The Effect of Microstructure on the HEAC Behavior of Monel(R) K-500

Monel K-500 is a Ni-Cu-Al superalloy commonly used in marine applications due to its high corrosion resistance, high strength, and fracture toughness. This alloy is known to be susceptible to sub-critical hydrogen environment assisted cracking (HEAC) when immersed in seawater and exposed to cathodic polarizations. This has resulted in long time (10 year) service failures of Monel K-500 components during field use, suggesting significant sub-critical cracking. Previous work has quantitatively established the influence of cathodic polarization on the HEAC kinetics for a single lot of Monel K-500. The goal of this work is to determine the extent to which the HEAC behavior varies for Monel K-500 lots that meet alloy specification (QQ-N-286G); four materials were investigated and termed Tie Rod (TR) 1, TR 2, TR 3, and Allvac. TR 1, 2, and 3 were taken from engineering components from the field and Allvac is a virgin material lot. First, fracture mechanics testing was performed to quantify the HEAC behavior of Monel K-500 from four different lots in various environmental conditions. Second, detailed characterization of the microstructure was completed to identify any lot-to-lot variations. Specifically, the grain size, yield strength, grain boundary impurity segregation, grain boundary character, crack deflection, and slip behavior were compared. Third, the quantitative HEAC parameters and microstructure characteristics from the previous two steps were coupled with results from a parallel study (executed by Prof. J.R. Scully at UVa) to quantify various aspects of the H-metal interaction. Finally, the micro-mechanical models were used to quantitatively evaluate how the observed changes in the microstructure may effect HEAC behavior.
Inert environment testing was completed and all materials showed similar behavior, assuring that differences in HEAC kinetics observed in charging environments are related to the material-hydrogen interaction. The HEAC kinetics for all materials were characterized at -950mVSCE. No significant difference was observed in HEAC behavior between the four material lots at this potential. HEAC characterization in the near-threshold potential regime (-850mVSCE) revealed slight variation in KTH and da/dtII between TR 1, TR 2, and TR 3. The Allvac material, on the other hand, did not suffer intergranular failure at this potential. Extensive material characterization was focused on identifying the microstructural feature(s) present in Allvac, but not in TR 1, TR 2, or TR 3, that can explain the observed difference in HEAC susceptibility at -850mVSCE.

It is concluded that the similarities in HEAC behavior across all four material lots observed at -950mVSCE is due to the high hydrogen concentrations achieved in the fracture process zone at such an aggressive potential. This was confirmed using micromechanical modeling. The small lot-to-lot differences in HEAC behavior observed between the tie rods at -850mVSCE appear to be due to variation in yield strength and grain size between the material lots. The variation in HEAC behavior between all material lots is observed to be independent of the fraction of low energy “special” boundaries and of the connectivity of high angle, random boundaries. No variation in crack deflection or slip behavior between the material lots was observed. The increased HEAC resistance in the Allvac material is shown to be a result of either (a) the decreased yield strength of Allvac and/or (b) less sulfur segregation at the grain boundaries in the Allvac material than in the three tie rods (supported by Auger depth profiling showing sulfur segregation in TR 2 not present in Allvac, a smaller bulk sulfur concentration in Allvac than in TR 1, TR 2, and TR 3, and by the smaller grain size of the Allvac material compared to the other material lots). Finally, micromechanical modeling is put forth that reasonably captures the effect of sulfur segregation on HEAC behavior in all four material lots.