Welding techniques and quality for titanium and nickel equipment

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In actual production, tungsten inert gas welding is the primary method for welding titanium-nickel pressure vessels, and the quality awareness and operational skills of tungsten inert gas welders are key to ensuring the quality of such vessels. As a TIG welder, it is necessary to develop four types of awareness regarding welding quality: cleanliness awareness ； Protection awareness ； Thorough penetration awareness ； A sense of fullness and aesthetics. 1. Pre-welding preparation: The degree of cleanliness has a significant impact on the welding quality of titanium-nickel materials. Impurities such as moisture, grease, rust, and oxide films, if they enter the weld metal, can cause defects such as pores, inclusions, and cracks. Special attention should be paid to ensuring that no sulfur- or lead-containing substances remain on the surface. Porosity is more likely to occur when welding titanium using TIG welding, as the welding wire does not contain deoxidizing elements such as Mn, Ti, and Al like those found in nickel welding wires. 2.1.1 Before welding nickel materials, the oxide film on the groove surface and over a width of 20 mm on each side of it can be removed using a stainless steel wire brush or a metal milling head; grease, dust, and other contaminants should be removed with acetone or alcohol. The oxide layer formed on the surface of the nickel groove as a result of thermal cutting must be polished off completely to reveal a metallic luster. The melting point of nickel’s oxide film is much higher than that of the base metal (for example, the melting point of nickel’s oxide film is 2090°C, while that of pure nickel is 1450°C, a difference of about 640°C). If the oxide film does not melt during welding, it can lead to the formation of inclusions in the weld metal. Therefore, this oxide film must be removed before welding. 2.1.2 The titanium and titanium alloy welding wires, as well as the surface of the groove and the area around it within 20 mm on each side, must be cleaned thoroughly. Degreasing or mechanical cleaning can be chosen depending on the degree of surface contamination. The cleaned weld wire and welded parts should not be touched directly with the hands anymore. For grooves fabricated by flame methods such as oxygen cutting and plasma cutting, 2–3 mm should be mechanically removed from the discolored portion. The surface of the machined groove should be flat and smooth, without cracks, delamination, inclusions, burrs, flash, or oxidation colors. The groove surface should have a silver-white metallic luster. It is particularly important to note that within 10 mm on either side of the groove on the sandblasted titanium plate, the oxide layer must be removed; welding should only be carried out after a metallic luster appears. Otherwise, pores will appear at the weld edges. 2.1.3 Before piping the heat exchanger, remove any debris attached to the surface of the entire heat exchange tube ； Remove surface oxide films, oil, water stains, and burrs from the pipe ends ； The cleaning length shall be not less than the outer diameter of the tube, and not less than 25 mm. The tube sheet and baffle plates are completely cleaned of oil and burrs. When the heat exchange tube extends too far beyond the tube sheet, it should be flattened using mechanical methods; grinding with a grinder is not allowed. 2.2 Argon shielding: The surfaces of the base material and welding materials must be clean to prevent contaminants from entering the weld seam ； The high-temperature molten pool and the high-temperature weld bead must be effectively protected by an inert gas to prevent the oxidation of the weld metal; these two factors are essential for obtaining high-quality weld joints. Therefore, argon shielding is the most basic requirement for welding titanium-nickel materials. Oxidation will weaken the mechanical properties and corrosion resistance of the welded joint. When welding titanium-nickel materials, it is necessary to have a sense of protection and master protection techniques. The higher the reactivity of a metal, the stricter the requirements for protection; titanium requires 99.99% pure argon, zirconium requires 99.999% highly pure argon, and tantalum should be welded in a sealed chamber filled with argon. Creating appropriate shielding fixtures (commonly known as shielding boxes) based on the actual shape of the welded joint is a skill that TIG welders should possess, and it is also an important preparatory step for welding titanium and nickel materials. Therefore, the welder has a dual task: to weld well and to provide protection. Welding and protection complement each other; it’s useless to weld well if the protection is inadequate, and it’s also useless to have good protection if the welding isn’t done properly. There are no appropriate protective measures to prohibit welding. There are two methods of protection: protection with a drag cover and argon filling protection; it is possible to use protection with a drag cover without argon filling. The space filled with argon is often large, which not only results in poor protection but also consumes a lot of gas. 2.2.1 As long as TIG welding is used, it is necessary to protect the molten pool or weld bead from high temperatures (above 400°C for titanium) with argon gas, even in the case of ordinary carbon steel. When using TIG welding for the root pass of carbon steel and stainless steel, the back side should also be protected with argon gas. Well-protected carbon steel is also silver-white. Radiographic testing of unprotected joints is not a problem, but the performance (especially back-bending) may not meet the requirements. 2.2.2 When using TIG welding to align and spot-weld the workpieces, if the back side can be protected by a shield, it should be protected with such a shield. For small-diameter joints, argon protection can be applied. 2.2.3 There are two methods for welding the heat exchange tubes to the tube sheet in heat exchangers: one is where the tube end is melted and folded over (break-end welding), and the other is where the tube end remains unmelted (keep-end welding). A cracked-head weld with a copper plug (as shown in Figure 3) is used to prevent and reduce the loss of argon gas from the welding torch nozzle during welding. The joint is welded inside the tube with a protective fixture (as shown in Figure 4), under argon gas protection. 2.3 Lack of penetration or incomplete penetration reduces the thickness capable of withstanding pressure; like cracks, these are welding defects that are not permissible as they affect the safety of pressure vessels. For Class A and B butt joints of pressure vessels, since radiographic inspection is required, full penetration is generally achievable in most cases. Underwelding often occurs at the Class D fillet joints between the nozzles and the shell, especially in cases with reinforcement rings. Since such welded joints generally do not require radiographic or ultrasonic testing, operators have not developed an awareness of achieving full penetration over the years. Unless otherwise specified, incomplete welding often occurs during takeover. Thorough penetration requires proper groove preparation, which facilitates penetration while preventing excessive alignment gaps. If possible, the groove can be machined; otherwise, plasma or flame cutting can be used, but a margin for grinding must be left. Testing methods are an important means of fostering welders’ awareness of full penetration. 2.4 Full and aesthetically pleasing welds: A full weld means that the filler metal in a butt joint is at least at the level of the base material surface, while in a fillet joint, the height of the weld legs meets the requirements specified in the drawings. Whether the weld is full depends on the number of welding passes; an incomplete weld is caused by insufficient welding passes. Full weld seams and proper weld leg height are the easiest issues to address, but they are often overlooked. Aesthetic appearance is achieved when the weld shape is consistent and the weld bead is uniform. Achieving aesthetic appearance of the weld while ensuring its internal quality is the highest level of welding skill. Being gorgeous both on the inside and outside is the ultimate goal of welding quality. A pass in the visual inspection of welds does not necessarily ensure an aesthetically pleasing appearance. What are the criteria for judging beauty? Here, the only thing that matters is that it looks \"pleasing to the eye\", just like admiring a work of art. Do not use weld repair as an excuse for an unsightly weld; it must be understood that welding repair causes the most severe damage to the surface quality of the weld, and every effort should be made to avoid it. Therefore, a welding philosophy of \"weld it well, strive for even better, and weld it perfectly in one go\" should be adopted. “\"Weld it properly\" is essential to ensure that the overall performance of the welded joint is satisfactory; this is the most basic requirement ； ““Seeking even better” means achieving an attractive weld bead appearance ； “\"Welding in one go\" means no need for rework, no damage to the appearance of the weld, and no waste of time or materials.