Stainless Steel

Stainless Steel: A Comprehensive Guide

Stainless steel, as an essential material, plays a significant role in modern life and production. This comprehensive guidance helps you learn more knowledge about  Stainless Steel Grades, Stainless steel applications, and suggestions for selecting the right materials.

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What is Stainless Steel?

Stainless Steel

Stainless steel is an alloy steel with iron as its base, known for its corrosion resistance and rust-prevention properties. It typically contains around 11% chromium, along with varying proportions of nickel, molybdenum, manganese, and other elements. Depending on its composition, structure, and properties, stainless steel can be classified into various types, with the most common being austenitic and martensitic stainless steel. Specific types of stainless steel are often designated by three-digit numbers, such as 304 stainless steel. Stainless steel can be rolled into sheets, plates, pipes, bars, and wires, making it widely utilized across various industries.

 

Types of Stainless Steel Materials

Stainless steel comes in five main types, classified by crystal structure: Austenitic, Ferritic, Martensitic, Precipitation Hardening, and Duplex Stainless Steel.

Austenitic Stainless Steel:

Austenitic Stainless Steel

Austenitic stainless steel refers to a type of steel with the same microstructure at all temperatures. This microstructure is achieved by alloying the steel with appropriate amounts of nitrogen, manganese, and nickel, maintaining an austenitic microstructure from low temperatures to the melting point. It does not harden at different temperatures.

Austenitic stainless steel mainly comprises the 300 series and the 200 series etc.

200 series

200 Series

 

The 200 series is a nickel-manganese-chromium alloy. It involves increasing the amount of nitrogen and manganese while reducing the amount of nickel. Due to the addition of nitrogen, their yield strength is nearly 50% higher than that of the 300 series stainless steel. Common examples include stainless steel 201 and stainless steel 202.

300 series

300 Series

 

The 300 series is a nickel-chromium alloy. Its austenitic structure is achieved solely through nickel alloying. The 300 series is the most widely used and largest group of stainless steels.

The most common grade is type 304, also known as 18/10 and 18/8 because of its composition of 8% to 10% nickel and 18% chromium, respectively.

The most famous austenitic stainless steel is type 316. The addition of 2% molybdenum provides superior acid resistance and resistance to localized corrosion caused by chloride ions.

304L or 316L are the grades with the least amount of carbon. With a carbon ratio below 0.03%, they are used to combat corrosion issues caused by welding.

 

Ferritic Stainless Steel:

Ferritic Stainless Steel

Ferritic stainless steel has a similar microstructure to carbon steel, containing 10.5% to 27% chromium and little to no nickel. Due to the addition of chromium, it cannot be hardened by heat treatment. Cold working cannot strengthen it as effectively as austenitic stainless steel and ferritic stainless steel is magnetic. Common grades include 430 (00Cr17), which contains 17% chromium and exhibits good formability, making it suitable for decorative and architectural applications, and 409/409cb, typically used in automotive exhaust systems.

 

Martensitic Stainless Steel:

Martensitic Stainless Steel

Martensitic stainless steel is a type of stainless steel that can be hardened through heat treatment, making it a class of stainless steel that can be hardened and is magnetic. Martensitic stainless steel offers high strength and corrosion resistance, making it suitable for manufacturing machine parts such as steam turbine blades (1Cr13), shafts, and rods for steam equipment (2Cr13), as well as parts working in corrosive media such as valves, bolts, etc. (4Cr13).

Steels with higher carbon content (such as 4Cr13 and 9Cr18) are suitable for manufacturing medical instruments, knives, measuring tools, springs, etc. They are similar to ferritic stainless steel. Other alloying elements can also be added to martensitic stainless steel to improve other properties:

1. Adding 0.07% S or Se improves machinability, for example, 1Cr13S or 4Cr13Se.

2. Adding approximately 1% Mo and 0.1% V can increase the wear resistance and corrosion resistance of 9Cr18 steel.

3.Adding approximately 1Mo-1W-0.2V can improve the thermal strength of 1Cr13 and 2Cr13 steel.

