» 218. 5. Three
ong or iy
eM oC cig
Wo tyes are
fig. 6 : High speed steel, A etching, carbides in white and matrix in black.
(a) at 5 mm from the surface roll. (b) at 55 mm from the surface roll.
Quantitative parameters
Three parameters are measured to characterise the carbides. After the 7A etching, they are
computed globally without distinction of nature. After a Groesbeck s etching, each carbide type can
be characterised separately. The first parameter is the area fraction of the carbide defined as the
ratio between pixels belonging to carbides and the total number of pixels in the image.
ah nim The intercept method (ASTM El 12-63) allows to calculate a number of intercepts. The same
MeL mE principle has been applied in digital images by Launeau (3). The algorithm allows to compute
DE SPAN of automatically the number of intercepts and the mean intercept length in several directions with a
vod) pine degrees angular resolution. The grain and carbide sizes can be evaluated from the
Ep 10 euch omnidirectional mean intercept length inside each phase. Although the grain boundaries are only
id does not affect partially delimited by the carbides, the mean intercept length is a meaningful parameter for the
ar im dark brown; characterisation of the grain size. As the MC carbides are preferentially located inside grains, the
ast between {hc mean intercept length of the martensite must be measured on images containing only the M6C and
the M2C carbides to avoid an underestimation of the value.
The carbide interdistance is characterised by the mean radius of the maximum inscribed discs
between carbides. First, every pixel of the matrix receives a value equal to the smallest distance
from the nearest carbide boundary. Afterwards, pixels equidistant from at least two carbide
boundaries are conserved and formed the SKIZ (Skeleton by influence zones). The value associated
to a pixel of the SKIZ is the radius of the maximum inscribed disc between carbides.
Fig. 7 : Intercept tength. Fig. 8: SKiZ. Maximum
oo inscribed discs between carbides.
ct of this cooling
, xt! martensite Results
und * teit The results were obtained from a sample submitted to the 7A etching. Five images were acquired
J gn ese every 5 millimetres, from 5 to 55 millimetres depth. These results confirm well our preliminary
observations, Fig. 9 to 12. The global area fraction of carbides does not vary a lot from the surface
to the core. Fig. 9. The mean global value is 13.4 % and the standard deviation is 0.6 %.
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