1. Welding of stainless steel pipes

1. TIG welding
The stainless steel pipe requires deep penetration, no oxide inclusions, and the heat affected zone is as small as possible. The tungsten inert gas shielded argon arc welding has good adaptability, high welding quality and good penetration performance. Its products are used in chemical industry. , nuclear industry and food industries are widely used. The low welding speed is the shortcoming of argon arc welding. In order to improve the welding speed, a variety of methods have been researched and developed abroad. Among them, the single-electrode and single-torch welding method has been developed into a multi-electrode and multi-torch welding method and is applied in production. In the 1970s, Germany first adopted multiple torches arranged in a straight line along the direction of the welding seam to form a long heat flow distribution and significantly improve the welding speed. Generally, argon arc welding with three-electrode torch is used, the wall thickness of the welded steel pipe is S ≥ 2mm, the welding speed is 3 to 4 times higher than that of the 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 thickness. In addition, 5-10% hydrogen in argon gas, and then high-frequency pulse welding power source can also increase the welding speed.
Multi-torch TIG welding is suitable for welding austenitic and ferritic stainless steel tubes.

2. High frequency welding
High-frequency welding has been used for carbon steel welded pipe production 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 architectural decoration, household appliances and mechanical structures. High-frequency welding has lower power supply, and can achieve higher welding speed for steel pipes with 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 high productivity.

Because of the high speed of high-frequency welding, it is difficult to remove the burrs in the welded pipe. At present, high-frequency welded stainless steel pipes cannot be 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 form welding methods have also successfully welded steel grades such as ferritic stainless steel AISI409.

3. Combined welding technology
Various welding methods of stainless steel pipes have their own advantages and disadvantages. How to make use of strengths and avoid weaknesses, combine several welding methods to form a new welding process, and meet people's requirements for stainless steel welded pipe quality and production efficiency, is a new trend in the current development of stainless steel welded pipe technology. After several 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 a certain combined welding technology.

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 increases the welding speed very significantly. For the combination welded steel pipe with high frequency preheating, the weld quality is comparable to that of conventional argon arc welding and plasma welding, the welding operation is simple, and the entire welding system is easy to realize automation. This combination is easy to connect with the existing high frequency welding equipment. Low cost and good benefit.


2. Heat treatment of stainless steel tube
Stainless steel tube heat treatment abroad generally adopts non-oxidation continuous heat treatment furnace with protective gas to carry out intermediate heat treatment in the production process and final heat treatment of the finished product. Since a bright surface without oxidation can be obtained, the traditional pickling process is cancelled. The adoption of this heat treatment process not only improves the quality of the steel pipe, but also overcomes the environmental pollution caused by pickling.

According to the current world development trend, bright continuous furnaces are basically divided into three types:

1. Roller hearth type 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 gases that can be used are high-purity hydrogen, decomposed ammonia and other protective gases. Can be equipped with a convection cooling system for faster cooling of the steel pipe.

2. Mesh belt type 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, processing steel pipes up to 40 meters in length, and can also handle coiled capillaries.

3. Muffle type bright heat treatment furnace. The steel pipe is mounted on a continuous handle frame and heated in a muffle tube, which can process high-quality small-diameter thin-walled steel pipes at a lower cost, and the hourly output is about 0.3 tons or more.

3. The effect of 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 it is often used to weld non-ferrous metals, stainless steel, ultra-high-strength steel and other materials. However, TIG welding has shortcomings such as shallow penetration (≤3mm) and low welding efficiency, and multi-pass welding is required for thick plates. Although increasing the welding current can increase the penetration depth, the increase of the penetration width and the volume of the molten pool is much greater than that of the penetration depth.

The activated TIG welding method has attracted worldwide attention in recent years. This technology is to apply a layer of active flux (referred to as active agent) on the surface of the weld before welding. Under the same welding specification, compared with conventional TIG welding, the penetration depth can be greatly improved (up to 300%). For 8mm thick plate welding, a large penetration depth or one penetration can be obtained at one time without groove, and for thin plates, the welding heat input can be reduced without changing the welding speed. Currently 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, so it has a very good application prospect. The key factor of A-TIG welding is the selection of active agent components. At present, the commonly used active agent components are mainly oxides, chlorides and fluorides. Different materials have different active agent components. However, due to the importance of this technology, the composition and formulation of active agents are subject to patent restrictions in both PWI and EWI, and are rarely reported in public publications. At present, the research on A-TIG welding mainly focuses on the research of the mechanism of the activator and the application technology of the activated welding.

At present, there are three main types of active agents developed and used at home and abroad: oxides, fluorides and chlorides. The active agents for titanium alloy welding developed by PWI in the early days are mainly oxides and chlorides, but chlorides are highly toxic, which is not conducive to popularization and application. At present, the activator used in the welding of stainless steel and carbon steel abroad is mainly oxide, and the activator for the welding of titanium alloy materials contains a certain amount of fluoride.

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

1. For the welds coated with SiO2 activator, as the amount of SiO2 coating increases, the width of the weld bead gradually narrows, and the arc crater becomes longer, narrower and deeper. The residual height at the rear of the weld bead becomes higher, and at the junction of the coated and uncoated activator, the weld metal accumulates a lot. Among all the activators, SiO2 has the greatest effect on the weld formation.

2. The effect of the active agents NaF and Cr2O3 on the formation of the weld bead is not obvious. With the increase of the coating amount, the weld width does not change much, and the arc crater does not change significantly. There is also no significant change in the bead width compared to the weld without activator, but the crater is larger than that without activator.

3. With the increase of TiO2 coating amount, the appearance of the weld bead has little change, and the arc crater has no obvious change, which is similar to that without the active agent. However, the formed weld surface is relatively smooth and regular, and there is no undercut phenomenon, which is better than the weld bead without activator.

4. The active agent CaF2 has a great influence on the formation of the weld bead. With the increase of the amount of CaF2 coating, the weld formation becomes worse, the arc crater changes little, and the weld width changes little. However, defects such as undercut appear with the increase of the amount of CaF2.

5. In terms of the influence on the penetration depth, compared with the non-active agent, the above five activators can increase the penetration depth of the weld, and with the increase of the coating amount, the penetration depth also increases accordingly. However, when the coating amount reaches a certain value, the penetration depth increases and reaches saturation, and when the coating amount is increased, the penetration depth decreases instead.