Welding and quality control of Ni200 pure nickel pipe

By summarizing the physical and chemical properties of Ni200 pure nickel pipe materials, this paper analyzes the weldability and common defects of pure nickel materials, and puts forward the corresponding quality control scheme on this basis, which ensures the smooth implementation of the project and provides a useful reference for similar projects.

Industrial pure nickel is an important rare metal, which not only plays an essential role in most alloys as an alloying element, but also plays an important role as an engineering structural material, which is widely used in chemical, pharmaceutical, nuclear, marine and other fields. In particular, pure nickel (Ni≥99%) has been an essential key material in caustic soda projects due to its excellent alkali corrosion resistance.
The material of this project is ASTM B161 UNS N02200, and the welding is carried out in accordance with AS-MEIX.

Chemical composition and physical properties of Ni200 pure nickel pipes

Nickel is a slightly yellowish silver-white exhibition metal, atomic number 28. pure nickel is single-phase austenite organization, the crystal structure of the face-centered cubic lattice, liquid solidification process does not occur in the phase change. the chemical composition of Ni200 pure nickel is shown in Table I:
Table I Chemical composition of Ni200 pure nickel

Grade

C

Mn

Si

S

Cu

Fe

Ni

Nickel 200

0.15 max

0.35 max

0.35 max

0.01 max

0.25 max

0.40 max

99.0 min

The density of Ni200 pure nickel is about 8.9g/cm3, Mohs hardness 5.0-6.0, yield strength 105MPa. the main physical properties of pure nickel and low carbon steel, austenitic stainless steel are compared in Table II:
Table II Ni200 pure nickel and low carbon steel, stainless steel 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

From the above table, it can be seen that the density of pure nickel than mild steel and stainless steel, but the yield strength is relatively low, thermal conductivity is much greater than mild steel and stainless steel. Therefore, pure nickel and low carbon steel and stainless steel, compared to the material is relatively soft, liquid viscosity is relatively large, poor mobility, fast solidification speed.
It is worth noting that the surface of nickel at room temperature will generate a dense oxide film, the main component is nickel oxide (NiO), its melting point is as high as 1984 ℃, more than the melting point of pure nickel nearly 500 ℃, and insoluble in alkali, nickel-based pipe alkali corrosion resistance is derived from the protection of this layer of oxide film.

The Ni200 pure nickel material weldability analysis

From the above analysis of the chemical composition and physical properties of nickel, it is clear that the melting point of pure nickel is higher than that of stainless steel, but lower than that of mild steel, so nickel can be welded by melt welding technology.

1. The role of nickel and hydrogen, oxygen, nitrogen and other gas elements.

At room temperature, the nature of pure nickel is relatively stable, but as the temperature rises, its ability to absorb hydrogen, oxygen and nitrogen rises significantly. Pure nickel starts to oxidize slightly in the air at a high temperature of 500℃, and oxidizes violently when the temperature reaches 750℃. Oxygen, hydrogen and other gaseous elements in the liquid nickel solubility, especially in the arc welding atmosphere, the molten metal superheat large, on the surface of the molten pool through the local active part and molten droplets absorb gas, the arc atmosphere has excited molecules, atoms and ions, which increases the activity of the gas, so that its solubility in the metal increases, so the amount of gas absorbed by the molten metal during arc welding often exceeds its equilibrium content (solubility). For example, the solubility of oxygen is 1.18% at 1720°C, but decreases sharply to 0.06% at 1470°C. The solubility of the gas decreases significantly with the temperature. Hydrogen in the liquid-solid phase transition when the solubility can quickly drop by two-thirds. Due to the pure nickel welding, the melt pool metal viscosity is relatively large, poor mobility, coupled with the large thermal conductivity of nickel, liquid-solid phase transition temperature range is small, the melt pool crystallization solidification speed, high temperature dissolved in the liquid nickel hydrogen can not escape, easy to remain in the melt pool to form hydrogen pores. In addition, melting and melt pool oxygen in the precipitation process will also react with nickel to produce nickel oxide NiO, which together with the nickel oxide in the oxide film can be reduced by hydrogen and generate water vapor H20, which is too late to escape in the solidification process and cause water vapor porosity.
Nitrogen itself is neither dissolved in nickel, nor does it react with nickel, therefore, theoretically, nitrogen can be used as protection to weld nickel. However, nitrogen in the oxidizing arc atmosphere to form NO and other nitrides, is the main factor causing the rise in hardness of the weld, plastic decline.

