Based on the factory’s many years of experience in welding duplex stainless steel, we’ve now summarized the following welding techniques for duplex steel. So, let’s take a closer look below! Duplex stainless steel pipe the welding process methods and operational steps!

　　I. Weldability of Duplex Stainless Steel Pipes

　　Duplex stainless steel pipes exhibit excellent weldability. They do not require preheating before welding, nor do they need post-weld heat treatment. They have low susceptibility to both cold and hot cracks and show a lower tendency toward embrittlement compared to ferritic steels. The microstructure of duplex stainless steel typically contains about 50% ferrite. Generally, duplex stainless steels are suitable for service at temperatures below 250°C. However, during welding of duplex stainless steel joints, intermetallic phases composed of chromium and iron may precipitate, leading to embrittlement. The temperature range for the formation of these phases is approximately 750–950°C. The formation of these phases takes a certain amount of time—typically occurring within 10–20 minutes—and once formed, they continue to grow over time. Therefore, during welding, it is preferable to use lower heat input, ensure rapid cooling, and perform stress-relief treatments at relatively low temperatures, such as 550–600°C.

II. Selection of Welding Methods

　　1. Electron-beam welding and plasma-arc welding are generally not used because their high heat input leads to coarse grains in the weld metal and HTHAZ, resulting in reduced toughness. On the other hand, electroslag welding, gas-metal arc welding (GMAW, MIG), and submerged arc welding (SAW) also have limitations on their applications.

　　2. The common welding methods for duplex stainless steel pipes include manual metal arc welding (SMAW) and gas tungsten arc welding (GTAW). Manual metal arc welding is suitable for all-position welding and is a versatile, flexible, simple, and widely used method. Gas tungsten arc welding is typically employed for root passes in pipe joints or for automated welding of pipelines; it is also frequently used for welding thin plates or for joining pipes to tube sheets. This method ensures excellent mechanical properties of the welded joints, particularly good low-temperature toughness. However, even in automated welding, the deposition rate tends to be relatively low. As a shielding gas, pure Ar or a mixture of 98% Ar and 2% N2 is usually adopted. Nevertheless, when performing single-sided welding with double-sided forming, regardless of the joint configuration used, back-side shielding gas (root gas) is required; again, pure Ar or a 98% Ar/2% N2 mixture is typically employed. It is customary to use multi-layer welding, multiple passes, and a low deposition rate during the welding process.

　　3. Selection of Welding Materials

　　Generally, for welding duplex stainless steel pipes, electrode rods are typically used in manual arc welding, while welding wires are employed in tungsten inert gas (TIG) welding. During welding, to achieve a phase ratio identical to that of the base metal, welding wires usually contain 2% to 4% more nickel than the base metal, thus ensuring the same phase ratio as the base material. The chemical composition and cooling rate can influence the proportion of phases present in the weld joint; however, increasing the nickel content during welding raises the phase transformation temperature, leading to a more complete transformation from one phase to another. Consequently, the effect of chemical composition becomes more pronounced. Regarding welding electrodes, titanium-coated (rutile-type) electrodes exhibit better weldability compared to basic electrodes. The former offer superior slag removal properties, which is particularly advantageous for weld beads at pipe ends. However, when high low-temperature mechanical properties are required for the weld, basic electrodes should be specifically considered. To prevent the formation of porosity, the electrodes must be preheated at 250–350°C for two hours before welding, or alternatively, use electrodes with enhanced moisture resistance.

4. Control of Welding Heat Input and Post-Weld Cooling Rate

　　To ensure the excellent overall performance of duplex stainless steel, it is crucial to maintain a balanced ratio between ferrite and austenite. However, due to the rapid cooling rates during the welding thermal cycle, this balance tends to become unstable in the weld zone. This instability arises because the ferrite content in the weld metal significantly exceeds 50%. To restore the balance, in addition to increasing the nickel content in the filler wire by 2% to 4%, it is also necessary to carefully control the heat input during welding and thereby regulate the cooling rate, which in turn promotes the formation of diffusional ferrite.

　　The above is an introduction to the welding process and operational steps for duplex stainless steel pipes. If you’d like to learn more, feel free to contact us anytime!