1439. ---------------. [HOFER, L. J. E., AND COHN, E. M.] Some Reactions in the Iron-Carbon System. Application to the Tempering of Martensite. Nature, vol. 167, 1951, pp. 977-978.

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Literature Abstracts

Precipitated Fe-Cu-K₂O (100:10:0.32) catalyst was reduced with H₂ at 220° C. and carburized with CO at 188° C. to a C:Fe weight ratio of 0.06966. This preparation, consisting of metallic Fe and hexagonal close-packed Fe carbide, was used as a model for the thermomagnetic study of the reaction that occur during the tempering of carbon-martensite. The results indicate that the 1^st tempering reaction (below 200° C.) may be the decomposition of martensite to hexagonal carbide and metallic Fe, and that the 2d reaction (at 200°-240° C.) is the decomposition of the retained austenite into the same products. Between 240° and 300° C., the hexagonal carbide is converted to Hägg carbide (see abs. 1446) and above 300° C., Fe and Hägg carbide react to form cementite (see abs. 1437). Above 470° C., the rate of decomposition of Hägg carbide into cementite and free Fe becomes appreciable (see abs. 586). Thus, the 3 stages of tempering are essentially confirmed by the reactions observed in the catalyst model, except that an intermediate reaction (formation of Hägg carbide) is postulated that has not yet been observed in martensite.