The main reason for the darkening of stainless steel welds is the oxidation of the weld surface and the high-temperature oxidation of the surrounding material.
The main causes of stainless steel darkening include:
1) Dirty plate and other welding parts before welding;
2) Insufficient argon gas protection;
3) Prolonged welding;
4) Excessive current;
5) Inadequate distance between the tungsten electrode and the welding;
6) Incorrect choice of tungsten electrode;
7) High pool temperature and lack of continuous protection after welding;
8) Inadequate protection of the back of stainless steel welding (back darkening).
A detailed analysis includes:
1) Dirty welding parts before welding, such as oil impurities in stainless steel welding, lead to darkening during welding. The solution is a thorough cleaning of the welding areas before welding.
2) Insufficient shielding of argon gas may be due to impure shielding gas containing oxygen and moisture. The solution is to guarantee the quality of the argon gas.
3) Damaged gas supply pipes, allowing other gases to mix, causing oxidation during welding. The solution is to check for leaks, block the welding gun's ceramic nozzle, press the welding gun's gas supply switch (be careful with electric shock) and wait to check for pressure. This can be seen on the manometer and pressure column installed in the argon gas cylinder.
4) Insufficient supply of argon gas. The solution is to monitor the pressure gauge and pressure column flow. When the gas pressure is insufficient, replace the argon cylinder.
5) Mastery of welding process parameters, such as argon gas flow during welding, gas delivery and stop time, control of welding change time, adjustment of welding current, distance between the tungsten electrode and the welding During welding, choice of tungsten electrode, weld pool temperature, welding wire and welding compatibility and welder technique are directly related to the level of welding technique.
For argon arc welding requirements:
1) Pre-flow: Send gas before welding to ensure argon protection of the welding area and prevent oxidation.
2) Post-flow: Continue supplying gas after lifting the welding torch, keeping the nozzle on the weld to ensure that the weld and surrounding area are not oxidized at high temperatures.
3) Welding torch switch control: For stainless steel, especially thin plates, avoid continuous and high current welding and use more spot welding. High current and slow welding speed cause darkening of the weld seam. Therefore, select the current based on the plate thickness, choose the tungsten electrode based on the current, choose the ceramic nozzle based on the electrode, and choose the gas volume based on the nozzle. The welding speed must be fast and the technique must be precise.
4) Technique: The distance between the tungsten electrode tip and the welding. If the tungsten electrode touches the weldment or is too far from it, the weld seam will turn black. The main reason is to ensure the stability and intensity of the welding arc during welding.
5) Choice of tungsten electrode:
- ① Always choose tungsten electrodes marked in yellow and white, and the quality of the electrode must be reliable;
- ② Keep the tungsten electrode sharp during use. The reason is that if the tungsten electrode is blunt, the current will not be concentrated and the stability of the arc during welding will not be easy to maintain, leading to dispersion.
6) Weld pool temperature control: This is related to the welder's technique and torch speed. The temperature of the weld pool is closely related to the welding fusion and current size, while ensuring no oxidation before cooling is related to the torch speed. Weld and cool at the same time, the speed of the torch must ensure the argon protection of the front weld pool, so that it is not oxidized at high temperatures.
7) Choose the welding wire material according to the welding material to ensure material consistency and avoid darkening due to material differences.
8) For protection of the welding backside and adjacent areas: Argon gas shielding or protective coating can be applied to the welding backside.
9) Welder's welding skills and their understanding and ability to improve: Understand the principle of electric welding, understand their interrelationships, and summarize, improve, elevate and make improvements in practice. For example, the length of the tungsten electrode coming out of the ceramic nozzle; the distance from the tungsten electrode to the welding, the current required for welding of different thicknesses and the argon flow required for different currents, etc.
Improvement example: Wrap copper wire around the tungsten electrode inside the welding torch. Its function is to create turbulent flow at the nozzle outlet, increasing the width of the gas shield, thus ensuring protection of the front welding point and anti-oxidation protection of the surrounding area during rapid torch movement.
In a good welding technique, the weld seam is gold or even white. But if the weld seam turns black, it needs to be treated.
Treatment for darkening of stainless steel weld beads:
The most common methods include:
1) Wool brush + polishing wax: Polish the weld seam. The weld seam and surrounding area feature stainless steel luster, more suitable for mirror housings;
2) Pickling Passivation Paste: Apply pickling passivation paste to the weld seam and surrounding area. After a few minutes, clean well with water. The weld seam and its surroundings restore the color of the stainless steel.
Observation:
1) Stainless steel welding seam blackening treatment should be carried out after welding cooling.
2) If pickling passivation paste is applied to remove blackening, in the case of low number of stainless steel material, thorough cleaning should be carried out to avoid excessive corrosion and color difference.
3) Removing the blackening of the weld seam must be carried out after removing the welding slag.
The best solution currently viable in our factory is to remove the browning while improving the argon arc welding technique. This can reduce the number of processes in box production. The column can be welded directly to the casing, eliminating the need for column drilling, casing bolt welding and bolt fastening of columns, reducing labor and material costs while increasing casing strength.
For diagonally cutting the box at 45 degrees, specialized molds can be purchased. This allows for quick and easy production, and the product looks beautiful.
