Basic Knowledge of Stainless Steel
Stainless steel is a type of high-alloy steel that resists corrosion in air or chemically aggressive environments. It has an attractive surface and excellent corrosion resistance, which means it doesn’t require plating or surface treatment to maintain its appearance. This makes it widely used in various applications. Common types include 13-chrome steel and 18-chrome nickel steel.
From a metallurgical perspective, stainless steel contains chromium, which forms a thin, protective oxide layer on the surface. This layer prevents oxygen from penetrating the metal and causing corrosion. To ensure this corrosion resistance, stainless steel must contain at least 12% chromium.
Numbering Systems for Stainless Steel
Different countries use various systems to classify stainless steel. In China and Russia, grades are denoted using chemical element symbols and numbers, such as 12CrNi3A. In the U.S. and Japan, they use series like 300, 400, and 200 series. Some grades are also identified by letters and numbers indicating their purpose.
In China, the numbering system includes elements like P (open hearth steel), F (boiling steel), B (killed steel), A (special steel), and T8 (special carbon). For example, 20CrMnTi represents alloy structural steel, while 1Cr18Ni9 indicates stainless steel with 0.1% carbon. Low-carbon grades like 0Cr18Ni9 have less than 0.08% carbon, and ultra-low carbon grades like 0Cr17Ni13Mo have even less.
The American Iron and Steel Institute uses a three-digit system to identify standard grades. Austenitic grades fall into the 200 and 300 series, such as 304, 316, and 310. Ferritic and martensitic grades are in the 400 series, like 430, 410, and 440C. Some special grades, especially those with high alloys, are named based on patents or trademarks.
Corrosion Resistance of Different Grades
304 is a versatile grade used in general applications due to its good formability and corrosion resistance. 301 is known for work hardening, making it suitable for high-strength applications. 302 is similar to 304 but with higher carbon content for increased strength. 303 and 303Se are free-cutting grades with sulfur and selenium, ideal for machining. 304L is a low-carbon version used in welding to prevent intergranular corrosion.
309, 310, and 330 have high nickel and chromium content for better high-temperature performance. 316 and 317 offer superior resistance to pitting corrosion, especially in marine environments. 321, 347, and 348 are stabilized grades with titanium, niobium, or tantalum, ideal for high-temperature welding.
Stainless Steel vs. "Stainless Iron"
Stainless steel refers to steel resistant to weak corrosive media like air and water, while acid-resistant steel is designed to withstand stronger chemicals like acids and alkalis. "Stainless iron" is not a true stainless steel; it's often recycled scrap mixed with other metals and can be mistaken for stainless steel with traditional testing methods.
Welding Stainless Steel: Tips and Precautions
When welding stainless steel, use a DC power source with reverse polarity. It’s best for thin plates under 6mm, offering good weld quality and minimal distortion. Argon gas with 99.99% purity is recommended, with flow rates adjusted based on current levels. The tungsten electrode should protrude 4–5mm, and the arc length should be kept short to avoid poor protection.
For MIG welding, use pure argon or Ar+2% O₂, with flow rates around 20–25 L/min. Wind protection is crucial, as even light breezes can cause porosity. Flux-cored wire welding typically uses CO₂ gas with a flow rate of 20–25 L/min.
Deep Drawing Challenges with Stainless Steel Sheets
Stainless steel sheets are difficult to deep-draw due to their high yield strength and cold work hardening. They tend to wrinkle, require large blanking forces, and may develop rebound issues. Adhesive tumors (stick-slip) are common during the process.
Solutions include using mold materials like cast aluminum bronze or tungsten carbide inserts. Proper heat treatment, such as soft nitriding, can reduce adhesion. Surface polishing plays a key role in reducing friction and improving adhesion resistance. New techniques like electrochemical and ultrasonic polishing help achieve smoother surfaces and better durability.
Overall, proper material selection, surface treatment, and process control are essential for successful deep drawing of stainless steel.
