Analytical life prediction methods are being developed for high-temperature turbine and valve studs/bolts. In order to validate the approach, the calculated results are compared to the results of uniaxial stress relaxation testing, bolt model testing, and service experience. Long time creep, creep-rupture, and stress relaxation tests were performed by the National Research Institute for Metals of Japan (NRIM) for 12 Cr-1 Mo-1 W-1/4V, Type 422 stainless steel bolting material, at 500, 550, and 600°C. Based on these results and limited tests for a service-exposed bolt, the creep behavior can be described using a two-parameter material model: ε/εr=1−1−t/trm+1δ where εr is the rupture strain, tr is the rupture time, and m and δ are material constants. For comparison with the measured uniaxial stress relaxation properties, the stress relaxation was calculated using the two-parameter creep equation and a strain-hardening flow rule. The rupture time data was correlated using time-temperature parameter methods. A power law was used for the rupture strain versus rupture time relationship at each temperature. The calculated stress versus time curves were in good agreement with the measured at all temperatures and for initial strain levels of 0.10, 0.15, 0.20, and 0.25 percent. [S0094-9930(00)01701-7]
Long-term stress relaxation properties of SUS 316 stainless steel.