What are the welding methods and precautions for stainless steel pipes

First, the welding of stainless steel pipes

(1) Argon arc welding

Stainless steel pipes require deep penetration, no oxide inclusions, and a minimal heat-affected zone. Argon arc welding with tungsten inert gas protection has 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 argon arc 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 of arranging multiple torches in a straight line along the weld direction to form a long heat flow distribution, significantly improving the welding speed. Generally, argon arc 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. Argon arc welding combined with plasma welding can weld steel pipes with thicker walls. Furthermore, adding 5-10% hydrogen to argon gas and using a high-frequency pulse 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

High-frequency welding has been used in the production of carbon steel welded pipes for over 40 years, but its application to stainless steel pipes is relatively new. Its economic efficiency makes its products more widely used in building decoration, household appliances, and mechanical structures. High-frequency welding has lower power consumption and can achieve higher welding speeds for steel pipes of different materials, outer diameters, and wall thicknesses. Compared to argon arc welding, its maximum welding speed is more than 10 times higher. Therefore, it has a higher productivity in producing general-purpose stainless steel pipes. Because of the high welding speed, removing burrs from the welded steel pipe is difficult. Currently, 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. Meanwhile, the development of new steel grades and advancements in forming and welding methods have also led to the successful welding of ferritic stainless steel grades such as AISI409.

(3) Combined Welding Technology

Various welding methods for stainless steel pipes have their own advantages and disadvantages. How to leverage strengths and mitigate weaknesses, and combine several welding methods to form new welding processes that meet people's requirements for the quality and production efficiency of stainless steel welded pipes, is a new trend in the development of stainless steel welded pipe technology. 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 good benefits.

Second, Heat Treatment of Stainless Steel Pipes

Internationally continuous heat treatment furnaces with protective gases are commonly used for the heat treatment of stainless steel pipes. These furnaces are used for intermediate heat treatment during production and for final finished product heat treatment. Because they produce a bright, non-oxidizing surface, the traditional pickling process is eliminated. This heat treatment process improves the quality of the steel pipes and overcomes the environmental pollution caused by pickling.

According to current global 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 the 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. A convection cooling system can be installed to cool the steel pipes quickly.

(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-type Bright Heat Treatment Furnace: Steel pipes are mounted on a continuous rack and heated inside the muffle tube. This process can process high-quality small-diameter thin-walled steel pipes at a relatively low cost, with an hourly output of approximately 0.3 tons or more.

Third, the Influence of TIG Welding Activator on Stainless Steel Weld Formation.

TIG welding has been widely used in production. It can produce high-quality welds and is commonly used to weld 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.

Activated TIG welding methods have attracted worldwide attention in recent years. This technology involves applying a layer of activated flux (referred to as an activator) to the weld surface before welding. Under the same welding specifications, compared with conventional TIG welding, it can significantly increase the penetration (up to 300%). For welding 8mm thick plates, a large penetration depth or complete weld can be achieved in one pass without beveling. For thin plates, the heat input can be reduced without changing the welding speed. Currently, A-TIG welding can be used to weld 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 welding deformation, showing great promise for future applications. A 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 patented in both PWI and EWI, and are rarely reported in public publications. Current research on A-TIG welding mainly focuses on the mechanism of action of activators and the application technology of activated welding.

Currently, the activators developed and used domestically and internationally mainly fall into three categories: oxides, fluorides, and chlorides. Early activators developed by PWI for welding titanium alloys were mainly oxides and chlorides. However, chlorides are highly toxic, hindering their widespread adoption. Currently, activators used abroad for welding stainless steel and carbon steel are primarily oxides, while those for titanium alloys contain a certain amount of fluoride.

The effect of single-component activators on stainless steel weld formation:

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. Activators NaF and Cr2O3 have little effect on weld formation. With increasing coating amount, the weld width does not change significantly, and the crater does not change noticeably. Compared with welds without activators, the weld width does not change significantly, but the crater is larger.

3. With increasing TiO2 coating amount, the weld appearance did not change significantly, and the arc crater showed no obvious change, similar to the case without activator. However, the resulting weld surface was relatively smooth and regular, without undercut, and the weld formation was better than that without an activator.

4. The activator CaF2 had a significant impact on weld formation. With increasing CaF2 coating amount, the weld formation worsened, with little change in arc crater and weld width. However, undercut and other defects appeared with increasing CaF2 content.

5. Regarding the effect on penetration depth, compared to the case without an activator, all five activators mentioned above could increase the weld penetration depth, and the penetration depth increased accordingly with increasing coating amount. However, when the coating amount reached a certain value, the increase in penetration depth reached saturation; further increases in coating amount resulted in a decrease in penetration depth.