Popular Science: Issues that need to be paid attention to when welding stainless steel pipes and copper pipes

Welding analysis of copper and austenitic stainless steel

The welding of copper and austenitic stainless steel belongs to the welding of dissimilar metals. The physical properties of the two are very different, and the melting point difference is more than 400℃, which makes welding more difficult. Whether a satisfactory welding joint can be obtained depends on the physical properties, chemical composition and welding method and process of the welded metal.

Analysis of physical property differences

The physical properties of steel and copper are very different. The thermal conductivity of copper is 2316 times that of steel. During welding, heat will be quickly transferred from the heating zone to the outside, and it is difficult for the fusion zone to reach the melting temperature, resulting in poor welding of the filler metal and the base material.

The linear expansion coefficient of copper is slightly larger than that of steel, and the shrinkage rate is 2135 times that of steel. In addition, copper has strong thermal conductivity and large deformation during cooling and solidification. The rigidity of the welded joint is large. After the welding deformation is blocked, a large welding stress will be generated, which becomes the mechanical cause of welding cracks.

Analysis of chemical composition differences

According to the principles of metallurgy, the compatibility between elements plays a decisive role in the weldability of dissimilar metals. The mutual solubility between chemical elements depends on the differences in the crystal lattice type, atomic size, and lattice constant between the solute elements. At high temperatures (912-1390°C), the crystal structures of Fe and Cu are both face-centered cubic (fcc), with similar atomic radius sizes and lattice constants, so the weldability of steel and copper is good [1]. However, when copper and steel are welded, a variety of low-melting eutectics such as (Cu+Cu2O), (Fe+Fe3S2), and (Ni+Ni3S2) are easily formed between the grains of the weld metal. In the later stage of the solidification and crystallization of the weld metal, these low-melting eutectics are continuously distributed in the form of "liquid films" on the grain boundaries of solid copper, cutting off the connection between solid crystals, weakening the intercrystalline bonding ability, and causing the plasticity of the weld metal to decrease significantly. At this time, the tensile stress generated more or less due to the significant difference in physical properties between steel and copper may cause thermal cracks in the fragile parts of the weld joint.

According to the analysis of the weldability of copper and steel, choosing the right welding method and suitable filling materials and formulating a reasonable welding process are the keys to obtaining excellent welded joints.

Welding method selection

Gas welding, argon arc welding and other welding methods can be used to weld copper and stainless steel. The temperature of the welding pool of gas welding is easy to control, and it is easy to achieve single-sided welding and double-sided forming. It is also convenient to preheat and post-heat the weldment before welding. However, due to the low temperature of the oxyacetylene flame and the dispersion of heat, it is difficult to overcome the defect of poor welding caused by the rapid heat dissipation of copper, and it is difficult to obtain good welding quality. Manual argon arc welding (TIG) has reliable argon protection, and the molten pool metal is not easy to oxidize; the welding temperature is high, the energy is concentrated, and the arc and molten pool are visible, the operation is convenient, and it is easy to control the shape of the molten pool and the formation of the weld; there is no slag, no need to clean the slag after welding, and the appearance quality of the welded joint is good, so the TIG welding method is determined.

Welding material selection

HS201 copper welding wire is selected as the filling material.

Preheating before welding

Since copper has a large thermal conductivity and dissipates heat quickly, one side of the copper tube must be preheated with a neutral flame before welding (temperature 400-450℃), and the temperature between the weld layers must be kept not lower than the preheating temperature during welding.

Key points of welding

Use short arc welding during welding, and the arc center should be biased toward the copper tube side by about 2-4mm. On the one hand, it can reduce the heat loss on the copper tube side to avoid defects such as incomplete welding and incomplete fusion; on the other hand, it can also prevent one side of the stainless steel tube from being overheated and causing burn-through and undercutting. Use continuous wire feeding, and do not use the method of directly fusing the two parent materials without adding welding wire.

Try to reduce the number of arc extinguishing during welding. Before arc extinguishing, in order to prevent the occurrence of arc pits and premature loss of protection, more filler metal should be added before arc extinguishing, and the arc should be extinguished after filling and argon gas supply should be stopped.

Heat preservation after welding

After welding, cover the weld with composite silicate felt to keep it warm and cool slowly to reduce welding stress and prevent cracks.