Section I working principle and characteristics of submerged arc welding

Submerged arc welding is also a welding method using arc as heat source. During submerged arc welding, the arc burns under the cover of a layer of granular fusible flux, and the arc is not exposed. Hence the name of submerged arc welding. The metal electrode used is a smooth welding wire fed continuously.

1、 Working principle

Fig. 1 is a schematic diagram of the weld formation process of submerged arc welding. The welding arc burns between the welding wire and the workpiece, and the arc heat melts the base metal and flux at the end of the welding wire and near the arc. The molten metal forms a molten pool, and the molten flux becomes molten slag. The molten pool is protected by slag and flux steam and is not in contact with air. When the arc moves forward, the arc force pushes the liquid metal in the molten pool behind the molten pool. In the subsequent cooling process, this part of the liquid metal solidifies into a weld. The slag solidifies into a slag shell and covers the weld surface. In addition to the mechanical protection of molten pool and weld metal, molten slag also has metallurgical reaction with molten metal during welding, which affects the chemical composition of weld metal. During submerged arc welding, the parts to be welded and the welding wire are respectively connected to the two poles of the welding power supply. The welding wire is connected with the power supply through sliding contact with the conductive nozzle. The welding circuit includes welding power supply, connecting cable, conductive nozzle, welding wire, arc, molten pool, workpiece and other links. The end of the welding wire is continuously melted under the action of arc heat, so the welding wire should be continuously sent in to maintain the stability of the welding process. The feeding speed of welding wire shall be balanced with the melting speed of welding wire. The welding wire is generally fed by the wire feeding roller driven by the motor. Depending on the application, the number of welding wires can be single wire, double wire or multi wire. In some applications, flux cored wire is used instead of solid wire, or steel strip is used instead of wire.

Figure 1

Submerged arc welding includes automatic submerged arc welding and semi-automatic submerged arc welding. The welding wire feeding and arc movement of the former are automatically completed by a special machine head, the welding wire feeding of the latter is completed mechanically, and the arc movement is carried out manually. During welding, the flux is spread in front of the arc from the funnel. After welding, the unmelted flux can be automatically recovered by the flux recovery device or manually cleaned and recovered.

2、 Advantages and disadvantages of submerged arc welding

(1) The welding current used is large and the corresponding input power is large. Coupled with the heat insulation effect of flux and slag, it has high thermal efficiency and large melting depth. The groove of the workpiece can be smaller, reducing the amount of filler metal. Single wire submerged arc welding can penetrate 20mm at a time without grooving the workpiece.

(2) The welding speed is high. Taking the butt welding of steel plate with thickness of 8 ~ 10mm as an example, the speed of single wire submerged arc welding can reach 50 ~ 80cm / min, while that of manual arc welding shall not exceed 10 ~ 13cm / min.

(3) The existence of flux can not only separate the contact between molten metal and air, but also make the molten pool metal solidify slowly. There is more time for metallurgical reaction between liquid metal and molten flux, which reduces the possibility of defects such as pores and cracks in the weld. Flux can also add some alloying elements to the weld metal to improve the mechanical properties of the weld metal.

(4) When welding in windy environment, the protection effect of submerged arc welding is better than that of other arc welding methods.

(5) During automatic welding, the welding parameters can be kept stable through automatic adjustment. Compared with manual arc welding, the dependence of welding quality on the welder's skill level can be greatly reduced.

(6) No electric arc radiation, good working conditions.

2. Main disadvantages of submerged arc welding

(1) Due to the use of granular flux, this welding method is generally only applicable to flat welding positions. Special measures shall be taken for welding at other positions to ensure that the flux can cover the welding area.

(2) The relative position between the arc and the groove cannot be directly observed. If the automatic weld tracking device is not adopted, it is easy to weld deviation.

(3) The electric field strength of submerged arc welding arc is large, and the arc is unstable when the current is less than 100A, so it is not suitable for welding thin plates with thickness less than 1mm.

