1. What is stainless steel and stainless acid-resistant steel?
A: The content of the main element "chromium" in the metal material (nickel, molybdenum and other elements need to be added)
It can make the steel in a passivated state and has the characteristics of stainless steel. Acid-resistant steel refers to steel that is resistant to corrosion in strong corrosive media such as acid, alkali, and salt.
2. What is austenitic stainless steel? What are the commonly used grades?
A: Austenitic stainless steel is the most widely used and has the most varieties. like:
<1> 18-8 series: 0Cr19Ni9 (304) 0Cr18Ni8 (308)
〈2〉18-12 series: 00Cr18Ni12Mo2Ti (316L)
<3> 25-13 series: 0Cr25Ni13 (309)
<4> 25-20 series: 0Cr25Ni20 etc.
3. Why is it difficult to weld stainless steel?
Answer: The main technological difficulties are: <1> The stainless steel material has strong thermal sensitivity, and the residence time in the temperature range of 450--850°C is slightly longer, and the corrosion resistance of the weld and heat-affected zone is seriously reduced. <2> Thermal cracks are prone to occur. <3> Poor protection and serious high temperature oxidation. <4> The linear expansion coefficient is large, resulting in large welding deformation.
4. Why should effective technological measures be taken for welding austenitic stainless steel?
A: The general process measures are: <1> We must strictly select welding materials according to the chemical composition of the base metal. <2> Small current., fast welding; small line energy, reduce heat input. <3> Thin-diameter welding wire, electrode, no swing, multi-layer multi-pass welding. <4> Forced cooling of the weld and heat affected zone to reduce the residence time at 450-850°C. <5> Argon gas protection on the back of TIG weld. <6> The welds in contact with the corrosive medium are welded last. <7> Passivation treatment of welds and heat-affected zone.
5. Why should 25-13 series of welding wires and electrodes be used for the welding of austenitic stainless steel, carbon steel and low alloy steel (dissimilar steel welding)?
Answer: When welding austenitic stainless steel, carbon steel and low-alloy steel connected dissimilar steel welded joints, 25-13 series of welding wires (309, 309L) and electrodes (Ao 312, Ao 307, etc.) must be used for the deposited metal of the weld. If other stainless steel welding consumables are used, the martensitic structure will be formed on the fusion line of carbon steel and low alloy steel, which will cause cold cracks.
6. Why is the shielding gas of 98%Ar+2%O2 used for solid stainless steel welding wire?
Answer: During MIG welding of solid stainless steel wire, if pure argon gas is used for protection, the surface tension of the molten pool will be large, and the welding seam will be poorly formed, and it will be in the shape of a "hunchback" weld. Add 1-2% oxygen to reduce the surface tension of the molten pool, and the welding seam is flat and beautiful.
7. Why is the surface of solid stainless steel wire MIG weld black?
Answer: MIG welding speed of solid stainless steel wire is fast (30-60cm/min), the shielding gas nozzle has run to the front molten pool area, the weld is still in a red-hot high temperature state, oxidized by the air, the surface is oxidized, and the weld is damaged. black. Pickling and passivation can remove black skin and restore the original surface color of stainless steel.
8. Why does the solid stainless steel wire need a pulsed power supply to achieve jet transition and spatter-free welding?
Answer: When MIG welding solid stainless steel wire, φ1.2 welding wire, when the current I ≥ 260—280A, can realize jet transfer; less than this value, the droplet is short-circuit transfer, the spatter is large, and generally cannot be used. Only when the MIG power supply with pulse is used and the pulse current is greater than 300A, the pulsed droplet transfer under the welding current of 80-260A can be realized without spatter welding.
9. Why is the flux-cored stainless steel wire protected by CO2 gas? Without a pulsed power supply?
Answer: At present, the commonly used flux-cored stainless steel welding wire (such as 308, 309, etc.), the flux formula in the welding wire is developed according to the chemical metallurgical reaction of welding under the protection of CO2 gas, so it cannot be used for MAG or MIG welding; Pulsed arc welding power source.