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 austenite stainless steel that is the most widely used and versatile of the various grades of stainless steel. It is a part of the T300 series stainless steels with a chromium content of 18% and nickel content of 8%, percentages that give it the designation of 18/8 stainless steel. The most notable characteristic of stainless steel 304 is its ability to be deep drawn, which means that it can be formed by compressive forces that bend, cut, and twist the metal.
The forms of stainless steel 304 delivered to manufacturers include plates, tubes, bars, billets, blooms, and slabs. Each form aligns with the manufacturing method a producer of stainless steel products requires. Aside from its ability to be shaped to produce several types of products, stainless steel 304 is known for its resistance to heat and corrosion, excellent tensile strength, and appearance.
The versatility of stainless steel 304 can be seen in the products it is used to produce, which include cutlery, flatware, tubing, springs, nuts, bolts, and electrical enclosures. It is processed by hot or cold working using a variety of techniques and methods and is part of the reason for its popularity.
The differences between the various grades of stainless is see in their chemical makeup. Stainless steel 304 has a high chromium content at 18%, which is unlike stainless steel grade 316 that has a chromium content of 16%. Additionally, stainless steel 304 has a very high melting point of 1398.9o C up to 1451.4°C (2550°F up to 2650°F) and can endure exposure to extremely hostile and stressful environments.
Stainless steel 304 and stainless steel 316 are both austenitic stainless steel grades, which is a characteristic of all T300 series stainless steels. The factor that differentiates austenitic stainless steels from other forms of stainless steel is their nickel content,a factor that gives the T300 series a unique crystalline structure. The element that differentiates stainless steel 304 from stainless steel 316 is 316’s 2% to 3% molybdenum content, an element that enhances 316’s corrosion resistance.
The popularity and wide use of stainless steel is due to its resistance to corrosion and very low temperatures. These two factors, and stainless steel being the most hygienic metal, has led to its commercial and industrial use in applications that range from household appliances to essential components in industrial equipment. These properties are due to stainless steel's passive layer, which is a thin layer of chromium oxide on the surface of the metal.
The passive layer on stainless steel forms when the chromium content of stainless steel reacts with oxygen to form chromium oxide. Although this is common for stainless steel, it does not apply to all metals containing chromium since the reaction requires a material to have a chromium content of more than 11%.
It may be assumed that the passive layer on stainless steel is like rust or iron oxide. Such an assumption would be incorrect since rust is due to a reaction between iron and oxygen in the presence of water and does not provide any passivational protection. The passive layer on stainless steel is inert and does not react with other elements. It is a factor that gives stainless steel its hygienic and anti-corrosive properties.
Aside from its resistance to corrosion, there are several other factors that distinguish stainless steel from other metals. The group of metals defined as stainless steel are divided by their chemical makeup, physical properties, metallographic structure, and characteristics. All solid metals and alloys have grains that have a crystalline structure or lattice. With stainless steel, the crystalline structures are divided into ferrite, austenite, martensite, or combinations of two or more of these structures. The properties of stainless steel depend on which crystalline lattice is present in their structure. The crystalline structures of stainless steel are further divided into five families, which are the austenitic family, ferritic family, duplex family, martensitic family, and precipitation hardened family.
Although of the similarities between all stainless steels, small variations in their crystalline structure greatly affects their mechanical properties. The alloys in each family are familiar to each member but vary between the families such that each form of stainless steel is able to perform a unique and distinct function.
When stainless steel solidifies during the manufacturing process, its atoms are arranged into a crystal structure. If the crystals have the same structure, they are regarded as being in a phase with each phase associated with its crystalline structure name. The differentiation between the stainless steels is how their atoms are arranged. The austenite phase has a face centered cubic (fcc) crystal structure while ferrite phase stainless steel atoms have a body centered cubic (bcc) crystal structure. Martensite is a completely different phase with a body centered tetragonal (bct) crystal structure, unlike the cubic crystal structure of the other phases.
The austenitic family has a microstructure that is a non-magnetic face centered cubic phase with some austenitic stainless steels containing small quantities of the ferrite phase. The characteristics of the austenitic family include low yield strength, a high work hardening rate, high tensile strength, good ductility, and exceptional low temperature resistance.
