Practical knowledge of stainless steel

1-3. The role of nickel in stainless steel is only played 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, 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.

1-4. Manganese and nitrogen can replace nickel in chromium-nickel stainless steel

Although the advantages of chromium-nickel austenitic steel are many, 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 the concentrated distribution in a few areas, so there is a contradiction between nickel supply and demand 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 has been widely 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. 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 role is greater than that of nickel. For example, in order to obtain austenitic structure of 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 have been used in industry. Some It has successfully replaced the classic 18-8 chromium-nickel stainless steel.

1-5. Titanium or niobium is added to stainless steel to prevent intergranular corrosion.

1-6. Molybdenum and copper can improve the corrosion resistance of certain stainless steels.

1-7. The influence of other elements on the performance and organization of stainless steel

The above nine main elements affect the performance and structure of stainless steel. In addition to the elements that have a greater impact on the performance and structure of stainless steel, stainless steel also contains some other elements. Some are the same as general steel as impurities, such as silicon, sulfur, and phosphorus. Others 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 non-essential compared to the nine elements discussed. Even so, they cannot be completely ignored because they also affect the performance and structure of stainless steel. influences.

Silicon is an element that forms ferrite, and it is an impurity element often present in general stainless steel.

As an alloying element, cobalt is not widely used in steel. This is because the price of cobalt is high and it is more important in other aspects (such as high-speed steel, hard alloy, cobalt-based heat-resistant alloy, magnetic steel or hard magnetic alloy, etc.) use. There are not many common stainless steels that add cobalt as an alloying element. Commonly used stainless steels such as 9Crl7MoVCo steel (containing 1.2-1.8% cobalt) add cobalt. The purpose is not to improve the corrosion resistance but to increase the hardness, because the main purpose of this stainless steel It is the manufacture of slicing machine cutting tools, scissors and surgical blades.