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Duplex stainless steel is becoming more and more common. They can be produced in all major steel mills. The reasons for its general application are as follows: higher strength leads to weight saving, stronger corrosion resistance, especially stress corrosion cracking, high price stability and relatively cheap price. However, even with these advantages, the market share of global duplex stainless steel is still limited. The purpose of this paper is to provide a simple guidance for this kind of stainless steel. The advantages and disadvantages will be described. Principle of duplex stainless steel The idea of duplex stainless steel can be traced back to 1920s, and was first made in 1930 in Avesta, Sweden (Avesta). However, in the past 30 years, duplex stainless steel has only been reused. This is mainly due to the progress of steelmaking technology, especially in controlling nitrogen content. Standard austenitic stainless steels such as 304 (1.4301) and 430 ferritic stainless steel are relatively easy to make. From their names, they are mainly composed of one phase, namely austenite or ferrite. Although these types are applicable to a wide range of applications, there are some important technical shortcomings in the two types: Austenite low strength (200MPa, 0.2%PS) under the condition of solid solution annealing, low stress corrosion cracking, ferrite low strength (slightly higher than austenite, 250MPa, 0.2%PS), poor thick section weldability and poor toughness at low temperature. In addition, the high nickel content of austenitic stainless steel leads to price fluctuation, which is unfavorable for many end users. The basic idea of duplex stainless steel is to produce the chemical composition of the mixture that causes the ferrite and austenite to be approximately equal. This balance phase provides the following: Higher strength – the current dual phase brand 0.2%PS has a range of 400-550MPa. This may cause the section thickness to be reduced, thereby reducing the weight. This advantage is particularly important for applications such as pressure vessels and tanks, as well as structural applications such as bridges; thick section weldability is not as simple as austenite, but better than ferrite; good toughness is better than ferrite, especially at low temperatures, as low as minus 50 degrees Celsius, extending to zero below zero. 80 degrees Celsius; stress corrosion cracking – standard austenitic steel is particularly prone to corrosion. The important applications include: hot-water tank, brewing tank, processing plant, swimming pool structure. Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Local corrosion such as pitting and crevice corrosion of stainless steel usually occurs in the presence of halide ions, usually chlorides (eg, coastal and deicing chloride salts – sodium chloride, calcium chloride or magnesium chloride; hydrochloric acid; bleach – sodium hypochlorite or Calcium hypochlorite; and other chloride compounds). Point corrosion Pit corrosion can occur when there is local destruction of the passivation layer on the exposed surface of the stainless steel plate. Once started, the growth rate of these pits will be relatively fast, resulting in deep holes and even penetration. Other metals such as aluminum may also cause pitting corrosion. Crevice corrosion Crevice corrosion occurs at sites where oxygen does not circulate freely, such as in tight joints, fastener heads, and other conditions in which metal sheets are in intimate contact. Chloride salts, pollutants, and moisture in the environment can accumulate in the gaps. The environment in the crevice becomes depleted of oxygen, and the chloride-rich acid is easily acidified, thereby promoting the destruction of the passivation film and the anodic dissolution. Envirnmental factor: An important environmental factor that contributes to local invasion is high chloride content. The higher the temperature, the lower the pH and the higher the corrosion potential.

Early development of duplex stainless steel: Duplex stainless steel, which combines many beneficial properties of ferritic and austenitic stainless steels, was originally developed in the early 1930s. The initial duplex stainless steel provides good performance characteristics but has limitations under welding conditions. The metallurgical process at the time was not suitable for producing grades with the correct austenite-ferrite balance. In addition, the carbon content of these early duplex stainless steels was relatively high because there was no effective decarburization process technology at the time. Therefore, the production of these materials is often the main production limited to some specific applications. Modern duplex stainless steel development: In the late 1960s and early 1970s, several factors led to the development of duplex stainless steel. First, the introduction of vacuum and argon deoxygenation (VOD and AOD) processes opened the door to the production of modern biphasic grades. These developments make it possible to achieve a good balance of low carbon content with high chromium content, high nickel content, and ferrite and austenite. This can produce materials with very good properties. Alloy content provides good resistance to local and uniform corrosion. The two-phase microstructure contributes to the high resistance to chloride stress corrosion cracking under many conditions and high strength. Modern duplex stainless steel also has good weldability. These modern duplex stainless steels appeared in the same period as the offshore industry increased its activities. The industry needs a stainless steel that can handle aggressive environments. Although austenitic steels can also withstand these aggressive environments, the lack of nickel at the time led to higher prices. All these factors combine to encourage the offshore oil industry to adopt duplex stainless steel. Duplex stainless steel 2205: Duplex stainless steel 2205 (UNS S31803 / 32205) is the first commercially developed “second generation” duplex steel. It was developed in the mid-1970s and was introduced by the Krupp Steel producer in Germany. Today is still the most common duplex stainless steel that is currently considered the key material for duplex stainless steels. Duplex stainless steel 2205 provides corrosion resistance in many environments superior to Type 304 (UNS S30400), 316 (UNS S31600) and 317 (UNS S31700) austenitic steels. In addition, the yield strength is about twice that of austenitic stainless steel. Interestingly, the composition range initially set to 2205 (S31803) was later determined to be too wide. According to the original composition specification, biphasic 2205 may form a harmful intermetallic phase at high temperatures. In order to achieve the best corrosion resistance and avoid these intermetallic phases, the content of chromium, molybdenum and nickel needs to be maintained within more than half of S31803. This modified 2205 is called S32205 and is a typical example of the commercial production of Duplex 2205 iv today. Precision duplex stainless steel: Although duplex stainless steel 2205 continues to gain momentum in various industries over time, in some cases extraordinary corrosion resistance is already higher than required. This has led to the development of many streamlined duplex stages such as LDX 2101 (S32101), ATI 2003 (UNS 32003) and Duplex 2304 (UNS S32304). These new duplex stainless steels not only contain less than 2205 alloying elements, but also can be used for alternative 304 or even 316 grade applications. For example, duplex alloys are used in many construction applications due to high strength, good corrosion resistance, and lower overall cost, commonly used stainless steel grade 316. Super duplex: In addition, since the 1980s, the petroleum industry has been one of the main driving forces for the development of even higher-alloy dual-phase materials, known as super duplex and super