Dawn Chapter — Discussion on the Weldability of Nickel and Nickel-Based Welding Wires

一、 Weld Heat Cracking
1. Crystallisation Cracks:
During welding, weld metal in nickel and nickel-based corrosion-resistant alloys, as well as nickel-based high-temperature alloys, exhibits significant susceptibility to crystallisation cracking. This is primarily associated with the following factors.
1.1: Industrial pure nickel, nickel-based corrosion-resistant alloys, and nickel-based high-temperature alloys contain numerous alloying elements. Their microstructure consists of single-phase austenite, which has limited solubility for these alloying elements. These elements react with nitrogen (N) and iron (Fe) in the matrix to form low-melting-point eutectic phases. These phases segregate at grain boundaries and, under welding stresses, induce crystallisation cracks.
1.2: Furthermore, during solidification, the weld metal forms highly directional, single-phase austenitic columnar crystals. When low-melting-point alloys segregate between these columnar crystals, intergranular cracking readily occurs under welding stresses.
1.3: S, P, and Si are the primary elements responsible for forming low-melting-point liquid films at grain boundaries.
2. Multilocular Cracks
Polyhedral cracks constitute another form of hot cracks, typically manifesting as microcracks. Under stress, they may propagate into macroscopic cracks under severe conditions. These cracks nucleate at secondary boundaries due to the migration and aggregation of vacancies and dislocations, subsequently propagating along polyhedral boundaries. They frequently occur in multi-pass welds subjected to repeated heating, often distant from the fusion zone.
3. Liquidisation Cracks
Liquidisation cracks represent a common thermal cracking defect in nickel and nickel-based alloy welds. These are minute cracks propagating along austenitic grain boundaries, predominantly occurring within the fusion zone of welds and the interpass overheated zones of multi-pass welds. Such liquidisation cracks are particularly sensitive in nickel-based alloys and warrant significant attention.
二、 Strain-ageing Cracks
Strain-ageing cracks frequently manifest in welded structures of precipitation-hardened nickel-based alloys. They arise during post-weld heating when residual stresses relax and secondary hardening phases precipitate, yet the joint’s strain capacity proves insufficient to withstand the internal stresses generated within the metal. Such cracks are termed strain-ageing cracks. In essence, they also fall within the scope of reheat cracks. Strain-age cracks in nickel-based alloy structural components similarly nucleate along grain boundaries and propagate progressively, exhibiting characteristics of intergranular cracking. Such cracks typically originate in the coarse-grained zone of the heat-affected region and gradually extend into the fine-grained zone of the joint. Their formation is related to the degree of intragranular strengthening and intergranular weakening.
三、 Equivalent Strength Issues in Welded Joints
3.1: Welded joints in nickel-based alloys generally fail to achieve equivalent strength to the base material in the as-welded condition. Whether in solution-strengthened or precipitation-hardened alloys, the joint’s high strength and ductility are reduced compared to the base material.
3.2: In solution-strengthened alloys with low aluminium or titanium content, precipitation hardening may occur in the high-temperature regions of the heat-affected zone during welding. Particularly under prolonged ageing conditions, this hardening phenomenon can adversely affect joint properties and warrants attention.
3.3: Overheating grain growth is commonly observed in the heat-affected zone of nickel and nickel-based alloy welds, leading to reduced ductility and toughness. This effect is generally not adequately remedied by heat treatment, and its severity increases with the width of the overheated zone.
3.4: When welding nickel-based alloys in the aged condition, partial re-solution occurs within the high-temperature overheated zone. This effectively softens the material, resulting in reduced strength.
四、 Porosity
Nickel and nickel-based alloys exhibit heightened susceptibility to porosity formation. Residual moisture, oil residues, or oxides on the weld surface, if inadequately removed, can impair the fluidity of the weld pool during nickel alloy welding, occasionally leading to significant porosity. Such porosity frequently localises near the fusion line. Production experience indicates that thorough preparation of the weldment, selection of suitable welding consumables, and application of appropriate welding parameters generally prevent porosity occurrence.