Basic knowledge of stainless steel

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Mengni special alloy stainless steel: Stainless steel is a high-alloy steel that resists corrosion. It has an attractive appearance, and no surface treatment such as plating or painting is required; therefore, many machinery manufacturers, in order to indicate that their products are made of stainless steel, refrain from applying any paint. In contrast, ordinary steel (what is commonly referred to as ‘black material’) requires anti-rust treatment. A simple classification of stainless steel: Stainless steel can be roughly divided into three categories – the 200 series, the 300 series, and the 400 series. Among these, the 300 series is the most commonly used. The 200 series and 400 series serve as substitutes to some extent for the 300 series. Strictly speaking, the 400 series isn’t considered stainless steel; it’s called rust-resistant iron, as it does not contain nickel, and therefore can be attracted by a magnet. The 200 series and 300 series contain nickel, so they are non-magnetic and cannot be attracted by magnets. 304 is the most common grade in the 300 series; therefore, changes in the price of 304 can generally be used to gauge the overall trend in stainless steel prices. The 200 series contains less nickel, the 400 series contains no nickel at all, while the 300 series contains the most nickel; therefore, it is the 300 series that is most affected by nickel prices. The 300 series can be simply divided into 304, 304L, 316, 316L, 321, 309S, 310S, etc. The distinction between them is based on the varying contents of various metal elements; stainless steels with different element concentrations have different properties. The difference between 304 and 304L lies in their carbon (C) content – the version with L indicates a lower carbon content, meaning it is low-carbon. The same principle applies to the difference between 316 and 316L. Classification of stainless steel forms: Stainless steel can be divided into five categories – coil sheets, sheet metal, profiles, steel pipes, and components – with coil sheets and sheet metal being the most important. Profiles are materials of various shapes made from sheets, including angle steel, flat steel, I-beam, and channel steel. Steel pipes mainly refer to seamless steel pipes; the counterpart to seamless steel pipes is welded pipes, and the difference lies in whether they are formed in one step or not. The components in question mainly refer to elbows, flanges, and other small parts. Coiled materials are essentially the same as sheet materials; the only difference is their form at the time of production – coiled materials come in rolls, while sheet materials come in sheets. If they are produced in sheet form, they are also known as original sheets, and these are usually thick sheets, as thick materials cannot be rolled up; generally, materials thicker than 16 millimeters cannot be rolled. There are both coiled and sheet forms, and most of the time it’s sheet material that is used. The prices of coiled and sheet materials differ as well: when purchasing coiled materials, the price is calculated based on the actual weight, which results in a higher price, whereas sheet materials are priced based on theoretical calculations, resulting in a lower price. This difference arises because the actual thickness may vary; for example, a material with a thickness of 10 mm might actually be 9.6 mm thick, and this difference leads to a price variation. Stainless steel sheets are divided into hot-rolled and cold-rolled types, depending on the rolling process used in the rolling mills (which are machines used for rolling sheets). Hot-rolled sheets are typically labeled as NO.1, while cold-rolled sheets are labeled as 2B or BA (BA has a better surface finish than 2B; it’s brighter and closer to a mirror-like surface. The best quality is a mirror finish, but such sheets are not available in ready-made form and require processing). For example, 304 steel is often purchased in sheet form, which means that the coil must be processed using a machine to turn it into sheets – such a machine is called a slitting machine. By using a slitting machine, it is possible to obtain sheets of different lengths. If the length is not standard, it is referred to as a custom cut, meaning that the size is determined according to specific requirements. Since customers often ask for sheets in different lengths based on their actual needs, coils become important in such cases; standard sheets cannot meet these requirements. Domestically produced cold-rolled steel is generally under 3 millimeters