Welding is a process that involves heating or pressurizing (or both) the material of the part to be welded, whether it is the same or different. The use of filler materials is also determined to facilitate the combination of atoms and create a permanent connection between the part materials.
Related Reading: The Ultimate Guide to Welding
What are the main points and precautions for stainless steel welding?
I. Why can't stainless steel be welded with carbon steel welding rods?
Stainless steel cannot be welded with carbon steel electrodes because this can lead to intergranular corrosion in the stainless steel.
What is intergranular corrosion?
It is a type of localized corrosion. When stainless steel has excessive carbon content, the expansion rate of carbon along the interstices of stainless steel grains is faster than that of chromium. Carbon extends into the grains of stainless steel and forms a compound with chromium at the grain boundaries.
The chromium content near grain boundaries decreases significantly, leading to the formation of chromium-depleted areas. In the presence of a corrosive medium, these areas lose corrosion resistance, resulting in intergranular corrosion.
Intergranular corrosion significantly reduces the mechanical strength and physical properties of stainless steel, making it unable to withstand even light impacts. It is a very dangerous type of corrosion.
Intergranular corrosion in stainless steel is caused by carbon. Generally, carbon steel contains more carbon than stainless steel, which is why welding stainless steel with carbon steel electrodes can lead to intergranular corrosion.
Therefore, for welding stainless steel, it is advisable to use low-carbon or even ultra-low-carbon stainless steel electrodes.
On the other hand, for important structures that require high impact toughness and crack resistance, it is not advisable to choose acidic electrodes. Acidic electrodes refer to welding rods that contain a large amount of acidic oxides (such as silicon dioxide, titanium dioxide, etc.) in their coating.
This is mainly because:
1. The oxides in acidic electrodes make them highly oxidative and the weld metal contains a lot of oxygen, resulting in significant burning of the alloy elements.
2. The coating of acidic electrodes contains more silicon dioxide, a part of which exists in the weld in the form of silicon dioxide impurities, leading to poorer mechanical properties of the weld, especially lower plasticity and impact resistance than basic electrodes.
3. The acid electrode coating does not contain fluorite (CaF2), resulting in low dehydrogenation capacity; the weld metal contains a larger amount of hydrogen, poor cold cracking resistance, and a smaller amount of the desulfurizing element manganese, leading to a worse desulfurization effect.
Considering the above, it is not advisable to use acid electrodes for welding crucial stainless steel structural parts. Instead, alkaline electrodes with a high alloy transition coefficient and lower levels of oxygen, sulfur and hydrogen should be selected.
II. Which electrode is used for welding stainless steel ?
Stainless steel welding rods can be categorized into two types: chromium stainless steel welding rods and chromium-nickel stainless steel welding rods.
All welding rods that meet the national standard must undergo inspection in accordance with GB/T983-2012.
Chromium stainless steel has a certain level of resistance to corrosion (in oxidizing acids, organic acids and cavitation) and heat, which makes it a suitable choice for equipment and materials used in power plants, chemicals, petroleum and other industries.
However, the weldability of chrome stainless steel is generally poor, requiring careful attention to the welding process, heat treatment conditions, and the selection of appropriate welding electrodes.
Chromium-nickel stainless steel electrodes offer excellent corrosion resistance and oxidation resistance, making them widely used in chemical, fertilizer and petroleum fields.
To avoid intergranular corrosion caused by heating, the welding current should not be too high, and should be around 20% lower than that used for carbon steel electrodes. Furthermore, the arc must not be too long and the interlayer must be cooled quickly, which is best achieved by narrowing the weld bead.
III. Key Points and Precautions for Welding Stainless Steel
1. The power supply with vertical external characteristics must be adopted, and the positive polarity must be adopted in DC (the welding wire must be connected to the negative electrode)
1. It is generally suitable for welding thin plates less than 6mm, resulting in beautiful weld formation and minimal welding deformation.
2. The shielding gas used is argon with a purity of 99.99%. The argon flow rate should be 8-10L/min when the welding current is between 50-150A and 12-15L/min when the current is between 150-250A.
3. The length of the tungsten electrode protruding from the gas nozzle should be 4-5 mm. However, it should be 2 to 3 mm in areas with little shielding, such as fillet welding, and 5 to 6 mm in areas with deep grooves. The distance between the nozzle and the workpiece should generally not exceed 15 mm.
4. To prevent the occurrence of welding pores, it is important to thoroughly clean the welding parts and remove any rust, oil stains, etc.
5. When welding common steel, the welding arc length should be 2 to 4 mm. For stainless steel welding, 1-3 mm is ideal, as a longer arc length may negatively affect the shielding effect.
6. During butt preparation, it is necessary to provide gas shielding at the back of the lower weld bead to prevent oxidation.
7. To ensure good protection of the argon welding puddle and facilitate the welding operation, the center line of the tungsten electrode and the workpiece at the welding point should be kept at an angle of approximately 80°-85°. The angle between the filler wire and the part surface should also be minimized, ideally around 10°.
8. Wind protection and ventilation are essential. In very windy areas, measures must be taken to block the wind. Adequate ventilation measures must be taken indoors.
2. Key points and precautions of stainless steel MIG welding
1. Flat welding power source is used and DC reverse polarity is employed (with the welding wire connected to the positive electrode).
2. Normally, pure argon (99.99% purity) or Ar+2% O2 is used, and the flow rate should be set at 20~25L/min.
3. Arc Length
MIG welding of stainless steel is generally carried out under the spray transfer condition, and the voltage should be adjusted to maintain an arc length of 4~6mm.
4. Windproof.
MIG welding is highly susceptible to wind and air pockets can easily form in windy areas. Therefore, wind protection measures must be taken in locations with wind speeds greater than 0.5m/s.
Related Reading: MIG vs TIG Welding
3. Key points and precautions for stainless steel flux-cored wire welding
1. Flat welding power supply is used, and reverse polarity is used in DC welding.
General CO2 welding machine can be used for welding, but the wire feeder pressure should be slightly adjusted.
2. Carbon dioxide is commonly used as a shielding gas, with a gas flow rate of 20 ~ 25L/min.
3. The distance between the welding nozzle and the workpiece should be between 15 ~ 25 mm.
4. Generally, the length of the dry extension is about 15mm when the welding current is below 250A and about 20~25mm when the welding current is above 250A.
