Stainless Steel 304
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...
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This article takes an in-depth look at Stainless Steel Fabrication
You will learn:
Stainless steel fabrication is a set of manufacturing processes that shapes, stresses, bends, cuts, and deforms the metal to produce a group of resilient stainless steel products. The purpose of the various stainless steel fabrication processes is to take advantage of the many positive characteristics and properties of stainless steel to enhance the endurance, appearance, strength, and longevity of a product.
Although steel has been produced for thousands of years due to its strength, durability, and structural soundness, stainless steel is a relatively new steel alloy that was first developed a little over a hundred years ago as a method for preventing the erosion of gun barrels. After its first discovery, improvements to its structure and use rapidly followed as manufacturers found different methods for producing stainless steel.
The many ways to produce stainless steel have led to the development of an array of stainless steel alloys, each of which has a unique structure and use. Depending on the combinations of base alloys, stainless steel can be extremely hard and exceptionally resistant to corrosion or soft and pliable. These variations have led to the three classifications of stainless steel, which are austenitic, ferritic, and martensitic, and five families, which are austenitic, ferritic, martensitic, duplex, and precipitation hardening.
The families and classifications of stainless steel has led to a complex array of stainless steels that have been invented to serve different purposes. Unfortunately, this complicated organization of stainless steel can be confusing and daunting to people unfamiliar with steel and its alloys.
To complicate the identification of stainless steel further, each family has a variety of grades, which are identified by a number that is an indication of the quality and properties of the metal. In the United States, there are two grading systems, which are the Society of Automotive Engineers (SAE) system and the American Society for Testing and Materials (ASTM). The SAE system uses a three digit identifier while the ASTM system has a six digit identifier that categorizes 200 different stainless steel alloys.
In addition to the two grading systems in the United States, the International Organization of Standardization (ISO), Japan, Europe, Germany, Great Britain, Sweden, and China each have their grading systems. The number of standards, grading systems, classifications, and families has made it difficult to get concise information regarding the composition of the forms of stainless steel alloys.
Stainless steel grade 304 SAE is graded as follows under the various systems:
EN numbers are a European standard that was created by the British and are used to identify specifications, grades, and chemical components of metals, mainly types of steel. It was developed during World War II to make it easier to identify the standards for products. Deutsches Institut für Normung is German for the German Institute for Standardization.
Stainless steel is one of the most popular metals used for the fabrication of products and is found in common everyday items and industrial applications. Although stainless steel has a highly appealing appearance, its strength and corrosion resistance are two of the reasons for its wide use.
All grades of stainless steel can be fabricated to some degree with austenitic grades being processed the most. The fabrication of stainless steel includes all of the traditional forming methods that are used to shape and form various metals into components, tools, parts, and products. The choice of the method of fabrication depends on the grade of stainless steel and its alloys.
Each grade of stainless steel has different characteristics and properties that have been engineered to comply with the specific needs of an application. This aspect of stainless steel may seem to make selecting a grade for fabrication daunting and difficult, which is alleviated by the different grading scales that provide descriptions regarding the attributes of each grade.
Stainless steel can be fabricated using any of the traditional forming and shaping methods. Although stainless steel has a great deal of strength and a high hardening rate, it is very ductile, which enables it to be cold formed. Stainless steel fabricators adapt their methods of fabrication to the grade of stainless steel being formed. The ingredients in each grade vary in how they are combined, which changes the properties of the metal.
The use of work hardening depends on the grade of stainless steel that can be cold formed with austenitic grades being able to be work hardened rapidly while 400 series stainless steels have a rate of hardening that is a little higher than carbon steel. Austenitic stainless steels have the best properties of hardening processes. Martensitic stainless steel can be work hardened using thermal treatments such as tempering and quenching. With ferritic stainless steels, cold working is acceptable but is a very slow process, which makes it difficult to reach high strength requirements.
