Investigação da causa da fratura na haste do cilindro hidráulico da comporta do vertedouro da UHE Tucuruí

Abstract

This research presents an investigation of the causes of the fracture in a piston rod of he hydraulic cylinder that moves the spillway floodgate of the hydroelectric power plant of Tucuruí, after 30 years of service. The chemical analysis and tensile tests confirmed that the piston rod material meets the design specification corresponding to AISI 410 martensitic stainless steel, with J-Factor and P-Factor indicating that the material is susceptible to temper embrittlement. The analysis of the fractured surface, using a scanning electron microscope, showed a totally martensite structure with δ-ferrite, promoting a strong negative effect on the mechanical properties of the steel, reducing impact resistance and crack propagation energy. The impact tests, on Charpy-V test specimens, indicated low tenacity for the studied piston rod. The metallographic analysis showed an intergranular fracture with cleavage signals, crack propagation along the outlines of the previous austenite and an intense intergranular corrosion along the surface of the piston rod. The stress measurement in the piston rods of the hydraulic cylinders, in a real service situation, showed that the stress in the section of larger diameter of the piston rod, during the floodgate opening, is lower than design values, and the stress amplitude variation, caused by water passing through the floodgate, has values less than 1% of the nominal stress. The results btained in the field test were used in numerical studies modeled by the Finite Element Method, observing a stress of approximately 63% of the yield stress of the material and infinite life for low cycle fatigue (opening and closing of the floodgate) and very high cycle fatigue (water passing through the floodgate) in the place where the fracture occurs. The piston rod failed due to low impact tenacity and moderate corrosion resistance, associated with the notch effect of the critical transition zone of the piston rod section. These conditions, together with the corrosive effect of the water, led to crack nucleation under stress corrosion and propagation through fatigue, until the total brittle failure. The performance of the piston rod comes from inadequate heat treatment during the manufacturing process, which resulted in high δ-ferrite content, temper embrittlement, low impact tenacity and susceptibility to stress corrosion.