Surface pretreatment before stainless steel flange laser welding is a crucial step in ensuring welding quality. Its core objective is to thoroughly remove surface contaminants and optimize the material's surface condition, providing a stable foundation for flange laser welding. This process involves multi-dimensional technical operations and requires a systematic approach considering both material properties and welding process requirements.
Surface cleaning is the primary task of pretreatment. During processing, transportation, and storage, stainless steel flanges easily accumulate organic contaminants such as oil, cutting fluid, and fingerprints, as well as inorganic contaminants such as dust and metal particles. If these contaminants are not removed, they will vaporize or decompose at high temperatures during flange laser welding, generating gases that lead to defects such as weld porosity and inclusions. Cleaning methods must be selected based on the type of contaminant: organic contaminants are typically removed by soaking in alkaline solutions or wiping with organic solvents, utilizing saponification or dissolution; inorganic contaminants are removed by compressed air blowing, ultrasonic cleaning, or wiping with acidic solutions. After cleaning, the flanges must be rinsed with pure water and dried to prevent residues from affecting the welding process.
Rust removal and oxide scale removal target the oxide layer formed on the stainless steel flange surface due to heat treatment or long-term exposure. The presence of oxide scale reduces laser energy absorption, leading to insufficient weld penetration or incomplete fusion. Pickling is a common method, involving immersion in a mixture of nitric acid and hydrofluoric acid or a specialized rust remover to dissolve the oxide scale. For thick oxide scale or localized rust, the surface layer can be removed first by mechanical grinding or sandblasting before pickling. After pickling, residual acid must be neutralized and rinsed immediately to prevent over-acid corrosion of the substrate.
Surface roughness adjustment needs to be based on welding process requirements. If machining marks, burrs, or scratches exist on the flange surface, they must be removed by mechanical polishing, grinding, or tumbling. Mechanical polishing uses sandpaper and polishing wheels to gradually grind the surface until the roughness reaches Ra 0.8μm or less, reducing laser reflection loss. Tumbling is suitable for small flanges, removing surface defects through friction media. After treatment, surface flatness must be checked to avoid laser focusing deviation caused by localized protrusions.
Activation treatment is a key step in improving flange laser welding efficiency. The naturally formed passivation film on stainless steel surfaces hinders laser energy absorption. Weak corrosion activation can break down the passivation layer, exposing the active metal crystalline structure. Common methods include immersion in dilute nitric acid or electrochemical activation, creating micro-corrosion pits on the surface and increasing laser absorption. Rapid drying is necessary after activation to prevent re-oxidation.
Pretreatment for specific applications requires enhanced protection. Flanges used in humid or corrosive environments must undergo passivation immediately after pretreatment, immersing in nitric acid or chromate solutions to form a dense oxide film on the surface, improving corrosion resistance. For food-grade or pharmaceutical-grade applications, environmentally friendly cleaning agents free of chlorine and heavy metals must be used to avoid product contamination by residues.
The effectiveness of pretreatment must be strictly controlled. Cleanliness can be verified through a water film rupture test; a continuous, unbroken water film indicates thorough cleaning. The degree of oxide scale removal can be tested using the blue dot method; the absence of blue spots indicates sufficient activation. Furthermore, the surface must be inspected for cracks, inclusions, and other defects to ensure that pretreatment has not damaged the substrate. Surface pretreatment before stainless steel flange laser welding is a systematic project integrating materials science, surface engineering, and welding technology. From cleaning and rust removal to activation, each step requires precise control to eliminate potential factors affecting weld quality. Through scientific pretreatment processes, the stability and weld performance of flange laser welding can be significantly improved, providing reliable assurance for high-end equipment manufacturing.
