Precipitation Characteristics and Mechanism of Vanadium Carbides in a V-Microalloyed Medium-Carbon Steel

doi:10.1007/s40195-018-0775-8

Abstract

Abstract:

The precipitation characteristics and mechanism of vanadium carbides during isothermal transformation at 650 °C in a V-microalloyed medium-carbon steel were investigated through scanning electron microscopy and transmission electron microscopy as well as dilatometry test. Five morphologies of vanadium carbides were found to precipitate at different nucleation sites during the transformation. Two kinds of interphase precipitation form simultaneously in both pro-eutectoid and pearlitic ferrites. The linear arrays of fine interphase precipitates are parallel to the γ/α interface, and the fine needles of interphase precipitates are perpendicular to the γ/α interface. The vanadium carbides of long or short fibers, coarse particles and fine particles form in both pro-eutectoid and pearlitic ferrites, displaying different precipitation distributions and orientation relationships with ferrite. The precipitation mechanisms of vanadium carbide precipitates with different modes were proposed, and the precipitation sequence of various vanadium carbide precipitates was finally ascertained.

Key words: Microalloyed steels, Carbide precipitation, Isothermal transformation, Precipitation sequence, Orientation relationship

Xiao-Lin Pan, Minoru Umemoto. Precipitation Characteristics and Mechanism of Vanadium Carbides in a V-Microalloyed Medium-Carbon Steel[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(11): 1197-1206.

Figures/Tables 13

Fig. 1 Microstructures of the as-received steel: a SEM micrograph of pro-eutectoid ferrite and pearlite; b dark-field TEM micrograph of VC precipitates in pro-eutectoid ferrite

Fig. 2 Dilatometry results of the steel during the isothermal transformation

Fig. 3 SEM micrographs after isothermal transformation at 650 °C for 1 h: a low magnification; b typical pearlite morphology; c, d short and long precipitates of VC in pro-eutectoid ferrite

Fig. 4 TEM micrographs of coarse VC precipitation in austenite along the ferrite grain boundaries (holding for 1 h): a bright-field image; b dark-field image

Fig. 5 TEM micrographs of planar interphase precipitation in pro-eutectoid ferrite (holding for 1 h): a bright-field image; b dark-field image; c SAED pattern; d corresponding analysis

Fig. 6 TEM micrographs of fine needles of interphase precipitates in pro-eutectoid ferrite (holding for 1 h): a dark-field image; b, c bright-field and dark-field images of higher magnification of rectangular area in a; d SAED pattern; e corresponding analysis

Fig. 7 TEM micrographs of co-precipitation of banded interphase precipitates and needlelike interphase precipitates in pro-eutectoid ferrite (holding for 1 h): a bright-field image; b dark-field image

Fig. 8 TEM micrographs of fibrous VC precipitation in pro-eutectoid ferrite (holding for 1 h): a, c bright-field image; b, d dark-field image

Fig. 9 TEM micrographs of matrix precipitation in pro-eutectoid ferrite (holding for 1 h): a bright-field image; b, c dark-field image showing different orientations

Fig. 10 TEM micrographs of VC precipitates in pearlite (holding for 1 h): a, c, e, g bright-field image; b, d, f, h dark-field image

Fig. 11 Schematic diagrams of VC precipitation: a planar interphase precipitation; b needlelike interphase precipitation; c matrix precipitation; d fibrous precipitation

Fig. 12 TEM micrographs of VC precipitates nucleated on dislocations near the ferrite grain boundary after isothermal transformation at 650 °C for 100 s: a bright-field image; b dark-field image

Table 1 Precipitation sequence of various morphologies of VC precipitates

Sequence	Morphology	Nucleation site
1	Coarse particles	γ/grain boundary
2	Fine particles on planar sheets	γ/α interface
3	Fine needles on planar sheets	γ/α interface
4	Long or short fibers	Dislocations in α
5	Fine particles	α matrix

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