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When the surface of the stainless steel plate is not annealed (for example, the surface after welding), drilling mud residue, rust-proof film, or ferrite impregnated with stainless steel after the molding process, the corrosion resistance of the stainless steel material will be directly affected. At this point you need to use acidic oil cleaning agent to do the appropriate cleaning and maintenance operations on the stainless steel surface. Stainless steel pickling passivation can completely remove impurities and promote the formation of a new passivation layer, which is a complete chemical reaction process. It can not only clean the surface of the stainless steel plate, but also can form a completely uniform silver white on the surface of the treated stainless steel plate. According to actual tasks, stainless steel plates with different structures and surface sizes can use different pickling products. First look at the stainless steel pickling passivation paste, this product is used to deal with the surface of the larger stainless steel plate or surface weld around the weld spot, oxide layer color, as long as the uniform smear on the surface of the stainless steel plate, wait until The surface of the stainless steel plate is completely uniform and white. Followed by the stainless steel pickling passivation solution, which is mainly used for the pickling of small serial parts, such products are easy to immerse, usually using plastic containers soaked at room temperature, and even can achieve high efficiency pickling and passivation done synchronously, such as stainless steel After the surface of the board has been pickled and passivated, the maximum salt spray resistance can reach about 500 hours. In addition to the pickling spray, this process is suitable for pickling large stainless steel surfaces (including wall panels, containers) as well as non-removable structures or difficult-to-access structures such as bridge components. Pickling sprays can increase application control. There is also ablation pickling. This technique can remove the surface tension and internal stress of the stainless steel plate material by specifically removing the 3-5 micron surface, and also remove surface microcracking. However, no matter which process is used, the treated surface acid must be completely cleaned. It is best to use high-pressure cleaning. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The grade of 904L stainless steel is 00Cr20Ni25M04.5Cu, which belongs to a high austenitic stainless steel with high carbon content and low carbon content. It has strong corrosion resistance in the dilute sulfuric acid environment, because the addition of copper makes it have strong acid resistance. It is designed for the harsh environment of corrosion. Therefore, this austenitic stainless steel is mainly used for the manufacture of corrosion resistant pressure vessels. The composition of 904L is mainly composed of chromium and nickel. It is difficult to machine and has poor machinability. The main reasons are as follows. First of all, 904L stainless steel is more powerful than other kinds of stainless steel. Because the hardness of 904L stainless steel is not high (70-90HRB), the plasticity is better, the elongation is more than 40%, the shrinkage of the section is more than 50%. The tensile strength is B more than 490MPa, and the yield strength is 0.2 or more 216MPa. Therefore, the plastic deformation is larger in the cutting process, and the cutting force is increased. The second is the low thermal conductivity. The thermal conductivity of 904L stainless steel (20 C) is 12.9W / (mmK), and its thermal conductivity is low, only about 1 / 4 of the 45 steel. The thermal conductivity of the 45 steel is 47.5W / (mmK). Thermal conductivity is one of the main factors that affect the heat conduction of cutting. The lower the thermal conductivity of the processed material, the less the heat taken by the chip and the workpiece, and the more heat accumulated on the cutting tool, making the tool very easy to wear. Third, it is easy to form the chip tumor. Because of the high toughness of 904L stainless steel, it has strong affinity with the tool material during the cutting process. The cutting tool face is strongly rubbed with the chip bottom metal during cutting. It will produce adhesion phenomenon under the action of high temperature and high pressure. It is not easy to get the surface with high surface roughness. Fourth, the chip is not easy to bend and break. The elongation of 904L stainless steel is high, so the chip is not easy to bend and break in the cutting process. Failure to take proper measures will affect the normal operation of the cutting process, and scratch the machined surface easily, and even cause the tool to collapse and damage. Because of the above features of 904L stainless steel, the cutting tool is very easy to wear and the machining efficiency is low, and the workpiece is difficult to achieve the surface roughness and machining precision required by the drawings. Source: China Pipe Fittings Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Several methods can be used to increase the strength of stainless steel plates, which are alloying, quenching, working hardening, and a very common form of heat treatment, namely, precipitation or aging hardening. The so-called age hardening is in