2. Sulfur, phosphorus and other impurity elements on the impact of pure nickel weld

Sulfur, phosphorus and other impurity elements at high temperatures are very easy to generate low melting eutectic with nickel, such as nickel sulfide melting point of 645 ℃, nickel phosphide melting point of 880 ℃, while the melting point of pure nickel is 1455 ℃, so when the pure nickel melt pool solidification, these low melting point eutectic is also in the liquid state, in the form of liquid film residual in the grain boundary area, the contraction stress generated during the cooling process of the weld, easy to crack the formation of hot Cracking. Therefore, the base material and filler metal must be controlled impurity elements such as sulfur, phosphorus content, the general standard stipulates that the amount of sulfur shall not exceed 0.01%.

3. Bevel preparation and process parameters on the impact of pure nickel welding

Pure nickel materials due to high thermal conductivity, welding state of the liquid metal viscosity of the melt pool, mobility and wettability is poor, and therefore has the characteristics of low penetration, should not be used to increase the linear energy to increase penetration. Generally use to increase the bevel angle, reduce the thickness of the blunt edge, and through the appropriate swing and other wire transport techniques to improve the liquid metal flow, to increase the effect of penetration. Generally, thin-walled pipes use V-shaped bevel, the bevel angle should reach 75-80. 0-1mm blunt edge height, 2-3mm bevel gap.

The welding process

Pure nickel pipe welding is generally used tungsten arc welding, which has the advantages of single-sided welding, double-sided forming, good melt pool protection, dense weld metal, high transition coefficient of deoxidized alloy elements in the wire, easy to operate, beautiful weld appearance, high passing rate of flaw detection. The composition of the filler wire used in pure nickel tungsten arc welding should make the weld have high crack resistance and weak oxidation to ensure the transition of the remaining alloying elements and infiltration into the weld metal. Research shows that the Ti content of 2%-3.5%, the best crack resistance; at the same time, the wire should be a certain degree of manganese, manganese and sulfide and can prevent the formation of low-fusing eutectic nickel sulfide, can effectively prevent the generation of thermal cracking; a small amount of aluminum elements in the wire can also play the role of deoxidizer. The chemical composition of ERNi-1 wire is shown in Table III:
Table III AWSA5.14/ERNi-1 wire chemical composition

Ni C Si Ti Fe Cu· Mn Al S P
96.1 0.03 .54 2.46 0.30 0.01 0.50 0.03 0.003 0.006

Pure nickel pipe tungsten arc welding uses argon as the shielding gas. Argon is an inert gas that neither dissolves nor reacts with nickel, and is very stable even at high temperatures. But the purity of argon gas has a large impact on the quality of welding, argon gas in the oxygen, nitrogen, water vapor and other impurities content exceeds the standard, will be when the welding arc is unstable, and the production of porosity. At the same time argon flow is also an important welding process parameters, argon flow is too small, the effect of protection; when the flow of argon is too large, not only cause waste, but will produce turbulence, the surrounding air into the protected area, so that the molten pool protection effect deterioration. Generally should use a level of argon, the purity of 99.99% or more, the total mass fraction of impurities does not exceed 0.02%, the dew point below -50 ℃, the relative humidity of the air does not exceed 5%, moisture content is not greater than 0.001mg / L. Argon flow can be determined by the following empirical formula: 

20220603091509 63693 - Welding and quality control of Ni200 pure nickel pipe

Formula Q for argon flow; K for the coefficient (generally taken K = 0.8-1.2); D for the nozzle diameter, D = (2.5-3.5d) (d for the diameter of the tungsten electrode, generally d = 3mm), so the argon flow is generally in 8-10L/min is appropriate; back shielding gas flow is generally 4-6L/min.
According to the U.S. ASMEIX welding process evaluation standard requirements for process testing, the development of Ni200 pure nickel pipeline welding process parameters as follows Table IV:
Table IV Welding process parameters
20220603092251 96953 - Welding and quality control of Ni200 pure nickel pipe

Common welding defects and preventive measures

As pure nickel has a large viscosity, poor mobility, high thermal conductivity characteristics, easy to appear in the welding porosity, welding heat cracks, not fused and other defects.

1. Porosity

As mentioned earlier, the generation of pores is mainly hydrogen, oxygen and other impurity gases in the welding due to changes in the solubility of pure nickel metal melt pool, to not escape and residual in the weld metal in the formation of saturated pores. Sources of impurity gases are mainly the following: First, the wire and welding bevel and its sides of the oil, moisture, dust and oxide film cleanup is not clean; Second, the welding argon protection is not in place, the surrounding air into the protected area; Third, the use of argon gas contains impurities, resulting in impurity elements with the argon gas into the protected area.

The main measures to prevent porosity are: careful cleaning of the wire and workpiece before welding, the wire can be cleaned with acetone, and then stored in a dry, airtight container. Weld bevel and its two sides with a special wheel or stainless steel wire brush to remove the oxide layer, and acetone or anhydrous ethanol to remove the surface of oil, water, dust and other harmful substances. (Special attention is paid to the thin-walled nickel tube is relatively soft, can not use the general beveling machine for cutting, otherwise it will squeeze the tube or cause deformation.) Choose a larger diameter torch gas protection nozzle so that it has sufficient protection area for the molten metal, and choose the appropriate gas protection device to prevent the air from oxygen, nitrogen, water vapor, etc. into the molten pool. If necessary, you can use the argon gas protection tow cover shown in Figure 1 to strengthen the protection.

