1. What are stainless steel and acid resistant stainless steel?
Answer: The content of the main element "chromium" in metal materials (with the addition of other elements such as nickel and molybdenum) can make steel in a passive state and have stainless characteristics. 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?
Answer: Austenitic stainless steel is the most widely used and has the largest variety. For example:
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 level of technical difficulty in welding stainless steel?
Answer: The main process difficulty is:
1)Stainless steel material has strong thermal sensitivity, with a slightly longer residence time in the temperature range of 450-850 ℃, resulting in a significant decrease in the corrosion resistance of welds and heat affected zones.
2)It is prone to thermal cracking.
3)Poor protection and severe high-temperature oxidation.
4)The coefficient of linear expansion is large, resulting in significant welding deformation.
4. Why are effective process measures necessary for welding austenitic stainless steel?Answer: General process measures include:
1)Strictly select welding materials based on the chemical composition of the base material.
2)Small current, Quick welding; Small line energy reduces heat input.
3)Thin diameter welding wire and electrode, non swinging, multi-layer and multi pass welding.
4)Forced cooling of welds and heat affected zones to reduce residence time at 450-850 ℃.
5)TIG welding seam back argon protection.
6)The weld seam in contact with the corrosive medium is finally welded.
7)Passivation treatment of welds and heat affected zones.
5. Why is it necessary to use 25-13 series welding wire and electrode for welding austenitic stainless steel, carbon steel, and low-alloy steel (dissimilar steel welding)?
Answer: For welding dissimilar steel joints connecting austenitic stainless steel with carbon steel and low-alloy steel, the deposited metal of the weld must use 25-13 series welding wires (309, 309L) and welding rods (Ao312, Ao307, etc.). If other stainless steel welding materials are used, martensitic structure will be generated on the fusion line of carbon steel and low-alloy steel, which will result in cold cracks.
6. Why is a protective gas of 98% Ar+2% O2 used for solid stainless steel welding wire?
Answer: When using solid stainless steel wire MIG welding, if pure argon gas protection is used, the surface tension of the molten pool is high, the weld formation is poor, and the weld shape is "hunchback". Add 1-2% oxygen to reduce the surface tension of the molten pool, resulting in a smooth and aesthetically pleasing weld formation.
7. Why does the surface of solid stainless steel welding wire MIG weld turn black?
Answer: Solid stainless steel wire MIG welding has a fast welding speed (30-60cm/min), and the protective gas nozzle has already run to the front molten pool area. The weld is still in a red hot high temperature state, oxidized by air, and the surface generates oxides, causing the weld to turn black. The pickling passivation method can remove black skin and restore the original surface color of stainless steel.
8. Why does solid stainless steel welding wire require a pulsed power supply to achieve jet transition and splash free welding?
Answer: When using solid stainless steel wire for MIG welding, with a diameter of 1.2 wire, the jet transition can only be achieved when the current I is ≥ 260-280A; Droplets below this value are considered short-circuit transition, with significant splashing and generally cannot be used. Only by using a pulsed MIG power supply with a pulse current greater than 300A can pulse droplet transition be achieved under welding currents of 80-260A without spatter welding.
9. Why is CO2 gas shielding used for flux cored stainless steel welding wire? Don't you need a power supply with pulses?
Answer: Currently, commonly used flux cored stainless steel welding wires (such as 308, 309, etc.) have a flux formula developed based on the welding chemical metallurgical reaction generated under CO2 gas protection, so they cannot be used for MAG or MIG welding; Pulse arc welding power sources cannot be used.