Popular Science on Hastelloy C276 Welding Technology
At present, Hastelloy has been widely used in many fields such as petroleum, chemical industry, and environmental protection abroad. It has also been used in a small amount in China. However, in the field of nuclear power, only the United States and other countries have been used in nuclear power generation. It is still blank in China. The main problem at present is the parent The remaining materials are imported, and the main materials are imitated by scientific research units. Their performance is not particularly clear, especially the corrosion resistance at high temperatures. The main direction of this research is to choose the C276 material that can be purchased from Hastelloy as a material. For the first phase of the project, by selecting appropriate welding materials and processes, we can figure out the welding specifications that meet the product requirements, so that they have operational value.
Hastelloy itself is a nickel-based alloy, but it is different from ordinary pure nickel (Ni200) and Monel. It uses chromium and molybdenum as the main alloy element, aiming to improve its adaptability to various media and temperatures, and to be used in different industries. Special optimization has been carried out. At present, the chemical industry generally pursues high productivity and constantly improves processes to increase temperature and pressure to speed up reaction. In this way, production is prone to unexpected situations, resulting in unplanned maintenance. Hastelloy is precisely aimed at such a demand, adapting to various unpredictable harsh environments and minimizing unplanned maintenance. At the same time, it also has good processing and welding performance and facilitates on-site maintenance.
C series alloys are nickel-chromium molybdenum alloys. Because chromium can form a dense oxide film (passivation) on the surface of the alloy, it provides resistance to oxidizing environments, while chromium mainly provides resistance to reducing environments. Therefore, C series alloys can be used in environments with both oxidizing and reducing media. C series alloys are the most widely used alloys, especially C-276 alloy. Since its invention in the 1960s, it has still shown strong vitality after more than 40 years of testing. Currently, C series alloys have been used in quite a few industries in China. Series alloys.
This research project uses domestic C276 alloy, selects two specifications of plate thickness, 2mm and 6mm, determines the corresponding welding process, and determines
Determine whether the welding test piece under the welding process conditions meets the micro-corrosion requirements under the high temperature state of nuclear power, formulate welding process specifications, and complete the welding process assessment report for the corresponding specifications and thickness, and formulate welding process regulations suitable for nuclear power requirements to guide the project construction.
2. Analysis and measures of technical difficulties in welding process
C276 (UNSN10276) alloy is a nickel-molybdenum-ferrochromium-tungsten alloy, which is currently the most corrosion-resistant alloy. C276 alloy has been used for many years in construction engineering related to vessels and pressure valves as per the ASME Standards. The alloy appears in various product forms in Sections 1 and 8 of the ASME Standard Regulations. Part Two.
Although C276 alloy will eventually become brittle under high temperature and form precipitates, it also has good high-temperature strength and moderate oxidation resistance. The high molybdenum content gives the alloy resistance to local corrosion. The low heat content of the alloy minimizes the precipitation of carbides during welding. In order to maintain the resistance to inter-product corrosion of the thermally deteriorated parts at the welding interface.
(1) Weldability analysis: The electrical conductivity and thermal conductivity of Hastelloy are much lower than that of low carbon steel, while the resistivity and expansion rate are much higher than that of low carbon steel. The molten pool has poor fluidity, poor wettability, and penetration The force is small and the melting depth is shallow. Therefore, defects such as pores, hot cracks, incomplete welding, and incomplete fusion are prone to occur.
Reasons for pores: The bevel treatment of Hastelloy alloy before welding is not clean, the weather is humid, the molten pool is not well protected during the welding process, and hydrogen, nitrogen and other gases easily penetrate into the molten pool. Due to the small temperature gap between the solid and liquid phases of the alloy and the low fluidity, the insoluble gas has no time to escape during solidification and remains in the weld to form pores.
Hot cracks; low-body intergranular liquid film formed by impurities such as phosphorus and welding tensile stress are metallurgical factors that cause welding hot cracks. Since the alloy weld has a dendritic structure, some low-melting-point eutectic and low-melting-point gold are concentrated on the boundaries of the coarse grains.
genus, especially Ni-S eutectic (melting point is 645°C) and Ni-P eutectic (melting point is 880°C). They are distributed in a thin film between grain boundaries and are prone to cracks under the action of welding stress.
Cleanliness is one of the most important aspects of welding corrosion-resistant nickel-based alloys. Contaminants from grease, corrosion products, lead, sulfur, and other elements with low baking points can cause severe cracking problems. In order to ensure welding quality, the welding protection zone and welding wire of the weldment must be strictly and carefully cleaned before welding.
It is easy to oxidize, and the Ni and Cr atoms in the alloy are very active. The weld seam is easily oxidized when the alloy is welded. In severe cases, it becomes like tofu, which causes the corrosion resistance of the metal to drop sharply. It is also the main cause of cracks. Therefore, chlorine protection should be strengthened during welding. At the same time, the welding wire should generally be as thin as possible (1.2~2.4mm). Small welding parameters are helpful to compensate for the burning loss of certain elements during the welding process and the damage to welding cracks and pores. control.
Clean surface dirt; any residual dirt such as metal debris, abrasive dust, dust, etc. on the surface of the wood within 40mm of the C276 alloy welded joint from the groove must be removed with an austenitic stainless steel wire brush and clean new cotton yarn. Clear it clean. The tools used must be special and sandpaper and carbon wire brushes are not allowed.
(2) Technical difficulties include welding deformation control, welding backside protection, and research on corrosion resistance of welded specimens.
Acetone (or alcohol) cleaning; use acetone or alcohol to clean the groove surface before welding to remove surface oil and other impurities, and measures should be taken to prevent secondary pollution.
(3) Technical measures In order to prevent grain growth and phosphide precipitation in the weld and heat-affected zone, low welding heat input should generally be used. However, the nickel-based alloy molten pool metal has poor fluidity and shallow penetration, which easily causes unwelded joints. The heat input of dispersion welding cannot be too small. The solution is to use medium welding current, high welding speed, and control the welding heat input by reducing the high temperature residence time.
(4) Selection of welding parameters: test plate specifications: 2mm, 6mm; welding wire model and specifications; ERNiCrMo-4, φ2.4mm; groove form; the groove form of the welding joint is shown in Figure 1.
In order to improve the crack resistance and corrosion resistance of the weld, special attention should be paid to the cleaning of the welding area during welding to prevent harmful impurities from melting into the weld.
Preheating is generally not required when welding. In order to prevent grain growth and carbide precipitation in the weld and heat-affected zone, the interlayer temperature should be controlled at a low level. Generally no more than 100℃
