Stainless Steel Knowledge (Series 3)

There are more than 100 kinds of chemical elements that are currently known in the properties and organization of stainless steel, and about 20 kinds of chemical elements can be encountered in steel materials commonly used in industry. For stainless steel, a special steel series formed by people's practice of long-term struggle against corrosion, there are more than a dozen of the most commonly used elements. In addition to the basic elements of steel, the composition of steel has the greatest impact on the performance and organization of stainless steel. The elements are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, tantalum, titanium, manganese, nitrogen, copper, cobalt, and the like. In addition to carbon, silicon, and nitrogen, these elements are transitional elements in the periodic table of chemical elements.

In fact, the stainless steel used industrially has several kinds or even more than ten kinds of elements at the same time. When several elements coexist in a unified body of stainless steel, their influence is much more complicated than when they exist alone, because in this In this case, it is necessary not only to consider the role of each element itself, but also to pay attention to their mutual influence. Therefore, the organization of stainless steel depends on the sum of the influences of various elements. 1). Effect and role of various elements on the performance and organization of stainless steel 1-1. The decisive role of chromium in stainless steel: It is decided that there is only one element of stainless steel, which is chromium, and each stainless steel contains a certain amount of chromium. To date, there is no chromium-free stainless steel. The reason why chromium has become the main element determining the performance of stainless steel is because the addition of chromium to the steel as an alloying element has promoted its internal contradictory movement to develop 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 electrons of iron to passivate the passivation of iron because the anode reaction is prevented and the corrosion resistance of metals and alloys is increased. . There are many theories that constitute the passivation of metals and alloys, mainly including thin film theory, adsorption theory, and electronic arrangement theory. 1-2. The dual carbon of carbon in stainless steel is one of the main elements of industrial steel. The properties and organization of steel are determined to a large extent by the content of carbon in steel and the form of its distribution. The effect 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 its degree of action is very large (about 30 times that of nickel). On the other hand, the affinity of carbon and chromium is very high. Large, formed with chromium - a series of complex carbides. Therefore, in terms of both strength and decay resistance, the role of carbon in stainless steel is contradictory. Recognizing the laws of this effect, we can select different stainless steels with different carbon contents starting from different requirements. For example, the industry's most widely used and the most basic stainless steel - 0Crl3 ~ 4Cr13 these five steel standards for the chromium content of 12 to 14%, is the carbon to chromium and chromium carbide formation factor into account after The purpose of the decision was to combine the chromium and chromium into chromium carbide, and the chromium content in the solid solution would not be lower than the minimum chromium content of 11.7%. For these five steel grades, strength and corrosion resistance are also different because of different carbon content. 0Cr13～2Crl3 steels have better corrosion resistance but lower strength than 3Crl3 and 4Cr13 steels. They are mostly used for manufacturing structural parts. Because of the high carbon content of the two steel grades, they can obtain high strength and are used to make parts that require high strength and wear resistance, such as springs and knives. In order to overcome the intergranular corrosion of 18-8 chromium-nickel stainless steel, the carbon content of the steel can be reduced to 0.03% or less, or an element with greater affinity for chromium and carbon (titanium or niobium) can be added to prevent carbonization. Chromium, if high hardness and wear resistance become the main requirements, we can increase the amount of chromium in the steel while increasing the amount of chromium, so as to meet both the hardness and wear resistance requirements, but also take into account Corrosion resistance function, industrial use as bearings, measuring tools and blades with stainless steel 9Cr18 and 9Cr17MoVCo steel, although the carbon content is as high as 0.85 to 0.95%, due to their chromium content is also increased accordingly, so it still guarantees corrosion resistance Claim. In general, the carbon content of stainless steel currently used in industry is relatively low, most of the carbon content of stainless steel is between 0.1 and 0.4%, and acid-resistant steel is mostly containing 0.1 to 0.2% of carbon. Stainless steels with more than 0.4% carbon content account for only a small part of the total number of steels. This is because the stainless steel is always primarily resistant to corrosion under most conditions of use. In addition, the lower carbon content is also due to certain process requirements such as ease of welding and cold deformation. 