Visualization of fractographic signs of operational degradation of heat-resistant steel for estimating its actual structural-mechanical state

Abstract

Fracture surfaces of specimens of the steel 15H1M1F long-term (approx. 2·105 h) operated on the main steam pipelines of a thermal power plant have been analysed after their mechanical testing. Fractographic analysis showed some distinctions resulted from different number of shutdowns (501 and 576) of power units during their operation. After impact toughness and fracture toughness tests, as expected, the fracture surfaces of the steel subjected to more shutdowns have less area of ductile (subcritical) fracture in the vicinity of notch or pre-crack, and more intergranular fragments against the background of typical transgranular relief in the zone of spontaneous fracture. These features are concerned with more intensive degradation of the steel after more shutdowns of power units. It can be explained by a larger pre-fracture zone in impact specimens and, accordingly, the possibility of a greater deviation of the crack from its main path, involving the boundaries of dozens of grains with operational damages. In contrast, pre-cracked specimens for fracture toughness evaluation have the narrower pre-fracture zone, therefore, there is significantly less grain boundaries weakened by degradation on the crack path. Some areas of intergranular fracture against the background of a classical, as a whole, transgranular fatigue fracture were also found at the fracture surfaces of the operated steel after fatigue tests, namely, in the near-threshold region of the crack growth. They are associated with grains encountered along the path of fatigue crack propagation, the boundaries of which were damaged as a result of creep during long-term operation of the steel on steam pipelines. Based on fractographic research of the long-term operated steel, the area of intergranular fragments in the unit area of the fracture surfaces is proposed as a quantitative fractographic indicator of changes in its structural and mechanical state due to degradation. The mechanical characteristics of brittle fracture resistance of the steel are consistent with the proposed fractographic indicator of the metal state.