Metal powders have gained increasing popularity in recent years mainly because of new, practical and commercially feasible methods for producing them. Metal powders can be produced by a number of processes including atomization of the molten metal by liquids or gases under pressure. A particularly advantageous method for the liquid atomization of molten metals, particularly iron or steel, is disclosed in my U.S. Pat. No. 3,334,408. Briefly, the method disclosed in the foregoing patent involves the use of pairs of high velocity, thin, solid flat streams of cooling liquid that angularly impinge upon a stream of the molten metal to disperse it into fine, irregularly shaped powder particles. The powder particles thus formed are quenched and may be subsequently molded or compacted into coherent forms having many commercial applications.
I have found that the techniques adopted for the production of optimum powder shapes (i.e., irregular, angular) are inherently conducive to rapid surface oxide formation. Thus, iron powders produced by the liquid atomization of molten iron or steel generally have an oxygen content of more than about 0.7% after quenching and between about 0.8% and 1.0% after being dried. In order to use such iron powders for high quality products, (i.e., those requiring a low oxide impurity grade iron), the oxygen content of the powder should be reduced to less than about 0.25%. The removal of such oxide impurities from iron powders can be accomplished by annealing the powder in a reducing atmosphere in accordance with well known procedures. However, the annealing process can have adverse effects on the powder, as by undesirably increasing the grain size. It also has been found that the annealing of iron powder relieves energy and internal stresses in the particles which I have found to be advantageous for the subsequent processing of wrought products.
The oxidation of iron powder particles produced by liquid atomization of the molten metal is a function of many variables, including the particle size, time at elevated temperature, and environment. Iron powder will oxidize very rapidly at temperatures down to about 300.degree. F. in an oxidizing environment. However, when cooled to below about 200.degree. F., the oxidation rate is relatively slow. Oxide formation also, of course, can occur during the drying of liquid atomized powder, which tends to compound the problem of high oxide formation.
Heretofore, where low oxide powders have been required, it has been conventional to utilize gas atomizing techniques, rather than liquid atomization, to derive the metal powders. However, gas atomizing techniques have many significant disadvantages. For one thing, the production capacity of a gas atomizing system is very low, as there is a relatively low rate of heat transfer between the hot metal and the atomizing gas. Additionally, the cost of the atomizing gas, which must be inert, is a significant factor in the economics of the system. Moreover, since the metal is cooled down at a relatively slow rate by gas atomization procedures, the atomized metal forms into particles of spherical shape, and particles of spherical shape are disadvantageous, as compared to irregular, angular particles produced by liquid atomization, for many end uses. Thus gas atomization has not provided a satisfactory answer to the production of low oxide atomized powder.