The Rehbinder effect in tests of superalloys in contact with molten salts

Abstract

The results of the Rehbinder effect manifestation during testing of superalloys in contact with corrosive media containing Na2SO4 + NaCl are discussed. We present the experimental study of the effect of salts containing chlorine and sulfur on the mechanical properties and long-term strength of single crystal and powder nickel-based superalloys at high temperatures. The practical value of the work is associated with the possible operation of gas turbine parts in conditions of ingress of the particles containing chlorine, sulfur and sodium into the flow path. A simplified (compared to a previously used) procedure of testing wrought alloys for long-term strength in molten salts is developed. A comparative study of the mechanical properties and long-term strength of a single crystal superalloy in the initial state (previously damaged by corrosion of different duration) and being in contact with salts during testing is carried out. Due to the large scatter of experimental data, the method of lower envelopes has been proposed and implemented to determine the guaranteed values of the long-term strength. It is shown that the guaranteed values of the long-term strength at different temperatures and test durations can decrease by 2.5 – 5.0 times in the presence of salts. Metallographic studies of the nature of damage and destruction of samples are carried out. The revealed decrease in the long-term strength of heat-resistant alloys in contact with salts is interpreted as the Rehbinder effect and not as a manifestation of the effect of stress corrosion cracking. A methodology for using the obtained test results with a duration of up to several thousand hours in highly aggressive environments is proposed to predict the long-term strength in relation to long-term operation (tens of thousands of hours) under conditions of relatively low salt loads.