Stainless Steel Overview

Stainless Steel is a shorthand for "stainless and acid-resistant steel." Steel that resists corrosion from weak corrosive media like air, steam, and water is called stainless steel. In contrast, steel that resists corrosion from chemical corrosive media (such as acids, alkalis, and salts) is referred to as acid-resistant steel. In practical applications, steel resistant to weak corrosive media is commonly referred to as stainless steel, while steel resistant to chemical corrosive media is called acid-resistant steel. Due to differences in chemical composition, the former may not resist chemical corrosion, while the latter generally possesses stainless properties. The corrosion resistance of stainless steel depends on the alloying elements within the steel. Typically, according to the metallographic structure, ordinary stainless steel is divided into three types: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. Based on these three primary metallographic structures, duplex steel, precipitation-hardening stainless steel, and high-alloy steel with less than 50% iron content have also been developed for specific requirements and purposes.

Classification by Metallographic Structure:

Austenitic Stainless Steel: Primarily has a face-centered cubic structure (CY phase) without magnetism. It can be strengthened mainly through cold working, which may lead to some magnetism. The American Iron and Steel Institute (AISI) uses numbers from the 200 and 300 series, like 304, to denote austenitic stainless steels.

Ferritic Stainless Steel: Primarily has a body-centered cubic structure (α phase) with magnetic properties. It generally cannot be hardened by heat treatment but can be slightly strengthened by cold working. AISI designates this type with numbers like 430 and 446.

Martensitic Stainless Steel: Its matrix has a martensitic structure (either body-centered cubic or cubic), with magnetic properties and the ability to adjust mechanical properties through heat treatment. AISI uses numbers such as 410, 420, and 440 to denote martensitic stainless steels. Martensite can exhibit an austenitic structure at high temperatures and transform into martensite when cooled at an appropriate rate to room temperature (known as hardening).

Austenitic-Ferritic (Duplex) Stainless Steel: Combines austenitic and ferritic phases, with the minority phase typically making up more than 15% of the structure, and exhibits magnetic properties. Duplex stainless steel can be strengthened through cold working, with 329 as a typical example. Compared to austenitic stainless steels, duplex stainless steels have higher strength and improved resistance to intergranular corrosion, chloride stress corrosion, and pitting corrosion.

Precipitation-Hardening Stainless Steel: Has an austenitic or martensitic matrix and can be hardened through precipitation-hardening treatment. AISI designates it with numbers in the 600 series, such as 630, or 17-4PH. Generally, austenitic stainless steel has superior corrosion resistance due to its alloying elements. Ferritic stainless steel is suitable for mildly corrosive environments, while martensitic and precipitation-hardening stainless steels are ideal for environments with light corrosion where high strength or hardness is required.

Thickness Differentiation:

Due to minor deformation in the rollers during the rolling process, the thickness of steel plates may vary slightly, often being thicker in the middle and thinner on the edges. When measuring thickness, the national standard mandates taking measurements from the middle section of the plate.

Tolerances are generally classified into large and small based on market and customer demands.

Factors Affecting Rust Resistance in Stainless Steel:

Alloy Element Content: Generally, steel with a chromium content above 10.5% is less prone to rust. Higher nickel and chromium content enhance corrosion resistance, as in the case of 304 stainless steel, which contains 8-10% nickel and 18-20% chromium, making it typically rust-resistant.

Refinement Processes: The corrosion resistance of stainless steel is also influenced by the production processes. High-quality stainless steel manufacturers with advanced equipment and techniques can ensure stable product quality by precisely controlling alloy elements, removing impurities, and maintaining optimal cooling temperatures for steel billets, thereby producing less rust-prone steel. In contrast, smaller manufacturers with outdated technology may fail to remove impurities effectively, leading to products that are more susceptible to rust.

External Environment: Stainless steel resists rust better in dry and ventilated environments. High humidity, prolonged rainy conditions, or environments with high acidic or alkaline content are more likely to cause rusting. Even 304 stainless steel may rust under harsh environmental conditions.

Rust Removal Techniques for Stainless Steel:

Chemical Methods: Use pickling paste or spray to re-passivate the rusted area, forming a chromium oxide film to restore corrosion resistance. After pickling, it's essential to rinse with clean water to remove all pollutants and acid residues thoroughly. Polishing with equipment and sealing with polishing wax afterward can also help.