1-3. The role of nickel in stainless steel is that it is an excellent corrosion-resistant material that is used only after it is combined with chromium. It is also an important alloying element of alloy steel. Nickel is an element that forms austenite in steel, but low-carbon nickel steel needs to obtain pure austenite structure, and the nickel content must reach 24%; and only when nickel content is 27%, the steel is resistant to certain media. Corrosion performance changes significantly. Therefore, nickel alone cannot constitute stainless steel. However, when nickel and chromium are present in stainless steel, 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 microstructure of the high-chromium steel so that the corrosion resistance and the process performance of the stainless steel are improved. 1-4. Manganese and nitrogen can replace the advantages of nickel-chromium-nickel austenitic steels in chromium-nickel stainless steels. Although there are many, but in recent decades due to nickel-based heat-resistant alloys and nickel-containing heat-intensive steel, the development and application of a large number of, and chemistry The growing demand for stainless steel from industrial development is increasing. Nickel deposits are small and concentrated in a few areas. Therefore, there are conflicts in the supply and demand of nickel in the world. Therefore, in the stainless steel and many other alloy fields (such as large-scale casting and forging steel, tool steel, heat-strength steel, etc.), especially in countries where the resources of nickel are relatively lacking, the science of nickel-reduction and nickel substitution with other elements has been widely carried out. Research and production practices, research and application in this area are more than manganese and nitrogen instead of nickel in stainless steel and heat-resistant steel. The effect of manganese on austenite is similar to that of nickel. But to be precise, the role of manganese is not in the formation of austenite, but in the fact that it lowers the critical quenching rate of steel, increases the stability of austenite during cooling, inhibits the decomposition of austenite, and causes the formation of high temperatures. Austenite is maintained at room temperature. In improving the corrosion resistance of steel, manganese has little effect. If the content of manganese in steel changes from 0 to 10.4%, the corrosion resistance of steel in air and acid does not change significantly. This is because manganese has little effect on increasing the electrode potential of the iron-based solid solution, and the protective effect of the formed oxide film is also very low, so although there are industrially austenitic steels alloyed with manganese (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steel Etc) but they cannot be used as stainless steel. The effect of manganese on the stabilization of austenite in steel is about one-half that of nickel, that is, the effect of 2% of nitrogen in the steel is also stable austenite, and the effect is greater than that of nickel. For example, to obtain 18% chromium-containing steel at room temperature to obtain austenite structure, nickel-nickel-nickel low-nickel stainless steel and elemental nickel-chromium-manganese-nitrogen-free steel are currently used in industry, and some Has successfully replaced the classic 18-8 chromium nickel stainless steel. 1-5. Titanium or niobium is added to stainless steel to prevent intergranular corrosion. 1-6. Molybdenum and copper can improve the corrosion resistance of some stainless steels. 1-7. Influence of other elements on the performance and organization of stainless steel The influence of the above nine main elements on the performance and organization of stainless steels. In addition to the elements that have a major influence on the performance and organization of stainless steels, stainless steels also contain some other elements. Some are the same as normal steel impurities, such as silicon, phosphorus, and so on. Some are added for specific purposes such as cobalt, boron, selenium, and rare earth elements. In terms of the main properties of corrosion resistance of stainless steels, these elements are non-essential relative to the nine elements already discussed. However, they cannot be completely ignored because they also have the same effect on the performance and organization of stainless steel. influences. Silicon is an element that forms ferrite and is a common impurity in normal stainless steel. Cobalt is not widely used as an alloying element in steel 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, hard magnetic alloy, etc.) use. In the general stainless steel plus cobalt as alloying elements is not much, commonly used stainless steel such as 9Crl7MoVCo steel (containing 1.2-1.8% cobalt) plus cobalt, is not intended to improve corrosion resistance but to increase hardness because of the main purpose of this stainless steel It is the production of slicer cutting tools, scissors and surgical blades. Boron: The addition of 0.005% boron to the high-chromium ferritic stainless steel Cr7Mo2Ti makes it possible to increase the corrosion resistance in boiling 65% acetic acid. Adding a small amount of boron (0.0006 to 0.0007%) can improve the hot plasticity