1 issues raised
Technology unified regulations usually consist of &ldquo austenitic stainless steel used in the container; may cause the intergranular corrosion environment, welding should be done after solid solution or stabilizing ”, make such a request, the rationality of its own existence. But even if the design personnel in drawing technical requirements proposed in this article. The requirements of factory of stainless steel containers (such as heat exchanger) of the heat treatment after welding, the process parameters of the heat treatment is difficult to control and some other unexpected difficulties, often difficult to achieve this ideal design requirements, actually in service of stainless steel the vast majority of states in the use of equipment after welding.
This prompted us to think: intergranular corrosion corrosion is the most common form of austenitic stainless steel, then what is the mechanism of intergranular corrosion in the media environment? What will cause the intergranular corrosion? To prevent and control method of intergranular corrosion of austenitic stainless steel containers have? For may cause the intergranular corrosion of welding environment after all heat treatment? This paper inspection monograph standards and norms related, combined with the actual production of some personal views.
2 intergranular corrosion mechanism
Intergranular corrosion is a common local corrosion, corrosion of metal or alloy along the grain boundary or near its regional development, while the grain corrosion is very slight, the corrosion is called intergranular corrosion, the corrosion of the binding force between grains greatly weakened. Severe intergranular corrosion can cause the metal to lose strength and ductility, and to break under normal load. The theory of intergranular corrosion is mainly based on the theory of chromium deficiency and the theory of grain boundary impurity selective dissolution.
2.1 chromium deficiency theory
Common austenitic stainless steel, in the oxidizing or weak oxidizing medium produced intergranular corrosion, mostly due to improper processing or use caused by heat. The so-called improper heating refers to the steel heating or slow cooling through 450 to 850 degrees Celsius temperature zone, the steel will be sensitive to intergranular corrosion. So this temperature is a dangerous temperature for austenitic stainless steel. Stainless steel material in the factory has been solid solution treatment, the so-called solid solution is to heat the steel to 1050 to 1150 DEG C after quenching, the purpose is to obtain homogeneous solid solution. There is a small amount of carbon in austenite steel, and the solid solubility of carbon in austenite decreases with the decrease of temperature. Such as 0Cr18Ni9Ti, at 1100 C, the solid solubility of carbon is about 0.2%, in the 500 ~ 700 DEG C, about 0.02%. Therefore, the carbon in the solid solution is supersaturated.
When the steel is heated or cooled through 450 ~ 850 DEG C, 23C6 (Fe, Cr) can be precipitated from austenite and distributed on grain boundaries. (Fe, Cr) 23C6 chromium content is much higher than the austenite amount of chromium, it consumes a lot of natural precipitation of chromium and chromium near the grain boundaries, not consumed by diffusion timely supplemented from the grains, because the diffusion rate of chromium is very slow, the chromium content is lower than that of the grain boundary passivation must be limited (12%Cr), the formation of chromium depleted zone, so the passive damage area near grain boundary potential was decreased, and the grain itself remains passive, potential is higher, grains and grain boundaries constitute a living — — — passive micro galvanic battery, battery with large cathode anode area ratio, thus lead to corrosion of grain boundary region.
2.2 grain boundary impurity selective dissolution theory
In the production practice, we also know that austenitic stainless steel can also produce intergranular corrosion in strong oxidizing medium (such as concentrated nitric acid), but the corrosion situation and the situation of oxidation or weak oxidizing medium. Usually occurs in the solution treatment of steel, after the sensitization of the steel generally does not occur. When the impurity in the solid solution is 100ppm or the silicon impurity is 1000 - 2000ppm, they will segregate on the grain boundary. These impurities dissolve under the action of strong oxidizing medium, leading to intergranular corrosion. The steel after sensitization, due to carbon and phosphorus can generate 23C6 (MP), or due to carbon segregation of phosphorus to the first limit grain boundary diffusion, these two cases will waive or reduce the impurity segregation in the grain boundary, can eliminate or weaken the sensitivity of steel to intergranular corrosion.
