The Influence of Different Elements on Stainless Steel
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 against corrosion, there are more than a dozen elements commonly used. In addition to the basic elements 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. Under the circumstances, not only the role of each element itself must be considered, but also their mutual influence. Therefore, the structure of stainless steel is determined by the sum of the influence of various elements.
1. The decisive role of chromium in stainless steel
There is only one element that determines the properties of stainless steel, and that is chromium. Each type of 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 iron electrons 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 the passivation of metals and alloys, mainly including film theory, adsorption theory, and electron arrangement theory.
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 the steel. The influence of carbon in stainless steel is particularly significant. The effect 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 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 steel with different carbon content from different use 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 stipulated to be 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 to form 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 high carbon content and are mostly used in the manufacture of 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 carbon content of the steel can be reduced to less than 0.03%, or an element (titanium or niobium) with greater affinity than chromium and carbon can be added to prevent it from forming carbonization.
Chromium, for example, when high hardness and wear resistance become the main requirements, we can increase the carbon content of steel while appropriately increasing the chromium content, so as to meet the requirements of hardness and wear resistance, but also take into account-fixed The corrosion resistance function of stainless steel 9Cr18 and 9Cr17MoVCo used as bearings, measuring tools and blades in the industry, although the carbon content is as high as 0.85 to 0.95% because their chromium content has been increased accordingly, it still guarantees corrosion resistance. Require.
In general, the carbon content of stainless steel currently used in the industry is relatively low. Most stainless steels have a carbon content of between 0.1% and 0.4%, and acid-resistant steels have a carbon content of 0.1% to 0.2%. Stainless steel with a carbon content of more than 0.4% only accounts for a small part of the total steel grades. This is because, under most conditions of use, stainless steel always has corrosion resistance as its main purpose. In addition, the lower carbon content is also due to certain technological requirements, such as easy welding and cold deformation.
3. The role of nickel in stainless steel is only played after it cooperates with chromium
Nickel is an excellent corrosion-resistant material and an important alloying element for alloy steel. Nickel is an element that forms austenite in steel, but for low-carbon nickel steel to obtain a pure austenite structure, the nickel content must reach 24%; and only when the nickel content is 27% can the steel be resistant to certain media. The corrosion performance changes significantly. Therefore, nickel cannot constitute stainless steel alone. But when nickel and chromium exist in stainless steel at the same time, nickel-containing stainless steel has many valuable properties.
Based on the above situation, the role of nickel as an alloying element in stainless steel is that it changes the structure of high chromium steel so that the corrosion resistance and process performance of stainless steel can be improved.
4. Manganese and nitrogen can replace nickel in chromium-nickel stainless steel
Although there are many advantages of chromium-nickel austenitic steel, in recent decades due to the large-scale development and application of nickel-based heat-resistant alloys and heat-strength steels containing less than 20% nickel, and the increasing development of the chemical industry, the demand for stainless steel has increased. The larger the size, the smaller the nickel deposits and the concentration distribution in a few areas, so there is a contradiction between the supply and demand of nickel in the world.
Therefore, in the fields of stainless steel and many other alloys (such as steel for large castings and forgings, tool steel, heat-strength steel, etc.), especially in countries where nickel resources are relatively scarce, the science of saving nickel and replacing nickel with other elements have been extensively carried out. In research and production practice, there are more researches and applications in this area that replace nickel in stainless steel and heat-resistant steel with manganese and nitrogen.
The effect of manganese on austenite is similar to that of nickel. But to be more precise, the role of manganese is not to form austenite, but to reduce the critical quenching rate of steel, increase the stability of austenite during cooling, inhibit the decomposition of austenite, and make it form at high temperatures. The austenite can be maintained to room temperature. In improving the corrosion resistance of steel, manganese has little effect. For example, the manganese content in steel changes from 0 to 10.4%, and it does not significantly change the corrosion resistance of steel in air and acid.
This is because manganese has little effect on increasing the electrode potential of iron-based solid solution, and the protective effect of the formed oxide film is also very low, so although there are austenitic steels alloyed with manganese (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steel Etc.), they cannot be used as stainless steel. The role of manganese in stabilizing austenite in steel is about one-half that of nickel, that is, the role of 2% nitrogen in steel is also stabilizing austenite, and the role is greater than that of nickel.
