Butt welding technology of nickel 201 plate
Nickel 201 is welded by manual argon tungsten arc welding. ERNi-1 welding wire with the same chemical composition and strength grade as nickel 201 is selected as the welding material. The physical properties of nickel 201 are characterized by low thermal conductivity, high expansion coefficient and poor weld metal fluidity. A larger V-groove and a groove angle of 70 ° should be used during butt welding. Small welding line energy and short arc welding shall be adopted in the welding process, and the interlayer temperature shall be less than 100 ℃. A series of tests shall be carried out after welding, mainly including nondestructive testing, mechanical property testing and micro metallographic testing. The results show that each performance index can meet the standard requirements with reasonable welding process.
In a research institute of molten salt production reactor manufacturing project, its core components need to be resistant to high temperature and corrosion, so a large number of applications with good corrosion resistance and high temperature resistance of nickel 201. but nickel 201 liquid weld metal fluidity is poor, its weldability is poor compared with carbon steel, need to develop a suitable welding process specifications to obtain a better welded joints to ensure the safe operation of the product.
Aiming at the metal materials involved in the manufacture of the product, this study takes nickel 201 plate as the test object to discuss the welding technology of nickel 201 plate.
1. Material properties, chemical composition and weldability
1.1 Characteristics of nickel 201
Nickel 201 solid has a face-centered cubic structure, no isomeric transformation, low chemical activity, and is one of the most corrosion-resistant metals in the atmosphere. physical properties of Ni201 are shown in Table 1, and mechanical properties are shown in Table 2.
Table 1 Physical properties of Ni201
Ni is not easily rusted in the atmosphere and is resistant to corrosion by caustic acids and to corrosion by aqueous solutions, molten salts or hot boiling caustic soda. Almost all organic compounds do not act with Ni. In air, NiO thin layer is formed on the surface of Ni to prevent Ni from continuing oxidation. Ni has no significant effect with C below 500°C, but S-containing gas has a greater corrosive effect on Ni, especially when Ni is eutectic with Ni3S2 above 635°C. Corrosion is serious.
Table 2 Mechanical properties of Ni201
Ni201 alloy is industrially pure wrought Ni, good mechanical properties, especially plasticity, excellent toughness; good hot working properties, the most suitable hot working temperature of 870 ℃ -1230 ℃; good ductility, easy cold working forming, its behavior is similar to soft steel. Can be annealed in a wide range of temperatures above the recrystallization temperature, annealing temperature 705 ℃ -925 ℃, the temperature is too high will make the grain easy to grow.
Nickel 201 alloy is corrosion resistant and usually maintains a bright metallic luster in indoor atmospheres, corrodes slowly in outdoor atmospheres, and has a very low corrosion rate in marine and rural atmospheres. It also has a very low C content and does not exhibit brittleness at high temperatures due to C or graphite. It is mainly used in handling equipment and parts of reducing halogen gases, alkali solutions, non-oxidizing salts, organic acids, etc. Its service temperature is preferably lower than 315℃ when in use. The material of low carbon nickel plate for this research project is Ni201, and its chemical composition is shown in Table 3.
Table 3 Chemical composition of Ni201 %
1.2 The weldability of Ni201
The weldability of Ni201 is good, equivalent to chromium-nickel austenitic stainless steel. However, thermal cracking and porosity of the weld metal and the weld heat affected zone has a tendency to grain growth, which is the main problem in the welding of nickel 201.
1.2.1 Thermal cracking
Nickel 201 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 the nickel 201 weld metal.
Measures to prevent thermal cracking: firstly, try to reduce the content of impurities such as S and Si in the weld metal, strictly clean the bevel area and welding wire before welding, etc., and strictly control the content of impurities in the base material; secondly, add appropriate amounts of Mn, Nb, Mo, Ti and other elements to the weld metal to offset the harmful effects of impurities such as S and 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.
