The main role of nickel in stainless steel is that it changes the crystal structure of the steel. One of the main reasons for the addition of nickel to stainless steel is the formation of an austenitic crystal structure, which improves stainless steel properties such as ductility, weldability and toughness, so nickel is known as an austenite former. The crystal structure of ordinary carbon steel is called ferrite, which is a body-centered cubic (BCC) structure, and nickel is added to promote the crystal structure from a body-centered cubic (BCC) structure to a face-centered cubic (FCC) structure, which is called for austenite. However, nickel is not an uncommon element with such properties. Common austenite-forming elements are: nickel, carbon, nitrogen, manganese, and copper. The relative importance of these elements in forming austenite has important implications for predicting the crystal structure of stainless steels. At present, many formulas have been developed 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 with 30 times the ability to form austenite than nickel, but it cannot be added to corrosion-resistant stainless steel because after welding it Can cause sensitized corrosion and subsequent intergranular corrosion problems. Nitrogen is also 30 times more capable of forming austenite than nickel, but it is a gas and only limited amounts of nitrogen can be added to stainless steel without causing porosity problems. The addition of manganese and copper can cause problems with reduced refractory life and welding in the steelmaking process.
As you can see from the nickel equation, the addition of manganese is not very effective in forming austenite, but the addition of manganese allows more nitrogen, which is a very strong austenite former, to dissolve into the stainless steel. In 200 series stainless steel, it is 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, Type 201 stainless steel contains only 4.5% nickel and 0.25% nitrogen. From the nickel equation, these nitrogens are equivalent to 7.5% nickel in their ability to form austenite, so a 100% austenite structure can also be formed. This is also the formation principle of 200 series stainless steel. In some 200 series stainless steels that do not meet the standard, due to the inability to add sufficient amounts of manganese and nitrogen, in order to form a 100% austenitic 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, two opposing forces act simultaneously: ferrite-forming elements keep forming ferrite, and austenite-forming elements keep forming 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 the stainless steel crystal structure. Because iron and chromium are both ferrite-forming elements, 400 series stainless steels are fully ferritic stainless steels that are magnetic. In the process of adding the 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 austenite structure , thus forming 300 series stainless steel. If only half the amount of nickel is added, 50% ferrite and 50% austenite are formed, a structure known as duplex stainless steel.
400 series stainless steel is an iron, carbon-chromium alloy. This stainless steel has a martensitic structure and iron, 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 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, a non-magnetic stainless steel material, which has better malleable properties than 400 series stainless steel. Due to the austenitic structure of the 300 series stainless steel, it has strong corrosion resistance in many environments, good resistance to fracture caused by corrosion caused by metal overstress, and its material properties are not affected by heat treatment. Influence.