Deoxidation
Deoxidation binds dissolved oxygen in the metal melt to elements with high oxygen affinity such as silicon, manganese, or aluminum. These oxides rise and are removed as slag. Without deoxidation, pores and brittle inclusions form in the weld.
Physicochemical Mechanism of Action
In liquid steel, oxygen is dissolved atomically. Upon solidification, the solubility of oxygen drops abruptly. The oxygen wants to escape as gas or combines with iron to form iron oxide. Both damage the weld. During deoxidation, elements with higher oxygen affinity are added, such as silicon, manganese, or aluminum. These elements immediately bind the oxygen as solid or liquid oxides. The oxides rise due to lower density and are skimmed off as slag. This leaves almost no free oxygen in the steel.
Metallurgical Relevance & Risks in Welding
Without sufficient deoxidation, gas bubbles form during solidification. These pores weaken the weld and can lead to leaks. At the same time, hard, brittle oxide inclusions are created that act as notches and reduce toughness. Under dynamic loading, such welds fail prematurely. Too much deoxidizer is also harmful: the excess silicon or manganese shifts the structure to higher hardness and can promote hot cracking.
Workshop Practice & Action Instructions
Praktitioners choose the filler material based on the base material and surface. For unalloyed steel, wire or stick electrode with sufficient manganese and silicon is used (e.g., G3Si1 with 1.5% Mn and 0.8% Si). For rusty or scaled sheets, increase the deoxidation content or clean the weld edge mechanically beforehand. Remove slag completely after each pass to avoid oxide inclusions.