Welding flux composition

In this article, we summary the influences of each welding flux component (agglomerated flux) on welding performance, especially when basicity is within 1.0-2.2. Below are the functions of oxidants in welding flux composition:

10-25% CaF₂

F₂ gas generated by decomposition of CaF₂, which has the same effect of carbonate, reduces the partial pressure of arc atmosphere. CaF₂ decomposes during submerged arc welding and reduces hydrogen content of weld metal. As a basic oxidant, CaF₂ could increase basicity of slag and improve low temperature impact toughness of weld metal.

15-25% SiO₂

SiO₂ is slag-forming agent, and important component that keeps arc stable. When SiO₂ content is lower than 12%, arc is unstable, even arc break occurs. When SiO₂ content is higher than 20%, submerged arc welding flux could transfer Si to weld metal. Slag becomes too viscous, fluidity declines. In addition, SiO₂ is surface active agent, which could reduce slag surface tension. It lowers adhesion between slag and liquid metal and improves slag detachability of agglomerated flux. SiO₂ is acid oxidant. It plays the roles of lowering slag basicity, reducing slag melting point, improving weld bead formation. SiO₂ is added in the form of mineral powders, silica, Ferrous alloy, water glass, etc. SiO₂ adding by different ways could improve comprehensive welding performance.

20-30% MgO

MgO is a very good slag-forming agent and it is used to adjust slag basicity. For basic welding flux, MgO could improve impact toughness of weld metal, and reduce diffusible hydrogen content in weld bead. Melting point of MgO is relatively high. It could increase viscosity of slag, and suppress fluidity of slag. When MgO content is higher than 30%, it makes weld bead formation bad.

Tests prove that, when MgO content is higher than 30%, slag detachability goes bad. It is because MgO can increase surface tension of slag. Especially in groove welding, slag can’t flow inside groove freely. Too early solidification makes weld metal can’t transit to groove smoothly, slag removal is also difficult. Welding defects like undercut, slag inclusion would occur.

15-25% Al₂O₃

Al₂O₃ is to adjust slag viscosity and fluidity. When Al₂O₃ content is higher than 20%, viscosity is too high. It is easy to form undercuts and pits on weld bead. Al₂O₃ could also increase surface tension of slag. If addition of Al₂O₃ is too high, it is hard for slag detachability improvement. Al₂O₃ is added in the form of corundum and bauxite.

5-15% CaO

CaO in agglomerated flux could increase slag basicity and improve weld metal mechanical properties. Generally CaO is added in the form of marble. Disadvantage is, during welding marble decomposes and produces much gases, which result in too high gas pressure in arc area, weld indentation on weld pass, fumes & smokes, air pollution.

1-3% TiO₂

TiO₂ proportion in welding flux composition is small, but it plays an important role. TiO₂ lowers melting point of slag, improves slag plasticity and fluidity. Thus, it is good for weld pass formation.

18-20% K-Na water glass

Water glass is used as binding agent. Most materials in water glass is SiO₂, K₂O, Na₂O. Water glass obviously improves arc stability, granular strength and resistance to water absorption.