of the austenitic stainless steel. A small amount of boron forms a low-melting point eutectic, which tends to increase the tendency for hot cracks to occur during the welding of austenitic steels. However, when a large amount of boron (0.5 to 0.6%) is contained, hot cracking can be prevented. Because when 0.5-0.6% of boron is contained, an austenite-boride two-phase structure is formed, and the melting point of the weld is reduced. When the solidification temperature of the molten pool is lower than that of the semi-dissolution zone, the tensile stress generated when the base metal is cooled is in the liquid state. The solid-state weld metal can withstand this, and it will not cause cracks. Even if cracks form in the near-seam area, it can be filled with liquid-solid pool metal. Boron-containing chromium-nickel austenitic stainless steels have special uses in the atomic energy industry. Phosphorus: In general stainless steel is an impurity element, but its hazard in the austenitic stainless steel is not as significant as in the general steel, so the content can be allowed to be higher, if some information is proposed up to 0.06% to facilitate Smelting control. Individual manganese-containing austenitic steels can contain up to 0.06% phosphorus (such as 2Crl3NiMn9 steel) up to 0.08% (such as Cr14Mnl4Ni steel). The use of phosphorus for the strengthening of steel, there are also phosphorus as an alloying element of age-hardening stainless steel, PH17-10P steel (containing 0.25% phosphorus) is PH-HNM steel (containing 0.30 phosphorus) and so on. * and selenium: In ordinary stainless steel is also often an impurity element. However, adding 0.2 to 0.4%* to stainless steel can improve the cutting performance of stainless steel, and selenium has the same effect. * and selenium improve the cutting performance of stainless steel because they reduce the toughness of stainless steel, for example, the impact value of general 18-8 chromium nickel stainless steel can reach 30 kg/cm2. Impact value of 18-8 steel containing 0.31%* (0.084% C, 18.15% Cr, 9.25% Ni) was 1.8 kg/cm2; 18-8 steel containing 0.22% selenium (0.094% C, 18.4% Cr, The impact value of 9% Ni) is 3.24 kg/cm2. Both selenium and selenium reduce the corrosion resistance of stainless steels, so practical application of them as alloying elements for stainless steels is rare. Rare-Earth Elements: Rare-earth elements are used in stainless steel and are currently mainly to improve process performance. If a small amount of rare earth elements is added to Cr17Ti steel and Cr17Mo2Ti steel, hydrogen-induced air bubbles in the steel ingot can be eliminated and cracks in the steel slab can be reduced. Austenite and austenite-ferritic stainless steels with 0.02 to 0.5% rare earth elements (barium alloys) can significantly improve forging performance. There has been an austenitic steel containing 19.5% chromium, 23% nickel, and molybdenum copper manganese. Because of the hot processing technology, in the past, only castings were produced. After adding rare earth elements, they were rolled into various shapes. 2). According to the metallurgical organization, the classification of stainless steel and the general characteristics of various types of stainless steel are based on the chemical composition (mainly chromium content) and its use. Stainless steel is divided into two major categories, stainless and acid-resistant. Industrially, stainless steels are classified according to the type of matrix structure of the steel after heated air cooling at a high temperature (900-1100 degrees Celsius), which is determined based on the characteristics of the influence of carbon and alloying elements on the stainless steel structure discussed above. Industrial application of stainless steel can be divided into three categories according to metallographic structure: ferritic stainless steel, martensitic stainless steel, austenitic stainless steel. The characteristics of these three types of stainless steels can be summarized (as shown in the following table), but it should be noted that martensitic stainless steels are not non-weldable, but are limited by certain conditions, such as high temperature tempering after welding should be preheated before welding. Etc., which makes the welding process more complicated. In actual production, some martensitic stainless steels such as 1Cr13, 2Cr13, 2Cr13, and 45 steel are still welded. Classification, main components and properties of stainless steel Comparative classification Classification (%) Hardenability Corrosion resistance Processability Weldability Magnetic CrNi Ferritic system 0.35 or less 16-27 - No good Good still available Martensitic 1.20 or less 11-15- Self-hardening Cocoa cannot have austenite 0.25 or less 16 or more 7 or more Excellent or not Excellent None None The above classification is based only on the steel matrix, due to the stabilization of austenite and the formation of ferrite elements in the steel. The effects cannot be balanced with each other, and because of the large amount of chromium, the equilibrium diagram S is shifted to the left. In addition to the three basic types mentioned above, the stainless steel used in the industry has martensite-ferrite, austenite-iron. Nucleus, transitional duplex stainless steel such as austenite-martensite, and stainless steel with martensite-carbide structure.