Introduction to Stainless Steel
All metals react with oxygen in the atmosphere and form an oxide film on their surface. Iron oxide, formed in common carbon steel, continues to oxidize and eventually causes corrosion holes. Carbon steel can be protected by electroplating with paint or oxidation-resistant metals such as zinc, nickel and chromium, but this protection is only temporary and can be easily damaged.
Stainless steel is resistant to weak corrosive media such as air, steam and water, as well as chemical corrosive media such as acid, alkali and salt. The term “stainless steel” is often used to refer to steel that is resistant to weak corrosion, while “acid-resistant steel” is used to describe steel that is resistant to chemical corrosion.
The distinction between the two is due to differences in chemical composition. Not all stainless steel is resistant to chemical corrosion, while acid-resistant steel is generally resistant to corrosion. The corrosion resistance of stainless steel depends on the alloying elements it contains, with chromium being the key element in achieving corrosion resistance.
When the chromium content in steel reaches about 1.2%, the chromium reacts with oxygen in the corrosive environment to form a thin oxide film on the surface of the steel, preventing further corrosion. Other commonly used alloying elements include nickel, molybdenum, titanium, niobium, copper and nitrogen, to meet the diverse needs and requirements of stainless steel in terms of microstructure and properties.
Stainless steel is generally divided into:
Ferritic Stainless Steel
Ferritic stainless steel contains between 12% and 30% chromium. Its corrosion resistance, toughness and weldability improve as the chromium content increases, and it has better resistance to chloride stress corrosion cracking than other types of stainless steel.
Austenitic Stainless Steel
Austenitic stainless steel has a chromium content greater than 18%, along with 8% nickel and small amounts of molybdenum, titanium, nitrogen and other elements. It has excellent comprehensive performance and can resist corrosion from various media.
Austenitic-Ferritic Duplex Stainless Steel
Austenitic-ferritic duplex stainless steel combines the benefits of austenitic and ferritic stainless steel and has superior ductility.
Martensitic Stainless Steel
Martensitic stainless steel has high strength but low plasticity and weldability.
Stainless Steel Grade Grouping
Precipitation-hardened stainless steel features good formability and weldability, making it a popular choice for ultra-high-strength applications in the nuclear, aviation and aerospace industries.
Based on its composition, precipitation hardening stainless steel can be classified into four categories: Cr system (SUS400), Cr-Ni system (SUS300), Cr-Mn-Ni (SUS200) and precipitation hardening system (SUS600) .
200 Series: Chromium-nickel-manganese austenitic stainless steel.
300 Series: Chromium-nickel austenitic stainless steel.
301: Known for its good ductility, this type of stainless steel is often used for molding products and can be hardened by machine speed. It has excellent weldability and better wear resistance and fatigue resistance compared with 304 stainless steel.
302: It has the same corrosion resistance as 304, but with a higher carbon content, which results in greater resistance.
303: Contains a small amount of sulfur and phosphorus, facilitating cutting.
304: Also known as 18/8 stainless steel and 0Cr18Ni9 in GB brand.
309: Offers better temperature resistance than 304.
316: The second most used stainless steel after 304, it is mainly used in the food industry and in surgical equipment. The addition of molybdenum gives it a special corrosion-resistant structure and improved resistance to chloride corrosion, making it a popular choice for “marine steel” applications.
SS316 is commonly used in nuclear fuel recovery units and is usually specified at grade 18/10.
321 Series – Similar to 304, except the addition of titanium reduces the risk of weld corrosion.
400 Series – Ferritic and martensitic stainless steels.
408 – Good heat resistance, but poor corrosion resistance. Composition: 11% chromium and 8% nickel.
409 – Cheapest model (British and American) used mainly as car exhaust. Ferritic stainless steel (chromium steel).
410 – Martensitic (high-strength chrome steel) with good wear resistance, but low corrosion resistance.
416 – The addition of sulfur improves the processability of the material.
420 – “Cutting tool grade” martensitic steel similar to early stainless steels such as Brinell high chromium steel. Also used for surgical tools which can become very shiny.
430 – Ferritic stainless steel used for decoration, such as automotive accessories. Good formability, but low temperature resistance and corrosion resistance.
440 – High-strength cutting tool steel with slightly higher carbon content. It can obtain higher yield strength with proper heat treatment and the hardness can reach 58 HRC. One of the hardest stainless steels. Commonly used in razor blades. Three common models: 440A, 440B, 440C and 440F (easy to process).
500 Series – Heat resistant chrome alloy steel.
600 Series – Precipitation hardenable martensitic stainless steel.
630 – The most commonly used precipitation hardening stainless steel model, also known as 17-4. Composition: 17% chromium and 4% nickel.
