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Alloy steel and stainless steel


Alloy steel is made of stainless steel with two or more substances (at least one of which is metal). It has the brightness close to the mirror, strong and cold touch. It is a relatively avant-garde material, which is in line with the cool aesthetic of the metal age. Alloy steel is still different from pure stainless steel. Today we will learn more about alloy steel and stainless steel.

What is alloy steel?

Alloy steel is called alloy steel by adding other alloy elements in addition to iron and carbon. An iron carbon alloy formed by adding an appropriate amount of one or more alloy elements on the basis of ordinary carbon steel. According to different added elements and appropriate processing technology, special properties such as high strength, high toughness, wear resistance, corrosion resistance, low temperature resistance, high temperature resistance and non magnetism can be obtained.

Different elements have different characteristics due to their different proportions in steel

1. Carbon (c): with the increase of carbon content in steel, the yield point and tensile strength increase, but the plasticity and impact reduce. When the carbon content exceeds 0.23%, the welding performance of steel deteriorates. Therefore, the carbon content of low alloy structural steel used for welding generally does not exceed 0.20%. High carbon content will also reduce the atmospheric corrosion resistance of steel, and high carbon steel in open stockyard is easy to rust; In addition, carbon can increase the cold brittleness and aging sensitivity of steel.

2. Silicon (SI): silicon is added as reducing agent and deoxidizer during steelmaking, so killed steel contains 0.15-0.30% silicon. If the silicon content in the steel exceeds 0.50-0.60%, silicon is regarded as an alloy element. Silicon can significantly improve the elastic limit, yield point and tensile strength of steel, so it is widely used as spring steel. The strength of quenched and tempered structural steel can be increased by 15-20% by adding 1.0-1.2% silicon. The combination of silicon and molybdenum, tungsten and chromium can improve corrosion resistance and oxidation resistance, and can make heat-resistant steel. Low carbon steel containing 1-4% silicon has very high permeability. It is used to make silicon steel sheets in electrical industry. The increase of silicon content will reduce the weldability of steel.

3. Manganese (MN): in the steelmaking process, manganese is a good deoxidizer and desulfurizer. Generally, the manganese content in steel is 0.30-0.50%. When more than 0.70% is added to carbon steel, it is called "manganese steel". Compared with ordinary steel, it not only has sufficient toughness, but also has higher strength and hardness, so as to improve the hardenability and hot workability of steel. For example, 16Mn steel is 40% higher than A3 yield point. Steel containing 11-14% manganese has high wear resistance and is used for excavator bucket, ball mill lining plate, etc. The increase of manganese content weakens the corrosion resistance of steel and reduces the welding performance.

4. Phosphorus (P): in general, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates the welding performance, reduces the plasticity and deteriorates the cold bending performance. Therefore, the phosphorus content in steel is usually required to be less than 0.045%, and the requirements for high-quality steel are lower.

5. Sulfur (s): sulfur is also a harmful element under normal circumstances. Make the steel produce thermal brittleness, reduce the ductility and toughness of the steel, and cause cracks during forging and rolling. Sulfur is also unfavorable to welding performance and reduces corrosion resistance. Therefore, the sulfur content is usually required to be less than 0.055%, and the high-quality steel is required to be less than 0.040%. Adding 0.08-0.20% sulfur to steel can improve machinability, which is usually called free cutting steel.

6. Chromium (CR): in structural steel and tool steel, chromium can significantly improve strength, hardness and wear resistance, but reduce plasticity and toughness at the same time. Chromium can also improve the oxidation resistance and corrosion resistance of steel, so it is an important alloy element of stainless steel and heat-resistant steel.

7. Nickel (Ni): nickel can improve the strength of steel while maintaining good plasticity and toughness. Nickel has high corrosion resistance to acid and alkali, rust prevention and heat resistance at high temperature. However, nickel is a scarce resource, so other alloy elements should be used instead of nickel chromium steel.

8. Molybdenum (MO): molybdenum can refine the grain of steel, improve hardenability and thermal strength, and maintain sufficient strength and creep resistance at high temperature (long-term stress and deformation at high temperature, called creep). Adding molybdenum to structural steel can improve mechanical properties. It can also inhibit the brittleness of alloy steel caused by quenching. The redness can be improved in tool steel.

9. Titanium (TI): titanium is a strong deoxidizer in steel. It can compact the internal structure of steel and refine the grain force; Reduce aging sensitivity and cold brittleness. Improve welding performance. Intergranular corrosion can be avoided by adding appropriate titanium to chromium 18 nickel 9 austenitic stainless steel.