A main feature of the austenitic stainless steel family is its ability to be fabricated into several different forms, configurations, and shapes, which makes it ideal for the manufacture of complex and intricate parts. The family has good weldability but cannot be hardened or strengthened by heat treatments. On the other hand, it can be strengthened by cold forming or work hardening.
The ferritic family consists of ferrite phase with small amounts of carbides and nitrides. Unlike the austenitic phase, the ferritic family has magnetic properties that are like carbon steel. The characteristics of the ferritic family are high strength and resistance to stress from chloride corrosion cracking. As with the austenitic family, the ferritic family cannot be hardened with heat treatment and has a limited toughness, which restricts its use to thin sheets and strips of stainless steel. Ferritic family products have a thickness of 4 mm (0.15 in) or less and are used for tubing and thin gauge applications.
Martensitic stainless steel can be strengthened by heat treatment. When heated to 1040°C (1904°F), it takes on an austenitic structure but transforms to a martensite structure when cooled to room temperature. To achieve the hardness and toughness of martensite stainless steel, it is tempered at 100°C up to 700°C (200°F up to 1300°F). Martensitic stainless steels have high strength, wear resistance, low toughness, and are difficult to weld.
The duplex family of stainless steel is made up of ferrite and austenite phases with a 50% to 55% austenite phase and 45% to 50% ferrite phase. The properties of the duplex phase include high strength, wear resistance, toughness, and resistance to chloride stress corrosion cracking. The exceptional strength of duplex grade stainless steel makes it ideal for pressure vessels, tanks, and structural components.
The strength of the precipitation hardened family is due to the stainless steel being subjected to a precipitation mechanism, which involves treating the metal with a solution at a high temperature that dissolves the solute atoms to form a single phase. Stainless steels of the precipitation hardened family contain a high percentage of chromium and molybdenum to make the metal corrosion resistant and capable of enduring high strength applications.
From the three broad categories of austenitic, ferritic, and martensitic, stainless steel is further divided into series and grades with each grade identifying the durability, quality, and heat resistance of the metal. Numbers that are placed after the grade indicate the chemical composition of the stainless steel with specific reference to its chromium and nickel content.
The SAE system, also known as the American Iron and Steel Institute (AISI) system, is a three-digit numbering system with the first number of each grade being 2, 3, 4, or 5. The first number indicates the grade of stainless steel with 2 and 3 standing for austenitic stainless steel and 4 for ferritic and martensitic. Stainless steel 500 series is a special type of stainless steel that is engineered for high temperature applications. Precipitation hardened stainless steel is identified as grade 600 series.
An ASTM identification number begins with a letter from A to G that indicates the type of material with A representing ferrous metals and B being nonferrous metals. The single letter is followed by one to four numbers, a dash, and the year the designation was issued. ASTM has issued over 12,000 standards for use by industry, which are related to a wide range of materials and products.
Stainless steel 304 belongs to the group of austenitic phase stainless steels that have a chromium content of 15% to 30% and nickel content of 2% to 20% as its major elements. As with all forms of steel, iron is the primary metal in stainless steel 304 and makes up most of its mass followed by chromium and nickel. The remaining alloys are at 2% or less but are the essential part of determining stainless steel 304’s properties.
| 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% |
The three types of stainless steel 304 are series 304, 304L, and 304H, which differ in regard to their carbon content with 304L having the lowest carbon content while 304H has the highest. The carbon content for stainless steel 304L is at 0.03% or lower while the carbon content of stainless steel 304H is between 0.04% and 0.1%. The higher carbon content of series 304H gives it greater resistance to heat and higher yield strength.
All stainless steels have an iron (Fe) content of at least 50%. In austenitic stainless steels, the percentage varies in accordance with the ductility and workability of the metal. As with all forms of steel, stainless steel is an iron based alloy that is adjusted and reconfigured by the addition of various alloys.
