Welding problems and solutions of pure nickel Ni201 plate
In the manufacture of a molten salt production reactor, the core component is a pure nickel NI201 plate with corrosion and high temperature resistance properties, and this process requires the welding of pure nickel materials.
The weldability of pure nickel Ni201 is good, equivalent to chromium-nickel austenitic stainless steel, but the liquid weld metal fluidity is poor, its weldability is poor compared to carbon steel, need to develop a suitable welding process specifications to obtain a more good welded joints. In particular, the weld metal thermal cracking and porosity and welding heat-affected zone has a tendency to grain growth, which are the three main problems in the welding of nickel 201.
Pure nickel welding the first problem: thermal cracking
Pure nickel NI201 welding, due to S, Si and other impurities in the weld metal segregation, S and Ni form a low melting point eutectic. During the solidification of the weld metal, the low melting point eutectic forms a liquid film between the grain boundaries and forms a solidification crack under the action of the welding stress. During the welding process Si and oxygen, etc. form complex silicates, which form a brittle silicate film between the grain boundaries and form high-temperature hypoplastic cracks during or after solidification of the weld metal in the high-temperature zone. Therefore S, Si is the most harmful elements in nickel 201 weld metal.
Measures to prevent thermal cracking methods: firstly, try to reduce the content of impurities such as S, Si in the weld metal, strictly clean up the bevel area before welding and welding wire, etc., and strictly control the content of impurities in the base material; secondly, add the right amount of Mn, Nb, Mo, Ti and other elements to the weld metal to offset the harmful effects of impurities such as S, Si. In addition, the use of small line energy welding is very necessary, no preheating before welding, the interlayer temperature should be as low as possible.
Pure nickel welding second problem: porosity
When welding pure nickel Ni201, porosity is a more difficult problem to solve, especially when welding pure nickel and nickel-copper alloys such as MONEL400 alloy more serious. This is due to the greater viscosity and tension of liquid nickel and nickel alloy weld metal, resulting in difficulties in the escape of gas upward, and therefore more opportunities for porosity. The pores of nickel alloy weld metal are H2O pores, hydrogen pores and carbon monoxide pores, of which H2O pores are dominant. As liquid nickel can dissolve a large amount of oxygen, and solidification when the solubility of oxygen significantly reduced, so that the excess oxygen during solidification will oxidize nickel into nickel oxide (NiO). Nickel oxide and liquid metal hydrogen reaction, nickel is reduced, and hydrogen and oxygen combined into H2O. H2O is too late to escape, and because of the fusion line and the closing arc, the arc is cooled quickly, in the place where the pores.
The solution to the porosity.
- (1) Through the welding rod or wire to the weld metal transition deoxidizer, such as titanium, aluminum, manganese, etc., to reduce the oxygen content of the weld metal to prevent the formation of nickel oxide (NiO). Welding rod arc welding using alkaline low-hydrogen welding rod to reduce the content of hydrogen and oxygen in the weld metal. The welding rod should be fully dried, using DC reverse connection, with a short arc welding.
- (2) Strictly clean the weld, wire before welding, remove the oxide film, grease, oil, coatings and pigments on the surface of the weld.
Pure nickel welding the third problem: welding heat-affected zone has a tendency to grain growth
Pure nickel Ni201 is a single-phase state, there is a tendency to grain growth, coupled with poor thermal conductivity, welding heat is not easy to dissipate, easy to overheat, resulting in coarse grains, so that the intergranular interlayer thickening, weakening the intergranular bond, reducing the plasticity of the weld and heat-affected zone, corrosion resistance, and lengthening the liquid and solid phase of the weld metal exists for a long time, which in turn enhances the formation of thermal cracks.
Measures to prevent grain growth: the use of small linear energy for welding, small current, welding fast, the welding rod does not do lateral oscillation, no preheating, interlayer temperature as low as possible, after welding can be forced cooling.
Source: China Ni201 Plate Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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