Classification and grades of nickel-based alloys (seven types):
- Industrial pure nickel (P41): 99.5% nickel. Such as Ni200, Ni201.
- Ni-Cu alloy (P42): such as: Monel 400 (Ni66Cu32) and so on.
- Ni-Cr alloy (P43): such as: 0Cr30Ni70, corronel230 (Cr35Ni65), inconel671 (Cr50Ni50).
- Ni-Cr-Fe alloy (P43): such as: inconel600 (Cr76Ni15Fe8), inconel625 (Cr61Ni21Mo9Fe3).
- Ni-Mo alloy (P44): such as: hastelloy A (Ni60Mo19Fe20), hastelloy B (0Ni65Cr28Fe5V), hastelloy B-2 (00Ni70Mo28).
- Ni-Cr-Mo alloy (P44): such as: hastelloy C (Ni60Cr16Mo16W4), hastelloy C-276 (000Ni60Cr16Mo16W4), hastelloy C-4 (000Ni60Cr16Mo16Ti).
The welding of nickel-based alloys has similar problems to the welding of austenitic stainless steels:
- (1) Welding hot cracks.
- (2) Porosity: Compared with low carbon steel and low alloy steel, porosity tends to be greater, especially for dirty grooves and welding wires.
- (3) Corrosion resistance: Most nickel-based corrosion-resistant alloys have little effect on corrosion resistance after welding. However, for some alloys of Ni-Cr, Ni-Mo, and Ni-Cr-Mo series, chromium depletion occurs near the heat-affected zone, resulting in intergranular corrosion and stress corrosion performance in some media.
- (4) Process characteristics: poor fluidity of liquid weld metal; shallow weld metal penetration.
Key points of the welding process:
(1) Correct selection of welding materials.
(2) Joint type: a joint type with a large bevel angle and a small pure edge.
(3) Cleaning near the groove and welding wire: the characteristics are important to prevent thermal cracks and pores.
(4) Preheating before welding: generally, preheating before welding is not required, and the temperature between layers should be controlled below 100 ℃. But when the temperature of the base material is lower than 15℃, it should be heated to 15-20℃ to avoid moisture condensation.
(5) Welding process:
- Limit the heat input, adopt a small amount of wire and keep the arc voltage stable, and adopt the operation method of short arc without swing or small swing.
- For the small diameter pipeline welding process, forced cooling measures should be taken to reduce the high temperature residence time of the weld and increase the cooling rate of the weld.
- Timely cleaning of slag and spatter on the surface of the weld after welding to prevent S and other impurities in the slag from embrittlement or deterioration of the weld.
(6) Post-weld heat treatment: Generally, post-weld heat treatment is not recommended. But sometimes heat treatment is required to ensure that no intergranular corrosion or stress corrosion occurs during use.
Classification and model of welding rod:
GB/T13814-92 "Nickel and Nickel Alloy Electrodes" standard is equivalent to American ANSI/AWS A5.11-89 "Nickel and Nickel Alloy Covered Electrodes":
- Industrial pure nickel electrode: ENi-0, ENi-1;
- Ni-Cu series electrode: ENiCu-7;
- Ni-Cr-Fe series electrodes: ENiCrFe-0 to ENiCrFe-4;
- Ni-Mo series electrodes: ENiMo-1, ENiMo-3, ENiMo-7;
- Ni-Cr-Mo series electrodes: ENiCrMo-0 to ENiCrMo-9;
- Coating type: titanium calcium type (03), alkaline (15, 16).
Classification and model of welding wire:
According to GB/T15620-1995 "Nickel and Nickel Alloy Welding Wire":
Industrial pure nickel welding wire:
- Ni-Cu series welding wire: ERNiCu-7;
- Ni-Cr series welding wire: ERNiCr-3;
- Ni-Cr-Fe series welding wire: ERNiCrFe-5, ERNiCrFe-6;
- Ni-Fe-Cr series welding wire: ERNiFeCr-1, ERNiFeCr-2;
- Ni-Mo series welding wire: ERNiMo-1, ERNiMo-2, ERNiMo-3, ERNiMo-7;
- Ni-Cr-Mo series welding wire: ERNiCrMo-1 to ERNiCrMo-4, ERNiCrMo-7 to ERNiCrMo-9.
(1) Selection of welding materials when welding the same nickel material:
- The same welding materials as the base metal alloy series should be selected;
- If there is no requirement for corrosion resistance, welding materials different from the base metal alloy system can also be used, but the joints should be guaranteed to have the performance required by the design.
- Monel 400: welding rod ENiCu-7; welding wire ERNiCu-7;
- Inconel600: welding rod ENiCrFe-1; welding wire ERNiCrFe-5;
- Incoloy 800H: welding rod ENiCrFe-2; welding wire ERNiCr-3;
- Hastelloy C-276: welding rod ENiCrMo-4; welding wire ERNiCrMo-4.
(2) The following factors should be considered for the welding of dissimilar nickel materials and between nickel materials and austenitic stainless steel:
- The strength of the weld (including high temperature durability) and the corrosion resistance meet the design;
- Use welding materials with a coefficient of linear expansion similar to that of the base metal.
Consider the sensitivity of welding materials to welding cracks and pores.
- Austenitic stainless steel and Monel 400 welding: welding rod ENiCu-7; welding wire ERNiCu-7.
- Monel 400 and Inconel600 welding: welding rod ENiCrFe-3; welding wire ERNiCr-3.
- Incoloy 800H and Hastelloy C-276 welding: welding rod ENiMo-3; welding wire ERNiMo-3.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)