When welding nickel 201, porosity is a more difficult problem to solve, especially when welding pure nickel and nickel-copper alloys 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 gases 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 at the fast cooling, in the place where the pores.
The methods to solve the porosity are as follows.
- (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.
1.2.3 Welding heat-affected zone has a tendency to grain growth
Nickel 201 is a single-phase alloy, there is a tendency to grain growth, coupled with the poor thermal conductivity of such alloys, welding heat is not easy to dissipate, easy to overheat, resulting in coarse grains, so that the intergranular interlayer thickening, weakening the intergranular bonding, reducing the plasticity of the weld and heat-affected zone, corrosion resistance, and lengthening the liquid and solid phases of the weld metal time, which 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 transverse oscillation, no preheating, interlayer temperature as low as possible, after welding can be forced cooling.
2. Welding methods and welding materials selection
2.1 Nickel 201 welding method
Nickel 201 welding methods are mainly electrode arc welding, manual tungsten arc welding, molten gas shielded welding and submerged arc welding. Manual tungsten arc welding has been widely used in the welding of nickel-based alloys, especially suitable for thin plates, small cross-section, joints can not be back welding bottoming and welding after the residual slag is not allowed structural parts. Manual tungsten arc welding has a strong protective effect and can effectively isolate the surrounding air, making the metallurgical reaction in the welding process simple and easy to control, providing good conditions for obtaining a higher quality weld, and the arc is very stable and can burn steadily even at very small currents (less than 10 A), making it particularly suitable for welding thin plate materials. In this study, manual tungsten arc welding of nickel 201 was chosen because the thickness of the plate was only 6 mm.
2.2 Nickel 201 welding material
The shielding gas for manual tungsten arc welding is Ar, and the welding wire is ERNi-1 wire with a diameter of φ1.6 mm. Most of the nickel-based alloy wire composition is comparable to the base material, but some more alloying elements will be added to compensate for the burnout of certain elements as well as to control welding porosity and thermal cracking. The chemical properties of the wire ERNi-1 are shown in Table 4 and the mechanical properties in Table 5.
Table 4 ERNi-1 chemical properties %
Table 5 ERNi-1 mechanical properties
|Yield strength, psi||Tensile Strength, psi||Elongation, %|
3. Form of welded joints
Nickel-based alloy welded butt joints recommended design form, design, the first consideration to have the appropriate accessibility. The root opening angle should be sufficient to allow the electrode, wire and torch to reach the bottom of the joint. Compared with carbon steel, the physical properties of Ni201 are characterized mainly by low thermal conductivity and high coefficient of expansion, all of which should be taken into account in the preparation of the weld bevel, including widening the bottom gap by 1-3 mm, while a larger bevel angle of 70° should be used in butt welding to offset material shrinkage due to poor fluidity of the weld metal, which does not flow easily to both sides of the weld. Sometimes in order to obtain good weld formation, the swing process will be used, but this swing is a small swing, swing distance of no more than three times the diameter of the welding rod or wire.
When the thickness of nickel-based corrosion-resistant alloy plate is greater than 2.4mm, the butt joint requires a V-bevel, U-bevel or J-bevel. Special attention should be paid to prevent unstable penetration and avoid unfusion, cracking and porosity. Since nickel-based alloys work in corrosive media at various temperatures, safe fusion-through welds are required.
The nickel plate size δ=6mm for this molten salt generator manufacturing project is manually tungsten arc welded with the welding parameters shown in Table 6.
Table 6 Ni201 welding parameters
The nickel plate butt weld is a V-shaped bevel with a bevel angle of 70°, and the joint form is a full fusion penetration bevel weld with a root gap of 2-3mm and no liner, as shown in Figure 1.
Figure 1 Nickel plate butt joint schematic
4. Welding operation process
4.1 Bevel cleaning before welding
- (1) Clean up the bevel, blunt edge and both sides of the weld channel within 30mm.
- (2) Clean up the oxide film at the bevel with files and grinding wheels.