Stainless steels with the highest rate of work hardening normally have a high magnetic permeability. Cold working increases the tensile properties of various grades, which can reach 2000 MPa. As the size of stainless steel workpieces increases, cold working becomes difficult and impractical.
All grades of stainless steel can be welded with each grade having different weldability characteristics. As with many other fabricating methods, austenitic stainless steel grades have the best weldability. Martensitic stainless steel has a high carbon content, which means that great care has to be used when welding it. Ferritic grades have exceptional weldability when performed properly.
The deep drawing process is completed at room temperature and is a cold working process but may be heat treated after being processed to improve the metal’s strength. The two types of stainless steels that are fabricated by deep drawing are series 400 of the ferritic group and series 300 of the austenitic group. As with other fabrication processes, series 300 is best suited for deep drawing.
Since stainless steel work hardens faster than carbon steel, it requires twice the amount of pressure to be stretched and formed. The pressure that is used for the process is custom controlled to avoid breakage and galling of the metal. The chromium oxide layer of stainless steel causes increased friction during deep drawing, which necessitates the use of tools that are coated and lubricated to minimize wear and tear.
The speed of the deep drawing process must account for high friction, high pressure, and the depth of the draw. Incorrect calibration of the speed can lead to breakage and wrinkles. The best practice for the process is to do it slowly, especially as the severity of the draw increases.
Punching involves placing a sheet of stainless steel between a punch and die that are made of hardened steel or tungsten carbide, which are exceptionally hard metals capable of making precise punches multiple times with little wear. The punch press moves down on the sheet of stainless steel and shears through to make a hole of the exact shape and size for a design. Aside from creating holes in stainless steel, punching may also be used to emboss patterns and dimples that do not completely puncture the workpiece.
It is very common to find computer numerical control (CNC) machining used for punching and embossing processes due to its exceptional accuracy and efficiency. The versatility of CNC makes it possible to perform multiple punches in a single cycle while performing a wide range of fabrication processes.
Cutting stainless steel involves shearing, sawing, and plasma or laser cutting. The same methods are used to cut stainless steel as are used to cut carbon steel with the one adjustment being the amount of force that is necessary. Ferritic stainless steel cuts very much like carbon steel since it has a similar level of strength. With austenitic stainless steel, the clearance of the punch should be kept low to minimize burrs.
The biggest issue with cutting stainless steel is the wear that the cutting tools have to endure as well as chipping since the cutting process has an impact on the quality of the edge of the tool. Burrs can be kept at a minimum by adjusting the clearance, cutting force, and cutting speed. With thermal cutting, shielding gas and energy input effects the number of burrs.
Stainless steel bending is used to create different shapes based on the angle and radius of the bend. The bending processes include V-bending, roll bending, U-bending, edge bending, and wipe bending.
Surface treatments take many forms for stainless steel due its toughness and resilience. Although surface treatments are designed to protect and enhance the metal, they are also decorative to improve the appearance of the metal. Cold rolling and pickling produce a dull durable finish that is appropriate for architectural applications. The processes produce smooth clear semi-reflective finishes.
The various methods for fabricating stainless steel described above are a sampling of the many ones manufacturers use to create different stainless steel products. When deciding on a stainless steel fabricating method, it is advisable to contact an experienced fabricator that can provide a selection of methods as well as custom and proprietary ones.
Every grade, series, and category of stainless steel has properties and characteristics that meet the specific requirements of an application. The many varieties of stainless steels are one of the reasons that it is so widely used. In many ways, when facing the need to select a stainless steel, the process may seem to be complex and overwhelming. Stainless steel producers around the world have developed classification systems that help users to select the correct stainless steel for products and parts.
Different types and grades of stainless steel have varying uses. When choosing one for a project, careful consideration has to be given to the type of product, method of forming, and equipment to be used.
Deciding on the type of stainless steel for a project requires an understanding of the different grades, which are divided into four families – austenitic, ferritic, martensitic, and duplex. The largest of the four groups is austenitic stainless steel, which is a combination of steel, nickel or manganese, and nitrogen.
