M3C and M7C3 carbides in 0.46C-5.22Cr steel

Figure 1: Eutectoid growing as isolated carbides sheathed in ferrite, at an immobile ferrite plate interface (arrows); Fe-0.13 pct C-2.99 pct Cr alloy reacted for 20,040 s at 570 °C (near the bay of the TTT diagram); brightfield TEM. Scale bar: 500 nm. (from Reference 1).

Figure 1: (a) Diffractogram showing both M3C and M7C3 peaks. (b) TEM observation of the extracted carbides, showing elongated M3C sheets and M7C3 rods, needles, and globules. Scale bar: 200 nm.

Carbide name: M3C, M7C3
Record No.: 778
Carbide formula: M3C, M7C3
Carbide type: M3C, M7C3
Carbide composition in weight %: No data
Image type: TEM, XRD
Steel name: 0.46C-5.22Cr steel
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: Chromium steels
Steel composition in weight %: 0.46% C, 0.02% Si, 0.02% Mn, 5.22% Cr, 0.01% Cu.
Heat treatment/condition: A ternary Fe-Cr-C (nominally 5 pct Cr, 0.5 C) alloy was vacuum induction cast at the Acxos Villares Research Center as a 60-kg ingot, then homogenized and hot rolled to 254 3 254 mm square section bars. A 500-mm-long bar was encapsulated in a quartz tube under partial argon pressure and homogenized at 1200 °C for 12 hours. Chemical analysis after homogenizing is described in upper text.
Cylindrical samples 2.3-mm diameter by 12-mm long were machined using spark erosion. Further heat treatments used an Adamel Lomargy quenching dilatometer. Each sample was austenitized at 1050 °C for 180 seconds and rapidly cooled to the isothermal treatment temperature, where it was kept for different times, followed by quick cooling to room temperature. A new sample was used for each heat-treatment cycle.
Note: The overall transformation kinetics of austenite isothermal decomposition above the bay of the time-temperature-transformation (TTT) curve and the eutectoid morphology of the resulting products have been studied in a Fe-0.46 pct C-5.2 pct Cr alloy. Classical lamellar pearlite was formed at high temperatures while complex ferrite plus carbide morphologies, sometimes described as spiky pearlite, arborescent structures, or nonclassical decompositions products of austenite in Fe-Cr-C alloys, formed at low temperatures. While X-ray diffraction of extracted carbides and selected area diffraction– transmission electron microscopy (TEM) showed evidence for a mixture of M3C and M7C3 carbides, thermodynamic calculations results obtained only M7C3 as the equilibrium carbide at the temperatures studied. A tentative explanation for the arborescent morphology is presented, based on the hypothesis of the existence of a drag force or free energy dissipation term that is locally relaxed by the partition of Cr into the carbides at the reaction front, consequently removing Cr from the interface.

The diffractogram (Figure 2(a)) showed the presence of more than one type of carbide, both M3C and M7C3 coexisting. The TEM observations of the extracted precipitate revealed distinct morphologies: thin elongated sheets coexisting with globular carbides and rodlike structures (Figure 2(b)). These observations agree with earlier investigations on Fe-Cr-C alloys with similar Cr/C ratios. The presence of the metastable M3C carbide is also in agreement with the literature results; the metastable eutectoid is kinetically favored since it needs lower Cr partition between the carbide and the ferrite.
Links: No data
Reference: Not shown in this demo version.

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