Abstract
Conventional sintering techniques for structural steels have been developed principally for Cu and Ni containing alloys. Applying these to Cr and Mn steels (successful products of traditional metallurgy) encounter the problem of the high affinity for oxygen of these elements. A solution is employing a microatmosphere in a semiclosed container which favours reduction reactions. This has already proved successful on a laboratory scale, especially with nitrogen as the furnace gas. Further modifications to the system, now described, include the use of two sintering boxes, one inside the other. Superior mechanical properties, even using air as the furnace gas, are attainable.
Highlights
Most of the structural PM steels, to date, have primarily used as alloying elements Cu, Ni, and Mo: metals with a low affinity for oxygen
Commercial powders containing Cr are available [1,2,3,4,5] and those containing Mn are being developed [6,7,8,9,10,11]. Such alloys are thermodynamically unstable at the sintering temperature and the process is often not repeatable
Whereas the oxides of copper, nickel and molybdenum can be reduced at conventional sintering temperatures in atmospheres without strict dew point control, the oxides of chromium [12,13], manganese [14], and silicon cannot
Summary
Most of the structural PM steels, to date, have primarily used as alloying elements Cu, Ni, and Mo: metals with a low affinity for oxygen. Into the local micro climate of a semi closed container, an element which oxidizes more readily than Cr, e.g. Mn, can be introduced as the oxygen binder, since it has a higher oxidation potential than chromium [18]. The thermodynamically calculated oxygen partial pressure in the semi closed container should be below ~ 10-12 Pa in order to have reducing conditions during sintering at 1120°C of e.g. steel powder pre-alloyed with 1.5% Cr. During cooling, the viscosity of the glass-forming liquid increases by many orders of magnitude, which acts as a “cut off valve” from the furnace atmosphere.
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