Welding process of industrial pure nickel pipe

The weldability of pure nickel and its influencing factors were analyzed. Combined with the application in practical work, the welding process of industrial pure nickel pipe is discussed. On the basis of TIG welding, the self-made argon protective cover is used to avoid the serious oxidation of nickel in the high temperature area during welding. At the same time, combined with other protective measures and reasonable welding specifications, the welding quality of nickel pipe is obviously improved.

The element nickel has a face-centered cubic lattice, which is characterized by a high melting point, corrosion resistance, high mechanical properties, good processing properties in the hot and cold state, and special physical properties. Nickel also has good chemical stability, in room temperature air, it is not corroded, in seawater and most acid and alkali salt media have excellent corrosion resistance, especially in hydroxide and alkaline solutions, its corrosion resistance is better than titanium and titanium alloys, and thus widely used in petroleum, chemical, shipbuilding and other industries. In production, we encountered a diameter of 108mm, wall thickness of 4mm industrial pure nickel pipe welding problems, through its welding properties and process research, the development of a special set of process measures, and used in actual production, and achieved good results.

1. Industrial pure nickel welding performance analysis

Industrial pure nickel is a rare metal, its weldability is good, but in the welding process, gas impurity contamination is prone to cause embrittlement of welded joints. At room temperature, industrial pure nickel is relatively stable, as the temperature increases, its performance begins to change, and its ability to absorb nitrogen, hydrogen and oxygen then rises significantly. Experiments have proved that nickel oxidizes slightly in high temperature air at 500℃, and oxidizes violently when the temperature reaches 750℃.
As the oxygen content of the weld rises, the tensile strength and hardness of the weld rises significantly, while the plasticity of the weld decreases significantly and the weld becomes brittle due to oxygen contamination. Nitrogen in the high temperature liquid metal solubility increases with the partial pressure of oxygen, nitrogen on the weld strength, hardness, plasticity than oxygen is more significant, that is, the pollution of nitrogen embrittlement effect than oxygen more strongly, nitrogen on the weld is mainly on the impact toughness impact toughness, nitrogen content increases, so that the impact toughness of the weld significantly reduced.
Due to the difficulty of preventing the embrittlement of welded joints caused by gas impurity contamination, the process of industrial pure nickel welding in its special requirements, the use of ordinary gas welding and manual arc welding is unable to meet the quality of welding. The use of tungsten arc welding is a more reasonable welding method, but it is worth noting that the usual torch structure and process is not enough to ensure the quality of the welded joint. Because this gun structure formed by the gas protection layer can only protect the welding pool from air pollution, has solidified and in the high temperature state of the weld and its heat-affected zone is no protection. In this state of nickel and strong ability to absorb nitrogen and oxygen in the air, will inevitably cause the weld to become brittle, and a serious decline in plasticity. If the back of the weld does not take effective protection, will also produce similar results, in the high-temperature melting pool and molten metal droplets are more susceptible to contamination by gas impurities. Therefore, a series of protective measures must be taken to improve the quality of nickel pipe welding.

2. Improve the quality of welding should take measures

2.1 The premise of ensuring the quality of welding

  • (1) impurities in the base material and welding wire must be within the range allowed by technical conditions.
  • (2) the use of high-purity argon gas for welding, welded joints should generally be guaranteed to be silvery white.
  • (3) the wire and workpiece must be carefully cleaned before welding.
  • (4) the correct choice of welding wire, welding specifications and the necessary welding heat treatment.
  • (5) strengthen the melt pool and heat-affected zone protection measures.

After the analysis of the wire, welding gun structure that it does not cause pollution to the weld, so mainly from the auxiliary protection device, welding fixture, pre-weld cleaning and other aspects to study.

2.2 Self-made argon gas-filled shield

To expand the protection of the weld seam, it is achieved through a homemade argon shield. This shield is selected according to the form of the joint, welding process parameters, the principle is: in the region above 500 ℃ can be effective protection, the protection effect to the weld surface is silvery white or light yellow for qualified, homemade shield structure is shown in Figure 1.

Drag hood structure and size by the geometry of the weld to determine, generally with 0.8 mm thick pure copper plate to make a long 40 “100 mm, 30 “60 mm wide hood, thin weld short, thick weld long, wide some. Argon gas from the inlet tube into the distribution tube, distribution tube made of φ6mm × 1mm stainless steel tube, there are two rows of diameter 0.8 “1.0mm holes drilled above the tube, the hole distance is 8 “10mm, filter mesh with 0.152mm × 0.152 (100 mesh) made of copper wire mesh, the distance from the filter mesh to the weld is not less than 8mm, there should be no dead space inside the inflatable chamber, in order to achieve argon gas will be all The air extrusion, protective cover as far as possible to make a smooth transition inside, to prevent argon gas flow is too large to form a vortex, so that in the welding process argon gas from the inlet pipe into the distribution tube, the spray from above to get a uniform distribution and buffer. When the airflow through the copper wire mesh, it will be distributed again, which improves the flow of argon, so that the molten pool of metal directly under the protection of argon stratosphere solidification. The welding gun and the towing cover are integrated in the process for easy operation.

