The Influence of Vanadium Additions on Isothermally Formed Bainite Microstructures in Medium Carbon Steels Containing Retained Austenite

Abstract

The influence of V additions on isothermally formed bainite in medium carbon steels containing retained austenite has been investigated using in-situ high energy X-ray diffraction (HEXRD) and ex-situ electron energy loss spectroscopy (EELS) and energy dispersive X-ray analysis (EDX) techniques in the transmission electron microscope (TEM). No significant impact of V in solid solution on the bainite transformation rate, final phase fractions or on the width of bainite laths was seen for transformations in the range 375-430 • C. No strong influence on the dislocation density could be detected, although quantitative analysis was impeded by ferrite tetragonality. A reduction in the carbon content of retained austenite C γ that is not believed to be due to competition with VC or cementite precipitation was observed. No influence of V on the carbon supersaturation in bainitic ferrite C b could be directly measured, although carbon mass balance calculations suggest C b slightly increases. A beneficial refinement of blocky MA and a corresponding size effect induced enhancement in austenite stability were found at the lowest transformation temperature. Overall, V additions result in a slight increase in strength levels.

Carbide-Free Bainite Transformations Above and Below Martensite Start Temperature Investigated by In-Situ High-Energy X-Ray Diffraction

“…For the sake of comparison, the values obtained on similar alloys after isothermal holdings by Pushkareva et al, 42 Garcia-Mateo et al, 29 and by Ranjan et al 54 have been compared with our values. Pushkareva et al 42 measured the carbon content in bainitic laths by EELS (electron energy loss spectroscopy) whereas Garcia-Mateo et al 29 and Ranjan et al 54 used APT (atom probe tomography).…”

“…These results are in agreement with results from the literature. 29,32,42,54,55 Ranjan et al 54 have recently explained this evolution by a correlation with the austenite carbon content, which also increases.…”

“…This has been clearly demonstrated by Pushkareva et al on two very similar alloys and transformation conditions. The final austenite carbon content measured by HEXRD has been compared with 42 measurements of the carbon content in austenite by EELS (electron energy loss spectroscopy). In nanobainitic steels, Garcia-Mateo et al 41 observed a strong dispersion in carbon content values using APT (atom probe tomography).…”

Thermomechanical Processing of Steels

“…The present Special Issue on Thermomechanical Processing of Steels includes eleven research papers [1][2][3][4][5][6][7][8][9][10][11]. A wide range of steel grades are covered in these papers.…”

Effect of V Addition on Microstructure and Mechanical Properties in C–Mn–Si Steels after Quenching and Partitioning Processes

“…The mechanical stability of austenite grains results from the competition between grain refinement and reduced C content, both of which are governed by V-alloying. By refining the grain size and enhancing the austenite stability, V addition improves the ductility of steels [15][16][17][18].…”