304 stainless steel pipe is a universal stainless steel material with stronger anti-rust performance than 200 series stainless steel materials. It is also relatively good in high temperature resistance and can maintain good corrosion resistance below 800 degrees. 304 stainless steel pipes have excellent stainless corrosion resistance and good resistance to intergranular corrosion.
During the use and production of 304 stainless steel pipes, the weld position will turn black. Blackening of the weld is one of the common welding defects. Why is the weld of 304 stainless steel pipe black? What is the reason? How to deal with this situation?
The main reasons for the blackening of the weld of 304 stainless steel pipes
The blackening of the weld of 304 stainless steel pipes is due to the oxidation of the weld surface and the high temperature oxidation of the surrounding materials.
The specific reasons are as follows:
1. The stainless steel pipe was not cleaned before welding, and there were impurities such as oil on it, causing the welding to turn black
2. Insufficient argon protection, impure protective gas, oxygen and moisture in the gas, insufficient argon supply
3. Long time welding
4. Excessive current
5. Improper distance between welding tungsten needle and weldment
6. Improper selection of tungsten needle
7. The molten pool temperature is high and no protection is continued after welding is completed.
8. The back of the stainless steel pipe welding is not properly protected (the back is black).
How to deal with the blackening of the welds of 304 stainless steel pipes?
1. Supply gas in advance: Supply gas before welding, and then weld the weldment to ensure that the argon gas protects the welding part and prevents oxidation.
2. Delayed gas supply: After welding, the gun muzzle should not leave the welding part of the weldment, and gas should continue to be supplied to ensure that the weld and surrounding parts of the weldment are not oxidized by high temperature.
3. Control of welding gun switch: For 304 stainless steel pipes, do not weld continuously for a long time with high current during welding, and use spot welding more often. Excessive current and slow welding speed will cause the weld to turn black.Therefore, when welding, you should select the current according to the thickness of the pipe, select the tungsten needle according to the current, select the porcelain nozzle according to the tungsten needle, and select the gas volume according to the porcelain nozzle. The welding speed should be fast and the technique should be accurate.
4. Technique: The distance between the tungsten needle tip and the weldment. If the tungsten needle touches the weldment, the weld will be black; if the tungsten needle is too far away from the weldment, the weld will also be black. The main reason is to ensure the stability and strength of the welding arc during welding.
5. Selection of tungsten needles: ①Tungsten needles with yellow and white markings should be used as much as possible, and the quality of tungsten needles should be reliable; ②The tungsten needle should be kept sharp during use. The reason is: if the tungsten needle is blunt, the current cannot be concentrated, and the stability of the arc is not easy to maintain during welding, and it is easy to diverge.
6. Control of the molten pool temperature at the welding point: It is related to the welder's technique and gun movement speed. The molten pool temperature that meets the requirements for welding of the weldment is closely related to the current size, while ensuring that it does not oxidize before cooling is related to the gun movement speed. Welding and cooling should be carried out at the same time, and the gun movement speed must ensure that the argon gas protects the front molten pool and does not oxidize it due to high temperature.
7. Select the welding wire material according to the 304 stainless steel pipe to ensure the uniformity of the material and avoid blackening due to material differences.
8. For the protection of the back of the weldment's welding point and surrounding parts: argon gas protection or coating of protective agent can be applied on the back of the weldment.
The above are the reasons and countermeasures for the blackening of the welds of 304 stainless steel pipes. In addition to the above, the welder's welding skills and his comprehension and improvement capabilities are more important, because it is necessary to understand the principles of electric welding, understand their interrelationships, and summarize, improve, enhance and improve in practice.
Read more: Treatment Methods of 304 Stainless Steel Pipes after Welding