3、 Scope of application of submerged arc welding

Due to the large penetration, high productivity and high degree of mechanized operation, submerged arc welding is suitable for welding long welds of medium and heavy plate structure. It is widely used in shipbuilding, boilers and pressure vessels, bridges, hoisting machinery, railway vehicles, engineering machinery, heavy machinery and metallurgical machinery, nuclear power plant structure, marine structure and other manufacturing departments. It is one of the most commonly used welding methods in today's welding production.

In addition to the connection of components in metal structure, submerged arc welding can also weld wear-resistant or corrosion-resistant alloy layer on the surface of base metal. With the development of welding metallurgy technology and welding material production technology, the materials that can be welded by submerged arc welding have developed from carbon structural steel to low-alloy structural steel, stainless steel, heat-resistant steel and some non-ferrous metals, such as nickel base alloy, titanium alloy, copper alloy and so on.

Section II structure and working principle of submerged arc welding equipment

1、 Submerged arc welding power source

Generally, thick wire is used in submerged arc welding, and the arc has horizontal static characteristic curve. According to the above requirements of arc stable combustion, the power supply shall have reduced external characteristics. When welding thin plates with fine wire, the arc has a rising static characteristic curve, and the flat characteristic power supply should be used. The submerged arc welding power source can be AC (arc welding transformer), DC (arc welding generator or arc welding rectifier) or both AC and DC. It shall be selected according to specific application conditions, such as welding current range, single wire welding or multi wire welding, welding speed, flux type, etc. General DC power supply is used for low current range, fast arc striking, short weld and high-speed welding. The arc stability of the flux used is poor and has high requirements for the stability of welding process parameters. When using DC power supply, different polarity will produce different process effects. When DC positive connection (welding wire connected to negative electrode) is adopted, the deposition rate of welding wire is the highest; When DC reverse connection (welding wire connected to positive electrode) is adopted, the weld penetration is the maximum. When AC power is used, the welding wire deposition rate and weld penetration are between DC positive connection and reverse connection, and the magnetic bias blow of arc is the smallest. Therefore, AC power supply is mostly used in high current submerged arc welding and occasions with serious magnetic bias blowing when DC is adopted. Generally, the no-load voltage of AC power supply is more than 65V. In order to increase penetration and improve productivity, multi wire submerged arc welding has been more and more industrial applications. At present, double wire welding and three wire welding are widely used. The power supply of multi wire welding can be DC or AC, or AC and DC. There are many combinations of welding power supply selection and connection in double wire submerged arc welding and three wire submerged arc welding.

2、 Submerged arc welding machine

Submerged arc welding machines are divided into automatic welding machines and semi-automatic welding machines.

（1） Semi automatic submerged arc welding machine

The main functions of semi-automatic submerged arc welding machine are:

(1) Continuously feed the welding wire into the arc area through the hose

(2) Transmit welding current;

(3) Control welding start and stop;

(4) Apply flux to the welding area. Therefore, it is mainly composed of wire feeding mechanism, control box, welding handle with hose and welding power supply. Hose type semi-automatic submerged arc welding machine has the advantages of automatic submerged arc welding and the mobility of manual arc welding. On workpieces that are difficult to realize automatic welding (such as welds with irregular centerline, short welds, workpieces with narrow welding space, etc.), this welder can be used for welding.

（2） Automatic submerged arc welding machine

The main functions of automatic submerged arc welding machine are;

(1) Continuously feed welding wire to the welding area;

(2) Transmit welding current;

(3) Move the arc along the joint;

(4) Main parameters of arc control;

(5) Control the start and stop of welding;

(6) Apply flux to the welding area;

(7) adjust the position of the wire end before welding. There are two kinds of automatic submerged arc welding machines: constant speed wire feeding and variable speed wire feeding. They are generally composed of machine head, control box, guide rail (or support) and welding power supply. Constant speed wire feeding automatic submerged arc welding machine adopts arc self-regulation system; Automatic submerged arc welding machine with variable speed wire feeding adopts automatic arc voltage regulation system. Automatic submerged arc welding machines are made into different forms according to work needs. Common are: welding car type, suspension type, machine tool type, cantilever type, gantry type, etc. MZ-1000 welding machine is the most widely used, which is of welding car type. MZ-1000 welding machine adopts automatic arc voltage regulation (variable speed wire feeding) system, and the wire feeding speed is directly proportional to the arc voltage.

