ASME STP-NU-037-2013 pdf download
ASME STP-NU-037-2013 pdf download.OPERATING CONDITION ALLOWABLE STRESS VALUES IN ASME SECTION III SUBSECTION NH.
3.3 Average Time to 1% Strain for 316H Stainless Steel
The minutes of the Code committee meetings indicated that 80% of stress to produce I % strain in a specified time was invoked as the third criterion for setting St and the time dependent values for Sm,, and (hat the stress values so determined controlled the allowable time-dependent allowable stress intensities for most of the temperature and time range covered in Table 1-14.4 for 3161-I stainless steel. The current rules stipulate IOO% of the average stress for 1% strain in a specific time. An inspection of the isochronous stress versus strain curves provided in Appendix T of BPV Ill-NH revealed that the time-dependent values in the isochronous curves at 1% strain were identical to the average values which were used for satisfying the 1% strain criterion for S,. It appeared as if the 80% factor was incorporated as a method to estimate the minimum stress for I % creep. Similar to the case for 304H stainless steel, the isochronous curves and the creep law that were used to calculate the time-dependent component of the isochronous curves were based on formulations by Blackburn in 1972 [51. Again, as with 304H stainless steel, considerably more creep data became available in the last 35 years, and several alternative creep laws were developed. However, it was beyond the scope of this assessment to introduce a replacement creep law. Rather, the assessment was limited to a denionstration that the time to 1% strain did not control the S, and Sm, values in the time-dependent range. To this end, the original and newer data were reviewed. Considerations regarding the development of the 1% strain criterion for 316H stainless steel were much the same as those identified in the section on 304H stainless steel discussed above and will not be repeated here.
The database used for the assessment consisted of 14 lots and 159 points. The initial approach was to analyze the data on the basis of the Larson-Miller parameter using the lot-centered method and the 3id order polynomial in log stress for f(S). The results of the regression analysis are shown in Figure 24. The average Lot Constant was found to be near 20.795 and the SEE was near 0.49 in log hours. The distribution of the residuals in log time is shown in Figure 25. A simple I order polynomial was also examined but offered no significant improvement.
3.4 Assessment of the Database for 316H Stainless Steel
Overall, the 316H stainless steel database is judged to be adequate for setting the allowable stress intensity values for BPV Ill-NH for temperatures to at least 649C (1200°F). Of course, additional test data covering very long times, especially above 649°C (1200°F) would help to eliminate some concerns regarding the extrapolation of data trends to limes beyond 100,000 hr. This is especially true for the estimation of values based on 1% strain and tertiary creep. The issues that could be addressed using the existing database are similar to those proposed for 3()4H stainless steel:
The creep curves produced for 316H stainless steel should be re-assembled and used to re-assess the creep laws. Many such curves were produced since the original collection that was used to produce the isochronous stress-strain curves in BPV Ill- NH. Several new models for creep have been advanced in connection with the Nuclear Systems Materials Handbook (NSMH), for example, and further study would improve the understanding of primary creep and tertiary creep. The information would he helpful in the development of continuum damage mechanics (CDM) models and creep-crack-growth models.
There is a need for a detailed review of the work that has been undertaken by NIMS to explain the lot-to-lot variability of the creep and rupture behavior. To some extent this variability has been attributed to the effects of “unspecified in the composition of the steels. The goal of the review would be to produce a restricted chemistry version of 3l6H stainless steel for the construction of critical nuclear components. The validity of the extrapolation of data to estimate the 500.000-hr stress allowables needed for the Generation IV reactors and the explanations offered for the fall-off in strength of the steels must he carefully considered.
The methods and procedures of data analysis for setting the allowahies for BPV 111- NH must be revisited and well-documented. Also, an assessment of procedures used in Europe. Japan and elsewhere should he reviewed with respect to the special requirements and criteria for setting stress allowables in ASME Ill-NH.
