Effect of process parameters on nitriding rate in obtaining powder metal by plasma centrifugal atomization

The improved performance properties of metals are ensured by introducing into them certain set and amount of alloying elements. Nitrogen, which is an area of growing interest, is one such element. Publications show that nitriding with gaseous nitrogen is also used for plasma-arc remelting. They provide data on metal alloying with nitrogen at the granules and powders production stage. This paper studies the process of nitriding in obtaining metal microgranules from EP741NP alloy by means of plasma centrifugal atomization. Metal powders are obtained by melting the end face of a rotating workpiece with a stream of ionized gas (gas mixture). The technology allows for nitrogen-alloyed fine metal powders of multicomponent alloys of spherical shape with a minimum number of satellites, which do not differ in size or chemical composition, to be obtained. The study of the nitriding rate is of great interest, especially in production of powder metal. One parameter which affects the degree of metal saturation with nitrogen is the residence time of the liquid melt under the nitrogen-containing plasma, and the crystallization time of a metal droplet. This paper presents a methodology which allows quantification of the role of these parameters on the absorption of nitrogen by the metal in obtaining powder. The kinetic parameters of the nitriding process are influenced by the interface area of two metal – gas phases. In the case of obtaining powder, this parameter depends on the size of the powder particle. In this regard, this paper presents a calculation method which allows the average fractional composition of metal powders to be estimated depending on a number of process factors. The values obtained are compared with the data of semi-industrial melting. It is demonstrated that the fractional composition of microgranules depends on the rotation speed and diameter of the workpiece to be remelted, as well as the alloy density and the surface tension force. It has been established that by increasing the rotation speed of the consumable electrode it is possible to achieve a decrease in the dispersiveness of metal powders.

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