottom dies and/or tools of each forming station assembled into one unit.

Cast (proof)

Any reproduction of a die cavity in any material, frequently lead, plaster or epoxy, used to confirm the exactness of the cavity. See Die Proof.

Cavity, die

The machined recess in a die that gives the forging its shape.


To break or remove sharp edges or corners of forging stock by means of straight angle tool or grinding wheel.

Charpy impact test

An impact test in which a specially V-notched specimen is broken by the impact of a falling pendulum. The energy absorbed in fracture is a measure of the impact strength or notch toughness of the sample.


Crack in a die impression, generally due to forging pressure and/or excessive die temperature. Die blocks too hard for the depth of the die impression have a tendency to check or develop cracks in impression corners.


A die forging defect; metal sheared from a vertical surface and spread by the die over an adjoining horizontal surface.

Chucking lug

A lug or boss to the forging so that “on center” machining and forming can be performed with one setting or chucking; this lug is machined or cut away on the finished item.


The process of removing scale, oxides, or lubricant—acquired during heating for forging or heat treating—from the surface of the forging. (See also Blasting, Pickling, Tumbling.)

Close-tolerance forging

One held to closer-than-conventional dimensional tolerances so that little or no machining is required after forging. See also Precision Forging.

Closed die forging

The shaping of hot metal completely within the walls or cavities of two dies that come together to enclose the workpiece on all sides. The impression for the forging can be entirely in either die or divided between the top and bottom dies. Impression-die forging, often used interchangeably with the term closed die forging, refers to a closed-die operation in which the dies contain a provision for controlling the flow of excess material, or flash, that is generated. By contrast, in flashless forging, the material is deformed in a cavity that allows little or no escape of excess material. See Impression Die Forging.


The forging operation that locally reduces diameters in hollow forgings.

Closure, die

A term frequently used to mean variations in thickness of a forging.


The reducing operation in which an ingot is worked into a billet by the use of a forging hammer or a forging press.


(1) A post-forging process—on hot or cold parts—used to attain closer tolerances or improved surfaces. (2) A closed-die squeezing operation in which all surfaces of a workpiece are confined or restrained, resulting in a well-defined imprint of the die on the work.

Coining dies

Dies in which the coining or sizing operation is performed.

Cold forging

Various forging processes conducted at or near ambient temperatures to produce metal components to close tolerances and net shape. These include bending, cold drawing, cold heading, coining, extrusion (forward or backward), punching, thread rolling and others.

Cold heading

Plastically deforming metal at ambient temperatures to increase the cross-sectional area of the stock (either solid bar or tubing) at one or more points along the longitudinal axis. See also Heading and Upsetting.

Cold lap

A flaw that results when a workpiece fails to fill the die cavity during the first forging. A seam is formed as subsequent dies force metal over this gap to leave a seam on the workpiece surface. See also Cold Shut.

Cold saw

Mechanical sawing machine used to produce cut pieces prior to the forging operation. Sawing is carried out on the material at ambient temperature.

Cold shut

Also known as lap or fold. A defect such as lap that forms whenever metal folds over itself during forging. This can occur where vertical and horizontal surfaces intersect.

Cold trimming

Removing flash or excess metal from the forging in a trimming press when the forging is at room temperature.

Cold working

Permanent plastic deformation of a metal at a temperature below its recrystallization point—low enough to produce strain hardening. Usually, but not necessarily, conducted at room temperature. Also referred to as cold forming or cold forging. Contrast with hot working.

Cold-coined forging

A forging that has been restruck cold in order to hold closer face distance tolerances, sharpen corners or outlines, reduce section thickness, flatten some particular surface, or, in non-heat-treatable alloys, increase hardness.


A concave condition applicable to the width of any flat surface.

Controlled cooling

Cooling from an elevated temperature in a predetermined manner to avoid hardening, cracking, or excessive internal stresses, or to produce a desired microstructure.

Conventional forging

A forging characterized by design complexity and tolerances that fall within the broad range of general forging practice.

Counterblow forging

One made by equipment incorporating two opposed rams, which simultaneously strike repeated blows on the workpiece.

Counterblow forging equipment

A category of forging equipment in which two opposed rams are activated simultaneously, striking repeated blows on the workpiece at a midway point. Action is vertical or horizontal.

