Selection of welding materials for nickel-based alloys

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Selection of welding materials for nickel-based alloys: 1. Classification and grades of nickel-based alloys (seven categories): ◆ Industrial pure nickel (P41): contains 99.5% nickel. Such as Ni200, Ni201. ◆Ni-Cu alloys (P42): Such as Monel 400 (Monel, Ni66Cu32), etc. ◆Ni-Cr alloys (P43): Such as 0Cr30Ni70 Corronel230 (Corronel nickel, Cr35Ni65), Inconel671 (Inconel nickel, Cr50Ni50). ◆Ni-Cr-Fe alloys (P43): Such as Inconel600 (Inconel nickel, Cr76Ni15Fe8), Inconel625 (Cr61Ni21Mo9Fe3). ◆Ni-Mo alloys (P44): Such as Hastelloy A (Hastelloy A, Ni60Mo19Fe20), Hastelloy B (0Ni65Cr28Fe5V), Hastelloy B-2 (00Ni70Mo28). ◆Ni-Cr-Mo alloys (P44): Such as Hastelloy C (Ni60Cr16Mo16W4), Hastelloy C-276 (000Ni60Cr16Mo16W4), Hastelloy C-4 (000Ni60Cr16Mo16Ti). ◆Ni-Fe-Cr alloys (P45): Examples include Incoloy 800 (Ni32Fe46Cr21) and Incoloy 825 (Ni42Fe30Cr21). 2. Welding characteristics of nickel-based alloys: Welding nickel-based alloys presents problems similar to those encountered in welding austenitic stainless steels: (1) Welding hot cracks; (2) Porosity. Compared with low-carbon steel and low-alloy steel, porosity is more likely to occur, especially in cases of unclean grooves and welding wires. (3) Corrosion resistance: Welding has little effect on the corrosion resistance of most nickel-based corrosion-resistant alloys. However, in some alloys of the Ni-Cr, Ni-Mo, and Ni-Cr-Mo series, chromium depletion occurs in the vicinity of the heat-affected zone, leading to intergranular corrosion in certain media and a decrease in stress corrosion resistance. (4) Process characteristics: Poor fluidity of the liquid weld metal ； The penetration depth of the weld metal is shallow. 3. Key points of welding process: (1) Proper selection of welding materials; (2) Joint design: Use joints with large groove angles and narrow root gaps; (3) Cleaning around the groove and the welding wire: This is crucial to prevent hot cracks and pores. (4) Preheating before welding: Preheating before welding is generally not required, and the interpass temperature should be kept below 100 °C. However, when the base material temperature is below 15°C, it should be heated to 15–20°C to prevent moisture from condensing. (5) Welding process: ◇ Limit heat input by using a low wire feed rate and maintaining stable arc voltage, and employ an operation method with a short arc that does not oscillate or only oscillates slightly. ◇During the welding of small-diameter pipes, forced cooling measures should be employed to reduce the high-temperature exposure time of the weld and increase its cooling rate. ◇Timely removal of slag and spatter from the weld surface after welding is necessary to prevent impurities such as S in the slag from causing brittleness or a decrease in the corrosion resistance of the weld. (6) Post-weld heat treatment: Post-weld heat treatment is generally not recommended. But sometimes heat treatment is required to prevent intergranular corrosion or stress corrosion during use. 4. Classification and types of welding electrodes: ★The standard GB/T13814-92 “Welding electrodes for nickel and nickel alloys” is equivalent to the American standard ANSI/AWS A5.11-89 “Specifications for coated welding electrodes for nickel and nickel alloys”. There are five types: ◆ Welding electrodes for pure industrial nickel: ENi-0, ENi-1 ◆ Welding electrodes of the Ni-Cu series: ENiCu-7 ◆ Welding electrodes of the Ni-Cr-Fe series: ENiCrFe-0 to ENiCrFe-4 ◆ Welding electrodes of the Ni-Mo series: ENiMo-1, ENiMo-3, ENiMo-7 ◆ Welding electrodes of the Ni-Cr-Mo series: ENiCrMo-0 to ENiCrMo-9 ★ Coating types: Calcium-titanium type (03), alkaline type (15, 16). 5. Classification and types of welding wires: ★According to the standard GB/T15620-1995 “Welding wires for nickel and nickel alloys”: ◆ Welding wires for pure industrial nickel: ◆ Welding wires of the Ni-Cu series: ERNiCu-7 ◆ Welding wires of the Ni-Cr series: ERNiCr-3 ◆ Welding wires of the Ni-Cr-Fe series: ERNiCrFe-5, ERNiCrFe-6 ◆ Welding wires of the Ni-Fe-Cr series: ERNiFeCr-1, ERNiFeCr-2 ◆ Welding wires of the Ni-Mo series: ERNiMo-1, ERNiMo-2, ERNiMo-3, ERNiMo-7 ◆ Welding wires of the Ni-Cr-Mo series: ERNiCrMo-1 to ERNiCrMo-4, ERNiCrMo-7 to ERNiCrMo-9. 6. Principles for selecting welding materials for nickel-based alloys: (1) Selection of welding materials when welding the same type of nickel material: ◇ Welding materials from the same alloy series as the base material should be used. ◇ If no corrosion resistance requirements exist, welding materials from a different alloy series can also be used, provided that the joint meets the required performance specifications. ■Example: Monel 400: Welding rod ENiCu-7 ； Welding wire ERNiCu-7 Inconel600: Weld rod ENiCrFe-1 ； Welding wire ERNiCrFe-5 Incoloy 800H: Weld rod ENiCrFe-2 ； Welding wire ERNiCr-3 Hastelloy C-276: Weld rod ENiCrMo-4 ； For welding wire ERNiCrMo-4 (2) in the welding of different nickel alloys as well as between nickel alloys and austenitic stainless steels, the following factors should be considered: ◇ The strength of the weld (including high-temperature creep strength), and corrosion resistance must meet the design requirements ； ◇Select welding materials with a linear expansion coefficient similar to that of the base material. ◇Consider the sensitivity of the welding material to welding cracks and pores. ■Example: ◇ Welding of austenitic stainless steel and Monel 400: electrode ENiCu-7 ； Welding wire ERNiCu-7. ◇Welding of Monel 400 and Inconel 600: Welding rod ENiCrFe-3 ； Welding wire ERNiCr-3. ◇Welding of Incoloy 800H and Hastelloy C-276: Welding rod ENiMo-3 ； Welding wire ERNiMo-3.