Research On Microstructure Evolution And Formation Of Fe-C Based Powders By Gas Atomization

High pressure gas atomization technique is widely used as an effective method for preparing ultra-fine metal powders with refined solidification structure to improve the properties of steel materials. Powders produced by using gas atomization exhibit excellent characteristics, including extended solubility of alloying elements, refined grains and reduced microsegregation. Most research works have focused on the preparation technology and its development of ultra-fine powder at home and abroad, while solidification process and microstructure of atomized powders have been paid a little attention to, relatively. In this paper, Fe-based alloy powders were fabricated by gas atomization through the equipment designed by our group. The studies on the solidification behavior, the phase constitution, the process of microstructure evolution and formation of powders are of great significance to improve properties of steel materials.The gas pressure distributions were obtained by using the nozzles with different spray angles, then the experiments under different atomization conditions were carried out on the basis of the former work in order to find out appropriate nozzle structure and atomization process parameters. Eventually the spherical high carbon Fe-based alloy powders were produced with 100μm in medium diameter.In the case of Fe-C binary alloy, microstructure evolution of the powders prepared by high-pressure gas atomization with different diameter was investigated. Network carbide existed in the conventional castings could not be found in the atomized powders of Fe-1.0wt%C alloy. It was shown that the lamellar pearlite existed in the majority of the powders and the irregular ferrite and the pearlite, as well as the acicular martensite, were presented in some powders with larger size. The microstruture was obviously refined in the powders, and the pearlite spacing decreased with the decrease of powder diameter. In the atomized powders of Fe-4.3wt%C alloy, graphite couldn't be observed. The powders ranged from 180 to 150μm in diameter were completely composed ofα-Fe phase and carbide. Furthermore, with decreasing powder diameter, the content ofα-Fe phase gradually decreased, while the content ofγ-Fe phase increased. The microstructure of the powders under 38μm in diameter was mainlyγ-Fe and carbide.Currently, chromium is an addition to improve the resistance to abrasion and heat in Fe-C binary alloy. Microstructure evolution of the atomized powders of Fe-25Cr-C ternary alloy was investigated in this paper. The atomized powders of Fe-25Cr-C alloy contained austenite and M₇C₃ type carbide, but the microstructure of the annealed powders consisted of a mixture of ferrite and carbide. With the decrease of the powder size, the amount of grains in the powders decreased. In the larger powders, a number of grains can be seen, while in the finer powders, no obvious nucleus was found. In hypoeutectic powders, the dendritic microstructure was found. The second dendrite arm spacing decreased with the decrease of the powder size. In the eutectic powders, some small rose-like eutectic colonies and the lamellar eutectic microstructure were developed in the larger powders, while the dendritic microstructure and network eutectic microstructure were obtained in the smaller powders. In the larger hypereutectic powders, the lamellar eutectic microstructure or the eutectic colony can be found near the fine lath-shaped primary carbide, however, there was no primary carbide in the powders with the size under 38μm, and network microstructure was presented in the finer powders.The phase selection of the undercooled Fe-25Cr-3.2C alloy melt was calculated by using steady state and transient state nucleation theories, respectively. The calculation results showed that the critical nucleation energy ofγphase was lower than that of (Fe,Cr)7C3 phase, which gave priority toγphase nucleation. At the same time, the nucleation incubation time ofγphase was shorter than that of (Fe,Cr)7C3 phase. So the priority was conducive to the nucleation ofγphase in the undercooled Fe-25Cr-3.2C alloy melt. According to theoretical calculation on the growth rate of two-phase coupled ofγand (Fe,Cr)7C3 and the dendritic growth rate ofγphase in Fe-25Cr-3.2C alloy, calculation results showed that the dendritic growth rate ofγphase was faster than that of two-phase coupled in the eutectic powders when undercooling was approximately larger than 159K. Therefore, the dendritic microstructure appeared in the eutectic powders under 38μm in diameter with the undercooling of 213.5K or more.