fact dependent on the formation of precipitates. In order to achieve the best combination of mechanical properties, the heat treatment cycle of stainless steel plates must be strictly controlled. In order to understand how sediment affects mechanical properties, some basic metallurgy needs to be understood. The mechanism of precipitation hardening requires that the alloy element, that is, the solute is in the metal, and the solubility in the solvent increases with the increase of temperature, and the solid solution line shows that the alloy element B decreases with the temperature of the solvent A. A metaphor is a salt in the water. As the temperature rises, more salt can be dissolved, but when the salt crystal begins to form or precipitate, it allows the solution to cool and vice versa. In addition to the dissolution and precipitation process that occurs in the solid and thus the atom is much slower because the atom is more difficult to move beyond the solid than the liquid solution, the same process occurs in the suitable alloying metal. As a result, once the deposit has been dissolved by a high enough temperature, that is, the wire is above the line line, it can be prevented by rapid cooling or quenching to prevent it from re forming. This heat treatment is called solid solution heat treatment and is formed to form an unstable supercooled solid solution. If it is reheated to a lower aging or precipitation hardening temperature, the precipitates will begin to be reformed. These precipitates are carried out with heat treatment. In a solid solution heat treated stainless steel plate, the atoms of the alloying elements are randomly distributed in the whole matrix, but once the temperature rises, the precipitation begins to form through the nucleation and growth process. At relatively low temperatures and shorter timescales, the solute atoms begin to gather together to form a very small, very small sediment called Guinier-Preston (GP), the two regions named after the first two metallurgists. The GP region is very small, which is not visible by ordinary optical microscope, but can be observed by electron microscope at about 100 thousand magnification. The GP region is described as coherent. In other words, they have the same crystal structure as solvent metals. However, they distort the lattice, that is, the framework of atoms. This makes the dislocation more difficult to move in the lattice and is the dislocation movement of the metal deformation; therefore, the tensile strength and hardness are increased, but the ductility and toughness are reduced. As the aging treatment continues or the temperature increases, the tensile strength continues to increase with the growth and thickening of the sediment. However, to some extent, the sediment began to lose consistency. Just before that, the alloy has the highest tensile strength. As the formation and size of these particles increase, the tensile strength decreases. It is said that the alloy is surplus, although the precipitates still contribute to the tensile strength of the alloy. High strength low alloy (HSLA) steel is a good example. In this case, incoherent, over aging precipitates are used to substantially increase the tensile strength. In order to achieve the best combination of properties, precipitates need to be uniformly distributed throughout the alloy grains and have the best size. The aging temperature and / or time can be significantly changed to adapt to the distribution and size of the sediment, and longer time and / or higher temperature usually lead to a decrease in strength, but the ductility increases, and the overused structure provides the lowest tensile strength but the highest ductility. Ferritic and nickel based alloys are usually used for over aging to ensure reasonable ductility. It can be seen that for some alloys, such as 17 / 4PH stainless steel, the precipitation mechanism is low enough to be able to cool the components in the stationary air or, like A286 stainless steel, requires long aging time. On the other hand, Al Cu alloy 2219 can be aged for several days at room temperature. Some of the 6000 (Al-Si-Mg) and 7000 series (Al-Zn-Mg) alloys will be similar to aging at ambient temperature. This is called natural aging, and high temperature aging is called artificial aging. In general, strict control of heat treatment time, temperature and cooling rate is essential if desired performance is required. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Hydrogen corrosion may occur in ammonia synthesis, hydrogen desulfurization and hydrogenation and petroleum refining units. Carbon steel is not suitable for high pressure hydrogen installations at temperatures above 232 C. Hydrogen can spread into the stainless steel plate and form methane at the grain boundary or in the pearlite zone with the iron carbide. Methane can not spread to the outside of the steel and gather together to form white spots or cracks in the metal. In order to prevent the formation of methane, cementite must be replaced by stable carbides. Stainless steel plates must be added chromium, vanadium, titanium and other elements. In fact, the increase of chromium content allows a higher use of temperature and hydrogen pressure to form chromium carbide in these steels, and the hydrogen element it meets is stable. Chromium steel and austenitic stainless steel with chromium content higher than 12% can