This article contains everything you need to know about stainless steel 304 and 304L as well as their use.
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Stainless Steel 304 – 18/8
Stainless Steel 304L - 18/9
Stainless steel grade 304 is an austenitic stainless steel known for its versatility and widespread use. It belongs to the T300 series, characterized by an 18% chromium and 8% nickel composition, which is commonly referred to as 18/8 stainless steel. One of its key features is its deep drawing capability, allowing it to be shaped through compressive forces that bend, cut, and twist the metal.
Stainless steel 304 is supplied to manufacturers in various forms such as "V" plates, tubes, bars, billets, coils, strips, and sheets. These forms are tailored to meet the specific manufacturing needs of producers. Renowned for its heat and corrosion resistance, excellent tensile strength, and attractive appearance, stainless steel 304 is highly versatile.
The adaptability of stainless steel 304 is evident in the wide range of products it is used to create, including cutlery, flatware, tubing, springs, nuts, bolts, and electrical enclosures. Its popularity is partly due to its ability to be processed through various hot or cold working techniques.
The differences among stainless steel grades are primarily reflected in their chemical compositions. Stainless steel 304 contains 18% chromium, whereas stainless steel 316 has a slightly lower chromium content of 16%. Additionally, stainless steel 304 has a notably high melting point, ranging from 1398.9°C to 1451.4°C (2550°F to 2650°F), allowing it to withstand exposure to extreme and challenging environments.
Both stainless steel 304 and 316 are austenitic grades, characteristic of the T300 series stainless steels. The key distinction between them lies in their nickel content, which imparts the T300 series with its unique crystalline structure. Stainless steel 316 is further differentiated by its 2% to 3% molybdenum content, which significantly enhances its corrosion resistance compared to stainless steel 304.
Stainless steel's widespread popularity and extensive use stem from its exceptional corrosion resistance and ability to perform well at very low temperatures. Additionally, stainless steel is recognized for its high level of hygiene, making it a preferred choice for both consumer goods and critical industrial components. These attributes are attributed to its protective passive layer, a thin chromium oxide film that forms on the metal's surface.
This passive layer develops when the chromium in stainless steel reacts with oxygen, creating chromium oxide. While this protective layer is a standard feature for stainless steel, not all metals with chromium exhibit this behavior. For effective passivation, the chromium content must exceed 11%.
It might be tempting to compare the passive layer on stainless steel to rust or iron oxide, but this is a misconception. Rust results from the reaction between iron and oxygen in the presence of water and does not offer any protective benefits. In contrast, the passive layer on stainless steel is inert and non-reactive with other elements, which contributes to its corrosion resistance and hygienic properties.
Beyond its corrosion resistance, stainless steel is distinguished from other metals by several key factors. It is categorized based on chemical composition, physical properties, metallographic structure, and characteristics. All metals and alloys have a crystalline structure or lattice, and stainless steel's crystalline structures are classified into ferrite, austenite, martensite, or combinations of these. The properties of stainless steel are determined by these crystalline lattices, which are further divided into five families: austenitic, ferritic, duplex, martensitic, and precipitation-hardened.
Despite their similarities, small variations in the crystalline structure of stainless steels can significantly affect their mechanical properties. Each family of stainless steel, while sharing common traits, varies in its unique properties, allowing each type to fulfill specific functions.
During the solidification of stainless steel in the manufacturing process, its atoms arrange into distinct crystal structures. These structures define the phases of stainless steel: the austenite phase has a face-centered cubic (fcc) structure, while the ferrite phase has a body-centered cubic (bcc) structure. Martensite, on the other hand, has a body-centered tetragonal (bct) structure, setting it apart from the cubic structures of the other phases.
The austenitic family features a non-magnetic face-centered cubic phase, and some austenitic stainless steels include small amounts of the ferrite phase. This family is known for its low yield strength, high work hardening rate, high tensile strength, excellent ductility, and exceptional resistance to low temperatures.
A significant advantage of the austenitic stainless steel family is its versatility in being fabricated into various forms, configurations, and shapes. This makes it particularly suited for the manufacture of complex and intricate parts. Although it has good weldability, it cannot be hardened or strengthened through heat treatments but can be enhanced by cold forming or work hardening.
The ferritic family is composed of ferrite phase with minimal carbides and nitrides. Unlike the austenitic phase, ferritic stainless steels have magnetic properties similar to carbon steel. The characteristics of the ferritic family include high strength and resistance to stress from chloride corrosion cracking. Like the austenitic family, ferritic stainless steels cannot be hardened by heat treatment and have limited toughness, which restricts their use to thin sheets and strips. Typically, these products have a thickness of 4 mm (0.15 in) or less and are used for applications such as tubing and thin-gauge components.