in thickness, while hot-rolled steel is usually over 3 millimeters. There is both hot-rolled and cold-rolled steel available in a thickness of 3 millimeters, but imported cold-rolled steel sheets can have a thickness of less than 4 millimeters or even less than 6 millimeters. Hot-rolled plates with a thickness of 3 millimeters to 12 millimeters are called medium plates; those with a thickness of over 12 millimeters are known as heavy plates, and their thickness can exceed 120 millimeters in such cases, at which point they are referred to as hot-rolled medium-heavy plates. Cold rolling, on the other hand, results in cold-rolled thin plates. A simple method for calculating the theoretical weight of stainless steel sheets: Sheet weight = specific gravity × thickness (mm) × width (m) × length (m). Here, mm is the unit for millimeters and m is the unit for meters. In the field of construction machinery, where precision in dimensions is important, millimeters are used as the unit worldwide; 1000 millimeters = 1 meter, and 1 millimeter = 100 microns. The weight calculated using this formula is in kilograms (KG), and it needs to be accurate to two decimal places. Since stainless steel is expensive and large quantities are traded, the digits after the decimal point are very important; therefore, the amount of money involved also needs to be precise to two decimal places. The specific gravity of stainless steel (in KG): Different types of stainless steel have varying specific gravities. For series A, 400, the specific gravity is 7.75; for 304, it is 3.21, while for 304N, 201, 202, and 304L, the specific gravity is 2.01. The specific gravity of 301 is 7.93. As for series C, 310S, 309S, 316, and 316L, the specific gravity is 7.98. Different countries around the world use various names for stainless steel, as indicated by steel grade tables. In the market, the terms commonly used are those from China and the United States, namely national standards and American standards. The 200 series, 300 series, and 400 series mentioned earlier belong to the American standard; since this standard is easy to understand, it is generally referred to as the American standard. National standard ============ American standard: 1Cr17Mn6Ni5N========201, 1Cr18Mn8Ni5N========202, 0Cr18Ni9============304, 0Cr19Ni10==========304L, 0Cr18Ni12Mo2T=======316Ti, 00Cr17Ni14Mo2=======316L, 0Cr17Ni12Mo2========316, 1Cr18Ni9Ti==========321, 0Cr18Ni10Ti=========321, 0Cr23Ni13===========309S, 1Cr20Ni14===========309S, 0Cr25Ni20===========310S, 1Cr25Ni20===========310S, 0Cr13===============405, 1Cr13===============410, 2Cr13===============420, 3Cr13===============4204, Cr3================430. The element contents specified in the national standard generally refer to intermediate values; they are theoretical values that can vary. What is tolerance? This is a very important term; tolerance refers to the accepted allowable deviation in length or thickness. This is because, during the production of sheets, it is difficult to achieve such precision for various reasons. The actual thickness obtained is often not as high as the theoretical thickness; for example, a plate that should be 6 millimeters thick may only be 5.6 millimeters thick. This is what is referred to as tolerance. In China, tolerances are usually negative, though positive tolerances do exist as well, but they are rare. When buying or selling sheets, the calculation is usually based on the theoretical thickness; this is also why the theoretical price is lower than the weight-based price. There are also differences in tolerances, and prices vary accordingly – those with larger tolerances have lower prices. For example, for a 6-millimeter thickness, if it is below 5.5 millimeters, then it falls under the category of large tolerance. 1. The tolerance ranges for 304/2B are shown in the table below (for reference only).
Specification: Maximum tolerance, Normal tolerance, Minimum tolerance, No tolerance
0.4MM: 0.4MM, 0.3–0.32, 0.33–0.37, 0.37–0.39, Above 0.39
0.5MM: 0.5MM, 0.4–0.42, 0.43–0.47, 0.47–0.49, Above 0.49
0.6MM: 0.6MM, 0.5–0.52, 0.53–0.57, 0.57–0.59, Above 0.59
0.7MM: 0.7MM, 0.6–0.62, 0.63–0.67, 0.67–0.69, Above 0.69
0.8MM: 0.8MM, 0.7–0.72, 0.73–0.77, 0.77–0.79, Above 0.79
0.9MM: 0.9MM, 0.8–0.82, 0.83–0.87, 0.87–0.89, Above 0.89
1.0MM: 1.0MM, 0.89–0.91, 0.91–0.94, 0.94–0.99, Above 0.99
1.2MM: 1.2MM, 1.01–1.08, 1.09–1.15, 1.16–1.18, Above 1.18
1.5MM: 1.5MM, 1.32–1.38, 1.39–1.45, 1.46–1.48, Above 1.48
2.0MM: 2.0MM, 1.71–1.79, 1.8–1.86, 1.87–1.9, Above 1.9
2.5MM: 2.5MM, 2.23–2.38, 2.39–2.42, 2.42–2.47, Above 2.47
3.0MM: 3.0MM, 2.7–2.77, 2.78–2.85, 2.85–2.9, Above 2.9