20220603112301 40086 - Welding and quality control of Ni200 pure nickel pipe

Figure.1 argon protective towing hood

Argon gas protection should be obtained to silver-white or light yellow weld surface for qualified.

2. Thermal cracking

Pure nickel welding thermal cracking is mainly solidification cracking and polygonization cracking. Solidification cracking is mainly caused by S, P and other impurity elements and nickel chemistry to generate low melting point eutectic. The source of the high content of sulfur and phosphorus elements in the weld is mainly the contamination of the base material and the surface of the wire, especially some contaminants containing sulfur and lead, which are transitioned into the weld during the welding process. In addition, the alloy composition of the welding wire desulfurization element manganese content is less is also one of the reasons, because improve the manganese content can expand the solubility limit of harmful elements such as sulfur, phosphorus, etc.. Therefore, before welding the base material and wire surface for strict cleaning, improve the manganese content of the welding material, improve the purity of the welding material, the correct choice of welding material to match the base material, reduce or inhibit the transition of harmful elements to the weld, is necessary to prevent the generation of solidification cracks.
Multi-variable cracks are usually micro-cracks, which are not easily detected by the general RT-ray detection, and should be given sufficient attention. In the pure nickel material in the single-phase austenite weld to add solid solution strengthening of the large atomic radius of Mo, W, Mn, Nb and other elements to impede the movement of vacancies, dislocations, merging, can effectively prevent the generation and development of polygonal cracks in pure nickel material welds, and these large atomic radius elements can also cause lattice dot distortion, so that the directional growth of the undesirable lattice is not easy to complete, which is also to prevent polygonal cracks This is also beneficial to prevent polygonal cracking.

3. Unfused

Caused by the pure nickel material weld is not fused mainly for two reasons: one is due to the pure nickel metal viscosity, poor mobility, fast thermal conductivity, liquid-solid phase temperature range is small, the base material is not enough to fully fuse with the molten pool metal and the local unfused; but due to the pure nickel material oxide film is not removed clean, due to the melting point of the oxide film than the melting point of pure nickel is nearly 500 ℃, therefore, the local oxide film when welding may not be fully melted, resulting in the molten pool of metal inside the unfused appear. The main measures to prevent unfused defects are: increase the bevel angle, reduce the thickness of the blunt edge, while using welding oscillation, improve the mobility and wettability of the molten pool metal, so that the molten pool metal and the base material fully integrated; in addition, the workpiece bevel and both sides of the 50 mm range to fully clean up before welding, the oxide film completely removed, and grinding must be welded within 4 hours after completion, otherwise we must re-clean.

4. Other precautions

When welding pure nickel pipes, do not use the scratch arc, and do not use the arc on the base material. High frequency arc starting device should be used to start the arc at a suitable place in the bevel. When closing the arc should use the current attenuation device, do not stop the arc on the base material and arc pit must be filled when closing the arc, after closing the arc should continue to keep the gas protection for 8-10 seconds to ensure that the high temperature metal does not contact with the atmosphere.

Conclusion

Practice shows that, through the summary of the physical and chemical properties of Ni200 pure nickel pipe material and its weldability analysis, the development of a scientific and reasonable welding process, while the welding process for its prone to problems in advance to prevent, fully able to meet the quality requirements of the U.S. ASME code for pure nickel pipe welding, the company’s first pure nickel pipe welding contributed to the successful completion, but also for future similar The project also provides a useful reference for similar projects in the future.

Author: Pei denian

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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Reference:

  • [1] Liaoyixiang, chenwenwei, Li Peng Welding of Ni200 pure nickel pipe [j] Chemical engineering construction engineering, 2004:26 (1): 30-31
  • [2] Ruan Xin, Ren Lijing, wangchengjun Welding defects and quality control of nickel equipment [j] Manufacturing and installation, 2005, 22 (5):32 - 34
  • [3] Jiangjiayi, zhengshifeng Welding process optimization of pure nickel material [j] Chemical engineering construction engineering, 2002, 24 (5):45-47
  • [4] Yangrongzun Welding, supervision and inspection of pure nickel pressure pipeline [j] Welding, 2006, 12:54-55
  • [5] Yangguohui Study on welding process of imported NiSB-162-200 pure nickel [j] Electric welding machine, 2005, 35 (10): 50 - 53
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  • [7] Wan Jun Welding of nickel and nickel alloys [j] Boiler manufacturing, 2004193 (3):32 - 34

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