Martensitic stainless steel can be strengthened through heat treatment. When heated to 1040°C (1904°F), it assumes an austenitic structure but transforms into a martensitic structure upon cooling to room temperature. To achieve the desired hardness and toughness, martensitic stainless steel is tempered at temperatures between 100°C and 700°C (200°F and 1300°F). This type of stainless steel is noted for its high strength, wear resistance, low toughness, and difficulty in welding.
The duplex family of stainless steel combines ferrite and austenite phases, with approximately 50% to 55% austenite and 45% to 50% ferrite. The duplex phase offers high strength, excellent wear resistance, toughness, and resistance to chloride stress corrosion cracking. Its outstanding strength makes it ideal for applications such as pressure vessels, tanks, and structural components.
The precipitation-hardened family gains its strength from a treatment process that involves heating the stainless steel with a solution at high temperatures, dissolving solute atoms to form a single phase. These stainless steels are enriched with chromium and molybdenum to enhance corrosion resistance and support high-strength applications.
Stainless steel is categorized into series and grades within the broad families of austenitic, ferritic, and martensitic. Each grade specifies the metal's durability, quality, and heat resistance, with numbers indicating the chemical composition, especially chromium and nickel content.
The SAE system, also known as the American Iron and Steel Institute (AISI) system, uses a three-digit numbering approach. The first digit denotes the grade of stainless steel: 2 and 3 represent austenitic stainless steel, 4 indicates ferritic and martensitic types, and the 500 series is designed for high-temperature applications. Precipitation-hardened stainless steel is identified by the 600 series.
An ASTM identification number begins with a letter from A to G, which denotes the type of material: A for ferrous metals and B for nonferrous metals. This letter is followed by one to four numbers, a dash, and the year the designation was issued. ASTM has issued over 12,000 standards covering a wide range of materials and products.
Stainless steel 304 is part of the austenitic phase stainless steels, which feature a chromium content ranging from 15% to 30% and a nickel content between 2% and 20%. Iron is the primary metal in stainless steel 304, comprising the majority of its mass, followed by chromium and nickel. The remaining alloying elements, although present in quantities of 2% or less, play a crucial role in defining the properties of stainless steel 304.
| Chemical Composition of Stainless Steel Series Grade 304 | |
|---|---|
| Elements | Weight Percentages |
| Carbon | 0.08% Max |
| Manganese | 2% Max |
| Phosphorus | 0.045% Max |
| Sulfur | 0.03% Max |
| Silicon | 0.75% Max |
| Chromium | 18% to 20% |
| Nickel | 8% to 12% |
| Nitrogen | 0.1% Max |
| Iron | 67% to 71% |
Stainless steel 304 comes in three variants: series 304, 304L, and 304H, each differing in carbon content. Series 304L has the lowest carbon content, capped at 0.03%, while 304H has a higher carbon content, ranging from 0.04% to 0.1%. The increased carbon content in series 304H enhances its heat resistance and yield strength compared to the other variants.
All stainless steels contain a minimum of 50% iron (Fe). In austenitic stainless steels, this percentage can vary, impacting the metal’s ductility and workability. Like all steel types, stainless steel is an iron-based alloy that is modified and enhanced through the addition of various alloys.
Carbon enhances the strength and hardness of stainless steel but reduces its corrosion resistance. Therefore, in austenitic stainless steels, the carbon content is kept low, typically below 0.1%. Excess carbon can affect the chromium content and lead to sensitization, where chromium carbide forms instead of the chromium oxide layer on the surface. This reaction impairs corrosion resistance. Lowering the carbon content, as seen in stainless steel 304L, improves the ductility of the steel.
Chromium is crucial in stainless steel 304 for forming the passivation layer that prevents rust and corrosion. Stainless steel is created when chromium content exceeds 10%, transforming ordinary steel into a corrosion-resistant material. When chromium reaches the surface, it forms a passive, non-reactive layer. In austenitic stainless steel, chromium is balanced with other alloying elements to create the austenitic microstructure.
The addition of nickel to austenitic stainless steel is to help form and stabilize the austenite structure such that the stainless steel has good strength, plasticity, and toughness. Nickel expands the austenite phase zones with a minimum amount being 8% to 9%. The effect of nickel on the mechanical properties of austenitic stainless steel is determined by how it helps maintain the stability of the austenite structure.
