Welding quality control for pipes and containers

Thread Content

Nickel possesses good mechanical, physical, and chemical properties. Adding appropriate elements can enhance its antioxidant properties, corrosion resistance, high-temperature strength, and improve certain physical properties. Welding characteristics and requirements for nickel and nickel-based alloys – Pre-welding cleaning: The cleanliness of the weld surface is an important requirement for successfully welding nickel-based corrosion-resistant alloys. The contaminants on the surface of welded parts are mainly surface scale and elements that cause brittleness. Elements such as S, P, Pb, Sn, Zn, Bi, Sb, and As, which can form low-melting eutectics with Ni, are all harmful elements. These harmful elements increase the hot crack susceptibility of nickel-based corrosion-resistant alloys. These impurities must be completely removed before welding preheating or welding. 1. Workshop dirt and grease can be removed by steam degreasing or using solvents such as acetone. 2. For paints and other contaminants that are insoluble in degreasers, cleaning agents such as chloromethane and alkalis, or special synthetic compounds can be used for cleaning. Marking ink is generally cleaned with methanol. 3. Impurities pressed into the surface of the welded joint can be removed by grinding, shot blasting, or cleaning with a hydrochloric acid solution (10% by volume), followed by rinsing with clean water. Limiting heat input: Using a high heat input method for welding nickel-based corrosion-resistant alloys can have adverse effects. A certain degree of annealing and grain growth occurs in the heat-affected zone (HAZ). High heat input can cause excessive segregation, precipitation of carbides, or other harmful metallurgical phenomena. This may cause thermal cracking or reduce corrosion resistance. The grain size of the base material must also be considered when selecting the welding method and welding process. Due to the presence of numerous carbides and intermetallic compounds at the grain boundaries of coarse grains, which promote liquidation cracks, the tendency for thermal cracking is increased. Process characteristics 1) Poor fluidity of liquid weld metal: The weld metal has poor fluidity and does not flow easily to the sides of the weld. Therefore, to achieve good weld formation, a weaving process is sometimes used. But this swing is a small one, with the swing distance not exceeding three times the diameter of the electrode or wire. The groove angle of the joint is larger. 2) The weld metal penetration is shallow; this is also an inherent characteristic of nickel-based corrosion-resistant alloys. Similarly, increasing the welding current does not increase the penetration depth. As mentioned above, excessive current is harmful to welding, as it causes cracks and pores. 3) Preheating and post-weld heat treatment: Nickel-based corrosion-resistant alloys generally do not require preheating before welding. However, when the base metal temperature is below 15°C, the area 250–300 mm wide on either side of the joint should be heated to 15–20°C to prevent moisture from condensing. Post-weld heat treatment is generally not recommended. Specific construction of nickel and nickel alloy pipes – Groove preparation: 1) Mechanical methods should be used for groove preparation; if plasma cutting is employed, the surface resulting from the cutting process must be cleaned. 2) The grooves should have a large angle and minimal bevel, and they must meet the requirements specified in the design documents and operating instructions. 3) After groove preparation, an visual inspection should be carried out to ensure that there are no defects such as cracks or delamination on the groove surface. 4) If the design requires non-destructive testing of the groove surface, this should be done in accordance with the design specifications. In the absence of such specifications, penetrant testing should be performed on cast pipes, pipes and fittings used for handling highly hazardous substances, as well as pipes or fittings designed for pressures equal to or greater than 10 MPa. Any defects found must be removed promptly. Bevel cleaning and alignment: Before alignment, the bevel and the areas within 20 mm on both sides thereof must be cleaned of oil, rust, etc., until a metallic luster becomes visible. The surfaces within the corresponding cleaning range should also be cleaned with an organic solvent prior to welding. After the solvent has evaporated, anti-spatter coating should be applied to the areas within 100 mm on both sides of the bevel. Key welding points: 1) Use a welding method with low current, minimal oscillation, and low wire energy during welding; 2) When using TIG welding, the inside