Chemical elements are known to have more than 100 kinds of industrial materials used in steel can be encountered in about more than 20 kinds of chemical elements. For people in the fight against corrosion for long term practice of this particular form ofstainless steel series, the most commonly used in a dozen elements, in addition to the basic elements of the composition of steel other than iron, the performance of stainless steel and organizations most affected.
The elements are: Carbon, Chromium, Nickel, Manganese, Silicon, Molybdenum, Niobium, Titanium and Miobium, Nitrogen,Copper, Cobalt, Aluminum, Sulfur and Selenium.
These elements, in addition to carbon, silicon, other than nitrogen, are located in the periodic table of chemical elements of transition. In fact the application of the stainless steel tube industry at the same time there are several elements as well as a dozen, when the number of elements co-exist in a continuum of stainless steel tube, they separate the impact of the presence of more much more complex, because in this cases not only have to consider the role of the various elements of their own, and they should pay attention to the impact of each other, so the organization decided to stainless steel pipe of various elements in the sum of the impact.
1. Various elements on the performance of stainless steel and the impact and role of organizations
1-1. Chromium in the stainless steel a decisive role in stainless steel is a decision of only one element, that is, chromium, stainless steel each contain a certain amount of chromium. To date, no non-chromium stainless steel. Chromium stainless steel performance decision has become the main element, the fundamental reason is to add chromium as an alloying element, the internal contradiction of campaign in favor of resistance to the development of corrosion damage.
Such a change can be obtained from the following description:
1. Chromium Fe-based solid solution so that the electrode potential to improve
2. Chromium electronic absorption of iron so that iron-passivated
Anodic passivation is due to be prevented from arising from reaction of metal and alloy corrosion resistance phenomenon can be improved. Passivation of metals and alloys constitute the theory of many major film theory, deals with the electronic order of adsorption.
1-2. Carbon in the stainless steel tube in the dual nature of Carbon steel is the industry one of the key elements, steel and organizational performance to a large extent determined by the carbon content in steel and its distribution in the form of the impact of carbon stainless steel is particularly significant. Carbon in the stainless steel on the impact of organizations mainly in two ways, on the one hand is stable austenite carbon element, and the extent of the role of a large (approximately 30 times for nickel), on the other hand, as a result of the affinity of carbon and chromium is large, with the formation of chromium – series of complex carbides. Therefore, the candle from the intensity and decay properties, both in terms of carbon in the role of stainless steel are mutually contradictory.
Recognizing the impact of the law, we can use from different requirements of different carbon content stainless steel. For example, most widely used in industry, but also the stainless steel at least – 0Crl3 ~ 4Cr13 five standard steel grade chromium amount is 12 ~ 14%, that is, the carbon to form chromium carbide and chromium factors taken into account after determined that the purpose is to make the combination of carbon and chromium as chromium carbide, the solid solution of chromium in the amount of not less than 11.7% of the minimum amount of chromium.
No. 5 on the steel is due to the different carbon content, strength and corrosion resistance is also differentiated, 0Cr13 ~ 2Crl3 better corrosion resistance of steel but lower than the 3Crl3 and 4Cr13 strength steel, used in the manufacture of the structure of many parts, after As the No. 2 steel with higher carbon intensity will be high and more used in the manufacture of springs, cutting tools, such as high strength and wear-resistant parts. Another example is in order to overcome the 18-8 Cr-Ni stainless steel intergranular corrosion, can be carbon steel to 0.03% below, or by adding chromium and carbon affinity than the larger elements (titanium or niobium), so that does not form a carbide chromium, Another example is when the high hardness and wear resistance as a major requirement, we can increase the carbon content of steel at the same time to suitably increase the amount of chromium so that not only satisfy the requirements of the hardness and wear resistance, but also take into account – will be corrosion-resistant function, used for industrial bearings, has a stainless steel blade measuring and 9Cr18 and 9Cr17MoVCo steel, although the carbon content as high as 0.85 ~ 0.95%, due to their chromium also increased accordingly, it is still guaranteed the corrosion resistance of requirements.
