Weldability of Alloy 825

825 alloy is a kind of austenitic metal material containing less AL and Ni-Cr-Fe. It has corrosion resistance, high strength, high temperature oxidation resistance and good weldability. When 825 alloy is welded, since S, P and other impurities are segregated in the weld metal, the Ni-NiS low-melting eutectic formed by S and Ni forms a thin film between the crystals, which easily causes intergranular cracks, so S and P are harmful elements. The alloy 825 has poor thermal conductivity, and the welding heat is not easily diffused, which tends to cause overheating, resulting in coarse grains, increasing the thickness of the intercrystalline interlayer and weakening the intergranular resultant force. It also enables the liquid solidification of the weld metal to be too long, which promotes the formation of hot cracks.
Alloy 825 has high sensitivity to hot cracking (crystal cracking, liquefaction cracking and high temperature deplasticization cracking). When welding, crystal cracks are most likely to occur in the weld crater, forming a crack in the fire. The crystal crack is formed at a slightly higher temperature above the solidus line. Liquefaction cracks mainly occur in the heat affected zone near the fusion line. Liquefaction crack is a microcrack that cracks along the austenite grain boundary. Its small size is mostly found in the recessed area of ​​the weld fusion line and the front layer weld of the multilayer weld. The cause of liquefaction cracks is generally considered to be that the low-melting eutectic on the austenite grain boundary is re-melted at the high temperature in the heat-affected zone or the multi-layer weld-welded layer. 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 plastic loss cracks generally occur in heat affected zones or welds.

The fluid metal of the weld of Alloy 825 is very poor. The weld is not like steel. The weld metal is easy to wet and expand. Even if the welding current is increased, the fluidity of the weld metal cannot be improved, but it is counterproductive. Due to the poor fluidity of the 825 alloy liquid weld metal, it is not easy to flow to both sides of the weld, so it has a good weld appearance. In the welding, the swinging process is required, but it should be a small amplitude swing, and the swinging distance should not exceed 2, 5 times the diameter of the welding rod and the welding wire. In order to better control the metal filling of the weld joint, the groove angle of the 825 alloy joint should be larger to achieve the swing welding process.
The pollutants on the surface of the 825 alloy groove are mainly surface oxide scale and the element causing embrittlement. 89. The surface oxide scale of the alloy material is much higher than that of the base material, and often forms inclusions or fine discontinuities. Since the presence of S, P and other elements will increase the tendency of hot cracking of Alloy 825, it is necessary to weld the inner and outer sides of the joint 25 mm before welding and the area is completely cleaned and polished. After the groove is finished, the 50mm area inside and outside the groove should be cleaned with acetone solution to thoroughly remove impurities such as grease and perform penetration test.