Many terms are used to describe steel and the forging process, which is why we created a glossary of terms that we use on a frequent basis.

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Aircraft Quality

A term that indicates the stock is of sufficient quality to be used for highly stressed aircraft parts or other critical purposes. This stock is of the highest quality and requires closely controlled and restricted manufacturing practices to ensure that it passes rigid requirements, such as magnetic particle inspection.

Air Quenching

A form of normalizing that involves more rapid air circulation. The process consists of heating steel above the transformation temperature range and then using agitated air for cooling.


Composed of two or more chemical elements, at least one of those being metallic, an alloy is a material that exhibits metallic properties. The word alloy is often used to indicate materials with relatively high-alloy grades e.g. “alloy” steels are differentiated from “carbon” steels. Materials can be alloyed in order to improve strength, ductility, and hardenability as well as other physical and mechanical properties.

Alloying Element

An element that is added to and then remains in a metal in order to change that metal’s properties.

Alloy Steel

Steel comprised of carbon with the addition of sufficient amounts of one or more other elements that significantly alter the mechanical or physical properties. This alteration of properties differentiates alloy steel from the properties found in carbon steel.

Aluminum-Treated Steel

Aluminum is added to steel in order to deoxidize it, control grain size, or to alter the steel’s mechanical properties through subsequent nitriding or precipitation hardening.


A process in which metal is heated to a temperature above its critical range, is held at that temperature until the metal reaches full crystallization, then is slowly cooled. This is done to decrease hardness, improve machinability, increase ductility, facilitate cold working, or to create a preferred microstructure.


A section that has been hot rolled from a billet into a round, hexagonal, octagonal, square, or rectangular form, containing corners that are either sharp or rounded. A bar has a cross-sectional area of less than 16” as well as a section that is solid and long in comparison to its cross-sectional measurements. The cross section is completely symmetrical section and the width or greater distance between parallel faces is 3/8” or more.


1) A metal ingot hot rolled into a rectangular semi-finished section with a cross section range of 16” to 36” and a width that is less than twice the thickness.  The term “bloom” is used when the cross section exceeds 36”, while “bar” is used for measurements under 16”. 2) A rectangular, semi-finished metal product that is cogged, hot-rolled, or continuous cast and contains radiused corners. Billets are comparatively larger than bars.


Often called a “slug” or “multiple” a blank is a piece of stock with which forging is to be made.

Blast Cleaning (Blasting)

A cleaning or finishing operation done on metal objects with an air jet or centrifugal wheel that propels abrasive particles such as grit, sand, or shot at a fast speed against the workpiece’s surfaces.


Creating a hole or expanding a current one.

Brinell Hardness Testing

A process that involves forcing a hard steel or carbide ball of specified diameter at a known pressure (10-mm ball, 500kg load for aluminum alloys) in order to determine the hardness of a metal made. The Brinell Hardness Number (BHN) is the unit in which the results are expressed. The BHN is obtained by dividing the applied load in kilograms by the surface area of the resulting impression in square millimeters.

Carbon Steel

Steel that contains usually under 1.0 percent, but up to 2.0 percent carbon, combined with only very small amounts of other alloying elements, except for elements added for composition control (silicon usually limited to 0.60 and manganese to 1.65 percent, maximum).

Case Hardening

A process or a combination of processes that involves hardening the surface layer of a ferrous alloy so that it is significantly harder than the interior. This is done to provide a hard, wear-resistant surface on forging, while simultaneously maintaining a soft, tougher core. Carburizing, cyaniding, nitriding, and heating and quenching techniques are generally used.

Charpy Impact Test

A test done to determine the impact strength or notch toughness of a special V-notched specimen. The process involves breaking the specimen with the impact of a pendulum, and the energy absorbed in fracture is measured.

Coarse Grain Size

An austenitic grain size generally less than ASTM 5.


A process to obtain closer tolerances, smoother surfaces, or to eliminate draft done by applying pressure to all or part of the surface of a forging.

Cold Working

Using a temperature low enough to induce strain hardening for plastic deformation of a metal.


Used for shaping metal in a hammer or press, a forging die is a steel block with a flat or contoured working face.

Drop Forging

The process of creating a metal form or shape in closed or impression dies under a drop or steam hammer.


A property of a metal that allows it to stretch before fracturing.


After a specimen has fractured during a tension test, the elongation is measured as the total permanent extension of the sample within the gauge length. Elongation can also be defined as the amount of drawing out during any stage of forging.


The hot mechanical forming of metals with hammers, presses, or forging machines.

Forging Quality

Describes the stock to be of sufficiently higher quality and is suitable for commercially satisfactory forgings.


The irregular surface created from a ruptured or broken piece of metal.

Fracture Test

A test to determine the structure of a metal or the presence of internal defects by intentionally breaking the metal and examining the fracture surface.


The characteristic crystalline structural unit for metals and alloys.