Mechanical Methods: Methods like sandblasting, shot peening with glass or ceramic particles, grinding, and polishing are effective. Mechanical cleaning can remove contaminants like removed materials, polishing residues, or particles that may contribute to corrosion, especially in moist conditions. Mechanical cleaning is most effective under dry conditions. However, it only cleans the surface and does not alter the material's inherent corrosion resistance. Therefore, it is recommended to polish and wax-seal after mechanical cleaning.

Common Stainless Steel Grades and Properties:

304 Stainless Steel: One of the most widely used austenitic stainless steels, suitable for making deep-drawing parts, acid transport pipes, containers, structural parts, and various instrument bodies. It can also be used for non-magnetic, low-temperature equipment and components.

304L Stainless Steel: Developed to solve the intergranular corrosion tendency of 304 stainless steel in certain conditions due to Cr23C6 precipitation. It offers superior sensitization resistance to intergranular corrosion compared to 304 stainless steel, with properties similar to 321 stainless steel but slightly lower strength. It is mainly used for corrosion-resistant equipment and parts requiring welding without solution treatment.

304H Stainless Steel: A subset of 304 with a carbon content of 0.04% to 0.10%, offering better high-temperature performance than standard 304.

316 Stainless Steel: Adds molybdenum to 10Cr18Ni12 steel, providing excellent corrosion resistance in reducing environments and superior pitting resistance, making it suitable for use in seawater and other media.

316L Stainless Steel: A low-carbon steel with good sensitization resistance and intergranular corrosion performance, suitable for thick-section welded components and equipment, such as corrosion-resistant materials in petrochemical equipment.

316H Stainless Steel: A subset of 316 with 0.04% to 0.10% carbon content, offering enhanced high-temperature performance.

317 Stainless Steel: Offers better pitting and creep resistance than 316L, suitable for making petrochemical equipment and equipment resistant to organic acids.

321 Stainless Steel: A titanium-stabilized austenitic stainless steel, providing enhanced intergranular corrosion resistance and good high-temperature mechanical properties. It is generally not recommended outside high-temperature or hydrogen-resistant applications.

347 Stainless Steel: A niobium-stabilized austenitic stainless steel with intergranular corrosion resistance similar to 321 in acidic, alkaline, and salt environments, with good welding properties. It is mainly used in power and petrochemical industries for containers, pipelines, heat exchangers, shafts, and furnace tubes in industrial furnaces.

904L Stainless Steel: A super-austenitic stainless steel with nickel content of 24%-26% and carbon below 0.02%, offering excellent corrosion resistance, especially in non-oxidizing acids like sulfuric, acetic, formic, and phosphoric acids. It is corrosion-resistant in sulfuric acid at temperatures below 70℃ and resistant to any concentration and temperature of acetic acid and formic-acetic acid mixtures. Some European instrument manufacturers use 904L for key parts, like the measuring tubes of mass flow meters from E+H and the cases of Rolex watches.

440C Stainless Steel: A martensitic stainless steel with the highest hardness among hardenable stainless steels, with a hardness of HRC57. It is mainly used for nozzles, bearings, valve cores, valve seats, sleeves, and valve stems.

17-4PH Stainless Steel: A martensitic precipitation-hardening stainless steel with HRC44 hardness, offering high strength, hardness, and corrosion resistance. It is not suitable for temperatures above 300°C and is commonly used for offshore platforms, turbine blades, valve cores, valve seats, sleeves, and valve stems.

300 Series - Chromium-Nickel Austenitic Stainless Steel:

301: Offers good ductility, suitable for forming products, and can harden quickly through mechanical processing. It has better wear resistance and fatigue strength than 304 stainless steel.

302: Essentially a variant of 304 with higher carbon content, achieving higher strength through cold rolling.

302B: Contains higher silicon for increased oxidation resistance at high temperatures.

303 and 303Se: Sulfur and selenium-containing stainless steels, respectively, designed for easy machining and high surface brightness. 303Se stainless steel is also used in applications requiring hot upsetting.

304N: Contains nitrogen to enhance strength.

305 and 384: Have higher nickel content with a low work-hardening rate, suitable for high cold-forming applications.

308: Used in welding rods.