The role of nickel in stainless steel is only played after it is combined with chromium.
1. Nickel as an Alloying Element in Stainless Steel
Nickel is an excellent corrosion-resistant material and a critical alloying element in alloy steel. As an austenite-forming element in steel, nickel needs to reach a content of 24% to obtain a purely austenitic structure in low-carbon nickel steel. It is only when the nickel content reaches 27% that the steel's resistance to corrosion in certain media changes significantly. Therefore, nickel alone cannot constitute stainless steel. However, when nickel and chromium coexist in stainless steel, the nickel-containing stainless steel exhibits many valuable properties.
From this, it is clear that the role of nickel as an alloying element in stainless steel is to alter the structure of high-chromium steel, thereby enhancing the corrosion resistance and processability of stainless steel.
2. Manganese and Nitrogen as Substitutes for Nickel in Chromium-Nickel Stainless Steel
Although chromium-nickel austenitic steel has many advantages, recent decades have seen a large-scale development and application of nickel-based heat-resistant alloys and heat-strength steels containing less than 20% nickel. Additionally, the growing development of the chemical industry has increased the demand for stainless steel. However, nickel's mineral reserves are limited and concentrated in a few regions, leading to a global supply-demand imbalance. Consequently, the science of conserving nickel and substituting it with other elements has become a focus of research and production, particularly in countries with scarce nickel resources. In this context, manganese and nitrogen are used to replace nickel in stainless steel and heat-resistant steel.
3. Manganese has a similar effect on austenite as nickel
but with some distinctions. Specifically, manganese does not form austenite; instead, it reduces the critical quenching speed of steel, increases austenite stability during cooling, inhibits austenite decomposition, and helps maintain austenitic structure at room temperature. Manganese has a minimal effect on enhancing steel's corrosion resistance. For instance, increasing the manganese content in steel from 0 to 10.4% does not significantly change its corrosion resistance in air or acidic environments. This is because manganese has a negligible impact on raising the electrode potential of iron-based solid solutions, and the protective effect of the oxide film formed is also quite low. Consequently, although austenitic steels alloyed with manganese exist in the industry (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steel, etc.), they are not used as stainless steel.
Manganese's role in stabilizing austenite in steel is about half that of nickel. In comparison, 2% nitrogen can also stabilize austenite in steel, and its effect is even greater than that of nickel. For instance, to obtain an austenitic structure at room temperature in steel containing 18% chromium, low-nickel stainless steel with manganese and nitrogen substituting for nickel, as well as chromium-manganese-nitrogen stainless steel with nickel, are now used in the industry. In some cases, these alternatives have successfully replaced the classic 18-8 chromium-nickel stainless steel.
