What Is Submerged Arc Welding?

Submerged arc welding (including submerged arc surfacing, electroslag surfacing, etc.) is a method of arc welding under the flux layer for welding. Its inherent welding quality is stable, welding productivity is high, no arc light and little smoke and so on, making it the main welding method in the production of important steel structures such as pressure vessels, pipe sections and box beams. In recent years, although many efficient and high-quality new welding methods have appeared in succession, the application areas of submerged arc welding remain unaffected. From the perspective of the weight of the deposited metal by various welding methods, submerged arc welding accounts for about 10%, and has not changed much over the years.

Submerged arc welding is one of today's high-efficiency mechanized welding methods. Its full name is submerged arc.

Currently it is mainly used for welding various steel plate structures. Weldable steel types include carbon structural steel, stainless steel, heat-resistant steel and composite steel. Submerged arc welding is most widely used in shipbuilding, boilers, chemical containers, bridges, lifting machinery, metallurgical machinery manufacturing, marine structures, and nuclear power equipment. In addition, submerged arc welding is used to overlay wear-resistant and corrosion-resistant alloys or for welding nickel-based alloys, and copper alloys are also ideal.

Automatic submerged arc welding process

During automatic submerged arc welding, the processes of igniting arc, wire feeding, arc moving in the welding direction, and welding receiving tail are completely completed by machinery.

The submerged arc automatic welding process is shown in Figure 2-11.

After the flux 2 flows out from the funnel 3, it is evenly stacked on the assembled workpiece 1, and the welding wire 4 is sent into the welding arc area by the wire feeding mechanism through the wire feeding roller 5 and the conductive tip 6. The two ends of the welding power source are respectively connected to the contact tip and the workpiece. The wire feeding mechanism, flux hopper and control panel are usually mounted on a trolley to achieve the movement of the welding arc.

The welding process is automatically controlled by operating a push-button switch on the control panel. During the welding process, the workpiece is covered with a layer of 30-50mm thick granular flux. The continuously fed wire generates an arc between the welding layer and the weldment. The heat of the arc causes the welding wire, workpiece and flux to melt and form a metal. Weld pools to isolate them from air. As the welding machine moves forward automatically, the arc continuously melts the weldment metal, welding wire and flux in front, and the edge behind the molten pool starts to cool and solidify to form the weld seam. The liquid slag also condenses to form a hard slag shell. As shown in Figure 2-12. Unmelted flux can be recycled.

The function of the wire and flux during welding is the same as the electrode core and electrode coating of manual arc welding. Welding wire and flux of different composition should be selected for different materials. For example, H08A welding wire is commonly used when welding low carbon steel, and high manganese and high silicon type flux HJ431 is used. Welding power sources usually use large-capacity arc welding transformers.

Advantages of submerged arc automatic welding

The main advantages of submerged arc welding are:

(1) The productivity of high-productivity submerged arc welding (the length of the wire from the end of the contact tip to the end of the arc) is much shorter than that of manual arc welding, generally about 50mm, and it is a light wire, which will not increase the current. As a result, the electrode skin becomes red, and a larger current can be used (5-10 times larger than manual welding). Therefore, the penetration depth is large and the productivity is high. For butt welding below 20mm, there is no need to open the groove and leave no gap, which reduces the amount of filler metal.

(2) The quality of the welding seam is perfect to protect the weld pool, and there are fewer impurities in the weld metal.

Submerged arc semi-automatic welding is mainly automatic welding of hoses, which is characterized by the use of a thinner diameter (2mm or less) welding wire, which is fed into the molten pool through a curved hose. The movement of the arc is done manually, and the feeding of the welding wire is automatic. Semi-automatic welding can replace automatic welding to weld some curved and shorter welds. It is mainly used for fillet welds and also for butt welds.

(1) The small wheels of the submerged arc automatic welding machine must have good insulation, the wires should be well insulated, and the wires should be straightened during work to prevent twisting and being burnt by slag.

(2) The control box and the welding machine shell shall be reliably grounded (zero) and prevent leakage. The terminal board cover must be closed.

(3) During the welding process, care should be taken to prevent the solder from suddenly being supplied and intense arc exposure may cause burns to the eyes. Therefore, ordinary protective glasses should be worn when welding.

(4) The handle of semi-automatic submerged arc welding shall have a fixed place to prevent short circuit.

(5) The components of submerged arc automatic welding flux contain manganese oxide and other harmful substances to the human body. Although welding does not generate visible smoke like hand arc welding, it will generate a certain amount of harmful gases and vapors. Therefore, it is best to have local exhaust ventilation at the work site.

Introduction

Preparation before welding: Submerged arc welding must be prepared before welding, including bevel processing of the weldment, surface cleaning of the part to be welded, assembly of the weldment, cleaning of the surface of the wire, and drying of the flux.

Bevel processing

The bevel processing requirements are implemented in accordance with GB 986-1988 to ensure that there is no unpenetration or slag inclusion at the root of the weld and reduce the amount of filler metal. Bevel processing can be done using edger, mechanized or semi-mechanized

Introduction

The welding parameters of submerged arc welding mainly include: welding current, arc voltage, welding speed, wire diameter and protruding length.

Welding current

When other parameters are unchanged, the effect of welding current on the shape and size of the weld is shown in the figure.

Effect of current on weld

Under normal welding conditions, the weld penetration is directly proportional to the welding current.

With the increase of welding current, the penetration depth and the remaining height of the weld have increased significantly, while the width of the weld has not changed much. At the same time, the melting amount of the welding wire also increases accordingly, which increases the remaining height of the weld. As the welding current decreases, both the penetration depth and residual height decrease.

Arc voltage

With the increase of arc voltage, the welding width increased significantly, while the penetration and weld height decreased. However, when the arc voltage is too large, it will not only make the penetration deeper and cause under-welding, but also lead to poor weld formation, difficult slag removal and even undercuts. Therefore, while increasing the arc voltage, the welding current should also be appropriately increased.

Welding speed

When other welding parameters are unchanged and the welding speed is increased, the welding heat input is correspondingly reduced, so that the penetration depth of the weld is also reduced. Too much welding speed can cause defects such as incomplete penetration. In order to ensure the welding quality, a certain amount of welding heat input must be guaranteed, that is, to increase the welding speed in order to improve productivity, the welding current and arc voltage should be increased accordingly.

Welding wire diameter and extension length

When the other welding parameters are unchanged and the diameter of the welding wire is increased, the diameter of the arc pillar is also increased, that is, the current density is reduced, which will cause the weld width to increase and the penetration depth to decrease. Conversely, the penetration depth increases and the weld width decreases.

When other welding parameters are unchanged and the wire length is increased, the resistance is also increased, the preheating effect of the protruding wire is increased, and the melting speed of the wire is accelerated. As a result, the penetration depth becomes shallow, and the remaining height of the weld is increased. The extension length of the welding wire must be controlled, and it should not be too long.

Welding wire inclination

The inclining direction of the welding wire is divided into forward leaning and backward leaning. The direction and size of the inclination angle are different, and the force and heat effect of the arc on the molten pool are also different, which affects the formation of the weld. When the welding wire is inclined backwards, the arc is directed to the welding direction, so that the weldment in front of the molten pool is preheated, and the discharge of the liquid metal from the molten pool is weakened, which results in a wide weld and shallow penetration. On the contrary, the width of the weld seam is small and the penetration depth is large, but it is easy to cause unfusion and undercut of the weld seam and make the weld seam poor.

Other

a. Bevel shape b. Root clearance c. Weldment thickness and welding heat dissipation conditions.