ASME STP-NU-019-1:2009 pdf free download

ASME STP-NU-019-1:2009 pdf free download.VERIFICATION OF ALLOWABL E STRESSES IN ASME SECTION III SUBSECTION NH FOR GRADE 91 STEEL.
5 CHARACTERISTICS OF THE CREEP DATABASE FOR ALLOY GR 91
The database assembled for the verification of allowable stresses in ASME Ill-NH provides information bearing on the three criteria in ASME Ill-NH that relate to time-dependent stress limits:
time to 1% to strain (t1%); time to initiate tertiary creep (t); and time to rupture (tR). With respect to the time to t%, the bulk of the data were extracted from the U.S. database with additional data from the NIMS report [13]. The data in the U.S. database were entered as the time to 1% creep strain rather than time to 1% total strain. The NIMS database reported both time to 1% creep strain and time to 1% total strain. The ratio of these two times was found to be proportional to the applied stress. This ratio was used to convert the times in the U.S. database to times to 1% total strain. The distribution of data is shown in Figure 2. Altogether. a total of 312 values for creep were available. These were distributed from 450 to 7X0’C (R40 to 1435F) with times to 25,000 hours. Most data fell between 500 and 650’C (930 and 1200’F) at times below 5000 hours.
A substantial database for £1, exists within the European database [14]. These were distributed more- or-less evenly at 550, 600 and 650CC (1020, 1110 and 1200F) with a few short time data at 700C (1292T). The European data were not included in the database used to validate the ASME Ill-NH stress intensity values.
7.3 Stress-Rupture, tR
As outlined in earlier section on available sources for creep-rupture data, the correlation of stress- rupture data to predict the long-time strength of Gr 91 steel has been an on-going activity at an international level for decades. The undertakings have been largely in support for the use of Gr 91 steel in ASME I3P’ Codes Section I and VIII. ASME Piping Codes B31.l and B31.3 and corresponding overseas construction codes. The objective has been to estimate accurately the allowable stresses at the upper limit of the use temperature for Gr 91 steel. Many parametric procedures have been developed and compared but there remains no consensus as to which is best. Techniques to improve” the accuracy of long time estimations include ‘censoring” data by not using data for times less than 3000 hours [24]. region splitting by not using data produced at stresses above a fraction of the hot yield strength [25] and adding more parametric constants to the time- temperature-stress models [26]. However, it should he recognized that the criteria for setting S1 in Ill- NH are conservative relative to the criteria in ASME lI-D Table 1-100, so the OflUS to produce accurate estimates from the same database is not as demanding.
Data corresponding to rupture lives less than 100 hours were not used in the analyses. This lefi nearly 1600 data covering temperatures from 450 to 780C (840 to l435F). The Larson Miller global fit to these data is shown in Figure 16 (leIl) and lot-centered fit is shown in Figure 16 (right). One fit appeared to be as good as the other, although there was a four point difference in the optimized parametric constant: —26 for the global lit and —30 for the lot centered Iit The SEE values were similar: 0.333 in log time for the global fit and 0.345 in log time for the lot-centered fit. The distribution of residuals for the two fits was similar, and information is shown in Figure 17 for the lot centered model. The plots show how the residuals were distributed about zero. The distributions with temperature and stress arc shown in Figure 18. These distributions show no strong bias (Figures lXa and lXb). When plotted against the observed rupture lives, the residuals tended to move from a negative bias to a positive bias with increasing life (Figure 18c). Also, the U.S. data tended to exhibit.