Fastener Terminology

Alloy Steel

A mixture (or alloy) of ordinary steel added to other metals besides carbon with the specific purpose of attaining certain characteristics such as higher strength. A few exceptions to this definition exist, however, so that a chromium content above 4% is not considered alloy steel and above 12% is considered stainless steel.


The most abundunt metal in the earth, aluminum is blueish and silvery-white, very ligth, malleable, and ductile with high heat and electrical connectivity. It is non-magnetic and one-third the weight of steel with good corrosion resistance against certain chemicals and acid and but weak resistance against other elements such as sea water.


Stands for Air Force-Navy.

ASTM 193-194

ASTM 193 are chemical and physical specifications for hex head cap screw, studs, and bolts made of steel and stainless steel. ASTM 194 refers to nuts. The commonly used stainless is called grade 8, referring to 304 material to certain specifications, and grade 8M referring to 316 material. The major differences between ASTM and commercial stainless fasteners are: (a) 304 material must be used for manufacturing grade 8, not simply 18-8; (b) ASTM 193-194 generally refers to heavy hex heads and heavy nuts, through semi-finished hex head and finished nuts may be supplied with the permission of buyer; (c) cold formed material will require carbide solution treatment or annealing to reduce hardness to meet ASTM requirements.


To heat metal in order to lower its hardness. The term anneal refers to the heat treatment given all 300 series stainless and most 400 series stainless by a steel mill after the raw material has been completed but before fasteners are manufactured. Anneal also refers to the heat treatment given 400 series stainless fasteners after their manufacture (also called hardening and tempering) to lower hardness and increase toughness. For example, fasteners of 410 stainless may score over 200,000 psi after manufacture and be too brittle. By annealing at 1000 degrees F. tensile strength would decrease to 125,000-15,000 psi; annealing the same material to 500 degrees F. would bring tensile to 16,000-19,000 psi.


A metallic solution of steel that is non-magnetic; has a face centered cubic structure. In carbon steels, this temperature range allows for higher absorption of carbon into the metal.


Refers to 300 series stainless, the most popular of the stainless alloys accounting for 85%-90% stainless fasteners sold. Named for Sir Robert Williams Austen, an English metallurgist, austentic stainless is a crystal structure formed by heating steel, chromium, and nickel to a high temperature where forms the characteristics of 300 series stainless steel. An "AUSTENITE" is a molecular structure where 8 atoms of iron surround one atom of carbon, thus limiting the corrosive effects of the carbon. Austenitic fasteners have the highest level of corrosion resistance in the stainless family, cannot be hardened by heat treatment, and are almost always non-magnetic. Sometimes heat and friction in cold forming can cause austenitic stainless to take on slight magnetism, but the corrosion-resistant properties remain the same. The most popular of austenitic grades is known generically as "18-8 stainless" and includes grades 302, 302HQ, 303, 304, 305, and XM-7. Typical industries using 18-8 fasteners include: food, Dairy, Wine, Chemical, Pulp & Paper, Pharmaceutical, boating, swimming pool, pollution control, electronic, medical and hospital equipment, computer, textile. Type 316 stainless has added nicked and especially molybdenum, the molybdenum (called moly) sharply increases corrosion resistance to chlorides and sulfates, including sulfurous acids in the pulp industry. It has superior tensile strength at high temperature compared to 18-8. Besides pulp and paper, typical industries using 316 are: photographic and other chemicals, ink, textile, bleach, rubber. Exotic metal in the 300 series include 309, 310, 317, 321, and 347. With superior corrosion resistance at elevated temperatures, these metals are used for furnace parts, high temperature containers and processing equipment, aircrafts parts such as collectors rings, exhaust system, and equipment for very corrosive compounds of sulfuric, nitric, citric, and lactic acids. Their usage compared to 18-8 and 316 is very low.


The process of uniformly heating a metal until the metal grain structure transforms from Ferrite to Austenite. For carbon steels, transformations begins at 13330F and becomes completly uniform throughout the metal at a higher temperature (varies by carbon content).


Plating metal that possesses superior corrosion resistance (compare to zinc) and lubricity which helps during installation. Highly toxic; restricted under RoHS.


Adds strength to stainless steel, but also lowers corrosion resistance. The more carbon there is, the more chromium must be added, because carbon offsets 17 times its own weight in chromium to form carbides, thus reducing the chromium available for resisting corrosion.

