CO2 gas shielded welding is a method that uses carbon dioxide gas as a shielding gas for arc welding. This technique is characterized by its simplicity, low cost, high efficiency, and good weld quality, making it widely used for welding low-carbon steel and low-alloy steel. This article will introduce the basic principles, operational procedures, and precautions of CO2-shielded welding for your reference.
I. Basic Principles of CO2 Gas Shielded Welding
The basic principle of CO2 gas-shielded welding involves using a welding wire as a consumable electrode to form an arc with the workpiece. The high temperature generated by the arc melts both the welding wire and the workpiece, creating a molten pool. At the same time, CO2 gas sprayed from the welding gun covers the arc and the molten pool, protecting them from the effects of oxygen and nitrogen in the air, preventing oxidation and nitriding, and enhancing the mechanical properties and corrosion resistance of the weld. At high temperatures, CO2 gas decomposes into carbon monoxide and oxygen. Carbon monoxide has a reducing effect that can eliminate oxides in the weld, while oxygen increases the weld penetration and thermal efficiency. The arc in CO2 gas-shielded welding is unstable, characterized by the welding wire tip frequently short-circuiting and breaking off in the molten pool, resulting in a short-circuit transition of the molten droplets. This method provides good weld formation but also produces more spatter and smoke. To know GMAW vs. CO2 Welding: A Comparison of Two Metal Arc Welding Processes.
II. Operation Procedures of CO2 Gas Shielded Welding
The CO2 gas-shielded welding operation procedure mainly includes the following steps:
1) Preparation Before Welding
Inspect the welding machine, gas cylinder, wire feeder, welding gun, and other equipment for integrity and correct connections. Adjust welding parameters such as current, voltage, wire feed speed, and gas flow. Select the appropriate welding wire and welding gun, ensuring the wire diameter and material match the workpiece, and the welding gun model and rated current suit the welding current. Clean the workpiece surface, removing oil, rust, paint, and other contaminants to ensure welding quality. Check How to Adjust the Current and Voltage of MIG Welding? 2 Methods for Precise and Fast Adjustment!
2) Arc Initiation
Arc initiation involves creating a short circuit between the welding wire and the workpiece to ignite the arc. Before striking the arc, trigger the control switch on the welding gun to feed the wire out of the nozzle, maintaining an extension length of 10-15 mm. Position the welding gun at the arc initiation point, making contact between the wire tip and the workpiece. The nozzle height is determined by the welding current, typically 10-20 mm. Turn on the power, the welding machine will automatically start the gas flow, delay the current, and maintain a high voltage with a slow wire feed speed. When the wire short-circuits with the workpiece, the arc ignites. During arc initiation, prevent the arc from becoming too long and extinguish it by applying slight pressure to the welding gun.
3) Welding
Welding involves using the arc's heat to melt the welding wire and the workpiece, forming a molten pool that moves along the weld seam. Maintain an appropriate tilt angle and nozzle height of the welding gun, generally 5-15° and 10-20 mm. The welding gun should move in the same direction as the wire feed, known as the left welding method. Keep a uniform movement speed of the welding gun, typically 300-500 mm/min. For wider grooves, move the welding gun laterally to increase the pool width and improve fusion. Continuously monitor the welding effect, including the pool size, shape, depth, arc stability, spatter amount, and weld formation, and adjust the welding parameters accordingly to ensure quality.
4) Arc Termination
Arc termination is the process of ending the welding and extinguishing the arc. Prevent crater defects at the weld end by filling the crater, which some welding machines do automatically by lowering the current and voltage and extending the gas flow time. If not equipped, manually stop the forward motion of the welding gun and repeatedly break and reignite the arc until the crater is filled. This operation should be quick to avoid defects like lack of fusion or porosity if the pool solidifies completely before reigniting.
III. Precautions for CO2 Gas Shielded Welding
Despite its simplicity, CO2 gas shielded welding requires attention to several precautions:
1) Preventing Gas Cylinder Overheating
Store gas cylinders in cool, dry places away from sunlight, heaters, stoves, and other heat sources. The ground temperature should not exceed 31°C to prevent CO2 from evaporating into high-pressure gas and causing an explosion. Cylinders should not be placed horizontally to avoid rapid CO2 liquid outflow and evaporation, which can rupture the ducts.