The theory of the two kinds of mechanisms to explain the intergranular corrosion can be applied to a certain alloy's microstructure and a certain medium. The most common corrosion of stainless steel in production practice occurs in the medium of weak oxidation or oxidation, so most of the corrosion examples can be explained by the theory of chromium deficiency.
3 medium environment for intergranular corrosion
Common austenitic stainless steel intergranular corrosion of the media, there are two main categories. One is the oxidation or weak oxidizing medium, one is a strong oxidizing medium, such as concentrated nitric acid, etc.. Common is the first category, the following list of common austenitic stainless steel intergranular corrosion caused by the media environment.
The 3.1 most common cause of &ldquo intergranular corrosion of austenitic stainless steel in G. A. medium Nelson establishment; corrosion data chart ” lists the common causes of intergranular corrosion of austenitic stainless steel medium: acetic acid, acetic acid and salicylic acid, ammonium nitrate, ammonium sulfate, chromic acid, copper sulfate, fatty acid, formic acid, ferric sulfate, hydrofluoric acid + ferric sulfate, lactic acid, nitric acid, nitric acid, hydrochloric acid, oxalic acid, phosphoric acid, water, salt, sodium hypochlorite, sodium bisulfate, sulfur dioxide (wet), sulfuric acid, sulfuric acid, copper sulfate, sulfuric acid and ferrous sulfate, sulfuric acid + methanol, sulfuric acid and nitric acid, phthalic acid, sodium hydroxide and sodium sulfide.
3.2 intergranular corrosion tendency test
When austenitic stainless steel is used in the environment which may cause intergranular corrosion, intergranular corrosion tendency test should be carried out according to GB4334. 1 ~ GB4334 stainless steel intergranular corrosion test method. The selection of test methods for intergranular corrosion tendency of austenitic stainless steels and their eligibility requirements shall comply with the following requirements:
(1) when the temperature is greater than or equal to 60 DEG C, and the concentration is greater than or equal toThe use of 5% austenitic stainless steel in nitric acid and concentrated nitric acid special stainless steel, 65% stainless steel should be GB4334. 3 "test method" nitric acid corrosion test, corrosion of the average corrosion rate of five cycle or three cycle rate should be less than 0 6g/ M2 • H (or the equivalent of 0 6mm /a). The state of the sample can be used or sensitized.
(2) Ni Cr austenitic stainless steel (such as 0Cr18Ni10Ti, 0Cr18Ni9, 00Cr19Ni10 and similar steel): General requirements: according to GB4334. 5 "stainless steel &mdash" test method of copper sulfate; sulfuric acid, bending test, the specimen surface shall not have intergranular corrosion cracks. Higher requirements: according to GB4334. 2 "stainless steel sulfuric acid — ferric sulfate corrosion test method", the average corrosion rate should not be greater than 1 1g/ M2 • H.
(3) molybdenum austenitic stainless steel (such as 0Cr18Ni12Mo2Ti, 00Cr17Ni14Mo2 and similar steel): General requirements: according to GB4334. 5 "stainless steel &mdash" test method of copper sulfate; sulfuric acid, bending test, the specimen surface shall not have intergranular corrosion cracks. Higher requirements: according to GB4334. 4 "stainless steel nitric acid — hydrofluoric acid corrosion test method", the corrosion ratio is not greater than 1.5. Also according to GB4334. 2 "sulfuric acid — ferric sulfate test method", the average corrosion rate should not be greater than 1 1g/m2 • H.
(4) when the media has special requirements, it can be carried out in addition to the above provisions of intergranular corrosion test, and provides the corresponding eligibility requirements.
4 measures to prevent and control intergranular corrosion
According to the corrosion mechanism, the measures to prevent and control the intergranular corrosion of austenitic stainless steel are as follows:
(1) the use of ultra low carbon stainless steel to reduce the carbon content of less than 0.03%, such as the use of 00Cr17Ni14Mo2, so that the steel is not formed (Fe, Cr) 23C6, there is no chromium depleted zone, to prevent the occurrence of intergranular corrosion. General strength is not high, the force is not large, require plastic parts, from an economic point of view, can choose 0Cr18Ni9, etc..