3、 Submerged arc welding auxiliary equipment

During submerged arc welding, in order to adjust the relative position between the welding head and the workpiece, make the joint in the best welding position or achieve the expected process purpose, it is generally necessary to have corresponding auxiliary equipment to cooperate with the welding machine. Auxiliary equipment for submerged arc welding can be divided into the following types:

（1） Welding fixture

The purpose of using welding fixture is to accurately position and clamp the workpiece for welding. In this way, the tack weld can be reduced or exempted, and the welding deformation can be reduced. Sometimes, in order to achieve other process purposes, welding fixtures are often combined with other auxiliary equipment, such as single-sided welding and double-sided forming device.

（2） Workpiece displacement equipment

The main function of this equipment is to rotate, tilt and turn the workpiece, so as to place the joint to be welded in the best welding position, so as to improve productivity, improve welding quality and reduce labor intensity. The type, structure and size of workpiece displacement equipment vary with the welded workpiece. The commonly used workpiece displacement equipment in submerged arc welding include roller frame, turnover machine, etc.

（3） Welding machine positioner

The main function of this equipment is to accurately send the welding head to the position to be welded, and it can be operated at this position during welding; Or move the welding head along the specified track at a certain speed for welding. This equipment is also called welding operator. Most of them are used together with workpiece positioner and welding roller frame to complete the welding of various workpieces. The basic forms are platform type, cantilever type, telescopic type, gantry type and so on.

（4） Weld forming equipment

The arc power of submerged arc welding is large. In order to prevent the loss and burn through of molten metal and promote the formation of the back of the weld, it is often necessary to add a pad on the back of the weld. In addition to the copper backing plate mentioned above, the most commonly used weld forming equipment also has flux pads. There are two basic types of flux pads for longitudinal seam and circumferential seam.

（5） Flux recovery and conveying equipment

It is used to automatically recover and transport flux in welding to improve the degree of welding automation. The suction flux recovery conveyor with compressed air can be installed on the trolley

Section III welding materials for submerged arc welding

During submerged arc welding, welding wire and flux directly participate in the metallurgical reaction in the welding process, so their chemical composition and physical properties will affect the welding process, and affect the chemical composition, microstructure and properties of weld metal through the welding process. It is an important content of submerged arc welding technology to correctly select welding wire and use it with flux.

1、 Welding wire

There are two types of welding wires used in submerged arc welding: solid core welding wire and flux cored welding wire. At present, solid core welding wire is widely used in production. The variety of welding wire increases with the increase of the type of metal to be welded. At present, there are carbon structural steel, alloy structural steel, high alloy steel, various non-ferrous metal welding wires and special alloy welding wires for surfacing. The selection of welding wire diameter depends on the purpose. The welding wire for semi-automatic submerged arc welding is thin, with a general diameter of 1.6, 2 and 2.4mm, so that it can pass through the hose smoothly, and the welder will not feel difficult due to the stiffness of the welding wire in operation. Automatic submerged arc welding generally uses welding wire with diameter of 3 ~ 6mm to give full play to the advantages of high current and high deposition rate of submerged arc welding. For a certain current value, welding wires with different diameters may be used. When welding wire with smaller diameter is used at the same current, the effect of increasing weld penetration and reducing weld width can be obtained. When the workpiece is poorly assembled, the thicker welding wire should be selected.

The surface of welding wire shall be clean and smooth, and can be smoothly fed during welding, so as not to interfere with the welding process. In addition to stainless steel welding wire and non-ferrous metal welding wire, the surface of various low carbon steel and low alloy steel welding wires is best plated with copper. The copper plating layer can not only prevent rust, but also improve the electrical contact between the welding wire and the conductive nozzle. In order to make the welding process stable and reduce the auxiliary welding time, the welding wire shall be neatly wound on the welding wire disc with a wire coiler. Each coil of steel welding wire shall be wound by one welding wire.