Manganese, as with nickel, is an austenite forming element that increases the strength, toughness, and hardenability of stainless steel 304. It acts as a deoxidizer that reduces the amount of oxygen during manufacturing and makes it easier to form strong intermolecular bounds during smelting and increases ductility. Manganese removes any sulfur from liquid iron to reduce the brittleness of stainless that the sulfur impurities cause. Additionally, the presence of manganese makes it possible to increase the hardness of stainless steel after heat treatment.
Phosphorus is added to stainless steel 304 to enhance its strength. While it positively contributes to the metal’s strength, it also negatively affects its corrosion resistance and increases the risk of cracking during welding.
Sulfur is included in stainless steel 304 to improve machinability. However, sulfur can make the metal brittle and reduce its weldability and resistance to high temperatures. It also decreases corrosion resistance, so its levels are carefully controlled.
Silicon acts as a deoxidizing agent and is present as a residual element in stainless steel alloys. In moderate amounts, it boosts the strength of stainless steel 304 and enhances its resistance to sulfuric acid and oxidation. Silicon also stabilizes ferrite, but excessive amounts at high temperatures can lead to the formation of intermetallics.
Nitrogen is an austenite-forming element that enhances the stability of the austenite phase in stainless steel 304. It significantly improves the yield strength and resistance to pitting corrosion. When combined with manganese, nitrogen supports the formation of the austenite microstructure, offering greater strength and reduced susceptibility to sensitization compared to carbon, nickel, and manganese alone.
More than half of the stainless steel produced globally is grade 304, which is available in various forms such as sheets, plates, bars, billets, blooms, slabs, and tubing. This grade is further classified into 304, 304L, and 304H, with distinctions based on their carbon content. The other elements in these grades remain consistent, differing only in carbon levels.
The variations in carbon content among grades 304, 304L, and 304H affect their specific properties, making them suitable for different functions and applications. Grade 304L contains less than 0.03% carbon, while 304H has up to 0.10%. Grade 304 typically has a carbon content of 0.08%.
The various grades of stainless steel 304 are created by altering the carbon content while keeping the percentages of other elements constant. Adjusting the carbon content increases the strength, hardness, and hardenability of the steel but can reduce its resistance to corrosion. High carbon levels not only diminish corrosion resistance but can also make the steel more brittle and decrease its weldability.
Grade 304 is the most widely used austenitic stainless steel. It contains 8% to 10.5% nickel and 18% to 20% chromium, making it suitable for applications that demand exceptional formability. Stainless steel grade 304 is known for its high corrosion resistance, electrical conductivity, and thermal conductivity. Its broad usage is attributed to its resistance to various environmental conditions and many corrosive chemicals. It offers excellent performance at a reasonable cost, making it ideal for both commercial and industrial applications.
One of the key features of stainless steel grade 304 is its high tensile strength of 621 MPa (90 ksi). It also exhibits oxidation resistance at temperatures above 870°C (1598°F) without adverse effects and has a melting point ranging from 1399°C to 1454°C (2550°F to 2650°F).
The key characteristic of stainless steel grade 304L is its low carbon content of 0.03%, which has a significant impact on its performance. While it is similar to grade 304, 304L exhibits distinct differences in mechanical properties. Specifically, grade 304L has a tensile strength of 586 MPa (85 ksi) and a yield strength of 0.2% of 241 MPa (35 ksi), making it structurally weaker compared to grade 304.
Despite these differences, grade 304L is used extensively, much like grade 304, due to its ability to prevent carbide precipitation during welding. This feature enables it to be employed in its as-welded state even in highly corrosive environments. Using grade 304L avoids the need for post-weld annealing, which can save time, money, and effort.
The elevated carbon content in stainless steel grade 304H provides enhanced strength at temperatures exceeding 427°C (800°F). Despite this, grade 304H maintains the same level of corrosion resistance as grades 304 and 304L, as its only altered chemical factor is carbon content. Although the increased carbon content improves strength, it also makes grade 304H more susceptible to carbide precipitation in the heat-affected zones during welding. Nevertheless, grade 304H remains weldable and easy to fabricate.
Grade 304H’s ductility allows for cold working, which enhances its strength and hardness. It is processed by hot working at temperatures ranging from 750°C to 1150°C (1652°F to 2102°F) and then annealed at 1038°C (1900°F). Machining grade 304H requires careful control to prevent chipping, and it is performed at slow speeds using lubrication, sharp tools, and high-powered equipment.
