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Stainless steel pipe heat treatment is commonly used in foreign countries as a non oxidizing continuous heat treatment furnace with protective gas for intermediate heat treatment during the production process and final product heat treatment. Due to the ability to obtain a bright surface without oxidation, the traditional pickling process is eliminated. The adoption of this heat treatment process not only improves the quality of steel pipes, but also overcomes the environmental pollution caused by acid washing.
Stainless steel pipe heat treatment is commonly used in foreign countries as a non oxidizing continuous heat treatment furnace with protective gas for intermediate heat treatment during the production process and final product heat treatment. Due to the ability to obtain a bright surface without oxidation, the traditional pickling process is eliminated. The adoption of this heat treatment process not only improves the quality of steel pipes, but also overcomes the environmental pollution caused by acid washing.
According to the current trend of world development, there are basically three types of bright continuous furnaces:
1) Roller bottom type bright heat treatment furnace. This furnace type is suitable for heat treatment of large specifications and batches of steel pipes, with an hourly output of over 1.0 tons. The protective gases that can be used are high-purity hydrogen gas, decomposed ammonia, and other protective gases. It can be equipped with a convective cooling system to quickly cool the steel pipe.
(2) Mesh belt type bright heat treatment furnace. This furnace type is suitable for small diameter thin-walled precision steel pipes, with an hourly production capacity of about 0.3-1.0 tons. It can handle steel pipes up to 40 meters in length and can also be processed into coils of capillary tubes.
(3) Muffle type bright heat treatment furnace. The steel pipe is installed on a continuous rack and heated inside a muffle tube. It can process high-quality small diameter thin-walled steel pipes at a lower cost, with an hourly production capacity of about 0.3 tons or more.
Stainless steel welded pipe requires deep penetration welding without oxide inclusion, the Heat-affected zone should be as small as possible, argon arc welding with tungsten inert gas protection has good adaptability, high welding quality and good penetration performance, and its products are widely used in chemical, nuclear industry, food and other industries.
The disadvantage of argon arc welding is that the welding speed is not high. In order to improve the welding speed, various methods have been researched and developed abroad. The development of single electrode single welding torch and the application of multi electrode multi welding torch welding method in production. In the 1970s, Germany first adopted multiple welding torches arranged in a straight line along the direction of the weld seam, forming a long heat flux distribution, significantly improving the welding speed. Generally, a three electrode welding torch is used for argon arc welding. The wall thickness of the welded steel pipe S ≥ 2mm, the welding speed is increased by 3-4 times compared to a single welding torch, and the welding quality is also improved. The combination of argon arc welding and plasma welding can weld steel pipes with larger wall thicknesses. In addition, using a high-frequency pulse welding power source in argon gas with 5-10% hydrogen gas can also improve the welding speed.
Multi torch argon arc welding is suitable for the welding of austenitic and ferritic stainless steel pipes.
High frequency welding has been used in the production of carbon steel welded pipes for over 40 years, but it is a relatively new technology for welding stainless steel pipes. The economy of its production makes its products more widely used in the fields of building decoration, household appliances, and mechanical structures.
High frequency welding has a higher power supply and can achieve higher welding speed for steel pipes of different materials and outer diameter wall thickness. Compared with argon arc welding, it is more than 10 times its maximum welding speed. Therefore, the production of stainless steel pipes for general purposes has a high productivity.
Due to the high speed of high-frequency welding, it is difficult to remove burrs inside the welded pipe. At present, high-frequency welding stainless steel pipe is not accepted by the chemical and nuclear industry, which is one of the reasons.
From the perspective of welding materials, high-frequency welding can weld various types of austenitic stainless steel pipes. At the same time, the development of new steel grades and the progress of forming welding methods have also successfully welded steel grades such as ferritic stainless steel AISI409.
Various welding methods for stainless steel welded pipes have their own advantages and disadvantages. How to leverage strengths and avoid weaknesses, combine several welding methods to form new welding processes, and meet people's requirements for the quality and production efficiency of stainless steel welded pipes, is a new trend in the current development of stainless steel welded pipe technology.
