Effect of temperature on the microscopic appearance of the fracture surface of alloy 690TT under SSRT testing

Brown, C.M. and Mills, W.J. reported that Alloy 690, a nickel based high chromium alloy, exhibited a considerable reduction in fracture toughness below 149 degrees C in hydrogenated water environments, and they proposed that the possible mechanism attributable to the toughness degradation was hydrogen-induced intergranular cracking. Based on experimental results of the compositional analysis of the fracture surface using energy dispersive x-ray spectrometry (EDS), Brown, C.M. et al. also reported that the cracks might have propagated in the matrix, very close to the grain boundary. The authors, however, had reported that intergranular cracking was confirmed in Alloy 690TT under a Slow Strain Rate Tensile (SSRT) test of a cathodically hydrogen-charged specimen. The objective of this study is to clarify the effect of temperature and hydrogen on the intergranular cracking of Alloy 690TT. Slow Strain Rate Tensile (SSRT) tests were performed at several temperatures in simulated Pressurized Water Reactor (PWR) primary water conditions and at room temperature in air. The appearance of the fracture surface after SSRT tests at about 50 degrees C showed "ductile intergranular fracture" which is characterized by the appearance of intergranular cracks on the fracture surface accompanied by dimples. No great difference was observed between the maximum stress after a SSRT test at 320 degrees C in simulated PWR primary water conditions and a tensile test at room temperature in air. In addition, no correlation was found between the appearance of the fracture surface after either the SSRT or tensile tests and the test conditions such as strain rate and the amount of dissolved hydrogen in the water. Brown, C.M. et al. observed hydrogen-induced intergranular cracking with a CT specimen where the crack penetrates through the high tri-axial stress field, while in this paper we observed ductile intergranular fracture with smooth SSRT specimens which represent a typical uni-axial stress state. Hydrogen, however, could be trapped in the high tri-axial stress state region where high stress, high strain and a high dislocation density coexist. Further experiments, including the high tri-axial stress of such cracks are needed in order to clarify in detail the mechanism for ductile intergranular fracture.