At present, there are more than 100 chemical elements known, and there are about 20 chemical elements that can be encountered in steel materials commonly used in industry. For the special steel series of stainless steel formed by people’s long-term struggle with corrosion, there are more than a dozen elements commonly used. In addition to the basic element of steel, iron has the greatest impact on the performance and structure of stainless steel. The elements are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, etc. Except for carbon, silicon and nitrogen, these elements are all elements in the transition group of the periodic table of chemical elements.
In fact, the stainless steel used in industry has several or even a dozen elements at the same time. When several elements coexist in the unity of stainless steel, their influence is much more complicated than when they exist alone. In this case, not only the role of each element itself must be considered, but also their mutual influence. Therefore, the structure of stainless steel depends on the sum of the influence of various elements.
1). The influence and effect of various elements on the performance and organization of stainless steel
1-1. The decisive role of chromium in stainless steel: There is only one element that determines the properties of stainless steel, which is chromium, and each stainless steel contains a certain amount of chromium. So far, there is no chromium-free stainless steel. The fundamental reason why chromium has become the main element that determines the performance of stainless steel is that the addition of chromium as an alloying element to steel promotes its internal contradictory movement to help resist corrosion damage. This change can be explained from the following aspects:
① Chromium increases the electrode potential of iron-based solid solution
②Chromium absorbs electrons from iron to passivate iron
Passivation is a phenomenon in which the corrosion resistance of metals and alloys is improved due to the prevention of anode reaction. There are many theories that constitute passivation of metals and alloys, mainly including film theory, adsorption theory and electron arrangement theory.
1-2. The duality of carbon in stainless steel
Carbon is one of the main elements of industrial steel. The performance and structure of steel are largely determined by the content and distribution of carbon in steel. The influence of carbon in stainless steel is particularly significant. The influence of carbon on the structure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is an element that stabilizes austenite and has a large effect (about 30 times that of nickel). On the other hand, because of the very affinity of carbon and chromium Large, formed with chromium—a series of complex carbides. Therefore, in terms of strength and corrosion resistance, the role of carbon in stainless steel is contradictory.
Knowing the law of this influence, we can choose stainless steels with different carbon content based on different usage requirements.
For example, the most widely used and the most basic stainless steel in the industry-the standard chromium content of the five steel grades 0Crl3″4Cr13 is set at 12″14%, which is to take the factor of carbon and chromium into chromium carbide. The purpose of the decision is that after carbon and chromium are combined into chromium carbide, the chromium content in the solid solution should not be lower than the minimum chromium content of 11.7%.
For these five steel grades, due to the different carbon content, the strength and corrosion resistance are also different. The corrosion resistance of 0Cr13″2Crl3 steel is better but the strength is lower than that of 3Crl3 and 4Cr13 steel. It is mostly used to manufacture structural parts. The two steel grades can obtain high strength due to their higher carbon content. They are mostly used to manufacture springs, knives and other parts that require high strength and wear resistance. For example, in order to overcome the intergranular corrosion of 18-8 chromium-nickel stainless steel, the steel The carbon content is reduced to less than 0.03%, or an element (titanium or niobium) with greater affinity than chromium and carbon is added to prevent the formation of chromium carbide. When high hardness and wear resistance become the main requirements, we can Increasing the carbon content of steel while appropriately increasing the chromium content, so as to not only meet the requirements of hardness and wear resistance, but also take into account the fixed corrosion resistance. It is used in industries as bearings, measuring tools and blades with stainless steel 9Cr18 and 9Cr17MoVCo Although the carbon content of steel is as high as 0.85″0.95%, because their chromium content has been increased correspondingly, the corrosion resistance requirements are still guaranteed.
Generally speaking, the carbon content of stainless steel used in the industry is relatively low. Most stainless steels have a carbon content of 0.1″ 0.4%, and acid-resistant steels have a carbon content of 0.1″ 0.2%. Stainless steel with a carbon content of more than 0.4% only accounts for a small part of the total number of steel grades. This is because under most conditions of use, stainless steel always has corrosion resistance as the main purpose. In addition, the lower carbon content is also due to certain technological requirements, such as easy welding and cold deformation.