The role of various elements in the stainless steel

image_pdfimage_print

At present, there are more than 100 varieties of the known chemical elements, and the chemical elements can be encountered in commonly used steel materials is about more than 20 varieties in industrial. There are a dozen types of the most common elements in the stainless steel, except iron which is the essential element of forming steel, the maximum impact on the properties and microstructure of stainless steel elements are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, etc.
The decisive role of chromium in stainless steel: chrome is the only element which decision the properties of stainless steel. Each stainless steel contains a certain amount of chromium. So far, there is no stainless steel without chromium. Why the chromium becomes the key element to decide stainless steel’s performance? The fundamental reason is that after adding chromium as an alloying element in steel, prompting its internal contradictions to the benefit of the development of resistance to corrosion damage aspect.
Carbon is one of the major elements of industrial steel, steel performance and organization largely depends on the content and distribution of carbon in steel. The impacts on the organization of carbon in stainless steel is mainly manifested in two aspects: on the one hand, carbon is stable austenite elements, and the extent of the role is large (about 30 times than nickel); on the other hand, due to the big affinity of carbon and chromium, it will form a series of complex carbide with chromium. So, judging from both strength and corrosion-resistant candle, the role of carbon in stainless steel are mutually contradictory.
Nickel is an excellent corrosion-resistant materials and an important alloying element in steel. Nickel is the element forming austenite in steel, but if the low-carbon nickel steel wants to obtain pure austenite, the content of nickel needs to reach 24%; while only containing 27% of nickel can make the corrosion-resistant of steel significantly changed in some medium. So nickel can not constitute a stainless steel alone. However, when nickel and chromium exist in the stainless steel at the same time, the nickel-containing stainless steel can have many valuable properties. Based on the above situation indicates that nickel’s role as the alloying elements in stainless steel, is that it makes organization changes of high chromium steel, making properties and corrosion resistance of stainless steel for certain improvements.
The role of manganese to austenitic is similar to the nickel. But to put it exactly, the role of manganese is not in forming austenite, but in reducing the critical quenching speed of the steel, to increase the stability of austenite during cooling, inhibit the decomposition of austenite, make the austenite formed at high temperatures can maintain at room temperature. In improving the corrosion resistance of the steel, the role of manganese is not large, such as the amount of manganese in the steel changes from 0 to 10.4 %, would not make the corrosion resistance of steel in the air have obvious change. This is because the role of manganese to improve the electrode potential of the iron-based solid solution is very limited, the protective effect of oxide film formed is very low, so although there are austenitic steels alloyed with manganese in industry, but they can not be used as stainless steel. The stable austenite role of manganese in the steel is about half of nickel, even if the content is 2% the role of nitrogen in steel is also a stable austenitic and the effect of degree larger than nickel.
Cobalt is not much used in steel as an alloying element. This is because the high price of cobalt and its application in other areas (such as high-speed steel, carbide, cobalt-based heat resistant alloy, magnetic steel or hard magnetic alloy, etc.) has a more important purpose. Add cobalt as alloy elements in general stainless steel is not common. Commonly used stainless steel add cobalt is not aimed to improve corrosion resistance but to improve the hardness, because the main use of this stainless steel is manufacturing surgical blade and slicing machine blades, scissors and so on.