Introduction to welding core knowledge

The metal core covered by the coating in the electrode is called a core. The core is generally a steel wire of a certain length and diameter. When welding, the core has two functions: one is to conduct welding current, the arc is used to convert electrical energy into heat, and the other is that the core itself is melted as a filler metal to fuse with the liquid base metal to form a weld.

When the electrode is welded, the core metal accounts for a portion of the entire weld metal. Therefore, the chemical composition of the core directly affects the quality of the weld. Therefore, the wire used as the electrode core has its own designation and composition. If it is used as a filler metal for submerged arc automatic welding, electroslag welding, gas shielded welding, gas welding, etc., it is called a welding wire.

The electrode coating refers to the coating layer applied to the surface of the core. The drug skin is decomposed and melted during the welding process to form gas and slag, which plays the role of mechanical protection, metallurgical treatment and improvement of process performance. The composition of the coating includes: minerals (such as marble, fluorite, etc.), iron alloys and metal powders (such as ferromanganese, ferrotitanium, etc.), organic substances (such as wood flour, starch, etc.), chemical products (such as titanium). White powder, water glass, etc.). The electrode coating is an important factor in determining the quality of the weld. It has the following effects in the welding process:

First, improve the stability of arc combustion. Photochemical strips without a coating are not susceptible to arcing. Even if it is ignited, it cannot be burned steadily. In the electrode coating, it generally contains substances with low ionization potential such as potassium, sodium and calcium, which can improve the stability of the arc and ensure the continuous welding process.

Second, protect the weld pool. During the welding process, oxygen, nitrogen and water vapor in the air are immersed in the weld, which will adversely affect the weld. Not only the formation of pores, but also the mechanical properties of the weld, and even cracks. After the electrode coating is melted, a large amount of gas is generated to cover the arc and the molten pool, which reduces the interaction between the molten metal and the air. When the weld is cooled, the melted skin forms a layer of slag covering the surface of the weld, protecting the weld metal and allowing it to cool slowly, reducing the possibility of creating voids.

Third, to ensure the weld deoxidation, desulfurization impurities. Although the welding process is protected, it is inevitable that a small amount of oxygen will enter the molten pool, causing oxidation of metals and alloying elements, burning alloying elements and reducing the quality of the weld. Therefore, it is necessary to add a reducing agent (such as manganese, silicon, titanium, aluminum, etc.) to the electrode coating to reduce the oxide that has entered the molten pool.

Fourth, add alloying elements to the weld. Due to the high temperature of the arc, the alloying elements of the weld metal are burned out by evaporation, which reduces the mechanical properties of the weld. Therefore, it is necessary to add appropriate alloying elements to the weld through the coating to compensate for the burning of the alloying elements and to ensure or improve the mechanical properties of the weld. For the welding of some alloy steels, it is also necessary to infiltrate the alloy into the weld through the coating, so that the weld metal can be close to the metal composition of the base metal, and the mechanical properties catch up even more than the basic metal.

Fifth, improve welding productivity and reduce splashing. The electrode coating has the effect of increasing the droplets and reducing the splash. The melting point of the electrode strip is slightly lower than the solder joint of the core, but because the core is in the center of the arc, the temperature is higher, so the core melts first and the coating melts slightly later. In this way, a short length of the drug sheath is formed at the end of the electrode, and the arc blow force is applied to make the droplet direct to the molten pool, which is advantageous for the back welding and the vertical welding. In addition, the arc heat is more concentrated after the core is coated with the coating. At the same time, since the metal loss caused by the splash is reduced, the welding coefficient is increased, and the welding productivity is also improved. In addition, the amount of dust generated during the welding process will also decrease.

(1) Effect of alloying elements in the core on welding

1) Carbon (C) carbon is the main alloying element in steel. When the carbon content increases, the strength and hardness of the steel increase remarkably, and the plasticity decreases. In the welding process, carbon plays a certain deoxidation effect, and cooperates with oxygen in the action of high temperature of arc to generate carbon monoxide and carbon dioxide gas, and removes the arc area and the air around the molten pool to prevent harmful gases such as oxygen and nitrogen in the air. The adverse effects on the molten pool reduce the oxygen and nitrogen content in the weld metal. If the carbon content is too high, the reduction is severe, causing large splashes and pores. 1%。 The carbon content of the low carbon steel core is generally 0.1%.

2) Manganese (Mn) manganese is a good alloying agent in steel. As the manganese content increases, its strength and toughness will increase. Manganese is also a good deoxidizer during the welding process, which can reduce the oxygen content in the weld. Manganese and vulcanization form manganese sulfide floating in the slag, thereby reducing the tendency of weld hot cracking. 5%。 The Mn content of the steel is 0. 30% ~ 0. 55%, the welding of certain special-purpose steel wire, the manganese content of up to 1.70% - 2. 10%.

3) Silicon (Si) silicon is also a good alloying agent. Adding proper amount of silicon to steel can improve the yield strength, elasticity and acid resistance of steel. If the content is too high, it will reduce plasticity and toughness. In the welding process, silicon also has a good deoxidizing ability to form silica with oxygen, but it will increase the viscosity of the slag and easily promote the formation of non-metallic inclusions.

4) Chromium (Cr) chromium can improve the hardness, wear resistance and corrosion resistance of steel. For low carbon steel, chromium is an accidental impurity. The main metallurgical feature of chromium is that it is susceptible to rapid oxidation, forming a refractory oxide of chromium oxide (Cr203), which increases the likelihood of weld metal inclusions. After the transition of chromium trioxide to the slag, the viscosity of the slag is increased and the fluidity is lowered.

5) Nickel (Ni) nickel has a significant effect on the toughness of steel. When the low temperature impact value is required to be high, some nickel is appropriately incorporated.

6%。 The sulfur (S) sulfur is a harmful impurity, as the sulfur content increases, will increase the hot cracking tendency of the weld, so the sulfur content of the core should not be greater than 0. 04%. The sulphur content of the sulphur content shall not be greater than 0.03%.

7) Phosphorus (P)

(2) Classification of the core

The welding core is classified according to the national standard "welding steel wire" (GB 1300-77). The special steel wire for welding can be divided into three types: carbon structural steel, alloy structural steel and stainless steel.

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