62 Prakt. Met. Sonderband 50 (2016)
As can be seen from the phase fractions of martensite obtained via image analysis
summarized in Table 3, a close correlation is established compared to XRD measurements.
Table 3: Comparison of martensite content obtained using XRD and image analysis
| Specimen-condition % Martensite XRD | % Martensite metallographically |
KAB Hi-Mn-annealed 19.7 21
[ KAB Hi-Mn- heat treated | 0 0.3
KAB 1-forged 0 3.7
ı_ KAB I-heat treated 0 0.6
It is known that bainite when etched with common attack etchants like Vilela, appears darker
due to a greater amount of interphases. It is usually explained with internal precipitation of
carbides within the bainitic constituent, but a similar effect is observed when etching carbide
free lower bainite-martensite microstructures. To illustrate the advantages of color etching,
the same sample of KAB-HiMn was conventionally etched using Vilela, and viewed in dark
field mode to obtain a sound contrast, where the martensite appears darkened. When I
analyzing the amount of martensite from the grayscale images the obtained results vary
. : . ek so
considerably depending on minor variations to the threshold value as small dark dots could 1
. . . . . . €
not be entirely avoided as can be seen in Fig.2. Consequently the estimated volume fractions N
of martensite were considerably higher with an average of about 26%, therefore only tint m
etchants were applied on the remaining samples. N
nn:
100 pm 100 um
Figure 2: Determination of martensite content based on grayscale contrast, from samples etched with Viella (micrograph
taken in dark field mode).
When the austenite grain size becomes refined, it is known to increase the required strain
energy for the growth of martensitic plates, resulting in a lower Ms temperature. This narrows
the gap between Ms and Bs in KAB Hi-Mn, and a consequently a much lower fraction of
martensite is formed in the forged condition and especially after heat treatment where a fine
grain size has been obtained.