 

Precipitation Hardening Stainless Steel:

Precipitation Hardening Stainless Steel

Precipitation-hardening stainless steel achieves similar corrosion resistance to austenitic stainless steel. However, they can rapidly harden to higher strengths than other martensitic steel grades. Precipitation hardening stainless steel is divided into three types:

1. Martensitic 17-4 PH (AISI 630 EN 1.4542) contains approximately 17% Cr, 4% Ni, 4% Cu, and 0.3% Nb.

2. Semi-austenitic 17-7PH (AISI 631 EN 1.4568) contains approximately 17% chromium, 7.2% nickel, and 1.2% aluminum.

3. Austenitic A286 (ASTM 660 EN 1.4980) contains approximately 15% Cr, 25% Ni, 2.1% Ti, 1.2% Mo, 1.3% V, and 0.005% B.

 

Duplex Stainless Steel:

Duplex Stainless Steel

Duplex stainless steel features a composite microstructure of ferrite and austenite, ideally in a 50:50 mixture. However, commercial alloys may have a ratio of 40:60. Compared to austenitic stainless steel, duplex stainless steel exhibits higher strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion cracking. Duplex stainless steel also boasts excellent resistance to pitting corrosion and is a cost-effective alternative to nickel-based stainless steel. The yield strength of duplex stainless steel is twice that of austenitic stainless steel. Classified by their chemical composition, duplex stainless steel can be divided into four types: Cr18, Cr23 (without Mo), Cr22, and Cr25. Among them, Cr22 and Cr25 are the most commonly used, with Cr25 further divided into regular duplex and super duplex stainless steel. Industries commonly utilizing duplex stainless steel include refining, chemical processing, and paper manufacturing.

 

Advantages of Stainless Steel

Stainless steel offers numerous desirable qualities, with its primary advantage being its corrosion resistance due to the presence of chromium. In addition, stainless steel is also known for its high-temperature resistance, high tensile strength, ease of shaping and manufacturing, low maintenance requirements, aesthetic appeal, and environmental friendliness. These outstanding characteristics make stainless steel widely applicable in both industrial and domestic settings. 

Excellent Oxidation Resistance: Corrosion Resistance, Anti-Rust

Excellent Oxidation Resistance

The chromium present in stainless steel acts as an alloying element that provides its corrosion resistance properties. It can withstand the corrosion from most chemical mediums such as acids and alkalis. Additionally, stainless steel exhibits excellent stability in humid and corrosive environments, prolonging its lifespan. Therefore, stainless steel finds widespread applications in marine engineering due to its outstanding resistance to oxidation and rust.

Fireproof and High-Temperature Resistant

Fireproof and High Temperature Resistant

Special high-nickel and chromium alloy types can prevent scaling and maintain high durability at high temperatures. Stainless steel is widely used in hot boilers, main pipelines, heat exchangers, water heaters, valves, and superheaters, as well as aerospace and aircraft applications.

High Strength

High Strength

Stainless steel exhibits high strength, making it widely used in structural engineering. By controlling the alloy element content and heat treatment, the strength of stainless steel can be adjusted to meet various application requirements. For example, in the construction industry, stainless steel is used for supporting structures, bridge components, and exterior wall decorations of high-rise buildings, bearing crucial structural functions.

Hygienic Properties

Hygienic Properties

Another advantage of stainless steel is its cleanliness and hygiene. Its smooth surface makes it resistant to bacterial adhesion and dirt accumulation, making it easy to clean and disinfect, meeting hygiene standards. Therefore, stainless steel finds widespread applications in fields such as healthcare, food processing, and pharmaceuticals. For instance, stainless steel is commonly used in critical equipment such as surgical instruments, food processing equipment, and medical devices, ensuring hygiene safety, and reliable quality.

Durability

Stainless steel is durable. Even thin stainless steel does not warp under heavy weight, making it one of the most sustainable metals on the market. It can withstand weight, cold and hot temperatures, as well as extreme weather conditions.

Long-Term Value

Stainless steel offers long-term value as it is 100% recyclable. Over 50% of new stainless steel is made from old, redesigned stainless steel scrap, completing the entire lifecycle. When considering total costs, it’s best to evaluate production and material costs along with lifecycle expenses. When considering the costs over the entire lifecycle, stainless steel is often the most cost-effective material choice—offering cost-saving advantages with corrosion-free products expected to have a longer lifespan.

Aesthetic Appeal

Aesthetic Appeal

Stainless steel offers various surfaces, allowing for customization through polishing, sandblasting, coloring, and other treatments to meet diverse design needs, providing broader adaptability for your projects. With its sleek appearance and modern feel, stainless steel is commonly used in architectural decoration, furniture manufacturing, and art production, enhancing the visual appeal of any space or product.

 

Characteristics of Stainless Steel

Characteristics of Stainless Steel

The performance characteristics of stainless steel are mainly reflected in its mechanical properties and chemical properties.

Stainless steel’s mechanical properties

Yield strength 

Yield strength refers to the point at which stainless steel undergoes plastic deformation after experiencing a certain amount of tension and compression. It reflects the material’s load-bearing capacity between elastic and plastic deformation. The yield strength of stainless steel plates typically ranges between 205-210 MPa.

