Stainless steel, a type of steel with a chromium content of at least 10.5%, is utilized in various industries, including traditional and marine constructions and household appliances. The demand for stainless steel is increasing at a 5% annual rate, with global production reaching over 52 million tons in 2019.
Despite its higher upfront costs, its excellent properties lead to increased service life and reduced cycle costs, making it a cost-effective choice for various applications. Well, in this reading, we’ll explore what stainless steel is, its applications, mechanical and chemical properties, composition, types, grades, and alloying elements. We’ll also discuss how stainless is made, as well as its benefits.
Let’s get started!
What is Stainless Steel?
Stainless steel, an alloy of iron, chromium, carbon, nickel, and manganese, is known for its corrosion resistance and strength. Its high content of chromium, at least 10.5%, creates a thin, transparent chromium oxide layer on the steel surface, protecting the underlying metal from corrosion.
This corrosion resistance is achieved when the chromium reacts with oxygen in the air, forming a rustproof layer. Stainless steel is a popular choice due to its ease of maintenance, long service life, and resistance to various corrosion types. Stainless steel is relatively inexpensive compared to other corrosion-resistant materials like titanium or nickel-based alloys.
The carbon percentage in stainless steel can range from 0.03% to 1.2%. Stainless steel’s high chromium content enhances its corrosion resistance and strength, allowing it to maintain a spotless appearance for extended periods under normal working conditions.
Applications of Stainless Steel
Stainless steel is a versatile material with numerous uses, including construction, kitchen appliances, medical equipment, food processing, transportation, and decorative applications.
Its durability and ease of maintenance make it a popular choice for kitchen appliances and cookware. Stainless steel’s corrosion resistance and sterilization ability make it a popular choice in the medical industry.
Its non-reactive properties prevent food contamination, making it a safe choice for food processing. Moreover, its strength and durability make it a popular choice for chemical processing equipment. Lastly, stainless steel is used for decorative purposes in public buildings, sculptures, and artwork.
Stainless steel, a versatile material with corrosion resistance, has gained popularity in various industries, including wristwatches and the chemical industry. Its resistance to corrosion has made it a preferred choice for various applications, from making small parts for wristwatches to large panels covering entire buildings. Its resistance to corrosion ensures the longevity and durability of the steel used in various applications.
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Mechanical Properties of Stainless Steel
Stainless steel mechanical properties include yield strength, strength at high temperatures, tensile strength, and cryogenic resistance. High yield strength is compared to carbon steels, while high temperatures provide better fire resistance and stiffness retention. Tensile strength is superior to materials like aluminium, brass, and mild steel, with precipitation hardening and martensitic grades having the highest tensile strength.
Cryogenic resistance is achieved by some stainless steel grades, such as austenitic steels, which display exceptional toughness and increased tensile strength at sub-zero temperatures. However, ferrite, martensitic, and precipitation hardening grades are less effective at cryogenic temperatures.
Ductility varies among different grades, with some having high ductility, allowing for strenuous deep drawing processes. Higher work hardening rate refers to a metal’s ability to increase strength through cold working processes, allowing the same grade to be used in multiple applications by varying its strength.
Electric conductivity and magnetism are also important aspects of stainless steel. Although it conducts electricity, it is extremely low, making it suitable for industries with high hygiene standards or electrical appliances exposed to corrosive or humid environments. Austenitic stainless steels are non-magnetic, but cold working can induce magnetic properties in some grades. All other types display magnetic properties.
Chemical Properties
Stainless steel is a unique material due to its chemical properties, including high oxidation resistance, biological inertness, and resistance to acids, bases, and organic materials. The high oxidation resistance is due to the presence of chromium, with some grades having up to 26% chromium content.
This resistance prevents corrosion deterioration and makes stainless steel an ideal material for medical equipment, cutlery products, and kitchen appliances.
Stainless steel is also resistant to a wide range of compounds, with different grades showing varying resistance to acids and bases. Its non-reactivity with basic and organic compounds makes it suitable for chemical industries for storage, handling, and other processes. Additionally, stainless steel can resist moisture, salt, sulphur, carbon dioxide, and chloride compounds, allowing it to survive in harsh environments for a longer period than most other metals.
