Stainless Steel Pipe Welding Methods and Precautions

First, Stainless Steel Pipe Welding
1. TIG Welding of Stainless Steel Pipe: Stainless steel pipe requires deep penetration, no oxide inclusions, and a minimal heat-affected zone. TIG welding with tungsten inert gas (TIG) offers excellent adaptability, high weld quality, and excellent penetration performance. Its products are widely used in the chemical, nuclear, and food industries. However, a drawback of TIG welding is its low welding speed. To increase this speed, various methods have been developed abroad. Among these methods, the evolution from a single-electrode, single-torch welding method to a multi-electrode, multi-torch welding method is now widely used in production. In the 1970s, Germany pioneered the use of multiple torches arranged in a straight line along the weld seam, creating an elongated heat flux distribution and significantly increasing welding speed. Three-electrode TIG welding is generally used for welding steel pipes with a wall thickness of 2mm or greater. This increases welding speed by 3-4 times compared to a single-torch welding method, and also improves weld quality. Combining argon arc welding with plasma welding can weld thicker steel pipes. Furthermore, using 5-10% hydrogen in argon and a high-frequency pulsed welding power source can also increase welding speed.

Multi-torch argon arc welding is suitable for welding austenitic and ferritic stainless steel pipes.

2. High-frequency welding of stainless steel pipes: High-frequency welding has been used in the production of carbon steel pipes for over 40 years, but its application to stainless steel pipes is a relatively new technology. Its economical production has led to its widespread use in architectural decoration, household appliances, and mechanical structures. High-frequency welding requires a lower power source and can achieve high welding speeds for pipes of varying materials, outer diameters, and wall thicknesses. Compared to argon arc welding, its maximum welding speed is over 10 times higher. Therefore, it offers higher productivity in the production of general-purpose stainless steel pipes. However, the high speed of high-frequency welding makes it difficult to remove burrs from the welded pipes. This is one of the reasons why high-frequency welding of stainless steel pipes has not yet gained acceptance in the chemical and nuclear industries. Regarding the welding material, 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. Combination Welding Technology for Stainless Steel Pipes: Each welding method for stainless steel pipes has its own advantages and disadvantages. A new trend in the development of stainless steel pipe technology is to leverage its strengths and mitigate its weaknesses, combining several welding methods to create new welding processes that meet the high-quality and production efficiency requirements for stainless steel pipes. After years of exploration and research, combined welding processes have made significant progress, and stainless steel pipe production in countries such as Japan and France has mastered certain combined welding techniques. Combination 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. Combination welding significantly increases welding speed. For steel pipes welded using high-frequency preheating, weld quality is comparable to conventional argon arc welding and plasma welding. The welding operation is simple, and the entire welding system can be easily automated. This combination method easily integrates with existing high-frequency welding equipment, offering low investment costs and high returns.

Second, stainless steel pipe heat treatment. Overseas, non-oxidizing continuous heat treatment furnaces with protective gas are commonly used for stainless steel pipe heat treatment. These furnaces perform intermediate heat treatments during production and final heat treatment of finished products. This produces a non-oxidizing, bright surface, eliminating the traditional pickling step. This heat treatment process not only improves the quality of steel pipes but also mitigates the environmental impact of pickling.

Based on current global trends, continuous bright heat treatment furnaces are generally classified into three types:
1. Roller-hearth bright heat treatment furnace: This type of furnace is suitable for heat treating large-sized, high-volume steel pipes, with an hourly output exceeding 1.0 tons. It can use high-purity hydrogen, decomposed ammonia, and other protective gases. 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 tubing. 3. Muffle-type bright heat treatment furnace: Steel pipes are mounted on a continuous frame and heated within the muffle tube. This furnace can process high-quality, small-diameter, thin-walled steel pipes at a low cost, with an hourly output of approximately 0.3 tons or more.

Third, the effect of TIG welding activators on the weld seam profile of stainless steel pipes. TIG welding has been widely used in production, producing high-quality welds, and is commonly used for welding non-ferrous metals, stainless steel, ultra-high-strength steel, and other materials. However, TIG welding has disadvantages such as shallow penetration (≤3mm) and low welding efficiency. For thick plates, multiple passes are required, requiring groove cutting. While increasing the welding current can increase penetration, the increase in weld width and molten pool volume is far greater than the increase in penetration.

Activated TIG welding has recently attracted worldwide attention. This technique involves applying a layer of activated flux (referred to as an activator) to the weld surface before welding. Under the same welding specifications, it can significantly increase penetration (up to 300%) compared to conventional TIG welding. For welding plates as thick as 8mm, greater penetration can be achieved in one pass without beveling. For thin plates, heat input can be reduced without changing the welding speed. Currently, 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 significantly improve productivity, reduce production costs, and minimize weld distortion, offering excellent application prospects. The key factor in A-TIG welding lies in the selection of the activator composition. Commonly used activators include oxides, chlorides, and fluorides, with different materials requiring different activator compositions. However, due to the importance of this technology, the composition and formulation of activators are subject to patent restrictions in both PWI and EWI, resulting in limited publications. Current research on A-TIG welding focuses on the mechanism of activator action and the application of activated welding technology.

Currently, three main types of activators are being developed and used both domestically and internationally: oxides, fluorides, and chlorides. Early activators developed by PWI for titanium alloy welding were primarily composed of oxides and chlorides. However, the high toxicity of chlorides hindered their widespread application. Currently, activators used internationally for welding stainless steel, carbon steel, and other materials are primarily oxides, while those used for welding titanium alloys contain a certain amount of fluoride.

The Effect of Single-Component Activators on Stainless Steel Weld Form:
1. For welds coated with SiO2 activator, as the SiO2 coating amount increases, the weld bead width gradually narrows, and the crater becomes longer, narrower, and deeper. The weld bead reinforcement increases, and significant weld metal accumulation occurs at the junction of the activator-coated and uncoated welds. Among all activators, SiO2 has the greatest effect on weld form.
2. The activators NaF and Cr2O3 have little effect on weld bead form. With increasing coating amount, the weld bead width and cratering do not change significantly. Compared to welds without an activator, the weld bead width also does not change significantly, but the crater is larger. 3. As the TiO2 coating amount increases, the weld bead appearance changes little, with no noticeable change in cratering, similar to the weld without an activator. However, the resulting weld surface is relatively flat and regular, with no undercutting, resulting in a better weld profile than the weld without an activator.
4. The activator CaF2 has a significant impact on the weld bead profile. With increasing CaF2 coating amount, the weld profile deteriorates, with little change in cratering and weld width. However, defects such as undercutting appear with increasing CaF2 content.
5. Regarding the effect on weld penetration, all five activators increase weld penetration compared to the weld without an activator, and this increase increases with increasing coating amount. However, at a certain coating amount, the weld penetration reaches saturation, and further increases in coating amount actually decrease weld penetration.