After years of exploration and research, progress has been made in the combination welding process, and stainless steel welded pipe production in countries such as Japan and France has mastered certain combination welding technologies.
The combination welding methods include: argon arc welding plus plasma welding, high-frequency welding plus plasma welding, high-frequency preheating plus three torches argon arc welding, high-frequency preheating plus plasma plus argon arc welding. Combination welding significantly improves welding speed. For the combination welding of steel pipes using high-frequency preheating, the quality of the weld seam is equivalent to that of conventional argon arc welding and plasma welding. The welding operation is simple, and the entire welding system is easy to achieve automation. This combination is easy to connect with existing high-frequency welding equipment, with low investment cost and good efficiency.
TIG welding has been widely used in production, as it can obtain high-quality welds and is commonly used to weld materials such as non-ferrous metals, stainless steel, and ultra-high strength steel. However, TIG welding has drawbacks such as shallow fusion depth (≤ 3mm) and low welding efficiency. For thick plates, multiple passes of welding need to be made with grooves. Although increasing the welding current can increase the penetration depth, the increase in penetration width and pool volume is much greater than the increase in penetration depth.
The activated TIG welding method has attracted worldwide attention in recent years. This technology involves applying a layer of active flux (referred to as active flux) to the surface of the weld before welding. Under the same welding specifications, compared to conventional TIG welding, it can significantly increase the penetration depth (up to 300%). For 8mm thick plates, larger penetration or penetration can be achieved at once without opening grooves. For thin plates, welding heat input can be reduced without changing the welding speed. At present, A-TIG welding can be used for welding materials such as stainless steel, carbon steel, nickel based alloys, and titanium alloys. Compared with traditional TIG welding, A-TIG welding can greatly improve productivity, reduce production costs, and reduce welding deformation. It has a very important application prospect. The key factor in A-TIG welding is the selection of active agent components. The commonly used active agent components currently include oxides, chlorides, and fluorides. Different materials have different applicable active agent components. However, due to the importance of this technology, the composition and formula of the active agent have patent restrictions in both PWI and EWI, and are rarely reported in public publications. At present, research on A-TIG welding mainly focuses on two aspects: the study of the mechanism of activating agent action and the study of activated welding application technology.
At present, there are three main types of active agents developed and used domestically and internationally: oxides, fluorides, and chlorides. The early active agents developed by PWI for titanium alloy welding were mainly oxides and chlorides, but the toxicity of chlorides was high, which was not conducive to promotion and application. At present, the active agents used for welding stainless steel, carbon steel, and other materials abroad are mainly oxides, while for the welding of titanium alloy materials, the active agents contain certain fluoride components.
The effect of a single component activator on the formation of stainless steel welds:
(1) For welds coated with SiO2 activator, as the amount of SiO2 coating increases, the weld bead width gradually narrows, and the arc crater becomes longer, narrower, and deeper. The residual height at the rear of the weld bead increases, and at the intersection of the coated and uncoated active fluxes, there is a lot of metal accumulation in the weld bead. Among all active fluxes, SiO2 has the greatest effect on weld formation.
(2) The effect of active agents NaF and Cr2O3 on the weld bead formation is not significant. As the coating amount increases, the width of the weld seam does not change significantly, and there is no significant change in the crater. Compared with welds without activating agents, there is no significant change in weld bead width, but the arc crater is larger than that without activating agents.
(3) As the amount of TiO2 coating increases, there is little change in the appearance of the weld bead, and there is no significant change in the arc crater, similar to when there is no activator. But the surface of the formed weld seam is relatively flat and regular, without any undercut phenomenon, which is better than the formation of the weld bead without activating agent.
(4) The active agent CaF2 has a significant impact on the formation of the weld bead. As the amount of CaF2 coating increases, the weld formation deteriorates, the arc crater does not change much, and the weld width does not change much. But as the amount of CaF2 increases, defects such as undercutting appear.
(5) In terms of the impact on weld penetration, compared to non active agents, all five active agents mentioned above can increase the weld penetration, and as the coating amount increases, the penetration also increases accordingly. However, when the coating amount reaches a certain value, the melting depth increases to saturation, and increasing the coating amount actually leads to a decrease in melting depth.