Tensile strength

Tensile strength refers to the maximum tensile force stainless steel can withstand during the stretching process, representing the maximum stress it can withstand under tension.

Stainless Steel Hardness

Stainless steel hardness varies depending on its material composition, processing methods, and type. It can be evaluated using Brinell, Vickers, and Rockwell scales, each employing different techniques with different-shaped indenters and applied forces. Hence, conversions between different scales are only approximate. Precipitation and martensitic hardening types can be hardened through heat treatment, while other types can be hardened through cold working.

Stainless steel’s ductility

Stainless steel’s ductility, often calculated as the percentage of elongation before fracture during a tensile test, is typically high in annealed austenitic stainless steel, with figures ranging around 60-70%.

Conductivity and magnetism

Conductivity and magnetism, Like all metals, stainless steel is conductive, but its conductivity is much lower compared to other steels. Austenitic stainless steel is non-magnetic; however, certain types may exhibit magnetic properties after cold working. All other types are currently magnetic.

 

Stainless Steel’s chemical properties

High oxidizability

Stainless steel’s chemical properties include high oxidizability, where its resistance to oxidation is primarily due to the chromium content, which can be as high as 26% in some types.

Biological inertness

Stainless steel lacks biological activity, making it the ideal choice for medical tools such as trauma screws, steel plates, and surgical instruments. This property also makes it perfect for kitchenware and tableware products.

Resistance to acids, alkalis, and organic materials

Stainless steel is resistant to acids, alkalis, and organic materials, as well as salt, moisture, sulfur, chlorides, and carbon dioxide. This resistance, compared to most other metals, contributes to its longer lifespan in many harsh environments.

 

Grades of Stainless Steel

Grades of Stainless Steel

Cr-Ni-Mn series (200 series)

Consists of nickel-manganese-chromium alloys,201, 202, etc.: With manganese instead of nickel, these grades have poorer corrosion resistance and are widely used domestically in China as inexpensive alternatives to the 300 series.

Cr-Ni series (300 series)

Chromium-Nickel Austenitic Stainless Steel: Cold working techniques can only harden this type of stainless steel. These stainless steel grades contain chromium (approximately 18% to 30%) and nickel (approximately 6% to 20%) as important additives in alloying.

301: Good ductility, used for formed products. Can also be rapidly hardened through machining. Good weldability. Wear resistance and fatigue strength superior to 304 stainless steel.

302: Corrosion resistance similar to 304, but with higher carbon content for better strength.

303: Easier to machine than 304 due to the addition of small amounts of sulfur and phosphorus.

304: Common type; the classic stainless steel known as 18/8 stainless steel. Used in products such as corrosion-resistant containers, cutlery, furniture, railings, and medical equipment. The standard composition is 18% chromium and 8% nickel. Stainless steel is non-magnetic and cannot change its metallographic structure through heat treatment methods. The GB standard grade is 06Cr19Ni10.

304 L: Similar characteristics to 304, but with lower carbon content for increased corrosion resistance and ease of heat treatment, though mechanical properties are poorer. Suitable for welded and products not easily heat-treated.

304 N: Similar characteristics to 304, but nitrogen-added for increased steel strength.

309: Better heat resistance than 304, withstanding temperatures up to 980°C.

309 S: High chromium and nickel content, hence excellent heat resistance and oxidation resistance. Used in products such as heat exchangers, boiler components, and jet engines.

310: Excellent high-temperature oxidation resistance, with a maximum usage temperature of 1200°C.

316: The second most widely used steel grade after 304, mainly used in the food industry, watchmaking, pharmaceuticals, and surgical instruments. The addition of molybdenum gives it a special corrosion-resistant structure. Used as “marine steel” due to its better resistance to chloride corrosion compared to 304. SS316 is commonly used in nuclear fuel recycling devices. 18/10 grade stainless steel typically also meets this application level.

316 L: Lower carbon content for increased corrosion resistance and ease of heat treatment. Used in products such as chemical processing equipment, nuclear power generators, and refrigerant storage.

321: Similar performance to 304 except for the reduced risk of material weld seam corrosion due to the addition of titanium.

347: Addition of stabilizing element niobium, suitable for welding aircraft parts and chemical equipment.