2、 Flux

The flux used in submerged arc welding is a granular fusible material, which is equivalent to the coating of welding rod.

1. Basic requirements for flux

(1) It has good metallurgical properties. Match with the selected welding wire to ensure that the weld metal can obtain the required chemical composition, mechanical properties and the ability to resist hot crack and cold crack through appropriate welding process.

(2) It has good process performance. That is, it is required to have good arc stabilization, weld forming, slag removal and other properties, and less toxic gas is generated in the welding process.

2. Classification of flux

Submerged arc welding fluxes are generally classified according to manufacturing method, chemical composition, chemical properties, particle structure, etc., except that they are divided into steel fluxes and non-ferrous metal fluxes according to their uses.

(1) According to the manufacturing method, it can be divided into three categories

1) Melting flux. The required raw materials are weighed according to the formula proportion, melted after dry mixing, then injected into cold water or on the quench plate to granulate, and then dried, crushed and screened. According to its particle structure, melting flux can be divided into glass flux (transparent particles), crystalline flux (granular with crystalline characteristics) and pumice like flux (granules are foamed).

2) Sintered flux. Various powder components are mixed evenly according to the formula proportion, mixed with water and glass to form wet material, sintered at 750 ~ 1000 ℃, crushed and screened.

3) Ceramic flux. Mix various powder components evenly according to the formula proportion, add water and glass to make a wet material, make the wet material into particles of a certain size, and dry it at 350 ~ 500 ℃.

(2) Classification by chemical composition

1) It can be divided into alkaline flux, acidic flux and neutral flux according to alkalinity.

2) The classification according to the content of main components is shown in table 4-1.

(3) Classified by chemical properties of flux,

1) Oxidizing flux flux flux containing a large amount of SiO2, MnO or FeO.

2) Weak oxidizing flux contains less oxides such as SiO2, MnO and FeO.

3) Inert flux contains Al2O3, Cao, MgO, CaF2, etc., and basically does not contain SiO2, MnO, FeO, etc.

3. Preparation method of flux model

(1) Smelting flux is composed of HJ indicating smelting flux, followed by three Arabic numerals.

1) The first digit indicates the content of manganese oxide in the flux, and 1, 2, 3 and 4 represent no manganese, low manganese, medium manganese and high manganese flux.

2) The second digit indicates the content of silicon dioxide and calcium fluoride in the flux. 1 ~ 9 successively represent low silicon and low fluorine, medium silicon and low fluorine, high silicon and low fluorine, low silicon and medium fluorine, medium silicon and medium fluorine, high silicon and medium fluorine, low silicon and high fluorine, medium silicon and high fluorine and other types of flux.

3) The third digit indicates different grades of the same type of flux, which are arranged in the order of 0, 1, 2 and 9.

4) When two particle sizes are produced for the same brand of flux, the word "X" shall be added after the brand of fine particle flux.

(2) Sintered flux is composed of SJ, which represents sintered flux, followed by three Arabic numerals. The first digit indicates the slag system of flux slag, and 1 ~ 6 successively represent fluorine alkali type, high aluminum type, silicon calcium type, silicon manganese type, aluminum titanium type and other types of flux. The second and third digits represent different brands of flux in the same slag type flux, which are arranged in the order of 01, 02 and 09.

3、 Selection of flux and welding wire

In order to obtain high-quality submerged arc welding joints, it is very important to correctly select flux and welding wire.

Low carbon steel can be welded with high manganese and high silicon flux combined with H08MnA welding wire, or low manganese and manganese free flux combined with H08MnA and H10Mn2 welding wire. Medium manganese medium silicon or low manganese medium silicon flux combined with welding wire matching with steel strength can be selected for welding of low alloy high strength steel.

Medium silicon or low silicon type can be selected for welding of heat-resistant steel, low-temperature steel and corrosion-resistant steel