The main role of nickel in stainless steel is that it changes the crystal structure of steel. One of the main reasons for adding nickel to stainless steel is to form austenite crystal structure, thereby improving the properties of stainless steel such as plasticity, weldability and toughness, so nickel is called austenite forming element. The crystal structure of ordinary carbon steel is called ferrite, which has a body-centered cubic (BCC)structure. Nickel is added to promote the transformation of the crystal structure from a body-centered cubic (BCC)structure to a face-centered cubic (FCC) structure. This structure is called It is austenite. However, nickel is not the only element with this property. Common austenite forming elements are: nickel, carbon, nitrogen, manganese, and copper. The relative importance of these elements in the formation of austenite is of great significance for predicting the crystal structure of stainless steel.

At present, people have developed many formulas to express the relative importance of austenite forming elements, the most famous one is the following formula:  
Austenite forming ability =Ni%+30C%+30N%+0.5Mn%+0.25Cu%

It can be seen from this equation that carbon is a strong austenite-forming element, its ability to form austenite is 30 times that of nickel , but it cannot be added to corrosion-resistant stainless steel, because after welding It will cause sensitized corrosion and subsequent intergranular corrosion problems. The ability of nitrogen to form austenite is also 30 times that of nickel , but it is a gas. If you want to avoid porosity problems, you can only add a limited amount of nitrogen to stainless steel. The addition of manganese and copper will cause reduction in refractory life and welding problems in the steelmaking process.

It can be seen from the nickel equation that adding manganese is not very effective for forming austenite, but adding manganese can dissolve more nitrogen into stainless steel, and nitrogen is a very strong austenite forming element. In 200 series stainless steels, it is precisely with enough manganese and nitrogen to replace nickel to form a 100% austenitic structure. The lower the nickel content, the higher the amount of manganese and nitrogen that needs to be added. For example, in type 201 stainless steel, it only contains 4.5% nickel and 0.25% nitrogen. It can be seen from the nickel equation that these nitrogens are equivalent to 7.5% nickel in the ability to form austenite, so it can also form a 100% austenite structure. This is also the formation principle of 200 series stainless steel. In some 200 series stainless steels that do not meet the standard , because sufficient amounts of manganese and nitrogen cannot be added, in order to form a 100% austenite structure, the amount of chromium added is artificially reduced, which will inevitably lead to a decline in the corrosion resistance of stainless steel. .

In stainless steel, there are two opposite forces acting simultaneously: ferrite-forming elements continue to form ferrite, and austenite-forming elements continue to form austenite. The final crystal structure depends on the relative amounts of the two types of added elements. Chromium is a ferrite-forming element, so chromium is in a competitive relationship with austenite-forming elements in the formation of stainless steel crystal structure.Because iron and chromium are both ferrite-forming elements, 400 series stainless steels are completely ferritic stainless steels with magnetic properties. In the process of adding austenite-forming element - nickel to the iron -chromium stainless steel, as the nickel content increases, the austenite formed will gradually increase until all the ferrite structure is transformed into the austenite structure , Thus formed 300 series stainless steel. If only half the amount of nickel is added, 50% ferrite and 50% austenite will be formed . This structure is called duplex stainless steel.

400 series stainless steel is an alloy of iron and carbon and chromium. This stainless steel has a martensitic structure and iron elements, so it has normal magnetic properties. 400 series stainless steel has strong resistance to high temperature oxidation, and compared with carbon steel, its physical and mechanical properties have been further improved. Most 400 series stainless steels can be heat treated.

The 300 series stainless steel is an alloy material containing iron, carbon, nickel and chromium. It is a non-magnetic stainless steel material with better forgeability than the 400 series stainless steel. Due to the austenitic structure of 300 series stainless steel, it has strong corrosion resistance in many environments, and has good resistance to fracture caused by corrosion caused by metal super stress, and its material characteristics are not affected by heat treatment. influences.