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Stainless steel is a type of high-alloy steel that resists corrosion in air or chemically aggressive environments. It has an attractive surface and excellent corrosion resistance, which means it doesn’t require plating or surface treatment to maintain its appearance. This makes it widely used in various applications. Common types include 13-chrome steel and 18-chrome nickel steel.
From a metallurgical perspective, stainless steel contains chromium, which forms a thin, protective oxide layer on the surface. This layer prevents oxygen from penetrating the metal and causing corrosion. To ensure this corrosion resistance, stainless steel must contain at least 12% chromium.
Numbering Systems for Stainless Steel
Different countries use various systems to classify stainless steel. In China and Russia, grades are denoted using chemical element symbols and numbers, such as 12CrNi3A. In the U.S. and Japan, they use series like 300, 400, and 200 series. Some grades are also identified by letters and numbers indicating their purpose.
In China, the numbering system includes elements like P (open hearth steel), F (boiling steel), B (killed steel), A (special steel), and T8 (special carbon). For example, 20CrMnTi represents alloy structural steel, while 1Cr18Ni9 indicates stainless steel with 0.1% carbon. Low-carbon grades like 0Cr18Ni9 have less than 0.08% carbon, and ultra-low carbon grades like 0Cr17Ni13Mo have even less.
The American Iron and Steel Institute uses a three-digit system to identify standard grades. Austenitic grades fall into the 200 and 300 series, such as 304, 316, and 310. Ferritic and martensitic grades are in the 400 series, like 430, 410, and 440C. Some special grades, especially those with high alloys, are named based on patents or trademarks.
Corrosion Resistance of Different Grades
304 is a versatile grade used in general applications due to its good formability and corrosion resistance. 301 is known for work hardening, making it suitable for high-strength applications. 302 is similar to 304 but with higher carbon content for increased strength. 303 and 303Se are free-cutting grades with sulfur and selenium, ideal for machining. 304L is a low-carbon version used in welding to prevent intergranular corrosion.
309, 310, and 330 have high nickel and chromium content for better high-temperature performance. 316 and 317 offer superior resistance to pitting corrosion, especially in marine environments. 321, 347, and 348 are stabilized grades with titanium, niobium, or tantalum, ideal for high-temperature welding.
Stainless Steel vs. "Stainless Iron"
Stainless steel refers to steel resistant to weak corrosive media like air and water, while acid-resistant steel is designed to withstand stronger chemicals like acids and alkalis. "Stainless iron" is not a true stainless steel; it's often recycled scrap mixed with other metals and can be mistaken for stainless steel with traditional testing methods.
Welding Stainless Steel: Tips and Precautions
When welding stainless steel, use a DC power source with reverse polarity. It’s best for thin plates under 6mm, offering good weld quality and minimal distortion. Argon gas with 99.99% purity is recommended, with flow rates adjusted based on current levels. The tungsten electrode should protrude 4–5mm, and the arc length should be kept short to avoid poor protection.
For MIG welding, use pure argon or Ar+2% O₂, with flow rates around 20–25 L/min. Wind protection is crucial, as even light breezes can cause porosity. Flux-cored wire welding typically uses CO₂ gas with a flow rate of 20–25 L/min.
Deep Drawing Challenges with Stainless Steel Sheets
Stainless steel sheets are difficult to deep-draw due to their high yield strength and cold work hardening. They tend to wrinkle, require large blanking forces, and may develop rebound issues. Adhesive tumors (stick-slip) are common during the process.
Solutions include using mold materials like cast aluminum bronze or tungsten carbide inserts. Proper heat treatment, such as soft nitriding, can reduce adhesion. Surface polishing plays a key role in reducing friction and improving adhesion resistance. New techniques like electrochemical and ultrasonic polishing help achieve smoother surfaces and better durability.
Overall, proper material selection, surface treatment, and process control are essential for successful deep drawing of stainless steel.
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