Introduction to Basic Knowledge and Technology of Submerged Arc Welding

Electric arc: a strong and persistent gas discharge phenomenon in which there is a certain voltage between the positive and negative electrodes, and the gas medium between the two electrodes should be in an ionized state. When igniting a welding arc, it is usually done by connecting two electrodes (one electrode being the workpiece and the other electrode being the filler metal wire or welding rod) to the power supply, briefly contacting and quickly separating. When the two electrodes come into contact with each other, a short circuit occurs, forming an arc. This method is called contact arcing. After the arc is formed, as long as the power supply maintains a certain potential difference between the two poles, the combustion of the arc can be maintained.

Arc characteristics: low voltage, high current, high temperature, high energy density, good mobility, etc. Generally, a voltage of 20-30V can maintain stable combustion of the arc, and the current in the arc can range from tens to thousands of amperes to meet the welding requirements of different workpieces. The temperature of the arc can reach over 5000K and can melt various metals.

Arc composition: cathode zone, anode zone, and arc column zone.

Arc welding power source: The power source used for welding arc is called arc welding power source, which can usually be divided into four categories: AC arc welding power source, DC arc welding power source, pulse arc welding power source, and inverter arc welding power source.

DC positive connection: When a DC welding machine is used to connect the workpiece to the anode and the welding rod to the cathode, it is called DC positive connection. At this time, the workpiece is heated more and is suitable for welding thick and large workpieces;

DC reverse connection: When the workpiece is connected to the cathode and the welding rod is connected to the anode, it is called DC reverse connection. At this time, the workpiece is less heated and suitable for welding thin and small workpieces. When using an AC welding machine for welding, there is no problem of positive or negative connection due to the alternating polarity of the two poles.

The metallurgical process of welding involves the interaction between liquid metal, slag, and gas in the arc welding process, which is the process of metal remelting. However, due to the particularity of welding conditions, the welding chemical metallurgy process has different characteristics from general smelting processes.

Firstly, the metallurgical temperature of welding is high, the phase boundary is large, and the reaction speed is high. When air invades the arc, the liquid metal will undergo strong oxidation and nitriding reactions, as well as a large amount of metal evaporation. The water in the air, as well as the hydrogen atoms decomposed from the oil, rust, and water in the workpiece and welding material at high arc temperatures, can dissolve into the liquid metal, leading to a decrease in joint plasticity and toughness (hydrogen embrittlement), and even the formation of cracks.

Secondly, the welding pool is small and cools quickly, making it difficult for various metallurgical reactions to reach equilibrium. The chemical composition of the weld is uneven, and gases, oxides, etc. in the pool cannot float out in time, which can easily form defects such as pores, slag inclusions, and even cracks.

During the arc welding process, the following measures are usually taken:

• During the welding process, mechanical protection is provided to the molten metal to isolate it from the air. There are three protection methods: gas protection, slag protection, and gas slag combined protection.

(2) Metallurgical treatment of the welding pool is mainly carried out by adding a certain amount of deoxidizer (mainly manganese iron and silicon iron) and a certain amount of alloying elements to the welding materials (electrode coating, welding wire, flux), in order to eliminate FeO from the pool during the welding process and compensate for the loss of alloying elements. Common arc welding methods

Submerged arc welding is a melting electrode welding method that uses granular flux as a protective medium and hides the arc under the flux layer. The welding process of submerged arc welding consists of three steps:

1. evenly deposit sufficient granular flux at the joint to be welded on the workpiece;
2. Connect two stages of welding power supply to the conductive nozzle and the welding piece respectively to generate welding arc;
3. Automatically feed the welding wire and move the arc to carry out welding.

The main characteristics of submerged arc welding are as follows:

1. Unique arc performance

• High weld quality, good slag insulation and air protection effect, the main component of the arc zone is CO2, the nitrogen and oxygen content in the weld metal is greatly reduced, the welding parameters are automatically adjusted, the arc walking is mechanized, the molten pool exists for a long time, the metallurgical reaction is sufficient, the wind resistance is strong, so the weld composition is stable and the mechanical properties are good;
• Good working conditions and slag isolation arc light are beneficial for welding operations; Mechanized walking results in lower labor intensity.

2. The arc column electric field strength is higher than that of gas metal arc welding, and it has the following characteristics:

• good equipment adjustment performance. Due to the high electric field strength, the sensitivity of the automatic adjustment system is higher, which improves the stability of the welding process;
• The lower limit of welding current is relatively high.

3. Due to the shortened conductive length of the welding wire, the current and current density are significantly increased, resulting in high production efficiency. This greatly improves the arc penetration ability and the deposition rate of the welding wire; Due to the thermal insulation effect of flux and slag, the overall thermal efficiency is greatly increased, resulting in a significant increase in welding speed.

Scope of application:

Due to the deep penetration, high productivity, and high degree of mechanical operation of submerged arc welding, it is suitable for welding long welds of medium and thick plate structures. It has a wide range of applications in shipbuilding, boiler and pressure vessel, bridge, overweight machinery, nuclear power plant structures, marine structures, weapons and other manufacturing sectors, and is one of the most commonly used welding methods in welding production today. In addition to being used for connecting components in metal structures, submerged arc welding can also weld wear-resistant or corrosion-resistant alloy layers on the surface of the 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 evolved from carbon structural steel to low-alloy structural steel, stainless steel, heat-resistant steel, and some non-ferrous metals such as nickel based alloys, titanium alloys, copper alloys, etc.

 Due to its own characteristics, its application also has certain limitations, mainly due to:

• welding position limitations. Due to the retention of flux, submerged arc welding is mainly used for welding horizontal and downward position welds without special measures, and cannot be used for horizontal, vertical, and upward welding.
• The limitation of welding materials is that they cannot weld highly oxidizing metals and alloys such as aluminum and titanium, and are mainly used for welding ferrous metals;
• Only suitable for welding and cutting long welds, and cannot weld welds with limited spatial positions;
• Cannot directly observe the arc;

(5) Not suitable for thin plate and low current welding.