First, welding of stainless steel pipes
Argon arc welding: Stainless steel pipes require deep penetration, no oxide inclusions, and the heat-affected zone is as small as possible. Tungsten inert gas-protected argon arc welding has good adaptability, high welding quality, and good penetration performance. Its products are widely used in industries such as chemical, nuclear, and food. The low welding speed is a shortcoming of argon arc welding. In order to increase the welding speed, foreign countries have developed a variety of methods. Among them, the welding method developed from a single electrode and a single torch to a multi-electrode and multi-torch welding method is used in production. In the 1970s, Germany first used multiple torches to be arranged in a straight line along the direction of the weld to form a long heat flow distribution, which significantly increased the welding speed. Generally, argon arc welding with a three-electrode torch is used. The wall thickness of the welded steel pipe is S≥2mm, and the welding speed is 3 to 4 times higher than that of a single 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, 5 to 10% hydrogen in argon gas and a high-frequency pulse welding power supply can also increase the welding speed. Multi-torch argon arc welding is suitable for welding austenitic and ferritic stainless steel pipes.
High frequency welding: High frequency welding has been used in the production of carbon steel welded pipes for more than 40 years, but it is a relatively new technology for welding stainless steel pipes. The economic efficiency of its production makes its products more widely used in the fields of architectural decoration, household appliances, and mechanical structures. High frequency welding has a lower power supply and can achieve a higher welding speed for steel pipes of different materials and outer diameters and wall thicknesses. Compared with argon arc welding, it is more than 10 times its maximum welding speed. Therefore, the production of general-purpose stainless steel pipes has a higher productivity. Because the high-frequency welding speed is high, it is difficult to remove burrs in the welded pipe. At present, high-frequency welded stainless steel pipes are not yet accepted by the chemical and nuclear industries, 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 advancement of forming welding methods have also successfully welded ferritic stainless steel AISI409 and other steel grades.
Combined welding technology: Various welding methods for stainless steel pipes have their own advantages and disadvantages. How to make the best use of strengths and avoid weaknesses, combine several welding methods to form a new welding process, 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, the combined welding process has made progress, and the production of stainless steel welded pipes in Japan, France, and other countries has mastered certain combined welding technologies. The combined welding methods are: argon arc welding plus plasma welding, high-frequency welding plus plasma welding, high-frequency preheating plus three-torch argon arc welding, and high-frequency preheating plus plasma plus argon arc welding. The combined welding speed is significantly improved. For the combined welded steel pipe with high-frequency preheating, the weld quality is equivalent to conventional argon arc welding and plasma welding. The welding operation is simple, and the entire welding system is easy to automate. This combination is easy to connect with existing high-frequency welding equipment, with low investment cost and good benefits.
Second, heat treatment of stainless steel pipes
For heat treatment of stainless steel pipes, non-oxidizing continuous heat treatment furnaces with protective gas are generally used abroad to carry out intermediate heat treatment in the production process and final heat treatment of finished products. Since a non-oxidizing bright surface can be obtained, the traditional pickling process is canceled. The adoption of this heat treatment process not only improves the quality of steel pipes but also overcomes the environmental pollution caused by pickling.
According to the current development trend of the world, bright continuous furnaces are basically divided into three types:
Roller bottom bright heat treatment furnace: This type of furnace is suitable for heat treatment of large-scale and large-volume steel pipes, with an hourly output of more than 1.0 tons. The protective gas that can be used is high-purity hydrogen, decomposed ammonia and other protective gases. It can be equipped with a convection cooling system to cool the steel pipe faster.
Mesh belt bright heat treatment furnace: This type of furnace is suitable for small-diameter thin-walled precision steel pipes, with an hourly output of about 0.3 to 1.0 tons, and the length of the processed steel pipe can reach 40 meters. It can also process rolled capillaries.
Muffle bright heat treatment furnace: The steel pipe is installed on a continuous rack and runs and heats in the muffle tube. It can process high-quality small-diameter thin-walled steel pipes at a lower cost, with an hourly output of about 0.3 tons or more.
Third, the influence of the TIG welding activator on the formation of stainless steel welds
TIG welding has been widely used in production. It can obtain high-quality welds and is often used to weld non-ferrous metals, stainless steel, ultra-high strength steel, and other materials. However, TIG welding has the disadvantages of shallow penetration (≤3mm) and low welding efficiency. For thick plates, grooves need to be opened for multiple passes. Although increasing the welding current can increase the penetration, the increase in the molten width and the volume of the molten pool is much greater than the increase in the penetration. The activated TIG welding method has attracted worldwide attention in recent years. This technology is to apply a layer of activated flux (referred to as activator) on the weld surface before welding. Under the same welding specifications, compared with conventional TIG welding, it can greatly increase the penetration (up to 300%). For 8mm thick plate welding, a larger penetration or one-time penetration can be obtained without groove opening. For thin plates, the welding heat input can be reduced without changing the welding speed. At present, A-TIG welding can be used to weld 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, and has very good application prospects. The key factor of A-TIG welding lies in the selection of activator components. At present, the commonly used activator components are mainly oxides, chlorides, and fluorides, and different materials have different activator components. However, due to the importance of this technology, the composition and formula of the activator are subject to patent restrictions in PWI and EWI and are rarely reported in public publications. At present, the research on A-TIG welding mainly focuses on the research on the mechanism of action of the activator and the application technology of activated welding.
At present, there are three main types of activators developed and used at home and abroad:
Oxides, fluorides and chlorides. The activators developed by PWI for titanium alloy welding in the early days were mainly oxides and chlorides, but the toxicity of chlorides was high, which was not conducive to promotion and application. At present, the activators used for welding stainless steel, carbon steel, etc. abroad are mainly oxides, while the activators for welding titanium alloy materials contain certain fluoride components.
The influence of 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 width of the weld gradually narrows, and the arc pit becomes longer, narrower, and deeper. The excess height of the rear part of the weld becomes higher. At the junction of the coated and uncoated activator, the weld metal accumulates more. Among all activators, SiO2 has the greatest effect on weld formation.
2. The effect of activators NaF and Cr2O3 on weld formation is not obvious. As the amount of coating increases, the weld width does not change much, and the arc pit does not change significantly. Compared with the weld without an activator, the weld width does not change significantly, but the arc pit is larger than that without an activator.
3. As the amount of TiO2 coating increases, the appearance of the weld does not change much, and the arc pit does not change significantly, which is similar to the case without an activator. However, the weld surface formed is relatively flat and regular, and there is no undercut phenomenon, which is better than the weld without an activator.
4. The activator CaF2 has a great influence on the weld bead formation. With the increase of CaF2 coating amount, the weld formation becomes worse, the arc crater does not change much, and the weld width does not change much. However, with the increase of CaF2 amount, defects such as undercut appear.
5. In terms of the influence on the penetration depth, compared with no activator, the above five activators can increase the penetration depth of the weld, and with the increase of coating amount, the penetration depth also increases accordingly. However, when the coating amount reaches a certain value, the penetration depth increases to saturation, and the penetration depth decreases when the coating amount is increased.
Post time: Apr-27-2025