Glossary

18/8 stainless steel is a common industry term that describes the chemical composition of austenitic stainless steel rather than a specific standardized grade. The designation refers to a composition containing approximately 18% chromium and 8% nickel, which provide good corrosion resistance.

Although 18/8 is not a formal grade defined by ASTM material specifications, it is widely understood and treated across the fastener industry as a grade designation. In practice, fasteners sold as 18/8 stainless steel are most often manufactured from Grade 304 stainless steel or a closely related alloy that falls within the same composition range.

Grade 304 stainless steel is an austenitic stainless steel and the most commonly used stainless steel alloy. It is specified under ASTM material specifications and contains approximately 18% chromium and 8% nickel, which provide good corrosion resistance and strength.

304 stainless steel performs well in indoor and many outdoor environments with exposure to moisture and mild chemicals but does not contain molybdenum, making it less resistant to chlorides and saltwater than Grade 316 stainless steel.

In fastener applications, 304 stainless steel is widely used for general-purpose assemblies where moderate corrosion resistance is required.

Grade 316 stainless steel is an austenitic stainless steel specified under ASTM material specifications and known for its superior corrosion resistance compared to Grade 304 stainless steel. Like other austenitic steels, it contains chromium and nickel, but it also includes approximately 2–3% molybdenum, which significantly improves resistance to pitting, crevice corrosion, and chloride exposure.

The typical composition of 316 stainless steel includes about 16–18% chromium, 10–14% nickel, and 2–3% molybdenum. This alloy is often referred to as “marine grade stainless steel” due to its durability in saltwater, chemical, and harsh outdoor environments.

In fastener applications, 316 stainless steel is commonly used in marine, coastal, chemical, and outdoor structural settings where maximum corrosion resistance is required.

Class A2 stainless steel is a metric fastener designation defined under ISO standards and commonly associated with austenitic stainless steel similar to Grade 304. It contains chromium and nickel and offers good corrosion resistance in many indoor and outdoor environments.

Class A2 stainless steel fasteners perform well in applications exposed to moisture and mild chemicals but do not contain molybdenum, making them less resistant to chlorides and saltwater than Class A4 stainless steel. They are widely used for general-purpose fastener applications where moderate corrosion resistance is required.

Class A4 stainless steel is a metric fastener designation defined under ISO material specifications and is typically associated with austenitic stainless steel similar to Grade 316. In addition to chromium and nickel, it contains molybdenum, which provides enhanced resistance to corrosion, particularly in chloride-rich environments.

A4 stainless steel fasteners are often referred to as marine-grade fasteners due to their durability in saltwater, chemical, and harsh outdoor environments. In fastener applications, A4 stainless steel is commonly used in marine, coastal, chemical, and outdoor structural settings where high corrosion resistance is required.

Austenitic stainless steel is a category of stainless steel characterized by its face-centered cubic (FCC) crystal structure, known as the austenitic phase. This structure is stable at both high and low temperatures and gives the alloy its defining properties—excellent corrosion resistance, high ductility, good weldability, and nonmagnetic behavior (in most conditions). Austenitic stainless steels are the most widely used type of stainless steel, accounting for roughly two-thirds of all stainless steel production.

The primary alloying elements in austenitic stainless steel are chromium (Cr) and nickel (Ni), though manganese (Mn) and nitrogen (N) can also be added to enhance strength and stability. Chromium, typically at levels of 16–26%, provides a passive oxide layer that protects the surface from oxidation and corrosion, while nickel (usually 6–22%) stabilizes the austenitic structure and improves toughness and formability.

This class of stainless steel is notable for its excellent resistance to corrosion and oxidation, even in aggressive environments such as marine, chemical, and food processing applications. It also maintains its strength and toughness at both cryogenic and elevated temperatures, making it suitable for a wide range of industrial uses—from fasteners, pressure vessels, and heat exchangers to kitchen equipment, architectural panels, and piping systems.

Austenitic stainless steels are grouped into several grades, with the most common being Type 304 (18% chromium, 8% nickel) and Type 316 (which adds about 2% molybdenum for improved resistance to chlorides and acids). While these steels are generally nonmagnetic, cold working can induce slight magnetism due to strain-induced transformation of some austenite into martensite.

