Abstract
In the paper results of fractal analysis are presented for fracture surfaces in Ni-Cr steel in two different microstructural states. Alternation of fractal dimension very well corresponds with characteristic regions ahead of the fatigue crack tip controlled by different micromechanisms of fracture. In the ductile damage region the fractal dimension reaches its maximum, in the brittle fracture region the fractal dimension is minimal, and it does not change too much with increasing distance from the fatigue crack tip. Competition effects of transgranular and intergranular brittle fracture can cause growth of surface roughness and its fractal dimension. The fracture toughness of Ni-Cr steel tested in the transition temperature domain is inversely proportional to the difference of the fractal dimension in the stable crack propagation region and the dimension in fracture instability area of the fracture surface.

Key words
tempered microalloyed steel, fractography, fracture toughness, ductile fracture, brittle fracture, intergranular fracture, fractal dimension

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