The roles of carbon and grain boundary carbides on the intergranular corrosion cracking behavior of Ni-16Cr-9Fe-xC alloys at 360 degrees C.

Abstract

The roles of carbon and grain boundary carbides on the creep and cracking behaviors of controlled purity Ni-16Cr-9Fe-xC alloys at 360$\sp\circ$C were both isolated and determined in order to better understand the effect of carbon distribution on intergranular stress corrosion cracking (IGSCC) behavior. Constant load tensile (CLT) tests were conducted in 360$\sp\circ$C argon and primary water in order to determine relative creep susceptibilities, while constant extension rate tensile (CERT) tests were performed in 360$\sp\circ$C argon and primary water environments to study relative cracking propensities. Solid solution carbon increases the creep resistance of Ni-16Cr-9Fe-xC alloys at 360$\sp\circ$C by delaying the recovery process of climb in the grain boundary. Grain boundary recovery rates were estimated by performing in situ TEM grain boundary dislocation spreading experiments to determine grain boundary diffusivities. The addition of 65 wppm carbon in solution serves to lower the grain boundary diffusivity and grain boundary recovery rate by over 4 orders of magnitude. As a result of lowering the grain boundary diffusivity, solid solution carbon suppresses both grain boundary sliding and cavitation. Grain boundary carbides decrease the creep resistance of Ni-16Cr-9Fe-xC alloys at 360$\sp\circ$C compared to a microstucture containing all carbon in solution, but increase IGSCC resistance in primary water environments containing 0, 1, and 18 bar hydrogen overpressures. The magnitude of the beneficial effect of the grain boundary carbides is strongly dependent upon hydrogen overpressure. The superior IGSCC resistance of a microstrucutre containing grain boundary carbides can be attributed to its highest overall resistance to both creep and environmentally induced cracking. The detrimental effect of hydrogen on the IGSCC resistance shows consistencies with both the film rupture/slip dissolution and hydrogen embrittlement cracking mechanisms. It is proposed that carbon distribution influences the cracking mechanism by affecting local surface film formation at the grain boundary.