An analysis of the decarburization and aging processes in 2 1/4 Cr−1 Mo steel

Abstract

Specimens of 2 1/4 Cr−1 Mo steel that had been decarburized in sodium or aged in an inert atmosphere for 26,500 h at 839 K were metallurgically examined with optical and transmission electron microscopy. The carbide particle density of the decarburized and aged specimens was considerably less after 26,500 h than after 10,000 h, with the density in the aged specimen being considerably greater than that in the decarburized specimen. For the aged specimen, the density decrease was a result of Ostwald ripening, and the total amount of carbide was not noticeably affected. Decarburization, however, resulted in a gradient in carbide density, with the smallest density near the surface. Carbides were electrolytically extracted and identified by X-ray diffraction. From the before-test material, which was in the annealed condition and had a ferrite-pearlite microstructure, 1.4 wt pct carbide was obtained. This sample contained M₃C, M₂₃C₆, and M₆C, with M₂₃C₆ constituting the major portion. In the aged specimen, 2.3 wt pct of precipitate was extracted and contained 60 pct M₆C, the balance M₂₃C₆. The decarburized specimen contained two regions with different carbide contents. The first 20 pct of the 1.6 mm thick (0.063 in.) specimen yielded 1.3 wt pct precipitate, which contained 94 pct M₆C, the balance M₂₃C₆. The interior of the specimen contained 1.6 wt pct precipitate, 86 pct M₆C.