Adding nickel in stainless steel will form austenitic crystal structure, can improve the plasticity of stainless steel, toughness, weldability and other properties, so the nickel is called austenite forming elements. Carbon steel structure is ferrite, the body-centered cubic (BCC) structure, by adding nickel can promote the crystal structure from the body-centered cubic (BCC) structure into a face-centered cubic (FCC) structure, known as the Austrian Body stainless steel.
However, this nature of nickel is not unique. Austenite formation of the more common elements are: nickel, nitrogen, manganese, carbon, copper. Now people have a lot of formulas to express the relative importance of austenite forming elements, the following is the most famous formula:
Austenite forming ability = Ni% + 30C% + 30N% + 0.5Mn% + 0.25Cu%
From this formula, it is seen that carbon is 30 times more potent for forming austenite than nickel, but it can not be added to corrosion-resistant stainless steels because after soldering this leads to sensitized corrosion and subsequent intergranular corrosion problem. The effect of nitrogen on the formation of austenite is also 30 times the nickel, but it is a gas, adding too much nitrogen will cause porous problems. In addition, the addition of manganese and copper can lead to problems of refractory life reduction and welding during the steelmaking process.
Although the addition of manganese is not conducive to the formation of austenite, but the addition of manganese can help more nitrogen dissolved in stainless steel, and nitrogen is very conducive to the formation of austenite. In the 200 series stainless steel, the amount of manganese and nitrogen is substituted for the nickel element to form a 100% austenitic structure, and the lower the nickel content is, the higher the amount of manganese and nitrogen is. For example, in 201 stainless steel, containing only 4.5% nickel, but also contains 0.25% nitrogen. By the above formula, it can be seen that the nitrogen element is equivalent to 7.5% of nickel in the ability to form austenite, and therefore can also form a 100% austenitic structure. 200 series of stainless steel is the resulting Health. Part of the non-compliance 200 series stainless steel, due to the inability to add a sufficient amount of manganese and nitrogen, but for 100% of the austenitic structure, by deliberately reducing the amount of chromium added, which led to the stainless steel corrosion resistance.
In stainless steel, ferrite-forming elements continue to form ferrite, austenite-forming elements continue to form austenite. And the final crystal structure depends on the relative amount of the two elements added. Where chromium is a ferrite forming element, so in the formation of stainless steel crystal structure of chromium and austenite formation element is competitive. Since both iron and chromium are ferrite-forming elements, the 400-series stainless steel is completely ferritic stainless steel and is magnetic. In the process of adding the austenite forming element-nickel to the iron-chrome stainless steel, the austenite formed gradually increases as the nickel element increases until all the ferrite structures are transformed into austenite Structure, and ultimately the formation of 300 series stainless steel. However, if only half of the amount of nickel is added, 50% of the ferrite and 50% of the austenite will be formed, that is, duplex stainless steel.
400 series stainless steel is an alloy of iron, carbon and chromium. It is a martensite structure and an iron element with normal magnetic properties. And also has a strong resistance to high temperature oxidation, but compared with carbon steel, 400 series stainless steel physical properties and mechanical properties have improved space. 400 series stainless steel heat treatment can be done.
300 series stainless steel contains iron, carbon, nickel and chromium and other elements, there is no magnetic, malleable features better than the 400 series of stainless steel. Because of the austenitic structure of the 300 series stainless steel, its corrosion resistance is strong in many environments and the fracture properties caused by the corrosion resistance caused by the metal over stress are also good, and the material properties of the 300 series stainless steel are not subjected to heat treatment Impact.
Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)
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