Nickel Knowledge: The Role of Nickel in Stainless Steel
The role of nickel in stainless steel comes into play after it is combined with chromium.
Nickel is an excellent corrosion-resistant material and an important alloying element for alloy steel. Nickel is an element that forms austenite in steel, but for low-carbon nickel steel to obtain a pure austenite structure, the nickel content must reach 24%; and only when the nickel content is 27% can the steel be resistant to certain media. The corrosion performance changes significantly. Therefore, nickel cannot constitute stainless steel alone. But when nickel and chromium exist in stainless steel at the same time, nickel-containing stainless steel has many valuable properties.
Based on the above situation, it can be seen that the role of nickel as an alloying element in stainless steel is that it changes the structure of high chromium steel so that the corrosion resistance and process performance of stainless steel can be improved to some extent.
Manganese and nitrogen can replace nickel in chromium-nickel stainless steel.
Although there are many advantages of chromium-nickel austenitic steel, in recent decades, due to the large-scale development and application of nickel-based heat-resistant alloys and heat-strength steels containing less than 20% nickel, and the increasing development of the chemical industry, the demand for stainless steel has increased. The larger the size, the smaller the nickel deposits and are concentrated in a few areas, so there is a contradiction between the supply and demand of nickel in the world.
Therefore, in the fields of stainless steel and many other alloys (such as steel for large castings and forgings, tool steel, heat-strength steel, etc.), especially in countries where nickel resources are relatively scarce, the science of saving nickel and replacing nickel with other elements have been extensively carried out. In research and production practice, there are more researches and applications in this area that replace nickel in stainless steel and heat-resistant steel with manganese and nitrogen.
The effect of manganese on austenite is similar to that of nickel. But to be more precise, the role of manganese is not to form austenite, but to reduce the critical quenching rate of steel, increase the stability of austenite during cooling, inhibit the decomposition of austenite, and make it form at high temperatures. The austenite can be maintained to room temperature. In improving the corrosion resistance of steel, manganese has little effect. For example, the manganese content in steel changes from 0 to 10.4%, and it does not significantly change the corrosion resistance of steel in air and acid.
This is because manganese has little effect on increasing the electrode potential of iron-based solid solution, and the protective effect of the formed oxide film is also very low, so although there are austenitic steels alloyed with manganese (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steel) Etc.), but they cannot be used as stainless steel.
The role of manganese in stabilizing austenite in steel is about one-half that of nickel, that is, the role of 2% nitrogen in steel is also stabilizing austenite, and the effect is greater than that of nickel. For example, in order to obtain an austenitic structure in steel containing 18% chromium at room temperature, low-nickel stainless steel with manganese and nitrogen instead of nickel and nickel-free chromium-manganese-nitrogen stainless steel has been applied in the industry at present, and some It has successfully replaced the classic 18-8 chromium-nickel stainless steel.
Titanium or niobium is added to stainless steel to prevent intergranular corrosion.
Molybdenum and copper can improve the corrosion resistance of certain stainless steel.
The influence of other elements on the performance and structure of stainless steel.
The above-mentioned main elements have an influence on the performance and structure of stainless steel. In addition to the elements that have a greater influence on the performance and structure of stainless steel, stainless steel also contains some other elements. Some are the same as common steel as impurities, such as silicon, sulfur, phosphorus, etc., and some are added for specific purposes, such as cobalt, boron, selenium, and rare earth elements. In terms of the main nature of the corrosion resistance of stainless steel, these elements are not the main aspects compared to the several elements that have been discussed. Even so, they cannot be completely ignored because they also affect the performance and structure of stainless steel. Influence.