Machine Design 2 Terms

Ultimate Stress or Tensile Strength

Highest point on the stress-strain curve. It is the maximum load divided by the original area before straining occurs.

Yield Strength

The stress for a specified deviation from the straight part of the stress-strain curve.

Elastic Limit

The maximum stress to which a standardized test specimen may be subjected without a permanent deformation.

Proportional Limit

Is coincident with the elastic limit, but is precisely defined as the stress at which the stress-strain curve deviates from a straight line.

Modulus of Elasticity

A measurement of stiffness.

Factor of Safety

A number that is divided into a criterion of strength in order to obtain a design criterion. Also called Design Factor.

Strain

Deformation per unit gage length.

Age Hardening (Precipitation Hardening)

Occurs in some metals, notably certain stainless steel, aluminum, and copper alloys, at ambient temperature after solution heat treatment, the process being one of the constituent precipitating from solid solution.

Age Hardening (Precipitation Hardening)

Aging at moderately elevated temperature expedites the process and is called artificial aging.

Alloy

A substance with metallic properties, composed of two or more elements of which at least one is metal.

Alloy

They are used to improve hardenability of steel, to reduce distortion from heat treatment, to increase toughness, ductility, and tensile strength, and to improve low-temperature or high-temperature properties.

Alloying Elements

In steel, are usually considered to be the metallic elements added for the purpose of modifying the properties.

Anisotropy

The characteristic of exhibiting different properties when tested in different directions (as tensile strength “with the grain” or “across the grain”).

Brittleness

A tendency to fracture without appreciable deformation.

Charpy Test

One in which a specimen, supported at both ends as simple beam, is broken by the impact of a falling pendulum.

Cold Shortness

Brittleness of metals at ordinary or low temperatures.

Cold Working

The process of deforming a metal plastically at a temperature below the recrystallization temperature and at a rate to produce strain hardening.

Damping Capacity

The ability of a material to absorb or damp vibrations, which is a process of absorbing kinetic energy of vibration owing to hysteresis. The absorbed energy is eventually dissipated to the surroundings as heat.

Decarburization

A loss of carbon from the surface of steel, occurring during hot rolling, forging, and heat treating, when the surrounding medium reacts with the carbon (as oxygen and carbon combining).

Ductility

That property that permits permanent deformation before fracture in tension. It is the opposite of brittleness. It is frequently a valuable property because, by virtue of it, a member may take an occasional exceptionally-high load without breaking.

Elasticity

The ability of a material to be deformed and to return to the original shape. Stress is proportional to the strain only during an elastic deformation.

Embrittlement

Involves the loss of ductility because of a physical or chemical change of the material.

Free Carbon

That part of the carbon content of steel or iron that is in free form of graphite or temper carbon.

Hard Drawn

A temper produced in wire, rode, or tube by cold drawing.

Homogeneous Materials

Have the same structure at all points.

Isotropic Materials

Have the same properties in all directions.

Izod Test

A test in which a specimen, supported at one end is a cantilever beam, is broken by the impact of a falling pendulum.

Killed Steel

Steel that has been deoxidized with a strong deoxidizing agent, such as silicon or aluminum, in order to eliminate a reaction between the carbon and oxygen during solidification.

Ingots of Killed Steel

Are sounder, containing fewer gas holes, and more homogeneous than non-killed or rimmed steel; these are desirable characteristics for forgings and heavy rolled sections.

Machinability

A somewhat indefinite property that refers to the relative ease with which a material can be cut.

Malleability

A material’s susceptibility to extreme deformation in rolling or hammering.

Mechanical Properties

Those that have to do with stress and strain: ultimate strength, and percentage elongation, for example.

Percentage Elongation

The extension in the vicinity of the fracture of a tensile specimen, expressed as percentage of the original gage length, as 20% in 2in.

Percentage Reduction of Area

The smallest area at the point of rupture of a tensile specimen divided by the original area.

Physical Properties

Exclude mechanical properties, and are other physical properties such as density, conductivity, coefficient of thermal expansion.