2. The tolerance ranges for hot-rolled sheets are shown in the table below (for reference only).
Specification: Maximum tolerance, Normal tolerance, Minimum tolerance, No tolerance
3.0MM: 3.0MM, 2.6–2.75, 2.75–2.85, 2.86–2.9, Above 2.9
4.0MM: 4.0MM, 3.2–3.6, 3.6–3.75, 3.75–3.95, Above 3.95
5.0MM: 5.0MM, 4.2–4.6, 4.6–4.75, 4.75–4.95, Above 4.95
6.0MM: 6.0MM, 5.1–5.5, 5.5–5.75, 5.75–5.9, Above 5.9
8.0MM: 8.0MM, 7.2–7.5, 7.5–7.75, 7.75–7.95, Above 7.9
10.0MM: 10.0MM, 9.1–9.3, 9.3–9.6, 9.6–9.85, Above 9.85
12.0MM: 12.0MM, 11.1–11.2, 11.2–11.6, 11.6–11.85, Above 11.85
14.0MM: 14.0MM, 13.1–13.2, 13.2–13.6, 13.6–13.85, Above 13.85

Characteristics and uses of major stainless steel grades:
304: As a widely used steel grade, it possesses good corrosion resistance, heat resistance, low-temperature strength, and mechanical properties. It is suitable for hot processing such as stamping and bending. It does not undergo hardening due to heat treatment, is non-magnetic, and can be used in temperatures ranging from -193 degrees Celsius to 800 degrees Celsius. Uses: tableware, kitchen utensils, water heaters, boilers, auto parts, medical devices, food processing machinery, wine storage containers, pressure vessels (chemical machinery, chemical equipment). As a low-carbon version of 304 steel, 304L has corrosion resistance similar to that of 304 under normal conditions. However, after welding and stress relief, it exhibits good resistance to intergranular corrosion. It can also maintain good corrosion resistance even without heat treatment. It is generally used at temperatures around 400 degrees, and its applications include the petrochemical industry and construction materials. 321: Ti is added to 304 steel to prevent intergranular corrosion; it is suitable for temperatures ranging from 430 to 900 degrees Celsius and is non-magnetic ； Applications: Automotive exhaust systems, heat exchangers, containers, and other products that are welded without heat treatment; due to the presence of Ti, they are not suitable for use in food processing equipment. 316 low carbon version, with MO element added; as a result, it boasts excellent corrosion resistance, resistance to atmospheric corrosion, and high-temperature strength. It can be used under harsh conditions, is suitable for use at temperatures below 900 degrees, and is non-magnetic. Uses: Equipment used in seawater, production equipment for the chemical industry, dyes, papermaking, acetic acid, fertilizers, etc., facilities in the food industry and coastal areas, products with special requirements to resist intergranular corrosion. The 309S/310S materials have high levels of nickel and chromium; additionally, an increased amount of silicon is present, which confers on them high-temperature resistance and corrosion resistance. 309S can withstand repeated heating up to 980 degrees, while 310S can operate at temperatures of up to 1200 degrees, with a continuous operating temperature of 1150 degrees. Neither of these materials is magnetic. Applications: Suitable for high-temperature furnace equipment, drying equipment and other critical components, as well as in furnace materials, aviation, petrochemicals, power generation, and other fields. The 200 series is similar to 304 in terms of lower price and cost-effectiveness. Uses: Food processing utensils, kitchen equipment, food processing devices, filters, milk containers, durable consumer goods, washing machine accessories, water heaters, steel furniture, building decoration, ornaments. In terms of fatigue resistance, 201 has a higher hardness but less toughness than 304; therefore, 304 has better fatigue resistance.