Burn-out of alloying elements
Burn-off is the loss of alloying elements from the weld pool by evaporation or oxidation in the arc. This makes the weld leaner in alloying elements and loses strength, toughness, or corrosion resistance. The welder must compensate for this loss with suitable filler materials or protective measures.
Physico-Chemical Mechanism of Action
In the arc, temperatures exceed 3000 degrees Celsius. Alloying elements with low boiling points evaporate. These include, for example, manganese (boiling point 2061 degrees Celsius) and zinc (907 degrees Celsius). Other elements like titanium or aluminum react with oxygen from the air or from oxides. They form oxides that float on the surface as slag or escape as fume. This reduces the proportion of these elements in the molten metal.
Metallurgical Relevance & Risks in Welding
The loss of alloying elements changes the properties of the weld. Less manganese and silicon means poorer deoxidation and lower strength. Less chromium means lower corrosion resistance. The steel can become softer or lose its toughness. Porosity, cracks, and insufficient mechanical properties can be the result. Burn-off is especially critical for high-alloy steels.
Workshop Practice & Instructions
Choose a filler material with a higher alloy content than the base material. This compensates for burn-off. Example: In MAG welding, manganese burn-off can be 10 to 20 percent. Therefore, use a welding wire with a correspondingly higher manganese content. Pay attention to the correct shielding gas composition. Too much active gas like CO2 increases oxidation. Work with a short arc or pulse technique. This reduces overheating of the weld pool. Electrodes with basic coating protect the weld metal better from oxidation.