VISUALIZATION OF FRACTOGRAPHIC SIGNS OF OPERATIONAL DEGRADATION OF HEAT-RESISTANT STEEL

Abstract

Fracture surfaces of specimens after testing for impact toughness, fracture toughness and
fatigue crack growth resistance of heat-resistant of 15Kh1M1F steel for a long time (a2·105
h) are
analyzed. Fractographic analysis of fracture surfaces revealed differences between the state of the
metal, which during operation suffered a different number of block shut-downs (501 and 576). Namely,
in a metal with a large number of shut-downs, the fraction of intergranular fragments against the
background of a typical transgranular relief in the zone of spontaneous fracture of specimens tested for
impact toughness and fracture toughness was high, which is associated with its more intense
degradation. In addition, after impact toughness tests, the fraction of intergranular facets was greater
than in tests for fracture toughness, which is associated with a large deviation of the crack (even by
several grains) from the main direction of its propagation due tu the bigger dimention of plastic zone in
the tip of stress concentrator than in the crack tip. Whereas in the fracture toughness tests, destruction
occurred only along those grain boundaries weakened by degradation that were encountered along the
path of the main crack front. During fatigue crack growth resistance tests (in the near-threshold
section of crack growth) on the fracture surfaces of exploited steel against the background of classical
transgranular fatigue fracture, fragments of intergranular fracture were found, which were associated
with grain boundaries damaged due to creep during long-term operation. The obtained results give
grounds to recommend the area of intergranular relief per unit area of fracture surfaces of specimens
from exploited heat-resistant steel, for a quantitative assessment of its structural and mechanical state.