Characteristics and uses of stainless steel:
- Austenitic nickel steel 1Cr17Mn6Ni5N is magnetic after cold processing and is used for railway vehicles. It is an alternative to 1Cr17Ni7.
- Nickel steel 1Cr18Mn8Ni5N is an alternative to 1Cr18Ni9.
- 1Cr17Ni7 has high strength after cold processing and is used in railway vehicles, conveyor belts, screws and nuts.
- 1Cr18Ni9 has high strength after cold processing, but its elongation is slightly worse than 1Cr17Ni7. It is used for decorative components in construction.
- Y1Cr18Ni9 improves cutting resistance and abrasion resistance and is more suitable for use in automatic lathes. It is used for screws and nuts.
- Y1Cr18Ni9Se improves cutting resistance and abrasion resistance and is more suitable for use in automatic lathes. It is used for rivets and screws.
- 0Cr19Ni9 is a heat-resistant stainless steel widely used for food equipment, general chemical equipment and atomic energy industry.
- 00Cr19Ni11 steel has lower carbon content than 0Cr19Ni9 and has superior intergranular corrosion resistance. It is used as a component without heat treatment after welding.
- 0Cr19Ni9N is a reinforced version of 0Cr19Ni9, with improved strength and unchanged plasticity. It is used to reduce the thickness of the material as a component of structural strength.
- 0Cr19Ni10Nb has the same characteristics and uses as 0Cr19Ni9N, with the addition of N and Nb.
- 00Cr18Ni10N is a reinforced version of 00Cr19Ni11, with higher strength and unchanged plasticity and better resistance to intergranular corrosion.
- Hardenability is used for spinning, special drawing and cold heading.
- 0Cr23Ni13 has better corrosion resistance and heat resistance than 0Cr19Ni9.
- 0Cr25Ni20 has better oxidation resistance than 0Cr23Ni13 and is mainly used as heat-resistant steel.
- 0Cr17Ni12Mo2 has better corrosion resistance than 0Cr19Ni9 in seawater and other media, and is mainly used as a pitting corrosion-resistant material.
- 0Cr18Ni12Mo2Ti is used in equipment resistant to sulfuric acid, phosphoric acid, formic acid and acetic acid and has good resistance to intergranular corrosion.
- 00Cr17Ni14Mo2 is an ultra-low carbon version of 0Cr17Ni12Mo2 and has better resistance to intergranular corrosion than 0Cr17Ni12Mo2.
- 0Cr17Ni12Mo2N is a reinforced version of 0Cr17Ni12Mo2, with greater strength and unchanged plasticity and reduced material thickness.
aluminum alloy
The general term for aluminum-based alloys refers to a group of materials composed primarily of aluminum and other elements such as copper, silicon, magnesium, zinc and manganese. Secondary alloy elements, including nickel, iron, titanium, chromium, and lithium, are also present in smaller amounts.
Aluminum alloys are known for their low density and high strength, which is comparable to or even greater than that of high-quality steel. In addition, aluminum alloys have good plasticity and can be easily molded into various shapes. These alloys also have excellent electrical and thermal conductivity, as well as superior corrosion resistance.
Due to its versatility and durability, aluminum alloy is widely used in various industries, second only to steel in popularity. The use of aluminum alloy can be divided into two categories: cast aluminum alloy and deformed aluminum alloy. Cast aluminum alloys are used as is, while deformed aluminum alloys can withstand pressure processing, resulting in higher mechanical properties.
Forged aluminum alloys are divided into two subcategories: non-heat treatable reinforced aluminum alloys and heat treatable reinforced aluminum alloys. Non-heat treatable alloys such as high-purity aluminum, high-purity industrial aluminum, pure industrial aluminum and anti-rust aluminum can only be strengthened through cold working deformation and not through heat treatment. Heat-treatable reinforced aluminum alloys, on the other hand, can be improved through heat treatment methods such as quenching and aging, and are divided into hard aluminum, wrought aluminum, super-hard aluminum and special aluminum alloy.
Cast aluminum alloys can be classified based on their chemical composition, including aluminum-silicon alloy, aluminum-copper alloy, aluminum-magnesium alloy, aluminum-zinc alloy and rare earth aluminum alloy. Within the aluminum-silicon alloy category, there are two subtypes: simple aluminum-silicon alloy and special aluminum-silicon alloy. The former has low mechanical properties but good casting properties, while the latter can be strengthened by heat treatment and has high mechanical properties and good casting properties.
As an example of its widespread use, the “Xiangyun” torch used at the 2008 Beijing Olympic Games was made of aluminum alloy.
