Welding of Ni200 pure nickel pipe

By analyzing the physical and chemical properties and weldability of pure nickel, the welding process of nuclear material is formulated and successfully applied in engineering. The key to ensure the welding quality of Ni200 pure nickel pipes is to adopt welding methods, high current and fast welding processes, and to carry out strict pre welding cleaning and pre welding preheating in low temperature environment.

1. Preface

The main principle is to make 99.9% molten caustic soda from 32% NaOH solution by first and second stage evaporation and third stage concentration, and then make flake soda by cooling and scraping through the flaking machine. The key equipment and piping components involved in the whole process are imported from Swiss Bote.
As the caustic soda solution becomes more and more corrosive with the increase of concentration and temperature in the process of evaporation and concentration, the metal material selection requirements for process equipment and piping are also higher and higher. When the concentration of caustic soda solution ≥ 50%, stainless steel materials can no longer meet the requirements of corrosion resistance, must use pure nickel materials with good resistance to caustic soda corrosion performance. A section of the device after the evaporation of equipment and caustic soda solution pipeline actually used Ni200 material, the total length of the pipeline more than 240m, more than 260 welding ports. The minimum pipe size is DN25mm, the maximum is DN800mm, and the wall thickness is 3.05 – 4.31mm.
Under the harsh conditions of strong corrosive use, the quality of pure nickel pipeline welding is related to the long-term safe and stable operation of the whole unit, and because of the special physical and chemical properties and weldability of pure nickel material, the welding operation is difficult and the welding technology is very demanding. The owner and the experts of Botek pay great attention to the welding of pure nickel pipeline, and all the welding quality inspection adopts Swiss standard. The welding of pure nickel pipeline also became the key to the construction of the project. 2.

2. Chemical composition and physical properties of pure nickel

2.1 Chemical composition

The chemical composition of Ni200 pure nickel is shown in Table 1.
Table.1 Chemical composition of Ni200 pure nickel

Common Trade Names	C	Cu	Fe	Mn	Ni	S	Si
Nickel 200, Nickel 201	≤0.15	≤0.25	≤0.40	≤0.35	≥99.0	≤0.01	≤0.35

2.2 Physical properties

The physical properties of pure nickel and carbon steel and stainless steel are compared in Table 2.

3. Analysis of the weldability of low-carbon nickel

Pure nickel materials welded mainly exist welding thermal cracking, porosity and poor weld appearance forming and other problems.

Table.2 Physical properties

Material	Specific gravity g/cm3	Melting point ° C	Coefficient of thermal expansion × 106 （m/m·℃）	Thermal conductivity W/m.K	Resistivity m Ω cm
Pure nickel	8.89	1453	4.1	91.96	6.8
Mild steel	7.85		11.4	46.89	15
18-8 stainless steel	7.93		17.3	16.29	72

3.1 Welding thermal cracking

Pure nickel materials containing ≥ 9% nickel, welding with a certain tendency to thermal cracking, and higher nickel content of pure austenitic stainless steel (δ < 0.6%) is more similar. The tendency of pure nickel materials to thermal cracking and whether the material contains sulfur, phosphorus and other harmful elements and content. A small amount of sulfur and phosphorus elements present may have a considerable impact on the welding of pure nickel, and with the increase in sulfur and phosphorus content, the tendency to thermal cracking is greatly increased. The reason is that the weld metal from liquid cooling to solidification process, sulfur, phosphorus and nickel formation of low-melting eutectic, to the weld metal grain boundary deviation and the formation of liquid film, the role of tensile stress in welding leads to the generation of thermal cracks. The harmful effects of impurity elements such as sulfur and phosphorus also have the tendency to superimpose. Therefore control the material sulfur, phosphorus impurities
Element content, to prevent external sulfur and phosphorus containing material pollution is very important.

3.2 Weld porosity.

Pure nickel is very sensitive to hydrogen porosity. Hydrogen is more soluble in the liquid metal and decreases significantly as the temperature drops. Due to the large thermal conductivity of pure nickel, the solid-liquid phase temperature range is small, the liquid metal viscosity, poor mobility, weld crystallization solidification is fast, the dissolved hydrogen in the liquid metal of the melt pool is not easy to precipitate, the melt pool is easily saturated with hydrogen and the formation of pores. The key to controlling weld porosity is to reduce the content of [H] in the liquid metal of the melt pool, on the one hand, should use suitable welding materials, on the other hand
On the other hand, it is necessary to ensure the cleanliness of the weld and the wire, to prevent outside air, grease, water and other pollution of the melt pool.
Welding climate conditions have a considerable impact on the weld porosity. When the ambient temperature is lower than 16 “C easy to produce porosity, which is due to the presence of condensation on the bevel surface, the use of acetone scrubbing can not be removed, must be preheated before welding; welding also requires air relative humidity < 65%.

3.3 Unfused and poorly formed weld seam

Due to the large thermal conductivity of pure nickel, molten pool cooling solidification is fast, low current welding is not easy to fully melt the base material, resulting in welded seams prone to unfused defects liquid metal mobility and wettability is poor, the weld surface pattern is coarse, easy to produce biting edge, depression, weld forming is not as beautiful as carbon steel and stainless steel; in addition, pure nickel welding pool color change is not easy to see is also one of the reasons for poor weld forming.

