How to avoid the porosity of self-shielded flux-cored welding wires
In the last article we introduced what is copper-free welding wire and its advantages. As we know, there are mainly two kinds of welding wire according to its protection: One is the welding wire that relying on flux or gas protection, the welding wire plays as filling metal and conducting electricity, such as submerged arc welding, solid cored welding wire and part of flux-cored welding wire used in CO2 gas shielded welding; The other kind is the flux-cored welding wire without external gas protection, it relies on the alloy elements of the wire itself and high temperature to prevent the invasion of oxygen, nitrogen and other gases in the air and adjust the composition of the weld metal, which is called self-shielding flux-cored wire, is a kind of a little expensive but potential welding wire.
At present, self-shielding flux-cored wire is widely used in pipeline construction, ocean engineering, outdoor large steel structure manufacturing, high-rise steel structure building, surface surfacing, especially the welding of light structures such as thin carbon steel and galvanized steel plate. The self-shielding flux-cored wire protects the droplet and molten pool by the gas and slag produced by the slag-forming and gas-forming agent in the arc core under the action of high temperature, and the welding porosity or welding pores are a common problem in the semi-automatic welding of self-shielding flux-cored wire, so we analyze and make some control measures to avoid them.
The cause of welding pores for the self-shielded flux-cored welding wires
Welding cooling rate
Due to the gravity of the liquid metal itself in the vertical welding section, the welding speed is faster and the weld pass melting depth is shallow, which speeds up the cooling rate of the liquid metal in the weld, reduces the gas escape, and causes more pores in the weld pass.
When the metal oxide spatter adhered to the front end of the conductive nozzle reaches a certain amount, it enters the molten pool with the moving welding wire. This becomes more serious with the increase of the amount of metal in the weld pass, resulting in the occurrence of porosity in the weld pass.
The weld joint of the hot welding layer, filling layer and cover layer is easy to superposition, which increases the chance of dense pores in the weld bead.
When the welding wire is placed in an open-air environment with high humidity, which is easy to cause the welding wire to be damp. In addition, if no wind protection measures are taken when the wind speed is greater than 8m/s, it is also an important reason for the occurrence of pores in the weld pass.
Welding process parameters
If there is a narrow adjusting range of welding process parameters of the semi-automatic welding of self-shielding flux-cored. Generally, the arc voltage is between 18 and 22V, and the wire feeding speed is between 2000 and 2300mm/min. Otherwise, the high voltage is easy to cause the slag protection effect on the weld pass surface is not good, easy to produce pores.
How to avoid the welding pores?
- Adjust the arc voltage and welding parameters before welding.
The welding power supply adopts DC and inverter power supply, DC direct connection (DC-): the welding parts are connected to the positive pole of the power supply, and the welding gun is connected to the negative pole of the power supply. The welding ground wire is close to the welding area as far as possible, and it should be confirmed that the conduction is good (whether the ground wire is oxidized, whether the connection is firm, and there can be no rust in the contact place between the ground wire and the base metal). If the conduction is not good, it will cause arc instability.
The welding parameters directly affect welding quality. Too small current is easy to cause the incomplete fusion, slag and other defects, while too large current is easy to cause the burn through, splash increase, down to the welding caused by slag and molten iron drip, can not be applied to welding, also easy to appear pores. Voltage is too low, it’s easy to cause arc instability, top wire, incomplete molten pool and slag inclusion. Voltage is too high, the arc is too far from the molten pool, air involved in the molten pool, and holes occurs.
|AWS A5.20 E71T-11|
AWS A5.20 E71T-GS
|DC- connection, positive grounding wire, negative welding gun|
- Angle of welding torch
Before welding the cover layer, if the filling layer in the vertical welding section is too low or too high, it shall be trimmed until the welding height of the filling layer is about 0.5~1.0mm lower than the base metal, before the welding of the next procedure can be carried out.
- Control the extension length and Angle of the welding wire
Generally should be controlled in 6 ~ 10 times the diameter of the welding wire, generally 15~20mm, such as dry elongation is too long, will make the welding wire melting too fast, reduce the arc blowing force. Too short will cause the metal oxide spatter at the front of the conductive nozzle to accumulate too fast; Too long will reduce the arc voltage and affect the quality of welding. In addition, you need to check and clean the conductive nozzle before welding. The Angle of the welding wire is generally required to maintain 800 ~ 900 between the welding wire and the workpiece to avoid the downward flow of molten slag and molten iron near the vertical position, which affects the smooth welding operation and is prone to defects such as slag inclusion and porosity.
- Necessary preparation before welding.
The surface of the welded parts should be uniform and smooth, and there should be no rust, slag, grease and other harmful substances that affect the welding quality.