 

400 Series

Ferritic and martensitic stainless steels, The 400 series stainless steels contain 11% chromium and 1% manganese, higher than the 300 series. The 400 series can rust and corrode under certain conditions. Heat treatment can harden the 400 series steel. The high carbon content of 400 series stainless steel gives it a martensitic crystal structure, providing high strength and corrosion resistance. Martensitic stainless steels are not as corrosion-resistant as austenitic stainless steels.

408: Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.

409: The cheapest grade (in the UK and US), commonly used for automobile exhaust pipes, belongs to ferritic stainless steel (chromium steel).

410: Martensitic (high-strength chromium steel), good wear resistance, relatively poor corrosion resistance.

416: Sulfur is added to improve the material’s machinability.

420: “Cutlery-grade” martensitic steel, similar to the earliest stainless steel such as the Brinell high-chromium steel. Also used for surgical instruments, can be made very bright.

430: Ferritic stainless steel, decorative, for example, used for automotive trim. Good formability, but poor heat resistance and corrosion resistance.

440: High-strength tool steel, with slightly higher carbon content, can achieve higher yield strength and hardness up to 58HRC after appropriate heat treatment, among the hardest stainless steels. The most common application example is “razor blades.” Common grades include three types: 440A, 440B, 440C, and there is also 440F (easy processing type).

500 Series: 

Made of heat-resistant chromium alloys, not very commonly used.

600 Series: 

Martensitic precipitation hardening stainless steel. The 600 series stainless steel alloys are typically used for applications requiring high temperature and corrosion resistance. These alloys in the series have excellent mechanical properties, providing a desirable combination of good machinability and high strength.

601 to 604: Low-alloy martensitic steel.

610 to 613: Martensitic hardening secondary steel.

614 to 619: Chromium martensitic steel.

630 to 635: Stainless steel hardened by precipitation of semi-austenite and martensite.

650 to 653: Hot/cold working strengthened austenitic steel.

660 to 665: Austenitic high-temperature alloys; all types except alloy 661 undergo second-stage precipitation hardening.

 

Applications of Stainless Steel

Applications of Stainless Steel

Building and Decoration: 

Stainless steel can be used to make various decorative materials such as pipes, sheets, screws, etc., and is widely used in the construction and decoration industries.

Chemical and Food Processing: 

Stainless steel has excellent corrosion resistance, making it widely used in chemical and food processing industries for manufacturing chemical containers, food processing equipment, food storage tanks, etc.

Medical Instruments: 

Stainless steel, with its non-toxic, tasteless, and antibacterial properties, is widely used in the manufacturing of medical instruments such as surgical instruments, medical needles, artificial bones, etc.

Aerospace: 

Stainless steel’s high-temperature stability and hardness make it widely used in the aerospace industry for manufacturing aerospace engine components, spacecraft structures, etc.

Energy and Environmental Protection:

Stainless steel’s corrosion resistance, high-temperature resistance, and radiation resistance make it widely used in the energy and environmental protection industries for manufacturing nuclear power equipment, solar power generation equipment, offshore oil and gas drilling platforms, etc.

Electronics: 

Stainless steel can be used to manufacture various electronic products, such as mobile phones, tablets, laptops, etc., as shell materials, offering aesthetic appeal, durability, and corrosion resistance.

In addition to these, stainless steel is also applied in industries such as automotive manufacturing, petrochemicals, water treatment, metallurgy, etc. Overall, the application of stainless steel is extremely extensive.

 

What are the Manufacturing Processes of Stainless Steel?

What are the Manufacturing Processes of Stainless Steel

Step 1: Melting

The manufacturing of stainless steel begins with melting scrap metal and additives in an Electric Arc Furnace (EAF). Using high-power electrodes, the electric arc furnace heats the metal to form a molten mixture within several hours. Stainless steel is 100% recyclable; many stainless steel orders contain up to 60% recycled steel. This not only helps control costs but also reduces environmental impact.

Step 2: Refining to Reduce Carbon Content

Carbon contributes to the hardness and strength of iron. However, excessive carbon can lead to issues such as carbide precipitation during welding. Before stainless steel is formed into shapes, its carbon content needs to be appropriately calibrated and reduced. In the refining furnace, deoxidation, desulfurization, and impurity removal are achieved through the introduction of gas such as argon, nitrogen, or oxygen.

Step 3: Forming or Casting

Once the molten steel is ready, the stainless steel melt is transferred to a continuous casting machine where it is poured into molds. The molten steel solidifies and cools within the molds, forming rectangular or round billets.

Step 4: Rolling

Depending on the desired grade or shape, the steel may undergo one or more of these stages to achieve the desired appearance or properties. The following stages are the most commonly used:

Hot Rolling: This step is carried out at temperatures higher than the recrystallization temperature of the steel. It helps adjust the rough physical dimensions of the steel. Precise temperature control during the process ensures the steel remains smooth enough to work with without altering its structure.

