Microstructure of Martensite in Fe–C–Cr and its Implications for Modelling of Carbide Precipitation during Tempering

Abstract

The microstructure of as-quenched martensite in four Fe-C-Cr alloys (0.15C-1Cr, 0.15C-4Cr, 1C-1Cr, 1C-4Cr, mass%) has been investigated. Moreover, the microstructures served as input for setting up modeling of carbide precipitation during tempering of martensite. The modelling was conducted using the Langer-Schwartz approach and the software TC-PRISMA, which retrieves thermodynamic data from the Thermo-Calc databank. It was found that the martensite in the low carbon steels is predominantly lath martensite with units arranged parallel to each other. On the other hand, the plate martensite dominates the microstructure in the high carbon steels. The ratio of high-angle to low-angle grain boundaries was found to increase with increasing Cr in the low carbon steels, which indicates that Cr has a similar effect as C on the lath martensite microstructure, however, the micro-hardness remained unaffected by the addition of Cr. Finally, the precipitation modeling clearly demonstrates the importance of proper definition of the initial microstructure for predictive modelling. Parameters such as dislocation density and frequency of high-angle grain boundaries have a drastic effect on e.g. the mean size of carbides.

Microstructure evolution during tempering of martensitic Fe–C–Cr alloys at 700 °C

“…This is also seen in the present work where the high-Cr alloy has a slightly higher dislocation density than the low-Cr alloy in the as-quenched condition and is in agreement with our prior observations that Cr has a similar effect to C on the lath martensite microstructure [17]. It may be related to the slight lowering of the Ms temperature (Table 1) when the Cr content increases [14,17]. It should also be noted that there is a difference in the prior austenite grain size between the two alloys of 40 ± 6 lm for 4% Cr alloy and 65 ± 8 lm for 1% Cr alloy [17].…”

“…It should, however, be mentioned that in addition to carbon, other alloying elements will also affect the dislocation density of martensite [9,25,28]. This is also seen in the present work where the high-Cr alloy has a slightly higher dislocation density than the low-Cr alloy in the as-quenched condition and is in agreement with our prior observations that Cr has a similar effect to C on the lath martensite microstructure [17]. It may be related to the slight lowering of the Ms temperature (Table 1) when the Cr content increases [14,17].…”

“…The chemical compositions of the steels studied are listed in Table 1, and the preparation procedures for these alloys can be seen elsewhere [17]. The M s temperatures were calculated using the model proposed by Stormvinter et al [18] and are also listed in Table 1.…”

“…Two model alloys both resulting in a lath martensitic microstructure [17], with varying additions of chromium-a slow diffusing substitutional element compared to carbon, were investigated. The evolution of the martensitic microstructure and precipitation during tempering was evaluated using X-ray diffraction (XRD), electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM).…”

Features of Carbide Precipitation During Tempering of 15H2NMFA and 26HN3M2FA Steels

“…Furthermore, the definition of the interface energy is a requirement for the calculation of precipitation and growth kinetics of carbides: based on an analysis of literature data [1][2][3] for 15H2NMFA and 26HN3M2FA steels the value of the interface energy was assumed to be: All calculations were carried out using TC-Prisma 2.0 program with thermodynamic and kinetic database of the materials properties TCFE7.0 and MOB2.0, respectively. Calculation shows that the volume fraction of the carbides increases with the passage of time and tends to asymptotes corresponding to the equilibrium phase content in the steel.…”