4. Welding process

4.1 The choice of welding method

Manual tungsten arc welding (TIG) has a good pool protection effect, dense weld metal, high transition coefficient of deoxidized alloy elements in the wire, easy to operate, beautiful weld appearance, high flaw detection rate, pure nickel pipeline field welding using TIG welding can ensure the quality of welding.

4.2 Selection of welding materials

Ni200 pure nickel pipe TIG welding using ERNi-1 2.5 mm wire, wire chemical composition is shown in Table 3.
Table.3 Welding wire chemical composition (%)

Brand	C	Si	Mn	Ni	Al	Ti
ERNi-1	≤0.15	≤0.75	≤1.0	≥93	≤1.5	2.0 – 3.0

This grade of wire contains a certain amount of manganese, manganese and sulfide together to prevent the formation of low-melting eutectic nickel sulfide, can effectively prevent the generation of thermal cracking, a small amount of aluminum, titanium elements in the wire plays the role of deoxidizer.

4.3 Pure nickel material storage and protection

Pure nickel is vulnerable to sulfur, when it is in contact with sulfur-containing substances, the temperature exceeds the critical value of 300C, the hot sulfur will penetrate the grain boundary into nickel sulfide. Sulfur exists in many forms in the material, easy to contact with pure nickel are oil, grease, dust, paint, marker ink, tape, fingerprints (sweat), footprints, and pollution air containing sulfide, etc. Therefore, in order to avoid contamination of materials, pure nickel pipes and fittings must be stored in a dry, clean and safe warehouse before construction, and designate a person responsible for storage. Before installation workshop doors and windows closed, the ground clean; installation process workers wear clean work clothes, wear clean gloves, do not directly touch the material with their hands; prefabricated materials on a clean work platform and covered with plastic film; materials do not contact with iron, ground; in the material surface shall not be marked with markers, etc.

4.4 Bevel preparation

Pure nickel weld metal is not like carbon steel and stainless steel weld metal as easy to flow and wet the base material, in order to facilitate the welding operation, the bevel should choose a larger angle: 75 – 80 °; V-shaped bevel root gap: 2.5 – 3.0mm, blunt edge height: 0 – 1.0mm.
Pipe beveling machine is used to process pipe bevels, and the bevels which are not easy to be machined can be ground by angle grinder first, and then the bevel surface can be trimmed by contouring knife.

4.5 Cleaning and grouping before welding

Use stainless steel wire brush to clean up the oxide film on the bevel surface, and then use acetone or alcohol to scrub the bevel and the surface of the base material within 50mm on both sides to remove water, oil and dirt. When the wire is not used, put it in the wire packaging tube so as not to be polluted by outside dust, etc. When welding, pull it out and use it after scrubbing with acetone or alcohol. Grouping the wrong side of the tube is not more than 10% of the wall thickness of the tube; grouping gap 2.5 – 3.0mm. spot solid welding seam should be subsequently completed, so as not to be placed too long need to clean up again.

4.6 Welding process parameters

Due to the high thermal conductivity of pure nickel, fast solidification, welding should be used when the larger welding current, otherwise prone to unfused, root not welded through, poor surface formation of the weld and other phenomena. Pure nickel pipe welding process parameters are selected in Table 4.
Table.4 N2.4068 pure nickel pipe welding process parameters

Welding	Welding current (A)	Arc voltage (V)	Type of power supply	Tungsten rod diameter (mm)	Nozzle aperture (mm)	Hydrogen flow (L/min)		Interlayer temperature (℃)
						Welding torch	In-pipe
Bottom up	120 – 130	14±2	DC+	ø2.5	ø12	12 – 14	8 – 10
Other layers	120 – 140	14±2	DC+	ø2.5	ø12	12 – 14	6 – 8	<100

4.7 Welding precautions

(1) Priming and the first layer of filler welding must be protected by argon filling on the back of the weld to prevent oxidation of the root of the weld and poor forming.
(2) Priming welding sealing tape can not be directly glued to the pipe, should be shown in Figure 1 counter-stick, to prevent the adhesives attached to the base material to contaminate the weld.

Figure.1 Adhesive tape counter-stick
(3) Welding shall not use the scratch method to start the arc, must use a special argon arc welding machine and high-frequency arc, to prevent the phenomenon of tungsten trapping.
(4) In the weld channel surface formed by the deoxidizer in the wire oxide, slag spots and nickel oxide film adhesion, multi-layer welding must be used to clean the weld channel surface with a stainless steel wire brush or angle grinder thousand net to prevent the generation of interlayer unfused.
(5) Prohibit in the base material. On the arc, the arc pit should be filled when the arc is closed.
(6) When the ambient temperature is lower than 16C, the application of dry hot air before welding the bevel and nearby areas for preheating, to remove the surface condensation to prevent porosity.

5. Conclusion

Practice shows that N2.4068 pure nickel pipe welding using TIG welding and matching UTP80Ni argon arc welding wire and large angle welding bevel, welding with high current fast welding, and pay attention to the protection of the root of the weld, can obtain high-quality welded joints.
Pay attention to the storage of pipe fittings and the protection of the construction process, and carefully clean up before welding is also the key to successful welding of N2.4068 pure nickel pipe. In winter low temperature environmental conditions, the bevel must be preheated before welding.

Authors: Liao Yixiang, Chen Wenwei, Li Peng

Source: China Ni200 Pipe Manufacturer – Yaang Pipe Industry (www.epowermetals.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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