Effect of chemical composition on root bead surface properties in TIG arc welding. Effect of chemical composition on weldability in fabrication of high-alloy steel (Report 4)

摘要

1 During TIG arc welding without back shielding, an increase in the Cr content impairs the root bead surface properties, whereas the Si content tends to improve the surface properties.2 The oxides tend to grow thicker with an increasing Cr content and conversely to grow thinner with an increasing Si content.3 The oxides formed on the root bead surface have a laminated structure, consisting of Si-rich oxides in the inside layer and composite oxides containing Fe, Mn, etc in the outside layer. When the root bead surface has superior properties, the inside Si oxide layer is uniformly and continuously formed.4 The results of the simulated welding thermal cycle tests suggest that Fe, Cr, Mn, Si, etc are simultaneously oxidised to form the composite oxide layer during the initial period of oxidation after completion of welding and that the inside Si oxide layer is thereafter formed. At a high Si content, the inside Si oxide layer forms and grows in a short time.5 Improvement of the root bead surface properties with an increased Si content in low-Cr steel is due to the role played by the Si oxide layer continuously and uniformly formed on the inside in obstructing diffusion, so that subsequent growth of oxides is suppressed, and the root bead surface is covered with a thin and uniform oxide layer.6 On the other hand, in high-Cr steel, the equilibrium oxygen pressure of Cr_2O_3 decreases, high-concentration Cr is oxidised simultaneously with the Si at the oxide layer/metal interface at reduced oxygen pressure, hampering uniform Si oxide layer formation and accordingly impairing the root bead surface properties.