Stainless steel grade 304 is widely used because of its versatile properties that make it suitable for various applications and environments. Often referred to as stainless steel 18/8 due to its chromium and nickel content, grade 304 is an austenitic stainless steel with excellent deep-drawing capabilities. This characteristic has made it the preferred choice for manufacturing sinks and saucepans.
The discussion of stainless steel invariably highlights its corrosion resistance, a key factor in its ability to endure diverse environments and conditions. For stainless steel 304, this corrosion resistance is primarily due to its chromium content, which forms a protective passive layer on the metal's surface. Despite its renowned resistance, stainless steel 304 can still be susceptible to pitting and crevice corrosion in environments with chlorides, and stress corrosion cracking may occur under specific temperature conditions.
Oxidation resistance is a crucial feature of stainless steel 304, which helps prevent rusting and degradation. Grade 304 performs well in intermittent service at temperatures up to 870°C (1598°F) and in continuous service up to 925°C (1697°F). However, continuous service between 425°C and 860°C (797°F and 1580°F) is not recommended, as it can negatively impact the corrosion resistance of stainless steel 304.
Creep strength, which reflects a material's ability to resist deformation at high temperatures, is a significant property of stainless steel 304. While standard and low-carbon versions of stainless steel 304 may experience reduced strength under elevated temperatures, grade 304H is specifically designed for high-temperature applications and maintains its strength better in such environments.
Certain stainless steels can develop a sigma phase when exposed to high temperatures, which compromises their corrosion resistance. This phase forms over time and is temperature-dependent. However, stainless steel grade 304 does not exhibit sigma phase formation, a condition more commonly found in stainless steels with higher chromium content.
Stainless steel 304 has good machinability and can be cut, welded, and shaped using various techniques. There are certain factors that have to be taken into consideration when machining stainless steel 304, which include the use of lubricants and the quality of the cutting tools. It is essential that cutting tools be sharp and clean and have the ability to make cuts that are light and deep to prevent hardening. The low thermal conductivity of stainless steel 304 results in heat concentration at the edge of a cut, which requires the use of a coolant or lubricant. In most cases, chip breakers are used to remove swarf at the location of a cut.
| Mechanical Properties of Stainless Steel Grade 304 | ||
|---|---|---|
| Mechanical Properties | Metric | English |
| Tensile Strength | 505 MPa | 73200 psi |
| Tensile Yield Strength | 215 MPa | 31200 psi |
| Rockwell Hardness B | 70 to 92 | 70 to 92 |
| Elasticity | 193 to 200 GPa | 28000 to 29000 ksi |
| Charpy V Notch Test | 325 J | 240 ft-lb |
Stainless steel 304's robustness allows for effortless sanitation and cleaning, making it perfect for food processing and kitchen applications. Its prevalent use in the food industry stems from its ability to resist rust and corrosion, thanks to its elevated nickel levels, which contribute to a sleek and non-porous surface.
The method of manufacturing stainless steel significantly influences its appearance, performance, and characteristics. Despite being a distinctive material, stainless steel is still an alloy and is produced through standard steel processing techniques, including the formation of a molten alloy.
The raw materials for the forming of stainless steel are melted together in an electric arc furnace where carbon electrodes make contact with the raw materials. As electric current passes through the electrodes, the temperature in the furnace increases to the point that the raw materials melt into a molten mass. The ratios of the various elements and chemicals placed in the electric arc furnace are carefully measured to produce the desired type of stainless steel.
In the production of stainless steel 304, the carbon content is regulated during the decarburization stage, which can take place in either a vacuum oxygen decarburization (VOD) system or an argon oxygen decarburization (AOD) system. This process removes carbon to achieve the specified concentration.
During the refinement process, the molten stainless steel is stirred to ensure an even distribution of its components. This technique helps maintain consistent quality throughout the material and adjusts the composition to meet specific metal requirements. Temperature and chemical balance are meticulously controlled to fulfill the grade specifications.
After the molten metal is thoroughly mixed and homogenized, it is transferred to be molded into various forms for delivery to stainless steel product manufacturers. Different techniques are employed to shape the molten stainless steel into blooms, billets, slabs, rods, or tubes. The specific forming and shaping processes depend on the metal's grade, intended final product, and its intended use.
The annealing temperature for grade 304 is between 1100°C and 1150°C (2012°F and 2102°F). The annealing process adds corrosion resistance and is critical for the machinability and formability of the metal. It relieves stress created during manufacturing and is easy to perform on stainless steel grades 304.