The use of carbon in stainless steel enhances its strength and hardness but lowers its resistance to corrosion. For this reason, the amount of carbon in austenitic stainless steel is kept low at less than 0.1%. The presence of carbon in austenitic stainless steel is strictly controlled since increased quantities of carbon have an effect on the chromium content and cause sensitization, which is the formation of chromium carbide. When that reaction occurs, the chromium content does not rise to the surface of the stainless to form the chromium oxide layer. As is found with stainless steel 304L, lowering the carbon content improves the ductility of stainless steel 304.
Chromium is an essential element in stainless steel 304 due its ability to form the passivation layer that keeps stainless steel 304 rust and corrosion free. The addition of chromium to steel transforms steel into stainless steel when the amount of chromium is over 10%. When chromium rises to the surface of the steel, it forms the passive non-reacting layer. With austenitic stainless steel, chromium is balanced with the other alloying elements to form 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.
As with a few of the other alloys, phosphorus is added to stainless steel 304 to increase its strength. The addition of phosphorus can be beneficial and detrimental. The positive factor with phosphorus is how it improves the strength of stainless steel 304. The downside of phosphorus is how it weakens the corrosion resistance of stainless steel 304 and increases its tendency to break during welding.
Sulfur is carefully added to stainless steel 304 to improve stainless steel 304’s machinability. As with all metals, sulfur is a residue from the production of stainless steel 304 and can cause the metal to be brittle and affect its weldability and high temperature resistance. Like phosphorus, sulfur decreases stainless steel 304’s resistance to corrosion, which is the reason that its presence is monitored and controlled.
Silicon is a deoxidizing agent and is found in alloys as a residue. In small quantities, it improves the strength of stainless steel 304 but, in high amounts at high temperatures, it can cause the forming of intermetallics. The addition of silicon improves corrosion resistance from the effects of sulfuric acid, oxidation resistance, and works as a ferrite stabilizer.
Nitrogen, as with nickel, is an austenite forming element that increases the stability of the austenite phase in stainless steel 304. With the addition of nitrogen, the yield strength of stainless steel 304 is significantly improved as well as its resistance to pitting from corrosion. Nitrogen with manganese assists in the formation of the austenite microstructure and is more powerful in the process than carbon, nickel, and manganese. An increase in the amount of nitrogen improves the strength of stainless steel and lowers its susceptibility to sensitization.
Over half the stainless steel produced in the world is stainless steel grade 304, which is manufactured in sheets, plates, bars, billets, blooms, slabs, and tubing. The three grades of stainless steel 304 are 304, 304L, and 304 H, which are differentiated by their carbon content. All of the other elements of the three grades are the same except for their carbon content.
The change in the carbon contents of 304, 304L, and 304H change the specific properties of the metals and make them adaptable to various functions and applications. Grade 304L has a carbon content of less than 0.03% while 304H has a carbon content of up to 10%. Grade 304 has a carbon content of 0.08%.
The grades of stainless steel 304 are produced by adjusting the carbon content of grade 304 without changing the percentages of the other elements. Changes to the carbon content of stainless steel increases its strength, hardness, and hardenability but makes it less resistant to the effects of corrosion. In addition to lowering the resistance to corrosion, high quantities of carbon can make stainless steel brittle and reduce its ability to be welded.
Grade 304 is the most commonly used austenitic stainless steel. It has a nickel content of 8% up to 10.5% and chromium content of 18% to 20% and is used in applications that require exceptional formability. Stainless steel grade 304 has high corrosion resistance, electrical conductivity, and thermal conductivity. Its wide use is due to its resistance to the effects of various environmental conditions and many forms of corrosive chemicals. Stainless steel grade 304 provides excellent performance at a reasonable cost and is used for commercial and industrial applications.
One of the most notable features of stainless steel grade 304 is its high tensile strength of 621 MPa (90 ksi). Additionally, it has oxidation resistance at temperatures over 870°C (1598°F) without showing any ill effects and has a melting point between 1399°C and 1454°C (2550°F and 2650°F).
The main characteristic of stainless steel grade 304L is its low carbon content at 0.03%, which produces a marked difference in the performance of grade 304L. Although grade 304 and 304L are very similar, there are important differences in their mechanical properties. The tensile strength of grade 304L is 586 MPa (85 ksi) with a yield strength of 0.2% of 241 MPa (35 ksi), which makes grade 304L structurally weaker than grade 304.
