The Role Of High Temperature Steam On SCC Crack Growth Rate Of Boiler Tube Material In Power Plant

High temperature performance of metal materials is the primary problem of the development of advanced ultra-supercritical units. And stress corrosion cracking(SCC) of boiler piping materials operating in supercritical water environment is especially critical. The structural material cracking problem can lead to serious safety accidents under extremely atrocious conditions, which directly affect the safe and reliable operation of the generator unit. Therefore, it is necessary to study stress corrosion cracking mechanism of boiler piping materials in high temperature steam environment.In this paper, a SCC crack growth rate test on a compact tension specimen of nickel based alloy Inconel 625 was conducted in high temperature steam at 700℃, 725℃,750℃, respectively. The dissolved oxygen content was controlled at 0～8000ppb. The crack growth rate during the test was measured using direct current potential drop (DCPD) technique. After the test, specimen were snapped and observed by scanning electron microscope(SEM). The results showed the fractures of all specimens presented typical intergranular stress corrosion cracking morphology. The crack growth rate was increasing with the temperature improved. So as with the dissolved oxygen (DO) content. When the DO content was under 4000ppb, the crack growth rate increased with the increasing of the dissolved oxygen, but when the DO content was more than 4000ppb the increasing rate slowed down. And crack growth rate of metal in high temperature steam was a little more than that in high temperature air. In the case of the higher temperature, steam environment compared to air environment in multiples of crack growth rate was smaller.At the crack tip strain theory, based on the high temperature oxidation, as one of the control factors of crack propagation, the nickel base alloy Inconel 625 crack growth rate model was derived, analyzed the sensitivity of the crack tip strain of material in different models. The crack growth rate at 700℃ and 750℃ was calculated for the materials under the condition of air and steam, the results showed that both temperature and steam environment accelerated the process of crack propagation. Meanwhile, compared with experimental data, the model based on GZH was well joint theoretical curve. Besides, the optimal values of characteristic length of crack tip ro was quite different at different temperature.