Generally speaking, the current industry access to the application of the carbon content of stainless steel pipe are relatively low, most of the carbon content of stainless steel in the 0.1 ~ 0.4%, and acid-resistant carbon steel with 0.1 to 0.2% of the majority. Greater than 0.4% carbon content of stainless steel grade is only a small fraction of the total, which is used because in most conditions, to corrosion-resistant stainless steel is always the primary purpose. In addition, the lower carbon content is also a process for some requirements, such as the ease of welding and cold deformation.
1-3. Nickel in the role of stainless steel and chromium in the play after the Nickel is an excellent corrosion-resistant materials, is also an important steel alloying elements. Nickel in the austenitic stanless steel pipe is the formation of the elements,such as 304,316,321.but the low-carbon steel to obtain pure nickel austenite, the volume of nickel to achieve 24%; and only when 27 percent nickel steel, in some medium resistance significant changes in corrosion. Thus alone can not constitute a nickel stainless steel. But at the same time the existence of nickel and chromium in the stainless steel, the nickel-containing stainless steel but has many valuable properties. Based on the above circumstances, we can see that nickel as alloying elements in the role of stainless steel is that it allows high-chromium steel changes, so that corrosion resistance of stainless steel and certain to improve process performance.
1-4. Manganese and Nitrogen can substitute for Ni-Cr-Ni stainless steel Cr-Ni austenitic steels Although many of the advantages, but in recent decades as a result of nickel-based heat-resistant nickel alloy and the heat below 20% of the large number of strong steel development and applications, as well as the growing chemical industry of the increasing demand of stainless steel The greater the amount of the nickel deposits less concentrated in a few areas, it appeared in the world and the need for nickel in the conflict area. Therefore, in stainless steel alloys and many other fields (such as a large forging steel, tool steel, heat strong steel, etc.), especially the lack of nickel resources of the country, carried out extensive section of nickel and nickel on behalf of other elements in the scientific research and production practice, in this regard the research and application is based on a relatively large number of manganese and nitrogen to replace the stainless steel and heat-resistant nickel steel.
For the role of manganese and nickel austenitic similar. But to be more exact, the role of manganese does not lie in the formation of austenite, but it reduced the critical quenching rate of steel in cooling to increase the stability of austenite and suppress the decomposition of austenite, so that the formation of high temperature austenite to room temperature is maintained. In improving the corrosion resistance of steel, the manganese plays a minor role, such as manganese steel increased from 0 to 10.4% change, do not make steel in the air with the acid corrosion resistance of significant change. This is because the manganese to iron-based solid solution to increase the electrode potential does not help the formation of the protective role of the oxide film is very low, so the industry although some of the austenitic manganese steel alloys (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steel, etc.), but they can not be used as the use of stainless steel. Manganese in steel is about the role of a stable austenitic nickel half, that is 2% of nitrogen in steel is the role of austenite
stability and the role of larger than nickel. For example, to save with 18% chromium steel austenitic at room temperature under the body to manganese and nitrogen on behalf of low-nickel stainless steel and nickel chromium nickel element nitrogen does not induce manganese steel has been applied in industry, and some has successfully replaced the classic chrome-nickel stainless steel 18-8.
1-5. Stainless steel Titanium or Niobium Canada is to prevent intergranular corrosion.
1-6. Molybdenum and Copper can increase some of the corrosion resistance of stainless steel.
1-7. Other elements on the performance of stainless steel and organizational impact
More than nine major elements of stainless steel performance and the impact of organizations, in addition to these elements and organizational performance of stainless steel elements of a greater impact, the stainless steel contains a number of other elements. Some, like steel and general for the regular deposit of impurity elements, such as silicon, sulfur and phosphorus. Also some specific purpose in order to join, such as cobalt, boron, selenium, and other rare earth elements. From the stainless steel corrosion resistance of the nature of the main, these elements have been discussed in relation to the nine elements are non-key aspects, although the case, but can not be completely ignored because their performance of stainless steel and organizations have also taken place in the same impact.
