Functions of elements contained in the hottest sta

The role of elements contained in stainless steel

at present, there are more than 100 known chemical elements, and about 20 kinds of chemical elements 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 against corrosion, there are more than a dozen kinds of elements that are most commonly used. In addition to the basic element iron, which constitutes the steel, the elements that have the greatest impact on the performance and structure of stainless steel are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, etc. In addition to carbon, silicon and nitrogen, these elements are in the transition group of the periodic table of chemical elements

in fact, stainless steel used in industry has several or even more than a dozen elements at the same time. When several elements coexist in the unity of stainless steel, their influence is much more complex than when they exist alone, because in this case, we should not only consider the role of each element itself, but also pay attention to their mutual influence. Therefore, the structure of stainless steel depends on the sum of the effects of various elements

1). The influence and role of various elements on the properties and microstructure of stainless steel

1-1. The decisive role of chromium in stainless steel: there is only one element that determines the property of stainless steel, which is chromium. 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 determining the performance of stainless steel is that after adding chromium to steel as an alloy element, its internal contradiction movement develops in favor of resisting corrosion damage. This change can be explained from the following aspects:

① chromium increases the electrode potential of iron-based solid solution

② chromium absorbs iron electrons to passivate iron

passivation is a phenomenon that the corrosion resistance of metals and alloys is improved due to the prevention of anodic 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 largely depend on the content and distribution of carbon in steel, especially in stainless steel. 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 plays a great role (about 30 times that of nickel); on the other hand, due to the great affinity between carbon and chromium, it forms a series of complex carbides with chromium. Therefore, in terms of strength and corrosion resistance, the role of carbon in stainless steel is contradictory

knowing the law of this effect, we can choose stainless steel with different carbon content from different use requirements

for example, the standard chromium content of the five steel grades 0crl3 ~ 4Cr13, the most widely used and minimum stainless steel in industry, is stipulated to be 12 ~ 14%, which is decided after taking into account the factors that carbon and chromium form chromium carbide. The purpose is to make the chromium content in the solid solution not less than the minimum chromium content of 11.7% after carbon and chromium combine to form chromium carbide

for these five steel grades, due to the different carbon content of Bayer's trademark that will no longer be used, the strength and corrosion resistance are also different. 0Cr13 ~ 2Crl3 steel has better corrosion resistance, but the strength is lower than 3Crl3 and 4Cr13 steel, which are mostly used to manufacture structural parts. The latter two steel grades can obtain high strength due to their high carbon content, and are mostly used to manufacture springs, knives and other parts requiring high strength and wear resistance. For another example, in order to overcome the intergranular corrosion of 18-8 chromium nickel stainless steel, the carbon content of the steel can be reduced to less than 0.03%, or elements (titanium or niobium) with greater affinity than chromium and carbon can be added to make it not form chromium carbide. For another example, when high hardness and wear resistance become the main requirements, we can appropriately increase the chromium content while increasing the carbon content of the steel, so as to meet the requirements of hardness and wear resistance, It also takes into account the corrosion resistance function. In industry, 9Cr18 and 9cr17movco stainless steels are used as bearings, measuring tools and blades. Although the carbon content is as high as 0.85 ~ 0.95%, their chromium content is also correspondingly increased, so the requirements of corrosion resistance are still guaranteed

generally speaking, the carbon content of stainless steel used in industry is relatively low. The carbon content of most stainless steel is between 0.1 ~ 0.4%, while the carbon content of acid resistant steel is mostly 0.1 ~ 0.2%. Stainless steel with carbon content greater than 0.4% only accounts for a small part of the total number of steel grades, because under most service conditions, stainless steel always takes corrosion resistance as the main purpose. In addition, the low carbon content is also due to some technological requirements, such as easy welding and cold deformation

1-3. The role of nickel in stainless steel is played only after it is matched with chromium.

nickel is an excellent corrosion-resistant material and an important alloying element of alloy steel. Nickel is an element forming austenite in steel, but in order to obtain pure austenite structure in low-carbon nickel steel, the nickel content should reach 24%; Only when the nickel content is 27%, the corrosion resistance of steel in some media is significantly changed. Therefore, nickel cannot form stainless steel alone. However, when nickel and chromium exist in stainless steel at the same time, stainless steel containing nickel has many valuable properties

based on the above situation, the role of nickel as an alloy element in stainless steel is that it changes the structure of high chromium steel, thereby improving the corrosion resistance and process performance of stainless steel