Regardless of its shortcomings, grade 304L is just as widely used as grade 304 due to its ability to eliminate carbide precipitation during welding, which makes it possible to use it in an as welded condition in severely corrosive environments. The use of grade 304L eliminates the need to anneal weld joints, saving time, money, and effort.
The high carbon content of stainless steel grade 304H gives it extra strength at temperatures over 427°C (800°F). Grade 304H has the same corrosion resistance as that of 304 and 304L since the factor in its chemistry that is adjusted is its carbon content. As beneficial as the high carbon content is in enhancing the strength of grade 304H, the drawback of the increase is 304H’s susceptibility to carbide precipitation in heat affected zones during welding. Grade 304H can easily be welded and fabricated.
Since grade 304H is ductile, it can be cold worked, which increases its strength and hardness. Grade 304H is formed by being hot worked at 750o C up to 1150°C (1652°F up to 2102°F) and annealed at 1038°C (1900°F). The process for machining of grade 304H is carefully controlled to avoid chipping and is done at slow speeds, with lubrication, sharp tools, and high-powered equipment.
The wide use of stainless steel grade 304 is due to its many positive properties that make it adaptable to different situations, conditions, and environments. Known for years as stainless steel 18/8 due the combination of chromium and nickel in its content, stainless steel 304 is an austenitic stainless steel that can be severely deep drawn, a property that has made it the first choice for the manufacture of sinks and saucepans.
All discussions of stainless steel begin with its corrosion resistance, which makes it capable of withstanding the effects of a wide variety of environments and conditions. The key to stainless steel 304’s corrosion resistance is its chromium content that forms a passive layer on the surface of the metal. Regardless of grade 304’s reputation of resistance to corrosion, conditions that contain chlorides can pit the metal and create crevices. Stress corrosion cracking can occur at certain temperatures.
Resistance to oxidation is an important characteristic of stainless steel 304, a factor that causes steel alloys to rust and become weak. Grade 304 has good oxidation resistance during intermittent service at 870°C (1598° F) and continuous service at 925°C (1697°F) with continuous service at 425°C up to 860°C (797°F up to 1580°F) not being recommended since it can be detrimental to stainless steel 304’s corrosion resistance.
Creep strength is related to heat resistance and is the ability of stainless steel to resist deformation when exposed to high temperatures and is a property that is used to define stainless steel 304. Low carbon versions of stainless steel 304 are susceptible to lower strength in high temperature environments while grade 304H is widely used in high temperature environments.
Some stainless steels suffer from the formation of a sigma phase when exposed to high temperatures, a condition that strips stainless steel of its resistance to corrosion. The formation of the sigma phase is dependent on time and temperature. It is a condition that is not found in stainless steel grade 304 but is prevalent in stainless steels with a high 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 |
The durability of stainless steel 304 makes it easy to sanitize and clean and ideal for use in food processing and kitchen equipment. The wide use of stainless steel 304 in food processing is due to its resistance to oxidation and corrosion, which is the result of its high nickel content, a characteristic that makes its surfaces smooth and impermeable.
The process used to manufacture stainless steel plays an important part in how it will look and perform and its properties. Although stainless steel is a unique and unusual metal, it is still a steel alloy and is produced using steel processing methods, which include the creation 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.
The amount of carbon content for stainless steel 304 is controlled during the decarburization process, which can occur in a vacuum of oxygen (VOD) or argon (AOD) system. During the process, carbon is removed to achieve the desired percentage.
During the tuning process, the molten stainless steel is stirred to evenly distribute the molten elements. This process has the dual effect of ensuring the uniform quality of the molten material and blending it to meet the specifications for the metal. Balancing and homogenization of temperature and chemistry take place to meet the requirements of the grade.
Once the molten metal is fully mixed and blended, it is sent on to be shaped in forms for shipping to stainless steel product manufacturers. Using a variety of methods, the molten stainless steel is shaped into blooms, billets, slabs, rods, or tubes. The steps used to form and shape the molten stainless steel vary according to the grade, final product, and function of the metal.
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.