Silicon is a ferrite forming element, in general, always keep the stainless steel for the impurity elements. Cobalt as alloying elements in steel by the application of small, this is because the high price of cobalt and in other ways (such as high-speed steel, carbide, cobalt-based heat-resistant alloys, magnetic or hard magnetic alloy, etc.) has a more important purposes. Stainless steel in the general increase in the cobalt alloy elements for not more commonly used stainless steel, such as 9Crl7MoVCo (including 1.2-1.8% cobalt) plus cobalt, the purpose is not to improve corrosion resistance and to improve hardness, which are mainly used for stainless steel slicing machinery manufacturing cutting tools, such as scissors and blades.
Boron high-chromium ferritic stainless steel Crl7Mo2Ti plus 0.005% of boron, can in boiling 65% acetic acid can enhance the corrosion resistance. Add small amount of boron (0.0006 ~ 0.0007%) austenitic stainless steel will enable the plastic to improve the thermal state. A small amount of boron due to the formation of low melting point eutectic, so that when austenitic steel welding hot cracking tendency to increase, but contains more boron (0.5 ~ 0.6%) when it prevents the emergence of hot cracking . When containing 0.5 ~ 0.6% of boron, the formation of austenite – two-phase boride organizations to lower the melting point of weld. Coagulation bath temperature is below half the melting zone, the base metal in the cooling of the tensile stress generated by the liquid is. Solid- state under the weld metal, is at this time without causing cracks even in the near seam zone formed a crack, it can be in liquid – solid metal by filling the pool. B-containing austenitic stainless steel of the Cr-Ni in the atomic energy industry has a special purpose.
Phosphorus in the general impurity elements are stainless steel, but its in danger of austenitic stainless steel in general is not as significant in steel, it allows a higher concentration, if the information up to 0.06%, to control in favor of smelting. Individual austenite manganese steel output of about 0.06% phosphorus (such as steel 2Crl3NiMn9) and 0.08% (for example, steel Cr14Mnl4Ni). The use of phosphorus on the strengthening of the role of steel as well as age-hardening increases phosphorus alloying elements of stainless steel, PH17-10P steel (containing 0.25% phosphorus) is a PH-HNM steel (containing 0.30 P) and so on.
Sulfur and selenium in the general stainless steel is also often of impurity elements. However, China and Canada to the stainless steel 0.2 ~ 0.4% of sulfur, can improve the cutting performance of stainless steel, selenium also has the same effect. Sulfur and selenium to improve the cutting performance of stainless steel because they reduce the toughness of stainless steel, such as the 18-8 Cr-Ni stainless steel in general the impact of the value of up to 30 kg / cm 2. Containing 0.31% sulfur 18-8 steel (0.084% C, 18.15% Cr, 9.25% Ni) the impact of the value of 1.8 kg / cm2; containing 0.22% selenium 18 -8 steel (0.094% C, 18.4% Cr, 9% Ni) the impact of a value of 3.24 kilograms / square centimeters. Both sulfur and selenium to reduce the corrosion resistance of stainless steel, so the practical application of them as a stainless steel alloy of the rare element.
Rare-earth element rare-earth element used in stainless steel pipe, the key is to improve the process performance. Crl7Ti such as the steel and steel plus Cr17Mo2Ti a small number of rare earth elements, can be eliminated in ingot caused by hydrogen bubbles and the reduction of cracks in the slab. Austenitic and austenitic – ferritic stainless steel in 0.02 ~ 0.5% increase in the rare earth elements (Ce-La alloy), can significantly improve the performance of forging. Had a 19.5 percent containing chromium, nickel 23% copper and molybdenum austenitic manganese steel, due to thermal processing performance in the past only the production of castings, after the increase of rare earth elements can be rolled into various sections.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)