(2) stabilized stainless steel containing titanium and niobium in steel, stainless steel, (that is, we often say that the stabilized stainless steel), adding a certain amount of steel smelting of titanium and niobium two components, and their carbon affinity, the TiC or NbC form steel, and the solid solubility of TiC or NbC and than (Fe, Cr) 23C6 is much smaller in the solid solution temperature is almost insoluble in austenite. In this way, although the Fe (Cr) 23C6 does not cause a large number of precipitation on the grain boundary, the tendency of intergranular corrosion of austenitic stainless steel is eliminated to a great extent. Such as 1Cr18Ni9Ti, 1Cr18Ni9Nb and other steel, can work in the range of 500 to 700 DEG C, there will be no intergranular corrosion tendency.
(3) when the austenitic stainless steel is welded, the temperature of the molten pool is as high as more than 1300 DEG C, and the temperature on both sides of the weld decreases with the increase of the distance. We should try to avoid sensitization of austenitic stainless steel in the temperature range of heating and slow cooling, if found to have the tendency of intercrystalline corrosion, the general non stabilized stainless steel is heated to 1000 to 1120 DEG C, holding per mm 1 to 2 minutes, and then quenching; stabilization of stainless steel by heating to 950 to 1050 DEG C is appropriate. After the solution treatment, the steel must be prevented from heating at the temperature of sensitization, otherwise the chromium carbide will be precipitated along the grain boundaries.
(4) using the correct welding method, if the operation is not skilled or welding material is too thick, the welding time is long to stay in the sensitized temperature zone opportunity more, the welded seam on both sides have sensitivity to intergranular corrosion. In order to reduce the sensitivity of the welded joint, the input of line energy should be minimized. Argon arc welding is generally lower than the input energy of the arc welding. Should be used for welding of ultra low carbon stainless steel or Ti, Nb stable elements of stainless steel, for the electrode should be selected for ultra low carbon electrode or Nb containing electrode. In order to avoid the overheating of the welding joint, it is necessary to use the argon arc welding, the operation is fast, and the welding should be cooled quickly.
5 the weld zone is not always stressed that the heat treatment after welding, the general solution to 1100 to 1150 DEG C after a certain time of insulation and cooling, three minutes to complete 925 to 540 DEG C temperature range on cooling, fast cooling to 425 degrees centigrade; stabilization in 850 ~ 880 DEG the range of a few hours after the air cooling heat insulation. The expected effect of heat treatment after welding, with all the key parameters of the whole process of heat treatment (such as the furnace temperature and heating rate, the heating process of the workpiece in different parts of the furnace atmosphere, temperature, time of heat preservation, thermal insulation in the process of the various parts of the temperature, cooling rate, temperature etc.) closely linked.
For the austenitic stainless steel vessel, which may cause the intergranular corrosion environment, the solid solution treatment or stabilization treatment of the general parts can be realized. And the welding heat treatment of the whole container (mostly heat exchanger) will face many difficulties. This kind of treatment is not part of the post weld heat treatment, but the entire welding parts or the whole container after welding heat treatment. Because of the complex structure of most chemical vessels (such as the shell and tube heat exchangers).
If the requirements of the whole shell and tube heat exchanger welding area after welding solid solution or stabilization treatment, the key process parameters can not be controlled at all, not to mention the quality of heat treatment after welding. Even if the treatment is often self defeating, not only failed to improve the weld structure, organization structure of parent material but was notSome deterioration. Therefore, even if the austenitic stainless steel chemical vessel used in the intergranular corrosion environment, more than 90% of the welding state is still used, rather than the use of post weld heat treatment.
