» 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 %. 
141