Free-machining agents have different effects upon the microstructures of the test premixes. These effects depend upon the nature of the base iron (sponge or atomized) and the premix composition (Ref 29) and are illustrated with reference to the F-0008 composition in Fig. 6 and 9. Sulfur appears to promote sintering of the test compositions and produces very round pores. A portion of the sulfur dissolves in the iron matrix and diffuses a short distance into the iron matrix. In sponge iron compositions, a portion of sulfur reacts with iron to form iron sulfides. Sulfur also appears to promote pore rounding during sintering.

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Fig. 9 Dimensional change of F-0008 sponge plus free-machining agents

Manganese sulfide (Ref 6) is considered to be stable in both iron-graphite and iron-copper-graphite premixes. Metallography indicates that the manganese sulfide occurs in pores or the fine pores remaining at prior particle boundaries. It appears to be almost inert during the sintering process with little evidence of diffusion into the iron matrix in the compositions examined.

The effects of molybdenum disulfide appear to be between those of sulfur and manganese sulfide and to depend upon the amount added to the test composition. It appears that a significant portion of the molybdenum disulfide reacts with the sintering atmosphere. At low MoS2 additions, almost all of the addition transforms to molybdenum, which remains within pores and at particle boundaries. Some sulfur evaporates during the sintering process and some dissolves in the iron matrix and can form sulfides on cooling from sintering temperature. At higher sulfur additions, it appears that an equilibrium is reached between the sulfur present as molybdenum disulfide and that in the particles, so that more typical slight pore rounding occurs.

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