The Effect of Hardening via Long Range Order on the SCC and LTCP Susceptibility of a Nickel-30Chromium Binary Alloy
The objective of this work is to evaluate the effect of hardening on the susceptibility of a Ni-30.7Cr wt% (Ni-33 at.%Cr) model alloy to stress corrosion cracking (SCC) and to low temperature crack propagation (LTCP). Unlike previous studies that employ cold work to induce susceptibility to SCC, this work utilizes isothermal ageing to produce the long range ordered Ni2Cr phase. Samples were aged at 475 °C for durations up to 3176 h in order to produce hardness values between 70-100 HRB. After ageing, precracked compact tension specimens were tested for susceptibility to primary water SCC and to LTCP. Samples aged for less than 200 h (70-80 HRB) showed very high resistance to SCC, while intergranular cracking was observed in samples aged for longer times. An activation energy of 129.7 ± 17.6 kJ/mol and a yield strength exponent of 6.4 ± 2.1 were measured for SCC growth at constant KI and ΔEcP conditions, consistent with observations for Alloy 690. Hardening via long range order had no measureable effect on the toughness of the alloy in air, but degraded the toughness and promoted intergranular fracture in hydrogen deaerated water (i.e. it caused susceptibility to LTCP). The similarity of the yield strength dependence to cold worked Alloy 690 and the common temperature dependence (~130 kJ/mol) to A600, X-750, A690, etc. suggests a common SCC mechanism for all these alloys. Hardening via long range order is a novel method to induce SCC susceptibility in Ni-30 wt%Cr alloys, which avoids some microstructural damage, inhomogeneity, and orientation effects that complicate testing of cold worked material.
KeywordsStress corrosion cracking Low temperature crack propagation Long range order Alloy 690 EN52
The Authors are indebted to John Mullen, who assisted with testing and data analysis; Bill Fichtner, Walt Fonda, Nelson Schwarting, and Rich Taylor who performed the SCC testing; Darryl Hull who performed the LTCP testing; and Reza Najafabadi for thoughtful insight and discussion throughout the testing and analysis.
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