Ferritic stainless steel contains carbon steel and chromium while martensitic stainless steels come in four different combinations of iron, chromium, and carbon. How the materials are combined, and depending on the addition of other alloys, determines the type of martensitic stainless steel. Duplex stainless steel is a combination of austenite and ferrite in various percentages and ratios.
A special form of stainless steel is precipitation hardening (PH) stainless steel, which refers to a heat treatment applied to stainless steel to increase its yield strength. The result of the process is stainless steel with high temperature strength. Precipitation hardening involves heating the metal to a very high temperature and rapidly cooling it and reheating it again to an intermediate temperature.
Grades of stainless steel are identified by a numbering system, which varies between Great Britain, the International Organization of Standardization (ISO), Japan, Europe, the United States, Germany, and China. Regardless of the system, all organizations base their system on the properties, characteristics, and standards for stainless steel.
The five general categories of stainless steel are separated into grades that are differentiated by the types of alloys that are combined to produce them. Of the five categories, the most used are ferritic, martensitic, and austenitic with austenitic being more prevalent than all of the other categories.
Ferritic stainless steel has over 12% chromium content with low carbon content, which makes it non-hardenable. It has high corrosion resistance and differs from other stainless steels in its molecular grain structure and chemical makeup. Ferritic stainless steel is magnetic with a body centered cubic (BCC) lattice structure. In addition to its chromium and carbon content, ferritic stainless steel has small quantities of manganese, molybdenum, niobium, and titanium.
The series number for ferritic stainless steel is type 400, a series that is divided into five groups.
Ferritic stainless steel is not suited for thermal hardening methods but can be shaped by cold working. Its properties are magnetism, ductility, and corrosion resistance with the most commonly used grade being type 430 that has a chromium content of 16 percent. Ferritic stainless steels are less expensive than austenitic and duplex grades. It is mainly used in the manufacture of automobile parts, industrial machinery, and kitchen utensils.
Austenitic stainless steel is a commonly used stainless steel that can be welded, formed, shaped, and reconfigured for multiple purposes. It is a face centered cubic (FCC) iron or steel alloy with 10.5% chromium, 5% to 12% nickel, and small percentages of nitrogen and carbon. Austenitic stainless steel can be cold worked to improve its hardness, strength, and stress resistance. It can be heat treated to be shaped, after which it returns to its original strength when cooled. Austenitic stainless steel is classified in the 200 and 300 series of stainless steel.
Series 300 austenitic stainless steel includes grade 304, which has an 18 percent chromium and 8 percent nickel content while grade 316 has a 16 percent chromium, 10 percent nickel, and 2 percent molybdenum content. Most grades of austenitic stainless steel can be hardened by cold working but not by heat treatment. Common characteristics of austenitic stainless steel include:
The subgroups of austenitic stainless steel are series 200 and series 300 with series 300 having nickel as the most prevalent alloy. In series 200, manganese and nitrogen are the leading elements with nickel being present. Many of the series 300 austenitic stainless steels are used for a wide range of products while series 200 has had limited use.
Top series 300 grades:
Martensitic stainless steel is a 400 series stainless steel with alloys of iron and chromium. Unlike ferritic stainless steel, martensitic stainless steel can be hardened by heat treatment and has good ductility, magnetism, and corrosion resistance. Type 410 is the most popular because of its ability to withstand stressful conditions.
The chromium content of martensitic stainless steel is between 11.5% and 18% with 1.2% carbon and some nickel. The high carbon content gives the metal a strong molecular structure while the small amount of nickel makes it less resistant to corrosion. Other alloying elements include manganese, molybdenum, and nickel.
Martensitic stainless steel is divided into low carbon and high carbon with low carbon martensitic stainless steel having 0.05% to 0.25% carbon while high carbon martensitic stainless steel has 0.61% to 1.5% carbon. The increase in the carbon content of martensitic stainless steel makes it stronger but brittle and unable to be welded or easily formed.