Duplex stainless steel is a type of stainless steel that combines the properties of both austenitic and ferritic stainless steels within a single microstructure. Its name comes from this “dual-phase” composition — roughly 50% austenite and 50% ferrite — which gives it an exceptional balance of strength, toughness, and corrosion resistance. This hybrid structure provides advantages that neither phase can achieve alone, making duplex stainless steels highly valued in demanding industrial environments.

Chemically, duplex stainless steels typically contain 19–28% chromium, 3–10% nickel, and 0.05–0.5% nitrogen, with possible additions of molybdenum, copper, and tungsten to enhance corrosion resistance. The nitrogen and molybdenum are especially important for improving resistance to pitting, crevice corrosion, and chloride-induced stress corrosion cracking, which are major weaknesses of austenitic and ferritic grades when used alone.

Mechanically, duplex stainless steels are about twice as strong as common austenitic grades like 304 or 316, while maintaining good ductility and impact toughness. They have excellent resistance to chloride stress corrosion cracking, a property inherited from their ferritic phase, and superior general and localized corrosion resistance due to their high chromium and molybdenum content. Their reduced nickel content also makes them more cost-stable, as they are less affected by fluctuations in nickel prices.

Common grades include 2205 (UNS S32205/S31803), the most widely used standard duplex stainless steel, and 2507 (UNS S32750), a “super duplex” grade with higher alloy content and even greater corrosion resistance. Duplex stainless steels are widely used in chemical processing, oil and gas production, desalination plants, pulp and paper manufacturing, marine environments, and structural applications where strength and corrosion resistance are critical.

In summary, duplex stainless steels offer a unique combination of high strength, corrosion resistance, and cost efficiency, bridging the gap between austenitic and ferritic grades. Their dual-phase structure gives them the ability to withstand harsh chloride environments, high mechanical loads, and thermal stresses, making them one of the most versatile materials used in modern industrial design.

Ferritic stainless steel is a type of stainless steel primarily composed of iron and chromium, with a body-centered cubic (BCC) crystal structure known as the ferritic phase. Unlike austenitic stainless steels, ferritic grades contain little to no nickel, which makes them more affordable but also less ductile. They are magnetic, have moderate corrosion resistance, and exhibit excellent resistance to stress corrosion cracking.

The chromium content in ferritic stainless steels typically ranges from 10.5% to 30%, which provides a protective oxide layer that resists oxidation and rust. However, since they lack significant amounts of nickel and other stabilizing elements, they are less resistant to highly corrosive or acidic environments compared to austenitic stainless steels. They also have lower toughness, especially at low temperatures, and are not hardenable by heat treatment, though they can be strengthened through cold working.

Ferritic stainless steels are known for their good thermal conductivity, resistance to scaling at high temperatures, and minimal thermal expansion, which makes them especially useful in automotive exhaust systems, heat exchangers, furnace components, and architectural trim. They are also favored in applications where magnetic properties are desirable or where cost reduction is important, since the absence of nickel significantly lowers material expense.

Common ferritic stainless steel grades include Type 409, used widely in automotive exhaust systems, Type 430, common in kitchen appliances and architectural panels, and Type 446, which offers excellent resistance to oxidation and scaling in high-temperature environments.

Martensitic stainless steel is a category of stainless steel known for its high strength, hardness, and moderate corrosion resistance, achieved through heat treatment that transforms its crystal structure into martensite. It is characterized by a body-centered tetragonal (BCT) crystal structure, which forms when steel containing sufficient carbon is rapidly cooled or quenched from a high temperature. This gives martensitic stainless steels their distinctive ability to be hardened and tempered, much like carbon steels.

The composition of martensitic stainless steels typically includes 11.5% to 18% chromium and 0.1% to 1.2% carbon, with small amounts of other elements such as molybdenum, nickel, or vanadium to improve strength, corrosion resistance, and toughness. The chromium content provides the protective oxide layer that resists rust and oxidation, while the carbon allows for the formation of martensite during quenching, leading to significant hardness and wear resistance.

Unlike austenitic or ferritic stainless steels, martensitic grades are magnetic and can be heat treated to a wide range of hardness levels. However, this comes at a tradeoff: their corrosion resistance is generally lower, especially in environments involving chlorides or acids. They also tend to have reduced ductility and weldability compared to other stainless steel families.

