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Properties and Microstructure of Stainless Steel

2019/01/17 09:40
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At present, there are more than 100 known chemical elements, and about twenty chemical elements can be encountered in steel materials commonly used in industry. For stainless steel, a special steel series formed by people's long-term struggle against corrosion, there are more than ten common elements. In addition to iron, which is the basic element of steel, the most influential elements on the properties and structure of stainless steel are carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper and cobalt. Except for carbon, silicon and nitrogen, these elements are all transitional elements in the periodic table of chemical elements.
In fact, the stainless steel used in industry has several or more elements at the same time. When several elements coexist in the stainless steel as a whole, their influence is much more complex than that when they exist alone. In this case, not only the effect of each element itself, but also the influence of each other should be considered. Therefore, the structure of stainless steel depends on all kinds of elements. The sum of the effects of elements.
1. The effect of various elements on the properties and structure of stainless steel
1-1. The decisive role of chromium in stainless steel
There is only one element that determines the nature of stainless steel, chromium. Each stainless steel contains a certain amount of chromium. So far, there is no stainless steel without chromium. The basic reason why chromium has become the main element determining the properties of stainless steel is that after adding chromium as alloy element to steel, the internal contradiction movement of chromium is promoted to develop in favor of corrosion resistance. This change can be explained in 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 because the anodic reaction is blocked. There are many theories for passivation of metals and alloys, such as film theory, adsorption theory and electron arrangement theory.
Duality of Carbon in Stainless Steel
Carbon is one of the main elements in industrial steels. The properties and structure of steels depend largely on the content and distribution of carbon in steels, especially in stainless steels. The effect of carbon on the structure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is an element of stable austenite, and its effect is very large (about 30 times that of nickel). On the other hand, because of the affinity of carbon and chromium, it forms a series of complex carbides with chromium. Therefore, the role of carbon in stainless steel is contradictory in terms of strength and corrosion resistance of candles.
Understanding the law of this effect, we can choose stainless steel with different carbon content from different requirements.
For example, the standard chromium content of the five stainless steel grades, 0Crl3-4Cr13, which is the most widely used and minimum in industry, is set at 12-14%, which is determined after taking into account the factors that carbon and chromium form chromium carbide. The purpose is that after the combination of carbon and chromium into chromium carbide, the chromium content in the solid solution should not be less than 11.7% of the minimum chromium content.
For these five steel grades, because of the different carbon content, the strength and corrosion resistance are also different. 0Cr13-2Crl3 steel has better corrosion resistance but lower strength than 3Crl3 and 4Cr13 steel, which are mostly used to manufacture structural parts. The latter two steel grades, because of their high carbon content, can obtain higher strength for manufacturing parts requiring high strength and wear resistance, such as springs and cutters. For example, in order to overcome the intergranular corrosion of 18-8 Chromium-Nickel Stainless steel, the carbon content of steel can be reduced to less than 0.03%, or the elements with greater affinity than chromium and carbon (titanium or niobium) can be added to prevent the formation of chromium carbide. When high hardness and wear resistance become the main requirements, we can increase the carbon content of steel while appropriately increasing the chromium content to meet both hardness and resistance. The requirements of abrasiveness and corrosion resistance are also considered. Industrial bearings, measuring tools and blades are made of stainless steel 9Cr18 and 9Cr17MoVCo. Although the carbon content is as high as 0.85-0.95%, the requirement of corrosion resistance is still guaranteed because their chromium content is correspondingly increased.
Generally speaking, the carbon content of stainless steel used in industry is relatively low. The carbon content of most stainless steel ranges from 0.1% to 0.4%, while that of acid-resistant steel ranges from 0.1% to 0.2%. Stainless steel with carbon content greater than 0.4% only accounts for a small part of the total number of steel grades. This is because under most operating conditions, stainless steel always aims at corrosion resistance. In addition, low carbon content is also due to some technical requirements, such as easy welding and cold deformation.
1-3. The role of nickel in stainless steel is played only after it cooperates with chromium.
Nickel is an excellent corrosion resistant material and an important alloying element in alloy steel. Nickel is an austenite forming element in steel, but the corrosion resistance of low carbon nickel steel in some media will change significantly only when the content of nickel is 27% in order to obtain pure austenite structure. So nickel can't be made of stainless steel alone. But when nickel and chromium exist in stainless steel at the same time, the stainless steel containing nickel 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, thus improving the corrosion resistance and technological properties of stainless steel.
1-4. Manganese and nitrogen can replace nickel in chromium-nickel stainless steel
Chromium-nickel austenitic steel has many advantages, but in recent decades due to