Other Functional Properties
Stainless steel has important properties beyond mechanical and chemical applications. Its recycling capabilities reduce environmental strain by requiring fewer raw materials and reducing waste. Its non-biodegradable nature prevents contamination of resources. Stainless steel is easy to work with, allowing designers to create complex shapes and products without special equipment.
Its non-toxic properties make it easy to clean, extending the service life of products. Stainless steel products have high luster, making them ideal for exposed surfaces, and come in various finishes, including bright, matt, brushed, engraved, embossed, and tinted. These properties make the initial purchase worth it in the long run, making the initial investment worthwhile.
Grades of Stainless Steel
The market today features hundreds of grades of stainless steel, each with unique properties. The American Iron and Steel Institute (AISI) system, which uses three-digit numbers starting with 2, 3, or 4, is still used in the industry to identify these grades.
Stainless steel grades are used to describe properties like toughness, magnetism, corrosion resistance, and alloy composition. Older grades often have a three-digit identifier established by the Society of Automotive Engineers (SAE), while North American grades use a six-character identifier set by the American Society for Testing and Materials (ASTM).
These grades often share the first three numbers with their AISI equivalent. Other grading systems include British Standards (BS), International Organization for Standardization (ISO), Japanese Industrial Standards (JIS), European Standard (EN), German Standard (DIN), and Chinese Standard (GB). Obtaining information about an alloy’s unique composition from its grades can be challenging due to the vast number of standards and grading systems.
The SAE standard and BS EN both have specific requirements for the composition of stainless steel. The SAE standard requires 304 stainless steel to contain 18% to 20% Chromium, 8% to 10.5% Nickel, 0.08% Carbon, 2% Manganese, 0.75% Silicon, 0.045% Phosphorus, 0.03% Sulfur, and 0.1% Nitrogen.
The composition of these alloys impacts the performance of the steel. To determine the appropriate grade and composition requirements, refer to the lists provided for SAE and UNS Chemical Compositions & Mechanical Properties, BS EN Stainless Steel Compositions, and MatWeb Metall Alloy UNS Number Search.
The 200 Series
The 200 series of austenitic grades, containing manganese, is utilized in various equipment such as cuttinglery, washing machines, food & drinks, automotive industry, and indoors, ensuring durability and longevity.
The 300 Series
The 300 series of austenitic stainless steels, containing carbon, nickel, and molybdenum, is commonly used in the food and beverage industry, automotive, medical instruments, and jewelry. These stainless steels, known as AISI 304 and 316, offer improved corrosion resistance and ductility. The addition of molybdenum enhances ductility, making them ideal for applications in the food & beverage, automotive, and critical environments.
400 Series and SAE Grades
The 400 series of stainless steel, consisting of ferritic and martensitic alloys, is suitable for heat treating and offering high strength and wear resistance. Its applications include agricultural equipment, motor shafts, and gas turbine parts.
The SAE system uses a 1-letter + 5-digit UNS number code to designate stainless steel, with AISI grade 304 having S30400 as its designation. Newly developed exclusive grades may not have an SAE code.
Types of Stainless Steel
The five common types of stainless steel are austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, duplex stainless steel, and precipitation-hardened stainless steel.
Austenitic Stainless Steel
Austenitic stainless steel, with a face-centered cubic crystal structure, is a non-magnetic material with high levels of chromium and nickel. Its structure provides good ductility and resistance to stress corrosion cracking, making it suitable for various applications such as kitchen appliances, medical instruments, and automotive parts.
Ferritic Stainless Steel
Ferritic stainless steel, with a body-centered cubic crystal structure, is resistant to stress corrosion cracking and high temperatures. Its lower chromium content and magnetic properties make it suitable for high-temperature applications like car exhaust systems and boilers and heat exchangers. Despite its lower resistance to corrosion, ferritic stainless steel remains a strong and durable choice.