Common martensitic grades include Type 410 (general-purpose, used for cutlery, valves, and turbine blades), Type 420 (high-carbon, used for surgical instruments and knives), and Type 440C (very high carbon, used for bearings, precision tools, and high-wear components). These steels are often used where mechanical strength, edge retention, and wear resistance are more critical than corrosion resistance.

Precipitation hardening stainless steel, often abbreviated as PH stainless steel, is a class of stainless steel that achieves its exceptional high strength, hardness, and corrosion resistance through a specialized heat treatment process known as precipitation hardening (or age hardening). This process involves the controlled formation of very fine particles (precipitates) within the metal’s microstructure, which strengthen the material without significantly reducing its toughness or corrosion resistance.

Chemically, PH stainless steels contain chromium and nickel as their main alloying elements, similar to austenitic grades, but they also include copper, aluminum, titanium, or niobium to enable the precipitation-hardening effect. The chromium provides corrosion resistance, while the nickel stabilizes the austenitic or martensitic structure, and the additional elements form the fine precipitates that dramatically increase strength during heat treatment.

The hardening process occurs in three main stages: solution treatment, quenching, and aging. During solution treatment, the alloying elements are dissolved at high temperature. Quenching rapidly cools the material to lock these elements in a supersaturated solid solution. Finally, during aging, the metal is reheated to a moderate temperature, allowing controlled precipitation of intermetallic compounds that impede dislocation motion — the mechanism that gives the steel its high strength.

Depending on their metallurgical structure, PH stainless steels can be martensitic, semi-austenitic, or austenitic. The most common grade is 17-4 PH (UNS S17400), which contains roughly 17% chromium and 4% nickel, along with copper and niobium for hardening. It offers excellent strength, moderate corrosion resistance (similar to 304), and can be hardened to yield strengths exceeding 1,000 MPa (145 ksi). Other notable grades include 15-5 PH, 17-7 PH, and 13-8 Mo, each designed for specific combinations of toughness, hardness, and corrosion resistance.

Precipitation hardening stainless steels are used extensively in aerospace, nuclear, and petrochemical industries, as well as for valves, shafts, turbine components, fasteners, and high-performance mechanical parts that must maintain strength at both elevated and sub-zero temperatures.

In summary, precipitation hardening stainless steels combine the corrosion resistance of austenitic grades with the strength of martensitic steels, made possible through precise heat treatment. Their ability to achieve very high mechanical performance while maintaining corrosion resistance makes them ideal for critical applications where strength, durability, and reliability are equally important.

Stainless steel is a corrosion-resistant alloy of iron that contains a minimum of 10.5% chromium (Cr) by mass, along with varying amounts of carbon, nickel, molybdenum, and other elements. The presence of chromium is what gives stainless steel its defining property — the ability to form a thin, invisible layer of chromium oxide on its surface when exposed to oxygen. This layer, known as the passive film, protects the metal beneath from rust and oxidation, and it self-heals if scratched or damaged.

Stainless steel is both strong and durable, with excellent resistance to corrosion, heat, and chemical attack, which makes it one of the most versatile materials in modern engineering, manufacturing, and architecture. Its composition can be modified to enhance certain properties — for example, adding nickel improves ductility and toughness, while molybdenum increases resistance to acids and chlorides (such as in seawater or salt-rich environments).

There are several major categories of stainless steel, each with distinct characteristics:

- Austenitic stainless steels (e.g., 304, 316): The most common type, containing high levels of chromium and nickel. They are non-magnetic, highly formable, and extremely corrosion-resistant. Grade 304 is widely used in kitchen equipment and architecture, while 316 includes molybdenum for added marine and chemical resistance.

- Ferritic stainless steels: Contain chromium but little or no nickel. They are magnetic, less ductile, and typically used in automotive exhaust systems and appliances.

- Martensitic stainless steels: Have higher carbon content, allowing them to be hardened by heat treatment. They are used for cutting tools, blades, and fasteners.

- Duplex stainless steels: Combine austenitic and ferritic structures for greater strength and stress corrosion resistance, often used in chemical plants and pipelines.

- Precipitation-hardening stainless steels: Can be heat-treated to very high strengths, used in aerospace and high-performance applications.

Stainless steel’s appeal lies not only in its performance but also in its aesthetic and hygienic qualities. Its smooth, reflective surface resists staining and bacterial buildup, making it ideal for food processing, medical instruments, architecture, and consumer products.