be corroded in all known applications under harsh conditions (temperature greater than 593 degrees Celsius). Most metals and alloys do not react with molecular nitrogen at high temperatures, but atomic nitrogen can react with many stainless steels. It penetrates into the surface of stainless steel to produce brittle nitride. Iron, aluminum, titanium, chromium and other alloying elements may participate in these reactions. The main source of atomic nitrogen is the decomposition of ammonia. Ammonia converter, ammonia plant heater and ammonia decomposition at 371 -593 C and -10.5Kg/mm2 under an atmospheric pressure. In these atmospheres, chromium carbide is produced in low chromium steel. It may be corroded by atomic nitrogen to form chromium nitride and release carbon and hydrogen to form methane, which, as mentioned above, may form white spots or cracks. However, when the chromium content is greater than 12%, the carbides in these stainless steel are more stable than chromium nitride, so the front reaction will not appear, so the stainless steel plate can now be used in the high temperature environment of the heat ammonia. The state of stainless steel plates in ammonia depends on temperature, pressure, gas concentration and chromium and nickel content. The results of field experiments show that the corrosion rate of ferrite or martensitic stainless steel (altered metal depth or carburizing depth) is higher than that of austenitic stainless steel. The higher the nickel content is, the better corrosion resistance is, and the corrosion rate increases with the increase of content. The corrosion of austenitic stainless steel in high temperature halogen vapor is very serious, and the corrosion of fluorine is more serious than chlorine. For high Ni-C r stainless steel, the upper limit of temperature in dry gas is 249, and chlorine is 316. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The biphase in the name of the duplex stainless steel is mainly derived from the microstructure of the alloy. The duplex stainless steel usually contains about 50/50 austenite and delta ferrite phase mixture. Compared to austenitic stainless steel (for example, 304 or 316 stainless steel), duplex stainless steel has better corrosion resistance, especially chloride stress corrosion and chloride pitting, and higher strength. Compared with ordinary austenitic stainless steel, the main difference is that the dual phase stainless steel has higher chromium content, 20-28%, and high content of molybdenum, up to 5%, the nickel content is low, about 9%, and the nitrogen content is about 0.05-0.5%. The low nickel content and high strength (dual thinner part) of duplex stainless steel have obvious cost effectiveness. Therefore, they are widely used in the pipeline system of offshore oil and natural gas industry, and are becoming more and more popular in the pipeline and pressure vessel industry of petrochemical industry. Duplex stainless steel has higher corrosion resistance and higher strength than 300 stainless steel. For example, 304 stainless steel has a 0.2% yield strength in the 280N / mm 2 region, and the minimum 0.2% yield strength of 22% chromium duplex stainless steel is about 450N / mm 2, and the minimum grade of super duplex stainless steel is 550N / mm 2. Although duplex stainless steel has strong corrosion resistance and oxidation resistance, it can not be used at high temperature. This is because the ferrite phase will form brittle phase at lower temperature, which has a disastrous effect on the toughness of the duplex stainless steel. Therefore, the ASME pressure vessel specification limits the use temperature of all grades to less than 315 C, other specifications and even lower use temperatures, for super dual phase steel may be as low as 250 C. The biphase alloys can be pided into three major categories: poor biphase, 22%Cr biphase and 25%Cr super double phase, and even higher alloying super biphase steel grades. This pision is mainly based on the alloying level of the duplex stainless steel, such as the resistance to pitting, and the resistance to pitting resistance of the alloy. PREN is calculated by a simple formula: PREN =%Cr + 3.3%Mo + 16%N, and sometimes the factor of W can be considered 1.65. The PREN of the biphase steel is less than 40; the PREN of the super complex is between 40-45, the super complex PREN is over 45, and the rarefied grade usually has the lower nickel, so the price is lower. Source: China Stainless Steel Flanges Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Mechanical testing of stainless steel plates are used to generate data that can be used for design purposes or as part of the material connection procedure or operator acceptance procedure. The most important function may be to provide design data, because it is important to understand the limit value of stainless steel sheet product structure and ensure that it will not fail. One other effect of this mechanical test is the tensile test, which can be used to determine the yield strength of the steel used for design calculations, or to ensure that the stainless steel plate meets the strength requirements of the material specifications. Mechanical tests can be pided into quantitative or qualitative tests. A quantitative test is a qualitative test that provides data for design purposes, and the results are used for qualitative tests such as hardness testing or bend