duplex. These higher alloy biphasic grades are designed to handle extreme environments such as higher corrosive conditions and higher pressures encountered at larger depths in oil and gas fields [V]. Super duplex grades have an equivalent pitting resistance (corrosion resistance, also known as PRE or PREN), higher than 40. The number of PREs at the super duplex level is 48 or higher [v]. Currently, the current product categories include the super-duplex SAF 2507 SD (UNS S32750) and super-duplex SAF 3207 HD (UNS S33207) and SAF 2707 HD (UNS S32707). Although the current duplex stainless steel market accounts for a very small part of the total stainless steel, duplex stainless steel is a developing industry and its prospects continue to grow. From the International Stainless Steel Forum ISSF, studies have shown that duplex output soared from 6,000 tons a month, increased to 10,000 tons in 2004 to 2005, and up to 22,000 tons in 2008. Duplex steel continues to gain popularity as various industries begin to consider the entire life cycle cost. In addition to the potential immediate material cost savings, the use of duplex stainless steel can also lead to longer life cycles and lower maintenance costs in many cases. Source: China Duplex Stainless Steel Pipe Fittings Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

The combination of tensile stress and a specific corrosive environment can cause the stainless steel plate to break. This type of attack is called Stress Corrosion Cracking (SCC). The most common environmental exposure condition that causes SCC in stainless steel is the presence of chloride. Although the degree of stainless steel immunity to chloride SCC is large, the relative resistance of stainless steel plates varies greatly. Effect of alloy composition: The relative resistance of chloride SCC depends on the stainless steel series. The austenitic stainless steel series is the most susceptible. The tolerance of SCC to austenitic stainless steels is related to the nickel content. The austenitic stainless steels that are most susceptible to SCC have a nickel content in the range of 8-10%. Therefore, standard brands such as 304, 304L and 316, 316L are very sensitive to this type of attack. Austenitic grades with higher nickel and molybdenum contents, such as alloy 20, 904L and 6% molybdenum superaustenitic grades have significantly better chloride SCC resistance. The ferritic stainless steel series includes grades 430 and 444 and is very resistant to chloride SCC. Duplex stainless steels with austenitic, ferrite microstructures have electrical resistance between the austenitic and ferritic grades. Corrosion test The relative resistance of the stainless steel to the chloride SCC is usually quantified by using a standard boiling salt solution. The following table summarizes the test results in 26% NaCl (sodium chloride), 33% LiCl (lithium chloride) and 42% MgCl 2 (magnesium chloride) in boiling salt solution. Boiling LiCl and MgCl 2 test solutions are very aggressive with respect to practical use, and only austenitic alloys with compositions close to that of nickel-based alloys will routinely resist cracks in these test solutions. Table 1: Relative chloride chloride SCC resistance measured in a standard boiling salt solution using a fully immersed U-bend specimen. (taken from producer data) Cracked appearance: The typical crack morphology of chloride stress corrosion cracking consists of branching nucleation cracks. Figure 1 shows the cracks that occur on 6Mo super austenitic stainless steel (N08367) exposed to 0.2% chloride at 500°F (260°C). Figure 1: Typical appearance of chloride stress corrosion cracking Envirnmental factor: Environmental factors that increase the susceptibility of stainless steel to cracking include higher temperatures, increased chloride content, lower pH, and higher tensile stress levels. Temperature is an important variable. When the stainless steel plate is completely submerged, chloride stress corrosion cracking of the stainless steel plate is rarely seen at temperatures below 60°C. There is a synergistic relationship between dissolved oxygen and chloride levels. If the oxygen content is reduced to the 0.01-0.1 ppm range, aqueous solutions containing low to moderate chloride levels cannot crack austenitic stainless steels, such as 304L and 316L stainless steels. At room temperature to moderate temperatures, the normal solubility of oxygen in water is 4.5-8 ppm at atmospheric pressure. In actual use environments, evaporation can cause local accumulation of corrosive substances such as chlorides and H+ ions, leading to severe corrosive conditions. Under severe evaporation conditions, stainless steel plates will crack at temperatures well below the threshold measured under full immersion conditions. Therefore, care must be taken when specifying materials for applications involving the evaporation of chlorine-containing solutions on the surface of hot stainless steel plates. Figure 1 shows the crack threshold for 304L and 316L stainless steel as a function of temperature and chloride content. The amount of chloride needed to produce cracks is relatively low. Failure has occurred in environments with as little as 10 ppm chloride. This is especially true of environments with a concentrated (evaporation) mechanism (eg, a wet/dry interface) or a solution film that is in immediate contact with a heat-resistant surface. In these cases, a few ppm of chloride in the bulk solution can be concentrated to several hundred ppm in the evaporation zone. Source: China Stainless Steel Plates Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

There are different types of stainless steel fasteners that provide different levels of corrosion resistance and strength. They are used in corrosive industrial and marine environments, equipment, construction and construction applications. These products are available in all stainless steel designs and are used to join other metals such as copper, aluminum, carbon steel and zinc. The specifications for a particular fastener grade should be based on the required corrosion resistance, strength and material to be fastened. The fastener should have corrosion resistance comparable to or higher than the corrosion resistance of the metal to be fastened. Specific application guidelines should be consulted to determine the most appropriate grade of stainless steel. SSINA Design Manual: Stainless Steel Fasteners can find general guidance. Some application-specific guidelines are provided in the Additional Design and Specification Resources section. Manufacturing method Stainless steel fasteners can be manufactured by machining or cold heading. Machining is the oldest method of producing fasteners, and it still stipulates very large diameters and small production runs. Cold heading is a more common method of production. It transforms the wire into the desired shape by applying sufficient pressure to plastically deform the metal into the mold and stamping cavity without preheating the material. The production of bolts, screws, nails and rivets is relatively cool, but special custom operations can also be carried out in this way. Cold heading can significantly increase the strength of 300 series stainless steel fasteners. ASTM standard ASTM International Standards for fastener materials and specific products (bolts, nuts, screws and bolts) are shown in the table below. There are general purpose and specialized standards for high temperature, high pressure and low temperature applications. It should be noted that stainless steel gaskets do not have ASTM standards.ASTM StandardTitleA 193 / A 193 Malloy steel and stainless steel bolt material for special applications such as high temperature or high pressure serviceA 194 / A 194MCarbon steel and alloy steel nuts (including stainless steel) for high or high temperature service or both boltsA774 / A774Maustenitic stainless steel joint (welded) for general corrosive service at low/medium temperaturesA951 / A951MStandard Specification for Wire and Stone ReinforcementA 962 / A 962Mfastener or fastener material or both are common requirements for any temperature from low temperature to creep rangeA1082 / A1082MStandard Specification for High Strength Precipitation Hardening and Duplex Stainless Steel Bolts for Special Applications, Note: For all high strength duplex and PH stainless steel fasteners – any sizeC1242Standard Guide for Design, Selection and Installation of External Size Stone Anchors and Anchoring Systems NOTE: This means that the metal in contact with the stone should be 300 series stainless steel, but other materials may be used if moisture and plating corrosion are properly prevented. Copper and stainless steel are used for the cable tie. Specify 316 instead of 304 if there is a typical coastal or deicing salt exposure. In harsh high salt environments, it is recommended to use more corrosion resistant stainless steel.F 593stainless steel bolts, hex screws and studsF 594Stainless Steel Nut SpecificationsF 738Mstainless steel metric bolts, screws and studs, metricF 788 / F 788MSurface discontinuities in bolts, screws and studs, inch and metricF 836MStainless Steel Metric Nut SpecificationsF 837 / 837MStainless Steel Hexagon Head Screw SpecificationsF 879 / 879Mstainless steel socket button and flat head skull head screwF 880 / 880MStainless Steel Socket Set Screw Specifications Source: China Stainless Steel Pipelines Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

In the temperature range of 950-1450°F, chromium carbide tends to precipitate at the grain boundary of austenitic stainless steel. Any exposure or thermal excursion within this temperature range during metal manufacture and use may cause sensitization of the austenitic stainless steel. Common practices such as welding, stress relief and thermoforming can make stainless steel reach the sensitizing temperature range. The formation of chromium carbide is easily reversed by solution annealing heat treatment. The test methods outlined in ASTM A262 have been developed to test the susceptibility of intergranular attack in austenitic stainless steels. The time and temperature required to produce intercrystalline corrosion susceptibility depends on the alloy composition in the material, especially the carbon content. The figure below shows the time-temperature sensitization curves of 304 stainless steel with different carbon contents.

The basic production process of iron and steel is to first obtain iron ore and coking coal and then to convert them into pig iron in an ironmaking blast furnace; the next step is to use pig iron as raw material, and use different steelmaking furnaces to smelt steel into steel; The shape of a steel ingot or continuous cast billet is then sent to a rolling mill for rolling or forging to finally become available in various shapes of steel. First, iron ore mining and processing From the day of the birth of the earth, the iron deposits were very unevenly distributed throughout the world. Iron is present in the natural state in the form of a compound, and is particularly present in the state of iron oxide. In theory, any ore containing iron or iron compounds can be called iron ore. However, industrially or commercially, iron ore not only contains iron components, but also must have a relatively high iron content before it can be used. There are two kinds of hematite and magnetite which have good smelting performance and utilization value. Dark red or brown is hematite. Its main component is iron oxide Fe2O3 with a specific gravity of about 5.26, most of which contain Fe below 70%, and O with more than 30%. It is the most important iron ore species. The magnetite is dark gray or metallic luster. Its main component is Fe 3 O 4 Fe3O4 with a specific gravity of about 5.15. The theoretical maximum Fe content is up to 72.4%, O is at least 27.6%, and it is magnetic. Iron ore species. Some iron ore is buried shallow and open-pit mining is used. It is like digging a large pit for the Earth to open a skylight, and the mining cost is relatively low. Some of the deeper iron ore can only be mined in underground mine roadways, similar to the methods used to extract underground coal. However, there are not many ores with more than 66% iron in the world. Many iron ores have low “grades” (that is, iron content), which may be between 30% and 50%. The stone composition in the ore is too high. Is not directly used for ironmaking. Therefore, in order to increase the iron content of ore raw materials, it is necessary to use mechanical equipment to remove some of the stone impurities and further enrich the iron-containing components. This is to carry out “crushing” and “concentration”, in which the beneficiation links must be heavily used. A small-scale hematite site can process 2,000 to 3,000 tons of iron-containing ore concentrates of about 60% per day. Iron ore concentrates are in powder form, commonly known as iron ore fines, with high iron content of 62-66%. Such smelting can not be directly put into the blast furnace, just as people only burn briquette in the heating furnace, they must be artificially “kneaded” into blocks to increase the permeability and reducibility. The internationally accepted method is secondary processing and manufacturing of “sintered ore” and “pellet ore”. These two types of “artificial block ore” increase the compressive strength at the same time, which is beneficial to blast furnace smelting. The fuel added to the iron concentrate raw material, after high-temperature ignition, burns a lot of heat to make the mineral in the material layer melt. With the combustion layer down and the cold air passing, the molten liquid phase generated at 1000-1100°C is Upon cooling and recrystallization, solidified into a massive sintered ore having a mesh structure. The concentrate powder mixing ingredients to “ball”, and then also on the machine roasting, its specifications are not as large as sinter. Sinters and pellets are ores that can be added to blast furnaces for smelting. Second, coal coke More than 95% of the world’s steel production is still using the coke ironmaking method invented by the Darby British 300 years ago. Therefore, ironmaking requires coke, mainly when it is used for fuel, and coke is also a reducing agent. Without it, it cannot. Iron is displaced from the iron oxide. Coke is not a mineral, but it is to be refined with certain types of coal. The general ratio is 25-30% for fat coal and 30-35% for coking coal, and then it is charged in a coke oven for 12-24 hours. , forming a hard porous coke. The appearance of coke is somewhat similar to that of coal, but its calorific value is very high. It is purer than coal, almost pure carbon, and weighs more than half as much as coal because most of the impurities are removed. Third, blast furnace ironmaking Blast furnace ironmaking is to iron ore and fuel (coke has a dual role, as a fuel, two as reducing agent), ingredients such as limestone, melting in the blast furnace, so that it occurs in the reduction reaction at high temperature, restore from the iron oxide Basically iron-based, part-carbon “pig iron”, that is, molten iron. The molten iron is poured into the ladle and transported to the steel plant or cast iron. If the molten iron is not sent directly to the steelmaking, it can be cast into pig iron, stored or sold on the market. However, because of its carbon content exceeding 2%, the pig iron blocks are brittle and break when they fall. Direct cast iron can be used to cast a variety of cast iron products, such as diesel crankshafts and cast iron pipes. Four, ironmaking into steel The fundamental difference between the nature of iron and steel is the carbon content, and carbon content below 2% is the real “steel”. The so-called “steelmaking” is to decarburize pig iron and transform it into steel during the high temperature smelting process. The commonly used steelmaking equipment is a converter or an electric furnace. Pour hot metal and