Case Hardening

A process that involves putting carbon (or a combination of carbon and nitrogen) into the surface of steel to make it a higher-carbon steel which can be hardenend by heat treatment.


A blue-white metal, chromium is the most important element providing corrosion resistance in stainless steel. By adding 12% chromium to ordinary steel, stainless steel is formed. Chromium offsets the corrosive effects of carbon found in steel and is the primary factor in the ability of stainless to form a passive film on its surface providing corrosion resistance.

Cold Forming or Cold Heating or Cold Working

When fasteners are produced without heating or small heat below the recrystallization temperature (so the raw material bond of stainless remains unchanged) by pressing metal wire against various dies at high speed to from a fastener's head or basic shape. Cold working causes an increase in tensile strength and hardness (know as work hardening) and a decrease in ductility.


The wearing away or alteration of a metal by an electrochemical reaction or by a direct chemical attack.

Corrosion, Galvanic

Corrosion involving two dissimiliar metals in the presence of electrolyte. The more active (anodic) of the two metals, sacrifices its ions to the less active, thereby causing breakdown in the active metal.

Corrosion, Pitting

(also called Cavitation Corrosion) Localized corrosion in which a small bubble of air gets trapped on the surface of the metal. This bubble deprives the metal's surface of fresh oxyzen supply, causing that area to become anodic. The change in the reactivity of the metal causes the formation of small, sometimes deep, pits of rust.


A chemical conversion coating commonly used on zinc electroplated fasteners. Helps prevent the corrosion of the zinc plating and may be dyed to add color to the fasteners.


The deposition of a metallic Coating onto an object by putting a negative charge onto the object and immersing it into a solution which contains a salt of the metal to be deposited. The metallic ions of the salt carry a positive charge and are attracted to the part. When they reach it, the negatively charged part provides the electrons to reduce the positively charged ions to metallic form.


The failure of a fastener due to cyclic loads.

Fatigue Strength

Measures the endurance of a fastener by showing the load it can accept without breaking under repeated load cycles.

Frictional Coefficient

A ratio: friction between adjacent surfaces as they slide across one another over force pressing the two surfaces together. Friction Perpendicular Pressure, Influenced by lubrication.


When two metal or fasteners stick together and cannot be easily loosened. In tightening fasteners, for example, pressure builds on threads as metals rub against each other, and the passive film preventing corrosion on stainless may not form due to lack of oxygen. Heat contributes to galling caused by high speed fasteners installation. A reduced wrench speed can help. Thread lubrication is the most effective treatment for galling.


Any one of a group of highly corrosion resistant high strength nickel based alloys produced and tredemarked by Haynes International Inc.

Heat Treating

Any heating or cooling process used to influence the mechanical properties of a metal. In fastener manufacture in typically refers to the process of austenizing, quenching, and tempering a product in order to form a martensitic grain structure(i.e. improve strength properties).

Hot Dip Galvanizing

A process to coat fasteners that involves dipping the fasteners to a prepared item bath of molten zinc.

Hot Forging

Heating metal to red-hot temperatures or temperatures above the recrystallization point to soften it before shaping a fastener. Hot forging is primarily used when the diameter of the metal is too large for cold forming or the quantity required is too small economically set up a cold forming machine.

Hydrogen Embrittlement

Hydrogen trapped under the surface of fastener can later cause ruptures. It is generally associated with carbon and alloy steels, not stainless. There may be not external signs of corrosion before a break occurs.


Nickel-Chromium-Iron alloy (not to be confused with 18-8 Stainless Steel). Exotic material with excellent corrosion resistance.


As related to stainless fasteners, 300 series stainlees is non-magnetic in its raw material condition. Cold working can sometimes induce traces of magnetism in 300 series, depending on the severity of cold working and chemical composition of the stainless. A rise in magnetism is related to an increase in tensile strength and working hardening caused by the heat and friction of cold forming and does not reduce corrosion resistance or cause any molecular change in austenitic raw material. A higher portion of nickel can increase stability in stainless, thus decreasing work hardening and any possibilites of magnetism. Brass and silicon bronze are non-magnetic. Many stainless specs including MS hex heap cap screws allow 2.0 magnetic permeability which translates to low/medium magnetism. Magnetic permiability of 1.0 translates to a very slight, glancing magnetism.