of the pipe should be protected with argon gas, with a purity of not less than 99.99%. During root welding, a small hole with a diameter of 2mm–3mm should be maintained at the front edge of the molten pool, and the welding wire should be fed into the molten pool in droplet form; 3) When performing fill and finish welding using manual arc welding, apply chalk powder to the base metal within a range of at least 100mm on each side of the groove to prevent welding spatter from damaging the base metal; 4) Thoroughly clean the areas between layers and the surface using an angle grinder or stainless steel wire brush. Grinding and cutting must be carried out using corundum grinding wheels, and these must not be used together with carbon steel grinding wheels; 5) Pipes and fittings should be placed on rubber or asbestos cloth pads, and they must not be stored together with carbon steel; 6) If there is concurrent work at the installation site, cover the pipes in areas prone to contamination with asbestos cloth; 7) For pipes with thicker walls, symmetric welding should be used to avoid welding deformation; 8) Socket welding must be completed in two passes; 9) After welding, clean the surface of the weld promptly and inspect its appearance. Once it meets the quality standards, write the welder’s code 20mm–50mm away from the weld using paint that does not contain elements such as zinc or sulfur. Shielded metal arc welding of nickel-based corrosion-resistant alloys. Classification of electrodes: Electrodes for nickel-based corrosion-resistant alloys are divided into five categories: commercially pure nickel, Ni-Cu, Ni-Cr-Fe, Ni-Mo, and Ni-Cr-Mo. (Details on the classification of welding electrodes are shown in Table 1.) Welding process: 1) Due to the shallower penetration depth of nickel-based corrosion-resistant alloys and the poor fluidity of the liquid weld metal, it is necessary to strictly control changes in welding parameters during the welding process. Nickel-based corrosion-resistant alloy electrodes generally use direct current, with the electrode connected to the positive pole. 2) Each type and specification of welding rod has an optimal current range. When setting the welding current for a specific joint, factors such as the thickness of the base material, the welding position, the type of joint, and the rigidity of the clamping should be taken into account. 3) Liquid nickel-based corrosion-resistant alloys have poor fluidity; to prevent defects such as lack of fusion and pores, it is generally necessary to move the welding rod appropriately during welding. 4) When breaking the arc, slightly reduce the arc height and increase the welding speed to reduce the size of the molten pool. Doing this can reduce crater cracks. When re-igniting the arc at a welded joint, reverse arc ignition technology should be used to help achieve a smooth weld seam at the joint and to prevent the formation of pores. Tungsten inert gas welding of nickel-based corrosion-resistant alloys: Tungsten inert gas welding is widely used for welding nickel-based corrosion-resistant alloys, and it is particularly suitable for thin plates, components with small cross-sections, those where root welding is not possible, and structures in which residual slag is not allowed after welding. Tungsten inert gas arc welding can be used not only for welding solution-strengthened nickel-based alloys but also for welding precipitation-hardening nickel-based alloys. Tungsten inert gas arc welding is the most common method for welding precipitation-hardening nickel-based alloys. Nickel-based alloy welding wires are used not only in tungsten inert gas arc welding but also in metal inert gas arc welding, plasma arc welding, and submerged arc welding. To control porosity and hot cracks, alloying elements such as Ti, Mn, and Nb are often added to the welding wire. (Details on the classification of welding wires are shown in Table 1.) Welding process: 1) For both manual and automatic welding, direct current with positive polarity is used, with the electrode connected to the negative pole. Welding machines are usually equipped with high-frequency arc initiation and current attenuation devices. 2) Since the welding wire needs to contain elements that improve crack resistance and control porosity, at least 50% of the weld must be made up of filler metal for these elements to be effective. During the welding process, the molten pool should remain calm to prevent the arc from disturbing it. 3) During welding, the heated end of the welding wire must be enclosed in a protective gas to prevent oxidation of the hot end and subsequent contamination of the weld metal. The welding wire should enter the molten pool at its front end to avoid contacting the tungsten electrode.