Effect of temperature on fatigue crack growth rate in inconel 690

Abstract

The effect of temperature on fatigue crack growth rate in Inconel 690 steam geneator tube was invastigaed at room temperature, 150℃, 300℃ and 400℃ in air environment. Fatigue tests was performed on compact tension(CT) specimens made to thichness 8mm. Heat treatment was performed to obtain various microstructures. Tests was conducted at load ratio 0.1, at frequency 10Hz using sinusoidal waveform. Crack length was monitored by compliance method using sensor probe. Tensile tests was performed to obtain yield strength for fatigue test at each testing temperature. Fracture modes were studied using scanning electon microscopy(SEM). Tensile properties of Inconel 690 were dependent on grain size and testing temperature. As temperature and grain size increased, yield strength decreased. At temperature above 300℃, the serrations which were an evidence of dynamic strain aging (DSA) were observed for all cases of materials. Fatigue crack growth rate decreased with increading grain size. At low Δ K, grain size effect was significant at room temperature, which was due to roughness induced crack closure. With increasing temperature, fatigue crack growth rate increased and grain size effect decreased. Chormium carbides which have large size and semi-continuous distribution in the grain boundaries decreased fatigue crack growth rate.