Stainless steel is the name of a family of iron-based alloys known for their corrosion and heat resistance. One of the main characteristics of stainless steel is its minimum chromium content of 10.5%, which gives it its superior resistance to corrosion in comparison to other types of steels. Like other steels, stainless steel is composed primarily from iron and carbon, but with the addition of several other alloying elements, the most prominent being chromium. Other common alloys found in stainless steel are nickel, magnesium, molybdenum, and nitrogen.
What are the properties of stainless steel?
Stainless steel has many desirable properties that contribute greatly to its widespread application in the making of parts and components across many industrial sectors. Above all, because of its chromium content, it is extremely resistant to corrosion. The 10.5% minimum content makes steel approximately 200 times more resistant to corrosion than steels without chromium. Other favorable properties for consumers are its high strength and durability, its high and low temperature resistance, increased formability and easy fabrication, low maintenance, long lasting, attractive appearance and it is environmentally friendly and recyclable. Once stainless steel is put into service, it does not need to be treated, coated or painted.
- Corrosion resistant
- High tensile strength
- Very durable
- Temperature resistant
- Easy formability and fabrication
- Low-maintenance (long lasting)
- Attractive appearance
- Environmentally friendly (recyclable)
Grading systems for stainless steel
There are many numerical grading systems for stainless steel, designated according to their composition, physical properties, and applications. Each type of stainless steel is classified by its series number and then assigned a numerical grade. The most popular series numbers are 200, 300, 400, 600, and 2000. The most common grades are type 304 and 316 that consist of austenitic chromium-nickel alloys. Cutlery grade stainless steels are found in the 400 Series, which is derived from ferritic and martensitic chromium alloys. Type 420 is known as surgical steel, and type 440 is known as razor blade steel.
Stainless Steel Classifications
The family of stainless steels is primarily classified into four main categories based upon their crystal microstructure.
Ferritic
Ferritic steels are the 400 Grade stainless steels noted for their high chromium content, which can range from 10.5% to 27%. They have magnetic properties, too, offers good ductility, tensile-property stability, and resistance to corrosion, thermal fatigue, and stress-corrosion cracking.
Ferritic Stainless Steel Applications
Typical applications for ferritic stainless steels include automotive components and parts, petrochemical industry, heat exchangers, furnaces, and in durable goods like appliances and food equipment.
Austenitic
Perhaps the most common category of stainless steel, austenitic grade steels are high in chromium, with varying amounts of nickel, manganese, nitrogen, and some carbon. Austenitic steels are divided into the 300 series and 200 series subcategories, which are determined by which alloys are used. The austenitic structure of the 300 series is distinguished via the addition of nickel. The 200 series primarily uses the addition of manganese and nitrogen. Grade 304 is the most common stainless steel.
Austenitic Stainless Steel Applications
Sometimes referred to as 18/8 because of its 18% chromium and 8% nickel, it is used in kitchen equipment, cutlery, food processing equipment, and structural components in the automotive and aerospace industries. Grade 316 is another common stainless steel. It is used in the making of a wide range of products such as food preparation equipment, laboratory benches, medical and surgical equipment, boat fittings, pharmaceutical, textile, and chemical processing equipment.
Martensitic
Martensitic stainless steels are in the 400 Grade series of stainless steels. They have a low to high carbon content, and contain 12% to 15% chromium and up to 1% molybdenum. It’s used whenever corrosion resistance and-or oxidation resistance are required along with either high strength at low temperatures or creep resistance at elevated temperatures. Martensitic steels are also magnetic and possess relatively high ductility and toughness, which make them easier to form.
Martensitic Stainless Steel Applications
Applications for martensitic stainless steels include a wide range of parts and components, from compressor blades and turbine parts, kitchen utensils, bolts, nuts and screws, pump and valve parts, dental and surgical instruments, to electric motors, pumps, valves, machine parts sharp surgical instruments, cutlery, knife blades, and other cutting hand tools.
Duplex
As the name implies, duplex stainless steels possess a mixed microstructure of ferrite and austenite. The chromium and molybdenum content is high, with 22% to 25%, and up to 5%, respectively, with very low nickel content. The duplex structure gives the stainless steel many desirable properties. For starters, it offers double the strength of ordinary austenitic or ferritic stainless steels, with excellent corrosion resistance and toughness.