A micro structure of carbon steel formed by rapidly quenching a steel from austenzing temperatures. Notable for high hardness and increased strength capabilities compared to ferrite or austenite.


Molybdenum(Moly) is a metal added to 316 stainless steel, sharply increasing its corrosion resistance to chlorides and sulfates, especially various sulfurous acids in the pulp industry. Molybdenum helps reduce hardness and increase tensile strength at higher temperatures. It is also added to Marutex self-drilling screws made of 410 stainless steel to signifiacantly increase corrosion resistance.


Nickel-Copper Alloy. Has excellent corrosion resistance in heat and salt water.


An annealing process that is used to remove strain and reduce coarse(eg.: martensitic) crystal structure.


The spontaneous formation of a hard non-reactive surface film that inhibits further corrosion. This layer is usually an oxide or nitride that is a few atoms thick. Under normal conditions of pH and oxygen concentration, passivation is seen in such materials as aluminum, magnesium copper, stainless steel, titanium, and silicon.

Proof Load

An applied tensile load that the fastener must support without permanent deformation and represents the usable strength of a certain standard of fasteners.

Proof Strength

The ideal amount of stretch that can be applied to a bolt in order for it to properly clam in a joint(i.e. must be functioning in the elastic range prior to yield).

Proof Stress

For internally threaded fasteners, the minimum ultimate tensile strength for the threads.


The process of rapidly cooling a metal that has been heated into the austenitic range.


The space of intervals over which a series of torque values occur when compared to one another. The difference between various torque values.

Shear Force

A force that is applied perpendicular to the fasteners axis. Caused by two adjacent parts moving parallel to one another, in opposite directions.

Shear Plane

The two dimensional plane through which two adjacent materials move parallel to one another, in opposite directions.

Shear Strength

The maximum load that can be supported prior to fracture when applied at a rigth angle to the fastener's axis. Defined in PSI as the load in pounds to cause rupture divided by the cross sectional area in square inches of the part along the rupture plane.

Stainless Steel

A family of iron-based alloys that must contain at leaat 10.5% chromium. The presence of chromium creates an invisible surface film that resists oxidation and makes the material "passive" or corrosion resistant. Other elements, such as nickel or molybdenum are added to increase corrosion resistance, strength or heat resistance.

17-4PH Stainless Steel

A chromium nickel grade of stainless steel. It may be hardened by single low-temperature heat treatment, which gives it excellent mechanical properites at a high-strength level while minimizing scaling and distortion. Often used in aircraft or similar parts requiring intricate machining , extensive welding, and/or where distortion in conventional hear treatment is a problem.

Stainless Steel, Auetenitic

Also called 18-8 or 300 series. Most common stainless steel (over 80% of stainless steel fasteners are 300 series). They are highly corrosion resistant due to the presence of 18% chromium and 8% nickel. Austenitic stainless steels have a FCC structure due to the concentration of nickel in the steel. Non-hardenable by heat treatment. Common grades all 300 series.

Stainless Steel, Ferritic

Corrosion resistant steel which contains 12%-18% chromium, little to no nickel, and less than 0.2% carbon. Magnetic and non-hardenable by heat treatment. Common Grades: 430.

Stainless Steel, Martensitic

Corrosion resistant steel which contains 12%-18% chromium, little to no nickel, and enough carbon to be heat treated. Is magnetic and hardenable by heat treatment. Common Grades: 410 & 416.

Stainless Steel, Precipitation Hardened

Stainless Steel that has the corrosion resistance of austenitic stainless steels and better strength characteristics than some of the martensitic grades. This is accomplished through complex and lengthy form of heat treatment called age-hardening (due to the time required to complete it). Common Grades: 17-4, 17-7, 630A, A286.

Strees Relieving

A heat treatment perfomed to remove residual stress in cold worked parts.


A silvery gray metal with high corrosion resistance against salt waters, chlorides, and many acids. It is strong, through light weight, and very expensive.


A silvery gray metal with high corrosion resistance against salt waters, chlorides, and amny acids. It is strong, through light weigth, and very expensive.

Yield Strength / Point

The point at which a fastener has become permanently elongated under stress and begins to loose its clamping force.