1.What are the difficulties in welding galvanized steel?
Influence of zinc layer: The melting point of zinc (419℃) is much lower than that of steel (1538℃). During welding, the zinc layer vaporizes rapidly, producing a large amount of smoke and pores, leading to slag inclusions, cracks and decreased mechanical properties of the weld.
Corrosion in heat-affected zone: High welding temperature will destroy the protection of the zinc layer, and the exposed carbon steel substrate is prone to rust, requiring additional anti-rust treatment.
Spatter and arc instability: Zinc vapor interferes with arc stability, causing spatter of molten pool metal and affecting weld formation.
2.What are the difficulties in stainless steel welding?
Intergranular corrosion risk: Austenitic stainless steel (such as 304) is prone to precipitate chromium carbide (Cr23C6) in the range of 450-850℃ (sensitization temperature zone), resulting in chromium depletion at the grain boundary and reducing corrosion resistance.
Hot crack tendency: Stainless steel with high nickel content (such as 316) is prone to form low-melting point eutectics during solidification, and hot cracks may occur under welding stress.
Burning of alloy elements: High welding temperature may cause oxidation of alloy elements such as chromium and molybdenum, weaken the ability to form a passive film, and reduce corrosion resistance.
3.What is the difference between pretreatment of galvanized steel and stainless steel before welding?
Galvanized steel pretreatment
Remove zinc layer: The welding area needs to be polished or mechanically cut to remove the zinc layer (width ≥ 50mm) to prevent zinc vapor from affecting the quality of the weld.
If it is impossible to completely remove the zinc, "skip welding" or intermittent welding can be used to reduce the local zinc layer heating area. Surface cleaning: Remove impurities such as oil and rust to avoid slag inclusions in the weld.
Pretreatment of stainless steel to avoid contamination: Wipe the surface with acetone or alcohol to prevent pollutants such as grease and iron filings from introducing impurities such as carbon and sulfur, which may cause intergranular corrosion or cracks.
Control stress: Preheat thick plates before welding (≤150°C) to reduce welding stress, but avoid sensitization temperature zones.
4.What are the commonly used welding methods for galvanized steel?
Manual arc welding, gas shielded welding, resistance welding (spot welding / seam welding)
5.What are the commonly used welding methods for stainless steel?
Tungsten arc welding, metal arc welding, plasma arc welding