Duplex Stainless Steel Applications
Designated in the 2000 Grade series, duplex stainless steel is ideal for applications in demanding environments such as in chemical, oil, and gas processing and equipment, marine, high chloride environments, pulp and paper industry, cargo tanks for ships and truck, and bio-fuels plants, chloride containment or pressure vessels, transportation, heat exchanger tubes, construction, the food industry, desalination plants, and components for FGD systems.
| A series of | ASTM | Performance | Application |
|---|---|---|---|
| 300 | 304 | As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties. Stamping, bending and other thermal processing, no heat treatment hardening phenomenon, no magnetic, the use of temperature -196°C~800°C | Food industry, household products (class 1, 2 tableware, kitchen cabinet, indoor pipeline, water heater, boiler, bathtub) |
| 304L | As 304 steel with low C, its corrosion resistance is similar to 304 steel under general conditions, but it has excellent resistance to grain boundary corrosion after welding or stress elimination. It can also maintain good corrosion resistance without heat treatment. Generally used below 400C, non-magnetic, using temperature -196°C to 800°C | It is used in outdoor open-air machines, building materials, heat-resistant parts and parts with difficulty in heat treatment in chemical, coal and petroleum industries where grain boundary corrosion resistance is highly required | |
| 321 | Add elements in 304 steel to prevent grain boundary corrosion, suitable for use at 430°C~900°C | Medical appliances, Building materials, chemistry, agriculture, ship parts, atomic energy industry, aircraft, exhaust pipe, boiler drum accessories (windshield wipers, mold products) | |
| 316 | Mo should be added, so its corrosion resistance, atmospheric corrosion resistance and high temperature strength is particularly good, can be used under harsh conditions. Excellent work hardening (non-magnetic) | Seawater equipment, chemicals, dyes, paper making. Oxalic acid, fertilizer and other production equipment; Photography, food industry, coastal facilities, ropes, CD bars, bolts, nuts | |
| 316L | As the low C series of 316 steel, it has the same characteristics with 316 steel, and has excellent resistance to grain boundary corrosion. | 316 steel is a product with special requirements for resisting grain boundary corrosion | |
| 309S | 23Cr-13Ni high alloy rigid stainless steel, excellent corrosion resistance and strength, suitable for the operating temperature of 1000°C parts | Exhaust machine, heat treatment furnace and heat exchanger, etc. | |
| 310S | Excellent oxidation resistance, corrosion resistance, high temperature resistance, because of the higher percentage of chromium and nickel, 310S has much better creep strength, can continue to operate at high temperatures, has good high temperature resistance | Boiler, exhaust machine, heat treatment furnace and heat exchanger, etc. | |
| 200 | 202 | Mn and N replaced part of nickel, so as to obtain good mechanical properties and corrosion resistance, it is a new type of nickel saving stainless steel, its room temperature strength is higher than 304, under 800 tons has better oxidation resistance and moderate temperature strength. | Mainly used for decorative pipe, industrial pipe, some shallow stretching products |
| 2205 | The range of Cr, Mo and N elements are relatively narrow, easy to achieve a relative balance (that is, the two phase each account for half), improve the strength of steel, corrosion resistance and welding performance, mostly used for high performance requirements and welding materials, such as oil and gas pipes. | Used in oil refining, chemical fertilizer, paper making, petroleum, chemical industry and other seawater resistant high temperature concentrated nitric acid heat exchanger and condenser and devices | |
| 400 | 410 | For martensite steel on behalf of steel, although high strength, but not suitable for harsh corrosion environment. Its good machinability, heat treatment surface hardening (magnetic) | Blade, Machine parts, Petroleum refining unit, Bolts, nuts, pump rod, Class 1 tableware (knife and fork) |
| 430 | As a representative ferritic steel, low thermal expansion rate, formability and oxidation resistance. | Heat resistant appliances, burners, household appliances, class 2 tableware, external decoration materials for kitchen sinks, bolts, nuts, CD rod, screen | |
| 444 | High corrosion resistance ferritic stainless steel, corrosion resistance comparable to 316 superior steel grade | Water tank water heater, solar water heater collecting plate | |
| titanium | Grade2 Grade3 Grade4 | Excellent stamping performance. Various forms of welding, good welding performance, welding joints can reach 90% of the strength of the matrix metal. Easy to saw and grinding wheel cutting, good machining performance. Excellent corrosion resistance | Used for parts under 350℃ with small force and parts with various complex shapes. Such as thermal power plant condenser; Marine seawater corroded piping system; Valves, pumps; Chemical heat exchanger, pump body, distillation tower; Seawater desalination system, platinum-plated anode; An airplane skeleton, skin, engine parts, beams, etc. |