6 point of view
Chromium nickel austenitic stainless steel is the most commonly used corrosion resistant material, and intergranular corrosion is the most common failure form of Cr Ni austenitic stainless steel vessel. Intergranular corrosion greatly weakens the bonding force between grains, and the mechanical strength is completely lost. The corrosion of stainless steel, the surface appears to be very bright, but can not afford to knock gently broken into fine particles. Due to the intergranular corrosion is not easy to check, so the sudden destruction of equipment, it is very harmful, we should pay enough attention to.
The chromium nickel austenitic stainless steel vessel is basically formed by welding, and the two sides of the welded joint are intergranular corrosion sensitive zone. Through the post weld heat treatment, to improve the resistance to intergranular corrosion of the weld zone, to achieve the same degree as the base material, which is our goal, is our original intention of the post weld heat treatment. But in practice, there are many factors to consider, such as welding structure of complex shape, heat treatment after welding process parameters is difficult to guarantee, therefore, in fact most of the Ni Cr austenitic stainless steel after welding using state service.
For whether the intergranular corrosion resistance of Cr Ni austenitic stainless steel vessel weld zone for solid solution treatment or stabilization treatment, can not simply be generalized, should shape concrete analysis of container, analysis whether it can guarantee the effect of heat treatment, otherwise even if we put forward the postweld heat treatment requirements, but often counterproductive. Can not achieve the desired effect, but will affect the structure of base metal.
In order to improve the resistance to intergranular corrosion of Cr Ni austenitic stainless steel vessel, according to specific corrosion environment, the corrosion mechanism of material basis, the first optional ultra low carbon stainless steel, stabilized stainless steel welding, selection of welding methods, the proper combination of the above several prevention and control measures, in order to achieve good results, can not rely on only after welding solid solution or stabilization treatment.
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Round steel weight (kg) =0.00617× diameter × diameter × length
Other steel (pipe) material formula weight (Tianjin Huashun transmission)
1, angle: =0.00785× per meter weight; (edge width + edge width — edge thickness) × edge thickness
2, pipe: =0.02466× per meter weight; wall thickness × (outside diameter — wall thickness)
3, round steel: m per =0.00617× weight; diameter × diameter (thread steel and round steel the same)
4, Fang Gang: m per =0.00786× weight; edge width × edge width
5 and six angles: =0.0068× per m, × to the opposite diameter, and to the opposite diameter
6, octagonal steel: m per =0.0065× weight; diameter × diameter
7, equal angles: m per weight = edge width × edge thickness × 0.015
8, flat steel: m per =0.00785× weight; thickness × width
9 seamless steel tube: m per weight =0.02466× wall thickness × (outside diameter - wall thickness)
10, welding steel: m per weight = seamless steel tube
11 square meters, each weight plate thickness of =7.85×
12, brass tube: =0.02670× per meter weight; wall thickness × (outside diameter - wall thickness)
13, copper tube: =0.02796× per meter weight; wall thickness × (outside diameter - wall thickness)
14, aluminum checkered sheet: =2.96× per square meter thickness;
15, non-ferrous metals: copper plate density formula of 8.9 yellow copper lead zinc plate 8.5 7.2 11.3716, non-ferrous metal plate, the weight per square meter density = × thickness
17, square tube: per meter weight = (length side + side length) × 2× thick × 0.00785
18, unequal angle iron: =0.00785× per meter weight; edge thickness (long side width + short side width - side thickness)
19, I-beam: =0.00785× per meter weight; waist thickness [high +f (leg width - waist thick)]
20, channel: =0.00785× per meter weight; waist thick [high +e (leg width - waist thick)]
All kinds of steel (wood) weight conversion formula (Tianjin Huashun transmission)
The weight of steel =0.25× π × (diameter square - diameter Square) × L× π the proportion of iron and steel: steel pipe length = 3.14 L= proportion up to 7.8 so the weight of steel tube (diameter =0.25× 3.14× × square square - diameter) L& times; 7.8 meters (if the size units take * M), calculated the weight of the kg (Kg)