2) Preventing Gas Leaks
Regularly inspect the connections between gas cylinders, regulators, gas hoses, and welding guns to prevent leaks that can affect welding quality and safety. Check connections by applying soapy water and observing for bubbles, indicating leaks to be repaired or tightened. Always control the gas cylinder valve to prevent gas wastage or accidental leaks.
3) Preventing Excessive Arc Length or Short Arc
Maintain the arc length between 1.5-2 times the wire diameter, typically 3-5 mm. Excessive arc length leads to instability, reduced weld penetration, increased porosity and spatter, and poor weld formation. Short arcs cause frequent short circuits, large current fluctuations, increased penetration, expanded heat-affected zones, and increased weld distortion. Control arc length by adjusting the nozzle height and welding voltage, generally, higher voltage results in a longer arc.
4) Preventing Porosity and Lacking of Fusion
Porosity refers to gas voids in the weld, and lack of fusion refers to unmelted metal between the weld and groove or welds. Both defects reduce weld strength and toughness, affecting quality. Causes include:
Poor Gas Shielding: If the gas flow rate is too high or too low, the gas pressure is too high or too low, the gas purity is insufficient, the gas contains moisture or oil, there are leaks in the gas line, the gas nozzle is clogged or damaged, the distance between the gas nozzle and the welding wire is too far or too close, the direction of the gas nozzle does not align with the weld seam, the angle of the gas nozzle is incorrect, the distance between the gas nozzle and the workpiece is too far or too close, the distance between the gas nozzle and the arc is too far or too close, the distance between the gas nozzle and the molten pool is too far or too close, the distance between the gas nozzle and spatter is too far or too close, or if the gas nozzle is not compatible with wind speed, ambient temperature, ambient humidity, ambient pressure, ambient oxygen content, ambient nitrogen content, ambient carbon dioxide content, or other environmental gas concentrations, poor gas shielding will occur. This allows oxygen and nitrogen from the air to enter the molten pool, forming porosity and nitrides. Therefore, it is essential to select the appropriate gas flow rate, pressure, and purity according to the welding conditions, maintain the integrity of the gas line, clean the gas nozzle, and adjust the position, direction, and angle of the gas nozzle to avoid external interference and ensure effective gas shielding.
Incorrect Welding Parameters: If the welding current is too high or too low, the welding voltage is too high or too low, the wire feed speed is too fast or too slow, the welding gun movement speed is too fast or too slow, the tilt angle of the welding gun is too large or too small, the oscillation amplitude of the welding gun is too large or too small, the oscillation frequency of the welding gun is too fast or too slow, or the oscillation pattern of the welding gun is unsuitable, incorrect welding parameters will affect the temperature, pressure, flow, and surface tension of the molten pool, leading to instability and causing gas to escape or become trapped, forming porosity. This will also affect the width, depth, length, and shape of the molten pool, resulting in poor fusion between the molten pool and the groove or weld seam, causing a lack of fusion. Therefore, select the appropriate welding parameters based on the welding material, thickness, groove, position, and method, and maintain stable parameters to control the molten pool state and ensure proper fusion and gas escape.
Substandard Welding Materials: If the diameter, material, composition, surface quality, or extension length of the welding wire is substandard, or if the thickness, material, composition, surface quality, groove form, gap, or preheat temperature of the workpiece is substandard, these factors will lead to abnormal melting, fusion, and gas escape of the welding wire and workpiece, resulting in porosity and lack of fusion. Therefore, choose welding wires and workpieces that meet standards, clean their surfaces to remove oil, rust, paint, moisture, and other contaminants, and ensure the quality of welding materials.
Non-standard Welding Operations: If arc initiation, arc termination, arc restarting, arc refilling, root pass, filler pass, or cap pass welding operations are not standard, this will affect the continuity, uniformity, and smoothness of the weld seam, causing porosity and lack of fusion. Therefore, follow the basic rules of welding operations, and pay attention to the sequence, direction, and methods of welding to ensure standard operations.
These are the methods for using CO2 gas-shielded welding. I hope this information is helpful to you. If you have any other questions about CO2 gas-shielded welding, please follow Megmeet Welding Technology News Center to learn more about welding knowledge and techniques.