20220603112301 40086 - Welding process of industrial pure nickel pipe

Figure 1. Self made protective cover structure

2.3 Other

Welding fixture should be strong enough and good thermal conductivity, with copper gasket plate water-cooled fixture can meet the above requirements. Fixture and protective cover to determine, select the purity of 99.99% or more argon gas (test the purity of argon gas, is in each bottle of argon gas before use, according to the normal specifications in the nickel plate spot welding several points, the weld point is silvery white, the surface such as a mirror surface that is qualified), adjust the flow of different gases, observe the protective effect, and finally determine the appropriate protective gas flow. Before welding, the nickel surface oxide film should be cleaned up, cleaned up as soon as possible after welding, the placement time is generally not more than 4h.

3. Welding process evaluation

  • (1) by X-ray inspection without porosity, slag, cracks and other defects, with reference to the GB3323-1987 standard, can reach Ⅱ level requirements. Weld appearance inspection are silver-white.
  • (2) The joint mechanical properties test, although lower than the parent material, but in line with the design requirements.
  • (3) Chemical composition is shown in Table 1.

Example – Ni200 pure nickel pipe welding 

A chemical group 50,000 tons of ion membrane caustic soda plant using pure nickel pipeline Ni200 material, the total pipeline more than 240 meters, more than 260 welding ports, the minimum specification DN25mm pipeline, the maximum specification DN800mm, wall thickness 3.05 ~ 4.31mm, Ni200 pure nickel chemical composition and chemical composition of welding wire is shown in Table 1.

Table 1 Ni200 pure nickel chemical composition and chemical composition of welding wire / %

Chemical element

C

Si

Fe

S

P

Ni

Cu

Mn

Al

Ti

Raw material Ni200

≤0.15

≤0.40

≤0.4

≤0.01

≤0.02

>0.99

<0.25

 

 

 

Welding wire ERNi-1

≤0.15

≤0.75

 

 

 

≥93

 

≤1.0

≤0.15

2.0-3.0

(1) the weldability of low carbon nickel steel analysis
For ease of analysis, pure nickel Ni200, mild steel Q235 and stainless steel 18-8 physical properties for a comparison (see Table 2), pure nickel welding mainly exists welded thermal cracks, porosity and poor appearance of forming.
Table 2 pure nickel Ni200, low carbon steel Q235, stainless steel 18-8 physical properties

Metal material

Density/g·cm-3

Melting point/℃

Coefficient of linear expansion

/10-6·℃1

Thermal conductivity

/W·m-1·K-1

Resistivity

/mΩ·cm

Pure nickel Ni200

8.89

1453

4.1

91.96

6.8

Mild steel Q235

7.85

 

11.4

46.89

15

Stainless steel 18-8

7.93

 

17.3

16.29

72

① Weld thermal cracking Pure nickel content greater than or equal to 99%, welding with a certain tendency to thermal cracking and whether the material contains sulfur, phosphorus and other harmful elements and content, even a small amount, will have a considerable impact, with S, P content increases, the tendency to thermal cracking greatly increased, so control the material S, P impurities, to prevent the pollution of S, P is very important.
② Weld porosity Pure nickel is very sensitive to hydrogen porosity, hydrogen solubility in the liquid metal, with the temperature drops significantly, due to pure nickel thermal conductivity, solid-liquid phase temperature interval is small, liquid metal viscosity, poor mobility, weld crystallization solidification is fast, the melt pool of dissolved hydrogen in the liquid metal is not easy to precipitate, they generate porosity. To reduce porosity must use suitable welding materials, and to ensure that the weld and wire clean, dry, to prevent outside air, grease, moisture and other pollution of the melt pool. When the ambient temperature is lower than 16 ℃ easy to produce porosity, which is the reason for the existence of condensation on the surface of the bevel, must be preheated before welding, welding also requires the relative humidity of the air less than 65%.
③ Unfused and poorly formed weld seam Due to the large thermal conductivity of pure nickel, the melt pool cooling solidification fast, low current welding is not easy to fully melt the base material, resulting in the weld prone to unfused defects. Liquid metal mobility and wettability is poor, the weld surface pattern is coarse, and easy to produce biting, depression, weld forming is not as beautiful as carbon steel, stainless steel.
(2) Welding process
① Welding method using tungsten arc welding on the molten pool protection effect is good, easy to operate, easy to ensure the quality of welding. Welding wire using ERNi-1, diameter 2.5mm, the wire contains Mn, Mn and S chemistry can prevent the formation of low-melting eutectic NiS, can effectively prevent the generation of thermal cracking, a small amount of aluminum, titanium elements in the wire can play the role of deoxidizer. In order to protect pure nickel from environmental pollution, Ni tubes must be stored in a clean, dry and safe warehouse and kept by a person in charge of storage, storage and construction personnel can not directly touch the material with their hands, must wear clean work clothes, wear clean gloves, in short, are not allowed to bring pollution to the material.
② Bevel should choose a larger angle (75 ° ~ 80 °), the root gap of 2.5 ~ 3.0 mm. clean up the bevel surface oxide film before welding, with acetone or alcohol scrubbing bevel both sides of the 50mm range. Spot welding immediately after welding, placed no more than 8h.
③ Welding process parameters See Table 3.
Table 3 N2 4068 pure nickel pipe welding process parameters