scrap into oxygen converter The raw materials for converter steelmaking include 85% of hot metal and 10-15% of scrap steel, and then they are blown into oxygen for combustion, without adding any fuel, and relying on the physical heat of hot molten iron to transform the steel within a short time. The electric furnace steelmaking relies on external energy (electric energy) to heat and melt the scrap steel and pig iron. It does not use molten iron to make steel. Fifth, casting billets At present, in addition to the production of special steels and large steel castings, a small amount of cast steel ingots are also required for forging. In general, domestic and foreign mass production of ordinary steel basically abolishes the old process of casting steel ingots—blanking—rolling. The method of casting molten steel into billets and rolling them is called “continuous casting.” If you do not wait for the billets to cool down, do not drop on the way, and go directly to the rolling mill, you can produce the steel products you need. If the billet is cooled halfway and stored on the floor, the billet can be sold as a commodity. A small number of companies are more advanced and can adopt advanced “near-end” continuous casting methods to directly cast molten steel into very thin steel strips or shaped steel billets to reduce the processing of steel rolling mills, saving energy and increasing profits. Six, billet rolled into wood There are many kinds of common steel products. There are various types of steels, rebars, steel plates, steel strips, steel pipes, etc., some of which are hot-worked and some are cold-worked. Therefore, equipment for rolling steel materials is used, which is called “rolling mills”. “. We will briefly describe two examples of hot-rolled bars (round bars) and hot-rolled steel coils. Under rolling of the rolling mill, the billets are coarser and narrower, getting closer and closer to the final diameter of the product, and are sent to a bar cooling bed for cooling. Bars are mostly used for machining mechanical structural parts. If pattern rolls are used on the last bar finishing mill, rebar can be produced, which is called “rebar” for building structural timber. Hot coil rolling mills are used to produce rolled steel. Hot coils are rolled out first, and then cold rolled coils with a higher degree of smoothness are further processed by cold rolling and further processed into coated coils. This kind of thin steel has no breakage in the middle, and it is very advantageous to make cuts and cuts on the steel plate as in the case of clothing, and to make complex products such as various household appliances steel plates and car shells. Seven, forged steel According to different needs, some steels with special properties or special shapes cannot be processed by rolling mills that produce large quantities of products, but they must be produced in small batches or even only one or two pieces. Therefore, it is necessary to cast a number of steel ingots after steelmaking, and then after heating, special-shaped special steels are formed on special processing equipment such as die forging machines and hydraulic presses. In particular, it should be noted that more than half of this type of steel is not produced in steel companies, and their production sites are large and medium-sized mechanical factories. For example, we forged large-scale wind turbine main shafts to illustrate the production of forged steel products.

1. Industrial pipeline on-line inspection, what parts should be focused on inspection? (1) the outlet parts of the compressor and pump; (2), compensator, three links, elbow (elbow), big head, branch pipe connection and dead angle of medium flow. (3) piping components and welded joints near the damaged part of the support hanger; (4) there have been parts of the problem that affect the safe operation of pipelines. (5) the pipe section at the key part of the production process and the pipe section connected with the important device or equipment; (6) severe working conditions or pipe sections subjected to alternating loads.

The production cost of pipe fittings should be taken into consideration in the production enterprises of stainless steel pipe fittings, but the factors of preventing corrosion and improving the quality of the products should be taken into consideration. Therefore, it is necessary to adopt the solid solution process in the production process of stainless steel pipe fittings parts to enhance the safety of the pipe fittings, and the heat treatment process plays a vital role in the quality and safety of the pipe fittings. It is understood that there are a few enterprises in the pipe production to save cost, not to take solid solution treatment, only using polishing and sand blasting methods to cover the defects and shortcomings of the pipe fittings, which will bring permanent potential for the use of pipe fittings. The austenitic stainless steel is softened by heat treatment, heating the stainless steel to about 950~1150 C and holding the heat for a certain time, so that the carbides and all kinds of alloy elements are dissolved in the austenite, which is called solid solution treatment. Effect of heat treatment on stainless steel pipe fittings After forming, welding and other processes, the metal molecular structure, magnetic properties and physical properties of stainless steel pipe fittingss have changed. The solid solution process by atmosphere protection can restore the corrosion resistance after processing and the hardness required for stainless steel to ensure the best use of stainless steel. The stainless steel pipe fittings treated by solid solution has good improvement. 1. To eliminate the modification of stainless steel pipe fittings in the process of processing, the hardness of stainless steel is reduced to less than 220HV, the plasticity and toughness of stainless steel can be improved, and the pipe fittings are more convenient and safe in installation. 2, restore the stress and intergranular changes in the production process of stainless steel pipe fittings, reduce the intergranular corrosion and stress corrosion of stainless steel, and enhance the corrosion resistance. 3. Remove the magnetism produced by stainless steel and stabilize the austenite structure. 4. Restore the natural brightness of stainless steel surface (natural brightness and polishing brightness). The characteristics of solid solution of stainless steel pipe fittings Stainless steel is a special kind of steel. Because of the existence of alloying elements such as nickel and chromium, the heat treatment of stainless steel is different from that of ordinary steel. 1, the thermal conductivity of stainless steel is low, and the thermal conductivity is only 27% of carbon steel at normal temperature. With the increase of heating temperature, the thermal conductivity of stainless steel gradually decreases. Therefore, when stainless steel is heated at low temperature, the heating process should be carried out slowly. 2. After heating the austenitic stainless steel to about 1100 C, it can inhibit the formation of carbides, and then quickly cool to room temperature, so that carbon can reach the state of supersaturation, and the corrosion resistance of stainless steel can be greatly improved. 3. In the solid solution process, the pipe adopts (hydrogen) gas protection to avoid the formation of sticky iron oxide on the surface of the stainless steel, reduce the brightness of the stainless steel surface, and improve the reputation of the appearance. Elements of solid solution for stainless steel pipe fittings The advantages and disadvantages of the solid solution process of stainless steel pipe fittings have a great influence on the corrosion resistance and luminance of the stainless steel, and play a decisive role in the processing performance of stainless steel. Therefore, the heat treatment process of stainless steel plays a very important role in the production of stainless steel pipe fittings. 