testing. Tensile testing is used to provide information used in design calculations, or to prove that materials conform to the requirements of corresponding specifications, so it may be quantitative or qualitative testing. The test is to grasp the end of the standard sample properly prepared on the tensile testing machine, and then increase the uniaxial load until failure occurs. Standardized test pieces so that the results are reproducible and comparable. The specimen is usually proportionate. When the gauge length is L 0, it is related to the original cross-sectional area, A 0, as L 0 =k A 0. In the EN standard, the constant k is 5.65, and the ASME standard is 5. The length of these measurements is about 5 times the diameter of the sample and 4 times the diameter of the specimen, although this difference may not be important technically, but it is very important to declare it in accordance with the specification. Load (stress) and specimen elongation (strain) are measured, and engineering stress / strain curves are constructed from the data. The following aspects can be determined from the curve. A) the tensile strength, also known as the ultimate tensile strength, is pided by the original cross section by the load at the ultimate tensile strength (UTS) and the maximum = P maximum / A 0 when the fracture is broken. The maximum P = maximum load, A 0 = the original cross section area. In the EN specification, the parameter is also identified as “R m”. B) the yield point (YP), that is, the stress from the elastic to the plastic deformation, that is, the yield point below the unloading specimen means that it is restored to the original length, the permanent plastic deformation at the yield point above the yield point, YP or sigma y = P YP / A 0, P YP = the yield point load. In the EN specification, the parameter is also identified as “R e”. C) in reassembling the broken sample, we can also measure the elongation, and the El% test piece has been El (%) = (L F L – 0 / L) = (%) = (L F L = 100), L F = break distance and L 0 = original distance length. In the EN specification, the parameter is also identified as “A”. D) A%= (A 0 -A f / A 0) x 100 and A f = part of the cross section area, in which the percentage of R is reduced, and the fracture of the sample in the degree of necking or decrease in diameter. In the EN specification, the parameter is also identified as “Z”. A) calculation of elongation, b) calculation of area reduction rate (a) and (b) measure the strength of materials, (c) and (d) indicate the ductility or capacity of materials without deformation. The slope of the elastic part of a curve is basically a straight line, which will give young’s modulus of elasticity, which is to measure the degree of elastic deformation of the structure when it is loaded. Low modulus means that the structure will be flexible, and the high modulus structure will be stiff and inflexible. In order to produce the most accurate stress / strain curve, additional extensometer should be added to the stainless steel plate to measure the elongation of the gauge length. The less accurate way is to measure the movement of the crosshead of the drawing machine. The above stress-strain curves show material with good yield point, but only annealed carbon steel shows this behavior. There must be other ways to determine the “yield point” by alloying, heat treatment or cold hardening of metal without obvious yielding. This is measured by measuring yield stress (yield strength in American terms), that is, a certain amount of stress required for plastic deformation in the specimen. The stress is measured by drawing a straight line parallel to the elastic part of the stress / strain curve at a specific strain, and the strain is the percentage of the original length of the standard distance, so 0.2% verification, 1% verification. For example, in the specimen with a gauge length of 100mm, the yield strength of 0.2mm is measured by using the permanent deformation of the 0.2mm. Therefore, it is proved that strength is not a fixed material property, such as yield point, but depends on the number of plastic deformation specified. Therefore, when considering the strength of proof, the percentage must always be quoted. Most steel specifications use 0.2% of the EN specification, R P0.2. Some materials such as annealed copper, gray iron and plastic have no linear elastic part in stress / strain curves. In this case, similar to the method of determining the strength of the verification, the usual practice is to define the “yield strength” as the stress that produces a specified number of permanent deformations. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