Welding position Welding current /a Arc voltage /v Type of power supply Tungsten rod diameter /mm Nozzle outer diameter /mm Argon flow /l · min-1 Interlayer temperature / ℃
Welding post In-pipe
Underlay 120-130 142 DC+ 2.5 12 12-14 8-10

④ Welding precautions

  • a. priming and the first layer of filler welding must be filled with argon to protect the back of the weld to prevent the root of the weld from oxidation and poor formation.
  • b. 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 tungsten trapping phenomenon.
  • c. multi-layer welding must use stainless steel wire brush or angle grinder to clean the surface of the weld channel to prevent the generation of inter-layer unfused.
  • d. prohibit the arc on the base material, the arc should be filled when closing the arc pit. When the temperature is lower than 16℃, dry hot air should be used to preheat the bevel and nearby areas before welding to prevent the production of porosity.
  • e. practice shows that Ni 4068 pure nickel pipe welding using tungsten argon arc welding and matching UTP80Ni wire and large angle bevel, using high current fast welding, and focusing on the root protection of the weld, can obtain high-quality welded joints. The storage of the workpiece, protection during construction, and careful pre-weld clearing are also the keys to successful welding.

Nickel and nickel alloy manual welding process 

  • (1) Welding wire is shown in Table 1, welding rod is shown in Table 4.
  • (2) Welding current selection is shown in Table 5. thickness greater than 1mm pure nickel welded components and solid solution reinforced nickel alloy welding is often used manual arc welding, the use of DC power reverse short arc welding, small line energy, the electrode generally does not swing laterally, must swing laterally, swing amplitude is not greater than two times the diameter of the electrode, the interlayer temperature should not be too high. Nickel and nickel alloy manual arc welding bevel size is shown in Table 6.

Table 4 nickel and solid solution reinforced nickel alloy deposited metal composition, electrode selection and mechanical properties

Alloy series Chemical composition of deposited metal /%
C Mn Fe Si Cu Ni Al Ti Cr
Pure Ni ≤0.03 0.6-1.0 ≤0.8 ≤1.0 Allowance 0.2 0.7-1.0
0.02 0.61 0.3 0.45 Allowance 0.4
NiCr 0.06 23 2-3 0.48 Allowance 0.21 0.71 25-30
0.02 2-3 1-5 67.5 Allowance
NiCrFe 0.05 7.9 ≤7 0.81 70 0.31 15
0.08 ≤8.3 Allowance 16.5
NiCrMo ≤0.07 0.7-1.7 6.1 ≤0.4 Allowance 13.5-16.5 15.3
0.06 0.7 0.46 Allowance
NiMo 0.07 0.81 6.3 0.61 Allowance 5.4
Alloy series Chemical composition of deposited metal /% Mechanical properties of weld metal Grade of welding rod used
Nb Mo W Co σb/MPa δ/ Ψ/
Pure Ni 1.92.2 490 45 65 Ni 102
471.4 44
NiCr 0.91 > 491 > 30 > 58 Ni 207
554.7 45
NiCrFe 35 2-6 > 784 3040 Ni 307
17 617.4 40
NiCrMo 0.3 9.6-11 5.7 607.6 ≥28
15.8 639.9 13

Table 5 Nickel and nickel alloy manual arc welding bevel size

Plate thickness/mm

Electrode diameter/mm

Welding current/A

Plate thickness/mm

Electrode diameter/mm

Welding current/A

≤2

2-2.5

2.5-3.0

ф2

ф2-3

ф3

40-50

50-60

70-100

3-5

5-8

8-12

ф3-4

ф4

ф4-5

80-140

90-150

110-170

NiMo alloy thick parts are also not recommended for submerged arc welding, because the large linear energy and slow cooling rate will reduce the plasticity of the weld.

Table 6 Groove dimensions of nickel and nickel alloy manual arc welding

20220603125654 97933 - Welding process of industrial pure nickel pipe

4. Conclusion

  • (1) Industrial pure nickel has good welding performance. Good welding joints can be obtained by TIG welding and homogeneous welding wire as filler metal.
  • (2) Reliable protection during welding and reasonable selection of welding specifications are the premise to ensure quality, and the type and protection effect of towing hood are the key technologies.

Source: China Nickel Pipe 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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