Optimum Thread Rolling Process that Improves SCC Resistance

Abstract

Abstract Accelerated testing in environments aggressive for the specific material have shown that fastener threads that are rolled after strengthening heat treatments have improved resistance to stress corrosion cracking (SCC) initiation. For example, intergranular SCC was produced in one day when machined (cut) threads of high-strength steel (ASTM A193 B-7 and A354 Grade 8) were exposed to an aggressive aqueous environment containing 8 wt % boiling ammonium nitrate and stressed to about 40 % of the steel's yield strength. In similar testing conditions, bolts that were thread rolled before heat treatment (quench and temper) had similar high susceptibility to SCC. However, threads rolled after the strengthening heat treatment exhibited no SCC after a week of exposure, even when stressed to 100 % of the B-7 alloy yield strength. Similarly, intergranular SCC was produced in less than one day when machined (cut) threads of nickel-base alloys (X-750 and aged 625) were exposed to an aggressive 750°F doped steam environment (containing 100 ppm of chlorides, fluorides, sulfates, and nitrates) and stressed to about 80 % of the alloy yield strength. In similar testing conditions, threads rolled after strengthening exhibited no SCC after 50 days of exposure. This beneficial effect of the optimum thread rolling process (i.e., threads rolled after the strengthening heat treatment) is due to the retention of large residual compressive stresses in the thread roots (notches), which mitigate the applied notch tensile stresses resulting from joint design preloads. Use of these material-specific aggressive environments—“chemical cracking” tests—can provide an accelerated test to verify that threads were, in fact, produced by the optimum thread rolling process. The chemical cracking tests could also support fastener acceptance criteria or failure analysis of fasteners with unknown or uncertain manufacturing processes. The achievement of the optimum process effects may not always be detected by more conventional methods (e.g., metallography or hardness testing).