The chemical elements are often used in the steel industry, there are about more than 20 species, isstainless steel, commonly used elements of a dozen, in addition to iron, can affect the microstructure and properties of stainless steel elements are carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, cobalt, copper, nitrogen etc.. When several elements coexist on the same stainless steel matrix, their effects are more complex than when they are alone, so we should not only consider the elements of its function, but also pay attention to their mutual influence, so the microstructure and properties of stainless steel is the sum by the effects of various elements.
Chromium plays a decisive role in stainless steel
There is only one element that determines the stainless steel property, that is chromium, and each stainless steel must have chromium. The fundamental reason is that the addition of chromium to the steel can promote the movement of the internal contradictions in the steel to help resist corrosion damage. That is, chromium increases the electrode potential of Fe based solid solutions, and chromium absorbs iron electrons to make iron passivated.
Passivation is the phenomenon that the corrosion resistance of metals and alloys is improved because the anodic reaction is prevented. There are many theories about passivation of metals and alloys, including thin film theory, adsorption theory and electronic arrangement theory.
The duality of carbon in stainless steel
Carbon is one of the main elements of industrial steel. Its performance and microstructure are largely determined by the content and distribution of carbon in steel, and the effect of carbon is very obvious in stainless steel. Effect of carbon on microstructure in stainless steel is mainly manifested in two aspects, one is the stable austenite carbon element, and its effect is very strong (about 30 times of nickel), on the other hand, because of carbon and chromium and chromium have great affinity, forming a series of complex carbide. Therefore, according to the strength and corrosion resistance of parts, carbon in stainless steel is contradictory. So from the use of different requirements, choose different carbon content of stainless steel.
The role of nickel in stainless steel
Nickel is the excellent corrosion resistance of elements, is an important alloying element of alloy steel. Nickel is the main element in the formation of austenite stainless steel, but low carbon nickel steel to obtain pure austenite and nickel content to achieve 24%; and only 27% of nickel to stainless steel in some medium corrosion resistance improved significantly. So nickel can not be made into stainless steel alone. However, nickel and chromium exist in stainless steel at the same time, nickel containing stainless steel has a lot of valuable properties. So that, effect of nickel in stainless steel, high chromium steel is the organization change, so that the stainless steel corrosion resistance was improved to a certain extent.
Manganese and nitrogen can replace nickel in chromium nickel stainless steel
Cr Ni austenitic steel has the advantages of many, but now because of the nickel base heat-resisting alloy and nickel containing less than 20% of the heat resistant steel of the application and development of chemical industry, and the growing need of more and more large amount of stainless steel, but less nickel deposits and concentrated in a few areas, so produced a contradiction the nickel in the supply and demand in the world. Therefore, in the field of stainless steel and many other alloys (such as forging steel, tool steel, heat resistant steel etc.), especially the lack of nickel resources in the country, a large number of development and generation of nickel nickel with other elements of scientific research and production practice, research and application is more to manganese and nitrogen instead of stainless steel and heat resistant steel of nickel in this area.
The effect of manganese on austenite is almost the same as that of nickel. But rather, the role of manganese is not generated is austenite, which can reduce the critical quenching speed of stainless steel, increase the stability of austenite during cooling, inhibit decomposition of austenite, the high temperature formed austenite to maintain the normal temperature. To improve the corrosion resistance of stainless steel, little effect of manganese.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)