1-4. Manganese and nitrogen can replace nickel in chromium nickel stainless steel. Although chromium nickel austenitic steel has many advantages, in recent decades, due to the large-scale development and application of nickel based heat-resistant alloys and hot strength steels containing less than 20% nickel, as well as the growing development of the chemical industry, the demand for stainless steel is increasing, while nickel deposits are small and concentrated in a few areas, Therefore, there is a contradiction between supply and demand of nickel in the world. Therefore, in the field of stainless steel and many other alloys (such as steel for large castings and forgings, tool steel, thermal strength steel, etc.), especially in countries that lack nickel resources, scientific research and production practice of saving nickel and replacing nickel with other elements have been widely carried out. In this regard, manganese and nitrogen are mostly used to replace nickel in stainless steel and heat-resistant steel

the effect of manganese on austenite is similar to that of nickel. But to be exact, the role of manganese is not to form austenite, but to reduce the critical quenching speed of steel, increase the stability of austenite during cooling, inhibit the decomposition of austenite, and maintain the austenite formed at high temperature to normal temperature. Manganese has little effect on improving the corrosion resistance of steel. For example, the change of manganese content in steel from 0 to 10.4% does not significantly change the corrosion resistance of steel in air and acid. This is because manganese has little effect on improving the electrode potential of iron-based solid solution, and the protective effect of the formed oxide film is also aimed at the new material industry. The research and development of key common technologies is low. Therefore, although there are austenitic steels alloyed with manganese in the industry (such as 40mn18cr4, 50mn18cr4wn, ZGMn13 steel, etc.), they cannot be used as stainless steel. The effect of manganese on stabilizing austenite in steel is about half of that of nickel, that is, the effect of 2% nitrogen in steel is also stabilizing austenite, and the degree of effect is greater than that of nickel. For example, in order to obtain austenite structure in steel containing 18% chromium at room temperature, low nickel stainless steel with manganese and nitrogen instead of nickel and chromium manganese nitrogen free steel with nickel have been applied in industry, and some have successfully replaced the classic 18-8 chromium nickel stainless steel

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

12 times of development requires a lot of work - 6. Molybdenum and copper can improve the corrosion resistance of some stainless steels

1-7. Effects of other elements on the properties and microstructure of stainless steel

the effects of the above nine main elements on the properties and microstructure of stainless steel. In addition to those elements that have a greater impact on the properties and microstructure of stainless steel, stainless steel also contains some other elements. Some are common impurity elements like ordinary steel, such as silicon, sulfur, phosphorus, etc. Some are added for some specific purposes, such as cobalt, boron, selenium, rare earth elements, etc. In terms of the corrosion resistance of stainless steel, these elements are non major compared with the nine elements discussed. However, they cannot be completely ignored, because they also affect the properties and microstructure of stainless steel

silicon is an element that forms ferrite. It is a common impurity element in rust steel, which has broken through the key raw material of ADI whole industry chain products and technical bottleneck for aircraft, advanced automobile and ship coatings

cobalt is rarely used in steel as an alloying element because of its high price and more important applications in other aspects (such as high-speed steel, cemented carbide, cobalt based heat-resistant alloy, magnetic steel or hard magnetic alloy, etc.). Cobalt is not often added to general stainless steel as an alloy element. The purpose of adding cobalt to common stainless steel such as 9crl7movco steel (containing 1.2-1.8% cobalt) is not to improve corrosion resistance, but to improve hardness, because the main purpose of this stainless steel is to manufacture cutting tools, scissors and surgical blades

boron: adding 0.005% boron to high chromium ferritic stainless steel crl7mo2ti steel can improve the corrosion resistance in boiling 65% acetic acid. Adding a small amount of boron (0.0006 ~ 0.0007%) can improve the hot plasticity of austenitic stainless steel. Due to the formation of low melting point eutectic, a small amount of boron increases the tendency of hot cracks in austenitic steel during welding, but when there is more boron (0.5 ~ 0.6%), it can prevent the generation of hot cracks. Because when it contains 0.5 ~ 0.6% boron, austenite boride two-phase structure is formed, which reduces the melting point of the weld. When the solidification temperature of the molten pool is lower than the semi melting zone, the tensile stress generated by the base metal during cooling is in the liquid state. The solid weld metal will not cause cracks at this time. Even if cracks are formed in the near seam area, they can also be filled by the molten pool metal in liquid solid state. Chromium nickel austenitic stainless steel containing boron has special applications in atomic energy industry

phosphorus: it is an impurity element in general stainless steel, but its harmfulness in austenitic stainless steel is not as significant as that in general steel, so the content can be allowed to be higher. For example, some data suggest that it can reach 0.06% to facilitate smelting control. The phosphorus content of some manganese containing austenitic steels can reach 0.06% (such as 2crl3nimn9 steel) to 0.08% (such as cr14mnl4ni steel). The strengthening effect of phosphorus on steel is also used as the alloying element of age hardening stainless steel. Ph17-10p steel (containing 0.25% phosphorus) is ph-hnm steel (containing 0.30 phosphorus), etc

sulfur and selenium: impurities are often found in general stainless steel. However, adding 0.2 ~ 0.4% sulfur to stainless steel can improve the cutting performance of stainless steel, and selenium has the same effect. Sulfur and selenium improve the cutting performance of stainless steel because they reduce the toughness of stainless steel. For example, the impact value of 18-8 chromium nickel stainless steel can reach 30 kg/cm2. The impact value of 18-8 steel containing 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 1.8 kg/cm2; The impact value of 18-8 steel (0.094%c, 18.4%cr, 9%ni) containing 0.22% selenium is 3.24 kg/cm2. Both sulfur and selenium reduce the corrosion resistance of stainless steel