In the heat treatment of stainless steel plates, tempering is mainly used to soften any hard microstructures that may be formed during the previous heat treatment, improve the ductility and toughness of the material. In addition, the tempering can also form precipitates, and the size of these precipitates can be controlled to provide the required mechanical properties. This is very important for creep resistant Cr Mo steel. Tempering includes heating the steel to a temperature below the critical temperature, which is affected by the type of alloying elements added to the stainless steel plate. The temper of tool steel can be tempered at a temperature of 150 degrees centigrade, but for the welding engineer, the tempering temperature of the structural steel is usually between 550-760 degrees centigrade, depending on the composition of the steel. As for post weld heat treatment, this is a specific term which includes both stress relief and tempering, and should not be confused with heat treatment after welding. These treatments can include aging of aluminum alloy, solution treatment of austenitic stainless steel, hydrogen release and so on. When meeting certain standards, post weld heat treatment is mandatory in many standard procedures. By reducing residual stress and improving toughness, it reduces the risk of brittle fracture and reduces the risk of stress corrosion cracking. However, unless the stress is mostly compressed, there is almost no beneficial effect on fatigue performance. The method of post welding heat treatment depends on a number of factors, such as the available equipment, the size and configuration of the components, the thermal insulation temperature, the equipment capable of providing uniform heating with the required heating rate, and so on. The best way is to use a stove. This may be a permanent fixed furnace or a temporary furnace erected around the components, which is especially useful for bulky large structures or large parts on site. A permanent furnace can carry a wheeled hearth with components on it or a top hat type furnace with fixed grate and removable cover. Typically, a furnace that can heat 150 tons of pressure vessels will have a size of about 20 meters long, 5 x 5 meters, and will consume about 900cu / meter gas per hour. Natural gas or oil can be heated by resistance or induction heating. If fossil fuels are used, attention should be paid to ensuring that there is no sulphur and other elements that may cause some alloy cracking in the fuel, especially if these alloys are austenitic stainless steel or nickel base corrosion resistant coatings. No matter which fuel is used, the atmosphere in the furnace should be closely controlled so as not to cause excessive oxidation and scaling or carbonization due to the unburned carbon in the furnace atmosphere. If the furnace is gas or fuel, flame contact elements or temperature monitoring thermocouples are not allowed, which will cause partial overheating or PWHT temperature. Monitoring the temperature of components during post weld heat treatment is critical. Most modern furnaces use zone control with thermocouples to measure and control the temperature in the furnace area, and automatically control temperature through computer software. Area control is particularly useful for controlling the heating rate when assembled with different thickness of steel. However, it is recommended not to use the monitoring furnace temperature to prove that the components have reached the correct temperature. Therefore, the thermocouples are usually attached to the surface of the element at a specific interval, and these elements are used to automatically control the heating and cooling rates and the soaking temperature to achieve a uniform temperature. As mentioned previously, the yield strength decreases with increasing temperature, and the component may not be able to support its excessive weight distortion at the post weld heat treatment temperature. It is very important for components to be adequately supported in the heat treatment process, and the bracket suitable for components should be regularly placed. The spacing depends on the shape, diameter and thickness of the item. Internal support may be required in the cylinder, such as pressure vessel; if so, the support should be similar material, so that the coefficient of thermal expansion will match. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The corrosion reaction is unavoidable during the use of stainless steel pipes, so how to alleviate the corrosion is a key aspect of maintenance process. The following article describes the methods of avoiding or alleviating corrosion of stainless steel pipes. Because stainless steel pipes are usually corroded in the circuit, corrosion prevention needs to be started from the angle of interference circuit. The first is cathodic protection, which is a good way to prevent corrosion of stainless steel pipes. It uses the external current from the fixed anode to interfere with the circuit in the corroded battery. For most forms of external stainless steel pipe corrosion is 100% effective. Galvanic CP connects high-energy metals (such as zinc or magnesium) to pipes (anodes). Zinc or magnesium is used as a sacrificial anode to protect the pipe. The sacrificial anode acts as a current anode and applies voltages up to -1.4 to -2.1V. If the reverse current applied from the sacrificial anode is more negative than -0.85V, the corrosion in the stainless steel will stop. If the reverse applied current is between -0.8V and -1.00V, the corrosion of aluminum will stop. If the current exceeds -1.00V, an alkaline solution will be formed on the aluminum and corrode the metal. In some cases, conversion to -0.003 V is enough. Using a DC rectifier to drive the current to a suitable stable anode such as carbon, plating platinum titanium or magnetite in an applied current system, a proper CP level can also be reached. One end of the rectifier is connected to the pipe, and the other end is connected to the bed of the piezoelectric current anode or anode. Then the rectifier current rises until the line voltage reading