Definition

A substance that can melt to form slag and/or gas during welding, providing protection and metallurgical physical-chemical effects to the molten metal.

Introduction

Welding Flux

A substance that can melt to form slag and gas during welding, providing protection and metallurgical treatment to the molten metal. The flux used for submerged arc welding is called submerged arc flux. For brazing, there are hard soldering flux and soft soldering flux.

Flux, also known as soldering agent, has a broad definition, including molten salts, organic substances, active gases, metal vapors, etc., referring generally to any substance used to reduce the interfacial tension between the base material and the filler material, excluding the base material and filler.

The wetting ability of the filler material with the base material is certainly related to the nature of the filler, but it is secondary compared to the role of the flux.

Functions of Flux

The functions of flux can be divided into three parts:

1. Remove oxides from the welding surface, lower the melting point and surface tension of the filler material, and reach brazing temperature as quickly as possible.

2. Protect the weld metal from harmful gases in the surrounding atmosphere while in a liquid state.

3. Ensure that the liquid filler flows at an appropriate speed to fill the joint.

The melting point of the flux should be 10-30°C lower than that of the filler; in special cases, it can be higher. If the melting point of the flux is too low compared to that of the filler, it may melt too early, causing loss of activity due to evaporation or interaction with the base material before the filler melts.

The choice of flux usually depends on the nature of the oxide film. For alkaline oxide films such as those from Fe, Ni, Cu, acidic boron oxide (B2O3) fluxes are commonly used. For acidic oxide films like those on cast iron with high SiO2 content, alkaline Na2CO3 fluxes are often used to generate easily meltable Na2SiO3 that enters the slag.

Some fluorinated gases are also commonly used as fluxes; they react uniformly and leave no residue after welding. BF3 is often mixed with N2 for brazing stainless steel at high temperatures.

Fluxes used for brazing below 450°C are called soft fluxes. Soft fluxes are divided into two types: one is water-soluble, usually composed of hydrochloride and phosphate solutions or their mixtures; they are highly active and corrosive and require cleaning after welding. The other type is water-insoluble organic fluxes, typically based on rosin or synthetic resin with added organic acids or amines or their HCl or HBr salts to enhance de-coating ability and activity.

Classification of Flux

Welding can generally be divided into acidic and alkaline fluxes. The most commonly used and economical for low carbon steel is acidic flux, such as HJ431 combined with H08A or H08MNA. For important low-alloy high-strength steels, alkaline fluxes like SJ101 or SJ301 can be used in combination with H08MNA welding wire to significantly improve mechanical properties and toughness indicators.