Martensitic Stainless Steel
Martensitic stainless steel, with its higher carbon content and difficult heat treatment, offers excellent wear resistance and magnetic properties. It is commonly used in cutlery and other applications requiring a hard, durable material, such as valve components and high-strength springs. However, it has poor corrosion resistance and is not suitable for use in acidic or humid environments.
Duplex Stainless steel
Duplex stainless steel, a blend of ferritic and austenitic stainless steel, offers excellent corrosion resistance and strength. Its two-phase microstructure combines ferrite and austenite, making it a popular choice in the oil and gas industry, particularly in chemical processing, exploration, and marine environments.
Precipitation-Hardened Stainless Steel
Precipitation-hardening stainless steel, a high strength material, is widely used in aerospace and military applications due to its corrosion resistance and high strength. This process, which involves aging the material at high temperatures, is high in chromium and other alloying elements, making it suitable for highly stressed environments.
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Composition and Alloying Elements
Stainless steel, a type of steel with properties derived from alloying elements like chromium, is known for its strength, ductility, and corrosion-resistant properties. This type of steel, with its composition of at least 10.5% chromium, is a popular choice for its applications. The alloying elements of stainless steel are chromium, nickel, copper, silicon, nitrogen, molybdenum, and manganese.
The properties of stainless steel can vary significantly depending on the alloying element added. Chromium is the defining alloying element, providing the steel’s ‘stainless’ property. It blocks oxygen diffusion and maintains a strong bond with steel molecules, preventing corrosion. A minimum of 10.5 per cent is required for a steel to be ‘stainless’, but more is common for increased corrosion resistance. Chromium also acts as a ferrite stabiliser, causing the formation of the ferrite microstructure in the alloy.
Nickel is added to improve corrosion resistance and austenite stability, resulting in austenite formation. Adding 8-9% of nickel results in a fully austenitic structure with excellent welding properties. Copper also acts as an austenite stabiliser, improving corrosion resistance and work-hardening properties.
Silicon enhances stainless steel’s resistance to highly concentrated nitric and sulphuric acids and oxidation. Nitrogen is an austenite stabiliser, improving strength and localized corrosion resistance. Molybdenum and tungsten enhance general and localised corrosion resistance properties, while manganese improves strength, toughness, and hardenability.
In conclusion, the properties of stainless steel depend on the alloying element added, such as chromium, nitrogen, chromium, nickel, copper, silica, nitrogen, molybdenum, and manganese. By selecting the most suitable cost-effective option, the most suitable alloying element can be achieved.
How is Stainless Steel Made?
The manufacturing process of stainless steel involves melting raw materials like iron ore, chromium, silicon, nickel, and molybdenum in an electric arc furnace to create a molten metal mixture. This mixture is reduced to produce low-carbon steel by adding a strong reducing agent like aluminum, which reacts with oxygen to form aluminum oxide.
The molten metal mixture is then “tuned” to the desired composition by adding other elements, such as manganese, nitrogen, and other metals, depending on the desired properties of the final stainless steel product.
The molten stainless steel is then poured into molds or cast into desired shapes, which can be accelerated by spraying the hot stainless steel with water or another coolant. Hot rolling is a process where molten steel is passed through a series of rollers at high temperatures to reduce its thickness and form sheets or bars.
Cold rolling is used for thinner and more precise steel products, such as sheet metal for the automotive industry or foil for packaging. Annealing and descaling or pickling are also processes used to remove impurities or scale from the steel. Cutting involves shaping the steel into the desired size and shape, and finishing is the final step, which involves giving the steel a final surface treatment to improve its appearance and performance.
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Benefits of Stainless Steel
Stainless steel has experienced phenomenal success in various industries for over 70 years, with more applications being discovered with every passing year. With increased demand, production has increased, making it more affordable and available in standard and non-standard sizes.
Stainless steel offers a wide range of finishes, including polished, patterned, and colored surfaces. Additionally, it is 100% recyclable, with half of all production coming from scrap metal, making it a relatively eco-friendly material.