Analysis of the Causes of Root Cracks in Low-Pressure Rotor Blades

Abstract As the core components for energy conversion in steam turbines, blades endure long-term combined loads from steam excitation forces and centrifugal stresses. This study investigated the batch cracking issue of 57 spring-shaped blade roots (material: 0Cr17Ni4Cu4Nb) on the outlet side of the 6th-stage A/B low-pressure rotor in Unit 3 of a power plant. Among these, five cracks penetrated the blade roots, all initiating at the edge of the first tooth root on the outlet side. Through macroscopic inspection, metallographic analysis, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) characterization, combined with blade-girband fit clearance measurements and mechanical property testing, the following conclusions were drawn: The crack propagation zones exhibited typical fatigue striations, while Cl, Na, and K element enrichment was detected at crack initiation sites, confirming a failure mode of high-cycle fatigue fracture under corrosion-fatigue synergy. Excessive fit clearance at the blade-tip girband significantly amplified the alternating stress amplitude at the blade root under steam excitation forces. Concurrently, machining defects on the blade root surface induced localized stress concentration, accelerating crack initiation. Based on these findings, two improvement measures were proposed: First, replace all cracked blades and implement surface shot peening treatment on blade roots; Second, optimize the installation process to ensure girband clearances strictly comply with design specifications. This research provides a critical theoretical basis for enhancing blade reliability in homologous turbine units.