Introduction to Stainless Steel

Stainless steel is short for stainless and acid-resistant steel. Steel that is resistant to weak corrosive media such as air, steam, and water, or exhibits rust resistance, is called stainless steel. Steel that is resistant to chemically corrosive media (such as acids, alkalis, and salts) is called acid-resistant steel.

Stainless steel refers to steel that is resistant to weak corrosive media such as air, steam, and water, as well as chemically corrosive media such as acids, alkalis, and salts. It is also called stainless and acid-resistant steel. In practical applications, steel that is resistant to weak corrosive media is often referred to as stainless steel, while steel that is resistant to chemical media is often referred to as acid-resistant steel. Due to differences in their chemical composition, the former is not necessarily chemically resistant, while the latter is generally rust-resistant. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Common Classifications:

Usually classified by metallographic structure:

Common stainless steel is generally divided into three categories based on its metallographic structure: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. Based on these three basic metallographic structures, duplex steel, precipitation-hardening stainless steel, and high-alloy steel with an iron content of less than 50% have been developed to meet specific needs and purposes.

1. Austenitic stainless steel.

This stainless steel has a predominantly face-centered cubic austenite (CY phase) matrix and is non-magnetic. It is strengthened primarily through cold working (and may develop some magnetism). The American Iron and Steel Institute designates grades in the 200 and 300 series, such as 304.

2. Ferritic stainless steel. This stainless steel has a matrix composed primarily of ferrite (a-phase) with a body-centered cubic crystal structure. It is magnetic and generally cannot be hardened by heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Institute designates these grades as 430 and 446.

3. Martensitic Stainless Steel.

This stainless steel has a matrix composed primarily of martensite (body-centered cubic or cubic), is magnetic, and its mechanical properties can be adjusted by heat treatment. The American Iron and Steel Institute designates these grades as 410, 420, and 440. Martensite has an austenitic structure at high temperatures, and when cooled to room temperature at an appropriate rate, the austenitic structure can transform into martensite (i.e., harden).

4. Austenitic-Ferritic (Duplex) Stainless Steel.

This stainless steel has a matrix composed of both austenite and ferrite, with the minor phase typically exceeding 15%. It is magnetic and can be strengthened by cold working. Type 329 is a typical duplex stainless steel. Compared to austenitic stainless steel, duplex stainless steel offers increased strength and resistance to intergranular corrosion, chloride stress corrosion, and pitting corrosion. Corrosion resistance is significantly improved.

5. Precipitation-hardening stainless steel.

This type of stainless steel has an austenitic or martensitic matrix that can be hardened through precipitation hardening. The American Iron and Steel Institute designates these stainless steels with numbers in the 600 series, such as 630, which corresponds to 17-4PH.

Generally speaking, austenitic stainless steels, with the exception of alloys, offer excellent corrosion resistance. Ferritic stainless steels can be used in less corrosive environments. In mildly corrosive environments, if high strength or hardness is required, martensitic and precipitation-hardening stainless steels can be used.

What kind of stainless steel is rust-resistant?

Three main factors influence stainless steel rust:

1. The content of alloying elements.

Generally speaking, steel with a chromium content of 10.5% is rust-resistant. The higher the chromium and nickel content, the better the corrosion resistance. For example, 304 stainless steel requires a nickel content of 8-10% and a chromium content of 18-20%. Such stainless steel generally does not rust.

2. The manufacturer's smelting process also affects the corrosion resistance of stainless steel.

Large stainless steel mills with advanced smelting technology, equipment, and processes ensure consistent control of alloying elements, impurity removal, and billet cooling temperature. Consequently, their products are stable, reliable, and inherently high in quality, making them rust-resistant. Conversely, smaller steel mills with outdated equipment and processes fail to remove impurities during the smelting process, making rust unavoidable.

3. External environment: Dry, well-ventilated environments are less prone to rust.

Regions with high humidity, continuous rainy weather, or areas with high pH levels are more susceptible to rust. Even 304 stainless steel can rust if the surrounding environment is too harsh.

How should rust spots on stainless steel be treated?

1. Chemical Methods

Use an acid pickling paste or spray to help repassivate the rusted areas, forming a chromium oxide film to restore corrosion resistance. After pickling, it is important to rinse thoroughly with clean water to remove all contaminants and acid residue. After all treatments, re-polish with a polishing machine and seal with polishing wax. Minor localized rust can be removed with a 1:1 gasoline:motor oil mixture and wipe with a clean cloth.