1. The temperature of solid solution. According to the characteristics of the chemical composition of stainless steel, the solid solution temperature of stainless steel should be 950 – 1150 degrees Celsius, so that the softening effect can be achieved, the hardness of stainless steel is reduced to less than 220HV, and the quality requirement of the pipeline installation card pressure is reached. If the temperature control is unreasonable, all kinds of quality defects are prone to occur. 2. The time of solid solution. During the heating process, the content of residual ferrite in austenitic stainless steel decreases with the prolonging of heating time. Therefore, the solid solution treatment of stainless steel pipe fittings should be controlled at about 1050 degrees, so that the carbon will reach saturation and improve corrosion resistance. And then cooled rapidly in air to achieve solid solution effect. 3, solid solution speed and heat preservation. The thermal conductivity of stainless steel is low, and the thermal conductivity at room temperature is only 27% of that of carbon steel. Therefore, the heating process of stainless steel should be carried out slowly at low temperature. If the heating speed is too fast, it is easy to produce deformation. In the solid solution, the heating speed of stainless steel should be controlled, and the holding time should be noticed. If 316L steel is kept at about 1100 C for a long time, the content of residual ferrite will decrease. In solid solution production, the solid solution of stainless steel pipe fittings must be heated – insulation – time cooling process, and the whole process will take about 40 minutes. Source: China Stainless Steel Pipe Fittings Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

1 Application of Stainless Steel in Petroleum and Petrochemicals The most commonly used classification of stainless steel is classified according to the structure of the steel and can generally be pided into ferritic stainless steel, austenitic stainless steel, martensitic stainless steel, duplex stainless steel, and precipitation hardened stainless steel. In petroleum and petrochemical applications, austenite stainless steel, ferritic stainless steel, and duplex stainless steel account for a large proportion. Ferritic stainless steel generally has a Cr content of between 13% and 30% and a C content of less than 0.25%. In general, ferritic stainless steels have lower corrosion resistance than austenitic stainless steels and duplex steels, but are higher than martensitic stainless steels. However, due to its lower production cost compared to other stainless steels, it has a wide range of applications in the areas of chemical and petrochemical applications where corrosion resistance and strength requirements are not high. Martensitic stainless steel generally has a Cr content of between 13% and 17%, and a high C content of between 0.1% and 0.7%. It has higher strength, hardness and wear resistance, but lower corrosion resistance. It is mainly used in petroleum and petrochemical fields in environments where corrosive medium is not strong, such as components requiring higher toughness and impact loads, such as turbine blades, bolts, and other related parts. Austenitic stainless steels have Cr contents between 17% and 20%, Ni contents between 8% and 16%, C contents generally below 0.12%, and the austenite transformation area is mainly expanded by the addition of Ni elements, thus at room temperature. Under the austenite structure. Austenitic stainless steels are superior to other stainless steels in terms of corrosion resistance, plasticity, toughness, processability, weldability, and low temperature performance. Therefore, their application in various fields is the most extensive. Their use accounts for approximately all stainless steels. About 70% of the volume. In the petroleum and petrochemical field, austenitic stainless steels have a greater advantage for strong corrosive media and low temperature media. Duplex stainless steel is developed on the basis of single-phase stainless steel. Its Ni content is generally about half that of austenitic stainless steel, which reduces the cost of the alloy. Austenitic stainless steel has excellent corrosion resistance and high overall performance. It solves the shortcomings of poor corrosion resistance of ferritic and martensitic stainless steels and insufficient strength and wear resistance of austenitic stainless steels. In the petroleum and petrochemical field, it is mainly used in offshore oil platforms that are resistant to seawater corrosion, acidic components and equipment, and particularly in components that are resistant to pitting corrosion. Precipitated-strengthened stainless steels mainly obtain high-strength properties through precipitation strengthening mechanism. At the same time, they sacrifice their own corrosion resistance. Therefore, they are used less in corrosive media and are generally used in petrochemical machinery mining and other industries. 2 Application of Stainless Steel Pipe in Petroleum and Petrochemicals In the past 20 years, both stainless steel pipes and welding pipes have been greatly improved in terms of production technology. Stainless steel pipes produced by some domestic manufacturers have reached the level that can completely replace imported products, and have achieved localization of steel pipes. In petroleum and petrochemical industries, stainless steel pipes are mainly used in pipeline transportation systems, including high-pressure furnace pipes, piping, petroleum cracking pipes, fluid transportation pipes, and heat exchange pipes. Requires stainless steel to perform well under wet and sour service conditions. 2.1 The application of stainless steel seamless pipe with large diameter thick wall high pressure hydrogen In order to meet the requirements for processing low-quality crude oil and meeting environmental protection requirements, domestic refining companies continue to optimize the processing structure of the refinery equipment and adjust the product structure. High-pressure hydrogenation units such as hydrocracking and hydrotreating have developed rapidly in recent years, and the processing capacity of the equipment has been improved. It is also constantly improving. Hydrogen pipelines are characterized by a large diameter and a thick wall. For the selection of high-pressure hydrogen-imparting materials, TP321/H, TP347/H, etc. are generally used at home and abroad as the material for high-pressure hydrogen-infiltrating pipes because of their high-temperature and high-pressure working conditions. Both stainless steel materials are stable due to the addition of Ti, Nb, etc. Chemical elements have high temperature corrosion resistance and high temperature mechanical properties. At present, domestic large-caliber thick-walled hydrogen-producing pipelines are mainly produced by hot perforation + cold rolling/cold drawing. The hot-perforated + cold-rolled/cold-drawing tubes are superior to the steel tubes produced by other methods in that they have a good surface, high dimensional accuracy, and uniform wall structure. For high-pressure hydrogen-free steel pipes, due to the special nature of the working medium, the requirements for the steel pipe raw materials are relatively high. Therefore, the design requirements for the high-pressure hydrogen-producing pipes are: S ≤ 0.015%, P ≤ 0.030%, Non-metallic inclusions A Classes B, C, and D are not higher than 1.5. Ultrasonic inspection is required for the finished tube, and the artificial contrast defect is not more than 5% of the nominal wall thickness of the tube. 2.2 Application of Stainless Steel Welded Pipe for Low Temperature LNG Due to the development of society, people’s awareness of environmental protection has increased and more and more attention has been paid to clean energy. LNG is a clean and efficient energy source and plays an important role in the production and life of the people. Therefore, LNG receiving