- (3) Clean the dirt, grease, paint, etc. at the bevel with acetone, alkaline solution or special synthetic agent.
4.2 Welding precautions
(1) Welding to ensure penetration and fusion under the premise of good, in the process parameters as far as possible within the range of small welding line energy, short arc, no swing or small swing operation method.
(2) When the weld is thicker multi-layer welding should be consistent with the provisions.
- ① In addition to the bottom welding, the rest of the welding layer should be used for multi-pass welding.
- ② Interlayer temperature is less than 100 ℃.
- ③ Each layer after each weld should be thoroughly removed after the welding channel surface slag, and eliminate various surface defects.
(3) The welded joints of each layer of each channel should be staggered.
The use of solid wire or not filler tungsten arc welding, the back of the weld should be filled with argon, can be designed to implement back protection, and meet the following requirements.
- ① Welding before the appropriate increase in the flow rate at the beginning of the filling argon gas, to determine the device to completely remove the air before welding.
- ② Welding gradually reduce the flow of argon gas filling, in order to avoid high argon gas filling pressure caused by the back of the weld in the formation of the internal or root not welded through the phenomenon.
- ③ Manual tungsten arc welding, the heating end of the wire should always be under the protection of argon gas. To strengthen the protection effect can be added to the rear side of the welding nozzle an auxiliary transport protective gas towing cover.
- ④ Arc abrasion is strictly prohibited on the surface of the welded parts, and forbid the arc on the surface of the welded parts, arc collection.
- ⑤ Welding power ground connected to the welded parts shall not directly contact the workpiece, should be used with the welded parts of the same material transition connection to avoid iron pollution.
- ⑥ Welding to ensure the quality of the arc lead and arc closing, should fill the arc pit of the arc closing.
- ⑦ Welding must be completed in a timely manner to clean up the surface of the weld slag and surrounding spatter, anti-spatter materials.
- ⑧ Welding construction to avoid pollution, the use of stainless steel hammer, stainless steel wire brush, special grinding wheel piece.
5. Nickel 201 welding experimental test data
According to ASME-Ⅱ QW191.2.2 standard assessment, 24 h after welding flawless inspection, the results are judged to be qualified. Tensile test standard value is greater than or equal to 345MPa, the measured value of 1 # 365MPa, 2 # 355MPa, the base material extended broken, bending test qualified. Metallographic test is based on the standard GB/T13298-1991 ASME SB-162: 2004 ASTM E 112 grinding and polishing, corrosion, placed under the metallographic microscope, the results are: the parent material nickel 201 microstructure for γ-phase, twin crystal distribution, grain size level 7; heat-affected zone microstructure for γ-phase, grain growth slightly, grain size level 6; weld zone microstructure for γ-phase, dendritic Organization for the γ-phase, a dendritic distribution. It shows that the welding process is better control of linear energy, in line with the material high temperature and corrosion resistance needs, as shown in Figure 2.
Base material tissue morphology
Heat-affected zone tissue morphology
Weld zone organization
Figure 2 metallographic organization
- (1) Nickel 201 weldability is good, equivalent to the chromium-nickel austenitic stainless steel. Organizational structure of single-phase austenite, low thermal conductivity, high expansion coefficient, welding heat-affected zone has a tendency to grain growth, weld and heat-affected zone plasticity, corrosion resistance will be reduced, the weld metal is prone to the formation of thermal cracks and porosity.
- (2) Nickel 201 welding method preferred tungsten arc welding, tungsten arc welding arc stability, easy to achieve single-sided welding double-sided molding, good weld formation, no spatter, suitable for thin plate welding. Welding material selection and the base material chemical composition, mechanical properties equivalent to nickel-based alloy wire.
- (3) Strictly clean nickel 201 plate and wire before welding, compared to carbon steel class to increase the bevel angle, group gap, the back side of the weld channel must take argon gas protection, while the welding line energy is not too large, the interlayer temperature shall not exceed 100 ℃.
Author: Maoqiqin, Yangyun
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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