1. What are stainless steel and stainless acid-resistant steel?
The content of the main element “chromium” in metal materials (other elements such as nickel and molybdenum also need to be added)
It can make the steel in a passivated state and has the characteristics of stainless steel. Acid-resistant steel resists corrosion in strongly corrosive media such as acid, alkali, and salt.
2. What is austenitic stainless steel? What are the commonly used grades?
Austenitic stainless steel is the most widely used and has the most varieties. Such as:
- 18-8 series: 0Cr19Ni9 (304) 0Cr18Ni8 (308)
- 18-12 series: 00Cr18Ni12Mo2Ti (316L)
- 25-13 series: 0Cr25Ni13 (309)
- 25-20 series: 0Cr25Ni20 etc.
3. Why is there a certain technological difficulty in welding stainless steel?
The main process difficulty is:
- The stainless steel material has strong thermal sensitivity, and the residence time in the temperature zone of 450-850℃ is slightly longer, and the corrosion resistance of the weld and the heat-affected zone is seriously reduced.
- Hot cracks easily occur.
- Poor protection, serious high-temperature oxidation.
- The linear expansion coefficient is large, resulting in large welding deformation.
4. Why should effective process measures be taken for welding austenitic stainless steel?
The general process measures include:
- We must strictly select welding materials based on the chemical composition of the base metal.
- Small current, fast welding; small wire energy, reduced heat input.
- Thin-diameter welding wire, welding rod, non-oscillating, multi-layer, and multi-pass welding.
- Forced cooling of welding seam and heat-affected zone reduces the residence time of 450-850℃.
- Argon gas protection on the back of the TIG weld.
- Welds in contact with corrosive media are finally welded.
- Passivation treatment of welding seam and heat-affected zone.
5. Why should we use 25-13 series welding wires and electrodes for welding austenitic stainless steel, carbon steel, and low-alloy steel (dissimilar steel welding)?
To weld dissimilar steel welded joints of austenitic stainless steel carbon steel and low alloy steel, the weld deposited metal must use 25-13 series welding wire (309, 309L) and welding rod (Austenitic 312, Austrian 307, etc.). If other stainless steel welding consumables are used, a martensite structure will be generated on the fusion line of carbon steel and low alloy steel, which will cause cold cracks.
6. Why use 98%Ar+2%O2 shielding gas for solid stainless steel welding wire?
When using solid stainless steel wire for MIG welding, if pure argon gas protection is used, the surface tension of the molten pool will be large, and the weld will be poorly formed, showing a “humpback” weld shape. Add 1-2% oxygen to reduce the surface tension of the molten pool, and the weld seam is formed smooth and beautiful.
7. Why is the surface of the solid stainless steel wire MIG welded seam black?
The MIG welding speed of solid stainless steel wire is faster (30-60cm/min). The shielding gas nozzle has run to the front molten pool area, and the weld is still in the red hot and high-temperature state. It is oxidized by the air and oxides are formed on the surface. black. The pickling passivation method can remove the black skin and restore the original surface color of stainless steel.
8. Why do solid stainless steel welding wires need to use a pulsed power source to achieve jet transition and spatter-free welding?
When solid stainless steel welding wire MIG welding, φ1.2 welding wire, when the current I ≥ 260-280A, can realize the jet transfer; the droplet is smaller than this value short-circuit transfer, and the spatter is large, and it is generally not usable. Only use with pulse
The MIG power supply, the pulse current greater than 300A, can realize the pulse droplet transition under the welding current of 80-260A and the spatter-free welding.
9. Why is the flux-cored stainless steel welding wire protected by CO2 gas? Do not use a pulsed power supply.
Currently commonly used flux-cored stainless steel welding wire (such as 308, 309, etc.), the welding flux formula in the welding wire is developed according to the welding chemical metallurgical reaction under the protection of CO2 gas, so it cannot be used for MAG or MIG welding; Pulsed arc welding power source.