Grades of martensitic stainless steel:
Duplex stainless steel (DSS) alloys have high levels of chromium, between 18% to 28%, with a nickel content of 1.5% to 8%. The name of duplex stainless steel comes from its two-phase microstructure with 50% austenite and 50% ferrite, which gives the metal the characteristics of austenite and ferrite.
The subgroups of duplex stainless steel are lean, standard, super, and hyper duplex stainless steels. The various groups are tougher than ferritic stainless steel and have double the strength of austenitic stainless steel. They are less resistant to corrosion than ferritic stainless steel with better resistance to corrosion than the commonly used austenitic stainless steels, such as 304 and 316. The grades of duplex stainless steel are numbered in the 2000s.
Subgroups of duplex stainless steel:
A unique variety of stainless steel is precipitation hardening stainless steel, which is three to four times stronger than austenitic stainless steel. It is made by combining copper, molybdenum, aluminum, and titanium and comes as low carbon martensitic, semi-austenitic, and austenitic. Each type varies according to its combination of alloys. The strength and corrosion resistance of precipitation hardening stainless steel are achieved by the precipitation hardening process. The martensitic version, 17/4PH, is the most commonly used and is composed of 17 percent chromium and four percent nickel.
Precipitation hardening stainless steel is a family of corrosion resistant alloys that are heated to achieve a tensile strength of 850 MPa to 1700 MPa and yield strengths of 520 MPa up to over 1500 MPa with some being three to four times greater than austenitic grades 304 and 316.
Groups of precipitation hardening stainless steel:
Modern appliances and industrial products are made of stainless steel due to its ability to adapt to extreme temperatures, corrosion and rust resistance, and superior strength. The main stipulations for stainless steel products are that they require limited care and can endure all forms of use. The popularity of stainless steel rests in its ability to meet stringent and exacting requirements, especially its ability to maintain its appearance in stressful conditions.
Stainless steel is non-porous, scratch resistant, and sturdy enough to make it ideal for sanitary and antiseptically clean conditions, such as large kitchens, where it is used as cutlery, and cookware. Counter tops, cookware, and utensils in the food processing industries are made from stainless steel to meet the exacting requirements of the Food and Drug Administration (FDA) and local health inspectors.
The oil and gas industries necessitate the use of corrosion resistant pipes due to the harsh conditions of offshore drilling sites. The corrosive conditions can cause pipes to burst and become dangerous. Sturdy stainless steel pipes ensure safe operations.
Desalination removes salt and minerals from water. The process converts salt water into fresh water for human use or irrigation. Desalination is used in submarines and ships and to bring fresh potable water to parts of the world where there is limited fresh water. Saltwater damages normal piping and makes them ineffective, which is the reason that stainless steel pipes are ideal for the desalination process.
Chemical plants deal with acids and oxidizers that are strong and toxic capable of damaging piping and materials. During cleaning, to ensure complete removal of toxic substances, storage tanks and piping undergo a bleach washing, which other types of metals cannot endure.
Duplex stainless steel became widely used in the chemical industry near the end of the 20th century. Its high strength and resistance has caused the rapid rise in its use due to innovations in the chemical industry that required a sturdier and more durable metal. Duplex stainless steel’s exceptional strength and resistance to chloride induced stress corrosion cracking have increased its demand.
Another area where duplex stainless steel is having increasing use is in bridges and other structures since it is twice as strong as ferritic stainless steel. Along with its increased strength, less material is required to increase the strength of a structure, which makes duplex stainless steels cost effective and weight efficient.
The stringent requirements of the medical field require that all instruments, equipment, and implements be antiseptically clean and sterile. Very few metals are capable of withstanding the cleaning processes demanded to meet the standards. Since stainless steel is able to withstand disinfecting treatments that health professionals demand, it is the first choice for the production of medical instruments and equipment.