Cold Rolling: Typically used when precision is required, cold rolling occurs below the recrystallization temperature of the steel. Multiple backup rolls are used to shape the steel. This process creates a more uniform and attractive surface finish. However, it also causes deformation of the steel structure, often requiring heat treatment to recrystallize the steel to its initial microstructure.

Step 5: Annealing

After rolling, most steels undergo an annealing process. This involves cooling cycles and controlled heating. These cycles soften the steel and relieve internal stresses. The exact time and temperature will depend on the type of steel, cooling, and heating rates, which ultimately influence the final product.

Step 6: Descaling or Pickling

After stainless steel sheets are rolled, some oxides and impurities may remain on the surface. This accumulation is not only unattractive but can also affect the steel’s resistance to contamination, weldability, and durability. Descaling or pickling, done by controlling cooling and heating in an oxygen-free environment or using acid baths (known as pickling), can eliminate this scale.

Step 7: Finishing

Stainless steel comes in various finishes, from flat to mirror-like. Finishing is one of the final steps involved in the production process. Common techniques include galvanizing, polishing, etching, and sandblasting.

Step 8: Cutting and Transportation

After inspection and quality control, the batch of steel is cut according to the requirements of the order. The steel is then collected in its final form and prepared for transportation to the customer.

 

How to Maintain Stainless Steel?

How to Maintain Stainless Steel

Maintenance of stainless steel is important despite its strong resistance to rust. Here are some basic methods for maintaining stainless steel:

Regular Cleaning:

Clean the surface of stainless steel regularly to prevent dust and stains from adhering. Use a soft cloth or sponge for wiping and avoid using abrasive materials.

Avoid Harsh Chemicals:

Refrain from using cleaning agents that contain acids, salts, or strong alkalis, as they may corrode the surface of stainless steel.

Keep Dry:

Store stainless steel products in a dry and hygienic environment to prevent oxidation. Avoid direct contact with other metal products to prevent potential reactions.

 

Conclusion

Stainless steel products, with their excellent characteristics,  are widely applied in industries such as industrial, architectural, household, and food machinery due to its high corrosion resistance, strength, and aesthetic appeal. If you have any other interests or questions about stainless steel, Please contact us.

 

FAQ: 

What are the differences between Stainless Steel 304 and 201?

What are the differences-between-Stainless Steel 304 and 201

The main differences between 304 and 201 stainless steel lie in their composition. In 304 stainless steel, the chromium content is around 18%, while in 201 stainless steel, it’s approximately 15%. The nickel content in 304 is about 9%, whereas in 201, it’s around 5%. Additionally, 201 stainless steel does not contain manganese, whereas 304 does. Due to its lower nickel content, 201 stainless steel has inferior corrosion resistance compared to 304. 304 stainless steel exhibits better corrosion resistance to acidic or alkaline solutions and high-temperature oxidation.

The application areas also differ: 201 stainless steel is suitable for dry environments, such as decoration and industrial fields, while 304 stainless steel is applicable in industrial, home decoration, and medical industries.

 

What are the differences between Stainless Steel 304 and 316?]

What are the differences between Stainless Steel 304 and 316

Both 304 and 316 stainless sheets of steel are food-grade materials with excellent corrosion resistance, but there are differences between them. Firstly, their compositions differ: 304 contains 18% chromium and 9% nickel, while 316 adds 2-3% molybdenum on top of this. Due to the addition of molybdenum, 316 stainless steel exhibits superior resistance to chloride corrosion compared to 304 stainless steel, making it suitable for more demanding environments such as seawater and chemical industries with strong corrosive conditions. Because 316 outperforms 304 in terms of performance and corrosion resistance, its price is higher than that of 304.

 

How to choose the right Grade of  Stainless Steel?

How to choose the right Grade of Stainless Steel

Although stainless steel is widely used in various industries, with over 150 grades available due to its strength, hygiene, and high corrosion resistance, there are many factors to consider when selecting a stainless steel grade. Factors such as the operating environment, required corrosion resistance, strength, quantity needed, whether magnetic response is required, and specific performance characteristics should all be taken into account. Choosing the appropriate grade of stainless steel will help you successfully complete your project while also saving money.

Stainless steel products, with their excellent characteristics, are virtually ubiquitous across various industries, including furniture, kitchenware, appliances, equipment, electronics, architectural components, medical parts, aerospace components, automotive parts, bicycles, sporting goods, and more.

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