stations and LNG carriers have mushroomed. LNG is to cool gaseous natural gas to -162°C under normal pressure and condense it into a liquid. Therefore, the pipeline for LNG transportation must have high low temperature performance. For low-temperature LNG conveying pipes, ultra-pure, low-carbon, low-sulfur, and low-phosphor stainless steels are mostly used at home and abroad. In recent years, the dual-grade stainless steels are very popular among LNG users, among which TP304/304L, TP316/316L and other applications are particularly extensive. Double-certified steel not only has L-level corrosion resistance and low-temperature properties, but also has high mechanical properties. At present, domestic and foreign mainstream welding stainless steel tubes for low temperature LNG are generally processed by using automatic unit welding forming process, UOE forming process, and JCO forming process. The automatic welding unit is a fast, efficient and automatic welding pipe production method used in the case of not thick wall thickness. At present, most of the rollers are used to form the plate, and then the welding and heat treatment are performed online. Some welding units also Integrated advanced technology such as on-board ultrasonics for plates, on-line weld ultrasonics, and automatic weld seam tracking technology can provide high-efficiency manufacturing operations for LNG long-distance pipeline manufacturing and reduce the production deadlines of manufacturers. UOE molding technology is currently the most widely used, most mature, and most recognized quality of a low-temperature LNG welded pipe production process, the main process technology has been established. The JCO molding process is a fresh molding process in recent years. This molding technology is an organic combination of step-wise pre-bending and pipe NC bending. For the LNG stainless steel welded pipe, because its use environment is in a low temperature environment of -162°C, it is necessary for the LNG tube to have a high low temperature impact performance. At present, most design institutes, research institutes, and manufacturers require LNG tubes to have low-temperature impact performance of not less than 80 J, and lateral expansion volumes of not less than 0.38 mm according to ASME B 31.3. For stainless steel welded pipes, as the weak link of the pipe, the quality of the weld directly affects the safety of the pipeline and even the pipeline. The welding coefficient is one of the important factors for evaluating the quality of the weld. For low temperature LNG welded pipes, the welding coefficient is Ej = 1.0 and the welded joint must be a full penetration welded joint. After welding joints are completed, all welds must undergo 100% ray inspection. The welds must have no defects such as incomplete penetration, no weld inclusion, no undercut, and no cracks to ensure the stability of welded joints at low temperatures. 3 Outlook Petroleum pipelines are the bulk of consumption of stainless steel pipes. Stainless steel pipes play an important role in equipment manufacturing, oil recovery, oil refining, and transportation in the oil industry. In recent years, the state has increased the development of petroleum resources. At the same time, as the world’s largest net oil importing country, as the rigid oil demand increases, the oil-related industries will further develop, and the demand for stainless steel pipelines will continue to increase. increase. In 2018, the Chinese steel industry has shown signs of recovery. Domestic stainless steel pipe leading companies have increased their cooperation with relevant oil pipelines between PetroChina and Sinopec to increase market share. At the same time, domestic steel pipe enterprises have also started activities with foreign oil companies to push China’s stainless steel pipe manufacturing to the world platform. Source: China Stainless Steel Pipes Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Metal hose and bellows compensator main performance similarities and differences: Only air tightness a bellows is better than metal hose. Because the bellows are made of a monolithic material, the metallic hose is a flexible member that is wound with a stainless steel band, so that the problem of slight air leakage cannot be prevented. However, with the advancement of metal hose technology, its airtightness will also increase. Applicable to steam, water, oil and various industrial gases, pharmaceuticals and other media transportation. The movement of the pipe system, metal hose has good flexibility and temperature, pressure, corrosion resistance. Thermal expansion absorption and vibration absorption play an important role. Due to its superior performance, the model length and connection method are based on user requirements. Metal hoses are widely used in aviation, aerospace, oil pipelines and other devices. The scope of its application has rapidly expanded. The missile transfer vehicle is an important guarantee equipment for the entire missile preparation stage. The performance is directly related to the maneuvering capability of the missile transfer vehicle, and provides a strong guarantee for the launch of the entire missile launch position. The metal hose is an important part of the missile transfer vehicle. It is mainly used in the connection between the engine and the exhaust system. It mainly plays the role of reducing noise, absorbing vibration, and flexible connection. Among them, stainless steel wire braided hoses, fluid-conveying metal bellows, stainless steel pumps with vibration-reducing hoses, stainless steel fire hoses, stainless steel corrugated compensators, and gas-fired mechanical stainless steel bellows fully comply with domestic industrial standards. Metal hose is characterized by corrosion resistance, high temperature resistance, low temperature resistance (-196°C～+420°C), light weight, small volume and good flexibility. Widely used in aviation, aerospace, petroleum, chemical, metallurgy, electric power, paper, wood, textile, construction, medicine, food, tobacco, transportation and other industries. Stainless steel metal hose (metal bellows) Division produced stainless steel metal hose, stainless steel bellows, stainless steel metal soft joints and so on. Source: China Stainless Steel Bellows Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)

Long-distance pipeline welding method: First, manual welding (1) Arc welding of electrodes is sensitive and cumbersome, and has strong conformability. At the same time, due to the continuous improvement of electrode performance, its deposition efficiency and mechanical properties can still meet the requirements of today’s pipelines. In particular, welding welding is more commonly used. . The electrodes are cellulose-type electrodes and low-hydrogen type electrodes. The organic separation of the two methods of down welding and upward welding and the good root welding compliance of cellulose electrodes are still not replaceable by other welding methods in many places. (2) Manual tungsten-arc welding has good welding quality and no back slag on the back. It is commonly used to stop the import and export of ballast machines, ball valves and other equipment, as well as the process pipelines with smaller diameter and thinner wall thickness. Welding device welding. The TIG welding method requires strict stop of groove settlement before welding and windproof measures during welding. Second, semi-automatic welding (1) Semi-automatic welding of self-shielded flux-cored wire This technology was first applied to the Kuqin Line Pipeline Project in 1996, and subsequently applied in pipeline projects in Sudan, Lancheng County, and Shibuyinlan City. This welding method is sensitive to operation, environmental compliance can be strong, welding deposition efficiency is high, welding quality is good, welders are easy to control, welding qualification rate is high, is an important filling, cap welding method in the current domestic pipeline