François Roch

Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions

The effect of the tempering heat treatment, including heating prior to the isothermal step, on carbide precipitation has been determined in a 2.25 Cr-1 Mo bainitic steel for thick-walled applications. The carbides were identified using their amount of metallic elements, morphology, nucleation sites, and diffraction patterns. The evolution of carbide phase fraction, morphology, and composition was investigated using transmission electron microscopy, X-ray diffraction, as well as thermodynamic calculations. Upon heating, retained austenite into the as-quenched material decomposes into ferrite and cementite. M7C3 carbides then nucleate at the interface between the cementite and the matrix, triggering the dissolution of cementite. M2C carbides precipitate separately within the bainitic laths during slow heating. M23C6 carbides precipitate at the interfaces (lath boundaries or prior austenite grain boundaries) and grow by attracting nearby chromium atoms, which results in the dissolution of M7C3 and, depending on the temperature, coarsening, or dissolution of M2C carbides, respectively.

Metallurgical characterization of coupled carbon diffusion and precipitation in dissimilar steel welds

Abstract The complex microstructures developed during post-welding heat-treatment in the vicinity of the fusion line between a ferritic and austenitic steel were examined in the case of submerged arc welded 18MND5/309L dissimilar joints. Quantitative measurements of the carbon distribution in the as-welded and post-weld heat-treated conditions were performed by both wavelength dispersive spectrometry and secondary ion mass spectrometry. The extent of carbon diffusion was confirmed by hardness profiles performed by nanoindentation. On the low-alloy ferritic side, decarburization resulted in cementite dissolution allowing the evolution of the bainitic structure toward a large-grained ferritic region. In the weld metal, the carbon content reached unusually high levels and an intense precipitation of chromium-rich carbides was observed in both the interfacial martensitic layer and the austenitic weld metal. The evolution of the precipitation as a function of the distance from the interface was analyzed in terms of crystallography, chemistry, volume fractions, and size distributions. Automated crystal orientation mapping in a transmission electron microscope allowed identification of the precipitates extracted on carbon replicas from both the martensitic and austenitic matrices. A 3D reconstruction of the carbides population in the martensitic layer was performed by serial cutting with a focused ion beam: M7C3 and M23C6 were found to coexist in the two carburized regions, but displayed different sizes, compositions, and morphologies, depending on their location with respect to the fusion line. This evolution in terms of precipitation was analyzed taking into account the local microstructure and composition.