For example, in order to obtain the austenitic structure of steel containing 18% chromium at room temperature, low-nickel stainless steel with manganese and nitrogen instead of nickel and nickel-free chromium-manganese-nitrogen stainless steel has been applied in the industry at present, and some It has successfully replaced the classic 18-8 chromium-nickel stainless steel.
5. Titanium or niobium is added to stainless steel to prevent intergranular corrosion.
6. Molybdenum and copper can improve the corrosion resistance of certain stainless steel.
7. The influence of other elements on the performance and organization of stainless steel
The above nine main elements have an impact on the performance and structure of stainless steel. In addition to the elements that have a greater impact on the performance and structure of stainless steel, stainless steel also contains some other elements. Some are the same as general steel as impurities, such as silicon, sulfur, phosphorus, and so on. Some are added for specific purposes, such as cobalt, boron, selenium, and rare earth elements. In terms of the main nature of the corrosion resistance of stainless steel, these elements are non-essential compared to the nine elements discussed. Even so, they cannot be completely ignored because they also affect the performance and organization of stainless steel. Influence.
Silicon” is an element that forms ferrite, and is an impurity element often present in general stainless steel.
Cobalt is not widely used as an alloying element in steel. This is because of the high price of cobalt and its importance in other aspects (such as high-speed steel, hard alloy, cobalt-based heat-resistant alloy, magnetic steel or hard magnetic alloy, etc.) use. There are not many common stainless sheets that add cobalt as an alloying element. Commonly used stainless steels such as 9Crl7MoVCo steel (containing 1.2-1.8% cobalt) add cobalt. The purpose is not to improve the corrosion resistance but to increase the hardness, because the main purpose of this kind of stainless steel in the manufacture of slicing machine cutting tools, scissors and surgical blades, etc.
Boron The addition of 0.005% boron to the high-chromium ferritic stainless steel Crl7Mo2Ti steel can improve the corrosion resistance in the boiling 65% acetic acid. Adding a small amount of boron (0.0006 to 0.0007%) can improve the thermal plasticity of austenitic stainless steel. A small amount of boron forms a low melting point eutectic, which increases the tendency of austenitic steel to generate hot cracks during welding, but when it contains more boron (0.5 to 0.6%), it can prevent the occurrence of hot cracks.
Because when it contains 0.5 to 0.6% of boron, the 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 material during cooling will be in the liquid state. The solid weld metal will not cause cracks at this time. Even if a crack is formed in the near-joint area, it can be filled with liquid-solid molten pool metal. The boron-containing chromium-nickel austenitic stainless steel has special applications in the atomic energy industry.
Phosphorus is an impurity element in general stainless steel, but its hazard in austenitic stainless steel is not as significant as in general steel, so the content can be allowed to be higher if some data suggest it can reach 0.06%. Conducive to smelting control. The phosphorus content of individual manganese-containing austenitic steel can reach 0.06% (such as 2Crl3NiMn9 steel) or even 0.08% (such as Cr14Mnl4Ni steel). Using phosphorus to strengthen steel, phosphorus is also added as an alloying element for age-hardening stainless steel. PH17-10P steel (containing 0.25% phosphorus) is PH-HNM steel (containing 0.30 phosphorus) and so on.
Sulfur and selenium are also common impurity elements in general stainless steel. But adding 0.2 to 0.4% sulfur to stainless steel can improve the cutting performance of stainless steel, and selenium also 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 with 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 1.8 kg/cm²; 18 with 0.22% selenium The impact value of -8 steel (0.094% C, 18.4% Cr, 9% Ni) is 3.24 kg/cm². Both sulfur and selenium reduce the corrosion resistance of stainless steel, so they are rarely used as alloying elements of stainless steel.
Rare earth elements The application of rare earth elements to stainless steel is currently mainly to improve process performance. For example, adding a small number of rare earth elements to Crl7Ti steel and Cr17Mo2Ti steel can eliminate the bubbles caused by hydrogen in the steel ingot and reduce the cracks in the billet. Austenitic and austenitic-ferritic stainless steel with 0.02-0.5% rare earth elements (cerium-lanthanum alloy) can significantly improve the forging performance. There used to be austenitic steel containing 19.5% chromium, 23% nickel and molybdenum, copper, and manganese. In the past, it could only produce castings due to the performance of the hot working process. After adding rare earth elements, it could be rolled into various profiles.