The two methods for pickling grade 304 include the use of different percentages of citric and nitric acids. With citric acid, the stainless steel is submerged in a 10% citric acid solution for 30 minutes at a temperature of 65.6°C (150°F). With the nitric acid method, the metal is submerged in a 20% nitric acid solution for 30 minutes at temperatures that vary between 49°C up to 60°C (120°F up to 140°F).
Bright annealed finishes are produced by heat treating the surface of the stainless steel. It is a precision process that is closely monitored and controlled. The goal of the process is to enhance the corrosion resistance of stainless steel products.
Stainless steel grade 304 is the most commonly used type and appears in a wide range of applications, from home kitchens to industrial and construction settings. Its robustness, durability, and resistance to corrosion and heat make it an excellent choice for manufacturing a variety of products.
In residential settings, stainless steel 304 is commonly used in appliances such as refrigerators, dishwashers, cooktops, and ovens. Beyond these visible applications, it is also present in faucets, sinks, garbage disposals, and plumbing. Its non-porous nature makes it resistant to bacterial growth and prevents germs from accumulating. Stainless steel 304 is often used as a stylish element in various home features, adding both functionality and visual appeal.
Stainless steel 304's durability and strength make it a top choice for construction applications. It is commonly used in building facades, guardrails, and structural beams. Architects often incorporate stainless steel 304 as a decorative element to enhance building features or manage sunlight. With its potential for a polished, glossy finish, it offers an attractive appearance that requires minimal maintenance or painting.
Stainless steel 304 is extensively used in the food processing industry, adhering to stringent Food and Drug Administration (FDA) regulations. Its applications span a wide range of processes, from milk production to vegetable washing. Often termed "food grade stainless steel," it is valued for its versatility and reliability in various stages of food production and handling.
Stainless steel 304 is commonly used in manufacturing nuts, bolts, and screws due to its flexibility and resistance to harsh conditions. Bolts made from this grade offer excellent heat resistance, are malleable, and are suitable for welding. They are ideal for environments exposed to various chemicals. The primary reasons for using stainless steel 304 in these applications are its outstanding corrosion resistance, protection against crevice corrosion, and resistance to stress corrosion cracking.
Stainless steel 304 is utilized in the automotive industry, particularly in exhaust systems, due to its ability to withstand high temperatures. Its durability makes it well-suited for automotive components, capable of enduring frequent use and harsh conditions. In the railroad sector, it is employed in the undercarriages of rail cars because of its resistance to wear and lightweight properties. Using stainless steel 304 helps ensure a smoother ride and reduces rail wear.
Source 21 specializes in producing both austenitic and martensitic stainless steel in coil and strip forms. Their services encompass length cutting, polishing, masking, and temper rolling of the final products. The company provides various shipping options, including boxing, crating, skidding, and export packaging, to accommodate their global clientele. Source 21 supplies stainless steel to manufacturers in diverse sectors such as food service, aviation, electrical equipment, and sheet metal fabrication.
Cleveland-Cliffs stands as a major producer of flat-rolled stainless steel in North America, with iron ore mining operations in Michigan and Minnesota. The company supplies stainless steel to the automotive sector and is committed to steel recycling and sustainability practices. Cleveland-Cliffs manufactures all five stainless steel types, including grades 304 and 304L, serving both the automotive and various other manufacturing industries.
Allegheny Technologies produces a diverse array of metals and specialty metals, including multiple grades of stainless steel. The company provides a comprehensive selection of stainless steels, including specialized types such as superaustenitic and superferritic stainless steels. ATI uses hot rolling to manufacture stainless steel in various sizes and thicknesses. The company is known for its ability to fulfill orders promptly with short lead times.
Acerinox, a Spanish company, produces a wide variety of stainless steel products for sectors such as transportation, industrial equipment, food processing, and environmental technology. Their product line includes stainless steel plates, hot and cold coils, hot and cold sheets, strips, and discs. With facilities across the globe, Acerinox serves a diverse range of industries and clients with its extensive stainless steel offerings.
Aperam, headquartered in Luxembourg, provides a full range of stainless steel products to 40 countries, with a total production capacity of 2.5 million tons across five global facilities. Their offerings include stainless steel coils, sheets, tubes, discs, flat bars, strips, and heavy plates. The company operates a service center in Sterling Heights, Michigan, specializing in flat stainless steel products. Aperam is committed to innovative, high-performance solutions for technological and manufacturing challenges, with all their stainless steel products being recyclable and produced with a notably low CO2 footprint.
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