Grain Flow

During forging, the orientation of the constituents of the metal in the direction of working can cause fiber-like lines to appear on the polished and etched sections of that metal. These lines can improve essential mechanical properties of forgings.

Grain Size

As determined by metallographic examination, the number of grains per unit area of cross section.


The procedure of preparing for forging by removing metal from bar or billet sock surfaces by abrasion. Sometimes utilized to remove surface irregularities and flash from forgings.

Hammer Forging

The mechanical process of forming metal by utilization of a hammer that inflicts instantaneous pressure through repeated blows.


The depth up to which a piece of steel is hardened after the heating process of being quenched from above the transformation temperature range.


Any process that is intended to increase the hardness of a metal. Hardening using heat treatment consists of heating an alloy to within or above the critical temperature range, then maintaining that temperature for a set period of time, then rapidly cooling or by definition quenching. For age-hardening alloys, there is a two-step process that involves solution heat treatment and aging.

Heat Treatment

A process that combines controlled heating, holding, and cooling that is done on solid metals or alloys to produce a desired result.

Impact Testing

A process to determine the amount of energy absorbed by fracturing a test bar at high velocity.

Jominy Test

A test to determine the hardenability of steel by quenching one end of a test specimen.

Killed Steel

Deoxidized steel as a result of reduced levels of oxygen content that prevent a reaction to occur between the carbon and oxygen within the steel during solidification. This level of oxygen is achieved by either a strong deoxidizing agent, such aluminum or silicon, or through vacuum treatment.


A process that involves covering a test specimen with an etching solution until deep etching displays the steel’s structure. This test is used to determine conditions such as porosity, inclusions, segregations, carburization, and flow lines from hot working.


The general structure and properties of metals that can visibly be seen on a polished or etched surface without a microscope.

Magnetic Particle Testing

A nondestructive process in which a metal is magnetized, then iron powder is applied. The iron powder reveals surface and near-surface discontinuities by adhering to lines of flux leakage. This test is done on raw material in both acceptance testing and product inspection. The producer and purchaser usually agree on the quality level in advance.

Mechanical Properties

Mechanical properties are dependent on chemical composition, forging, and heat treatment. Those properties that reveal the reaction, either elastic or inelastic, of a metal to an applied stress or that are involved in the relationship between stress and strain e.g. the modulus of elasticity, tensile strength, and fatigue limit.


The internal structure that is observed when a polished, ground, or etched specimen of metal is viewed in an optical microscope at magnifications in range of approximately x25 to x1500.


Porous alloys are heated to approximately 100 degrees Fahrenheit above the critical range, maintain that temperature for a period of time, and then are cooled to room temperature in still air.

Physical Properties

Those properties discussed in physics (exclusive of those described as mechanical properties), for example, density, electrical conductivity, coefficient of thermal expansion.


A heat treatment that consists of the rapid cooling of alloys (the rate is important as it controls the degree of hardening) by contact with liquids, gases, or solids.

Reduction of Area (Contraction of Area)

The difference between the size of the original sectional area of a tension specimen and the area at the point of rupture. The results are stated as a percentage of decrease of cross-sectional area of a tension specimen after rupture.

Residual Alloys

Found within steel and not added intentionally, small quantities of unspecified alloy elements.


At forging temperatures, the heavy iron oxide layer that forms; at heat treating temperatures, the lighter oxide layer that forms.


On the surface of metal, an elongated defect usually obtained in casting or in working.


The mechanical cutting of metal bars to a necessary stock length for forging a specific product.


A form of annealing that involves heating an iron-based alloy to a temperature around, but slightly below the critical range, and then is cooled usually at a relatively slow rate. This process forces the iron carbide into a spheroidal shape.

Stress Relieving

A process that is applied to relieve stresses induced by quenching, normalizing, machining, cold working, or welding. The process involves heating the steel forgings to a temperature below the tempering temperature, maintaining this specific temperature for a period of time, and then slowly and uniformly cooling.


Reheating a quench-hardened or normalized ferrous alloy to a temperature below the transformation temperature range.

Tensile Properties

Data such as tensile strength, yield of point or yield strength, percent elongation in gauge length, and percent reduction in area determined from a tension test.

Tensile Strength

During a tension test and based on the original test cross-sectional area, the tensile strength is the maximum unit of stress carried to rupture.


The amount of variation permitted on dimensions, surfaces or size of part.


Hollowing out the core of metal by using a hollow punch while at forging temperatures or with a hollowing cutting tool at ambient temperatures.


Using a tool in a lathe or similar machine tool to remove metal from the outside of a part.

Ultrasonic Testing

The nondestructive use of an ultrasonic beam on sound-conductive materials with elastic properties to locate inhomogeneities or structural discontinuities.

Yield Point

The point where in the original cross-section an increase in the deformation of the specimen occurs without increase of load. In other words, the stress in the material where there is an increase in strain without an increase in stress.

Yield Strength

Offset from the modulus slope, stress corresponding to some fixed permanent deformation such as 0.1 or 2.0%.