Phase transformation in Fe-Mo-C and Fe-W-C steels. 1: The structural evolution during tempering at 700 C
- Max-Planck Inst. fuer Eisenforschung GmbH, Duesseldorf (Germany)
Mechanism and kinetics of carbide transformation during tempering at 700 C have been studied in Fe-Mo-C and Fe-W-C steels (with up to 2.5% W or Mo) by transmission electron microscopy (TEM) and X-ray diffraction. The sequence of carbide formation is Fe{sub 3}C{yields}Mo{sub 2}C{yields}(Fe{sub 2}MoC, M{sub 23}C{sub 6}) in molybdenum steels and Fe{sub 3}C{yields}M{sub 6}C{yields}M{sub 23}C{sub 6} in tungsten steels. Increasing the alloying element level increases the rate of carbide replacement reaction. In Fe-Mo-C steels the Fe{sub 2}MoC carbides nucleate preferentially at the Fe{sub 3}C-{alpha} interface and grow into cementite, whereas in Fe-W-C steels the Me{sub 2}C carbides usually precipitate inside cementite giving rise to the in situ transformation Fe{sub 3}C{yields}M{sub 6}C. The software DICTRA and THERMO-CALC were used to simulate cementite growth and to show the possibility of the in situ transformation. The M{sub 23}C{sub 6} carbide is first confined to prior austenite grain boundaries and penetrates into the grains with increasing tempering time. During growth the M{sub 23}C{sub 6} carbide absorbs surrounding pre-existing carbides. As a result, after tempering for 500 h, patches of two-phase areas with (M{sub 23}C{sub 6} + {alpha}) are observed in tungsten steels, and patches of (M{sub 23}C{sub 6} + {alpha}) or (Fe{sub 2}MoC + {alpha}) in molybdenum steels. The alloying element partitioning between {alpha} and precipitated carbides was determined using TEM-EDS. It was established that the M{sub 23}C{sub 6} carbide is stable at 700 C in both investigated steels. The Fe{sub 2}MoC carbide is stable in the Fe-Mo-C system at this temperature. The MC carbide was not observed even after tempering for 3,000 h.
- OSTI ID:
- 524542
- Journal Information:
- Acta Materialia, Vol. 45, Issue 7; Other Information: PBD: Jul 1997
- Country of Publication:
- United States
- Language:
- English
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