Structural-fractographic signs of operational damage of steel of the steam turbine rotor disc
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The technical condition of the metal of a steam turbine rotor disk was analysed, and the causes of its failure were determined. It was shown that one of the rotor stages was damaged even though the mechanical properties of 30KhN3M1FA steel under tension and impact met regulatory requirements. Metallographic analysis revealed an excessive amount of non-metallic inclusions in the steel structure. Their high corrosive activity contributed to the dissolution of the matrix in their vicinity and created structural defects as sites for initiating fatigue cracks. High stresses arising along the contact surface of rivets and their holes in the rotor disk, and their ever-increasing relative displacement, contributed to fretting fatigue and the initiation of a network of microcracks in the layers adjacent to the surface of the holes. The combined effect of both inclusions and these cracks intensified the formation of a main fatigue crack, which spread across the entire cross-section of the intersections between the holes in the disk via a fatigue mechanism, forming a characteristic relief of parallel fatigue grooves.
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