During metal welding, the key to preventing weld oxidation lies in scientifically adjusting the shielding gas flow and composition to create a stable and efficient inert gas environment, thereby isolating reactive gases such as oxygen and nitrogen from the air. This process requires a systematic protection strategy that integrates material properties, welding process, and environmental conditions.
The selection of shielding gas composition is fundamental to preventing oxidation. Inert gases such as argon are the most commonly used shielding gases in metal welding due to their stable chemical properties and moderate ionization potential, enabling stable arc formation and uniform weld pool coverage. For example, in welding easily oxidizable metals such as aluminum alloys and stainless steel, pure argon shielding can effectively reduce oxide formation and improve weld corrosion resistance. For mild steel and low-alloy steel, carbon dioxide is often mixed with argon due to its low cost and controllable oxidizing properties. This not only reduces spatter with argon but also reduces oxygen content in the weld through the oxidative properties of carbon dioxide. The gas mixture ratio should be adjusted according to the material thickness. For example, a higher proportion of argon can be used for welding medium-thick plates, while a higher proportion of carbon dioxide can be used for arc stability when welding thin plates.
Controlling shielding gas flow must balance shielding effectiveness and process stability. Too low a flow rate can lead to inadequate shielding, exposing the molten pool to air and causing oxidation, nitriding, and porosity. Excessive flow can create turbulence, entraining air that weakens shielding effectiveness and even collapses the molten pool in thin plates. For example, when welding thin stainless steel plates, using a low-flow gas flow prevents molten pool collapse while maintaining a silvery-white weld luster. The flow rate range should be determined based on plate thickness, current level, and joint type. Typically, the weld surface should exhibit a uniform metallic luster and lack of oxidation color. For welding large structures, a local sealing method combined with low-flow gas flow can be used to conserve gas while ensuring shielding effectiveness.
Coordinated adjustment of welding process parameters and shielding gas is crucial. Excessively fast welding speeds shorten the molten pool's lifespan, reducing gas shielding time and increasing the risk of oxidation. Excessively slow speeds prolong high-temperature exposure, exacerbating oxidation. Current and voltage must be matched to the gas flow rate. When welding at high currents, the flow rate should be increased to accommodate more intense arc and molten pool activity. For example, in laser welding, the shielding gas must cover both the molten pool and the newly solidified area. Side-blowing is used to expand the shielding range and prevent oxidation of high-temperature hydrogen- and oxygen-absorbing materials such as titanium alloys.
Specific materials and working conditions require tailored shielding strategies. Titanium-stabilized steels have extremely high shielding requirements. When ambient wind speeds reach a certain level, even with high-purity argon, titanium loss can significantly increase. In these cases, a windbreak or a vacuum chamber welding setup is necessary. Adding nitrogen to ultra-low carbon steels can cause chromium nitride precipitation. Therefore, an argon-hydrogen mixture should be used to utilize the reducing effect of hydrogen to convert chromium oxide into metallic chromium. For pipelines that are extremely long or have limited space, direct argon filling of the weld joint or local sealing with water-soluble paper and copper liner can be used to reduce argon consumption while ensuring shielding effectiveness.
Operating procedures and environmental control are fundamental to ensuring shielding gas effectiveness. Before welding, thoroughly clean the weldment's surface of oil and rust, and purge with inert gas if necessary. During welding, maintain a sealed system to prevent gas leaks that could lead to unstable flow rates. Operators must receive training and master the techniques for adjusting gas flow rates, feed position, and angle. For example, when performing argon arc welding on stainless steel, the back of the weldment should be filled with argon beforehand. Continuous argon filling should be maintained during welding, and the argon hose should be disconnected after welding to ensure the weld is fully protected.
Optimizing equipment and testing methods can improve the accuracy of shielding gas control. Using high-precision flow meters and digital gas controllers automatically adjusts the flow rate based on welding parameters, reducing human error. Regularly inspect the gas system to ensure leaks in tanks, pipelines, and valves to avoid interruptions or unstable gas quality. Use gas analyzers to monitor the purity of the shielding atmosphere and adjust the gas supply strategy promptly.
Systematic adjustment of shielding gas composition and flow, combined with optimized process parameters and standardized operating procedures, can effectively prevent weld oxidation during metal welding. This process requires comprehensive consideration of material properties, welding methods and environmental conditions, and the establishment of a full-process guarantee mechanism from pre-weld cleaning, gas ratio, parameter monitoring to post-weld inspection, ultimately achieving the stability of weld metal properties and improved corrosion resistance.
