What are the common welding methods and precautions for stainless steel pipes

First, what are the welding methods for stainless steel pipes?

(1) Argon Arc Welding: Stainless steel pipes require deep penetration, no oxide inclusions, and a minimal heat-affected zone. Tungsten inert gas (TIG) shielding provides good adaptability, high welding quality, and good penetration performance. Its products are widely used in the chemical, nuclear, and food industries. The low welding speed is a drawback of TIG welding. To improve welding speed, various methods have been developed abroad. Among them, the welding method using multiple electrodes and multiple torches has evolved from a single electrode and single torch to a multi-electrode, multi-torch method used in production. In the 1970s, Germany first adopted a method where multiple torches are arranged in a straight line along the weld direction, forming a long heat flow distribution, which significantly improved the welding speed. Generally, TIG welding using a three-electrode torch is used for steel pipes with a wall thickness S≥2mm. The welding speed is 3-4 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 thicker walls. Furthermore, adding 5-10% hydrogen to the argon gas and using a high-frequency pulse welding power source can also increase the welding speed.

(2) High-frequency welding: High-frequency welding has been used in the production of carbon steel welded pipes for over 40 years, but its application to welding stainless steel pipes is a relatively new technology. High-frequency welding has lower power consumption and can achieve high welding speeds for steel pipes of different materials, outer diameters, and wall thicknesses. Compared to argon arc welding, it is more than 10 times faster. Therefore, it has a high productivity in the production of general-purpose stainless steel pipes. However, the high welding speed makes it difficult to remove burrs from the welded pipe. Currently, high-frequency welded stainless steel pipes are not yet accepted by the chemical and nuclear industries, which is one of the reasons for this. In terms 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 advancements in forming and welding methods have also successfully welded ferritic stainless steel grades such as AISI409.

(3) Combined welding technology: Each welding method for stainless steel pipes has its own advantages and disadvantages. How to leverage the strengths and mitigate the weaknesses of several welding methods to form new welding processes that meet the demands for both quality and production efficiency in stainless steel welded pipes is a new trend in the development of stainless steel welded pipe technology. After several years of exploration and research, combined welding processes have made progress, and stainless steel welded pipe manufacturers in countries such as Japan and France have mastered certain combined welding technologies. Combined welding methods include: 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. Combined welding significantly improves welding speed. For combined welded steel pipes using high-frequency preheating, the weld quality is comparable to that of conventional argon arc welding and plasma welding. The welding operation is simple, the entire welding system is easily automated, and this combination is easy to integrate with existing high-frequency welding equipment, resulting in low investment costs and high efficiency.

Secondly, what are the heat treatment methods for stainless steel pipes?

Internationally, stainless steel pipe heat treatment commonly uses non-oxidizing continuous heat treatment furnaces with protective gases for intermediate heat treatment during production and final finished product heat treatment. Because a bright, non-oxidizing surface can be obtained, the traditional pickling process is eliminated. The adoption of this heat treatment process improves the quality of steel pipes and overcomes the environmental pollution caused by pickling. According to current global development trends, bright continuous heat treatment furnaces are basically divided into three types:

(1) Roller hearth bright heat treatment furnace: This type of furnace is suitable for heat treatment of large-scale, high-volume steel pipes, with an hourly output of over 1.0 tons. High-purity hydrogen, decomposed ammonia, and other protective gases can be used. It can be equipped with a convection cooling system for faster cooling of the steel pipes.

(2) 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 approximately 0.3 to 1.0 tons. It can process steel pipes up to 40 meters in length and can also process coiled capillary tubes.

(3) Muffle bright heat treatment furnace: Steel pipes are mounted 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 output of approximately 0.3 tons or more.

Third, what is the effect of TIG welding activators on the weld formation of stainless steel?

TIG welding has been widely used in production, producing high-quality welds, and is commonly used for welding non-ferrous metals, stainless steel, and ultra-high-strength steel. However, TIG welding has disadvantages such as shallow penetration (≤3mm) and low welding efficiency. For thick plates, beveling and multi-pass welding are required. While increasing the welding current can increase the penetration, the increase in weld width and weld pool volume is much greater than the increase in penetration. Currently, the main types of activators developed and used domestically and internationally are oxides, fluorides, and chlorides. The effect of single-component activators on the weld formation of stainless steel:

1. For welds coated with SiO2 activator, as the SiO2 coating amount increases, the weld width gradually narrows, and the crater becomes longer, narrower, and deeper. The weld reinforcement at the rear increases. At the junction of the activator-coated and un-activator-coated areas, there is more weld metal accumulation. Among all activators, SiO2 has the greatest effect on weld formation.

2. The activators NaF and Cr2O3 have little effect on weld bead formation. With increasing coating amount, the weld width and crater do not change significantly. Compared to welds without activators, the weld width is not significantly different, but the crater is larger.

3. With increasing TiO2 coating amount, the weld appearance does not change much, and the crater does not change significantly, similar to the case without activators. However, the resulting weld surface is relatively smooth and regular, without undercut, and the weld formation is better than that without activators.

4. The activator CaF2 has a significant impact on weld bead formation. With increasing CaF2 coating amount, the weld formation deteriorates, but the crater and weld width do not change significantly. However, with increasing CaF2 content, defects such as an undercut appear.

5. Regarding the effect on penetration depth, compared to welds without activators, all five activators can increase the weld penetration depth, and the penetration depth increases accordingly with increasing coating amount. However, when the coating amount reaches a certain value, the melt depth increases to a saturation point, and further increases in coating amount will actually decrease the melt depth.