engineering. (2) Semi-automatic welding of CO2 gas protection With the improvement of the characteristics of the welding power source, the splash problem of CO2 gas shielded arc welding has been fundamentally dealt with after controlling droplets and arc patterns, and has begun to play an important role in pipeline welding, such as STT type CO2 inverse. Variable welding machine applications. This kind of welding method is sensitive to operation, welders are easy to control, have strong conformability to different grooves, good welding quality, high welding efficiency, and weld bead lubrication, but the welding process is greatly affected by the ambient wind speed. The STT semi-automatic root welding request nozzle group insists on the average disagreement of the counterpart gap during the process, otherwise it will produce defects such as the edge of the groove not being fused and the slag inclusion in the subsequent filling and capping bead. Third, automatic welding Automatic welding can be applied to root welding and filling and cap welding. The automatic root welding method adopts automatic internal welder or external welder single-side weld double-sided molding. (1) The automatic welder used in the “West-East Gas Pipeline” pipeline project of the internal welder is a pipe root welding machine for Φ1016mm pipe diameter. The internal welder and the CNPC Natural Gas Pipeline Bureau, which were purchased by British NOREAST Company, respectively. PIW3640 internal welder. Its characteristic is that the range of applicable pipe diameter is narrow, the equipment investment is larger at one time, but the welding efficiency is very high, the root welding of the Φ1016mm steel pipe takes about 70 seconds. Since the welding is stopped in the steel pipe, the influence of the ambient wind speed during the welding process is relatively small, as shown in FIG. 1 . (2) Automatic external welder single-sided welding double-sided forming root welding single-sided welding double-sided forming root welding equipment is mainly Italian PWT’s CWS.02NRT automatic external welding machine, and the United States LINCOLN’s STT power matching automatic external welding machine. The double-sided root welding equipment for single-side welding of automatic external welders handles the problem of single-sided double-sided root welding without backing. The root welding thickness reaches 4.5 mm, which is much higher than the welding thickness of internal welding machines (1 to 1.2mm), high welding efficiency, Φ1016mm steel root welding takes about 8 minutes. Due to the use of gas maintenance, the welding process is sensitive to the wind speed of the environment and windbreaks and other wind protection measures should be taken during construction. Welding process 1, Welding preparation 1.1 Selection Because the West-East Pipeline X is a DN813×9.5mm spiral steel pipe, the caliber is larger than the steel pipe normally used, so it is easier for the two steel pipes to have different circumferences, and the radius of curvature is not the same in some directions. This is relatively easy to produce a large amount of error in the construction, which directly affects the welding quality of the entire weld, so before the steel into the site it is recommended to measure the circumference of each pipe at both ends, and then have a rough distribution of the steel pipe The plan should try to make the difference between the circumferential lengths of the two steel pipes to be welded not exceed 3mm, which can reduce the defects caused by the wrong side. 1.2 Groove processing Groove cleaning is very important for root welding, otherwise it is easy to produce defects such as internal bite, concave, and air hole. Groove cleaning requires removal of all dirt within 25 mm from the inside of the groove and both sides of the groove. 1.3 Groove grinding Groove polishing technology requirements: grinding the inside of the groove to the emergence of metallic luster while not hurting the groove angle. Spiral Weld Grinding Technology Requirements: It is required that the spiral welding seam be flush with the base metal within 10mm on both sides of the grinding groove and gently transition at an elevation of 30 degrees. The higher requirements for the grinding of the spiral bead are due to three-dimensional stress and notch effects caused by unwelded spiral welds and pipeline welds that create cross welds, which can cause cracks, and the angles between unsanded spiral welds and the parent metal With rust, pores are easily formed on the cover. In addition, spiral welds of spiral steel pipes for welding excuses should be at least 100 mm away to avoid stress concentration and defects. 1.4 Groove preparation In the groove preparation process, special attention should be paid to the angle and blunt edge of the groove. If the size of the groove is too large or too small, the weld quality will be affected. Excessive blunt edges will generally show incomplete penetration, and too small blunt edges will easily burn. Wear will affect the quality of root welding. Therefore, it should be strictly in accordance with the process requirements, generally choose between 1.5 ~ 2.0mm. 2, Root welding The purpose of down welding for long-distance pipelines is to use large welding specifications and relatively low consumption of welding materials in order to achieve improved ergonomics and cost savings, and many welders still use the customary pipe all position up-welding large gaps, small blunt The use of edge-to-edge parameters as a downhole welding technique is neither scientific nor economical. Such counterpart parameters not only increase the unnecessary consumption of welding consumables, but also increase the consumption of welding consumables and increase the probability of occurrence of welding defects. Moreover, the repair of the root defect is more difficult than the defects produced in the filler cap. Therefore, the choice of the root joint parameter is quite important. Generally, the gap is between 1.2-1.6 mm, and the blunt edge is between 1.5-2.0 mm. When performing root welding, the electrode is required to make an angle of 90 degrees with the axis of the pipe and point to the axis. The correct electrode position is the key to ensure that the back surface of the root welding is formed, especially to ensure that the root bead is located at the center of the weld and eliminated. Bite and unilateral incomplete penetration, when the adjustment of the longitudinal angle of the electrode can change the penetration of the electrode, because generally can not get a complete groove gap and blunt edge, so the welder must be adjusted by the welding rod longitudinal angle adjustment to adjust the arc Penetration force to adapt the joint groove and welding position. The electrode should remain in the center of the joint unless an arc blow occurs. The welder can eliminate the arc blow by adjusting the angle between the welding rod and the axis of the pipe and keeping the short arc. Otherwise, the inner side of the single-sided groove where the arc is blown will be bitten inside, and the other side will be incompletely welded. For control of the weld pool, in order to obtain a well-formed root weld, it is critical to maintain a small, visible weld pool during the root weld. If the weld pool becomes too large, it will immediately lead to Bite or burn. In general, the size of the bath is 3.2 mm long, which is ideal. Once a small change in the size of the bath is found, the proper bath size should be maintained by adjusting the angle and current of the welding rod. Root rooting is the key to ensure the quality of root welding in the entire weld. The point of root rooting is to clearly define the convex weld bead and the track line. If the clear root is excessive, it will cause the root weld to be too thin, making it easy to heat weld. Burning and burning is not enough, and slag and pores are easily generated. Qing root should use 4.0mm thick disc wheel. Our welders usually like to use 1.5 or 2.0mm reworked cutting discs as welding slag removal tools, but 1.5 or 2.0mm cutting discs are often prone to deep grooves, resulting in incomplete fusion or slag inclusion in the subsequent welding process, resulting in rework, At the same time, the 1.5-mm or 2.0-mm cutting discs are not as good as the 4.0 mm thick disc-shaped discs in terms of slag loss and slag removal efficiency. For clearing requirements, the track line is cleared and the fish back is trimmed to be flat or dimpled. 