Basicity is one of the main factors that describe welding flux properties. It not only has impact on the welding performance, but also on weld metal mechanical properties. Many articles have reported the impact on impact toughness within a wide basicity range. However, no report about impact on welding performance and weld metal mechanical properties within a narrow basicity range is found. Especially under conditions of high speed welding of pipes, mixed welding flux of agglomerated flux and fused flux is used. Moreover, pipes for different purpose require different impact toughness. A study of basicity impact is necessary.

Impact on welding performance

In study of impact on performance, we set undercuts, slag detachability, weld bead smoothness, weld bead color as criteria to evaluate basicity impact.

As basicity changes from 2.2 to 0.7, undercut numbers decrease gradually, and finally disappear. This is caused by two reasons:

1. As basicity goes down, matters of high melting point, such as MgO, Al₂O₃, CaO decrease, matters of low melting point, such as MnO, SiO₂, increase. This makes flux melting point relatively lower. Slag length also grows longer. Under the same welding parameters, welding flux of low basicity shows large melting amount, good flowability, long existence time of weld puddle, which make the edge of weld metal react thoroughly and eliminate undercuts.
 

2. As content of MgO, Al₂O₃, CaO decreases, and content of MnO, SiO₂ increases, surface tension of slag decreases. This also aids to reduce undercuts.

For the same reason, basicity has impact on weld bead smoothness, weld bead color, slag detachability.
 

When basicity is between 1.0 and 1.6, slag detachability and welding performance is the best.

Impact on tensile strength

When basicity is 2.2 or 0.7, tensile strength is at the maximum level. When basicity is 0.7, it is acid welding flux. When basicity is 2.2, it is basic welding flux. If used together with H08C alloy SAW wire, the combination could improve strength greatly.

Impact on impact toughness

When welding flux basicity changes from 2.2 to 0.7, weld metal toughness goes down. This is a general rule between basicity and toughness. When basicity is within the range of 2.2-1.9, 1.6-1.3, 1.0-0.7, toughness won’t change too much. As known, under the same welding conditions (base metal, wires, parameters), toughness is determined by weld metal structure, impurities in weld metal and its content & distribution. All these factors are mainly decided welding flux. Within the basicity range, structure, impurities content and distribution don’t change too much. This is possibly due to neutral Al2O3 contained in welding flux. It shares both the properties of acid oxide and basic oxide. Within the range, metallurgy activeness of slag, impurities size & content & distribution doesn’t change or just slightly.

Impact on structure of weld metal

When basicity is 2.2-1.9, weld metal structure is mainly made of acicular ferrite, proeutectoid ferrite and pearlite. It is because within this range, slag contains much Al₂O₃. When basicity is 1.6-1.3, weld metal structure composition doesn’t change. However, proeutectoid ferrite content increases due to basicity decline. When basicity is 1.0-0.7, it forms acid slag system, CaO-SiO₂-MnO, MnO-SiO₂. Weld metal consists of mainly coarse grain boundary allotriomorphs and some pearlite. These structure has bad toughness.

Conclusions

1. When basicity changes from 2.2-0.7, undercuts would decrease and disappear, weld bead color changes from grey to blue, weld metal surface becomes smooth, slag detachability also changes.

2. Tensile strength is at maximum level between 2.2-0.7.

3. As basicity changes from 2.2-0.7, impact toughness goes down. Within the range of 2.2-1.9, 1.6-1.3, 1.0-0.7, toughness doesn’t change too much.

4. When basicity changes from 2.2-0.7, microstructure of weld metal changes obviously.