Welding Slag Essentials: Improve Your Metalwork

The Role of Slag in Welding Process

The core of welding rods, fluxes, and core wires in the welding process undergo a series of chemical and metallurgical changes after being heated and melted, forming non-metallic substances covering the surface of the weld, known as welding slag. Welding slag plays a crucial role in the metallurgical process of welding. The main functions of welding slag are as follows:

Mechanical Protection

During welding, the flux is heated, releasing gas and forming slag, which isolates the liquid metal from the air, protecting the liquid weld pool and preventing harmful gases such as nitrogen from entering the welding area. The solidified slag shell covering the weld surface can prevent the oxidation of the high-temperature weld metal and slow down the cooling rate of the weld metal.

Metallurgical Treatment

The slag formed by the melting of the flux (or flux) undergoes a series of metallurgical reactions with the liquid metal, significantly affecting the chemical composition of the weld pool.

When combined with the welding core, slag can remove harmful impurities (such as O, N, H, S, P, etc.) from the molten weld, protect or introduce beneficial alloying elements, and provide the weld metal with strong resistance to porosity, crack resistance, and mechanical properties that meet usage requirements.

Improvement of Welding Process Performance

Good welding process performance is crucial to ensuring smooth welding chemical reactions and obtaining high-quality welds.

The flux of the welding rod generally contains easily ionizable substances, forming slag with suitable composition and properties, making the arc easy to ignite, ensuring stable burning of the welding arc, reducing spatter, achieving aesthetically pleasing weld bead formation, facilitating slag removal, and enabling welding in various spatial positions.

Under certain conditions, welding slag may also have adverse effects, such as burning alloying elements in the weld metal, creating porosity, slag inclusions, welding defects, and causing difficulties in slag removal, thereby affecting welding productivity.

To ensure that welding slag functions as intended, it is crucial to adjust and control the chemical composition and mass fraction of the slag, enabling it to possess suitable physical and chemical properties.

Composition, Classification, and Characteristics of Slag

(1) Composition of Slag

The chemical composition of commonly used flux welding slag is shown in Table 1-3.

Table 1-3 Chemical Composition of Common Welding Slag (Mass Fraction)

Medicated Patch TypesSiO2TiO2Al2O3FeOMnOCaOMgONa2OK2OCaF2AlkalinitySlag Types
Titanium Iron Type29.2141.115.627.68.71.31.41.11.26Oxide Type
Titanium Type23.438.7106.911.73.70.52.22.90.45Oxide Type
Titanium Calcium Type25.130.23.59.513.78.85.21.72.30.74Oxide Type
Cellulose Type34.717.55.511.914.42.15.83.84.30.81Oxide Type
Iron Oxide Type40.41.34.523.321.31.34.61.81.51.22Oxide Type
Low Hydrogen Type24.171.543.537.50.80.820.81.44Salt-Oxide Type
Note: The formula for basicity calculation is BL = 6.05CaO + 4.8MnO + 4.0MgO + 3.4FeO – 6.31SiO2 – 4.97TiO2 – 0.2Al2O3

(2) Classification and Characteristics of Welding Slag

Salt-based Slag: Primarily composed of metal fluorides, chlorides, and oxygen-free compounds, such as CaF2-NaF, CaF2-BaCl2-NaF, KCl-NaCl-Na3AlF, BaF2-MgF2-CaF2-LiF, and other slag systems. These slags exhibit minimal oxidizing properties and are mainly used for welding aluminum, titanium, and other chemically reactive metals and alloys. At times, they can also be used for welding high-alloy steels containing reactive elements.

Salt-oxide-based Slag: Mainly composed of fluorides and strong metal oxides, such as commonly used CaF2-CaO-SiO2, CaF2-CaO-SiO2, CaF2-CaO-Al2O3-SiO2, CaF2-CaO-MgO-Al2O3, and other slag systems. Due to their relatively low oxidizing properties, these slags are primarily used for welding high-alloy steels.

Oxide-based Slag: Primarily composed of metal oxides, such as widely used MnO-SiO2, FeO-MnO-SiO2, CaO-TiO2-SiO2, and other slag systems. These slags are mainly used for welding low carbon steels and low-alloy steels.

The slag formed by flux and flux-cored welding wire can be classified into two major categories based on alkalinity: acidic slag and alkaline slag. The different alkalinity results in distinct structures of the slag, which in turn reflect different physical and chemical properties, significantly impacting welding process performance.

Observations of the solidified slag fracture surface reveal that the alkalinity greatly affects the fracture state of the solidified slag. Alkaline slag fractures exhibit a distinct crystalline or stony appearance, while acidic slag fractures display a distinct glassy appearance. Neutral slag fractures are a mixture of crystalline and glassy, with the glassy appearance predominating.

(3) Melting Points and Densities of Primary Compounds in Welding Slag

The melting point and density of welding slag directly impact the process performance of the welding rod and also have significant effects on the welding metallurgy process. The melting temperature of the coating on the welding rod is referred to as the slag-making temperature, which differs from the melting point of the slag by approximately 100-200°C.

Generally, it is required that the slag-making temperature of the coating is 100-250°C lower than the melting point of the welding rod. The melting point of the slag depends on the chemical composition of the slag. The melting points and densities of several primary compounds in welding slag are listed in Table 1-4.

Table 1-4 Melting Points and Densities of Several Key Compounds in Welding Slag

CompoundsMelting point (°C)Density (g/cm3)CompoundsMelting point (°C)Density (g/cm3)
CaO 29243.32B2O34503.33
CaF214182.8ZrO227005.56
CaS 2.8BaO1925
MgO 28003.5KF857
MgF2 1260Na2O9202.27
Al2O320503.37NaF997
Cu222306NbO1935
CuO 1447NbO22083
FeO 13705.9P2O56802.39
Fe2O315605.2V2O320004.87
Fe3O41597V2O5670
FeS 11934.6WO21573
MnO 15855. 11La2O36.51
MnO21650BeO3.03
MnS 1620CeO27.13
SiO217132.26PbO9.21
TiO218254.24CaO·SiO21540
TiO41920(CaO)2 – SiO21540
Cr2O322975.21MnO·SiO212703.6
NiO 1960(MnO)2 ·SiO213264.1
MoO3795(FeO)2 ·SiO212504.3
ZnO20755.47

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