Failure analysis and repair welding of 17–4 PH stainless steel lacing wire in low-pressure steam turbine blades

A lacing wire fracture was reported on the last-stage low-pressure blade of a 300 MW steam turbine after four years of continuous service. The wire, fabricated from precipitation-hardened 17–4 PH stainless steel with a 14 mm diameter, failed in the weld region, necessitating a full metallurgical investigation. Fractographic analysis using scanning electron microscopy (SEM) revealed beach marks and transgranular brittle fracture features in the weld area, confirming a fatigue-initiated failure mechanism. The root cause of this failure was identified as weld metal resulting from the use of ER410 filler metal without adequate preheating, which produced a brittle martensitic microstructure highly susceptible to fatigue cracking under cyclic service loads. To address this vulnerability, a comparative welding study was conducted using three filler metals: ER630, ER410, and ER309. These fillers were selected to represent the extremes of the Schaeffler diagram (martensitic and austenitic) and a matching composition (martensitic-austenitic), providing a comprehensive comparison of microstructural and mechanical properties. Tensile and bend testing demonstrated that ER410 produced brittle martensitic welds with a reduction in area of only 13.76%, while ER309 yielded highly ductile but low-strength joints. ER630, when applied with preheating at 100 to 150 $${}^\circ$$ C, achieved a tensile strength of 978.97 N/mm $${}^2$$ with a reduction in area of 23.74%, offering the most favorable balance of mechanical properties for field deployment. Microstructural characterization via optical microscopy, SEM, and energy-dispersive X-ray spectroscopy (EDS) confirmed that the weld metal phases aligned with Schaeffler diagram predictions. Vickers microhardness profiles across the heat-affected zone (HAZ) showed significant softening compared to the base metal, indicating a phase transformation or grain coarsening effect in the HAZ, while weld metal hardness correlated directly with the martensite fraction. The findings establish ER630 with preheating as the recommended filler metal for lacing wire repair welding and provide a metallurgical framework for preventing similar failures in steam turbine blade assemblies.