Optimum Thread Rolling Process proves SGC Resistance

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 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 weight % boiling ammonium nitrate and stressed to about 40 % of the steel's yield strength (120 ksi, 827 MPa). In similar testing conditions, fasteners that had threads rolled before heat-treatment (quench and temper) had similar susceptibility to SCC. However, threads rolled after strengthening, 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 IO0 ppm of chloride, fluoride, sulfate, nitrate and a controlled hydrogen overpressure) and stressed to about 80% of the alloy yield strength (1 17 ksi, 807 m a ) . In similar testing conditions, threads rolled after strengthening exhibited no SCC aRer 50 days of exposure, This beneficial effect of the optimum thread rolling process (Le., threads rolied after strengthening) 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 pre-loads. Use of these material specific aggressive environments can provide an accelerated test to verify that threads were produced by the optimum thread rolling process. These tests could support fastener acceptance criteria or failure analysis of fasteners with unknown or uncertain manufacturing processes. The optimum process effects may not aIways be detected by more conventional methods (e.g., metallography or hardness testing). BACKGROUND The capacity of the rolled thread process to provide improved fatigue life performance has been recognized for a considerable period of time and is the basis for aerospace threaded fastener process controls. This property improvement occurs only when the threads are rolled after the material strengthening heat treatment is performed. In contrast to the availability of improved fatigue performance data, only limited data exist to assess the performance improvements to stress corrosion cracking (SCC) of fasteners of specific materiais in a variety of susceptible environments. Most of the test environments are mildly SCC sensitive; e.g. 3.5% NaCl solutions for steel. A few test environments are aggressive; e.g., magnesium chloride for austenitic (3 00 series) stainless steel. These limited experiences show improved resistance of rolled threads to susceptible environments when rolled after final heat treatment. However, demonstration of the rolling process improvements is lacking for environments that are extremely aggressive to specific materials. Although fatigue test acceptance criteria has been the backbone of aerospace rolled thread fastener quality, similar acceptance testing for corrosion resistance to hostile environments has not been standardized. Thus, it remains possible that cost incentives may dictate the selection of the rolling process rather than use of controlled rolling processes that provide maximum resistance to stress corrosion cracking. Currently, the commercial (non-aerospace) fastener industry tends to prefer rolling of threads before a strengthening heat treatment so that the rolling die life is maximized and