of the copper sulphate half cell is less than -0.85V. We must pay attention to not too high, otherwise the coating will be stripped. The second is electrical connection, which is used to prevent the corrosion of AC and DC current. This is a simple application of the illegal current source between the conduit and the grounding system. The bonding line provides a safe way to return to other utilities’ grounding systems, not from the surface of pipes. It also provides a security element to protect workers from fatal shocks that may occur on insulated pipes near high voltage wires, trams and trolley systems. The electrical connection can also be achieved by the free use of the electrified anode, which places the pipeline at the same voltage as the illegal application. Care must be taken to ensure that all pipe flanges are in electrical contact. If they are not in contact with each other, the installation of jumpers from the bottom up ensures that the entire pipeline is protected. The third is the coating. The coating can be used to stop or reduce the corrosion of inner diameter (ID) and outer diameter (OD). In order to select the appropriate coating system, engineers need to consider the design, environment, content, pressure, external impact, design life and cost of the piping. Traditional external coatings such as mastic, as well as many new epoxy resins and polymers have been proven to be successful inhibitors. The internal coating can be used for corrosion control of contents, traction and friction reduction of cables, and erosion (impact) corrosion control. Source: China Stainless Steel Pipelines Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Brinell test was designed by Swedish researchers in early twentieth Century. As shown in Fig. 1 (a), the test involves pressing the hardened steel ball indenter into the surface of the sample using standard load. Select the diameter / load ratio to provide impressions of acceptable diameter. The diameter of the ball can be 10,5 or 1mm, the load can be 3000750 or 30kgf, the load P is related to the diameter D, and the ratio of the relational P / D 2 has been standardized to different metals, so that the test results are accurate and repeatable. For steel the ratio is 30:1 – for example, a 10mm ball can use 3000kgf load or 1mm ball 30kgf load. The proportion of aluminum alloy is 5:1. A fixed period of loading is usually 30 seconds. When the indenter is retracted, the indentation d 1 and D 2 of the two diameters are measured by microscope, and the calibration scale is used, and then the average value is shown, as shown in Figure 1 (b).

Stratified tears may sometimes occur in rolled steel plates with poor thickness ductility. The main characteristic of lamellar tear is that it appears in the base metal parallel to the weld fusion boundary and the surface of the plate, usually in the T weld and fillet welds. Cracks can occur at the toe or root of the weld, but are always associated with the point of high stress concentration. The fracture surface of the lamellar tear is fibrous and has a long parallel part, which indicates that the ductile ductility is low in the thickness direction. Because lamellar tearing is associated with high concentration extended inclusions parallel to the surface of the plate, tearing will be transgranular with stepped appearance. The occurrence of lamellar tearing must satisfy three conditions: the first is the transverse strain. The shrinkage strain in the welding must be used in the short direction of the plate, that is, through the thickness of the plate; the second is the welding direction, the fusion boundary will be roughly parallel to the plane of the inclusions; the third is the material sensitivity, and the plate must be in the direction of thickness. It is necessary to have poor ductility. Therefore, if the stress generated during welding acts in the thickness direction, the risk of lamellar tear will be greater. The risk also increases. The factors that can effectively reduce the risk of welding tear are material, joint design, welding process, consumables, preheating and coating selection. Lamellar tear occurs only in rolled steel plates instead of forgings and castings. Generally, the steel with low transverse shrinkage area (STRA) associated with high concentration of rolling sulphides or oxide inclusions is more susceptible. In general, more than 20% of STRA steel can basically be torn apart, while STRA less than 10%-15% steel plates can only be used for lightly constrained joints. Especially when the thickness is greater than 25mm, the steel with higher strength has greater risk. The risk of low sulphur content (<0.005%) aluminum treated steel is low. Lamellar tear also occurs at joints where high pass thickness strain is generated, such as T joint or corner joint. In T or cross joints, full penetration butt welds are particularly susceptible. The cruciform structure that can not be bent in the welding process will also greatly increase the risk of tear. In butt joints, there is almost no risk of lamellar tear due to the welding stress will not function through the thickness of the plate. Because the angle deformation will increase the strain at the root and / or toe of the weld, tear will also occur at the thick section joint with high bending constraint. Because tearing is more likely to occur at fully permeable T joint, if possible, use two fillet