6 hours of aging, hardness values in the range of 57-59 HRC are developed in alloys 330 and 333. With the progress of aging at the optimum temperature, the austenitic matrix gets transformed into a dark matrix embedded with fine and uni- formly dispersed globular carbides (Fig. 7).


may occur according to the aging temperature selected. At temperatures higher than the A3


On aging near the A3


temperature, two types of reactions temperature, the carbide


precipitation does not provoke any allotropic transformation during aging and the austenitic matrix is merely depleted of


carbon and other alloying elements, due to the precipitation of carbides. The reaction can be written as, γ → γ1


+ carbides,


where γ1 the A3


is alloy depleted austenite. At temperatures below temperature, the carbide precipitation is accompanied by a partial transformation of austenite into pearlite or fer-


elements present (Cr, Mn, Si, and Cu) obtained by EPMA in


rite-carbide aggregate. The reaction can be written as γ → γ1 + (α + carbides) where α is ferrite. The electrolytically ex- tracted carbides from the ausaged samples when subjected to x-ray diffraction analysis, are identified mostly as chromium carbides of M7


C3 types. The x-ray mappings for the alloying


Figure 8. SEM image and corresponding x-ray map of ausaged alloy 333: a) SEM micrograph, b) Cr Kα x- ray map, c) Mn Kα x-ray map, d) Si Kα x-ray map and e) Cu Kα x-ray map.


International Journal of Metalcasting/Winter 11 53


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