Causes and preventive measures of pitting on the surface of precision castings
1. Reasons for the formation of pitting on the surface of precision castings
1.1 Metal oxide inclusions brought in due to poor deoxidation of molten steel;
1.2 The conditions for complete deoxidation are: select dry and clean furnace materials, add ferromanganese first, then ferrosilicon for deoxidation after melting, then add silicon calcium for deoxidation, then power off and stand for 2 minutes, add aluminum for final deoxidation, and then heat-preserve pouring. Add sawdust or waste wax immediately after pouring, and then cover the box to seal and cool.
In the case of incomplete deoxidation, if the cover box is sealed and cooled, because the cooling time is long, a large amount of oxygen in the molten steel has sufficient reaction time, and the formation of a large number of pits is inevitable. In the case of complete deoxidation, the cooling of the cover box ensures that no external oxygen is absorbed into the molten steel, avoiding the secondary oxidation of the molten steel and preventing the formation of pitting.
When the shell is impervious to firing, a small amount of gas will be generated at high temperature during casting, which will promote the occurrence of interfacial reactions and form pits.
b The increase of impurities in refractory materials, especially the high content of Fe2O3, will directly lead to its participation in the interface reaction between the mold shell and molten steel, causing oxygen to enter the molten steel to cause secondary oxidation, thereby increasing the tendency of pit formation.
2. Preventive measures
2.1 The most important factor affecting pitting is the quality of molten steel; (that is, whether deoxidation and scum are complete)
a It should be operated in strict accordance with the smelting process.
That is to say, it is carried out according to the process of bottom slag→covering during melting→pre-deoxidation after melting→power off and static slag removal→final deoxidation before pouring.
b The choice of deoxidizer should not only fully deoxidize the molten steel, but also make the oxides formed after deoxidation have a low melting point and be easy to gather and float. The amount of final deoxidizer aluminum added must be strictly controlled, too much aluminum will promote the formation of pitting. (The residual amount of aluminum should be controlled between 0.015-0.02, ideally, if it is too low, it will easily produce pores and pits, if it is too high, there will be white spots and black spots after processing.)
③The steel material should be very clean, and it is not suitable to use too much returned material to increase the original inclusions of the steel material. During the melting process, try to prevent the time when the molten steel surface is exposed, and prevent the oxidation of Cr, Fe, and Si elements.
2.2 Since the temperature of the shell during pouring is about 800°C-900°C, and the temperature of the pouring molten steel is about 1600°C, the temperature contacting the surface of the casting is extremely high at this time, and the metal will not solidify instantly, especially for thick-walled parts. Oxygen in the atmosphere easily reacts with the metal surface through the mold shell to form oxides, and aggregates with inclusions in molten steel to form pits. Therefore, when pouring large pieces, add wood chips (or charcoal) to the insulation. After the module is poured, immediately add waste wax blocks to cover the box to keep warm, which is an important measure to prevent pitting.
2.3 The calcination temperature of the shell should not be lower than 1180°C, and the holding time should not be less than 45 minutes. The shell that is not completely burned through will have a small amount of gas generation during pouring, and will participate in the interface reaction between the molten steel and the shell, bringing oxygen into the molten steel, causing secondary oxidation. Carbon steel has a high content of C and does not contain Cr, so when it is cooled in the air, the interface reaction can only form a decarburized layer without pitting; if there are pitting, the shell is not burnt.
2.4 Refractory materials, especially surface refractory materials, in addition to the main components should meet the requirements, the impurity content mainly Fe2O3 must be low. Excessively high Fe2O3 content will intensify the oxidizing atmosphere of the interface reaction and lead to pitting.
3.1 Pitting is the aggregation of complex oxide inclusions of iron, chromium, silicon and aluminum in molten steel on the casting surface.
3.2 The main measures to prevent pitting are to fully and completely deoxidize during smelting, and to make the deoxidized products easy to float; to prevent secondary oxidation on the casting surface during the cooling process of the casting mold.
3.3 Strictly implement the firing process of the shell.
3.4 Effective measures to prevent pitting of castings: purchase graphite sand (petroleum coke particles) as transition layer shell.
For more than ten years, we have been improving the quality of the casting surface (surface pitting) by adding graphite powder to the back coating, the addition amount is about 0.3-0.5%. Adding graphite electrode (carburizer) to the layer of sand, the effect is very significant.