Peculiarities of fatigue fracture of high-alloyed heat-resistant steel after its operation in steam turbine rotor blades

•Two types of failures of steam turbine blades (premature and after long-term operation) are analysed.•Mechanical and fractographic indicators are used to evaluate the current state of operated steel.•An approach to assessing the degree of operational degradation of high-chromium steel is proposed.•The critical state of the steel operated in steam turbine blades is substantiated. The article focuses on the causes of failures of steam turbine rotor blades made of high-alloyed heat-resistant steel 15Kh11MF. Premature failure of blades is caused by technological defects (intensive corrosion-erosion wear of the blade edges, uneven structure and distribution of micro damages) that occurred because of imperfections of welding and/or surface hardening. Cracks initiated from these defects change the natural oscillation frequency of the blades, leading to their resonance and thus overload. The corrosive steam-water mixture in the phase transition zone contributes to the fracture process. Long-term operation of steel in steam turbines leads to the precipitation of carbides along the grain boundaries that weaken the cohesion between adjacent grains with subsequent pore formation and their merging into defects comparable to grain size. Fatigue crack growth resistance of steel reduces as a result of its operation. The effective threshold values of the SIF range decrease more significantly than the nominal ones due to the crack closure effect. It results from the increased roughness of the fracture surface caused by intergranular fragments protruding above the flat fatigue relief. They have been formed during steel’s operation along the damaged grain boundaries, and their number progressively increased with the operation time. To assess the current state of operated steel, the structural (the part of intergranular fragments S in the unit area of the fracture surface) and mechanical (a relative loss in fatigue crack growth resistance due to operation) indicators are proposed. The basic relationship between them makes it possible to substantiate the conditions for the safe operation of 15Kh11MF steel in STR blades.