As the most widely used form of stainless steel, stainless steel grade 304 can be found in every aspect and walk of life from residential kitchens to industrial and construction projects. Its strength, durability, and resistance to the effects of corrosion and heat make it the ideal choice for the manufacture of products.
In the home, most of the appliances are made of stainless steel 304 including the refrigerator, dishwasher, cooktops, and ovens. Aside from the visible presence of stainless steel 304, it is also found in faucets, sinks, garbage disposals, and piping. As a non-porous metal, stainless steel 304 is resistant to the effects of bacteria and does not provide a place for various germs to hide or grow. It can be found in several locations in homes as an attractive accent piece to offer a pleasant aesthetic appearance.
The durability and strength of stainless steel 304 makes it an ideal metal for construction projects. It is used for building facades, guard rails, and beams. Stainless steel 304 is used by architects as an accent piece to highlight an aspect of a building or to block sunlight. Since stainless steel 304 can have a shiny glossy finish, it provides a positive appearance that does not need painting or extra care.
One of the prevalent uses of stainless steel 304 is in the food processing industry, which has to comply with the strict regulations of the Food and Drug Administration (FDA). The use of stainless steel 304 in food processing covers a wide range of applications and uses from the production of milk to the cleaning of vegetables. It is referred to as the food grade stainless steel due to its many uses throughout food production and cultivation.
A common industrial use for stainless steel 304 is in the production of nuts, bolts and screws due to its malleability and resistance to the effects of hostile environments. Bolts produced from stainless steel 304 have a high heat carrying capacity, are pliable, and support welding. They are used in conditions where there is continual attack from various chemical substances. The essential reason for the use of stainless steel 304 for the production of bolts is their superior corrosion resistance, protection from crevice corrosion, and resistance to stress cracking.
Stainless steel 304 is used in the manufacture of automobiles and is found in exhaust systems due to its tolerance of high temperatures. The durability of stainless steel 304 is ideal for the manufacture of automobiles since it is capable of withstanding the constant use of a vehicle and extreme conditions. In railroads, it is used for the undercarriage of railroad cars due to its wear resistance and lightweight. The use of stainless steel 304 ensures a smooth ride and a reduction of wear on rails.
Source 21 is a leading manufacturer of austenitic and martensitic stainless steel in coil and strip form. The services of the company include cutting to length, polishing, masking, and temper rolling of the final product. Source 21 offers a wide range of shipping methods including boxing, crating, skid, and export packaging to meet the needs of their worldwide customer. The company supplies stainless steel to manufacturers of products for food services, aviation, electrical equipment, and sheet metal fabrication.
Cleveland-Cliffs is one of the largest producers of flat rolled stainless steel in North America with operations for mining iron ore in Michigan and Minnesota. The company provides stainless steel to the auto industry and is dedicated to steel recycling and sustainability. Cleveland-Cliffs produces all five types of stainless steel including grades 304 and 304L and provides steel for the auto industry and other manufacturing industries.
Allegheny Technologies is a producer of a wide range of metals and specialty metals including various grades of stainless steel. The company offers a complete line of stainless steels as well as specialty forms of stainless steel such as superaustenitic and superferritic stainless steels. ATI produces stainless steel using hot rolling in a wide range of product sizes and thicknesses. The company is capable of fulfilling customer orders quickly with short lead times.
Acerinox is a Spanish company that manufactures all forms of stainless steel for the transportation industry, industrial equipment manufacturing, food processing, and environmental technology industry. The company produces stainless steel plates, hot and cold coils, hot and cold sheets, strips, and discs. With locations around the world, Acerinox supplies its various forms of stainless steel to a wide range of industries and customers.
Aperam is located in Luxemburg and supplies all forms of stainless steel to 40 countries with a production capacity of 2.5 tons at five production facilities around the world. The company supplies stainless steel iron coils, sheets, tubes, discs, flat bars, strips, and heavy plates with a service center in Sterling Heights, Michigan that provides flat stainless steel. Aperam prides itself on its innovative and high performance solutions to technological and manufacturing problems. All of its stainless steel products are recyclable, which are produced with an exceptionally low CO2 footprint.
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