One of the main reasons for adding nickel to stainless steel is to form an austenitic crystal structure to improve properties of stainless steel such as plasticity, weldability and toughness, so nickel is called an austenitic forming element. The crystal structure of ordinary carbon steel is called ferrite, which is body centered cubic (BCC) structure. The addition of nickel causes the crystal structure to change from body centered cubic (BCC) structure to face centered cubic (FCC) structure, which is called austenite.
However, nickel is not the only element with this property. Common austenite forming elements are nickel, carbon, nitrogen, manganese, copper. The relative importance of these elements in the formation of austenite is important for predicting the crystal structure of stainless steel. At present, many formulas have been developed to describe the relative importance of austenite forming elements, the most famous of which is the following formula: Austenite forming capacity =Ni%+30C%+30N%+0.5Mn%+0.25Cu%.
It can be seen from this equation that carbon is a strong austenite forming element, with 30 times more austenite forming ability than nickel, but it cannot be added to corrosion resistant stainless steel because it will cause sensitization corrosion and subsequent intergranular corrosion problems after welding. Nitrogen is also 30 times more capable of forming austenite than nickel, but it is a gas and only a limited amount of nitrogen can be added to stainless steel without causing porosity problems. The addition of manganese and copper can cause fire resistance loss and welding problems during steelmaking.
As can be seen from the nickel equation, the addition of manganese is not very effective for austenite formation, but it causes more nitrogen to dissolve into the stainless steel, and nitrogen is a very strong austenite formation element. In the 200 series stainless steel, it is precisely 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 needed to be added. For example, in type 201 stainless steel, it contains only 4.5% nickel and 0.25% nitrogen. The nickel equation shows that the nitrogen is equivalent to 7.5% nickel in the ability to form austenite, so it can also form 100% austenite structure. This is also how the 200 series stainless steel is formed. In some of the 200 series stainless steel does not meet the standard, due to cannot add sufficient amount of manganese and nitrogen, in order to form 100% austenite structure, artificially reduced the amount of chromium, which inevitably leads to the decline of stainless steel corrosion resistance.
In stainless steel, two opposing forces operate 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 additive elements. Chromium is a ferrite forming element, so chromium in the formation of stainless steel crystal structure and the austenite forming element is a competitive relationship. Because iron and chromium are ferritic forming elements, the 400 series stainless steels are fully ferritic stainless steels with magnetic properties. In the process of adding the austenite forming element nickel to fe-Cr stainless steel, with the increase of nickel composition, the formation of austenite will gradually increase, until all the ferrite structure is transformed into austenite structure, thus forming 300 series stainless steel. If only half the amount of nickel is added, it forms 50% ferrite and 50% austenite, a structure known as duplex stainless steel. 400 series stainless steel is a kind of iron, carbon and chromium alloy. This stainless steel has a martensitic structure and iron, and thus has normal magnetic properties. 400 series stainless steel has a strong resistance to high temperature oxidation, and compared with carbon steel, its physical and mechanical properties are further improved. Most 400 series stainless steels can be heat treated.
300 series stainless steel is an alloy material containing iron, carbon, nickel and chromium. It is a non-magnetic stainless steel material with better malleability than 400 series stainless steel. Due to the austenitic structure of 300 series stainless steel, it has strong corrosion resistance in many environments, excellent resistance to fracture caused by corrosion caused by metal overstress, and its material properties are not affected by thermal treatment.
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