2. Mechanical Methods

Sandblasting, shot blasting with glass or ceramic particles, annihilation, brushing, and polishing. Mechanical methods can remove contamination from previously removed materials, polishing materials, or annihilation materials. All types of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should ideally be cleaned properly under dry conditions. Mechanical methods only clean the surface and cannot alter the material's inherent corrosion resistance. Therefore, it is recommended to re-polish with a polishing machine and seal with polishing wax after mechanical cleaning. Common Stainless Steel Grades and Properties for Instruments

1. 304 Stainless Steel. This is one of the most widely used austenitic stainless steels. It is suitable for deep-drawn parts, acid pipelines, containers, structural parts, various instrument bodies, and more. It can also be used to manufacture non-magnetic and low-temperature equipment and components.

2. 304L stainless steel. This ultra-low carbon austenitic stainless steel was developed to address the severe intergranular corrosion tendency of 304 stainless steel under certain conditions due to Cr23C6 precipitation. Its intergranular corrosion resistance in the sensitized state is significantly superior to that of 304 stainless steel. Aside from slightly lower strength, its other properties are similar to those of 321 stainless steel. It is primarily used for corrosion-resistant equipment and components that require welding and cannot undergo solution treatment, and can be used in the manufacture of various instrument bodies.

3. 304H stainless steel. This is an internal branch of 304 stainless steel, with a carbon content of 0.04% to 0.10%. Its high-temperature performance is superior to that of 304 stainless steel.

4. 316 stainless steel. This steel is based on 10Cr18Ni12 steel and has molybdenum added to it, giving it excellent resistance to reducing media and pitting corrosion. Its corrosion resistance is superior to that of 304 stainless steel in seawater and other media, and it is primarily used for pitting-resistant materials.

5. 316L stainless steel. Ultra-low carbon steel offers excellent resistance to sensitized intergranular corrosion and is suitable for manufacturing thick-section welded components and equipment, such as corrosion-resistant materials in petrochemical equipment.

6. 316H stainless steel. An internal variant of 316 stainless steel, with a carbon content of 0.04% to 0.10%, it offers superior high-temperature performance to 316 stainless steel.

7. 317 stainless steel. It offers superior pitting and creep resistance to 316L stainless steel and is used in the manufacture of petrochemical equipment and equipment resistant to organic acid corrosion.

8. 321 stainless steel. This titanium-stabilized austenitic stainless steel has titanium added to improve intergranular corrosion resistance and offers excellent high-temperature mechanical properties. It can be substituted for ultra-low carbon austenitic stainless steel. Its use is generally not recommended except for specialized applications such as high temperatures or hydrogen corrosion resistance.

9. 347 stainless steel. Niobium-stabilized austenitic stainless steel, with the addition of niobium to improve its resistance to intergranular corrosion, offers comparable corrosion resistance to 321 stainless steel in corrosive media such as acids, alkalis, and salts. It also boasts excellent weldability and can be used as both a corrosion-resistant and heat-resistant steel. It is primarily used in the thermal power and petrochemical industries, such as in the manufacture of containers, pipes, heat exchangers, shafts, industrial furnace tubes, and furnace tube thermometers.

10. 904L stainless steel. Super fully austenitic stainless steel, invented by Outokumpu of Finland, has a nickel content of 24% to 26% by mass and a carbon content of less than 0.02%. It exhibits excellent corrosion resistance, particularly in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid, and phosphoric acid. It also exhibits excellent resistance to crevice corrosion and stress corrosion. It is suitable for use in sulfuric acid of various concentrations below 70°C. It also exhibits excellent corrosion resistance in acetic acid of all concentrations and temperatures at atmospheric pressure, as well as in mixtures of formic and acetic acids. The original ASME SB-625 standard classified it as a nickel-based alloy, while the new standard classifies it as stainless steel. China only has a similar grade, 015Cr19Ni26Mo5Cu2. A few European instrument manufacturers use 904L stainless steel as a key material. For example, the measuring tube of E+H's mass flowmeters is made of 904L stainless steel, and Rolex watch cases also use 904L stainless steel.

11. 440C stainless steel. This martensitic stainless steel has the highest hardness among hardenable stainless steels, reaching HRC57. It is primarily used in nozzles, bearings, valve cores, seats, sleeves, and stems.

12. 17-4PH stainless steel. This martensitic precipitation-hardening stainless steel has a hardness of HRC44 and offers high strength, hardness, and corrosion resistance, but cannot be used at temperatures exceeding 300°C. It has good corrosion resistance to the atmosphere and dilute acids or salts. Its corrosion resistance is the same as that of 304 stainless steel and 430 stainless steel. It is used to manufacture offshore platforms, turbine blades, valve cores, valve seats, sleeves, valve stems, etc.