AIR ATOMIZING OF PURIFIED CAST IRON AT “SEVERSTAL” CHEREPOVETS, NORTH-WEST REGION.
Figure 1 Map of the Cherepovets, North-West region
WHAT HAD BEEN DONE?
Experimental studies of the influence of carbon content in the melt and air nozzles parameters on raw powder characteristics had given satisfactory information to adjust the atomizing process and to predict raw powder characteristics such as apparent density which is very significant for the following stage of the technology – annealing.
Summarizing the melt and nozzle parameters influence as follows:
- The higher the carbon content in the melt the lower the apparent density of raw powder. It may be explained by the higher overheating of the melt above the liquidus line that provides better fluidity of the melt and lowers its viscosity. Both of them are very positive during the atomizing process.
- The less angle between nozzles the lower apparent density. It may be explained by the effect of interaction between jets and droplets within the gas-metal spray. Probably, the sharper angle of the air jet attack on the metal stream creates tangential force spinning surface of the metal droplets. Such spinning force dominates above spheroidising force created by the droplets’ surface tension and the particles have resulted in a more irregular shape. A bigger angle at the same time opposite jets of the air may counteract and exhaust their impact energy.
- The less angle between air nozzles also creates a longer distance for free fall of the metal jet before the focal point, which may provide a condition of preliminary destruction of the metal jet for ligaments.
- The less angle between air nozzles creates higher suction of the air jets and better condition for metal jet destruction before the focal point.
- The longer distance between the nozzle the higher the apparent density due to the lower kinetic energy of the jets till the focal point. That may create the condition for a longer time of spheroidising resulting in more smooth and rounded particles.
Stage 1 is intended to purify high carbon iron alloy in the existing 3 tons ARC melting furnace. Initially, it was liquid cast iron transferred from nearby blast furnaces workshop, later it was pig iron or a mixture of liquid cast iron and solid pig iron. The main task was to remove gang elements like Si, Mn, P, S down to an acceptable level and the same time to preserve high carbon content in the alloy and overheat the molten metal up to 1650°C before atomization. It is one of the key stages to obtain the final iron powder of bi-structural characteristics.
Stage 2 is intended to atomize overheated high carbon alloy by compressed air into water. By regulating the atomizing process parameters it was claimed the possibility to vary the final apparent density of the raw powder as integral characteristics of the shape and surface of the powder particles. It is also one of the key stages.
Stage 3 includes standard operations to collect water and iron powder pulp, separate water and powder, purify water and return to the atomizer chamber, dry up the humid powder and accumulate dry raw powder before the next stages.
Stage 4 According to the patented technology the raw powder should contain carbon within 2.2% – 3.5% by mass and oxygen up to 12%. So, stage 4 is intended to decarburize the raw powder by the interaction between carbon and oxygen, which raw powder already contains. The reaction between carbon and oxygen of solid-phase proceeds via carbon gasification and consequent reaction between gaseous agent and oxygen of the powder. In continuous band type furnace process begins in the hot reaction zone and continues in the cooling zone to cool down the decarburized powder (here and further self-annealed powder) down to at least 50°C. Under a self-generated atmosphere in a hot zone, oxygen reduction is possible, but in the cooling zone under such conditions the secondary oxidation takes place. To prevent secondary oxidation the internal furnace space (muffle of the furnace) purges continuously with inert nitrogen. So, this key stage is named decarburizing under an inert gas furnace.
It was also claimed that during this stage the critical bi-structural properties of the particle surface were created. An integral indicator of this feature is the apparent density of the self-annealed powder.
Stage 5. After intermediate crushing self-annealed power goes to final purification under hydrogen. The stage is realized by conveyor band muffle furnace under hydrogen atmosphere. The target is to remove remnant oxygen down to 0.2-0.3% by mass according to demands on ready-made commercial iron powder.
Stage 6 is the standard operation of crushing, milling, and sieving of the sponge sintered cake after hydrogen annealing furnace.
Stage 7 is the standard operation of mixing the separate fraction of the powder according to the request on fraction composition etc.
THE TECHNOLOGY CAN BE REALIZED WITH MODERNIZATION AND MODIFICATION AGAINST AN ORDER.