Carbon Atom Re-Distribution during Quenching and Partitioning

Abstract

Quenched and Partitioned (Q&P) steels have been shown in previous literature to exhibit attractive mechanical properties due to austenite and martensite present in the microstructure. However, questions have remained regarding the mechanisms at work during the partition step. In this work, x-ray diffraction and atom probe tomography were utilized to gain insights into phenomena that occur (such as carbon partitioning from martensite to austenite, carbon atom clustering/dislocation trapping, and carbide formation). Evidence of all of these mechanisms was observed.KEY WORDS: quench and partition; carbon diffusion; martensite; austenite; atom probe tomography.

Effects of Q&P Processing Conditions on Austenite Carbon Enrichment Studied by In Situ High-Energy X-ray Diffraction Experiments

“…Carbon enrichment in austenite was shown to be the result of the competition between the direct partitioning process and the carbide-free bainitic transformation. They also showed that a significant fraction of carbon remains trapped in martensite, supporting the conclusions of Thomas et al [15].…”

“…Their observations are sustained with an original reassessed thermo-kinetic model. Carbon clustering in martensite has also been reported by Thomas et al [15] in a highly-alloyed system. Even if the nature of the carbide remains the bone of contention, all these recent studies lead to the conclusion that a certain fraction of carbon must also be trapped in recovered martensitic laths, preventing a complete carbon partitioning between martensite and austenite, thus limiting austenite carbon enrichment.…”

“…The carbon contents in austenite were deduced from the measured austenite lattice parameter at RT using the Toji et al formula [18]. Nevertheless, this method, which is commonly used when only post-mortem XRD measurements [15,16,18] are available, grants access to relevant trends, as the hydrostatic internal stresses inherited from martensitic phase transformations and thermal eigenstrains are not very sensitive to processing conditions (please refer to Table 1). …”

“…Reference As already highlighted by HajyAkbari et al [12] and Thomas et al [15], carbon content remaining trapped in the bcc phases (martensite and bainite) was far from being negligible, which jeopardized the possibility of verifying strictly the original Speer's assumption (almost carbon-free bcc phases). The measured phase fractions are plotted in Figure 6 as functions of the QT and are compared to the calculations of Figure 2.…”

Influence of Microsegregation on the Onset of the Martensitic Transformation

“…Predicting the development of the martensite volume fraction is of interest, for example, in the case of quenched and partitioned steels, where the amount of austenite remaining at the quench stop temperature is an important parameter in process design. [3][4][5] For plain iron -carbon alloys, Koistinen and Marburger (K-M) 6) showed that the transformation of austenite to mar-…”