Cross forging

Preliminary working of forging stock in alternate planes, usually on flat dies, to develop mechanical properties, particularly in the center portions of heavy sections.


The removal of carbon from the surface of steel as a result of heating in a medium that reacts with the carbon. Decarburization is usually present to a slight extent in steel forgings. Excessive decarburization can result in defective products.

Die holder

Also known as bolster, insert holder, can. Used to locate, clamp and support dies, die assemblies or die inserts.

Die impression

The portion of the die surface that shapes the forging.

Die lubricant

A material sprayed, swabbed, or otherwise applied during forging to reduce friction and/or provide thermal insulation between the workpiece and the dies. Lubricants also facilitate release of the part from the dies and provide thermal insulation. See also Lubricant.

Die match

Also known as mismatch. The alignment of the upper (moving) and lower (stationary) impression in the die.

Die proof (cast)

A casting of the die impression made to confirm the exactness of the impression.

Die set

The assembly of the upper and lower die shoes (punch and die holders), usually including the guide pins, guide pin bushings, and heel blocks. This assembly takes many forms, shapes, and sizes and is frequently purchased as a commercially available unit. Also, two (or, for a mechanical upsetter, three) machined dies used together during the production of a die forging.

Die shift

The condition that occurs after the dies have been set up in a forging unit in which a portion of the impression of one die is not in perfect alignment with the corresponding portion of the other die. This results in a mismatch in the forging, a condition that must be held within the specified tolerance.

Die shoes

The upper and lower plates or castings that constitute a die set (punch and die holder). Also a plate or block upon which a die holder is mounted, functioning primarily as a base for the complete die assembly. This plate or block is bolted or clamped to the bolster plate or the face of the press ram.

Die sinking

The process of machining impressions in die blocks.

Die straighten

A straightening operation performed in either a hammer or a press using flat or cavity dies to remove undesired deformation and bring the forging within the straightness tolerance.

Dies (die blocks)

The metal blocks into which forging impressions are machined and from which forgings are produced.

Dies, forging

Forms for the making of forgings; generally consist of a top and bottom die. The simplest will form a completed forging in a single impression; the most complex, made up of several die inserts, may have a number of impressions for the progressive working of complicated shapes. Forging dies are usually in pairs, with part of the impression in one of the blocks and the balance of the impression in the other block.

Dies, gripper

Clamping or lateral dies used in a forging machine or mechanical upsetter.

Directional properties

Properties whose magnitude varies depending on the relation of the test axis to a specific direction within the metal or alloy.

Disc (disk)

“Pancake” shaped forging (flat with a round cross-section); e.g., a blank for gears, rings and flanged hubs. Abbreviation is “D.”


Includes cracks, laps, folds, cold shuts, and flow-through, as well as internal defects such as inclusion, segregation, and porosity; internal discontinuities can be detected and evaluated using ultrasonic testing equipment.

Double forging

A forging designed to be cut apart and used as two separate pieces.


The necessary taper on the side of a forging to allow removal from the dies; also applies to the die impression. Commonly expressed in degrees as the draft angle. As applied to open die forging, draft is the amount of relative movement of the dies toward each other through the metal in one application of power.

Draft angle

The angle of taper, expressed in degrees (usually 5° to 7°), given to the sides of the forging and the side walls of the die impression.


(1) A forging operation in which the cross section of forging stock is reduced and the stock lengthened between flat or simple contour dies. See also Fullering. (2) in heat treating, the same as tempering.

Drawing out

The forging operation in which the length of a metal mass (stock) is increased at the expense of its cross section; no “upset” is involved. The operation covers converting ingot to pressed bar using “V,” round, or flat dies.


A condition where the dimensions of a part or forging are changed by local grinding or machining to remove one or more defects thereby causing a localized imperfection of a maximum depth. The depth is the dimension of the dressout.


In forging, the operation of forming or enlarging a hole by use of a tapered punch.

Drop forging

A forging made in closed or impression dies under a drop or steam hammer.

Drop hammer

A term generally applied to forging hammers wherein energy for forging is provided by gravity, steam, or compressed air. See also Air-Lift Hammer, Board Hammer, Steam Hammer.


The property of a metal that enables it to stretch before rupturing.


Portion of a press cycle during which the movement of a member is zero or at least insignificant. Usually refers to the interval between the completion of the forging stroke and the retraction of the ram.