welds. Double sided welding is more difficult than large-area single side welding, and balance welding to reduce stress, which will further reduce the risk of tear at the root. Large single sided fillet welding should be replaced by smaller double sided fillet welding. Redesigning the joint structure to make the fusion boundary more perpendicular to the sensitive plate surface is especially effective for reducing risk. Typically, when the weld foot length of fillet weld and T type joint is longer than 20mm, the flake tear is more likely to occur in large weld. Because constraints can cause this problem, the thinner sections of the truncated panels which are less susceptible to tear may still be at risk under high constraints. Because the design of material and joint is the main cause of welding tear, the selection of welding process has little influence on risk. However, it may be advantageous to generate low stress higher heat input processes through larger HAZ and deeper penetration. Because welding metal hydrogen can increase the risk of tear, low hydrogen technology should be used when welding sensitive steel. Where possible, the selection of lower strength consumables can usually reduce risk by accommodating more strain in the weld metal. A smaller diameter electrode has been used to prevent the tear. Low hydrogen consumables will reduce risk by reducing the level of diffused hydrogen in welded metal. Consumables must be dried according to the manufacturer’s recommendations. Preheating will have a beneficial effect on reducing the level of diffused hydrogen in welding metal. However, it should be noted that, at the restricted joint, excessive preheating may have adverse effects by increasing the constraint level produced by the shrinkage of the weld during the cooling process. Therefore, preheating should be used to reduce the hydrogen level, but preheating should be used so that it does not increase the shrinkage of the weld. In addition, coating the surface of sensitive boards with low strength welding metal has been widely used. As shown in the T butt weld, the surface of the plate can have grooves, so that the butter coated layer will extend beyond 15 to 25mm of each weld toe, and the thickness is about 5 to 10mm. In situ bonding, that is, low strength welding metal is first deposited on the induction board and then filled with joints, has also been successfully applied. However, the design calculation should be carried out before the docking technology is adopted to ensure that the overall welding strength is acceptable. Because the lamellar tear is a linear defect with sharp edges, therefore, according to the requirements of BS EN ISO 5817:2007, the welding of quality grade B, C and D is not allowed. Using visual inspection, liquid osmotic or magnetic powder detection techniques can easily detect lamellar tearing on the surface of the stainless steel plate, but the internal cracks require ultrasonic inspection, but there may be some problems in distinguishing the lamellar tearing of the inclusion zone. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Creep is a slow failure mechanism, which may occur when stainless steel plates are exposed to higher than the elastic limit for a long time. Creep is very slow and not obvious at most ambient temperatures. For stainless steel plates, increasing the ambient temperature increases the deformation rate of the stainless steel plate under the load. If the stainless steel plate element is safely used in the high temperature environment, it is very important to understand the speed of the deformation at a given load and temperature. Failure to do so may lead to premature failure of pressure vessels or scaling of gas turbine blades on turbine shells. In order to use fuel more efficiently in power plants and gas turbines, it is required that components can be used for higher operating temperature than design, so new creep resistant stainless steel plate alloys are developed. In order to study the design data of these stainless steel plate alloys, creep tests are needed. In stainless steel plates, creep failure occurs at grain boundaries, resulting in intergranular fracture. Fig. 1 illustrates the void formed on the grain boundary at the early stage of creep. The fracture appearance may be similar to brittle fracture, except for a small amount of elongation in the applied stress direction.

The causes of hydrogen crack formation in welding arc welding of stainless steel are mainly caused by three factors, namely, the brittle and brittle hard and brittle structure of hydrogen produced by the welding process, the tensile stress acting on the welded joint, and often when the temperature is up to the normal environment. In practice, for a given situation (material composition, material thickness, joint type, electrode composition and heat input), the risk of hydrogen cracking can be reduced by heating the joint. Reducing the cooling rate by preheating can diffuse some hydrogen and reduce the hardness of the stainless steel plate, that is, the hardness of the microstructural area sensitive to the crack, which causes the plate to break easily. When thick walled steel with high carbon equivalent (IIW CE) value is welded, the preheating level can be as high as 200 degrees. Because there are few cracks above the ambient temperature, it is also important to keep the temperature of the weldment