There are mechanical, chemical and electrochemical methods for removing scale from stainless steel pipes. Due to the complexity of the scale composition of stainless steel pipes, it is not easy to remove the scale on the surface and make the surface highly clear and smooth. The removal of scale on stainless steel welded pipes generally takes two steps, one is pretreatment, and the other is ash removal.
The pretreatment of the scale of the stainless steel welded pipe makes the scale loose, and then pickling is easy to remove. Preprocessing can be divided into the following methods:
1. Alkaline nitrate melting treatment method.
The alkaline melt contains 87% hydroxide and 13% nitrate. The ratio of the two in the molten salt should be strictly controlled so that the molten salt has strong oxidizing power, low melting point and low viscosity. In the production process, only analysis of sodium nitrate containing halo not less than 8% (wt). The treatment is carried out in a salt bath furnace at a temperature of 450-470°C, and the time is 5 minutes for ferritic stainless steel and 30 minutes for austenitic stainless steel. Similarly, iron oxides and spinels can also be oxidized by nitrates to become loose trivalent iron oxides, which are easily removed by pickling. Due to the high temperature, the formed oxides partly peel off and sink to the bottom of the bath furnace in the form of sediment.
2. Alkaline nitrate melting pretreatment process: steam degreasing → preheating (150-250°C, time 20-30min) → molten salt treatment → water quenching → hot water washing.
3. Molten salt treatment is not suitable for steel pipes with weld gaps or curled edges. When the steel pipe is taken out from the molten salt furnace and water-quenched, an irritating alkali and salt spray will be splashed, so a deep-bowl type anti-splash water quenching tank should be used for water quenching. When water quenching, hang the steel pipe basket into the tank first, stop above the water level, close the tank cover, and then lower the steel pipe basket into the water until it is submerged.
The pretreatment of the scale of the stainless steel welded pipe makes the scale loose, and then pickling is easy to remove. Preprocessing can be divided into the following methods:
1. Alkaline nitrate melting treatment method.
The alkaline melt contains 87% hydroxide and 13% nitrate. The ratio of the two in the molten salt should be strictly controlled so that the molten salt has strong oxidizing power, low melting point and low viscosity. In the production process, only analysis of sodium nitrate containing halo not less than 8% (wt). The treatment is carried out in a salt bath furnace at a temperature of 450-470°C, and the time is 5 minutes for ferritic stainless steel and 30 minutes for austenitic stainless steel. Similarly, iron oxides and spinels can also be oxidized by nitrates to become loose trivalent iron oxides, which are easily removed by pickling. Due to the high temperature, the formed oxides partly peel off and sink to the bottom of the bath furnace in the form of sediment.
2. Alkaline nitrate melting pretreatment process: steam degreasing → preheating (150-250°C, time 20-30min) → molten salt treatment → water quenching → hot water washing.
3. Molten salt treatment is not suitable for steel pipes with weld gaps or curled edges. When the steel pipe is taken out from the molten salt furnace and water-quenched, an irritating alkali and salt spray will be splashed, so a deep-bowl type anti-splash water quenching tank should be used for water quenching. When water quenching, hang the steel pipe basket into the tank first, stop above the water level, close the tank cover, and then lower the steel pipe basket into the water until it is submerged.
