FAST DIFFUSION ALLOYING FOR POWDER FORGING USING A LIQUID PHASE

Abstract

Prealloyed powders are dominant items in the economics of powder forgings today. Present estimates predict a maximum powder consumption that is too low to bring about essential reductions in the cost of these materials. Long homogenization discourages the use of plain iron powders with admixed alloying elements, especially where induction heating is employed, and the use of cheap alloying elements such as manganese is hampered by oxidation problems. A way of overcoming these difficulties is described. If the alloying additions needed for hardenability are made in the form of a low-melting master-alloy powder, diffusion times can be very much reduced. A condition is that the molten prealloy wets the iron particles, reducing the diffusion distance to the order of one particle radius. It is also desirable that the master alloy should penetrate quickly along the grain boundaries of the iron, further reducing the diffusion distance. Low-melting alloys of manganese with copper satisfy these conditions. Forgings can be produced from plain iron powders with copper-borne manganese additions without excessive oxidation of the manganese and preforms can be sufficiently homogenized within a heating time of a few minutes to give hardenability and tensile properties similar to those of conventional quenched and tempered steels. Since only small amounts of master-alloy powder are needed, ‘solid-liquid alloyed’ plain iron powders appear to offer great flexibility in alloy composition at a cost substantially below that of conventional prealloyed powders.