Plasticity

The ability of a metal to be deformed considerably without rupture. In a plastic deformation, the material does not return to its original shape.

Poisson’s Ratio

The ratio of the lateral strain (contraction) to the longitudinal strain (extension) when the element is loaded with a longitudinal tensile force.

Precipitation Heat Treatment

Brings about the precipitation of a constituent from a supersaturated solid solution by holding the body at an elevated temperature, also called artificial aging. In some alloys, precipitation may also occur at ambient temperatures, a process called aging.

Proof Stress

Stress which causes a specified permanent deformation of a material, usually 0.01% or less.

Red Shortness

A brittleness in steel when it is red hot.

Relaxation

Associated with creep, is the decreasing stress at a constant strain; important for metals in high-temperature service.

Residual Stresses

Those not due to applied loads or temperature gradients; they exist for various reasons, as unequal cooling rates, cold working, etc.

Rimmed Steel

Incompletely deoxidized steel.

Ingots of Rimmed Steel

Have a surface layer quite free of slag inclusions and gas pockets, which results in the optimum surface on rolled sheets.

Solution Heat Treatment

The process of holding an alloy at a suitably high temperature long enough to permit one or more constituents to pass into solid solution and then cooling fast enough to hold the constituents as a supersaturated solution.

Stiffness

The ability to resist deformation. It is measured by the modulus of elasticity in the elastic range, the higher the modulus, the stiffer the material.

Strain Hardening

Increasing the hardness and strength by plastic deformation at temperatures lower than the recrystallization range.

Temper

A condition produced in a non-ferrous metal by mechanical or thermal treatment; for example, annealed temper (soft), hard temper, spring temper.

Toughness

The capacity of material to withstand a shock load without breaking.

Transverse Strength

Refers to the results of a transverse bend test, the specimen being mounted as a simple beam; also called rupture modulus. It is frequently applied to brittle materials, especially cast iron.

Work Hardening

The same as strain hardening.

Wrought Steel

Steel that has been hammered, rolled, or drawn in the process of manufacture; it may be plain carbon or alloy steel.

Heat Treatment

An operation or combination of operations involving heating and cooling of metal or an alloy in the solid state for the purpose of altering the properties of the material

Aging (and Age Hardening)

A change in a metal by which its structure recovers from an unstable or metastable condition that has been produced by quenching or cold working. The process consists often of submicroscopic precipitation.

Aging (and Age Hardening)

The result is a change of mechanical and physical properties, a process that may be accelerated by using a temperature slightly higher than room temperature.

Annealing

A comprehensive term, is a heating and slow cooling of a solid metal, usually done to soften it.

Critical Range

Has the same meaning as transformation range.

Drawing

Often used to mean tempering, but this usage conflicts with the meaning of the drawing of a material through a die and is to be avoided.

Graphitizing, and Annealing Process

Causes the combined carbon to transform wholly or in part into a graphitic or free carbon; it is applied to cast iron, sometimes to high-carbon steel.

Hardening

The heating of certain steels above the transformation range and then quenching, for the purpose of increasing the hardness. In the general case, hardening is any process of increasing the hardness of a metal.

Malleablizing

Is an annealing process whereby combined carbon in white cast iron is transformed wholly in part of temper carbon. Temper carbon is free (graphitic) carbon in the form of rounded nodules, characteristic forms in graphitizing and malleablizing.

Normalizing

The heating of an iron-base alloy to some 100°F above the transformation range with subsequent cooling below that range in still air at room temperature. The purpose to produce a uniform structure.

Spheroidizing

Any heating and cooling of steel that produces a rounded or globular form of carbide.

Stress Relieving

The heating of a metal body to a suitable temperature and holding it at that temperature for a suitable time for the purpose of reducing internal residual stresses.

Tempering

A reheating of hardened or normalized steel to a temperature below the transformation range, followed by any desired rate of cooling.