10. Vanadium (V): vanadium is an excellent deoxidizer for steel. Adding 0.5% vanadium to the steel can refine the grain structure and improve the strength and toughness. The carbide formed by vanadium and carbon can improve the hydrogen corrosion resistance under high temperature and high pressure.

11. Tungsten (W): tungsten has a high melting point and a large ratio. It is a precious alloy element. Tungsten and carbon form tungsten carbide, which has high hardness and wear resistance. Adding tungsten to tool steel can significantly improve the red hardness and thermal strength, and can be used as cutting tools and forging dies.

12. Niobium (NB): niobium can refine grains, reduce overheating sensitivity and tempering brittleness of steel, improve strength, but reduce plasticity and toughness. Adding niobium to ordinary low alloy steel can improve the resistance to atmospheric corrosion and hydrogen, nitrogen and ammonia corrosion at high temperature. Niobium can improve welding performance. Adding niobium to austenitic stainless steel can prevent intergranular corrosion.

13. Cobalt (CO): cobalt is a rare precious metal, which is mostly used in special steels and alloys, such as thermal strength steels and magnetic materials.

14. Copper (Cu): steel made from Daye ore in WISCO, which often contains copper. Copper can improve strength and toughness, especially atmospheric corrosion. The disadvantage is that it is easy to produce thermal embrittlement during hot processing, and the plasticity is significantly reduced when the copper content exceeds 0.5%. When the copper content is less than 0.50%, it has no effect on weldability.

15. Aluminum (AL): aluminum is a common deoxidizer in steel. Adding a small amount of aluminum to steel can refine grain and improve impact toughness, such as 08Al steel for deep drawing sheet. Aluminum also has oxidation resistance and corrosion resistance. The combination of aluminum with chromium and silicon can significantly improve the high-temperature peeling resistance and high-temperature corrosion resistance of steel. The disadvantage of aluminum is that it affects the hot working performance, welding performance and cutting performance of steel.

16. Boron (b): adding a small amount of boron to the steel can improve the compactness, hot rolling performance and strength of the steel.

17. Nitrogen (n): nitrogen can improve the strength, low temperature toughness and weldability of steel and increase the aging sensitivity.

18. Rare earth (XT): rare earth elements refer to 15 lanthanide elements with atomic numbers of 57-71 in the periodic table of elements. These elements are all metals, but their oxides are very like "Earth", so they are traditionally called rare earth. Adding rare earth into steel can change the composition, morphology, distribution and properties of inclusions in steel, so as to improve various properties of steel, such as toughness, weldability and cold working properties. Adding rare earth into plowshare steel can improve wear resistance.

What is the difference between alloy and stainless steel?

The biggest difference between alloy and stainless steel is their hardness, price and surface treatment; The hardness of the alloy is lower than that of stainless steel, the price of the alloy is cheaper than that of stainless steel, and the surface treatment methods of the alloy are richer than that of stainless steel.

1. The hardness of alloy is lower than that of stainless steel: for example, the hardness of 6061-T6 alloy is 47hrb and that of 304 stainless steel is 92hrb. Due to the high hardness of stainless steel, it is usually used to make anti-theft doors, and the alloy plays a beautiful role.

2. The price of alloy is cheaper than that of stainless steel: the material of alloy is mainly aluminum with a small amount of manganese and other metals, and the main material of stainless steel is iron; The supplier's quotation for 6061 alloy is about 18rmb / kg, and that for 304 stainless steel is about 28-30rmb / kg; In addition, the density of alloy is about 2700kg / mm ^ 3, while that of stainless steel is 7900kg / mm ^ 3. Obviously, the price of stainless steel is much more expensive than that of alloy.

3. The surface treatment methods of alloy are richer than that of stainless steel: the surface treatment of alloy includes spraying, electrophoresis, anodizing, etc., while the surface treatment of stainless steel generally only has the surface natural color whitening treatment, so the alloy can play a more beautiful role more often.

4. The surface treatment of the alloy is not good, and oxidation and spots will appear after use for a period of time. The surface treatment of stainless steel is relatively single, so the stability is relatively good, but rust will also occur in the later maintenance.

Conclusion: the construction cost of stainless steel is higher than that of alloy. Therefore, stainless steel is basically used for those who are not short of money or government public works. Alloy is selected for those who pay more attention to beauty in the owner's home.

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