Weldability of alloy 825

Alloy 825 is a Ni-Fe-Cr austenitic metal material containing a small amount of Al and Ti. It has corrosion resistance, high strength, high temperature oxidation resistance, and good weldability. When the Alloy 825 is welded, due to the segregation of impurities such as S and P in the weld metal, the Ni-NiS low-melting eutectic formed by S and Ni forms a thin film between the crystals, which is easy to cause intercrystalline cracks, so S and P are harmful elements. 825 alloy has poor thermal conductivity, welding heat is not easy to diffuse, and overheating is easy to occur, causing coarse grains, increasing the thickness of the intercrystalline interlayer, and weakening the intercrystalline bonding force. It can also make the liquid solid of the weld metal exist for too long, which promotes the formation of hot cracks.

Alloy 825 has high sensitivity to thermal cracks (crystal cracks, liquefaction cracks and high-temperature deplasticization cracks). During welding, crystal cracks are most likely to be generated in the weld bead crater, forming a fire crack. Crystal cracks are formed at slightly higher temperatures above the solidus. Liquefaction cracks mainly appear in the heat affected zone near the fusion line. Liquefaction crack is a kind of micro crack that cracks along the austenite grain boundary, and its size is small, and it mostly appears in the depression area of ​​the weld line and the front layer weld of the multi-layer welding. The cause of liquefaction cracks is generally considered to be the melting of the low melting point eutectic on the austenite grain boundary at high temperature in the heat-affected zone or the multi-layer welding seam during welding, and the plasticity and strength of the metal drop sharply. It is formed by cracking along the austenite grain boundary under the action of tensile force. High-temperature loss-of-plasticity cracks generally occur in heat-affected zones or welds.
The liquid metal of the weld metal of 825 alloy is very poor. The weld seam is not like steel. The weld metal is easy to wet and spread. Even if the welding current is increased, the fluidity of the weld metal cannot be improved, but it has a counterproductive effect. Due to the poor fluidity of 825 alloy liquid weld metal, it is not easy to flow to both sides of the weld, so it has a good weld morphology. When welding, the swing process is required, but it should be a small swing, and the swing distance cannot exceed 2.5 times the diameter of the electrode and the wire. In order to better control the metal filling of the weld joint, the groove angle of the Alloy 825 joint should be larger in order to realize the swing welding process.
The contaminants on the groove surface of alloy 825 are mainly surface oxide scale and elements that cause embrittlement. The melting point of the oxide scale on the surface of alloy 825 is much higher than that of the base material, and often forms inclusions or fine discontinuous oxides. Due to the presence of elements such as S and P that will increase the thermal cracking tendency of Alloy 825, the 25mm area on both sides of the weld must be completely cleaned and polished before welding. After processing the bevel, the 50mm area inside and outside the bevel should be cleaned with acetone solution to completely remove impurities such as oil and grease and conduct penetration testing.