3, Hot welding Under the premise of ensuring the quality of the root welding roots, thermal welding can be performed. Usually, the gap between hot welding and root welding cannot be longer than 5 minutes. Semi-automatic welding usually uses a trailing angle of 5 degrees to 15 degrees, and the angle between the welding wire and the management axis is 90 degrees. The principle of hot bead is not to make or make a small pair of lateral swings. Under the condition of ensuring that the arc is located in the front of the weld pool, it is carried down from 4 o’clock to 6 o’clock with a puddle; the 8 o’clock to 6 o’clock position should be appropriate. Swing in the lateral direction to avoid excessive convex welds in the upside-down position. For the clearance of arc starting and arc collecting holes, a pause can be made at the arc starting point to facilitate the gas floating out of the weld pool, or the use of overlapping arc starting and arc closing is the most effective method to solve arc starting and arc collecting holes; After completion, the convex bead was removed using a 4.0 mm thick disc wheel. If root burnout occurs during hot welding, semi-automatic welding must not be used for repair, otherwise dense holes will appear in the repair bead. The correct process is to find that the semi-automatic protection welding is stopped immediately after burn-through, and the two ends of the root welding burn-through, especially the burn-through place, are grinded into a gentle slope transition. According to the requirements of the root welding process, the hand-made cellulose electrode is used for the burn-through. The repair welding is performed, and when the temperature of the weld at the repair weld is reduced to 100 degrees to 120 degrees, the normal hot welding semi-automatic welding process is continued for welding. The selection principle of the hot bead process parameters is based on the principle of no burn-through of the root bead. High wire feed speeds and welding voltages matching the wire feed speed should be used as much as possible. The advantage is that high welds can be obtained. Speed, high wire feed speed can achieve large penetration depth, large arc voltage can obtain a wide molten pool, which can make the residual residue after cleaning the residue in the root pass, especially the hidden slag melted in the root bead wire. Out, floating to the surface of the bath, and can get concave bead to reduce the hot bead clearance slag labor intensity. The slag of the hot-bead slag in principle requires that the wire-wheel be cleaned, and the slag that cannot be partially removed must be removed by the grinding wheel. The local convex bead requires grinding with a 4.0 mm thick disc-shaped grinding wheel to remove the protruding parts (mainly occurring at 5:30-6:30 o’clock), otherwise columnar blow holes are likely to occur. It is not allowed to have welding slag on the weld bead because the presence of welding slag will affect the conductivity of the filling welding arc, causing momentary arcing and forming local dense pores. 4, Fill welding The welding bead can only be filled under the premise of guaranteeing the quality of hot bead welding. The welding requirements of the filler welding are basically the same as those of the thermal welding. The feeding speed is slightly higher than the hot welding, and the voltage is slightly higher than that of the hot welding. After the completion of the filled bead, 2 to 4 points are required, and the 8 to 10 point filler weld is basically flush with the surface of the base metal, and the maximum remaining edge of the groove must not exceed 1.5 mm to ensure vertical welding when the surface weld is applied. Positions will not have air holes or be lower than the parent material. If necessary, filler welds will be added to add a vertical weld. The vertical filler weld only in the filler bead between 2-4 o’clock and 10-8 o’clock. When the filler weld is completed, the filling surface at the above position is much different from the groove surface, such as direct cover, complete weld bead Later, in the above position, there will be an increase in the number of vertical welds when the surface of the weld is lower than the surface of the base metal. The stand-up welding must be completed once after starting the arc, and the arc cannot be interrupted during the welding process because the welded joint at this location is prone to dense joints. The vertical fill welding usually does not swing laterally. With the molten pool down, the surface of the weld bead can be slightly convex or flat in the vertical welding position. This can avoid the concave shape of the surface of the cover weld and the center of the bead is lower than the base material. The selection principle of the welding process parameters for the vertical welding is relatively high welding wire feeding speed and a relatively low welding voltage, so that air holes can be avoided. 5, Cover welding Cap welding can only be performed on the premise of guaranteeing the quality of the filler weld. Due to the high deposition efficiency of the semi-automatic protection welding, special attention must be paid to the selection of the welding process parameters during cap welding. The key to the selection of process parameters is the wire feed speed, voltage, back drag, dry extension and welding speed. In order to avoid the generation of blowholes, higher wire feed speeds should be used, lower voltages (voltages lower than the normal and wire feed speeds should be about one volt lower), longer dry stretches, and welding speeds to ensure welding arcs Always in front of the weld puddle is the principle. At 5 o’clock – 6 o’clock, 7 o’clock – 6 o’clock, it can increase the dry elongation and change the welding to push welding, so that a thinner bead layer can be obtained so as to avoid excessive height in the upside welding position of the welding bead. In order to eliminate the occurrence of joint blowholes on the surface of uphill and upright welds, it is usually necessary to weld once at the upright welding position. It is forbidden to generate welding joints at 2 o’clock, 4:30 o’clock, 10 o’clock and 8:30 o’clock. In order to avoid the generation of stomata. In order to avoid the occurrence of cracks in the joints at the site of the upward slope, welds are to be made between 4 and a half minutes – 6 o’clock and 8 o’clock and 6 o’clock, followed by 12 o’clock – 4:30 o’clock, 12 o’clock. The weld between the 8 o’clock and 8 o’clock positions can effectively avoid the occurrence of joint pores on the climbing site. The welding process parameters of cap welding are basically the same as those of heat welding, but the wire feeding speed is slightly higher. 6, Control of semi-automatic welding welding defects The key to the operation of semi-automatic welding is to take advantage of the situation. Always keep the welding arc in the front of the welding bath and the thin layer of fast multi-pass welding is the key to overcome all welding defects. Never avoid a large single-layer thickness. , and pay attention to the stability of the welding process, welding quality is mainly related to the wire feeding speed, welding voltage, dry elongation, rear drag angle, welding walking speed of the five major welding process parameters, change any one, the remaining four parameters have to be done Corresponding adjustments. Source: China Stainless Steel Pipelines Manufacturer – wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)