The chromium of stainless steel increases the scratch and corrosion resistance of equipment, which are part of cleansing and sterilization processes. The nickel content provides a smooth even surface while the hardness of molybdenum helps medical tools maintain their strength and shape. Cutting tools for surgery are made from stainless steel due to their ability to retain their edge and cleanliness after sterilization.
The many positive properties of stainless steel make it ideal as a foundational material for structural load bearing. Sheets, plates, bars, and tubes of austenitic and duplex grades are hot rolled and welded as structural sections while other structural sections are cold formed. Additionally, bolts, screws, and other fasteners are made of stainless steel due to stainless steel’s strength and durability.
Stainless steels used in structures include austenitic series 304 and series 316 and duplex stainless steel grades 321, 322, and 323. Ferritic stainless steels are used for special structural applications in thicknesses of 3 mm with a nickel content of 2% to 4%.
Storage and cargo tanks on ships are made of duplex stainless steel while austenitic grades are used for navigation tankers. Piping, pulpits, hinges, propeller shafts, railings, accents, fasteners, and shackles on ships are all made of stainless steel. Although stainless steel is too expensive to be used for the construction of ship hulls, it is widely used in other areas of ships due to its resistance to corrosion and rust, especially in essential parts of a ship.
Power generation plants require materials that can endure high temperatures and extreme pressure. Traditional plants, powered by coal, have used stainless steel for many years due to its durability. With modern power plants, stainless steel is used for cooling, heat exchange, combustion, boiler superheaters, reheater tubing, and water panels. Stainless steel is in high demand for nuclear power plants for tanks, chimneys, and containment canisters.
One of the more popular uses for stainless steel is in the manufacture of storage tanks due to its resistance to corrosion and rust. Stainless steel tanks are used to hold corrosive substances such as chemicals, gases, and bulk materials, like food and water since the metal does not add flavor or odor to foods and water.
Agriculture, fire protection, fuel transport and storage, and other industries that require strong and sturdy storage tanks depend on stainless steel to safely store their products in durable and rugged tanks. This is especially true for the chemical industry that requires metal tanks that do not react to stored substances.
A popular use for stainless steel is the production of drawn wire that is produced by pulling or pushing stainless steel through a die under great stress and force. The diameter of the wire determines its tensile strength and hardness. Depending on the type of stainless steel, only grades of stainless steel that can be formed into wire are used. The most common wire grades are from series 300, which are 302, 304, and 316. The strength of these grades increase by cold drawing.
The many grades and families of stainless steel can make selecting the right grade for a project difficult. There are several factors to consider when designing a stainless steel product, such as the required strength and necessary work hardening as well as other characteristics that play a part in the fabrication process.
Since stainless steel is known for its strength and corrosion resistance, it is natural to assume that all stainless steels have these properties. Each grade and type of stainless steel have these characteristics, to varying degrees. The grade and type determine the percentage and amount of corrosion resistance, strength, and durability of a certain type. Before deciding to manufacture a product using stainless steel, it is best to study the properties of each type and grade.
The appearance and strength of stainless steel has made it a common part of construction projects and can be seen as exterior ornamentation such as railings, siding, fixtures, countertops, and backsplashes. Aside from its attractive appearance, the durability and strength of stainless steel ensures its longevity.
Since the 1930‘s, stainless steel has played a major part in the production of automobiles. Initially, it was used for exhaust systems, trim, and other non-structural purposes. With the advent of increased emissions standards, stainless steel has become an important component in the structure and design of automobiles.
Stainless steel has had a significant impact on the medical instrument industry due to its ability to be easily sterilized and its corrosion resistance. Aside from surgical instruments, implants, such as hip joints, are made from stainless steel. Pins and plates used to repair broken bones depend on stainless steel.
High carbon grades of stainless steel are popular and widely used in a variety of industries from the chemical processing of paper to the production of foods and beverages. Depending on the product and industry, the 400 series, with less corrosion resistance, may be used while the 300 series with good corrosion resistance may be preferred due to its lower cost and surface finishes.
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