Effect of heat treatment on fatigue crack propagation of inconel 690

Abstract

Inconel 690 is an austenitic nickel-based alloy, which has been proposed as a substitute material for Inconel 600 in steam generator (S/G) tubing in pressurized water reactor (PWR). There are some data indicating that the thermally treated Inconel 690 is a better corrosion resistant tubing material than other alloys, such as 304 SS, Incolloy 800 and Inconel 600. This alloy is immune to stress corrosion cracking (SCC) in primary water, all volatile treatment (AVT) environment, chloride solutions and sulfate solutions. However, more fatigue data of Inconel 690 are needed for better steam generator design. So, the effects of heat treatment on yield strength and fatigue crack propagation of Inconel 690 in air at room temperature were investigated. For the experimental purpose, the specimens were first solution-annealed (SA) at 1070℃ and 1150℃ for 1 hour to dissolve all chromium carbides into the matrix and to increase the grain size. These specimens were then isothermally heat treated at 700℃ and 750℃ respectively for 5 and 15 hours to vary the grain boundary carbide morphologies. Then, tensile tests were performed with strain rate of $5×10^{-4}$/sec. The fatigue tests were conducted at stress ratio of 0.2, frequency of 10 Hz.. From the test results, it was shown that the final grain size and the solution annealing temperature controlled the tensile properties, and it was found that chromium carbides at the grain boundaries and large grains reduced fatigue crack growth rate of Inconel 690 in the low stress intensity factor range region.