of the stainless steel plate in the manufacturing process. For susceptible steels, it is appropriate to keep the preheating temperature at a given time, usually between 2 and 3 hours, so that hydrogen diffuses from the welding area. Under the condition of sensitive crack, such as high IIW CE steel welded or under high constraint conditions, temperature and heating time should be increased, usually at 250-300 C three to four hours. For many kinds of steel, post weld heat treatment (PWHT) can be used immediately after welding, i.e., no preheating temperature is allowed. But in fact, since inspection can only be carried out at ambient temperature, it is only after PWHT that the “disposable” defect can be found. In addition, for high hardness steel, second heat treatments may be needed to adjust the hard microstructure after the first PWHT. In some cases, in order to avoid cracking, more stringent procedures are required (higher preheating temperature and / or lower hydrogen content in welding metal). It includes the following: height constraints, including the weld of section thickness above 50mm, and the operation of the root in the double inclined joint; the thick part (more than 50mm); the low carbon equivalent steel (C < 0.1% and IIWCE < 0.42 C-Mn steel); "clean" or low sulfur steel (S less than 0.008%) because of low sulfur and oxygen content It will increase the hardenability of steel. The alloyed weld metal, where the preheating level to avoid HAZ cracking may not be enough to protect welding metal. Low hydrogen technology and consumables should be used. For predicting the preheating requirements to avoid welding metal cracking, the diffusion hydrogen level of welded metal and tensile strength of welded metal are usually required as input. The use of austenitic stainless steel and nickel alloy welding metal can also effectively prevent cracking. Austenitic consumables should be used in case of poor preheating or no cracking prevention. Austenitic stainless steel and nickel alloy electrodes will produce a welding metal, and the solubility of hydrogen to iron at ambient temperature is higher than that of ferritic stainless steel. Therefore, any hydrogen formed during welding is locked in the welding metal and rarely diffuses to HAZ when cooled to ambient temperature. The commonly used austenitic MMA electrode is 23Cr:12Ni, such as EN 1600:1997. However, because nickel alloy has lower thermal expansion coefficient compared with stainless steel, nickel alloy electrode is preferred to reduce shrinkage strain. When welding up to 0.2%C steel, it is usually not necessary to preheat. However, at a temperature above 0.4%, the minimum temperature of 150 C will be needed to prevent HAZ cracking. The best way to avoid hydrogen cracking is to reduce the hydrogen content of weld metal, that is, to reduce the amount of hydrogen generated by consumables, that is, using low hydrogen technology or low hydrogen electrode. According to the amount of welding metal hydrogen produced in the standard test block, the welding process can be pided into high, medium, low, extremely low and ultra-low. MMA is likely to produce more hydrogen content. Therefore, in order to achieve a lower value, the base electrode must be used and roasted according to the manufacturer’s advice, or removed from the special package immediately before use, and the time of exposure to the environment is not more than the specified time by the manufacturer. For the MIG process, cleaner wires are needed to achieve very low hydrogen content. The following general guidelines are recommended for all types of steel, but the requirements for specific steels should be inspected according to EN 1011-2:2001. Low carbon steel is easy to weld. If low hydrogen technology or electrode is used, it usually does not need preheating. When welding thick cross section material, it may need preheating, high binding force and more hydrogen. The thin part of medium carbon and low alloy steel can be welded without preheating, but the thicker part needs lower preheating level. Low hydrogen technology or electrode should be used. Higher carbon and alloy steels require preheating, low hydrogen processes or electrodes, post weld heating and required slow cooling. It is recommended that the following practical technologies be used to prevent hydrogen cracking, clean joint surface, remove paint, cutting oil, grease and other pollutants. If possible, use the low hydrogen process, roast electrode (MMA) or flux (submerged arc), then store them, or limit the time exposed to environmental conditions. The stress on the weld is reduced by avoiding large root gap and height constraints. If preheating is specified in the welding procedure, it should also be applied when the temporary attachment is glued or used, and the joint is preheated to at least 75mm from the seam line to ensure that the thickness of the material is heated evenly and the preheating temperature of the measurement and the reverse side of the heating phase is measured. In acceptance, because the hydrogen crack is a linear defect with sharp edges, it is not allowed to conform to the weld of the quality grade B, C and D according to the requirements of EN ISO 5817. There are several main aspects of detection and remedial measures, because hydrogen cracks are usually very fine and may be subsurface. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)