Transformation Range

The temperature interval during which austenite is formed during heating; it is also the temperature internal during which austenite disappears during cooling.

Hardness

A measure of its resistance to indentation, and is one of the most significant properties because, properly interpreted, it says much about the condition of the metal.

Brinell Hardness Number (BHN)

Determined by a standard pressure applied to a 10mm ball which presses for 10sec or more on the surface of the material being tested. This hardness number is closely related to the ultimate tensile stress of steel.

Rockwell Tester

Faster than the Brinell and widely used commercially, utilizes several different indenters and, in effect, measures the depth of penetration by the indenters.

Rockwell Superficial Tester

A different machine, is used for a piece of material too thin for the standard tester.

Vickers Tester

Has a square-base, diamond pyramid indenter, and the Vickers number is the load in kilograms divided by the impressed area in square millimeters.

Shore Scleroscope Number

Obtained by letting a freely falling hammer with a diamond point strike the object to be tested and measuring the height of rebound. This height is the Shore Number; the higher the rebound, the harder is the material.

Shore Machine

Can be used on large parts and is often used as a quick inspection aid, but it is less accurate than the other tests.

ASTM

American Society for Testing Materials

AISI

American Iron and Steel Institute

SAE

Society of Automotive Engineers

Acid Bessemer

In AISI prefixes system, B means

Basic, open-hearth

In AISI prefixes system, C means

Acid open-hearth

In AISI prefixes system, D means

Electric-furnace

In AISI prefixes system, E means

Boron

In AISI middle letters, B means

Lead

In AISI middle letters, L means

Wrought Alloy Steel

Steel that contains significant quantities of recognized alloying materials, the most common being aluminum, chromium, cobalt, copper, manganese, molybdenum, nickel, phosphorus, silicon, titanium, tungsten, and vanadium.

Low-Alloy Structural Steel

Developed for structural uses where light weight is important, such as in the transportation industry, but they are also used in other structures.

Low-Carbon Alloy Steel

Used chiefly for carburizing.

Medium-Carbon Alloy Steel

Usually quenched and temped to hardness between 250 and 400 Brinell.

High-Carbon Alloy Steel

Ordinarily heat treated to hardness between 375 and 500 Brinell, for use as springs, wear resisting parts, etc.

High-Alloy Steel

Such as stainless steels.

Aluminum (Al)

An efficient deoxidizer, an alloy in nitriding steels (nitralloys), and it promotes fine grain size.

Boron (B)

In very small amounts (0.001% or less) is an economical hardenability agent in low- or medium-carbon deoxidized steels. It has no effect on tensile strength.

Chromium (Cr)

Improves hardenability economically, resistance to corrosion (with other alloys), strength at high temperatures, and wearing properties (high carbon).

Cobalt (Co)

Improves red hardness.

Columbium (Cb)

Often used to “stabilize” stainless steel (that is, it preempts the carbon and forestalls the formation of undesired carbides).

Copper (Cu)

Improves steel’s resistance to atmospheric corrosion; up to 4% Cu, it increases the fluidity of the melt; it improves tensile strength and the yield strength ratio in the normalized condition.

Lead (Pb)

Improves machinability, but affects different alloys differently.

Manganese (Mn)

Improves strength and increases hardenability moderately, counteracts brittleness from sulfur.

Molybdenum (Mo)

Increases hardenability markedly and economically, tends to counteract temper brittleness, improving creep strength and red hardness; it improves wear by forming abrasion-resistant particles. It is the most effective alloy for improving strength at high temperatures.

Nickel (Ni)

Strengthens unquenched and annealed sheets, toughens steel, and simplifies heat treatment by lessening distortion. It is the most effective element for reduction brittleness of steel at very low temperature. It is one of the principal alloys for stainless steel.

Phosphorus (P)

Increases hardenability, strengthens low-carbon steels, improves machinability of free-cutting steels, and improves resistance to corrosion.

Selenium (Se)

Improves machinability of stainless steel; also added to leaded resulfurized carbon steels for the same purpose.